Near-bottom observations of 4 kHz acoustic reflectivity and attenuation

Abstract A quantitative 4-kHz seismic profiling system has been developed as part of the Deep Tow Instrumentation System of the Marine Physical Laboratory at Scripps Institution of Oceanography. The system utilizes a sea-going digital computer system aboard ship for real-time processing and display of the quantitative near-bottom reflectivity data. Preliminary results from the system are presented for four diverse areas of the Pacific Ocean floor: the Samoan Passage, the Carnegie Ridge, the San Diego Trough, and, in particular, an abyssal hill region several hundred miles west of San Diego where the system was first operated in real time during a survey in May 1974.These results exhibit a number of substantial variations in observed reflectivity on a small horizontal scale. To be more specific, they show (1) several db variations in layer reflectivity for horizontal distances less than 100 m for well-defined reflectors; (2) abrupt changes of more than 7 db in subsurface reflectivity, which are generally coincident with topographic breaks in slope; (3) several db variations in reflectivity resulting from topographic focusing by buried reflectors, suggesting larger variations for surface observations; and (4) consistently diffuse returns from volcanic basement at relatively low energy levels which are substantially reduced by thin sediment cover, suggesting significant scattering at this frequency.In addition, calculations of acoustic attenuation in the sediment column have been made from the quantitative data by determining reflectivity differences for a well-defined layer as a function of depth of burial. These calculations give a value of 0.36 db/m for clayey silt in the San Diego Trough, which is consistent with measured values. A value of 0.12 db/m is obtained for calcareous ooze on the Carnegie Ridge. This low value for attentuation suggests that significant lithification may have occurred in these relatively young, high-porosity sediments. Sediment physical property analyses show relatively high velocities in these calcareous sediments, suggestive of greater rigidity.