Acoustic shadowing by an isolated seamount

Abstract

Acoustic shadowing by an isolated seamount has been studied by examining the multipath propagation measurements obtained in a shot run that passed over the seamount peak. Source depths of 24 and 196 m were used in the experiment. In the acoustic shadow, the propagation loss for the shallow 24-m shots increased by 10–15 dB over the loss expected in the absence of the seamount. Examination of the pressure-time history for shots deployed in the shadowing region revealed that the signals consisted of two components. The first and dominant pulse was determined to be a diffracted wave which passed over the seamount by rough-surface forward scattering and diffraction. The subsequent group of weaker pulses was attributed to the energy which had passed over the seamount by a series of surface–bottom interactions. The shadowing loss increased by 3 dB per octave for frequencies greater than 50 Hz, in agreement with theory, but is appreciably greater than the predicted values at lower frequencies. The shadowing loss for the 196-m shots was about 5 dB less than that observed for the shallower shots.