Evaluation of spatial resolution and estimation error of seafloor displacement observation from vessel-based bathymetric survey by use of AUV-based bathymetric data

Abstract

A repeated bathymetric survey reveals seafloor displacement between before and after geodynamic events. We evaluated the less-known spatial resolution and estimation error of the seafloor displacement observation from a vessel-based multi-narrow beam bathymetric survey. In this evaluation, bathymetric data from vessel-based and near-seafloor high-resolution autonomous underwater vehicle (AUV)-based surveys in the same area were used. Simulated vessel-based bathymetric “before and after” data of the seafloor displacement were made using AUV-based bathymetric data. The displacement was verified by comparing these simulated data using the analysis conditions that no locational errors of beam sounding points exist, a footprint effect is uniform, depth accuracy is constant in the analysis area, and there are no depth offset between two data. As a result, we found that the smallest vertical seafloor displacement that can be detected occurs when the horizontal extent of the deformation is larger than several times the size of the footprint (area of the narrow sounding beam projected onto the seafloor) of the used vessel’s multi-narrow beam echo sounder, and in the situation that the amplitude of the depth difference is greater than the accuracy of the vessel-based depth measurement (standard deviation of measuring error). When local slopes of the bathymetry are gentler than those of the artificial variation appeared in the depth differences between two data, the horizontal seafloor displacement seems to be difficult to resolve accurately. The local slope of the artificial depth variation is derived from the wavelength and the amplitude which are equivalent to ~1–3 times of the footprint size and the accuracy of the depth measurement, respectively.