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A newly distinguished marine magnetotelluric coast effect sensitive to the lithosphere-Asthenosphere boundary

A newly distinguished marine magnetotelluric coast effect sensitive to the lithosphere-Asthenosphere boundary
A newly distinguished marine magnetotelluric coast effect sensitive to the lithosphere-Asthenosphere boundary

The marine magnetotelluric (MT) method is a useful tool for offshore studies aimed at, for example, hydrocarbon exploration and the understanding of Earth's tectonics. Marine MT data are often distorted by coastlines because of the strong resistivity contrast between the conductive ocean and the resistive land. At mid ocean ridges, the resistivity of Earth's structure can be assumed to be two-dimensional, which allows MT data to be decomposed into a transverse electrical (TE) mode, with electric current flowing approximately along the ridge, and a transverse magnetic (TM) mode, with electric current flowing perpendicular to the ridge. We collected marine MT data at the middle Atlantic Ridge which exhibited highly negative TM-mode phases, as large as -180°, at relatively high frequencies (0.1-0.01 Hz). Similar negative phases have been observed in other marineMTdata sets, but have not been the subject of study.We show here that these negative phases are caused by a newly distinguished coast effect. The TM-mode coast effect is not only a galvanic effect, as previously understood, but also includes inductive distortions. TM-mode negative phases are caused by the turning of the Poynting vector, the phase change of electromagnetic fields, and vertically flowing currents in the seafloor. The findings provide a new understanding of the TM-mode coast effect, which can guide our ability to fit the field data with the inclusion of coastlines, and reduce misinterpretation of the data in offshore studies. The study also shows that the TMmode coast effect is sensitive to the depth and conductivity of the asthenosphere, an important feature of the Earth's interior that was the object of our Atlantic Ocean study.

Electrical properties, Electromagnetic theory, Magnetotellurics, Marine electromagnetics, Mid-ocean ridge processes.
0956-540X
978-987
Wang, Shunguo
f935a6b8-a8c1-46f0-975a-1d4aa56b5f11
Constable, Steven
f2ffd9c4-3738-435b-8a88-38dee97de7cc
Reyes-Ortega, Valeria
005d3245-d2bb-4e97-84ff-6a930ad1ce17
Rychert, Catherine A.
70cf1e3a-58ea-455a-918a-1d570c5e53c5
Wang, Shunguo
f935a6b8-a8c1-46f0-975a-1d4aa56b5f11
Constable, Steven
f2ffd9c4-3738-435b-8a88-38dee97de7cc
Reyes-Ortega, Valeria
005d3245-d2bb-4e97-84ff-6a930ad1ce17
Rychert, Catherine A.
70cf1e3a-58ea-455a-918a-1d570c5e53c5

Wang, Shunguo, Constable, Steven, Reyes-Ortega, Valeria and Rychert, Catherine A. (2019) A newly distinguished marine magnetotelluric coast effect sensitive to the lithosphere-Asthenosphere boundary. Geophysical Journal International, 218 (2), 978-987, [202]. (doi:10.1093/gji/ggz202).

Record type: Article

Abstract

The marine magnetotelluric (MT) method is a useful tool for offshore studies aimed at, for example, hydrocarbon exploration and the understanding of Earth's tectonics. Marine MT data are often distorted by coastlines because of the strong resistivity contrast between the conductive ocean and the resistive land. At mid ocean ridges, the resistivity of Earth's structure can be assumed to be two-dimensional, which allows MT data to be decomposed into a transverse electrical (TE) mode, with electric current flowing approximately along the ridge, and a transverse magnetic (TM) mode, with electric current flowing perpendicular to the ridge. We collected marine MT data at the middle Atlantic Ridge which exhibited highly negative TM-mode phases, as large as -180°, at relatively high frequencies (0.1-0.01 Hz). Similar negative phases have been observed in other marineMTdata sets, but have not been the subject of study.We show here that these negative phases are caused by a newly distinguished coast effect. The TM-mode coast effect is not only a galvanic effect, as previously understood, but also includes inductive distortions. TM-mode negative phases are caused by the turning of the Poynting vector, the phase change of electromagnetic fields, and vertically flowing currents in the seafloor. The findings provide a new understanding of the TM-mode coast effect, which can guide our ability to fit the field data with the inclusion of coastlines, and reduce misinterpretation of the data in offshore studies. The study also shows that the TMmode coast effect is sensitive to the depth and conductivity of the asthenosphere, an important feature of the Earth's interior that was the object of our Atlantic Ocean study.

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Manuscript_WANG_etal_clean_R2 - Accepted Manuscript
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More information

Accepted/In Press date: 3 May 2019
e-pub ahead of print date: 4 May 2019
Published date: August 2019
Keywords: Electrical properties, Electromagnetic theory, Magnetotellurics, Marine electromagnetics, Mid-ocean ridge processes.

Identifiers

Local EPrints ID: 433078
URI: http://eprints.soton.ac.uk/id/eprint/433078
ISSN: 0956-540X
PURE UUID: 9a00440b-0694-42b3-bd77-c69dc1d5b8e8

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Date deposited: 07 Aug 2019 16:30
Last modified: 07 Oct 2020 01:00

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