DETECTION OF SUBSURFACE VOIDS IN STRATIFIED MEDIA USING SEISMIC WAVE METHODS
Open Access
- Author:
- Srivastaa, Ashutosh
- Graduate Program:
- Civil Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- April 03, 2009
- Committee Members:
- Jeffrey A Laman, Thesis Advisor/Co-Advisor
- Keywords:
- Wave propagation; Void detection; Continuous wavel
- Abstract:
- The primary objective of this study is to investigate the effect of sub-surface anomalies such as voids in the stratified soil media on surface wave propagation. A data processing protocol was developed for processing seismic wave data for void detection by studying the signal simultaneously in the time and frequency domain using continuous wavelet transformation (CWT). The effect of voids in the soil media was examined by qualitatively comparing the signal properties acquired from the controlled laboratory experiments on the soil media, both with and without voids. For the controlled experimental study, a wooden box of dimensions 4.5m x 1.67m x 1.37m ( ), was constructed and filled with sand and gravel in two layers. A void of known dimension was excavated in the soil mass in the box at a known location. Micro seismic waves were produced using a 7.25kg (16-lb) sledge hammer and a rubber mallet. The vertical response of the soil mass surface was recorded using the SignalCalc®620 Dynamic Signal Analyzer and was processed using the MATLAB® 7.0 wavelet toolbox. Time-frequency plots of the seismic wave signals obtained from the unvoided soil mass experiment indicate that damped, uniform undulations are due to the surface wave dispersive behavior. Also, data obtained from the voided soil mass experiment indicate that the void anomalies cause low strength ripples in the time-frequency plots, usually in the low frequency region of the time-frequency plots. This observation has been used to study the properties of voids. In addition to the experimental study, a numerical study was also conducted. The wave propagation phenomenon was simulated for voided and stratified regions using the finite difference method in the Wave2000pro software. Thus, a refraction test was performed in the soil box to determine the shear wave velocity profile. The receiver data was processed with the same protocol that was used for analyzing the experimental test data conducted in the soil box with void. The time-frequency maps constructed using the experimental data confirm the numerical results. Finally, the time-frequency maps using different types of wavelets for the same set of experimental data were compared. From this analysis it was concluded that the wavelets that correlates with the properties of the original signal produce time-frequency plots with all the signal features distinctively so that all the signal properties can be separately studied. Thus, wavelet analysis of the seismic wave signals obtain from the micro-seismic tests can effectively investigate the sub surface void anomalies.