IG Geophysics
IG Geophysics
IG Geophysics
GEOPHYSICS
Initial
P-wave
S-wave
Mechanical Body Waves
Amplitude
S R
Time
P
Oscilloscope
Source
Receiver (Geophone)
R S
Mechanical Waves (Compression)
P - Wave Velocities
Steel
Intact Rocks
Weathered Rocks
Ice
Till
Sand
Clay
Sea Water
Fresh Water
S - W ave V elocities
Steel
Intact Rocks
Weathered Rocks
Ice
Till
Sand
Clay
Sea Water
Fresh Water
}V s =0
Determine depth t1
to rock layer, zR t2
Vertical Geophones
Source t3
(Plate) t4
x1
x2
x3
Soil: Vp1
zR x4
Rock: Vp2
Seismic Refraction
T r a v e l T im e (s e c o n d s )
0.020
xc V p2 V p1
zc
2 V p2 V p1
0.015
1
Vp2 = 4880 m/s
0.010
xc = 15.0 m
0.005
1 Depth to Rock:
Vp1 = 1350 m/s zc = 5.65 m
0.000
t values
0 10 20 30 40 50
Distance From Source (meters)
x values
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.
Crosshole
Seismic
Testing
Equipment
Oscilloscope
Crosshole Testing
ASTM D 4428
Pump
x = fctn(z)
t from inclinometers
Paul Mayne/GT
x Paul Mayne/GT
t Hammer
z1
z2 packer
Horizontal
Test
Depth Velocity
Interval Transducers
(Geophone
Receivers)
R12 = z12 + x2
Shear Wave Velocity:
R22 = z22 + x2
Vs = R/t Cased
Borehole
In-Situ Surface Wave Testing
Signal
Analyzer
Accelerometer
Source Sensors
Layer 1
Rayleigh Layer 2
Surface
Waves Layer 3
Layer 4
Shear Wave Measurements
Seismic Piezocone Test (SCPTu)
Seismic Piezocone Test
Wave, ts qc
Automated Seismic Source
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
Downhole Shear Wave Velocity
Anchoring System
Automated Source
Polarized Wave
Downhole Vs with
excellent soil
coupling.
Complete Set of Shear Wave Trains
Mud Island Site A, Memphis TN
Sounding Memphis, Shelby County, TN
5 5 5
5
10 10 10
10
15 15 15
Depth (m)
15
20 20 20 20
25 25 25 25
30 30 30 30
35 35 35 35
Seismic Flat Dilatometer (SDMT)
Seismic DMTs at UMASS, Amherst
SDMT1
SDMT4
2 2 2 SDMT5
4 4 4
Depth (m)
6 6 6
6 SDMT
DMT 21
DMT23
DMT
8 8 SDT 3
DMT 4 8
8
SDMT 4
SDMT 5
10 10 10
10
12 12
12 12
More Measurements is
More Better
Geophysical Methods
Electromagnetic Wave
Techniques
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).
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
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)
Ground Penetrating Radar (GPR)
Geostratigraphy
Examples of Ground Penetrating Radar (GPR)
Depth of ER survey:
i.e., greater spacing
influences deeper
Electrical Resisitivity Measurements
Electrical Resisitivity Measurements
Weathered Rocks
Glacial Till
Loose Sands
Loam
Clay
SPT
In-Situ Test Method
CPT
DMT
PMT
VST
Geophysics
0.0001 0.001 0.01 0.1 1 10 100 1000