Measuring lateral variability of sediment geoacoustic properties

Dispersion analysis of interface waves (Scholte/Stoneley/Rayleigh) has been used extensively to determine geoacoustic models of near-bottom ocean sediments. In the traditional approach, a single seismic trace is analyzed to obtain group velocity dispersion using the multiple filter technique. A dispersion curve obtained in this manner reflects the averaged influence of the sediment acoustic properties over the entire path from source to receiver. Thus a geoacoustic model derived from this dispersion curve is not a true vertical section in the vicinity of the receiver unless the sediment properties do not change over the entire propagation path. However, when a linear array of receivers is used to obtain data simultaneously at a number of different ranges, it is possible to derive both group and phase velocity dispersion curves that correspond to propagation over a short portion (typically 5 m) of the total path at many different ranges. With this kind of data, the range-dependent variation of geoacoustic properties in the sediment may be assessed. Results are presented based on data obtained with a 24-receiver array and an explosive source, both deployed on the seafloor. Analysis of data from a number of shallow water sites suggests that there is a considerable amount of lateral inhomogeneity even over very short ranges.