Remote Sound Source Localization Using a Horizontal Array

Seismic reflection surveys use low frequency acoustic energy by rapidly releasing compressed air downwards (air gun signals) to image the structure of the seafloor. The reflected signals from and beneath the ocean bottom are received by an 8 km seismic horizontal array (called streamer) containing 636 hydrophone channels with 500 Hz sampling rate towed at a constant depth. The purpose of this research is to develop and evaluate a technique to locate an underwater sound source using a long horizontal array of sensors (in this case the seismic streamer) and expand it to whale localization in the vicinity of seismic surveying.

Figure 1: Cartoon showing the seismic streamer towed behind the research vessel.

The method for locating the sound source with the data recorded by a hydrophone streamer uses the normalized root-mean-square (rms) travel time residual. It is a grid search technique based on calculating the propagation time between a trial point in the search grid and a hydrophone of the streamer. Since the speed of sound decreases with depth in the ocean thermocline, there is no direct path between a shallow source and a receiver at large ranges. So, the propagation time cannot be directly calculated by dividing the range by the average speed of sound. In this case, the horizontal velocity, the velocity of rays travelling on paths along the sea surface, should be used instead of the average speed of sound.

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