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Marine dipole-dipole controlled source electromagnetic and coincident-loop transient electromagnetic experiments to detect seafloor massive sulphides: effects of three-dimensional bathymetry

Marine dipole-dipole controlled source electromagnetic and coincident-loop transient electromagnetic experiments to detect seafloor massive sulphides: effects of three-dimensional bathymetry
Marine dipole-dipole controlled source electromagnetic and coincident-loop transient electromagnetic experiments to detect seafloor massive sulphides: effects of three-dimensional bathymetry
Seafloor massive sulphides (SMS) are regarded as a potential future resource to satisfy the growing global demand of metals including copper, zinc and gold. Aside from mining and retrieving profitable amounts of massive sulphides from the seafloor, the present challenge is to detect and delineate significant SMS accumulations, which are generally located near mid-ocean ridges and along submarine volcanic arc and back-arc spreading centres. Currently, several geophysical technologies are being developed to detect and quantify SMS occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock. Here, we use a short, fixed-offset controlled source electromagnetic (CSEM) system and a coincident-loop transient electromagnetic (TEM) system, which in theory allow the detection of SMS in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings.

In 2016, CSEM and TEM experiments were carried out at several locations near the TAG (Trans-Atlantic Geotraverse) hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor. Measurements were conducted in an area that contains distinct SMS sites located several kilometres off-axis from the Mid-Atlantic ridge, some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased. Based on the quality of the acquired data, both experiments were operationally successful. However, the data analysis indicates bias caused by three-dimensional (3D) effects of the rough bathymetry in the study area and, thus, data interpretation remains challenging. Therefore, we study the influence of 3D bathymetry for marine CSEM and TEM experiments, focusing on shallow 3D conductors located beneath mound-like structures. We analyse synthetic inversion models for attributes associated with 3D distortions of CSEM and TEM data that are not sufficiently accounted for in conventional 1D (TEM) and 2D (CSEM) interpretation schemes. Before an adequate quantification of SMS in the region is feasible, these 3D effects need to be studied to avoid over/under-estimation of SMS using the acquired EM data. The sensitivity of CSEM and TEM to bathymetry is investigated by means of 3D forward modelling, followed by 1D (TEM) and 2D (CSEM) inversion of the synthetic data using realistic error conditions. Subsequently, inversion models of the synthetic 3D data are analysed and compared to models derived from the measured data to illustrate that 3D distortions are evident in the recorded data sets.
0956-540X
2156–2171
Haroon, Amir
05a1f75d-ff5a-40cf-a6cf-c2c437e0058a
Hölz, Sebastian
bba70872-1962-4018-8c7e-04ce86e2105e
Gehrmann, Romina
1ee547b2-aa53-4d38-9d36-a2ccc3aa52e2
Attias, Eric
abf34bba-f99f-47f9-ba89-92df1c488a5e
Jegen, Marion
c4a9eace-f47f-4a5d-a4ac-47aa3477e374
Minshull, Timothy
bf413fb5-849e-4389-acd7-0cb0d644e6b8
Murton, Bramley
9076d07f-a3c1-4f90-a5d5-99b27fe2cb12
Haroon, Amir
05a1f75d-ff5a-40cf-a6cf-c2c437e0058a
Hölz, Sebastian
bba70872-1962-4018-8c7e-04ce86e2105e
Gehrmann, Romina
1ee547b2-aa53-4d38-9d36-a2ccc3aa52e2
Attias, Eric
abf34bba-f99f-47f9-ba89-92df1c488a5e
Jegen, Marion
c4a9eace-f47f-4a5d-a4ac-47aa3477e374
Minshull, Timothy
bf413fb5-849e-4389-acd7-0cb0d644e6b8
Murton, Bramley
9076d07f-a3c1-4f90-a5d5-99b27fe2cb12

Haroon, Amir, Hölz, Sebastian, Gehrmann, Romina, Attias, Eric, Jegen, Marion, Minshull, Timothy and Murton, Bramley (2018) Marine dipole-dipole controlled source electromagnetic and coincident-loop transient electromagnetic experiments to detect seafloor massive sulphides: effects of three-dimensional bathymetry. Geophysical Journal International, 215 (3), 2156–2171, [ggy398]. (doi:10.1093/gji/ggy398).

Record type: Article

Abstract

Seafloor massive sulphides (SMS) are regarded as a potential future resource to satisfy the growing global demand of metals including copper, zinc and gold. Aside from mining and retrieving profitable amounts of massive sulphides from the seafloor, the present challenge is to detect and delineate significant SMS accumulations, which are generally located near mid-ocean ridges and along submarine volcanic arc and back-arc spreading centres. Currently, several geophysical technologies are being developed to detect and quantify SMS occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock. Here, we use a short, fixed-offset controlled source electromagnetic (CSEM) system and a coincident-loop transient electromagnetic (TEM) system, which in theory allow the detection of SMS in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings.

In 2016, CSEM and TEM experiments were carried out at several locations near the TAG (Trans-Atlantic Geotraverse) hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor. Measurements were conducted in an area that contains distinct SMS sites located several kilometres off-axis from the Mid-Atlantic ridge, some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased. Based on the quality of the acquired data, both experiments were operationally successful. However, the data analysis indicates bias caused by three-dimensional (3D) effects of the rough bathymetry in the study area and, thus, data interpretation remains challenging. Therefore, we study the influence of 3D bathymetry for marine CSEM and TEM experiments, focusing on shallow 3D conductors located beneath mound-like structures. We analyse synthetic inversion models for attributes associated with 3D distortions of CSEM and TEM data that are not sufficiently accounted for in conventional 1D (TEM) and 2D (CSEM) interpretation schemes. Before an adequate quantification of SMS in the region is feasible, these 3D effects need to be studied to avoid over/under-estimation of SMS using the acquired EM data. The sensitivity of CSEM and TEM to bathymetry is investigated by means of 3D forward modelling, followed by 1D (TEM) and 2D (CSEM) inversion of the synthetic data using realistic error conditions. Subsequently, inversion models of the synthetic 3D data are analysed and compared to models derived from the measured data to illustrate that 3D distortions are evident in the recorded data sets.

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More information

Accepted/In Press date: 22 September 2018
e-pub ahead of print date: 25 September 2018
Published date: 1 December 2018
Additional Information: This article has been accepted for publication in Geophysical Journal International ©: 2018 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

Identifiers

Local EPrints ID: 423707
URI: http://eprints.soton.ac.uk/id/eprint/423707
ISSN: 0956-540X
PURE UUID: d1a73c1c-b182-42e8-95ce-793ef4c19138
ORCID for Romina Gehrmann: ORCID iD orcid.org/0000-0002-3099-2771
ORCID for Timothy Minshull: ORCID iD orcid.org/0000-0002-8202-1379

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Date deposited: 27 Sep 2018 16:30
Last modified: 07 Oct 2020 02:09

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