Dmitry Chizhik
Naval Undersea Warfare Ctr., New London, CT 06320
Wideband bottom loss measurements were conducted at two sites in the eastern Mediterranean. The data were processed incoherently to determine bottom loss in one octave bands at center frequencies ranging from 200 to 4000 Hz and spanning grazing angle range of 10(degrees)--90(degrees). Geological literature indicated that the bottom at Herodotus abyssal plain location was a ``fast'' turbidite bottom, while the Nile cone site was characterized by a ``slow'' silty-clay bottom. The bottom loss model was formulated for each site and was found to be in good agreement with the data. A geometric ray model is often used for bottom loss prediction which includes the refracted (diving) wave arrival at the proper grazing angle. This approach works well at high frequencies and high grazing angles, but gives erroneously low bottom loss values at low frequencies and low grazing angles. A hybrid ray/plane wave model was developed to account for the additional loss due to penetration of the wave into the sediment in excess of that predicted by the geometric ray theory. This correction is most important for ``slow'' bottoms at low grazing angles and low frequencies, as demonstrated by comparison to data.