Abstract
Natural source electromagnetic methods have the potential to recover rock property distributions from the surface to great depths. Unfortunately, results in complex 3D geo-electrical settings can be disappointing, especially where significant near-surface conductivity variations exist. In such settings, unconstrained inversion of magnetotelluric data is inexorably non-unique. We believe that: (1) correctly introduced information from seismic reflection can substantially improve MT inversion, (2) a cooperative inversion approach can be automated, and (3) massively parallel computing can make such a process viable. Nine inversion strategies including baseline unconstrained inversion and new automated/semiautomated cooperative inversion approaches are applied to industry-scale co-located 3D seismic and magnetotelluric data sets. These data sets were acquired in one of the Carlin gold deposit districts in north-central Nevada, USA. In our approach, seismic information feeds directly into the creation of sets of prior conductivity model and covariance coefficient distributions. We demonstrate how statistical analysis of the distribution of selected seismic attributes can be used to automatically extract subvolumes that form the framework for prior model 3D conductivity distribution. Our cooperative inversion strategies result in detailed subsurface conductivity distributions that are consistent with seismic, electrical logs and geochemical analysis of cores. Such 3D conductivity distributions would be expected to provide clues to 3D velocity structures that could feed back into full seismic inversion for an iterative practical and truly cooperative inversion process. We anticipate that, with the aid of parallel computing, cooperative inversion of seismic and magnetotelluric data can be fully automated, and we hold confidence that significant and practical advances in this direction have been accomplished.
Highlights
Introduction and BackgroundCo-location of seismic and magnetotelluric surveys is becoming increasingly viable for shallow to mid-depth exploration programmes like those completed by the minerals, hydrocarbon, groundwater and hydrothermal industries
The links we described between seismic and EM information required for joint or cooperative inversion should not be confused with the seismo-electromagnetic phenomena in which a tiny portion of the seismic wavefield is converted to an electromagnetic wavefield or vice versa (Dupuis et al 2009; Garambois and Dietrich 2002)
Where there is a lack of drill-hole information, one approach that we have developed is to compute a first-pass unconstrained inversion, based on the outcome, statistically assign a single resistivity to each subvolume within the model framework generated from seismic information
Summary
Co-location of seismic and magnetotelluric surveys is becoming increasingly viable for shallow to mid-depth exploration programmes like those completed by the minerals, hydrocarbon, groundwater and hydrothermal industries. It is increasing routinely for deeper crustal research. The challenge we identify and address is the creation and testing of practical cooperative inversion strategies intended to link information from seismic and magnetotelluric (MT) surveys to recover detailed subsurface rock property distributions. The goal of the MT method is to recover subsurface electrical conductivity distributions from the measurement of naturally occurring electromagnetic fields. The information content residing within a co-located seismic data set should be capable of improving outcomes from magnetotelluric inversion and vice versa. We should develop and if possible automate methods able to extract suitable and consequential information from the seismic data, which has a higher resolution than the MT data
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