FY 1992

A technique for combining SeaMARC I sidescan sonar and gridded bathymetric data to display undistorted seafloor images

Lau, T.-K.A., and C.G. Fox

In Proceedings, OCEANS '91 Conference, Honolulu, HI, October 1991, 1140–1145 (1991)

SeaMARC I is an advanced sidescan sonar system capable of producing high-resolution digital images of large areas of the seafloor. Each scan line, which can represent swath widths from 0.5 to 5 km, consists of 2048 1-byte pixels of acoustic backscatter strength. After merging the towfish position and altitude with the acoustic backscatter data, a navigated sidescan image of the seafloor can be automatically generated. The position of each pixel within a scan line is calculated from the two-way acoustic travel time, the towfish altitude, and assumption of a flat terrain over the extent of the swath. This flat-bottom assumption necessarily introduces distortions into images collected in rough or sloping terrains. The flat-bottom assumption can be corrected in post-time to more accurately determine the pixel positions by using gridded bathymetric data from any source. Techniques are presented to correct SeaMARC I sidescan imagery based on gridded bathymetry from SeaBeam multibeam surveys. The depth used in the original flat- bottom representation is assumed to be that recorded by Sea Beam at the towfish position. Sea Beam recorded depths beneath the corresponding scan line are used in the correction process. Computations involve 1) defining a transit line formed by a scan line, 2) calculating the intersections between the transit line and the bathymetric grid lines, 3) transforming all the intersections from 3-D to 2-D coordinates, 4) forming circles in 2-D using the mid-point of the transit and the towfish altitude as the x and y coordinates of the origin, 5) locating the correct line segments which intersect the circles, 6) solving for the intersections, and 7) using these intersections to obtain the new terrain-corrected positions in 3-D coordinates. Examples of corrected and uncorrected images from the northeast Pacific Ocean are presented.