Abstract:
The bidirectional reflectance distribution function (BRDF) of a typical highly lineated deep-ocean ridge on the western flank of the Mid-Atlantic Ridge is estimated by applying classical radiometric theory to long-range bistatic reverberation measurements obtained during the ARSRP Main Acoustics Experiment of 1993. The BRDF of a surface is defined as the ratio of the reflected radiance toward the direction of the receiver to the irradiance in the direction toward the source. Therefore, such effects as two-way transmission loss, the areal resolution of the measurement, and its foreshortening are accounted for in computation of the BRDF. (For example, the BRDF for a Lambertian surface is simply 1/(pi).) In particular, high-resolution bathymetry of 5-m resolution, roughly the scale of the acoustic wavelength for the low-frequency measurements, is used to determine surface orientation and foreshortening. The parabolic equation is used to compute two-way transmission; a ray trace is used to determine incident and scattered angles at the surface; and a spatial convolution is used to account for the spatially varying areal resolution of the measurement system. Statistical fluctuations due to nonlinear speckle noise inherent in the measurements are reduced by stationary averaging and are stabilized by logarithmic transformation.