Hatteras Abyssal Plain Low Frequency Bottom Loss Measurements

Bottom-loss measurements were made for frequencies less than 800 Hz at three stations in the Hatteras Abyssal Plain. The stations were each separated approximately 55 km along 70 deg. 30W longitude. The data acquisition procedure was to launch two self-recording submersibles AUTOBUOYS and then maneuever from and to the launch site detonating explosives as the AUTOBUOYS recorded data. Because of the experimental geometry, the direct and bottom-interacting signals were time-separated as they arrived at the AUTOBUOYS. Thus, a self-calibrating processing procedure could be used to compute bottom loss values. Bottom loss versus grazing angles 5-66 deg was obtained over a wide bandwidth 90-790 Hz and for a number of narrow bandwidths 13 octave and less. Broadband results were found to be homologous between stations, whereas the narrowband results were less correlated. Negative bottom loss values were also observed. Analyses of the impulse responses for the southern station also aided in developing a geophysical description of the sediment structure. The derived structure consists of a thin 21 m, constant sound-speed layer overlaying a thick layer having a linear depth-dependent sound speed. This structure is representative of a class of documented subbottom sediment structures that can support the propagation of low-frequency acoustic signals traversing non-plane wave reflected and refracted acoustic paths.