Application of Seismic Refraction Tomography to Karst Cavities

For three years we have used synthetic and field data to investigate the effectiveness of commercial refraction tomography codes on both simple and complex subsurface velocity structures, with the ultimate goal of determining the suitability of the method for karst problems. The results of these studies indicate that refraction tomography is able to resolve karst features under some conditions. The analysis of field data acquired on the Oak Ridge Reservation, TN shows low velocity zones on three parallel seismic lines. These zones are located at similar depths and fall on a line that is parallel to geologic strike, leading to an interpretation of a possible karst conduit. This feature has velocities of about 1500-2000 m/s in a matrix of 30004000 m/s, reasonable velocities for a mud filled void in saprolite at these depths. Drilling of this feature is anticipated in the near future. Analysis of a seismic line taken over the known mud-filled cavity shows a low velocity feature with a location consistent with drilling results. The velocity of the feature is about 1000 m/s, a value that is a little lower than that found for the features discussed above. Synthetic modeling sometimes generates results similar to the field results, but often fails to image cavities as well, or at all. Ongoing investigations are aimed at refining our understanding of the circumstances where these methods can be successful, and investigating the relevance of model results to actual field conditions.