Mapping offshore sedimentary structure using electromagnetic methods and terrain effects in marine magnetotelluric data

Abstract

SUMMARY Marine magnetotelluric (MT) and marine controlled-source electromagnetic (CSEM) soundings can be used to study sedimentary structure offshore. In an example of this application, we collected MT and CSEM data in the 1-km deep water of the San Diego Trough, California. The Trough is a pull-apart basin and part of the complex Pacific/North American tectonic plate boundary, and is flanked by the Thirtymile Bank to the west and the Coronado Bank to the east. Our MT data are highly distorted by seafloor topography and the coast effect, which is largely 2-D and can be modelled using 2-D finite element codes. The distortion includes a strong (several orders of magnitude) static depression of TM mode resistivities (electric field perpendicular to structure), upward cusps in the TE mode resistivities (electric field parallel to structure) and negative TE mode phases. The depressed TM mode resistivity is a well-known consequence of galvanic interruption of coastperpendicular electric fields. The TE mode distortion is an inductive effect associated with currents flowing along the edge of the deep ocean basins, steepening the magnetic field and even causing a phase reversal in the horizontal field used for MT impedance calculations (and thus generating negative phases). The land-side enhanced vertical magnetic field is well known as the geomagnetic coast effect, but the ocean-side consequences have been less well documented. Although the MT data are dominated by coast effect and topographic distortion, inclusion of accurate bathymetry in the inversion model’s finite element mesh allows the subseafloor geological structure to be recovered. This shows the Trough sediments to be about 3 km thick, bounded to the west by resistive basement, but to the east by conductive clastic sediments forming Coronado Bank. Amplitudes and phases of five frequencies of CSEM data (from 0.1 to 1.0 Hz) collected along the axis of the Trough are well fit with a simple, 1-D layered model, indicating that sediment resistivities increase with depth from 1.5 to 2.3 � -m and are no more than 3300 m thick, thinning to the north, in good agreement with the MT model. An existing density model generated by fitting surface and deep-towed gravity is in good agreement with the EM interpretations. In particular, combining sediment densities and CSEM resistivities allows us to estimate pore water conductivity and temperature, which follows a geothermal gradient of 25.4 ± 8Kk m −1 .