Passive Localization of Calling Animals and Sensing of their Acoustic Environment Using Acoustic Tomography

When monitored by several acoustic receivers, the sounds emitted by terrestrial and marine animals can be used to localize their positions. The localization can proceed quickly and automatically with the use of computers when cross-correlation of the acoustic records is used to optimally estimate the travel-time difference between signals at different receivers. The time-bandwidth product (call duration x acoustic bandwidth) of the call is an important parameter. When the time-bandwidth product of the acoustic call equals or exceeds 10 (which is the case for many animals), localization is possible even when the signal-to-noise ratios of the call are below the noise level, as long as cross-correlation is used to estimate the travel-time difference. Use of cross-correlation significantly increases the range at which a calling animal may be detected over that possible by visual inspection of acoustic records. The error maps for the localization can be improved by factors from 2 to 100 if tomographic techniques are used to account for the fluctuation in winds (currents) and sound speed and the errors in the receiver positions. In addition, useful tomographic maps of the atmospheric wind and the sound-speed fields can be obtained from the sounds emitted by the animals. Tomographic maps of the currents and the sound speed in water are much less precise because the fractional change in the acoustic propagation speed from currents and sound-speed fluctuations is much smaller in water than in the air. Studies in ecology, behavior, and conservation could be significantly enhanced by using acoustic tomography.