Nicholas C. Makris
Naval Res. Lab., Washington, DC 20375
Narrow-band waveforms in the frequency range 200--300 Hz were transmitted from a vertical source array during the Main Acoustics Experiment (SRP) of July 1993. The resulting backscatter from the ocean bottom was received on a horizontal array. Two-way travel times are computed using a ray trace that includes bottom and surface interacting paths. Combining these results with beamforming, backscatter returns are charted to respective scattering sites on the western flank of the Mid-Atlantic Ridge (MAR). A histogram of the backscatter level as a function of frequency is obtained for each grid point in the survey area. Each histogram is then assigned a color to represent that particular frequency response. A chart of the frequency responses or acoustic colors of the survey area is then composed by charting the acoustic color derived for each grid point. The acoustic color chart is compared with charts of two-way transmission loss, geomorphology, and mean acoustic intensity or brightness to elucidate the observed spatial characteristics. For example, deterministic facets much larger than a wavelength backscatter sound uniformly with frequency and are acoustically white. Surfaces with roughness scales comparable to an acoustic wavelength are characterized by acoustic colors that vary with the incident wavelength.