Scales of time and space variability of sound fields reflecting obliquely from underwater slopes

Abstract

Ocean flows impart a time dependence to interference patterns of sound reflecting at low horizontal angle from sloping bathymetry. This is a higher level of complexity than interference patterns within vertical slices in spherically symmetric environments. The time-space statistics may be co-dependent and non-normal, making conventional methods inapplicable. Here, patterns that occur when 100-1000 Hz sound reflects at slopes are simulated with three-dimensional methods. Multiple statistics of sound at virtual arrays are computed over the domain, including incoherent power, beam power, spatial correlation length, and array gain assuming idealized noise conditions. These depend on source and receiver locations with respect to bathymetric features, and can be computed for instantaneously sampled ocean conditions to study their evolution within the time frame of the dominant flow, or computed via averaging over a few periods of the dominant flow features (tides, for example). Here, the spatial complexity of patterns found in 100-Hz (and upward) simulations at a slope area off San Diego, CA in a time varying flow are linked to the imposed seafloor roughness as well as geometry, with mean intensity, scintillation index, and correlation scale used to quantify the effect. Implications for statistics employed in detection and tracking algorithms are discussed.