Measurements of two-dimensional spatial coherence of normal-incidence seafloor scattering.

Measurements have been made near normal incidence of the two-dimensional spatial coherence of the acoustic field scattered from the lakebed in Seneca Lake, New York. In the test region, the lakebed consists of a series of sediment layers created by a sequence of distinct depositional processes. The spatial coherence length of the scattered field is shown to be dependent on the structure of the underlying sediment sequences. Significant ping-to-ping variability in the spatial coherence surface was also observed for each sediment sequence. This variability is quantified by a two-dimensional spatial coherence metric that measures the coherence lengths and asymmetric coherence surface orientation. The ping-to-ping variation of the surface asymmetry appears to be linked to the spatial isotropy of the sediment scattering strength. The scattering strength of the deepest observed sequence in the sub-bottom is the most spatially isotropic and the ping-to-ping variability of the coherence lengths and surface orientations are random. The scattering strength of the shallower sequences is spatially anisotropic and the coherence lengths and surface orientations show intervals of non-random ping-to-ping behavior.

A metric for characterization of two-dimensional spatial coherence.

A metric is developed providing a quantitative measure of the two-dimensional spatial coherence of scattered fields. The metric is based on fitting a function similar to bivariate Gaussian to measured two-dimensional coherence surfaces. This function provides a robust fit to the measured data for a range of coherence lengths and surface asymmetries. Through an eigendecomposition of the bivariate Gaussian covariance matrix, it is possible to define surface orientation as well as coherence lengths along the major and minor axes. The metric is applied to normal-incidence scattering data collected in recent field trials at Seneca Lake, NY.