Sensitivity of Predicted Shallow Water Propagation Loss to Empirical andExtrapolated Bottom Loss Values

摘要

Historically, the majority of measured bottom loss values were obtained in deepwater at grazing angles of 20 deg or greater. An empirical fit to these data, if extended smoothly down to small angles, results in a value greater than 2 dB per bounce at 0 deg. Geophysical models suggest that for hard bottoms, a critical angle would be reached in this low-grazing angle region and that the bottom loss would then drop sharply, reaching a zero value at 0 deg. Under downward refracting conditions in shallow water, low-grazing angle paths may provide, in many cases, the dominant propagation mode. Hence, the value of low-grazing angle bottom loss is critical especially under strongly downward refracting conditions. Following the example of Urick, an empirical bottom loss formula developed by Bell has been modified. At low-grazing angles, its regular value at 1 0 deg has been linearly extrapolated to intersect a zero value at 0 deg. An analysis is conducted for each formula (empirical and extrapolated) for several shallow-water locations and source-receiver configurations. Bottom bounce, Propagation loss, Bottom loss, Shallow water environment.