REFRACTION TOMOGRAPHY OVER A BURIED WASTE-DISPOSAL SITE

Determining the depth and geometry of a landfill's lower boundary is a difficult task. Potential field methods generally lack the necessary depth resolution, and seismic reflection data are usually contaminated by source-generated noise in the time range of interest (<50 ms). To address this problem, we have developed a surface 2-D tomographic refr action scheme that is based on a fast finite-difference eikonal solver and an inversion method that incorporates appropriate damping and smo othing constraints. This new scheme has been applied to a first-arriva l traveltime data set collected across adjacent landfills in northern Switzerland. High-quality seismic data were collected along five profi les that crossed the landfills and two that sampled undisturbed natura l sediments. Seismic waves generated from multiple shots were recorded on large numbers of closely spaced receivers during quiet evening per iods. Reliability of the resultant velocity tomograms was estimated on the basis of (1) ray diagrams, (2) plots of synthetic and observed tr aveltimes, (3) traveltime residual analyses, (4) comparisons of coinci dent velocity-depth profiles computed from intersecting profiles, (5) inversions with diverse input models, and (6) quantitative error analy ses using a bootstrap technique. At our study site, the base of the ne ar-surface natural layer and the lower boundaries of the landfills wer e defined by rapid increases in velocity from < 1000 m/s to > 1500 mis , with velocities in the upper parts of the models determined to withi n about +/- 100 m/s. The thickness of the near-surface natural layer v aried between 2 and 6 m, with occasional thickening to similar to 7 m. In contrast,low velocities associated with the landfills could be tra ced to 9 to 11 m depth. Although our results have demonstrated that th e tomographic refraction scheme may be an efficient and cost-effective means of studying the very shallow subsurface (<20 m depth), compleme ntary geological and other geophysical data were required to discrimin ate between velocity anomalies attributed to the landfills and those a ttributed to natural variations in the near-surface geology.