Detection of subsurface cavities using the spectral-analysis-of-surface-waves method.

Abstract

The purpose of this research was to investigate the potential of the spectral analysis of surface waves (SASW) method to detect buried objects in general and subsurface cavities in particular. A systematic experimental investigation under well- controlled conditions in an indoor facility which consists of a 22 ft diameter by 5.5 ft deep sand bin. The bin is filled with dry silica sand compacted to a uniform density, and a three-cell void (4 ft long by 2 ft wide by 1 ft deep) was buried in the center of the bin with a cover depth of 1 ft. Tests were performed with all three cells empty, center cell empty, and all cells filled. SASW tests were performed using an electro-mechanical shaker as a source, new testing geometries, and segmental frequency bands. Dispersion curves obtained from the SASW tests show substantial differences for over-the-object lines compared with free-field, with a maximum difference of about 40%. To achieve this maximum difference, it is necessary to optimize the spacing of the receivers. The optimum spacing was found to vary with the test direction with respect to the object as well as with the object condition. Larger voids produced greater perturbations in the SASW dispersion curve, but for wavelengths large compared to void dimensions, the dispersion curve rejoins the free-field dispersion curve. SASW results were compared with other experimental methods for detection of a buried object under different conditions. The crosshole test was used to determine the shear wave velocity profile. The ground penetrating radar (GPR) technique was used to compare the SASW results (seismic method) with another geophysical but nonseismic method (electromagnetic method) at a site where both are applicable. The potential for further development of the SASW as a void detection method is suggested by the results obtained. However, rapid data processing and enhanced data presentation, and three-dimensional inversion program are necessary for the development of SASW tomographic technique.