Sub-bottom scattering: A modeling approach

Bottom scatter strength data exhibit artifacts when the sub-bottom plays a role in the scattering process. The artifacts arise from the classical assumption in the data processing that the scattering process occurs at the water-sediment interface. Many of the current bottom scattering models, even those that explicitly treat sub-bottom scattering, do not address these artifacts. A new bottom scatter modeling approach is proposed. The essence of the approach is to (1) employ a geoacoustic model capable of predicting the sub-bottom insonified field, (2) avoid the usual plane-wave assumption, and (3) compute the received level in the time domain. The first condition suggests the analysis of bottom loss data concurrent with bottom scatter data and leads naturally to the idea of a self-consistent geoacoustic basis. The model is employed to analyze a data set that showed surprising discrepancies between low angle (<20°) and intermediate angle (280-50°) bottom scattering strength. The proposed acoustic model predicts this discrepancy, which arises from the fact that the two angular regimes are controlled by entirely different scattering mechanisms. The difference in mechanisms suggests caution in the current and common practice of extrapolation from survey (intermediate angle) measurements to the low angles of interest for system performance prediction.