Using complex intensity to detect signals masked by shipping noise

Detecting weak signals amidst strong interfering sources remains a relevant ocean acoustics research problem, especially in regions with significant shipping activity and/or fast-moving sources. Acoustic vector sensors, which measure particle velocity along with pressure, provide an opportunity to detect weak signals under circumstances where hydrophone arrays are impractical. These sensors can instantaneously estimate a field’s predominant directionality by constructing a complex intensity vector. Plotting this directionality as an “azigram,” analogous to a spectrogram, provides new avenues for signal detection in an acoustic field dominated by a spatially compact source. Here, we use theory and data to examine how a vector sensor can enhance the detection of whale calls in the presence of shipping noise strong enough to mask the calls on a conventional hydrophone without employing computationally intensive conventional beamforming. We investigate how the presence of a secondary source influences the direction and magnitude of the active/reactive intensity and the polarization of the velocity field, as well as analyze the combinations of signal-to-interferer ratio (SIR) and angular separation over which weak source detection may be feasible. These approaches have applications for passive acoustic monitoring of biological activity in the presence of anthropogenic noise. [Work sponsored by ONR TFO.]