Geo-imaging: An Introduction to Engineering Geophysics

Transcription

1 Geo-imaging: An Introduction to Engineering Geophysics Chih-Ping Lin Distinguished Professor, Department of Civil Engineering & Natural Hazard Mitigation Research Center National Chiao Tung University, Taiwan Ex. Considerations in landslide assessment Geological model and material properties Slip surface/geometry of the sliding block Detection and monitoring of slope movement

2 The Role of Geophysics Investigation of geological structure Measurement of related physical properties Geotechnical Testing Geophysical Methods Sampling SPT,CPT DMT, PMT VST Mechanical response Small sample volume Small coverage High spatial resolution 1D2D3D Non-destructive Complementary Seismic wave Electrical EM wave Physical properties Large sample volume Large coverage Low spatial resolution Geo-imaging methods Resistivity (ERT) P-wave velocity (Traveltime) Reflection S-wave velocity (Surface Wave)

3 Geophysics for Slope Stability (Hack 2000) Geophysics for Slope Stability (Bell et al. 2006)

4 Geophysics for Slope Stability (Jongmans & Garambois 2007) Part I Seismic Methods Seismic waves and properties Seismic methods Borehole seismic Down-hole Cross-hole Suspension Cross-hole Tomography Surface seismic Refraction Reflection Surface Wave Principle Applications Limitations Guideline

5 Seismic waves Surface waves Compressional waves Shear waves Body Wave in Layered Media 10

6 Seismic Wave Seismic wave properties Elastic modulus Young s modulus (E) Poisson s ratio () Shear modulus (G) Vs Constrained Vp modulus (M) Damping Viscous damping D Deformation Strength Dynamic

7 Seismic wave velocity saturated Seismic Properties P-wave velocity vs. Unconfined compressive strength (Ookubo and Terasaki, 1971)

8 Seismic wave velocity Seismic Properties (Hada, 1984)

9 Seismic Methods Borehole/Penetration Methods Down-hole Cross-hole Suspension Cross-hole Tomography Surface Methods Refraction Reflection Surface Wave Seismic Downhole Method 震源 震測儀 裸孔或 PVC 套管 孔內受波器 ASTM Standard: ASTM D Standard Test Method for Downhole Seismic Testing

10 Borehole Seismic Borehole Seismic

11 Characteristics of Downhole Test Requires only one borehole Work in cased holes Less expensive Some space is required to place the source S/N decreases with increasing depth Spatial resolution also decreases with depth 21 Seismic Crosshole Method ASTM Standard: ASTM D4428 Standard Test Method for Crosshole Seismic Testing

12 Characteristics of Crosshole Test Most reliable Work in cased holes Accuracy and resolution does not decrease with increasing depth Requires at least 2 boreholes Borehole distance < 9 m Borehole deviation 23 Suspension PS-Logger Suspension 0 H1 0 H Depth (m) Depth (m) Time (ms) Time (ms) Limitations: - Casing? - water required - V Sensitivity

13 Characteristics of PS Logging Fast Accuracy and resolution does not decrease with increasing depth Water-filled borehole required Results may be affected by casing Receiver spacing is short (1 m) Cross-hole Tomography

14 Surface Seismic Methods 1 2 N-1 N x 1 dx :Source :Receiver x 1 :Near offset L dx:receiver interval L :Receiver spread Seismic Equipment 28

15 Seismic Refraction Method Snell s Law i r V 1 t V 2 29 Determining Earth Structure From Travel Time Curve Low Velocity Over High Velocity Halfspace

16 Refraction Method Multiple Subsurface Model Refraction Method Hidden Layers Low Velocity Layer

17 Thin, large velocity contrast layer Interpretation of Dipping Layers

18 More General Interpretation -The Reciprocal Method Depth term: t 0 =T AP +T BP -T AB Velocity term: T AP -t 0 /2 = 1/2T AB -1/2(T AP -T BP ) t 0 /2 t 0 /2 Refraction Tomography

19 Refraction Method Surface+Borehole Traveltime Tomography (Optim, 2001) Planning of Survey Geophone spread and direction Geophone spread Investigation depth No. of Channel & Receiver interval 24 channel are the most common Tyically 5 meter for shallow & 10 m for deep Source location 7 points method 9 points method source receiver / geophone 38 use 1/3 or ¼ line length

20 Refraction Method Data Acquisition Traveltime Curve Tomography Inversion 39 GRM Analysis Limitations of Seismic Refraction Depth: Typically < 30 m. Deeper depth requires explosives and long geophone spread Resolution: Typically resolve 3~4 layers, vertical resolution is limited by the accuracy of travel time (1 ms error induces ~1m depth error for surface layers and ~2-10 m for deeper layers) and depends on velocity contrast, lateral resolution = f (geophone spacing) Hidden layers Seismic velocity must increases with depth Will not detect thin layers Narrow shear zone with low velocity Lateral refraction in the horizontal plan Measurement sensitive to acoustic noise and vibration A source to geophone distance of 5 (or more) times the desired depth of investigation is needed

21 Seismic Reflection Method Principle Applications: Characterize geological structures Common Midpoint Survey Normal moveout correction Velocity Analysis Note: Before stacking, several data processing may be involved.

22 Walkaway noise test Survey Plan

23 Limitations of Seismic Reflection Depth: ten s m~ hundred s m Resolution: vertical resolution depends on wavelength (as good as 1m for 500 Hz), lateral resolution depends on geophone spacing (typically 0.3~3m) Operations are difficult in areas of steep topography or uneven surfaces. Measurements labor intensive, data processing complicated Measurement sensitive to acoustic noise and vibration Difficult to perform shallow depth (< 30 m) seismic reflection

24 Seismic Surface Wave Method Frequency component Geophones V S V S1 V S2 Time (s) z V S3 V Inversion Dispersion curve f Field Testing Dispersion analysis Dispersion of Rayleigh Wave

25 Surface Wave Methods Two-Station Method (SASW) Multi-Station Method (MASW) Surface Wave SASW Method x (rad) Unwrapping in f domain V ph (m/s) X=8 X=16 X= f (Hz) f(hz) Low frequency important, esp. for large x v( f ) 2f ( f ) x

26 MASW Method MWTSW MSASW Time (s) frequency (Hz) Frequency (Hz) Frequency, f (Hz) Offset (m) Offset (m) R Phase Velocity, v ph (m/s) k N 1 ( fi, v) n0 U ( f i, x n ) exp j 2f i v x n Surface Wave Tomography Surface Wave Methods Survey Line Data Acquisition National Chiao Tung Unversity

27 Limitations of Surface Wave Method It s basically a 1-D method Resolution and accuracy decreases with increasing depth Investigation depth is typically less than 30 m Inversion considering effect of multiple modes needs further study Part II Electrical/Electromagnetic methods Electrical Properties Electrical Methods Electromagnetic Wave Method Time Domain Reflectometry (TDR) Ground Penetration Radar (GPR) Electrical methods (Potential measurement) DC resistivity Spontaneous Potential Electromagnetic Induction Frequency Domain EM Time Domain EM Principle Applications Limitations Guideline

28 Electrical Properties Electrical Conductivity: dc EM wave Attenuation Dielectric Permittivity: r EM wave Velocity + O H (a) H - (b) E (c) Geo-electrical properties Type of geomaterial, water content, pore water properties, 55 Electrical Properties Electrical Properties Dielectric Permittivity r ( f ) r '( f ) jr ''( f ) Electrical Conductivity dc Equivalent Dielectric Permittivity * ' j r r ii r dc r ' j r " 2f 0 (Hilhorst, 1998)

29 Electrical vs. Geo Properties High-frequency dielectric permittivity is mainly a function of water content Dielectric dispersion is related to water content, soil density, and soil type Electrical conductivity is related to pore water characteristics, water content, soil density, and soil type Soil/Rock conditions vs. resistivity Conditions Changes of rock/soil resistivity Low High Remarks Resistivity of groundwater and pore water Low High Salinity, salt water wedge Degree of water saturation High Low Ground water Porosity (saturated) Large Small Clay fraction Many Few Soil type Degree of weathering and Strong Slight Strength alteration Temperature High Low Geothermal

30 Electrical Methods Borehole/Penetration Methods Time domain reflectometry (TDR) Down-hole resistivity/radar Cross-hole resistivity/radar tomography Surface Methods Ground penetration radar DC resistivity Self potential Frequency domain Electromagnetic Time domain electromagnetic TDR: Measurement of Electrical Properties TDR Step pulse generator v 0 Coaxial cable Sampler Waveguide V s /2 V f Apparent dielectric constant v 2L / T c / Ka Ka = f( r d ) Electrical conductivity EC=f( r d water EC) Lin et al. (2008&2009)

31 Water Content Measurement based on Ka sqrt(k a ) w / d K a (Lin, 1999) Measured Linear Regression Volumetric Water Content Empirical Relation (Lin, 1999) R 2 = y = x R 2 = w Topp's Equation Topp s equation K a K a K a Refractive Index Linear K K a d a a ASTM D6780 w Equivalently, b c a bw Ka= f( r d soil type) d K a (Topp, 1980) (Ledieu et al., 1986) (Siddique and Drnevich, 1995) d a b Note: 0 and d 0, Ka=0 instead of 1 (air), But fit as good as Ledieu et al. (1986) for soils. w Borehole Radar 62 Limitations: - penetration distance

32 Cross-hole Radar Tomography Velocity Dielectric Constant Attenuation Conductivity (Jhou et al. 2001) Cross-hole Radar Mapping moisture content changes due to tracer input (Binley, 2004)

33 Ground Penetrating Radar Limitations: -Penetration depth -Velocity analysis -Reflection pattern Survey Type 66

34 25 MHz, up to 40 depth (Otto & Sass, 2006) DC Resistivity Method 1 Resistivity vs. Resistance 2 Single point current source in half space r P

35 A practical way of measuring resistivity I V K r r r r I V V p P a Depth of current penetration vs. current electrode spacing DC Resistivity

36 Electrode Array (M.H. Loke., 2015) 71 Vertical Electrical Sounding Schlumberger and Wenner array are commonly used for sounding

37 Electrical Resistivity Tomography (ERT) 2D Electrical Resistivity Tomography (ERT) 74

38 Electrical Resistivity Tomography(ERT) 1D 2D 3D (Bichler et al. 2004) 76

39 DC Resistivity Time-lapse ERT Limitations of DC Resistivity Method Measurement susceptible, but less so than EM measurements, to interference from nearby metal objects Obtaining a good connection with the ground can sometimes be a problem Sensitivity (resolution) decreases significantly with increasing depth and depends on resistivity distribution

40 Borehole Resistivity 79 Cross-borehole ERT 80

41 Self-Potential Method Principle Applications: Investigation of subsurface water movement Electromagnetic Induction A suite of techniques to measure resistivity, complementary to DC resistivity method. Frequency Domain EM Time Domain EM Very Low Frequency (VLF) Pipe/cable locator Metal Detector

42 Geophysical logging Borehole caliper Velocity Density Resistivity Self Potential Natural Gamma Televiewer Hydrophysical Logging Optical/Acoustic Televiewer 84

43 Acoustic Televiewer Travel time Magnitude 東勢坑溪 Probe Acoustic televiewer measures: Acoustic travel time and reflection magnitude Sonde inclination and azimuth Optical Televiewer BOTV Camera LED 86

44 Televiewer data analysis Borehole deviation Breakout/ovalisation Log Structural/Fracture Analysis Borehole cross-section 87 Summary of Geophysical Methods Seismic waves and properties Seismic methods Borehole seismic Down-hole Cross-hole Suspension Cross-hole Tomography Surface seismic Refraction Reflection Surface Wave Electrical Properties Electrical Methods Electromagnetic Wave Method Time Domain Reflectometry Ground Penetration Radar Electrical methods (Potential measurement) DC resistivity Spontaneous Potential Electromagnetic Induction Frequency Domain EM Time Domain EM

45 Thank You! Chih-Ping Lin Distinguished Professor, Department of Civil Engineering & Natural Hazard Mitigation Research Center National Chiao Tung University, Taiwan