A passive fathometer technique for imaging seabed layering using ambient noise

TLDR

Correlating ambient noise measured by two sensors yields a function resembling the two‑point Green’s function, which represents the impulse response between the sensors. The study presents a passive acoustics technique that uses ambient noise to determine water depth and identify seabed sub‑bottom layering, illustrated through numerical simulations. By cross‑correlating signals from one or more hydrophones—especially all pairs in a vertical array—the method generates a passive fathometer that maps bottom depth and detects significant reflectors, with reduced averaging time when the array drifts. Experiments with a fixed array (200–1500 Hz) and a drifting array (50–4000 Hz) demonstrated that the technique can produce depth maps and locate sub‑bottom layers.

Abstract

A passive acoustics method is presented that uses the ocean ambient noise field to determine water depth and seabed sub-bottom layering. Correlating the noise field measured by two sensors one can recover a function that closely resembles the two-point Green’s function representing the impulse response between the two sensors. Here, a technique is described that is based on noise correlations and produces what is effectively a passive fathometer that can also be used to identify sub-bottom layers. In principle, just one or two hydrophones are needed—given enough averaging time. However, by combining the cross correlations of all hydrophone pairs in a vertical array a stronger signature can be obtained and this greatly reduces averaging time. With a moving (e.g., drifting) vertical array, the resulting algorithm yields both a map of the bottom depth (passive fathometer) and the locations of significant reflectors in the ocean sub-bottom. In this paper, the technique is described and illustrated using numerical simulations. Results are also shown from two experiments. In the first, ambient noise is taken on a fixed array in the 200–1500Hz frequency band and the second experiment uses a drifting array in the 50–4000Hz band.