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.