crp/csp profile 1001 n line cw6 resistivity,

INTERPRETATIONS AND SUMMARY

F

Geophysical methods used to better characterize surface water, alluvial aquifer, and bedrock aquifer

interaction in the Cedar River Valley, IowaAdel E. Haj Jr.1, Eric A. White2, Lance R. Gruhn1, S. Mike Linhart1, and John W. Lane Jr.21: USGS Illinois-Iowa Water Science Center; 2: USGS Office of Groundwater Branch of Geophysics

• CRP and CSP methods delineate common

sub-bottom features across multiple survey

lines

• Shallow subsurface reflectors observed in

CSP correlate with resistive zones

observed in CRP.

• HVSR useful for interpreting depth to

bedrock in this area, with clear resonance

peaks for bedrock mapping in most

attempted surveys

• HVSR sensor coupling key to success

• ERT results match well to HVSR data

• Study area bedrock depths range from 5 to

28m

• ERT shows clear unconsolidated/bedrock

transition

• ERT shows some shallow high-resistivity

zones overlying less resistive materials

(attributed to saturation, grain-size, or

cementation differences)

• ERT and CRP resistivity ranges and

structures similar

• Results will be used to calibrate AEM

• Shallow CSP reflectors interpreted as hard

surface or fine/coarse transition

• Electrically resistive zones interpreted as

hard/cemented materials or coarse-grained

materials

• Results suggest possible buried valleys

and regions of bedrock connectivity

BACKGROUNDBetween July 2011 and February 2013, Iowa experienced severe

drought conditions (generally referred to as the 2012 drought)

that critically affected water availability throughout the state

where communities rely on alluvial aquifers to supply their water

needs. The City of Cedar Rapids saw pumping levels drop to

near cavitation levels in some wells. The City is interested in

evaluating prolonged drought condition water-supply availability

to determine if surface-water intakes need to be included in their

water-treatment capabilities.

The USGS Iowa Water Science Center is charged with building a

new groundwater model of the Cedar River alluvial aquifer that

can simulate stresses and drawdowns experienced during

extreme drought and demand. The new groundwater model

considers river connectivity to the aquifer, alluvial aquifer

connectivity to the bedrock aquifer, subsurface recharge sources

such as tributary inflows and buried valleys, and wetland and

oxbow lake connectivity to the alluvial aquifer. A suite of

geophysical methods is being employed to efficiently and non-

invasively map aquifer thickness and bedrock connectivity for

parameterization of this model. These geophysical methods

include waterborne and land-based seismic and resistivity surveys

and a forthcoming airborne electromagnetic survey.

BACKGROUND Future workFuture work

• Airborne electromagnetic

(AEM) survey spring 2017

• AEM will be calibrated to

previous geophysical

work and well data

• AEM will provide far

more comprehensive and

continuous data for an

improved model

• AEM will be

complemented with

additional land-based

seismic and ERT surveys,

as well as new boreholes

Example AEM

data from

Sioux Falls

aquifer study.

Warm colors

represent

more resistive

material,

preliminarily

interpreted as

quartzite.

Cedar Rapids

model area

(yellow) and

AEM survey

area (red)

FUTURE WORK

Passive Seismic Horizontal-to-Vertical Spectral Ratio (HVSR)

Electrical Resistivity Tomography

(ERT)

• Unit records ambient seismic wavefield

using 2-horizontal and 1-vertical

component; peak H/V frequency ratio is

resonant frequency (Fr) of surficial material

• Fr related to seismic shear wave velocity

(Vs) and depth to bedrock (Z) by the

equation 𝑉𝑠 = 4𝐹𝑟𝑍• Cedar River alluvium average Vs (300 m/s)

calculated from measurements at 5 wells

with known depth to rock

• Results depend on strong acoustic contrast

between sediment and bedrock, good

coupling of unit to ground surface, absence

of anthropogenic noise

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RESISTIVITY,IN OHM-METERS

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W E

Distance (meters)

fr = 16.01 HzZ = 4.7 m

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E W

Distance (meters)

Dep

th b

elo

w g

rou

nd

(m

eter

s)

fr = 3.88 HzZ = 19.3

fr = 3.59 HzZ = 20.9

fr = 3.72 HzZ = 20.2

fr = 3.81 HzZ = 19.7

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26

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133

159

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IN

Distance (meters)

S N

fr = 3.67 HzZ = 20.4 m

fr = 3.44 HzZ = 21.8 m

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W E

Distance (meters)

fr = 3.02 HzZ = 24.8m

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10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270

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80

106

133

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N SPipeline

f0 = 4.42 HzZ = 17.0 m

f0 = 3.99 HzZ = 16.8 m

f0 = 6.00 HzZ = 12.5m

Dep

th b

elo

w g

rou

nd

(m

eter

s)D

epth

bel

ow

gro

un

d

(met

ers)

Dep

th b

elo

w g

rou

nd

(m

eter

s)

Distance (meters)

Line CW6

Line CW1

Seminole Valley Park

Line S10

Line CW3Dep

th b

elo

w g

rou

nd

(m

eter

s)

LAND-BASED METHODS

Sample HVSR data:

(a) horizontal and

vertical component

frequency spectra,

(b) horizontal to

vertical frequency

ratio.

a

b

• Survey performed

with Supersting R8

• Used dipole-dipole,

Schlumberger, and

inverse Schlumberger

arrays

• Modeled resistivity ranges from 25 to

325 ohm-m

• Data affected by pipeline in line CW3Horizontal black lines indicate depth

to bedrock from HVSR measurements.

Land-based surveys help characterize the extent and geophysical properties

of floodplain sediments.Waterborne surveys provide information about channel sub-bottom, an area

inaccessible to other methods.

Space

E

• 8-channel resistivity system and 11-electrode

streamer with 10-m spacing

• Subsurface penetration ~16m

• Resistivity range: 30 to 300 oh

• m-m

WATERBORNE METHODS

50

75

100

125

150

200

335


S

8

3

13

0

8

16

CRP/CSP Profile 1000

L1000 S L1000 SNS

0

8

16

CRP/CSP Profile 1001 NS

CRP/CSP Profile 1002 NS3D Composite of CRP data

AREAOFNOCRP

DATARECD

L1003A S L1003A S L1003B S L1003B S



FFf

EdgeTech Sub-bottom Profiler

• 4 - 24 kHz Chirp

system

• Subsurface

penetration ~5m

• Water bottom

multiples present in

all records

• 8-channel resistivity

system and 11-

electrode streamer

with 10-m spacing

• Depth of

investigation ~16m

• Resistivity range: 30

to 300 ohm-m

Continuous Resistivity Profiling

(CRP)

Continuous Seismic Profiling

(CSP)

3

8

13

0

8

16

3

8

13

0

8

16

3

8

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0

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16

3

8

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8

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0

5

E

N

PROJECT AREA

References

Advanced Geosciences, Inc., 2005, Instruction manual for EarthImager 2D, version 1.9.0, Resistivity and IP inversion software: Advanced Geosciences, Inc.,134 p., accessed at http://www.agiusa.com/.

Day-Lewis, F., White, E., Johnson, C., Lane, J., and Belaval, M., 2006. Continuous resistivity profiling to delineate submarine groundwater discharge-example limitation, The Leading Edge, June, 724-728.

Loke, M.H., 2000, Electrical imaging surveys for environmental and engineering studies - a practical guide to 2D and 3D surveys, unpublished short course training notes: Penang, Malaysia, University Sains Malaysia, 67 p.

Reynolds, J.M., 2011, An introduction to applied and environmental geophysics, John Wiley and Sons, 712p.

Telford, W.M., Geldart, L.P., Sheriff, R.E., 1990, Applied geophysics, 2nd

edition, Cambridge University Press

Todd, D.K., 1980, Groundwater hydrology 2nd edition, John Wiley and Sons

REFERENCES

Black lines in CRP profiles indicate interpreted bedrock surface.

L1001 S L1001 S

crp/csp profile 1001 n line cw6 resistivity,

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