Correcting for static shift of magnetotelluric data with airborne electromagnetic measurements: a case study from Rathlin Basin, Northern Ireland

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 present here a possible method to employ frequency–domain electromagnetic data in order to correct static-shift effects, illustrated by a case study from Northern Ireland. In our survey area, airborne frequency domain electromagnetic (FDEM) data 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 suborder of magnitude resistivity contrasts induced by the 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

Geophysics for Mineral Exploration

This Special Issue contains ten papers which focus on emerging geophysical techniques for mineral exploration, novel modeling, and interpretation methods, including joint inversions of multi physics data, and challenging case studies. The papers cover a wide range of mineral deposits, including banded iron formations, epithermal gold–silver–copper–iron–molybdenum deposits, iron-oxide–copper–gold deposits, and prospecting forgroundwater resources

Andean Hydrothermal and Structural System Dynamics: Insights from 3D Magnetotelluric Inverse Modelling

In an active volcanic arc, magmatically sourced fluids are channelled through the brittle crust by structural features inherent in the lithological setting. This interaction is observed in the Andean volcanic mountain chain, where volcanoes, geothermal springs and major mineral deposits are spatially coherent with first-order NNE oriented thrust fault systems, and convergent-margin oblique WNW striking Andean Transverse Faults (ATF). The volcanic and hydrothermal activity at Tinguiririca and Planchón-Peteroa volcanoes demonstrate this relationship, as both volcanic complexes and their spatially associated thermal springs show strike alignment to the outcropping NNE oriented El Fierro thrust fault system. This study aims to constrain the 3D architecture of this fault system in the proximity of the volcanoes and its interaction with volcanically sourced hydrothermal fluids from a combined magnetotelluric (MT) and seismic field study. Data from a 24 station broadband magnetotelluric survey were interpreted using 3D inversion. Over 700 seismic hypocentres from a 12 station coeval seismic survey are also presented in support of the final 3D conductivity model. The combined results show a correlation of conductivity anomalies with seismic clusters in the top 10 km of the crust, including a distinct seismogenic WNW oriented feature that occurs at an abrupt electrical conductivity contrast, which is most apparent at a 6 km depth. It is concluded that this discrete feature is an Andean Transverse Fault (ATF), and that the conductors are signatures of either geothermal fluid reservoirs or fluid saturated lithologies at depth. The associated fluids are channelled parallel to the margin-oblique ATF plane and cause fault reactivation due to increased pore fluid pressure acting on the fault plane. Seismicity induced by this mechanism is limited to the east of the El Fierro fault system, as fluids are compartmentalized along the footwall due to the low permeability fault core that prevent cross-fault fluid migration. This study thus contributes novel insight into how WNW oriented AFT systems interact with local volcanic, structural and hydrothermal systems