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 coast‐perpendicular 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 Ω‐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 ± 8 K km−1.

Electrical structure of the central Cascadia subduction zone: The EMSLAB Lincoln Line revisited

“…This peak has been seen in other similar data sets and is explained by anomalous currents flowing in the ocean (Constable et al, 2009;Key and Constable, 2011;Worzewski et al, 2012). The frequency at which the peak in TE mode apparent resistivity occurs and the distance from the shore of the furthest station where the peak is seen suggests a moderately resistive seafloor with bulk resistivity of around 500 Ω m (Worzewski et al, 2012), consistent with a resistive incoming oceanic lithosphere overlain by 3 km of sediments and porous oceanic crust.…”

Marine Electromagnetic Studies of Seafloor Resources and Tectonics

“…These distortions arise from a combination of galvanic and inductive effects that operate on local, regional and global scales (e.g., Fischer and Weaver 1986;Heinson and Constable 1992;Heinson and Lilley 1993;Key 2003;Schwalenberg and Edwards 2004). Constable et al (2009) report on broadband MT data collected in the San Diego Trough offshore California that exhibit extreme distortions. They observe TE apparent resistivity cusps and negative phases, and a strongly depressed TM mode apparent resistivity (Fig.…”

“…Right 10-20,000 s response obtained with 19 days recording in 2,900 m water for the SERPENT project offshore Nicaragua (http://marineemlab. ucsd.edu/Projects/SERPENT) Example applications include mid-ocean ridge topography distortions (Key and Constable 2002) and continental shelf distortions (Heinson et al 2005;Constable et al 2009). …”

Approximations for the 2-D coast effect on marine magnetotelluric data

“…Analysis of TE mode distortions was not addressed systematically until negative phases and peaks were reported offshore California by Constable et al (2009). They were able to qualitatively reproduce these special features with a 2-D bathymetric model and found that the distortion in the TE mode depends on the host resistivity.…”

“…Alekseev et al (2009) also modelled TE mode apparent resistivities peaks and phases that systematically progress through all four quadrants. Currently, coast-distorted marine curves are taken into account for the inversion processes and can be well fitted, as has been presented by Constable et al (2009), Wheelock et al (2010), Naif et al (2010) and also of the Costa Rican data set (Worzewski et al 2011). Currently, coast-distorted marine curves are taken into account for the inversion processes and can be well fitted, as has been presented by Constable et al (2009), Wheelock et al (2010), Naif et al (2010) and also of the Costa Rican data set (Worzewski et al 2011).…”