Geophysical Site Investigation (Seismic methods) Amit Prashant Indian Institute of Technology Gandhinagar Short Course on Geotechnical Aspects of Earthquake Engineering 04 08 March, 2013 Seismic Waves Energy travelling through earth layers Record 2 1
Types of Waves Body waves Through interior of earth Surface waves Travelling along the earth surface like on water ripples 3 Body Waves Compression wave Compression and Expansion Volume change Velocity-Moisture content dependent Fastest body wave Shear wave Do not travel through fluids No Volume change 4 2
Surface Waves Rayleigh wave Amplitude decreases exponentially with depth. Love wave Faster than Rayleigh waves 5 Body waves Seismic Waves Surface waves 6 3
Wave Velocities P-wave velocity V p Shear Wave velocity V s V p > V s 7 Soil Properties from Wave Velocity Shear Modulus G 2. V s Density of soil Constrained Modulus, M 2. Vp V 3V 4V Young s Modulus, E 2 2 V V V Poisson s Ratio, 2 V 2 2 2 s p s 2V p 2 2 p s V 2 2 p s s 8 4
Typical Values of Wave Velocities Material P-wave Velocity (m/s) S-wave Velocity (m/s) Air 332 Water 1400-1500 Petroleum 1300-1400 Steel 6100 3500 Concrete 3600 2000 Granite 5500-5900 2800-3000 Basalt 6400 3200 Sandstone 1400-4300 700-2800 Limestone 5900-6100 2800-3000 Sand (Unsaturated) 200-1000 80-400 Sand (Saturated) 800-2200 320-880 Clay 1000-2500 400-1000 Glacial Till (Saturated) 1500-2500 600-1000 9 Wave propagation and Soil Properties - Need, G,, G,, G,, G,, G, 10 5
Wave Velocities in Geomaterials 11 Preparation for Investigation Always visit site first. AVOID SURPRISES Database Information Maps Topographical, Air photos, Geological maps (bedrock and surficial geology), Soil survey maps, Oil company logs Water Well Logs Previous Reports Internal studies, Old reports, Previous consultant s reports Local practice, foundation types for similar structures nearby Representative samples, nearby boreholes 12 6
Steps in Investigation Reconnaissance : Ground surface profile, Rock outcrop, Locality and constraints, Utilities, Interviewing residents, etc. Method selection : Technical, Cost considerations Designing the survey : Configurations to serve the objective Date Acquisition Data Processing : Signal processing, Modeling Interpretation : Soil properties with depth 13 Seismic System Computer Source Receiver Seismic waves 14 7
Seismic Measurement-Systems 1. Geophone 2. Cable 3. Hammer (Source) 4. Processing and Control Unit 15 Geophone 16 8
Seismic Source Hammer Vibroseis Betsy Gun Dynamite Air Guns (In water) 17 Seismic Source 18 9
Seismic Methods Seismic Reflection Method Seismic Refraction Method Cross-Hole Test Down Hole Test & Up-Hole Test Spectral Analysis of Surface Wave (SASW) Multichannel Analysis of Surface Waves (MASW) method Bender Element Test in Laboratory 19 Waves from point source 20 10
Snell s Law Critical Angle of Refraction A V 1 1 sin V2 21 Seismic Refraction Method Depths less than ~ 30 m Cost Effective as compared to Reflection method (<3to5 times) Used for computation of layer thickness of soil http://www.geologicresources.com/seismic_refraction_method.html 22 11
Measurement at a Geophone 23 Shot Record uniform deposit 24 12
Time (s) Shot Record real deposit Source 25 Two Layer System h Vt 1 i L 2 1 V 1 A sin V2 h L.cos A 26 26 13
Multi-Layer System 27 27 Seismic Reflection Method Depths greater than ~15 m Particularly suited to marine applications (e.g. lakes, rivers, oceans, etc.) The inability of water to transmit shear waves makes collection of high quality reflection data possible even at very shallow depths that would be impractical to impossible on land. 28 14
Differences in Seismic Reflection and Seismic Refraction Method Seismic Reflection uses field equipment similar to seismic refraction, but field and data processing procedures are employed to maximize the energy reflected along near vertical ray paths by subsurface density contrasts. http://www.enviroscan.com/html/seismic_refraction_versus_refl.html Seismic Refraction involves measuring the travel time of the component of seismic energy which travels down to the top of rock (or other distinct density contrast), is refracted along the top of rock, and returns to the surface. 29 Cross-Hole Test Sensors are placed at one elevation in one or more boring. Source is triggered in another boring at the same elevation. S wave travels horizontally from source to receiving hole, and the arrivals of S waves are noted Shear wave velocity (Vs) is calculated by dividing the distance between the bore holes and the travel time. 30 15
Cross-Hole Test geo.cv.nctu.edu.tw/enggeo/download/d4428d4428m.pdf 31 Cross-Hole Record http://www.structuremag.org/article.aspx?articleid=994 32 16
Down Hole Test Sensors are placed at various depths in the boring. Source is located above the receivers, at the ground surface. Only one bore hole is required. A source rich in S wave should be used (P wave travels faster than S wave) Up-Hole method: source of energy is deep in boring and http://www.geophysics.co.uk/mets3.html the receiver is at the ground surface 33 Seismic Cone Penetration Test (SCPT) A Down-Hole Test Seismic cone is pushed into the ground Shear wave is generated at the top and the time required for the shear wave to reach the seismometer in the cone is measured Computer in the SCPT rig collects and processes all the data & shear wave velocity is measured 34 http://geoprobe.com/how-seismic-cone-penetration-equipment-works 17
Down-Hole Test Record SPT Velocity (m/s) Time (s) http://www.belirti.com/english/downhole.htm 35 SASW Test 36 18
Surface wave dispersion Approximate distribution of vertical motion in particles with depth for two surface waves of different wavelengths 37 Sensor Array: Midpoint Array Source C L S d Near Receiver Far Receiver 38 19
Dispersion Curve 39 Inversion Analysis for Interpretation of data The shear wave velocity profile that generates a dispersion curve that most closely matches the field dispersion curve is then presented as the shear wave velocity profile for the site. 40 20
Bender Elements Piezoelectric Sensors Bender element is a 2-layer system of the piezoelectric sensors. Piezoelectric sensors change their dimensions when electrically charged by a voltage. They can generate electric charge when mechanically stressed by a force. The element produces curvature, when one layer expands and the other layer contracts. With alternating electric charge, the element can vibrate and work as a wave generator. 41 Source Receiver Receiver Source 42 21
Bender Element System (BES) Sensors Signal http://www.sciencedirect.com/science/article/pii/s0267726104001563 43 BES Measurements P-Wave velocity: V p L 2l t p S-Wave velocity: L = Distance between source and receiver element V s L 2l t s l = Length of the element 44 22
Damping Ratio using Half-Power Method By varying the frequency with constant input voltage amplitude f f 2 1 2 f m Or, sometimes it is preferred to use f f 2 2 2 1 2 4 f m 45 Thank You 46 23