Sediment Sound Speed and Critical Angle Estimations Derived from In Situ Acoustic Measurements

摘要

Understanding the basic physics of sound penetration into ocean sediments is essential to the design of sonar systems which can detect, localize, classify, and identify buried objects. The sediment sound speed is a crucial parameter as the ratio of sound speed at the water-sediment interface determines the critical angle. Sediment sound speed is typically measured from core samples using high frequency (100's of kHz) pulsed travel time measurements. Earlier work on subcritical penetration into sandy sediments has suggested that the effective sound speed in the 2-20 kHz range is significantly lower than the core measurement results. Simulation using Biot theory for propagation in porous media confirmed that sandy sediments may be highly dispersive in the range 1-100 kHz. It is shown that a direct and robust estimate of the critical angle and therefore the sediment sound speed, at lower frequencies can be achieved by analyzing the grazing angle dependence of the phase delays observed on a buried array. A parametric source with secondary frequencies in the 2-16 kHz range was directed toward a sandy bottom similar to the one investigated in the earlier study. An array of 14 hydrophones was used to measure penetrated field. The critical angle were estimated by analyzing the variations of signal arrival times versus frequency, burial depth, and grazing angle. Matching the results with classical transmission theory yielded a sound speed estimate in the sand of 1626 m/s in the frequency range 2-5 kHz, significantly lower than the 1720 m/s estimated from the cores at 200 kHz. This dispersion is consistent with Biot theory.