GEOPHYSICAL INVESTIGATIONS FOR IDENTIFICATION OF SUBSURFACE. Arindam Dey Assistant Professor Department of Civil Engineering IIT Guwahati

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1 GEOPHYSICAL INVESTIGATIONS FOR IDENTIFICATION OF SUBSURFACE STRATIGRAPHY AT IIT GUWAHATI Arindam Dey Assistant Professor Department of Civil Engineering IIT Guwahati

2 What is MASW NDT Procedures SRS, GPR, NMR/PMR, SASW, MASW Seismic exploration method Evaluating ground stiffness 1D, 2D or 3D formats 19 December

3 Various Types of MASW Procedure of generation of waves Active MASW method Waves generated due to impact controlled by the user Passive MASW methods Utilizes the waves generated passively from natural and anthropogenic activities such as traffic, thunders, tidal motion, waves, air pressure changes and Microtremors Passive remote method Passive roadside method Combined Active and Passive Technique 19 December

4 MASW Survey: Overall Procedure Common procedure of 1D, 2D or 3D MASW surveys Data acquisition Acquiring multichannel field records Commonly referred as Shot Gathers in seismic exploration Dispersion analysis Extracting dispersion curves One from each record Inversion analysis Back-calculation of shear wave velocity (V s ) variation with depth 1D V s profile from each curve Indian Geotechnical Conference IGC2014 Theoretical dispersion curve closest to the extracted one 19 December

5 EXPERIMENTATION PROGRAM

6 Location of the site Academic complex, Department of Civil Engineering, IIT Guwahati 26 11'05.5"N 91 41'31.4"E Experimentation site 19 December

7 MASW SURVEY Active MASW survey at the site Hammer Plate 19 December

8 Experimentation Program Geophones in a linear array Geophones spacing and offset: 2m Data Acquisition System Sampling frequencies 15000Hz, 3750Hz, 500Hz, and 50Hz 19 December

9 CROSS-HOLE SURVEY In-situ estimation of P-wave and S-wave velocities Two boreholes at 4m c/c distance 5D Sensor, Ballard energy generator and the borehole for insertion 19 December

10 CROSS-HOLE SURVEY Experimental procedure Striking of the Ballard shearwave energy generator at some specified depths Recording the signals using the 5D sensor located at the same depth in the other borehole Analysis of wave signatures recorded at various depths First arrival time of P- and S-waves (First arrival time-picking method) Estimation of the wave velocities using the distance between the boreholes Analysis of both upstrike and downstrike records Identify the arrival of S-waves based on polarity comparison of the particle motion 19 December

11 Data Processing and Analysis

12 DATA PROCESSING USING EASYMASW EASYMASW Commercial software Characterization of subsurface in terms of shear wave velocity Dispersion analysis technique Wavenumber-frequency spectra Phase velocity-frequency spectra (Greater resolution of results Chosen for the present study) Shear wave velocity profile for the top 30m of soil (V s-30 ) Provides idea about geotechnical parameters based on an inbuilt database Shear modulus, elastic and edometric modulus of the soil. 19 December

13 Dispersion Analysis Typical traces obtained from field records Selection of proper trace record Time cutting (if necessary) Typical dispersion image and curve Manual selection of dispersion curve Choice of selecting unimodal or multimodal features 19 December

14 Inversion Analysis Inversion analysis for shear wave velocity Optimization procedure to arrive at the final profile Optimization over depths of layers Optimization over shear wave velocity No optimization over unit weight and Poisson s ratio Final soil Stratigraphic profile as obtained from Easy MASW 19 December

15 Cross Hole Survey Polarity of Particle Motion Identification of P- and S-waves based on polarity reversal of particle vibration Identification based on the basis of the polarity comparison Direction of particle motion changes for S-wave propagation subjected to upstrikes and downstrikes Upstrike- impact energy generated by pulling the striker. Downstrike- impact energy generated by release of the striker 19 December

16 Results and Discussions

17 3D and 2D Dispersion Image Space Typical 3D dispersion image as obtained in the case of sampling frequency Hz Typical 2D dispersion image as obtained in the case of sampling frequency Hz 19 December

18 Limitation on Sampling Frequency Typical 2D dispersion image as obtained in the case of sampling frequency 50 Hz Maximum frequency processed by EasyMASW ~ 60 Hz Nyquist frequency (F nf ) Sampling frequency > 2F nf Minimum sampling frequency required for reasonable dispersion image 120 Hz 19 December

19 Effect of Sampling Frequency on Dispersion Image 50Hz 500Hz 3750Hz 15000Hz 19 December

20 Inversion Analysis Selection of unimodal or multimodal dispersion points for inversion scheme Guided by the 3D dispersion image space Shear wave velocity profile of subsurface as obtained from inversion scheme Defines a range of possibility of shear wave profile 19 December

21 Comparative V s Profiles Shear wave velocity profile of subsurface as obtained from analysis using various sampling frequencies during MASW test 19 December

22 CROSS-HOLE SURVEY Picking up of P- and S-wave first arrival Polarity reversal concept to identify SH wave arrival Energy jump concept to determine P wave arrival P- and S-wave velocity profiles obtained within the depth of 5m from Cross-Hole survey 19 December

23 Velocity profiles from MASW and CH surveys Shear wave velocity profile from MASW Survey Polarity reversal concept to identify SH wave arrival Energy jump concept to determine P wave arrival Shallow depth information not obtained due to chosen optimization bounds Can be chosen in thin layers to get shallow surface information Shear wave velocity profile from cross-hole survey Provided shallow surface information for this study 19 December

24 CONCLUSION Subsurface investigation of stratigraphic profile at IIT Guwahati in terms of shear wave velocity profiles Active MASW surveys Choice of sampling frequency affects the resolution of dispersion image space Sampling frequencies lesser than twice the Nyquist frequency fails to provide reasonable dispersion image Identification of range of possible shear wave profiles Shallow depth shear wave profile not obtained due to optimization bounds Cross Hole surveys Arrival of P- and S-wave velocities are identified through Polarity reversal and Energy shift approaches Identification of shallow depth shear wave profile 19 December

25 REFERENCES Heisey, J.S., Stokoe II, K.H. and Meyer, A.H. (1982), Moduli of pavement systems from Spectral Analysis of Surface Waves, Transportation Research Record, 852, ( ) Park, C.B., Miller, R.D., Xia. J. and Ivanov, J. (2007). Multichannel analysis of surface waves - Active and passive method. The leading edge. Louie, J.N. (2001). Faster better: Shear-wave velocity to 100 meters depth from refraction microtremor arrays. Bulletin of the Seismological Society of America, Okada, H. (2003). The microtremor survey method. Geophysical monograph series 12, Society of Exploration Geophysicists, Tulsa. Gosar, A. Stopar, R., and Roser, J. (2008). Comparative test of active and passive MASW methods and micro tremor HVSR method. Materials and Geo-environment, Park, C.B., Miller, R.D., and Xia. J., (1998), Ground roll as a tool to image near surface anomaly, 68th Annual International Meeting of SEG, Expanded Abstracts December

26 Workgroup Jumrik Taipodia Research Scholar, IIT Guwahati Assistant Professor, NIT Arunachal Dipjyoti Baglari Research Scholar, IIT Guwahati Assistant Professor, JEIST Jorhat Budha Ram Former UG Student, IIT Guwahati A. Murali Krishna Associate Professor Department of Civil Engineering IIT Guwahati 4 December

27 Thank You 18 December