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Solid Earth An interactive open-access journal of the European Geosciences Union

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© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
12 Dec 2016
Review status
A revision of this discussion paper was accepted for the journal Solid Earth (SE) and is expected to appear here in due course.
Correcting for Static Shift of Magnetotelluric Data with Airborne Electromagnetic Measurements: A Case Study from Rathlin Basin, Northern Ireland
Robert Delhaye1,2, Volker Rath1, Alan G. Jones1,3, Mark R. Muller1,4, and Derek Reay5 1Dublin Institute for Advanced Studies (DIAS), 5 Merrion Square, Dublin 2, Ireland
2National University of Ireland, Galway, University Road, Galway, Ireland
3Complete MT Solutions, Ottawa, Canada
4Geophysical Consultant, Cambridge, United Kingdom
5Geological Survey of Northern Ireland (GSNI), Belfast, United Kingdom
Abstract. Galvanic distortions of magnetotelluric (MT) data, such as the static shift effect, are a known problem that can lead to incorrect estimation of resistivities and erroneous modelling of geometries with resulting misinterpretation of subsurface electrical resistivity structure. A wide variety of approaches have been proposed to account for these galvanic distortions, some depending on the target area, with varying degrees of success. The natural laboratory for our study is a hydraulically permeable volume of conductive sediment at depth, the internal resistivity structure of which can be used to estimate reservoir viability for geothermal purposes, however static shift correction is required in order to ensure robust and precise modelling accuracy.

We propose a method employing frequency–domain electromagnetic data for static shift correction, which in our case are regionally available with high spatial density. The spatial distributions of the derived static shift corrections are analysed and applied to the uncorrected MT data prior to inversion. Two comparative inversion models are derived, one with and one without static shift corrections, with instructive results. As expected from the one–dimensional analogy of static shift correction, at shallow model depths, where the structure is controlled by a single local MT site, the correction of static shift effects leads to vertical scaling of resistivity-thickness products in the model, with the corrected model showing improved correlation to existing borehole wireline resistivity data. In turn, as these vertical scalings are effectively independent of adjacent sites, lateral resistivity distributions are also affected, with up to half a decade of resistivity variation between the models estimated at depths down to 2000 m. Simple estimation of differences in bulk porosity, derived using Archie’s Law, between the two models reinforces our conclusion that the sub–order of magnitude resistivity contrasts induced by correction of static shifts correspond to similar contrasts in estimated porosities, and hence, for purposes of reservoir investigation or similar cases requiring accurate absolute resistivity estimates, galvanic distortion correction, especially static shift correction, is essential.

Citation: Delhaye, R., Rath, V., Jones, A. G., Muller, M. R., and Reay, D.: Correcting for Static Shift of Magnetotelluric Data with Airborne Electromagnetic Measurements: A Case Study from Rathlin Basin, Northern Ireland, Solid Earth Discuss., doi:10.5194/se-2016-168, in review, 2016.
Robert Delhaye et al.
Interactive discussionStatus: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version      Supplement - Supplement
RC1: 'RC', Andreas Junge, 06 Feb 2017 Printer-friendly Version Supplement 
RC2: 'FDEM static shift significance?', Anonymous Referee #2, 09 Feb 2017 Printer-friendly Version 
AC1: 'Response to Referees' comments', Robert Delhaye, 03 Mar 2017 Printer-friendly Version Supplement 
Robert Delhaye et al.
Robert Delhaye et al.


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Publications Copernicus
Short summary
We have modelled airborne electromagnetic data in order to correct bias in magnetotelluric data caused by very near-surface resistivity variations. Doing so recovers structures that match boreholes in the area more closely. This research is part of an exploration project looking at geothermal resources, and improved accuracy in modelling translates directly to more confidence in resources assessments.
We have modelled airborne electromagnetic data in order to correct bias in magnetotelluric data...