This presentation will summarize the technique of tomography and its application in characterizing a contaminant plume in the shallow subsurface. Hydrocarbons from an undetermined source migrated into a storm water sewer and leaked into a sensitive environment. Emergency measures were implemented to contain the spill on the water and included cementing a liner within the sewer to seal off the hydrocarbon seepage. Efforts to identify the source of the release were conducted concurrently. A soil boring program was performed to delineate a suspected hydrocarbon plume that potentially originated from an off-site source. The drilling disproved the presence of a contaminant plume and indicated that the hydrocarbons were possibly migrating along unidentified conduits within a massive subsurface concrete and stabilized sand structure. A field decision was made to place capped PVC riser pipe within the borings for utilization in a cross-hole tomography survey. The tomography survey was conducted between the boreholes to delineate the hydrocarbon source without extensive excavation.

Hydrophones were placed within three of the PVC pipes while an air gun was used as a signal source in the fourth well (centrally located). All hydrophone and air gun leads were wired to a geophysical recording truck, which was used to record and pre-process the field data. The receivers and air gun were alternately placed within all the PVC pipes which resulted in complete three-dimensional coverage of the suspected leak area.

The field data was commercially processed and interpreted. The printed data revealed two velocity anomalies that represented potential accumulations of hydrocarbons. This data was used to direct excavation activities, which later confirmed the presence of the subsurface hydrocarbons and the conduit through which the contaminants were migrating.

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