Near-bottom 4-kHz reflectivity observations in real time

A sea-going digital computer system has been utilized to process and display quantitative near-bottom 4-kHz sounding data in real time for an abyssal hill region of the sea floor 200 miles west of San Diego. Over a region approximately 50 miles square, variations in bottom reflectivity of as much as 10 dB were observed, with abrupt changes in sediment character and reflectivity (as much as 7 dB). Four precisely navigated cores were taken from the area for physical property analysis, one of which sampled a buried 3-cm-thick manganese layer. Quantitative 4-kHz reflectivity data from the above area as well as several others in the Pacific are being further analyzed in an attempt to infer acoustic attenuation values for sedimentary layers from variations in layer reflectivity with burial depth. Preliminary results are in good agreement with published values of directly measured attenuation in ocean sediments. A value of 0.12 dB/m at 4 kHz obtained for calcareous ooze of approximately 75% porosity from the Carnegie Ridge represents a relatively low value in comparison with published attenuation values, suggesting significant lithification of this high-porosity sediment.