hydrogeophysical methods at the test-site nauen ......2 basics and theory of used measurement...

Hydrogeophysical Methods at the Test-site Nauen -

Evaluation and Optimisation

Jeanette Goldbeck

Diplomarbeit January 2002

Technical University Berlin

Institute of Applied Geosciences

Department of Applied Geophysics

Ackerstraße 71 – 76

13355 Berlin

Content

Abstract 3

Introduction 4

1 The test-site Nauen 5

1.1 Location............................................................................................................................ 5

1.2 Regional geological background...................................................................................... 5

2 Basics and Theory of used measurement methods 6

2.1 DC Geoelectrics ............................................................................................................... 6

2.2 Ground Penetrating Radar (GPR) .................................................................................... 8

2.3 Refraction seismics ........................................................................................................ 10

2.4 Petrophysical properties ................................................................................................. 11

2.5 Nuclear Magnetic Resonance......................................................................................... 14

2.6 Electric conductivity and Archie exponents................................................................... 18

3 Field measurements and results 19

3.1 General information ....................................................................................................... 19

3.2 DC Geoelectrics ............................................................................................................. 19

3.3 Ground Penetrating Radar .............................................................................................. 25

3.4 Refraction seismics ........................................................................................................ 29

3.5 Well logging................................................................................................................... 30

3.6 Induced Polarisation....................................................................................................... 32

3.7 Surface Nuclear Magnetic Resonance (SNMR)............................................................. 33

1

4 Laboratory measurements and results 35

4.1 General information ....................................................................................................... 35

4.2 Grain Size Analysis ........................................................................................................ 35

4.3 Rock density and porosity .............................................................................................. 37

4.4 Specific internal surface ................................................................................................. 37

4.6 Electric conductivity and Archie exponents................................................................... 46

5 Interpretation 50

5.1 Hydrological stratigraphy............................................................................................... 50

5.2 Evaluation and effectiveness of used methods............................................................... 53

6 Conclusion 56

7 Outlook 57

Bibliography 58

Acknowledgements 59

Appendix 60

2

Abstract

3

Abstract

Hydrogeophysical Methods at the Test-site Nauen – Evaluation and Optimisation

Goldbeck, J., 2000, Diplomarbeit, Technical University Berlin

During the last few years a geophysical test-site to the northwest of Berlin was established by

the Department of Applied Geophysics of the Technical University Berlin in cooperation with

the Berlin Office of the Federal Institute for Geosciences and Natural Resources (BGR). Basic

geological and widely known hydrogeological conditions form a good fundament for testing

geophysical techniques.

Extensive measurements, using various geophysical techniques, have been carried out at the

test-site earlier. Within this thesis these were completed by additional surface geophysical

measurements such as 2D geoelectrics, Ground Penetrating Radar and refraction seismics. A

continuously cored and logged 60m borehole is available for purposes of calibration. Detailed

laboratory measurements of core samples took place to complement the field measurements.

Special attention was turned to petrophysical measurements such as determination of density

and porosity, laboratory NMR and determination of geoelectrical parameters through Spectral

Induced Polarisation.

By comparing laboratory specific values to borehole logs and surface geophysical data it was

analysed in what way a characterisation of the subsoil in regard to petrophysical and hydraulic

properties is possible. Furthermore an optimisation of results could be achieved by suitably

combining the various techniques.

Introduction

4

Introduction

For the last few years a site to the northwest of Berlin has been used be by members of this

department to test and evaluate different hydrogeophysical techniques. Based on these

measurements a fundamental geologic and hydrologic stratigraphy could be defined for this

test-site.

During the course of this thesis the following additional surface geophysical measurements

have been carried out to deepen the general geophysical information about the test-site:

Ground Penetrating Radar , 2D DC geoelectrics and refraction seismics. The objective was to

characterise the hydrologic structures above the first aquiclude.

These will be related to earlier results and can be calibrated with newly accessible borehole

data. Further on core samples are available for detailed laboratory investigations. In this

context special importance has been attached to hydrologically relevant petrophysical

parameters such as permeability and porosity which can be determined by laboratory Nuclear

Magnetic Resonance. This is a recent field of research for this department and the

transferability of laboratory parameters to respective field measurements shall be verified.

Additionally electric parameters such as conductivity, Archie parameter and formation factor

can be used to determine rock characteristics.

As a variety of techniques were used to obtain the different parameters, it will be evaluated in

what way they can be utilised to complement each other and the field measurements.

Limitations of these methods, in a hydrogeophysical sense, shall be pointed out and proposals

for optimisation will be made.

A short geographic and regional geologic background on the test-site will be given in

chapter 1. In the following section the basics of used techniques will be illustrated, theoretic

elaborations will nevertheless be restricted to direct applications. Chapters 3 and 4 display the

actual measured data from field and laboratory including a brief description of measurement

sequence and instrument as well as a short evaluation of each individual method. The

combination and evaluation of all results, based on the intention of this thesis, will take place

in chapter 5. Conclusions will be given in chapter 6 and an outlook to possible future projects

will be presented in chapter 7.

1 The test-site Nauen

5

1 The test-site Nauen

1.1 Location

The test-site Nauen is located to the

northwest of Berlin, about 20 km west of

Nauen. Directly to the south of the village

Barnewitz a field path branches off the main

road, reaching the edge of woods after about

300m.

test-site

The site, covering an area of roughly

60000m², consists mainly of an untilled

field surrounded by woods to the south and

east and bordered by the road to the west

and by the field track to the north.

The topography is overall plane, though

there are small unevenness’ and the terrain

rises for approximately 1m in the woods to

the east and south. Fig. 1.1. Map selection from topographic map: region

Barnewitz

1.2 Regional geological background

Due to 3 major ice advances during the last 650000 years, the Brandenburg geology consists

nearly solely of quaternary sediments.

The prequarternary set-up correlates to the eastern branch of the North German depression

including variscan blocks and marginal deposits of the Mesozoic.

Today’s structure and morphology though is predominated be the latest Weichsel Glacial

Period about 16 – 20000 years ago. It left glacial sediments and surface forms with slight

relief.

The region to the west and north of Berlin is subdivided into tabular plates and partially water

filled low grounds, which have been glaziofluvially buried. Valley sands and gravels form the

principal sedimentary structure for those filled morphologic depressions.

The test-site is situated at the southern brink of the Nauen plate. The transition from solid

glacial till in the north of the site to sandy material to the south is recognizable. The valley

sands form an excellent aquifer with a base of glacial till.

2 Basics and Theory of used measurement methods

6


2.1 DC Geoelectrics

In DC geoelectrics a direct current is applied to the subsoil and the resulting difference in

potential is measured.

There is a linear relation between generated current density j and electric field intensity E:

Ej ⋅= σ (2.1)

In an isotropic homogeneous medium the conductivity σ is a scalar. The reciprocal of σ is

called resistivity ρ.

With Ohm’s law the electrical resistance R can be calculated from electric current I and

voltage U as:

IUR = (2.2)

Additionally resistance can be determined from a samples’ material properties:

ALR ⋅= ρ (2.3)

where L is the samples’ length and A is the cross-section. Following equation (2.2) and (2.3)

the resistivity ρ can calculated as:

LA

IU

⋅=ρ (2.4)

If the point electrode delivering I is located at the surface of a homogeneous isotropic medium

(ρ=constant) the potential V can be calculated in dependence to distance r of the point

electrode (the return current electrode is at a great distance):

rIV

⋅⋅

=π

ρ2

(2.5)

When the distance between two current electrodes is finite, the potential at any surface point

N will be affected by both electrodes

⋅⋅

⋅=

BNAN rrIPV 112

)(πρ (2.6)

with rAN and rBN being the distances electrode A – point N and electrode B – point N.

When adding a second potential electrode at a point M the difference in potential between N

and M will be (M, N: potential electrodes; A, B: current electrodes)[Telford et al. 1990]:


7

+−−

⋅=−==∆

BNANBMAM rrrrINVMVUV 11112

)()(πρ (2.7)

Electrode positions and distances are summarized into the geometry factor k:

+−−=

BNANBMAM rrrrk 1111

21π

(2.8)

So resistivity ρ can be calculated from:

IUk ⋅=ρ (2.9)

Several different electrode arrays or spreads are known in today’s geophysics. Generally the

electrodes don’t have to be arranged in a certain geometry or linear spread (equation (2.7))

though interpretation becomes more and more complicated the more complex the electrode

layout is.

One of the most commonly used electrode arrays is the Wenner-spread where all electrodes

are spaced in line and equidistant. With unit electrode spacing a, equation (2.9) changes into:

IUa ⋅⋅= πρ 2 (2.10)

Fig. 2.1. Scheme of Wenner-section [RES2DINV Manual]

As the medium is not homogeneous under most geophysical conditions the measured apparent

resistivity is an integral value of individual layer or object resistivities. Through inversion and


8

modelling during data-processing this apparent resistivity can be counted back to original

resistivities or resistivity distributions.

Two different inversion programs where used to model resistivity data during this thesis:

RES2DINV and RESIX IP2DI.

RES2DINV is a so called “Smooth-Inversion” program based on the smoothness-constrained

least-squares method. The optimisation method basically tries to reduce the difference

between the calculated and measured apparent resistivity values by adjusting the resistivity of

the model blocks. A measure of this difference is given by the root-mean-squared (RMS)

error. However the model with the lowest possible RMS error can sometimes show large and

unrealistic variations in the model resistivity values and might not always be the "best" model

from a geological perspective. The users influence to the inversion routine is minimal which

is the major drawback of that program. On the other hand RES2DINV enables a quick first

overview of the acquired dataset and it’s resistivity distribution.

RESIX IP2DI is block-inversion program based “ridge-regression” which is a variation of

least square inversion. The subsoil is subdivided into discrete cells whose form and

distribution can be influenced directly by the user. This default distribution is used for

forward modelling. As the main difference to RES2DINV the user can now influence the

inversion routine directly by restricting or enabling the different body or layer parameters.

Higher RMS errors from RESIX IP2DI are due to threshold values which are difficult to

model exactly.

2.2 Ground Penetrating Radar (GPR)

GPR or Georadar is an electromagnetic method based on radiation and reception of short

electromagnetic pulses.

Electromagnetic pulses of short wavelengths are emitted in to the subsoil where they are

reflected or dispersed from boundaries or objects underground back to the receiver.

Propagation velocities depend on the dielectric constant ε of the medium, while the resistivity

ρ controls damping and therefore the penetration depth.

Electromagnetic wave propagation is based on the Maxwell equations from which the wave

equations for electric and magnetic fields can be derived as:

Et

Et

E rrr

rrr2

2

0002

δδεεµµ

δδσµµ +=∇ (2.11)


9

Ht

Ht

H rrr

rrr2

2

0002

δδεεµµ

δδσµµ +=∇ (2.12)

To solve these equations plane-waves, spreading in z direction, may be used:

)(0

kztieEE +⋅= ωrr (2.13)

k is called complex wave number and can be calculated from:

σωµµωεεµµ 02

002

rrr ik −= (2.14)

or

βα ik −= (2.15)

If eq.(2.13) is solves with this approach, then:

)(0

aztiz eeEE +⋅⋅= ωβrr (2.16)

The result is a damping term (involving β) and a wave propagation term (involving α).

Additionally it can be stated that damping is frequency and resistivity dependent. The later is

the reason for poor resolution in or below high conductivity layers (e.g. aquifers).

From the wave equation and it’s solution, α and β can be derived directly as:

+

+⋅= 11

2

2

0

00ωεε

σεεµµωα

r

rr (2.17)

−

+⋅= 11

2

2

0

00ωεε

σεεµµωβ

r

rr (2.18)

In the so-called radar case, if measuring with high frequencies, conductivity σ is very low

compared to εω. In that case eq. 2.17 and 2.18 can be developed to:

00 εεµµωα rr= (2.19)

0

02 εε

µµσβr

r= (2.20)

For non-magnetic media (µr=1) and ε0µ0=c (velocity of light) phase velocities merge to

propagation velocities:

vcvrr

p ====εεεµα

ω

00

1 (2.21)


10

2.3 Refraction seismics

Like most geophysical methods refraction seismics aims for identifying structures and

physical properties of the underground from above ground level. In this case seismic waves

refracted along boundaries are used. Refracted waves traverse the underground primarily

horizontal and are therefore observed farther from the source point then for instance reflected

seismic waves.

Wave propagation is based on Snell’s law:

.sinsin 2

2

1

1 consti

vi

v== (2.22)

This means there is a critical

angle ic where the angle of

emergence will be 90°, so that

the refracted wave will propagate

along the boundary in the lower

layer, where head waves are

generated. Based on the

Huygens’ principle every point

on a wavefront can be regarded

as a new source of waves. These

will superpose to form a new

wavefront arriving at the surface

under the critical angle. At the

crossover-distance the refracted

wave will overtake the direct

ground wave. This can be seen as

a break in traveltime curves. Fig.2.2 Timetravel curves for reflected and refrac ted seismic waves

Seismic velocities can be determined successively from the first arrivals of the seismogram,

the slope is inversely proportional to the traversed layer’s velocity.


11

2.4 Petrophysical properties

2.4.1 Density

The quotient of a samples mass m and it’s volume V is defined as the samples density ρ. The

SI-unit of density is kg m-3, but g cm-3 is more commonly used.

Additionally it is necessary to distinguish between different densities that describe the various

aspects of the material [Schön 1996]:

- bulk density: mean density including all components of the analysed sample volume

- individual rock component density: in sandstones e.g. quartz

- mean density of the pore filling (fluid or gas)

- mean density of the solid matrix with out pores.

2.4.2 Porosity and Saturation

For any material the porosity Φ describes the volume fraction of voids, e.g. pores, relative to

the complete sample volume:

sample

poreVV

=Φ (2.23)

To further characterize the pores the saturation S related to the pore fluid is defined as the

ratio of the fluid volume to the pore volume [Schön 1996].

Porosity is especially influenced by grain properties as grain size, sorting and grain shape.

0.2

0.3

0.4

0.5

0.6

0.01 0.1 1 10

medium grain size [mm]

med

ium

por

osity

Fig. 2.3. Medium porosity versus grain size for Nauen samples


12

Porosity will increase with increasing degrees of sorting and shows a tendency to decrease

with growing grain size (see figure 2.3.). Different grain shapes change the packing structure

of materials and consequently it’s porosity.

2.4.3 Specific Internal Surface

The specific internal surface forms the interface of pores and matrix. Similar to the

description of density it is advisable to distinguish between different types of specific internal

surfaces, relating the surface area of pores to:

- the total rock volume Stot

- the pore volume Spor

- the volume of the solid matrix Sm

- the mass of the dry rock Sma [Schön 1996]

The SI unit of Stot, Spor and Sm is m2 m-3=m-1. More commonly µm-1 is used. The SI unit of

Sma is m2 g-1.

2.4.4 Permeability

The fluid flow through porous media can be described using the permeability k to characterise

the material and the fluid.

puk

∇⋅−= η (2.24)

with u: volume flow density, η: dynamic viscosity of the fluid, p: fluid pressure

Permeability depends on porosity and pore space geometries. The SI unit is m2=10-12d.

If the streaming fluid is water the hydraulic conductivity coefficient kf can be used instead,

which is defined only for the constant viscosity and density of the medium water. The relation

between permeability k and hydraulic conductivity kf is as follows: 1−⋅⋅⋅= ηρ gkk flf (2.25)

where ρfl is the density of the streaming fluid (rH2O=1 g cm-3), g is the gravity acceleration and

η is the dynamic viscosity of water (η=10.02kg m-1 s-1 (20°C)). Usually it is sufficient to use

the relation that a permeability of 105d corresponds to a kf of 1m s-1.


13

Different estimations exist to calculate permabilities or hydraulic conductivities. For this

thesis 4 of them were used:

- from grain size analysis the kf value can be calculated by using Beyer’s equation: 2

10dCk f ⋅= (2.26)

where d10 is the grain size (mm) of 10 percentile from the grain size distribution and

the coefficient C is a function of U=d60/d10 [Hölting 1996]

- the PARIS equation is a modification of the Kozeny-Carman equation [Schön 1996]

and links permeability to specific internal surfaces and formation factors derived from

electrical measurements:

1085.3

1475][porSF

dk⋅

⋅= (2.27)

- to estimate permeability from NMR decay times and porosity the following equation

can be used [Kenyon, 1997]: 42][][ Φ⋅⋅= imsTCmdk i=1,2 (2.28)

The factor C is a rock parameter which is about 4.5 for sediments and sedimentary

rocks.

- instead of porosity in the above equation the formation factor calculated from

electrical measurements can be used if Φ4 is replaced by an approximation of F-2. 22][][ −⋅⋅= FmsTCmdk i i=1,2 (2.29)


14

2.5 Nuclear Magnetic Resonance

2.5.1 Basic concept

Magnetic resonance is a phenomenon related to any

magnetic system possessing an angular momentum.

Elementary particles such as protons and neutrons

possess a spin. Spin comes in multiples of ½ and can be

positive or negative. Unpaired protons, neutrons and

electrons all possess a spin of ½. Two or more particles

with spins having opposite signs can pair up to eliminate

the observable manifestations of spin. In nuclear

magnetic resonance, it is unpaired nuclear spins that are

of importance.

A nucleus rotating about it’s spin axis generates a

magnetisation along the same axis. If it is placed in an

external static magnetic field B0 having a different

orientation, the nucleus will precess along the orientation axis of B0 with Lamor frequency ω0.

B0

spinning

proton

Fig. 2.4. Precessing nucleus

00 B⋅= γω (2.30)

γ is the gyromagnetic ratio, characteristic to each isotope. For hydrogen, γ= 42.58 MHz / T.

There are two different energy states the nucleus can align with: a stable, low energy state

parallel to B0 and an instable high energy state antiparallel to B0. The particle can undergo a

transition between the two energy states by the absorption of a photon. A particle in the lower

energy state absorbs a photon and ends up in the upper energy state. The energy of this photon

must exactly match the energy difference between the two states and is related to the

frequency ω0 by Plank's constant (h = 6.626x10-34 J s).

hE ⋅= 0ω (2.31)

2.5.2 Decay times

At equilibrium, the net magnetisation vector lies along the direction of the applied magnetic

field B0 and is called the equilibrium magnetisation M0. In this configuration, the z component

of magnetisation MZ equals M0. MZ is referred to as the longitudinal magnetisation.

It is possible to change the net magnetisation by exposing the nuclear spin system to energy of

a frequency equal to the energy difference between the spin states. The time constant which


15

describes how MZ returns to its equilibrium value is called the spin lattice relaxation time

(T1). The equation governing this behaviour as a function of the time t after its displacement

is:

)1( 1/0

Ttz eMM −−= (2.32)

T1 is therefore defined as the time required to change the z component of magnetisation by a

factor of e.

If the net magnetisation is placed in the xy plane it will rotate about the z axis at a frequency

equal to the frequency of the photon which would cause a transition between the two energy

levels of the spin. The time constant which describes the return to equilibrium of the

transverse magnetisation, MXY, is called the spin-spin relaxation time, T2.

2/0

Ttxyxy eMM −⋅= (2.33)

T2 is always less than or equal to T1.

2.5.3 Pulse sequences

Three pulse sequences were uses for this thesis: FID to obtain T2*, CPMG for T2 and

INVREC for T1.

For the FID (Free Induction Decay) pulse sequence, net magnetisation is rotated down into

the xy plane with a 90o radio frequency pulse (RF). The net magnetisation vector begins to

precess about the z axis. After the pulse the relaxation process starts and in the coil producing

the pulse a current is induced due to the precessing moving charges. The received signal is

called FID and it’s amplitude is proportional to the number of protons causing the signal.

Fig. 2.5. Signal versus time for FID sequence Fig. 2.6. Signal versus time for spin-echo pulse sequence


16

Another commonly used pulse sequence is the spin-echo pulse sequence (e.g. CPMG). Here a

90o pulse is first applied to the spin system. The 90o degree pulse rotates the magnetisation

down into the xy plane. At some point in time after the 90o pulse, a 180o pulse is applied. This

pulse rotates the magnetisation by 180o about the x axis and causes the magnetisation to

produce a signal called an echo.

An inversion recovery pulse sequence (INVREC) can also be used to record an NMR

spectrum. In this sequence, a 180o pulse is first applied. This rotates the net magnetisation

down to the -z axis. The magnetisation undergoes spin-lattice relaxation and returns toward its

equilibrium position along the +z axis. Before it reaches equilibrium, a 90o pulse is applied

which rotates the longitudinal magnetisation into the xy plane. Once magnetisation is present

in the xy plane it rotates about the z axis and gives a FID. The amplitude of the FID is

correlated to the time difference between the 180° and the 90° pulses.

2.5.4 Petrophysical parameters from NMR relaxation

The amplitude of an NMR signal is proportional to the number of the signal producing

protons. Referring to a calibration sample with known water content, the water content of any

sample can be calculated from:

ncalibratio

onOcalibratiHsampleamplitude

VamplitudeOsampleHV 2

2

⋅= (2.34)

When measured with full saturation and the sample’s volume is known, the sample’s porosity

can be deduced from:

lV

V

tota

por=Φ (2.35)

This porosity is sometimes called NMR or effective porosity. It depends on measurement

resolution especially for very low relaxation times. In laboratory NMR studies capillary water

will be registered and can be distinguished from free / mobile water by characteristic (material

related) cutoff values.

In addition to porosity and internal surface, pore radii can be used to characterise pore

structure. Due to surface relaxation effects water inside pores will relax a lot faster than for

instance water in a test-tube [Kenyon 1997]. The surface / volume ratio of a specific pore is

proportional to the relaxation time constant, small pores will result in faster relaxation thus

lower decay times. The distribution of pore radii can therefore be demonstrated as summation


17

of single pore size relaxation constants. These can be displayed as a discrete spectrum, where

the amplitude of each pore (~size) is assigned to it’s respective relaxation time. Nevertheless a

direct conversion of relaxation times to exact pore sizes has not been attempted for this thesis.

Additionally permeability can be estimated from decay times. Various assumptions exist, 2 of

the were used for this thesis. They are discussed in more detail in chapter 2.4.4.


18

2.6 Electric conductivity and Archie exponents

Conductivity is a physical property of rocks and sediments and can be explained by 3

individual mechanisms which add linearly:

- matrix or metallic conductivity σm

- electrolytic conductivity σe

- excess or interface conductivity σq.

The matrix conductivity of most minerals except metals is small. Mean values are in the range

of 10-14 to 10-10Sm-1[Knödel 1997]. That means σm can mostly be neglected.

The electrolytic conductivity of saturated rocks is controlled by the electrical conductivity of

the fluid i.e. ion concentration, by the amount of fluid i.e. porosity and saturation and by the

connectivity of the pores i.e. pore structure.

Interface conductivity results mainly from the electric bilayer at the grain boundaries and

includes all conductivity mechanisms that are not matrix or electrolytic dependent [Knödel

1997]. This is a special characteristic of clays or clay containing sediments.

Conductivity of saturated rocks is well described by the empirical equation of Archie:

qwnm SS σσσ ν ⋅+⋅⋅Φ= (2.36)

with: Φ: porosity, S: degree of water saturation (S=Vwater/Vpore), σw: conductivity of the pore

fluid, σq: interface conductivity, n, ν: saturation exponents and m: Archie exponent.

An additional parameter can be derived from Archie’s 1. equation:

σσ w

mF =

Φ=

1 (2.37)

F is the formation resistivity factor and describes the conductivity change related to the pore

fluid as a result of the presence of a non-conductive matrix. [Schön 1996]

3 Field measurements and results

19


3.1 General information

The geophysical field measurements were carried out during fall 2001. The following

measuring techniques were used: DC geoelectrics, GP radar and refraction seismics. Most

profiles were aligned on the main profile line (MPL) that crosses the test-site from northeast

to southwest with the origin located to the far northeast. The borehole is located on the MPL

as well, just before it enters the woods to the southwest.

To provide more detailed information about the test-site additional earlier measurements will

be considered. These have been carried out by other members of the department before this

thesis: Induced Polarisation measurements and Surface Nuclear Magnetic Resonance.

Furthermore borehole logs are available as well.

field

woods

grassland

Main profile

GP radar

DC geoelectrics

borehole

seismics

IP

SNMR

0-100-200-300

0

-100

-200

-300

Distance [m]

Dis

tanc

e [m

]

IP3

IP2

IP1

B5

B8

B7

B2

B6

Fig. 3.1. Map of field measurements at Nauen


20

3.2 DC Geoelectrics

3.2.1 Measuring instrument and method

Measuring instrument during the field work in Nauen was a “Campus Tigre” of the University

of Kassel. It is a multi-electrode resistivity equipment for up to 64 electrodes. For controlling

and data recording a laptop was used.

Fig. 3.2. Outline of geoelectric profiles at the test-site

The used electrode array was a Wenner-spread with an electrode spacing of 3.5m

main3 and 5m for main1 and main6. For profile main1 a roll-along method was u

initial section only a part of the electrodes is moved along the profile and the nex

include only those data points not already covered by the first. As the first step o

the main section and all associated roll-alongs are merged into one file which

processed.

The complete inversion results including measured apparent resistivities can be

appendix.

N
(a=5m)
(a=5m)

(a=3.5m)

for profile

sed: after the

t section will

f processing

can now be

found at the


21

3.2.2 Results

Profile main1

Main1 is the longest of the geoelectric profiles and extends nearly completed across the test-

site. It starts about 80m southwest of the borehole (see figure 3.1.) in the woods and has a

length of 314m with a spacing of 5m. main6 borehole

Fig. 3.3. Smooth inversion results main1

The RES2DINV inversion results show a quite homogenous resistivity distributio

Principally a two to three layered underground can be identified.

The first layer from the ground level to about 2m depth is a high resistivity layer wi

apparent resistivities of approximately 1000 to 4000Ωm. This is the vadose zone above th

first aquifer. The extremely high value from the uttermost southwest to –260m are due to th

high contact resistances in the woods. The coupling of the electrodes to the soil was high

difficult in that area, as the ground is covered with about 0.5 to 1m of underwood.

Below that is a seemingly uniform layer of about 100 to 250Ωm where the resistivity is n

varying much. This can be identified as the first aquifer.

From other measurements, geologic background and borehole measurements it is known th

the first aquiclude, consisting of mainly of silts, should follow the aquifer at a depth of abo

20m near the borehole. Another drop in resistivity can be seen in the lowest levels though th

data basis is to thin to assume a third layer from these measurement alone.

With this background information as a starting model an inversion using RESIX IP2DI w

attempted. During this inversion different parameters were fixed or released in alternation

gain a geological reasonable result. First all parameters were unlocked to get a gener

overview of the resistivity range and the layer distribution. Then the individual resistiviti

were fixed so the error balancing was achieved only through adjusting the position of th

layers.

NE
SW
n.

th

e

e

ly

ot

at

ut

e

as

to

al

es

e


22

As a result the resistivities were set to 3500 Ωm for the weathered zone and 300 Ωm for the

aquifer. The seeming drop of the groundwater table in the woods to the southwest is explained

by a rise in topography of about 1 to 1.5m which was not considered during inversion.

distance [m]

0

20

10

dept

h [m

]

3500 mΩ

SW NE

300 mΩ

90 mΩ

borehole main6

Fig. 3.4.Block inversion model main1, RMS error 24 %

Below 20m again the aquiclude is modelled, though as in the RES2DINV model the data is

stretched quite thin in the lowest levels and the dataset contains no information about the

lower boundary areas.

Profile main3

Main3 is the shortest profile located at the northeast end of the test site (see figure 3.1). The

complete length was 171m with a spacing of 3.5m. The reason for this special spread was the

preliminary geologic information about this area, so a better resolution was wanted for.

SW NE


The change from main1 is quite obvious. Although the topmost layer seems identical to the

weathered zone of main1 in resistivity and depth, below that an identification is not so simple.

There seems to be a very low resistivity layer coming from the northeast, which appears to

drop in the middle of the profile and then rises again at the end. The RMS value for this


23

model is quite low nevertheless it doesn’t seem very reasonable. From the measured data

(figure 3.6) the rise to the southwest is not evident.

SW NE

Fig. 3.6. Apparent resistivity section main3

There it rather seems like the low resistivity layer has an outcrop to the northeast which is

coherent with the already mentioned preliminary information. So the effects to the southwest

are probably artefacts from the inversion process of RES2DINV.

The resistivities from the inversion match the ones from profile main1.

Based on the resistivity values from for the main1-model a block-model was calculated for

main3. As the to profiles lap for about 100m the resistivities should be the same.

distance [m]

3500 mΩ

300 mΩ

90 mΩ

0

20

10

dept

h [m

]

SW NEmain6

Fig. 3.7. Block inversion model main3, RMS error 22 %

The outcrop now is quite obvious, the resistivities match those in main1. It is not explicit from

this model if the outcrop comes though the weathered zone, there are not enough data points

at the far sides. The resistivity of 90Ωm corresponds to the implied aquiclude / silt layer in

main1 but this time it can be modelled quite well even at the lowest level to the southwest.

The lapping area of both models matches perfectly.

Profile main6

Main6 is a cross-profile to main1 and main6 (see figure 3.1). The spacing was 5m, full profile

length is 245m. Main6 crosses the MPL at a length of –100m.


24

he RES2DINV section of that profile resembles main1 a lot though it is not as undisturbed.

The RESIX IP2DI model was rather difficult to obtain and took longer than both of the other

NW SE


T

Especially along the edges the data seems to be a bit unsettled which was obvious during the

field work already, where it resulted in number of necessary repeated measurements. The

measured resistivities again match those of the other profiles with 1000-6000Ωm for the dry

cover and about 50-150Ωm for the lower aquifer. From the RES2DINV model it is possible to

assume that there is another outcrop or at least upward arching of the silt layer. This can be

observed from the measured apparent resistivity section also. distamain1 / main3

0

20

10

dept

h [m

]

3500 mΩ

350 mΩ

70 mΩ

Fig. 3.9. Block inversion model main6, RMS error 28 %

NWnce [m]

SE

models, probably due to the higher noise level in this section. Most of the calculated models

were not reasonable or simply geological nonsense, additionally the resistivity difference

from aquifer to aquiclude could not be modelled without accepting high RMS errors. Figure

3.9 was the only reasonable model for the given conditions, although the resistivities don’t

match to well and the RMS error is above 25 %.


3.3 Ground Penetrating Radar


For radar field measurements a RAMAC / GPR instrument of MALA Geoscience was used.

The instrument consists of a processing unit, a receiver and a transmitter antenna and a laptop

for data handling and storing.

For profile measurements the antennas

are arranged at a constant offset. By

moving slowly along the profile

individual traces for each location are

recorded, which can be displayed as

radargrams. They present two-way

traveltimes depending on the location

along the line.

T T T TR R R R

tProfilfortschritprofile line

Fig. 3.10. Scheme of travel-ways for radar profile

To determine propagation velocities the common midpoint (CMP) method is used: receiver

and transmitter are moved apart from a mutual midpoint thus constantly increasing

traveltimes of the transmitted wave in

ε1

ε2

Fig. 3.11. CMP

The main prof

woods to the

0.1m. Due to

restarted from

disturbance of

the same datu

both CMPs a

processed and

For processin

processing flo

were applied.

at balancing t

R

layout

ile was again laid out a

southwest. Antenna f

problems with the equ

this location at a late

layers at this point, w

m point 25m to the no

ppear to be equal in

will be presented here

g and presentation th

w was applied: the fil

The last “technical” co

echnical features of th

T

25

the medium. Velocities for each layer

can be determined based on reflection

curves.

long the MPL (see figure 3.1.), starting about 30m in the

requency was 200MHz and traces were recorded every

ipment the profile had to be interrupted at –100m, it was

r time and both files were merged. The result is a slight

hich will not be interpreted. Two CMPs were realised for

rtheast of the borehole along the MPL. As the results for

data range and content, only one of them was further

.

e program REFLEX was used. The following general

es were start-time corrected, then declipping and dewow

rrection was to subtract the DC shift. These 4 steps aim

e instrument. After this a frequency filter was applied to


26

suppress unwanted frequency bands. The last processing step was a gain function. To improve

data quality a background removal was attempted, but this proved to be non effective.

Fig. 3.12. Radar CMP measurement Nauen

3.3.1 Results

First the CMP shall be discussed (figure 3.12).

The dent in the direct ground and the air wave is due to irregular movement (trace increment)

during the measurement. Nevertheless the velocities can be adapted to the front part (0 to

20m) of the CMP. Below 200ns the reflections from trees of the nearby wood is very

noticeable. 4 decreasing velocities were adapted, though in my opinion only the first is an

actual boundary, that is the ground water table. All other modelled boundaries rather appear to

“mirror” each other: all modelled layers have the same thickness and velocity difference.

They are in my opinion due to multiples of the ground water table. Overall velocities seem

too low. In my opinion this CMP is rather featureless, maybe because of the wet conditions on

that day. The modelled velocities were not used for depth adjustment of the main profile.

The main radar profile line is displayed in figure 3.13..

The most prominent feature is obviously the descending reflector to the right of the

radargram. For the first about 200ns the descent is quite steep, after that the slope is more

flattened with little depressions along the line. This reflector is interpreted as the outcropping

glacial till.

Apart from this reflector the appearance below the groundwater-table (20ns) is homogeneous,

the layers are horizontally bedded. At about 180ns a “layer” appears to have rather stronger


27

echoes than the upper horizons. It can be traced along the complete line up to the outcropping

till. From the stratigraphic profile at the borehole (-265m) a gravel layer is known from the

depth of about 12 to 14 m. The transition from overlaying fine sands into the gravel should

produce a rather strong reflector, due to the change in material consistency. That this will not

be as smooth a layer as e.g. the till’s upper boundary is obvious.

To the left of the profile (-270 to –230m) a kind of lineament with a ~45° angle appears. As

the profile run into the woods at –268m to the southwest, these can be interpreted as

reflections from the trees.

For time-depth conversion a medium velocity of 0.15m/ns was used. This value was chosen to

adjust the groundwater table to a depth of about 1.5 to 2m.


28

Fig. 3.13. Radargram for the MPL Nauen


29

3.4 Refraction seismics

3.4.1 Measurement instrument and method

For seismic measurements a 12 channel Bison instrument was used. Geophone spacing was

2m along the Main Profile Line. Shots were arranged every 12m, the first shotpoint was set

1m in front of the first geophone. The used source was an accelerated weight drop mounted

on a trailer.

Resulting files were first merged into shot-point-based files, then evaluated and presented

with REFLEX.

3.4.2 Results

Although the measured data appeared to be of good quality during the field day, it wasn’t

possible to evaluate the time-travel curves (they will therefore not be displayed here).

From preliminary information it is known that the velocity of the weathered layer is very low,

with about 150 to 500m/s sometimes even lower than sound velocity. That’s why the

accelerated weight drop was used as a source, to get a distinct signal and improve the signal-

noise ratio. This proved to be unsuccessful, rather the noise seems to have been increased

even more. A velocity analysis was not possible, because shot and reverse shot travel-times

could not be balanced.

From the different seismograms (example figure 3.14.) little information could be gained: as

already known velocities of the first layer (weathered zone) vary between 200 to 450 m/s. For

the second layer velocities of 1000 to 3000m/s can be presumed.

Fig. 3.14. Example seismogram for shot point 191 / Nauen


30

3.5 Well logging

3.5.1 General information

In January 2001 a 60m bore hole was drilled by an external company at the end of the Main

Profile Line (see figure 3.1). A few days after drilling extensive borehole logs where

recorded, thereafter the well was supported and additional control logs were measured (see

[Richter, 2000]).

The following logging techniques were used:

- Gamma-Ray-Log (before and after support)

- GG-Log (Dual Spacing) (before and after support)

- NN-Log (Dual Spacing) (before and after support)

- FEL (before support)

- Short / Long Normal (Resistivity) (before support)

- Caliber (before support)

- Sal-Temp-Log (before and after support)

- Microlog2 (after support)

- Microcaliber (after support)

Some of those logs will be discussed in detail, all of them are displayed at the appendix.

3.5.2 Resistivity Logs

Normal logs are used for resistivity logging, the configuration being similar to those in

surface resistivity methods (dipole-dipole-configuration). For a short normal log (sondes 16

inch apart) the measured apparent resistivity depends mainly on the beds close to the hole

wall and are influences by the mud in the borehole and penetration of drilling fluid into the

formation. The long normal (sondes 64 inch apart) measures intermediate resistivities,

containing information about the invaded zone resistivities and true formation resistivities.

The third used electric log is a focused current log (FEL). A sharply focused current is used to

measure resistivity with a resolution of few centimetres. The depth of penetration is very

shallow, the device is used to measure the resistivity of the flushed zone.

The normal logs are quite inconspicuous, dropping to a level of about 150 to 200Ωm for long

and 80 to 100Ωm for short below the first aquifer. The only major feature is in my opinion the


31

rise in resistivity for the short normal at about 20m. This can be probably attributed to the

change in formation from sands to silts. This anomaly is not very prominent though, as it is

covered by the influence of the drilling fluid and mud.

The resistivity distribution for the FEL log is more distinct, with a few noticeable maxima and

jumps. The sand-silt transition at the bottom of the upper aquifer is 2m early and overall the

distribution seems a little erratic.

3.5.3 Gamma Ray Logs

For sediments γ logs reflect mainly shale content as natural radioactive elements tend to

concentrate in clays and shales. γ ray logs are calibrated in API units, shales having average

values of 100API. γ ray logs are used quantitatively to estimate shale / clay content.

Gamma-gamma logs are known as density logs and may be used to determine porosity if

matrix and formation fluid density are known. The detected γ ray intensity is an exponential

function of rock density, with a maximum depth of investigation of about 15cm. [Telford]

The measured gamma ray log shows, as was expected, strong reactions to the different layers

with high clay content. Additionally it displays a maximum for 12-14m which was

interpreted, by the driller, as a mica containing level. From the stratigraphic profile this can

not be verified. Rather it contains a higher fraction of fine material additional to the gravel

fraction, thus also explaining the observed higher bulk

densities. GWT

aquifer

aquiclude

aquifer

aquiclude

aquifer

The resolution of even smallest layers of silt is very good, in

particular notable for the about 1m thick coarse silt layer at 8

– 9m or the small interbedding of silt at 42.5m.

3.5.4 Main stratigraphy from logging

Based on the borehole logs a basic, overall hydrological

stratigraphy was obtained as follows:

- 0 - 21m: aquifer

- 21 – 31m: aquiclude

- 31 – 39m: semi-permeable aquifer

- 39 – 47m: aquiclude

- 47 – 60m: aquifer Fig. 3.15. Hydrological stratigraphy

for borehole Nauen


32

3.6 Induced Polarisation

During an earlier thesis [Hertrich, 2000]

extended IP measurements were conducted for

3 different locations on and near the test-site.

The first is located in the field to the northeast

of the actual site thus measured results should

be mainly influenced by the solid glacial till.

The second point is right on the field track in

the transition area from till to fluvial sands

and the third is situated farther inside the

measurement field (see figure 3.1).

Figure 3.16. displays inversion results for

resistivity and phase. The complete dataset

can be found at the appendix.

At the southernmost location IP1 the expected

3 layered case can be observed: to a depth of

2m resistivities are very high (~3000Ωm)

which resembles DC geoelectric results

(chapter 3.2) from this campaign, clearly

identifiable a the vadose zone. Below that, to a

depth of about 8m, follows a 300Ωm layer.

Subsequently a very low resistivity layer

(~100Ωm) was modelled. The phase

adjustment is rather poor for this assumed

case, though the general trend matches the

data. The second layer can be identified as the

aquifer followed by the glacial till.

IP3 is located to outside of the actual test-site and is o

Resistivities are below 100Ωm, but surprisingly a segm

a depth of ~6m another drop in resistivity occurs.

Fig. 3.16. IP models for resistivty (left) and

phase (right) / Nauen

verall implicat

entation is vis

The reason fo

IP3

IP2

IP1

ed with undisturbed till.

ible. Near the surface to

r his distribution is not


33

completely clear. It was assumed, that this implied layer is generated by 3 dimensional

influences due to the large spread. I find this rather hard to accept.

For the upper layers the electrode spread is small thus reducing effects from 3D influences.

The measurement location is about 50m remote of any anticipated changes in stratigraphy and

should therefore display the natural habit of the subsoil to a depth of at least 20m. For lower

depths the model concurs with the properties of the till in the test-site itself and if

discrepancies should appear due to 3D effects I would expect them in those levels.

It is difficult to explain the lower resistivities. The phases match my expectations for loosened

ground due to cultivation. From comparison to laboratory measurement it is known that

phases tend to rise with decreasing consolidation. A depth of 6m may be a bit low but isn’t

totally out of range. Loosening due to constant cultivation may well reach a few meters.

At the middle location IP2 influences of the northern exposed till and of the fluvial sediments

mix. The result is an implied 3 layer case with abrupt resistivity changes for the first 3m.

Below that again the undisturbed till is displayed with resistivities of ~100Ωm and phases of –

0.2 degrees.

3.7 Surface Nuclear Magnetic Resonance (SNMR)

5 SNMR soundings have been conducted along a secondary profile during an earlier

measuring campaign [Hertrich, 2000]. They were arranged linearly with a spacing of 25m

(see figure 3.1).

Similarly to the measurement principle for laboratory NMR an external alternating field with

the Lamor frequency is generated. For surface applications this is realised with an antenna

loop on the ground which, after turning of the external field, will measure the relaxation

signal as well. Again the initial amplitude correlates to the number of excited protons i.e. the

water content and decay times relate to mean pore sizes. Measured ambient noise is used to

examine signal quality and the phase shift between incitement and measured signal contains

information about subsurface electrical conductivity [Hertrich]. In SNMR, contrary to

laboratory NMR, irreducible water will not be detected due to measurement restriction in

early response times. Water content will therefore include only mobile water.

Figure 3.17. displays the water content (from amplitude) and decay times for all 5 locations.

Complete datasets can be found at the appendix.


34

60

50

40

30

20

10

00 10 20 30 40

de

pth

[m]

watercontent [%]

60

50

40

30

20

10

00 100 200 300 400

de

pth

[m]

decaytime[ms]

60

50

40

30

20

10

00 10 20 30 40

de

pth

[m]

watercontent [%]

60

50

40

30

20

10

00 100 200 300 400

de

pth

[m]

decaytime[ms]

60

50

40

30

20

10

00 10 20 30 40

de

pth

[m]

watercontent [%]

60

50

40

30

20

10

00 100 200 300 400

de

pth

[m]

decaytime[ms]

60

50

40

30

20

10

00 10 20 30 40

de

pth

[m]

watercontent [%]

60

50

40

30

20

10

00 100 200 300 400

de

pth

[m]

decaytime[ms]

60

50

40

30

20

10

00 10 20 30 4

de

pth

[m]

watercontent [%]

60

50

40

30

20

10

00 100 200 300 400

de

pth

[m]

decaytime[ms]

S N

0

B5 B8 B2 B7 B6

Fig. 3.17. Results of SNMR for 5 locations at the test-site Nauen

Inversions show a clear separation of vadose and saturated zone for the upper aquifer for all

soundings. Though the vertical resolution is insufficient the till interface for B8 and B7 can

probably be expected at 15m depth, recognisable from the drop in “free” water content. In

depths below 30m, water volume increases again pointing to the second aquifer. A more

specific segmentation based on water content below that level is not possible because the

vertical resolution is not high enough. From decay times more information can be gained. For

B8 and B7 decay times drop from about 300ms to 100ms in a depth of about 10 to 15m. This

is probably due to the stratigraphic change from medium sands to finer material including silt.

Below the till layer, which at this location should be situated in 10-15m depth, decay times

first rise and then decrease again, pointing to first a sandier (15-30m) and then finer facies

(below 30m). The low free water content is coherent with information from drilling, where a

low-permeable layer is presumed. Below 30m the vertical resolution for water content is to

low to gain additional information.

Inversions show a clear separation of vadose and saturated zone for the upper aquifer for all

soundings. Though the vertical resolution is insufficient the till interface for B8 and B7 can

probably be expected at 15m depth, recognisable from the drop in “free” water content. In

depths below 30m, water volume increases again pointing to the second aquifer. A more

specific segmentation based on water content below that level is not possible because the

vertical resolution is not high enough. From decay times more information can be gained. For

B8 and B7 decay times drop from about 300ms to 100ms in a depth of about 10 to 15m. This

is probably due to the stratigraphic change from medium sands to finer material including silt.

Below the till layer, which at this location should be situated in 10-15m depth, decay times

first rise and then decrease again, pointing to first a sandier (15-30m) and then finer facies

(below 30m). The low free water content is coherent with information from drilling, where a

low-permeable layer is presumed. Below 30m the vertical resolution for water content is to

low to gain additional information.

B6 and B5 are difficult to interpret as the data contradicts the expected structural information.

With the glacial tilt exposed here, low water content would be expected. Actually it is only

low for shallow depth and then rapidly increases to about 30%. This is probably due to the

large sphere of influence for the loop, so that the 3D effect is significant and the high values

are supposed to be caused by the aquifer beside.

B6 and B5 are difficult to interpret as the data contradicts the expected structural information.

With the glacial tilt exposed here, low water content would be expected. Actually it is only

low for shallow depth and then rapidly increases to about 30%. This is probably due to the

large sphere of influence for the loop, so that the 3D effect is significant and the high values

are supposed to be caused by the aquifer beside.

4 Laboratory measurements and results

35


Depth [m] Facies Sample Name0.7 medium sand B1_011.7 medium sand B1_022.7 medium sand B1_033.7 medium sand B1_04

4.75 fine sand B1_055.6 medium sand B1_066.7 medium sand B1_077.7 fine sand B1_08

8.75 interbed. Fs / Gu B1_099.6 fine sand B1_10

10.7 fine sand B1_1111.7 medium sand B1_1212.7 fine gravel B1_1313.7 fine gravel B1_1414.7 medium sand B1_1515.7 fine sand B1_1616.7 fine sand B1_1717.7 fine sand B1_1818.6 fine sand B1_1919.7 fine sand B1_2020.7 fine sand B1_21

21.45 medium sand B1_2226.2 coarse silt B1_23

31.23 medium sand B1_2436.3 fine sand B1_25

41.45 interbed. Fs / Gu B1_2647.35 medium sand B1_2756.3 fine sand B1_28

Fig. 4.1. Core segment photos

for segment 13 to 14m (left) and

segment 17 to 18m (right)

18m

17

m

14m

13

m

4.1 General information

During the well-logging a continuous drill core was

gained for the 60m-borehole on the test-site. The

individual 1m core segments were photographed to

preserve the in-situ facial character of the layers. Two

examples are displayed in figure 4.1.

For a detailed geological analysis a stratigraphic profile

was compiled by Mr. Jorzig of the Geological Survey

Berlin / Brandenburg. The complete profile can be found

at the appendix.

To gain a better overview the stratigraphic profile was

translated into medium grain sizes for each layer, which

was then displayed versus depth (figure 4.2.).

For all following laboratory analysis’ 28 samples were

taken throughout the core. Table 4.1. presents those

samples with depth and stratigraphical units.

The following laboratory measurements were used and

are described in detail during the next subchapters: grain

size analysis, rock density analysis, determination of

internal surface, laboratory NMR and Spectral Induced

Polarization for the determination of geoelectrical

parameters.

4.2 Grain Size Analysis

The first step for the petrophysical analysis was a grain

size analysis for the individual samples. By that a

classification for the samples can be received which not

only includes grain size distributions but also the

reference values for hydrologic conductivities.

The distribution curves and sum curves for the individual

samples are displayed in the appendix. Table 4.1. Laboratory sample list

Fig. 4.2. Stratigraphic profile (grain sizes) versus depth after Jorzig, Geological
Survey Berlin/Brandenburg (2001)

36


37

4.3 Rock density and porosity


The matrix density of the different core samples was analysed with a “micromeritics”

Pycnometer ‘AccuPyc 1330’. The pycnometer determines the volume of a specific sample by

measuring the pressure change of helium in a calibrated volume. From volume density can be

derived directly if the sample weight has been entered. Multiple measurement were carried

out to determine a medium density for each sample.

Additionally the pycnometer measurements allow an estimation of sample porosity by

determining the actual sample volume before pycnometer analysis which then is compared to

the measured matrix volume.

4.3.2 Results

As the measured samples consist mainly of sands and silts the expected grain densities were

in the range of 2.65 to 2.75 g/cm3.

These values were confirmed with a minimum value of 2.67 g/cm3 and the maximum being

2.74 g/cm3. The scattering probably results from temperature differences while measuring

different samples. A calibration for the instrument seems necessary for more extreme

temperature ranges.

Individual sample porosities were also determined from pycnometer analysis. As the material

consists of unconsolidated sediments there can be no exact porosity rather only a plausible

range. The obtained values should be compared to the NMR and log measurements.

Porosities from pycnometer analysis range from a minimum of about 30% for sandy gravel to

nearly 50% for sandy silts.

The exact values for density and porosity can be found at the appendix.

4.4 Specific internal surface


The specific internal surface in relation to the mass of the dry rock Sma was measured with a

“micromeritics” ‘Gemini 2360 Surface Area Analyser’. It’s primary function is to determine

the surface area of solid samples by measuring the differential pressure between a balanced


38

tube and a tube containing the sample. As the sample adsorbs the analysis gas the pressure

will drop in the sample tube. By maintaining a constant pressure of gas over the sample and

varying the rate of analysis gas, the rate at which the sample can adsorb the gas can be

determined.

The dry rock specific internal surface

was then calibrated with the

measured matrix density and the

porosity to gain the more commonly

used solid matrix specific internal

surface Sm and the pore specific

internal surface Spor.

4.4.2 Results

Values of internal surfaces can vary

over a wide range of magnitudes and

are directly dependent of

measurement resolution. With higher

resolution the absolute value will

increase, so the gained values can

only be an orientation and should be

used carefully. In general decreasing

grain sizes result in increasing

internal surfaces, which was

confirmed during the measurement

though the exact values seem a bit

low, especially for the sandy silts

(25m–end) were I expected more extreme values. On the other hand the internal surfaces of

the gravel samples (12-14m) are about as high as values for the sands, so the fine grained

fraction seems to balance out the expected low values. Overall the trends for all samples seem

right but the error range for the exact values is assumed to be 30% (due to resolution limits of

the instrument for small surfaces).

0

5

10

15

20

25

0 5 10 15 20

specific internal surface

dept

h [m

]

25

30

35

40

45

50

55

600 5 10 15 20

Sma [m²/g] Sm [1/µm] Spor [1/µm]

Fig. 4.3. Specific internal surface for Nauen samples


39

4.5 Laboratory NMR


For all laboratory NMR measurements a Maran2 of Resonance Instruments was used. The

magnetic flux density of the integrated permanent magnet is 0.047T, which corresponds to a

Lamor frequency of 2MHz for protons.

Before starting a measurement cycle a

ll samples were measured in full saturation. To eliminate effects due to ferromagnetic ions

amplitudes and relaxation times

needed. To

4.5.2 Results

careful selection of sequence

parameters is necessary. To receive

comparable results for all samples,

most of the parameters need to be

fixed for a complete sequence. For a

more detailed account on measurement

parameters and necessary adjustments

see [Krüger].

Fig.4.4. NMR instrument Maran2 (Resonance Instruments)

A

of any kind desalted samples were used. Additionally an individual porosity was determined

for each NMR sample to be compared to the measurements.

Two different pulse sequences were used to determine NMR

(see chapter 2.5.3): INVREC for T1 and CPMG, which is a spin-echo pulse sequence, for T2.

At first an FID sequence for measuring T2* was used as well. But the results proved to be

quite unstable, even during a sequence, so I decided to concentrate on T1 and T2.

For determining porosities from maximum amplitudes a calibration sample is

reduce relaxation times for the calibration sample it was mixed with copper sulphate, which is

paramagnetic. Relaxation acceleration will be dependent of the solution’s concentration.

Porosities from amplitudes

ces maximal amplitudes were determined by averaging over the From the two pulse sequen

first 3 measurement points. Based on equations 2.34 and 2.35 those amplitudes were then

converted into sample’s water content and later into NMR porosities. These were compared to


40

individual and medium porosities. The latter is an average value from all porosity

determinations (except NMR) for the sample material. Results are displayed in figure 4.5.

Measured porosities range from

NMR porosities

compared to medium

consolidated sediments.

ibutions. Generally a cut-off time

20% for the gravel layer (12-

14m), about 30–40% for

medium to fine sands and

nearly 50% for the silty beds.

The match for NMR porosities

to individual sample porosities

is very good.

As expected

tend to be slightly lower then

porosities determined for

individual samples.

Nevertheless the effect is

minimal. Due to the resolution

of the instrument even

irreducible water can be

resolved, one of the main

differences to SNMR, where

main effects result from “free”

water.

When

porosities large differences can

occur. This once again verifies

the importance of specifying

individual sample porosities for un

Irreducible water can be estimated from relaxation time distr

0

5

10

15

20

25

0 0.2 0.4 0.6

porosity

dept

h [m

]

25

30

35

40

45

50

55

600 0.2 0.4 0.6

T1 porosity T2 porosityindiv. porosity med. porosity

Fig. 4.5. Porosities from NMR relaxation amplitudes

of 33ms is assumed, relaxation times faster then that are attributed to adhesive water. A more

detailed description will be delivered by means of examples in the next subchapter.


41

Pore size distributions from relaxation

As seen in chapter 2.5.4, assumptions can be made about pore size distributions from decay

times. There is a direct relation from grain sizes to pore sizes (see Figure 4.6.). Though there

is no direct conversion, estimations are possible from comparison of different samples.

sand), B1_14 (fine gravel), B1_23 (coarse silt) in relation to grain size distributions.

Figure 4.7. shows the grain size distribution and T2 spectrum for sample B1_06, which is a

very good sorted medium sand. Despite the good sorting the T2 spectrum is not simple, that is

containing only one absolute maximum. Rather it shows 3 partial maxima with the latest at

B1_06 T2 distribution

0

20

40

60

80

100

120

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07tim e [µs]

B1_06 (5.6 m)

0

20

40

60

80

100

0.01 0.1 1 10

grain size [m m ]

Fig. 4.7. Grain size distribution (left) and T2 spectrum (right) for sample B1_06

T1 and T2 spectra for all samples can be found at the appendix. As it would be to extensive to

explain every result in detail, evaluation will be explained for 3 samples: B1_06 (medium

0.010.01 0.1 1 10

medium grain size [mm]

T1 T2

Fig. 4.6. Medium grain sizes versus decay times for Nauen samples

0.1

1

deca

y tim

e [s

]


432ms (85%) being the overall maximum and the lesser two at 0.3ms (5.7%) and 2.5ms

(9.3%) being about equal. This can be related to effects of packing which obviously results in

To estimate irreducible water any relaxation time amplitudes below the cut-off of 33ms have

to be considered. For B1_06 this would result in an irreducible water saturation of ca. 15%.

7%

about

small

maxim

B1_0

Again

That

usual

“usua

a very low percentage of very small pore radii throughout the sample. This effect is often

observed when good sorting i.e. narrow grain size distributions are involved (see [Krüger]

results for synthetic samples).

B1_14 (13.7 m)

0

20

40

60

80

100

0.01 0.1 1 10

grain size [mm]

mas

s %


01020304050607080

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07time [µs]

re. a

mpl

itude

4

The n

From

for th

6

notice


42

es about

of the samples pores and due to the larger grains the pore diameters are wider, as

80ms (22.5%) and at 2ms (9.9%). The later one is very broad, probably including 2

er maxima at about 1 and 4ms. The two (or three, depending on fitting parameters) local

a nevertheless contain a greater percentage of pores than i.e. the smaller ones of

6.

a cut-off value of 33ms is applied to display adhesive water, resulting in about 33%.

means, one third of the pore fluid is immobile. So even if the hydraulic conductivity is

ly higher for gravels, in this special case kf will probably be rather low compared to

l” gravel layers.

ext example displays the grain size and T2 distribution of B1_14, a fine gravel sample.

the grain size distribution the poor sorting is obvious, the curve has two maxima: one

e gravel size and another one for fine sands. The T2 maximum at 742ms includ

able from the higher values of T2 compared to B1_06. There are 2 more maxima: at


43

The third example is B1_23, a coarse silt sample. The grain sizes are very small, though

60

80

100


120

inside the silty band quite broad. The small dent at 0.045mm might be due to the conversion

from sieving to hydrometer analysis. In this case the T2 relaxation spectrum

straightforward: there is only one maximum at 19ms which of course is very low compared to

B1_06 and B1_14.

40

grain size [m m ] 0

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07tim e [µs]


B1_23 (26.2 m)

80

100

0

20

0.01 0.1 1 10 20

40

60

is

If an irreducible water cut-off of 33ms is assumed, B1_23 contains little to no mobile water.

This result was expected as the sample comes from the impermeable bed below the first

aquifer.

For all samples T spectra were determined as well. Although the exact values vary from T

the general distributions compared by sample are about the same. Differences in distribution

(additional or lost peaks) result in my opinion from the algorithm of the fitting program. One

of the main parameters of the program RI WinDXP is the “weight” parameter, which is

difficult to stabilize. I found that especially for T spectra this parameter was hard to maintain

for the complete sequence, resulting in artefacts or lost data. An example for this behaviour is

displayed in figure 4.10, where distribution spectra are compared for sample B1_19 (fine

sand).

1 2

1


50

150

200

0

100

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

tim e [µs]


150

200

0

50

100

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

tim e [µs]

Fig. 4.10. T1 (left) and T2 (right) distributions for B1_19


44

The overall maxima are about the same with 329ms for T1 and 296ms for T2. Nevertheless the

The second non-NMR-related permeability can be estimated from the PARIS equation (eq.

2.27). The values base on formation factors F from laboratory geoelectric measurements (see

chapter 4.6) and specific internal surfaces. The later has major relevance for the result as it is

highly error afflicted

T2 distribution is in my opinion more significant as it wasn’t possible to separate the smaller

maximum from the large without producing a main artefact below 0.1ms for T1.

Permeabilities from decay times

As mentioned in chapter 2.4.4 four different estimations for permeability were used.

From grain size analysis hydrologic conductivities will be calculated from the equation of

Beyer cted method, these

values will be used as reference.

.

s, the second again on formation factors. Additionally this will be a good error

stimation for the laboratory geoelectric measurements because even though the 2 parameters

) are most exemplary for those stratigraphic regions. I was a bit puzzled about the

intermediate region of about 30 to 40m. Stratigraphically this area consist of medium and fine

sands (samples B1_24 (31.2m) and B1_25 (36.3m)), so at least for B1_24 I expected a higher

permeability. But obviously the fine grained ratio is sufficient to lower k to nearly silty levels.

Below the interbedding region the material turns again to medium and fine sands (samples

B1_27 (47.35m) and B1_28 (56.3m)). This time permeability rises as well, resulting in values

equal to the upper aquifer.

(eq. 2.26). As this was the most basic and therefore least error affli

Two equations for NMR permeabilities were used (eq. 2.28 and 2.29) thus resulting in 4

different NMR values (two for each relaxation time). The first is based on individual sample

porositie

e

(porosity and formation factor) are related theoretically they were determined independently.

Permeabilities range from about 5 to 100d for the upper aquifer consisting of primarily

medium sands. There are smaller breaks for finer layers (e.g. 5m or 8-9m) where k drops to 1

to 10d. The gravel layer between 12 and 14m is not exceptionally noticeable, values tend to

be in the higher range but not really far above the medium sands. As expected the lowest

results of 0.01 to 1d were obtained for the silty layers, the samples B1_23 (26.2m) and B1_26

(41.45m


45

From [Kenyon] it is to be assumed that NMR permeabilities should come into the range of

true permeabilities with a factor of about 3. This comes very close to the results of my

measurements, but as [Kenyon]’s

assumptions were realised for

sandstones rather then unconsolidated

sediments an adjustment of the

atch

n the PARIS

alues. On the other hand

empirical factor C should be allowed. 5

The reference from NMR

permeabilities to “real” (Beyer) values

is surprisingly good, but the best m

was obtained based on a combination

of relaxation times and formation

factor.

The model based o

20

dept

h [m

]

equation seems to be the poorest,

although the results are better for the

layers below 25m. For this region

internal surfaces are higher, thus

probably resulting in more stable

25

30

35

40

50

10

15

25

45

55

k PaRiS from T1 & porosity from T1 & formation f.

Fig. 4.11. Permeabilities based on NMR measurements in

reference to Beyer and PARIS

00.01 0.1 1 10 100 1000

permeability k [d]

v

discrepancies are high for fine

materials at the top (e.g. ~5m or ~9m)

so the results will not simply get better

if the material is finer.

600.01 0.1 1 10 100 1000

k Beyer from T2 & porosity from T2 & formation f.


46

4.6 Electric conductivity and Archie exponents


Laboratory measurements for electric cond

1260 Impedance/Gain Phase

Analyzer”.

As the samples consist mainly of

uctivity or resistivity were done with a “Solatron

otential electrodes

meters to be determined to fully describe a samples

- q0

- exponent

-

To gain all 4 parameters with a minimum of

used:

1. interface conductivity

The samples were fully saturated with aqua dest. to exclude electrolytic conductivity. With

S=1 and σw→0

(4.1)

Because of the pore space of the sample is naturally filled with various ions electrolytic

conductivity can never be fully neglected. But it can be reduced to a minimum by desalting

the samples before the measurement process is started. This is a very drawn-out procedure

which can, depending on the sample, well take weeks.

unconsolidated sediments a new

measurement cell had to be designed

to prevent the pore fluid from

leaking. The cell is now completely

self-contained with the 4 connections

for current and p

included. It is well suited to small

sample volumes, containing about

70g of material.

Following equation (2.36) there are 4 para

conductivity:

σ : the interface conductivity

m: Archie exponent or cementation

n, ν: Saturation exponents

Fig. 4.12. Solatron Analyzer with measurement cell (front)

measurements, the following measuring plan was

00 qσσ =


47

2. Saturation exponent ν

The second step would be to use a partially aqua dest. saturated sample. The conductivity will

etermined from step 1. If the samples saturation S

ill be:

the be determined from:

0qo S σσ ν ⋅= (4.2)

with σq0 being the interface conductivity d

is known ν w

0

0

q )log(log S

=σσ

ν

wm

0

and

)log(log 0 Φ

=

w

mσσ

(4.3)

3. Archie exponent m

To determine m the samples will now be fully saturated with a highly conductive electrolyte

so interface conductivity can be neglected. With S=1:

(4.4) σσ ⋅Φ=

(4.5)

. Saturation exponent n

(4.6)

and

4

If m is known n can be determined from a partially (high conductive) saturated sample: nm Sσ ⋅⋅Φ=0 wσ

)log(log 0 Sn = mw

Φσ

σ (4.7)


48

4.6.2 Results

Interface or

ple. That is the higher the clay

or silt fraction of a sample is, the higher the

interface conductivity

These assumptions were verified with my

measurements. For the finer and medium sands

conductivity varies in a range of 0.002 - 0.008Sm .

Some samples in the upper layers contain a certain

fraction of finer sometimes even silty materials.

s probably why the conductivity has no

definite value for all t

up and down.

lower layers that consist

conductivity rise

gh this is higher than for the sandy samples it

is still lower than to be expected for silts. But as the

researched samples are o

definition, this had to be expected.

Exact values for individual samples can be found in

the appendix.

Archie exponent

The Archie exponent is an empirical material

constant and is often related to the cementation of

sediments and rocks. It is expected to be in the

range of 1.3 to 1.5 for unconsolidated sediments and

Interface conductivity

0

surface conductivity is phenomenon

that is known to be caused by mainly clay

components of a sam

is expected to be. 20

dept

h [m

]

5

10

15

25

0.001 0.01 0.1

interface conductivity [S/m]

25

35

40

50

55

Fig. 4.13. Interface conductivity for

Nauen samples

-130

45

That’

hose layers, but rather jumps 600.001 0.01 0.1

For the samples from the

primarily of silts and very fine sands the 1 1.5 2

Archie-exponent m

s to about 0.02 to 0.04Sm-1. 0

Thou

nly coarse silt by geologic 10

5

15

20

25

2.5

dept

h [m

]

25

30

35

40

45

50

55

601 1.5 2 2.5

Fig. 4.14. Archie exponent m for Nauen

samples


49

will rise with cementation to about 2.2 for highly

cemented sandstones [Schön].

values of 1.5 to 1.7 should

Both saturation exponent tend to be quite stable

over the variety of samples with n and ν having a

value of about 1 to 1.5 .

Exact value for individual samples can be found in the

appendix.

nce to porosity:

ified for the Nauen measurements.

here the formation factor varies little for most of

ly the gravel samples with low

porosities show increased values.

Exact value for individual samples can be found in

the appendix.

0

5

10

15

20

25

0.5 1 1.5 2 2

saturation exponents

dept

h [m

]

25

30

35

40

45

50

55

600.5 1 1.5

.5

For the Nauen samples

be expected and most of the values for the sands are

in that range. Nevertheless the overall trend is for

slightly too high values. The m values for the silty

sand is obviously to high though no explanation can

be given.

Exact value for individual samples can be found in

the appendix.

Saturation exponents

Fig. 4.15. Saturation expo

Formation factor F

The formation factor F can be calculated from

individual porosities and the Archie exponent m.

There is a direct inverse depende

0

5

10

15

20

25

2 4 6

formation facto

dept

h [m

]

25

30

35

40

55

602 4 6

Fig. 4.16. Formation facto

samples

45

50

with decreasing porosities F should rise.

This was ver

T

the samples. On

n

ν

2 2.5

s n and ν nent

8

r F

8 10

r for Nauen

10

5 Interpretation

50

5 Interpretation Related to the intention of this thesis there are two objectives for interpretation: first all results

a hydrogeophysical point of view, then

o each other, to effectiveness and to the

esults.

logical stratigraphy

have so far been orientated mainly at the

ing of the borehole to the southwest and the outcropping area of glacial till to

e northeast. These individual areas are linked by the measurements along the main profile

line (radar, DC geoelectrics). A concentration of data exists for both ends of this line. Due to

ssociated core samples a very detailed petrophysical characterisation of

ed on the general geologic structure it

units and therefore their hydrological

n area of the test-site. The stratigraphic profile at the borehole can be

si /

tline.

Starting from the basic model from well logging the following substructure can be assumed:

an open aquifer is followed by a impermeable layer at a depth of ~21m. Situated between this

ifer (31-39m). The third aquifer follows

r boundary can not be set.

edominantly consist of medium grained sands with

an be set at ~1.8 – 2.2m, depending on

rea can be identified by resistivities of

3000-4000 m for the vadose zone and ~400 m below the GWT. The percentage of

irreducible water can be assumed to be about 10-15% of the total water content (based on

laboratory NMR).

From 7 to 11m the aquifer is characterised through fine sands and the first occurrences of

coarse silts. For the latter porosities rise to 40% while at the same time permeabilities

decrease to 5d. The fine sands possess intermediate porosities (35%) and permeabilities of 20-

will brought together to characterise the test-site from

the numerous methods shall be valuated in relation t

possibility of combining them for an optimisation of r

5.1 Hydro

The measurement target-areas for the test-site Nauen

close surround

th

the borehole and the a

the subsurface is possible for the borehole area. Bas

should be possible to trace back the lithological

parameters to the norther

characterised in detail, based on boreholelogs, grain

geologic ou

ze analysis’ and the lithologic

aquiclude and the next at about 39m is the second aqu

from ~48m to the final depth of the borehole, the lowe

The first 7m of the uppermost aquifer pr

porosities of about 30 – 35%. The groundwater table c

season and rainfall. In DC geoelectric sections this a

Ω Ω

5 Interpretation

51

40d. From geoelectrics a medium resistivity of 150Ωm can be assumed for the lower part of

he poor sorting porosity drops to

0% and the increase in permeability is minimal. The rise in γ intensity can not be attributed

to mica, as done by the logger, but has to be ascribed to silt-bearing inclusions.

tom of the first aquifer the facies again changes to fine sands.

ayer from the rest of the aquiclude. The NMR porosities are at ~45% though

ll other laboratory measurements list at least 50%. This discrepancy can not be explained as

re immobile.

rom geoelectrics a medium resistivity of 70-90Ωm is implied.

ot as high as for the upper aquiclude. Laboratory relaxation times propose

this aquifer.

Located underneath to about 14.5m is a layer of fine gravel with medium grain sizes of 2-

4mm. From grain size analysis this layer can be characterised as unevenly grained, as it

contains a 20-30% fraction of fine to medium sands. Due to t

2

Below the gravel layer to the bot

Petrophysical parameters correlate to the upper fine sand layer, including porosities of 35%

and permeabilities of 10-50d.

Summarising the first aquifer consists of medium to fine grained sands with an intermediate

layer of fine gravel. Porosities vary in the range of 30-40%. The aquifer can be characterised

as medium permeable [Hölting], the irreducible water saturation can be assumed to be 15-

25% of total porosity.

The first aquiclude consists of mainly coarse silt with a layer of medium silt at the bottom.

The latter can be clearly distinguished in the γ log, but there is no further information to

differentiate this l

a

yet. Permeability is at the lowest value, 0.07d, corresponding to a hydraulic conductivity

below 10-6ms-1. From a hydrological point of view, this layer can still be considered “low

permeable” [Hölting]. Nevertheless laboratory NMR measurements show, that, due to the

small grains and pore throats, all water is bound to the grain structure and therefo

F

The second aquifer again is made up of mainly fine sands and silts with a layer of medium

sands at the top. Porosities for the fine sands are in the range of 45-50% with a permeability

of only ~0.2d. The percentage of fine grained material is nearly twice as high compared to

resembling sands from the upper aquifer. The irreducible water saturation will probably be

very high, though n

a ratio of about 50%. By hydrological standards this a medium to low permeable aquifer.

5 Interpretation

52

The second aquiclude from 39 to 48m is characterised by an interbedding of primarily coarse

silts with fine sands and silts. The stratigraphic sequence is varying quickly though the finer

s of medium

uifer.

hern part of the test-site, characterised by the outcropping till.

that vertical resolution is too

w for deeper interpretation.

to

5m thus rising with the upper aquifer / aquiclude.

material is concentrated at the top. Porosity and permeability match those of the first

aquiclude although the intermediate sand layers probably rather concur with the second

aquifer. Decay time distributions show a very low percentage of mobile water (~10-30%).

The lowest aquifer again consists of mainly fine sands with a few insertation

sands. Porosities are well in range for this stratigraphy (medium sands: 35%, fine sands 45%)

with an irreducible water content of 25-35%.Permeabilities again are higher (1-5d), qualifying

this as a medium permeable aq

All the above stratigraphical and petrophysical classification is keyed to the borehole and it’s

nearest surrounding area. But the radar and geoelectric profiles allow to link the deduced

information to the nort

For most of the main profile line the stratigraphic sequence is horizontal, only for the last

~100m the lithology is disturbed by the ascending aquiclude.

From the southernmost SNMR sounding the depth of the first aquifer can be seen at 12-15m

as indicated by water content. This concurs with the radar profile along the MPL, for a

location of about –75m. I nevertheless think that rather decay times should be considered as

the vertical resolution seems better. There I would place the till interface at 10-12m which

seems too shallow compared to radar, but is in an acceptable range. From radar no additional

information below this interface can be gained, but decay times place the aquiclude basis at

~18m with the second aquifer following to about 30m. Below

lo

For the next sounding location data of IP3 and SNMR B7 can be taken into account. In the IP

model an interface is displayed for 8m but there is only a small drop in free water content for

B7, rising again below. If decay times are examined the aquifer’s lower boundary can be

placed at again 10m. This difference to IP is within error tolerance due to vertical resolution

of both techniques. Again at radar the interface is a little deeper, so probably a variation

perpendicular to the main profile exists. This time the second aquifer stretches from ~14m

2

5 Interpretation

53

Due to the structural inhomogeneities and 3D effects the SNMR and IP soundings to the north

are difficult to explain and mostly contradict each other. That’s why they are not considered

in detail any further.

The following can be summarised: the rising outcrop of glacial till to the northeast of the test-

ite can be clearly identified as the first aquiclude from the borehole. Furthermore the basis of

5.2 Evaluation and effectiveness of used methods

Various laboratory and field technique have been used to gain as much geophysical and

hydrological information as possible about the test-site. Some have proved to be more

effective than others, some failed completely.

The most information about the lateral extent of hydrogeophysicaly relevant structures still

yields radar. Though it was limited to the upper aquifer and its interface with the till it

nevertheless provided detailed information which can be correlated to the punctual

information of soundings or logs. DC geoelectrics can provide similar details but data point

resolution is lower and needs to be balanced by inversion and modelling. An adequate starting

model e.g. from IP soundings is needed to improve inversion quality.

For this thesis refraction seismics proved to be the least effective method. Despite extensive

field work and data analysis no additional information was received. This might be due to the

s

the aquiclude as well as the second aquifer can be observed. Additionally can be stated that

the thickness of the lower layers is increasing to the southwest.

-250 -200 Borehole

Main Profile Line [m] SW

Fig. 5.1. Hydrogeological out-line of Nauen stratigraphy

0

10

40

]

-150 -100 -50 0

?

?

NE

aquiclude

20

30Dep

th [m

? aquifer

aquiclude

aquifer

50 aquifer

5 Interpretation

54

special circumstances for the test-site (very low velocities for the weathered zone). If

successful, measured seismic velocities can be used to characterise groundwater-bearing

layers in connection to porosity and saturation. The field effort is extensive compared to radar

r geoelectrics that’s why a pure seismic groundwater exploration is in my opinion obsolete

d can be efficiently combined

with lateral techniques. Especially radar and SNMR seem complementary as the general

ctural inf

porosity. From empirical relations an additional estimation of hydraulic permeabilities is

possible which is in general coherent with laboratory data.

reholes and ~logs provide an ional source of vertical structural information and can be

put to an effective use in groundwater exploration. γ gs are a valuable tool concerning

hydrogeological structures as they contain direct information on the percental distribution of

fine grained material (i.e. clay, shale, silt) and can be used to determine formation density and

porosity. If possible a borehole should be included in hydrogeophysical examinations though

is might prove difficult under normal conditions.

Laboratory investigations proved to be more time-consuming than field measurements.

Especially the electric measurement, with desalting, dragged on for more than 2 months.

Moreover, the measurements at partial saturations with low conductivities are highly unstable.

Partial saturations are used to determine saturation exponents, which are not necessarily

needed to access hydrologically relevant parameters. They shouldn’t be measured for large

groups of samples as the prove to be quite stable in value. If they are needed e.g. for new

inversion routines an exemplary determination for few samples might be sufficient.

Sample preparation for NMR is not as time consuming, especially as one sample is sufficient

for any number of pulse sequences. T takes the longest to measure and there is little to no

additional information compared to T . T * should not be used for laboratory evaluation as,

due to instability, results can not be compared to field data.

The measurement of rock density is a presupposition for electric and NMR measurements as

it is needed for porosity determination. The results are quite stable and again exemplary

determinations for the different stratigraphical units should probably be sufficient if no

o

and should be used only when all other lateral methods fail.

Both sounding methods used for this thesis, IP and SNMR, were highly effective. They

provide 1D vertical information about the sounding location an

stru ormation is upgraded with punctual data about stratigraphy (grain sizes) and

Bo addit

lo

th

1

2 2

5 Interpretation

55

material change is suspected (e.g. all samples consist of sands and silts, therefore matrix

densities should all correlate to quartz). For further intensive investigations the instrument

needs a temperature calibrated surrounding.

The determination of specific internal surface proved to be difficult as the surfaces involved

are very small for sands. This leads to unstable results near instrument resolution. The

easurement instrument has special set-ups for these situations, though they don’t seem to

ummarising, for the test-site Nauen the optimal combination of measurement techniques

m

improve data quality much.

The correlation of formation factor and decay times provided the best estimation of hydraulic

conductivities which can be used to specify aquifer properties from SNMR soundings.

Though radar isn’t very effective below the first aquifer, identification of relevant structures is

precise and transferable.

S

seems to be involving Ground Penetration Radar profiling, SNMR sounding and a combined

NMR and electric laboratory analysis.

6 Conclusion

56

6 Conclusion

The intention of this thesis was to evaluate and optimise different geophysical methods

trophysics) were successful and could be integrated into a more

etailed description of the test-site. The earlier stratigraphic model was expanded to the layers

n with SNMR soundings in correlation to laboratory

tudies. For SNMR vertical resolution needs to be improved for depths below 30m, though.

or laboratory measurements I would propose a reduced measurement program. Some of the

arameters determined for the Archie equations are not essential to access rock parameters in

a hydrological sense but nevertheless take the most time to determine. From only 2

measurements the necessary interface conductivity and Archie exponent could be calculated.

Additionally it wouldn’t be obligatory to desalt the samples, thus saving about 3-5 weeks of

measurement time. For laboratory NMR the use of only one pulse sequence seems

appropriate. There is no additional information from analysing both T1 and T2, nevertheless

T2* should be omitted as those decay times seem highly instable in the lab.

related to hydrological problems. These implementations should be based on the

determination of hydrogeophysical properties for the test-site Nauen.

With the exception of seismics, all field (DC geoelectrics, radar, IP, SNMR) and laboratory

measurement (NMR, SIP, pe

d

below the first aquiclude and the hydraulic parameters like porosity, irreducible water

saturation and permeability could be determined for the known 3 aquifers as well as for the

aquicludes. Earlier measurements could be correlated to the new data and interpretation was

improved.

By comparing the results of different techniques conclusions could be drawn about

effectiveness of laboratory and field measurements. Especially promising proves to be a

combination of lateral radar exploratio

s

F

p

7 Outlook

57

7 Outlook

On the subject of the failed seismic exploration, maybe another attempt should be made. The

accelerated weight drop failed to provide an appropriate signal to noise ratio. But within a

smaller scope there have been improved results with a sledgehammer and very high rates of

opographic grid should be established.

stacking. The measurement and evaluation effort for a small layout of maybe 4 shotpoints is

justifiable, even if the results should prove to be as unsuccessful as within this thesis.

To further analyse and model the measured data it seems necessary to improve the

topographic information about the test-site. Even though the field appears to be quite plain,

there are significant depressions and a rise in topography at the margins to the south and east.

A detailed t

Moreover an improved investigation of the area directly to the northeast of the test-site seems

promising. The lower boundary of the glacial till has not been accessed , the same applies to

the second aquifer and aquiclude. As IP or SNMR soundings prove to be difficult in that

region due to 3D effects, a drop-penetration-test might yield additional information.

Bibliography

Bibliography

Hertrich, M., 2000, Joint Inversion of Surface Nuclear Magnetic Resonance and Geoelectric

Sounding, Diplomarbeit, Technical University Berlin.

Kenyon, W. E., 1997, Petrophysical Principles of Applications of NMR Logging: The Log

Analysist.

nödel, K., Krummel, H., Lange, G., 1997, Handbuch zur Erkundung des Untergrundes von

Krüger, U., 2001, Untersuchungen mit der nuklearmagnetischen Resonanz (NMR) an

Gesteinsproben und synthetischen Materialien, Diplomarbeit, Technical University

Berlin.

oke, M. H., 1998, RES2DINV v3.4 Manual.

Richter, C.,2000, Ergebnisbericht zur Anlage und geophysikalischen Vermessung einer

Forschungsbohrung mit Ausbau zur Grundwassermessstelle im Bereich Nauen,

HGN (unpublished report).

Schön, J. H., 1996, Physical Properties of Rocks: Fundamentals and Principles of

Petrophysics.

Telford, W., Geldart, L., Sheriff, R., 1990, Applied Geophysics, Cambridge University Press

Dannowski, G., 1998, Abschätzung hydrologisch relevanter gesteinsphysikalischer Merkmale

durch kombinierte Radar- und Geoelektrikerkundung, Diplomarbeit, Technical

University Berlin.

Hölting, B., 1989, Hydrogeologie, 3-rd edition, Stuttgart.

K

Deponien und Altlasten, Band 3: Geophysik, Springer Verlag.

L

58

Acknowledgements

59

Acknowledgements

constant su

All membe ave been extremely helpful, a special thanks to Mr. Schmarsow

nd Mr. Schenkluhn from the department’s workshop, who supported me in the field and the

laboratory and to Mr. Hertrich, who uncomplainingly answered all my numerous questions

about the test-site and earlier used methods. Additionally a special thanks to Mrs. Krüger who

can never be found in textbooks.

ast but not least I would like to acknowledge my parents for their never-ending support

during the long progression of this study.

First I would like to thank Prof. Yaramanci for introducing me to the topic and for the

pport and encouragement during the course of this work.

rs of the institute h

a

explained NMR and Archie and all those little knacks that

L

Acknowledgements

60

Appendix

hydrogeophysical methods at the test-site nauen ......2 basics and theory of used measurement...

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