Three-dimensional inversion of seafloor magnetotelluric data collected in the Philippine Sea and the western margin of the northwest Pacific Ocean

We report a result of three-dimensional (3-D) upper mantle electrical conductivity inversion of seafloor magnetotelluric data. We used existing data at 25 sites in the Philippine Sea and the western margin of the Pacific Ocean. In order to obtain a reliable model by 3-D inversion, we evaluated the large and small-scale topographic effects. We also conducted a comprehensive search of the one-dimensional (1-D) profiles of the study area in order to determine the best initial and prior models. A two-phase inversion method was applied so that the error floors for the diagonal and off-diagonal elements of the impedance tensor could be separately controlled. Through this first attempt at inverting real data, we obtained basic knowledge about tuning the inversion parameters and conditions. We also proposed a procedure to evaluate the reliability of the 3-D conductivity anomalies imaged by the inversion by conducting checkerboard and sensitivity tests. After the iterations converged, 13 distinct anomalies were found in the inverted 3-D conductivity model; four conductive and two resistive anomalies were confirmed to be resolved enough by the data through the checkerboard test. Then the sensitivity tests were conducted to quantify how each anomaly was required by the observed data, and we confirmed that the intensities of three conductive anomalies and one resistive anomaly were statistically significant. This paper presented an example of possible approach in 3-D seafloor electromagnetic inversion procedure for imaging reliable electrical conductivity structure of the oceanic mantle, which will be useful in understanding dynamics and evolution of solid Earth.