Correction of Bathymetric Artifacts Resulting from Surface-Wave Inducted Modulations of an AUV Pressure Sensor
Recent increases in the capability and reliability of autonomous underwater vehicles (AUVs) have provided the opportunity to conduct bathymetric seafloor surveys in shallow water (< 50 m). Unfortunately, surveys of this water depth may contain artifacts induced by large amplitude wave motion at the surface. The artifacts occur when an onboard pressure sensor determines the depth of the AUV. Waves overhead induce small pressure fluctuations at depth, which modulate the AUV’s pressure sensor output without causing actual vertical movement of the AUV. Since bathymetric measurements are made with respect to the AUV’s depth, these pressure fluctuations, in turn, modulate the measurement of the seafloor. The result is a periodic across-track, vertical offset of the seafloor profile (similar to a heave artifact sometimes common in surface vessel surveys). In this paper we describe our experience with the “Gavia” model AUV (Hafmynd EHF, Iceland) in a recent bathymetric survey during which wave action overhead induced such an artifact with a peak-to-peak amplitude as large as 1 meter. A method for removing the artifact as well as recommendations for modifications to the sonar, INS and AUV to mitigate the effect in the future are provided. A brief note on terminology: The term “heave” as used in this paper is defined as the vertical displacement of the AUV with respect to the surface (the AUV’s depth). This convention is used in surface ship surveying and in the AUV’s Geoacoustics bathymetric sonar. Unfortunately, the term “heave” is defined by the AUV’s inertial navigation system and internal logs as vertical velocity of the AUV. Every effort will be made to reduce confusion between these definitions.