Extracting the rayleigh reflection coefficient from the passive fathometer

Mapping and profiling the ocean floor has, until recently, relied on the use of active sonar. These systems use transducers to transmit a known waveform toward the seabed, and hydrophones to listen for its reflections. In recent years, it has been shown that passive sonar systems, which use only hydrophones, can also be used for such purposes. Instead of artificial sounds, these techniques utilize naturally occurring noise on the ocean surface as a sound source. One of these techniques, called the passive fathometer, is able to produce a vertical profile of the seabed from breaking wave and wind noise. The time series it produces is intimately related to the acoustic properties of the seabed, as well as a quantity called the Rayleigh reflection coefficient (RRC). This paper presents a technique for estimating the normal incidence RRC using the passive fathometer. This is accomplished by time-gating the output and scaling the result using an incoherent estimate of the total power loss. Determination of the time-gate range is based off of a closed form solution that approximates the output of the passive fathometer. Using an idealized model of a vertical line array beamformer, it is shown that when surface noise undergoes several bottom-surface bounces, the correlation of end-fire beams produces a time domain representation of the RRC along with reverberations. Moving to a more realistic beamformer, it is shown how leakage at the opposite end-fire direction of the array produces additional artifacts in the time series output. In addition, leakage at oblique angles is used to explain artifacts that appear at the start of the time series that span twice the length of the array. By being able to predict the output of the passive fathometer for a given RRC, it is shown how an estimate of that coefficient can be extracted from ambient noise data. A simulation using the Ocean Acoustics and Seismic Exploration Synthesis (OASES) tool is presented which illustrates this technique.