Identifying shallow subsurface stratigraphy and voids using dispersive characteristics of electromagnetic and surface waves

This dissertation presents the results of three manuscripts on spectral analysis of electromagnetic and seismic surface waves to detect subsurface stratigraphy and voids. In the first manuscript, a new technique was developed by utilizing spectral analysis of surface waves to detect subsurface openings. This technique applied the concept of group delay to evaluate the effects of subsurface voids on the phase shift domain. Seismic data sets were acquired at different sites where the shape, size, and depth of the void varied. In all surveys, the time delay technique precisely identified the locations of the subsurface openings. The second manuscript presents the results of attenuation analyses of seismic surface waves to identify buried concrete conduits. A conduit of known size and depth was chosen for the study. Vertical and horizontal component geophones with different frequencies (14-, and 100-Hz) were employed for this study. The third manuscript presents the results of a study on dispersive electromagnetic signals to identify the thickness and dielectric permittivity of thin soil layers using Ground Penetrating Radar (GPR) techniques. Data for this experiment were acquired in a large experimental tank, where the permittivity and thickness of each soil layer was controlled. Six different experiments were carried out with three soil types, and variable-offset GPR data were acquired using four frequencies (100-, 250-, 500-, and 1000-MHz). Dispersive GPR signals were analyzed using inversion of dispersion curves to estimate the permittivity and thickness of the overlying soil layers. The accuracy of these estimates were analyzed as a function of layer thickness, antenna frequency, and permittivity. Both the thickness and permittivity estimates were most accurate when the overlying layer had a low permittivity, and higher frequencies usually had more accurate results --Abstract, page iv