Sub-meter Range Precision of Seafloor Deformation Obtainable from Correlation of Repeated Raw Sidescan Sonar Surveys

Abstract:

Despite recent advances in the field of seafloor geodesy, our ability to collect data to monitor for marine hazards or increase our understanding of offshore geologic processes remains limited by the exorbitant cost of data collection. We propose to measure horizontal seafloor displacements using raw sidescan data from repeated multibeam surveys. We grid the data in a natural range and azimuth coordinate frame, and estimate displacement from the peak offset of their normalized cross correlation. This method allows us to obtain sub-pixel accuracy in our displacement calculation as it involves averaging the data over a wide area. Our overall objective is to establish the accuracy of this method and determine how it depends on factors such as: ship speed, repeat track separation, seafloor characteristics, sound speed variations, and ping orientation. We analyze sidescan data archived at the NGDC database that vary over these parameters, finding that under optimal conditions we may obtain decimeter to meter level precision in the range direction and meter level precision in the azimuthal direction. The most important parameter is the ping orientation because small (~2-3 degrees) variations can drastically lower the maximum correlation value. Differing ship speed can cause large reductions in the correlation accuracy, although these effects are more pronounced in the azimuthal direction. Surprisingly, variations in sound speed are partially mitigated by data averaging over both sides of the ship. In addition, the characteristics of the seafloor seem to have minimal influence over the displacement accuracy.