ig geophysics
DESCRIPTION
geophysical explorationTRANSCRIPT
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Chapter 5
GEOPHYSICS
Mechanical Wave
Measurements
Electromagnetic Wave
Techniques
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Geophysical Methods
Mechanical Wave Measurements• Crosshole Tests (CHT)• Downhole Tests (DHT)• Spectral Analysis of Surface Waves• Seismic Refraction• Suspension Logging
Electromagnetic Wave Techniques• Ground Penetrating Radar (GPR)• Electromagnetic Conductivity (EM)• Surface Resistivity (SR)• Magnetometer Surveys (MT)
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Mechanical Wave Geophysics
Nondestructive measurements (s < 10-4%)
Both borehole geophysics and non-
invasive types (conducted across surface).
Measurements of wave dispersion:
velocity, frequency, amplitude,
attenuation.
Determine layering, elastic properties,
stiffness, damping, and inclusions
Four basic wave types: Compression (P),
Shear (S), Rayleigh (R), and Love (L).
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Mechanical Wave Geophysics
Compression (P-) wave is fastest wave;
easy to generate.
Shear (S-) wave is second fastest wave. Is
directional and polarized. Most
fundamental wave to geotechnique.
Rayleigh (R-) or surface wave is very close
to S-wave velocity (90 to 94%). Hybrid P-S
wave at ground surface boundary.
Love (L-) wave: interface boundary effect
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Mechanical Body Waves
Initial
P-wave
S-wave
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Mechanical Body Waves
SourceReceiver (Geophone)
Oscilloscope
P
S RTime
Amplitude
R S P
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Mechanical Waves (Compression)
0 1000 2000 3000 4000 5000 6000 7000 8000
Compression Wave Velocity, Vp (m/s)
Fresh Water
Sea Water
Clay
Sand
Till
I ce
Weathered Rocks
Intact Rocks
Steel
P - Wave Velocities
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Mechanical Waves (Shear)
0 1000 2000 3000 4000
Shear Wave Velocity, VS (m/s)
Fresh Water
Sea Water
Clay
Sand
Till
I ce
Weathered Rocks
Intact Rocks
Steel
S - Wave Velocities
Resistivity, (ohm- meters)
Resistivity Values ( ICE, London, 1976)
} V s = 0
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Geophysical Equipment
Seismograph Spectrum Analyzer
Portable Analyzer Velocity Recorder
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Seismic Refraction
Vertical GeophonesSource(Plate)
Rock: Vp2
ASTM D 5777
Soil: Vp1
oscilloscope
x1x2x3x4
t1 t2 t3 t4
Note: Vp1 < Vp2
zR
Determine depthto rock layer, zR
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Seismic Refraction
0.000
0.005
0.010
0.015
0.020 T
rav
el
Tim
e (
se
co
nd
s)
0 10 20 30 40 50 Distance From Source (meters)
Horizontal Soil Layer over Rock
Vp1 = 1350 m/s
1
Vp2 = 4880 m/s
1z
x2
V VV V
cc p2 p1
p2 p1
Depth to Rock: zc = 5.65 m
xc = 15.0 m
x values
t valu
es
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Shear Wave Velocity, Vs
Fundamental measurement in all solids
(steel, concrete, wood, soils, rocks)
Initial small-strain stiffness represented
by shear modulus: G0 = Vs2
(alias Gdyn = Gmax = G0)
Applies to all static & dynamic problems
at small strains (s < 10-6)
Applicable to both undrained & drained
loading cases in geotechnical
engineering.
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CrossholeSeismic Testing
Equipment
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Crosshole TestingOscilloscope
PVC-cased
Borehole
PVC-cased
Borehole
DownholeHammer (Source) Velocity
Transducer (GeophoneReceiver)
t
x
Shear Wave Velocity:Vs = x/t
TestDepth
ASTM D 4428
Pump
packer
Note: Verticality of casingmust be established by
slope inclinometers to correctdistances x with depth.
SlopeInclinometer
SlopeInclinometer
© Paul Mayne/GTx = fctn(z)
from inclinometers
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Downhole SeismicTesting Equipment
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Downhole TestingOscilloscope
Cased Borehole
TestDepth
Interval
HorizontalVelocity
Transducers (GeophoneReceivers)
packer
PumpHorizontal Plank
with normal load
Shear Wave Velocity:Vs = R/t
z1z2
t
R12 = z1
2 + x2
R22 = z2
2 + x2
x
Hammer
© Paul Mayne/GT
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SensorsSource
SignalAnalyzer
Accelerometer
RayleighSurfaceWaves
In-Situ Surface Wave Testing
Layer 1
Layer 2
Layer 3
Layer 4
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Shear Wave Measurements
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Seismic Piezocone Test (SCPTu)
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60o
fs
qc
Vs
u1
u2
Cone Tip Stress, qt
Penetration Porewater Pressure,u Sleeve Friction, fs
Arrival Time of Downhole Shear Wave, ts
Cone Tip Stress, qt
Penetration Porewater Pressure,u Sleeve Friction, fs
Arrival Time of Downhole Shear Wave, ts
Obtains Four Independent Measurements with Depth:Hybrid of Penetrometerwith Downhole Geophysics
Seismic Piezocone Test
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• Electronically-actuated
• Self-contained
• Left and right polarization
• Modified beam uses fin to enhance shear wave generation
• Successfully tested to depths of 20m
• Capable of being used with traditional impulse hammer
Automated Seismic Source
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Downhole Shear Wave Velocity
Anchoring System Automated Source Polarized Wave Downhole Vs with excellent soil coupling.
Anchoring System Automated Source Polarized Wave Downhole Vs with excellent soil coupling.
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Complete Set of Shear Wave TrainsMud Island Site A, Memphis TN
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Sounding – Memphis, Shelby County, TN
0
5
10
15
20
25
30
35
0 10 20 30 40
qt (MPa)
Dep
th (
m)
0
5
10
15
20
25
30
35
0 100 200 300
fs (kPa)
0
5
10
15
20
25
30
35
0 1000 2000 3000u2 (kPa)
0
5
10
15
20
25
30
35
0 100 200 300 400
Vs (m/sec)
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Seismic Flat Dilatometer (SDMT)
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Seismic DMTs at UMASS, Amherst
0
2
4
6
8
10
12
0 2 4 6 8
Lift-off Pressure po (bars)
Dep
th (
m)
0
2
4
6
8
10
12
0 20 40 60 80
Travel Time of Shear Wave (ms)
SDMT1
SDMT4
SDMT5
6
8
10
12
DMT 2
DMT 3
SDT 4
0
2
4
6
8
10
12
0 5 10 15
Expansion Pressure p1 (bars)
SDMT 1
DMT 2
DMT 3
SDMT 4
SDMT 5
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More Better
More Measurements is
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Geophysical Methods
Electromagnetic Wave
Techniques
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Electromagnetic Wave Geophysics
Nondestructive methods
Non-invasive; conducted across
surface.
Measurements of electrical & magnetic
properties of the ground: resistivity
(conductivity), permittivity, dielectric,
and magnetic fields.
Cover wide spectrum in frequencies
(10 Hz < f < 1022 Hz).
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Electromagnetic Wave Geophysics
Surface Mapping Techniques:
• Ground Penetrating Radar (GPR)
• Electrical Resistivity (ER) Surveys
• Electromagnetic Conductivity (EM)
• Magnetometer Surveys (MS)
Downhole Techniques
• Resistivity probes, MIPs, RCPTu
• 2-d and 3-d Tomography
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Ground Penetrating Radar (GPR)
GPR surveys conducted on gridded areas
Pair of transmitting and receiver antennae
Short impulses of high-freq EM wave
Relative changes in dielectric properties
reflect differences in subsurface.
Depth of exploration is soil dependent (up
to 30 m in dry sands; only 3 m in wet
saturated clay)
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Ground Penetrating Radar (GPR)
Xadar Sensors & Software GeoRadar
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Illustrative Results from Ground Penetrating Radar (GPR)
Crossing an underground utility corridor
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Illustrative Results from Ground Penetrating Radar (GPR)
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Illustrative Results of Ground
Penetrating Radar (GPR)
Geostratigraphy
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Examples of Ground Penetrating Radar (GPR)
Useful in Locating Underground Utilities
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Results from Ground Penetrating Radar (GPR)
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Results from Ground Penetrating Radar (GPR)
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Electrical Resisitivity Measurements
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Electrical Resistivity (ER) Surveys
Resisitivity R (ohm-m) is an electrical
property. It is the reciprocal of
conductivity
Arrays of electrodes used to measure
changes in potential.
Evaluate changes in soil types and
variations in pore fluids
Used to map faults, karst features (caves,
sinkholes), stratigraphy, contaminant
plumes.
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Electrical
Resisitivity
Measurements
What will be gained by changing electrodespacing?
Depth of ER survey:i.e., greater spacing influences deeper
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Electrical Resisitivity Measurements
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Electrical Resisitivity Measurements
1 10 100 1000 10000
Bulk Resistivity, (ohm- meters)
Clay
Loam
Loose Sands
Sands & Gravels
Glacial Till
Weathered Rocks
Resistivity Values (ConeTec & GeoProbe, 1997)
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Electrical Resistivity
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Electromagnetic Conductivity (EM)
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Magnetometer Surveys (MS)
Measure relative changesin the earths' magneticfield across a site.
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Applicability of In-Situ Tests
0.0001 0.001 0.01 0.1 1 10 100 1000
Grain Size (mm)
In-S
itu
Test
Meth
od
SPT
CPT
DMT
PMT
VST
Geophysics
CLAYS SILTS SANDS GRAVELS Cobbles/ Boulders
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In-Situ Testing - Objectives
Select in-situ tests for augmenting, supplementing, and even replacing borings.
Realize the applicability of various in-situ methods to different soil conditions.
Recognize the complementary nature of in-situ direct push methods with conventional rotary drilling & sampling methods.
Recognize values for utilizing these methods and quality implications for their underuse.
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A.P. Van den Berg Track Truck