Estimating low-frequency sediment sound speed dispersion from the horizontal coherence of the head wave excited by a light helicopter

Abstract

A series of shallow-water acoustic experiments has been conducted off the coast of southern California using a Robinson R44 helicopter as a low-frequency (≈ 13–3500 Hz) sound source. The aim of the experiments was to recover the sound speed of a fine to very-fine sand sediment from the horizontal coherence of the head wave excited in the water column by the helicopter. Two hydrophones, separated horizontally by approximately 15 m and situated 0.5 m above the seabed, received the head-wave signals, allowing the coherence function to be formed over the bandwidth of the airborne source. The sediment sound speed was recovered by matching the zero crossings of the measured coherence function to those predicted from a recently developed theory of head-wave generation in shallow water. Using this technique, the dispersion in the sediment sound speed can be estimated over a frequency range extending between 27 Hz, the lowest zero crossing of the coherence function, and 3.5 kHz, the bandwidth of the source. In the middle of the frequency band, the sound speed of the sediment was estimated to be 1682 ± 16 m/s, consistent with the known sediment type. [Research supported by ONR, SMART(DOD), NAVAIR, and SIO.]A series of shallow-water acoustic experiments has been conducted off the coast of southern California using a Robinson R44 helicopter as a low-frequency (≈ 13–3500 Hz) sound source. The aim of the experiments was to recover the sound speed of a fine to very-fine sand sediment from the horizontal coherence of the head wave excited in the water column by the helicopter. Two hydrophones, separated horizontally by approximately 15 m and situated 0.5 m above the seabed, received the head-wave signals, allowing the coherence function to be formed over the bandwidth of the airborne source. The sediment sound speed was recovered by matching the zero crossings of the measured coherence function to those predicted from a recently developed theory of head-wave generation in shallow water. Using this technique, the dispersion in the sediment sound speed can be estimated over a frequency range extending between 27 Hz, the lowest zero crossing of the coherence function, and 3.5 kHz, the bandwidth of the source. In the...