Measurements of the Acoustic Backscatter of Selected Areas of the Deep Seafloorand Some Implications for the Assessment of Manganese Nodule Resources

摘要

On the expeditions Echo 1 and Echo 2 the acoustic backscatter from four differentseafloor environments was measured at various frequencies and grazing angles for 5 to 90 deg. Five transducer arrays had been mounted on the stern of the Deep Tow vehicle, similar sidescan sonars, but looking aft. At frequencies above 15 kHz the backscattering strength increases as one goes from deep-sea clay (14 deg N, 126 deg W)to silty clay (Patton Escarpment), manganese nodules (14 deg N, 126 deg W), and phosphorite nodules and slabs (Thirtymile Bank). This order was found to be preserved at all grazing angles. It is thus possible to distinguish acoustically between the four environments. Detailed studies of the backscattered sound over the nodule field have shown that the returned energy is coherent at normal incidence, while at other grazing angles it is not. At frequencies above 15 kHz it is possible to deduce the percentage of seafloor covered by manganese nodules directly from the measured backscattering strength. At 9 kHz, nodules smaller than 4cm can be distinguished from nodules larger than 6 cm. Keywords: Manganese/natural resources; Ocean bottom/minerals; Underwater sound equipments; Side looking sonar; Sonar transducers; Transponders/hydrophones; Reprints; Underwater acoustics; Acoustic backscattering; Mathematical models. (mm)