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 i s analyzed to obtain group velocity dispersion using the multiple filt er 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 t his dispersion curve is not a true vertical section in the vicinity of the receiver unless the sediment properties do not change over the en tire propagation path. However, when a linear array of receivers is us ed to obtain data simultaneously at a number of different ranges, it i s possible to derive both group and phase velocity dispersion curves t hat correspond to propagation over a short portion (typically 5 m) of the total path at many different ranges. With this kind of data, the r ange-dependent variation of geoacoustic properties in the sediment may be assessed. Results are presented based on data obtained with a 24-r eceiver array and an explosive source, both deployed on the seafloor. Analysis of data from a number of shallow water sites suggests that th ere is a considerable amount of lateral inhomogeneity even over very s hort ranges.