Meter-Scale Seafloor Geodetic Measurements Obtained from Repeated Multibeam Sidescan Surveys

Abstract

We calibrate a technique to use repeated multibeam sidescan surveys in the deep ocean to recover seafloor displacements greater than a few meters. Displacement measurements from seafloor patches (3?km by 20?km) on the port and starboard side of the ship are used to estimate vertical and across-track displacement. We present displacement measurements from a survey of the Ayu Trough southwest of the Marianas Trench using a 12?kHz multibeam. Vertical and across-track displacement errors for the 12?kHz multibeam sonar are typically 0–2?m with RMS uncertainties of 0.25–0.67 m in the across-track and 0.37–0.75 m in the vertical as determined by 3-way closure tests. The uncertainty of the range-averaged sound velocity is a major error source. We estimate that variations in the sound velocity profile, as quantified using expendable bathythermographs (XBTs) during data collection, contribute up to 0.3?m RMS uncertainty in the across-track direction and 1.6?m RMS uncertainty in the vertical direction.     

Oceanic sound speed profile inversion

A modal (full-wave) method has been developed to predict ocean sound speed profiles from propagated acoustic field data. The method assumes a point source of sound in the ocean and uses as data the values of the transmitted acoustic field at an array. The formalism for depth-dependent sound speeds consists of the standard Hankel integral transform of the depth solution. In the travel length coordinate, the latter is written exactly, using the Green's function, in terms of an integral equation whose kernel includes the sound speed profile correction. A Born approximation to this equation is used. This is just the WKB solution, and permits the use of a nontrivial input (or guess) profile, here chosen as bilinear. The use of asymptotic methods enables us to write the data as an integral transform over the profile correction. The transform can be inverted. An example is presented for full-bandwidth inversion.