Measurements and predictions of transmission loss at two sites in the Southwest Approaches to the English Channel

Abstract

As part of a continuing series of measurements being made by

SACLANTCEN in representative shallow water areas an experiment was conducted in the southwest approaches to the English Channel. Measurements were made at two sites with different bottom characteristics. At one site there were long NE-SW trending ridges while at the other site the nearly flat bottom was chalk, with little or no sediment cover. Transmission loss was measured along two tracks at the first site, one along and the other perpendicular to the line of crests and troughs of the sand waves. At the other site, measurements were made along a single track. At both sites explosive sources and a four element receiving array were used. The transmission loss data were processed in 113 octave bands and analysed at fresuencies of 100, 200, 400, 800, and 1600Hz. Attempts to make direct measurements of bottom acoustic parameters were largely unsuccessful; instead the bottom acoustic parameters were adjusted to obtain a good fit to the transmission loss data with the SAFARI prediction model. The resulting geoacoustic models were reasonably consistent with knowledge of the bottom obtained from other sources. It was found that there was no significant difference in transmission loss measured along and perpendicular to the crests and troughs of the ridges at the first site. Transmission loss was much higher at the second site due to the highly absorbing chalk bottom. Unusually high loss was measured at the second site for the shallow receiver. This can be understood in terms of the characteristics of the modal structure of the acoustic field.