COMPENSATION FOR CHANGING BEAM PATTERN AND RESIDUAL TVG EFFECTS WITH SONAR ALTITUDE VARIATION FOR SIDESCAN MOSAICING AND CLASSIFICATION

Sidescan sonar images characteristically display across-track intensity variations resulting from the transducers' beam pattern. Along-track these changes are complicated by intensity variations introduced by changing sonar altitude. The effects are particularly apparent in low altitude surveys displaying strong sidelobe returns from regions closest to the transducers. The fall off in intensity with range for main lobe returns is also more noticeable in these missions. A time-varying gain (TVG) is usually applied to sonar signals to compensate for this range-dependent effect, but cannot adequately compensate for beam pattern or altitude variations. In this paper methods are presented for correction of beam pattern and residual TVG effects in the presence of altitude change. Estimation of correction factors is performed directly from the data. It is preferable to seek an estimate for these factors over a seabed region with relatively even, small-scale texture, a few hundred lines of data being sufficient. The approach to the beam pattern problem uses time-scaled representations of each line of data to account for geometrical variations introduced by altitude change. The applied TVG function is assumed to be dominant towards the end of each line of recorded data and can usually be approximated using a smooth function. Multiplicative correction factors are calculated for the TVG function and scaled beam pattern. These are applied to the sonar data to produce a representation of the ensonified seafloor topography with intensities adjusted relative to some user defined datum point.