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

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Presentation on theme: "Chapter 5 GEOPHYSICS  Mechanical Wave Measurements  Electromagnetic Wave Techniques."— Presentation transcript:

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2 Chapter 5 GEOPHYSICS  Mechanical Wave Measurements  Electromagnetic Wave Techniques

3 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)

4 Mechanical Wave Geophysics  Nondestructive measurements (  s < %)  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).

5 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

6 Mechanical Body Waves Initial P-wave S-wave

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

8 Mechanical Waves (Compression)

9 Mechanical Waves (Shear)

10 Geophysical Equipment Seismograph Spectrum Analyzer Portable Analyzer Velocity Recorder

11 Seismic Refraction Vertical Geophones Source (Plate) Rock: V p2 ASTM D 5777 Soil: V p1 oscilloscope x1 x2 x3 x4 t1 t2 t3 t4 Note: V p1 < V p2 zRzR Determine depth to rock layer, z R

12 Seismic Refraction V p1 = 1350 m/s 1 V p2 = 4880 m/s 1 Depth to Rock: z c = 5.65 m x c = 15.0 m x values t values

13 Shear Wave Velocity, V s  Fundamental measurement in all solids (steel, concrete, wood, soils, rocks)  Initial small-strain stiffness represented by shear modulus: G 0 =   V s 2 (alias G dyn = G max = G 0 )  Applies to all static & dynamic problems at small strains (  s < )  Applicable to both undrained & drained loading cases in geotechnical engineering.

14 Crosshole Seismic Testing Equipment

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

16 Downhole Seismic Testing Equipment

17 Downhole Testing Oscilloscope Cased Borehole Test Depth Interval Horizontal Velocity Transducers (Geophone Receivers) packer Pump Horizontal Plank with normal load Shear Wave Velocity: V s =  R/  t z1z1 z2z2 tt R 1 2 = z x 2 R 2 2 = z x 2 x Hammer © Paul Mayne/GT

18 Layer 1 Layer 2 Layer 3 Layer 4

19 Shear Wave Measurements

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21 Seismic Piezocone Test (SCPTu)

22 60 o fsfs qcqc VsVs u1u1 u2u2  Cone Tip Stress, q t  Penetration Porewater Pressure,u  Sleeve Friction, f s  Arrival Time of Downhole Shear Wave, t s Obtains Four Independent Measurements with Depth: Hybrid of Penetrometer with Downhole Geophysics Seismic Piezocone Test

23 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

24 Downhole Shear Wave Velocity  Anchoring System  Automated Source  Polarized Wave  Downhole V s with excellent soil coupling.

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

26 Sounding – Memphis, Shelby County, TN

27 Seismic Flat Dilatometer (SDMT)

28 Seismic DMTs at UMASS, Amherst DMT 2 DMT 3 SDT 4

29 More Better More Measurements is

30 Geophysical Methods Electromagnetic Wave Techniques

31 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 < Hz).

32 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

33 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)

34 Ground Penetrating Radar (GPR) Xadar Sensors & Software GeoRadar

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

36 Illustrative Results from Ground Penetrating Radar (GPR)

37 Illustrative Results of Ground Penetrating Radar (GPR) Geostratigraphy

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

39 Results from Ground Penetrating Radar (GPR)

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41 Electrical Resisitivity Measurements

42 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.

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

44 Electrical Resisitivity Measurements

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46 Electrical Resistivity

47 Electromagnetic Conductivity (EM)

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

49 Applicability of In-Situ Tests

50 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.

51 A.P. Van den Berg Track Truck


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