Near Surface Void Detection Using Seismic Q-factor Waveform Tomography

Several shallow, engineering-scale seismic surveys were carried out in Seven Sisters, Manitoba, between 1999 and 2001. The surveys were designed to locate three buried riprap targets, located within a constructed clay embankment as a blind test of geophysical imaging methods. While previous studies used near-offset reflection methods to image the targets, in this paper we use Waveform Tomography of the long-offset, refracted arrivals to image the seismic velocity and the seismic attenuation. The refraction surveys were carried out with a 2 m spacing on both source and receiver positions, with 70 source positions and 48 receiver positions respectively. The targets were buried approximately 7 m below the surface of the constructed embankment. Using weight-drop data, with frequencies between 20 and 130 Hz, we were able to resolve the sub-wavelength targets using the seismic-Q images. A very good fit between model and observed data is an indication of the reliability of the results.