Quantitative Backscatter Measurements With a Long-Range Side-Scan Sonar


N. C. Mitchell and M. L. Somers

Side-scan sonars have been used successfully to construct qualitative images of the sea floor for many years. However, if the sonar is calibrated, it is possible to estimate the backscatter strength from the transducer voltages using a simple acoustic propagation model. The backscatter values can then be compared with published results and scattering models. Backscatter strength estimates are desirable if we wish to make quantitative comparisons between returns from different bottom types and slopes. This calculation has been performed on GLORIA side-scan sonar data collected during 1987 in the southern Indian Ocean. GEOSECS hydrographic information was used to assess the effects of refraction (ray bending and aspherical spreading signal losses). Sea Beam bathymetry was used to correct the effective insonified area and compute the grazing angle. A major difficulty in performing this calculation over the terrain chosen (mid-ocean ridge topography) was one of adjusting navigation so that small features in Sea Beam and GLORIA data matched. Preliminary results show a 10-dB fall-off in backscatter strength with grazing angle (10û-40û) at 6.5 kHz over what must presumably be a rough surface (extruded basalts and breccias). On rugged topography such as at a mid-ocean ridge, it appears that acoustic shadowing produces much of the variation seen on GLORIA images.

Mitchell, N. C., and Somers, M. L., "Quantitative backscatter measurements with a long-range side-scan sonar." IEEE J. Oceanic Engineering, 14, 368-374, 1989.


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