Publication 187
Correcting for Static Shift of Magnetotelluric Data with Airborne Electromagnetic Measurements:
A Case Study from Rathlin Basin, Northern Ireland
Delhaye, R.D., V. Rath, A.G. Jones, M.R. Muller, and D. Reay
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.
Source
Solid Earth, , accepted, 19 April, 2017.
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Alan G Jones / 19 April 2017 /
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