Seafloor massive sulphide exploration using deep-towed controlled source electromagnetics: Navigational uncertainties
Seafloor massive sulphide exploration using deep-towed controlled source electromagnetics: Navigational uncertainties
Deep-towed geophysical surveys require precise knowledge of navigational parameters such as instrument position and orientation because navigational uncertainties reflect in the data and therefore in the inferred geophysical properties of the subseafloor. We address this issue for the case of electrical conductivity inferred from controlled source electromagnetic data. We show that the data error is laterally variable due to irregular motion during deep towing, but also due to lateral variations in conductivity, including those resulting from topography. To address this variability and quantify the data error prior to inversion, we propose a 2-D perturbation study. Our workflow enables stable and geologically reliable results for multicomponent and multifrequency inversions. An error estimation workflow is presented, which comprises the assessment of navigational uncertainties, perturbation of navigational parameters, and forward modelling of electric field amplitudes for a homogeneous and then a heterogeneous subseafloor conductivity model. Some navigational uncertainties are estimated from variations of direct measurements. Other navigational parameters required for inversion are derived from the measured quantities and their error is calculated by means of error propagation. Some navigational parameters show direct correlation with the measured electric fields. For example, the antenna dip correlates with the vertical electric field and the depth correlates with the horizontal electric field. For the perturbation study each standard deviation is added to the navigational parameters. Forward models are run for each perturbation. Amplitude deviations are summed in quadrature with the stacking error for a total, laterally varying, data error. The error estimation is repeated for a heterogeneous subseafloor model due to the large conductivity range (several orders of magnitude), which affects the forward model. The approach enables us to utilize data from several components (multiple electric fields, frequencies and receivers) in the inversion to constrain the final model and reduce ambiguity. The final model is geologically reasonable, in this case enabling the identification of conductive metal sulphide deposits on the seafloor.
Controlled source electromagnetics (CSEM), Electrical properties, Instrumental noise, Marine electromagnetics
1215-1227
Gehrmann, Romina A.S.
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Haroon, Amir
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Morton, McKinley
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Djanni, Axel T.
af73a5c3-9429-4a95-8988-fdb9740ad5e9
Minshull, Timothy A.
bf413fb5-849e-4389-acd7-0cb0d644e6b8
1 February 2020
Gehrmann, Romina A.S.
1ee547b2-aa53-4d38-9d36-a2ccc3aa52e2
Haroon, Amir
05a1f75d-ff5a-40cf-a6cf-c2c437e0058a
Morton, McKinley
7b76d25d-9813-4671-a2cd-0bb4648b780f
Djanni, Axel T.
af73a5c3-9429-4a95-8988-fdb9740ad5e9
Minshull, Timothy A.
bf413fb5-849e-4389-acd7-0cb0d644e6b8
Gehrmann, Romina A.S., Haroon, Amir, Morton, McKinley, Djanni, Axel T. and Minshull, Timothy A.
(2020)
Seafloor massive sulphide exploration using deep-towed controlled source electromagnetics: Navigational uncertainties.
Geophysical Journal International, 220 (2), .
(doi:10.1093/gji/ggz513).
Abstract
Deep-towed geophysical surveys require precise knowledge of navigational parameters such as instrument position and orientation because navigational uncertainties reflect in the data and therefore in the inferred geophysical properties of the subseafloor. We address this issue for the case of electrical conductivity inferred from controlled source electromagnetic data. We show that the data error is laterally variable due to irregular motion during deep towing, but also due to lateral variations in conductivity, including those resulting from topography. To address this variability and quantify the data error prior to inversion, we propose a 2-D perturbation study. Our workflow enables stable and geologically reliable results for multicomponent and multifrequency inversions. An error estimation workflow is presented, which comprises the assessment of navigational uncertainties, perturbation of navigational parameters, and forward modelling of electric field amplitudes for a homogeneous and then a heterogeneous subseafloor conductivity model. Some navigational uncertainties are estimated from variations of direct measurements. Other navigational parameters required for inversion are derived from the measured quantities and their error is calculated by means of error propagation. Some navigational parameters show direct correlation with the measured electric fields. For example, the antenna dip correlates with the vertical electric field and the depth correlates with the horizontal electric field. For the perturbation study each standard deviation is added to the navigational parameters. Forward models are run for each perturbation. Amplitude deviations are summed in quadrature with the stacking error for a total, laterally varying, data error. The error estimation is repeated for a heterogeneous subseafloor model due to the large conductivity range (several orders of magnitude), which affects the forward model. The approach enables us to utilize data from several components (multiple electric fields, frequencies and receivers) in the inversion to constrain the final model and reduce ambiguity. The final model is geologically reasonable, in this case enabling the identification of conductive metal sulphide deposits on the seafloor.
Text
ggz513
- Accepted Manuscript
More information
Accepted/In Press date: 13 November 2019
e-pub ahead of print date: 14 November 2019
Published date: 1 February 2020
Additional Information:
Funding Information:
We would like to thank the European Commission for funding the Framework 7 project Blue Mining (Grant number 604500). We thank all cruise participants and crew from M127 and JC138 for their support in data acquisition especially Ian Tan and Laurence North for their engineering expertise, Sebastian Hölz for his real-time navigation script, and Eric Attias and chief scientist Bram Murton for their proactive involvement. We thank Karen Weite-meyer and Steven Constable for their advice, and Steven for internally reviewing the manuscript. We also thank editor Ute Weck-mann, reviewer Rob Evans and one anonymous reviewer for their constructive feedback. We thank Kerry Key for the inversion code MARE2DEM and supporting matlab scripts as well as David Myer for processing routines. Tim Minshull was supported by a Wolf-son Research Merit award. Amir Haroon was in part funded by the German Research Foundation, Grant number 389727048. Data are available from Gehrmann (2019).
Publisher Copyright:
© 2020 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society.
Keywords:
Controlled source electromagnetics (CSEM), Electrical properties, Instrumental noise, Marine electromagnetics
Identifiers
Local EPrints ID: 436120
URI: http://eprints.soton.ac.uk/id/eprint/436120
ISSN: 0956-540X
PURE UUID: 054b6218-9dfe-4e52-9db8-fd7cf5e73aa4
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Date deposited: 29 Nov 2019 17:30
Last modified: 17 Mar 2024 02:50
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Contributors
Author:
Romina A.S. Gehrmann
Author:
Amir Haroon
Author:
McKinley Morton
Author:
Axel T. Djanni
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