Long Range Sediment Tomography

A scheme for the estimation of compressional wave speed and attenuation has been developed using broadband explosive sources. Sediment compressional speeds were estimated using hybrid methods based on the modal dispersion behavior of broadband acoustic propagation. In this hybrid scheme, we have used Genetic Algorithm (GA) as the main search tool. Local optimization tools were used to finetune the inversion. This scheme is discussed in detail by Potty, Miller, Lynch and Smith. Attenuation values were estimated by another inversion scheme based on modal amplitude ratios. In addition to sediment properties other parameters such as bathymetry, source-receiver range, source depth, receiver depth and source level were also treated as unknowns in this inversion scheme. Source receiver range was also estimated using the travel time differences of different modes and the calculated group speed values. This inversion scheme as applied to the data from the Shelf Break Primer Experiment conducted in summer of 1996 is described in detail by Potty, Miller and Lynch. The results of this inversion were successfully compared with core data from Atlantic Margin Coring (AMCOR) project and gravity cores. When we have a number of explosives deployed over a large area this inversion scheme can be applied along each sourcereceiver path. Hence we can come up with a 3-D description of the sediment properties over an area.