1. Mechanical Wave Measurements Electromagnetic Wave Techniques
Chapter 5 GEOPHYSICS
Mechanical Wave Measurements
Electromagnetic Wave Techniques

2. 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)

3. Mechanical Wave Geophysics
Nondestructive measurements (gs < 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).

4. 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

5. Mechanical Body Waves
Initial
P-wave
S-wave

6. Mechanical Body Waves R S P Amplitude S R Time P Oscilloscope Source
Receiver (Geophone)
R S P

7. Mechanical Waves (Compression)

8. Mechanical Waves (Shear)

9. Geophysical Equipment
Seismograph Spectrum Analyzer
Portable Analyzer Velocity Recorder

10. Soil: Vp1 Rock: Vp2 zR x1 x2 x3 x4 Note: Vp1 < Vp2 t1
oscilloscope
Seismic Refraction
ASTM D 5777 t1 t2 t3 t4
Note: Vp1 < Vp2 zR
Determine depth
to rock layer, zR
Vertical Geophones
Source
(Plate) x1 x2 x3 x4
Soil: Vp1
Rock: Vp2

11. 1 xc = 15.0 m 1 Depth to Rock: zc = 5.65 m Vp2 = 4880 m/s
Seismic Refraction
Vp2 = 4880 m/s 1
xc = m
Vp1 = 1350 m/s 1
Depth to Rock:
zc = 5.65 m
t values
x values

12. Shear Wave Velocity, Vs
Fundamental measurement in all solids (steel, concrete, wood, soils, rocks)
Initial small-strain stiffness represented by shear modulus: G0 = rT Vs2 (alias Gdyn = Gmax = G0)
Applies to all static & dynamic problems at small strains (gs < 10-6)
Applicable to both undrained & drained loading cases in geotechnical engineering.

13. Crosshole Seismic Testing Equipment

14. Crosshole Testing t x Shear Wave Velocity: Vs = x/t PVC-cased
Oscilloscope
Crosshole Testing
ASTM D 4428
Velocity
Transducer
(Geophone
Receiver) t x
Shear Wave Velocity:
Vs = x/t
Downhole
Hammer
(Source)
Pump
packer
x = fctn(z)
from inclinometers
© Paul Mayne/GT
Test
Depth
Slope
Inclinometer
Slope
Inclinometer
Note: Verticality of casing
must be established by
slope inclinometers to correct
distances x with depth.
PVC-cased
Borehole
PVC-cased
Borehole

15. Downhole Seismic Testing Equipment

16. Downhole Testing z1 z2 Shear Wave Velocity: Vs = R/t x t
Oscilloscope
Downhole Testing
packer
Pump
Horizontal Plank
with normal load
Horizontal
Velocity
Transducers
(Geophone
Receivers)
Shear Wave Velocity:
Vs = R/t z1 z2 t
R12 = z12 + x2
R22 = z22 + x2 x
© Paul Mayne/GT
Hammer
Test
Depth
Interval
Cased
Borehole

17. In-Situ Surface Wave Testing
Sensors
Source
Signal
Analyzer
Accelerometer
Rayleigh
Surface
Waves
In-Situ Surface Wave Testing
Layer 1
Layer 2
Layer 3
Layer 4

18. Shear Wave Measurements

20. Seismic Piezocone Test (SCPTu)

21. Seismic Piezocone Test
Obtains Four Independent
Measurements with Depth:
Hybrid of Penetrometer
with Downhole Geophysics Vs fs
Cone Tip Stress, qt
Penetration Porewater Pressure,u
Sleeve Friction, fs
Arrival Time of Downhole Shear Wave, ts u1 u2
60o qc

22. 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 impulse hammer

23. Downhole Shear Wave Velocity
Anchoring System
Automated Source
Polarized Wave
Downhole Vs with excellent soil coupling.

24. Complete Set of Shear Wave Trains Mud Island Site A, Memphis TN

25. Sounding – Memphis, Shelby County, TN

26. Seismic Flat Dilatometer (SDMT)

27. Seismic DMTs at UMASS, Amherst6
DMT 2
DMT 3
SDT 4 8 10 12

28. More Measurements is
More Better

29. Electromagnetic Wave Techniques
Geophysical Methods
Electromagnetic Wave Techniques

30. 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).

31. 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

32. 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)

33. Ground Penetrating Radar (GPR)
Xadar Sensors & Software GeoRadar

34. Illustrative Results from Ground Penetrating Radar (GPR)
Crossing an underground utility corridor

35. Illustrative Results from Ground Penetrating Radar (GPR)

36. Illustrative Results of Ground Penetrating Radar (GPR)
Geostratigraphy

37. Examples of Ground Penetrating Radar (GPR)
Useful in Locating Underground Utilities

38. Results from Ground Penetrating Radar (GPR)

39. Results from Ground Penetrating Radar (GPR)

40. Electrical Resisitivity Measurements

41. Electrical Resistivity (ER) Surveys
Resisitivity rR (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.

42. Electrical Resisitivity Measurements
What will be gained by changing electrode
spacing?
Depth of ER survey:
i.e., greater spacing
influences deeper

43. Electrical Resisitivity Measurements

44. Electrical Resisitivity Measurements

45. Electrical Resistivity

46. Electromagnetic Conductivity (EM)

47. Magnetometer Surveys (MS)
Measure relative changes
in the earths' magnetic
field across a site.

48. Applicability of In-Situ Tests

49. 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.

50. A.P. Van den Berg Track Truck