Controlled-source Magnetotelluric Data Research Articles

Magnetic and electromagnetic data interpretation for delineating geological contacts in the Tomelilla area, Sweden

This paper describes an application of diverse geophysical methods to detect the geological contacts in the Tomelilla area, Sweden. Airborne magnetic data and the tipper and peaker maps derived from tensor very-low-frequency electromagnetic (TVLF-EM) data are interpreted in order to indicate most geological boundaries. The inverted airborne TVLF data along adjacent profiles over each contact show the resistivity variations at depth and laterally localize the geological contacts while delineating resistive bedrocks in all cases. To control the resistivity images across these profiles, the ground-based measurements of the radio magnetotelluric data combined with controlled-source magnetotelluric data (RMT/CSMT) have been performed to plot resistivity variations with higher resolution. The obtained resistivity images are in good agreement with the ones acquired from airborne TVLF data. A relatively conductive layer with an average thickness of 20 to 30 m of the Quaternary deposits, increasing in thickness from east to the west, and a resistive basement with much higher resistivity on the granites than the shales are resolved from the geoelectric sections. This paper confirms that the combined geophysical surveys (i.e., airborne magnetometry/TVLF along with ground-based RMT/CSMT) reduce the uncertainty arising from geological mapping. It is shown that the airborne TVLF data have high capability for image resistivity variation along geological contacts in the study area while its resolution has been confirmed by the ground-based RMT/CSMT survey.

Imaging of CO 2 storage sites, geothermal reservoirs, and gas shales using controlled-source magnetotellurics: Modeling studies

To balance the steady decrease of conventional hydrocarbon resources, increased utilization of unconventional and new energy resources, such as shale gas and geothermal energy, is required. Also, the geological sequestration of carbon dioxide is being considered as a technology that may temporarily mitigate the effects of CO 2 emission. Sites suitable for shale gas production, geothermal exploration, or CO 2 sequestration are commonly characterized by electrical resistivities distinctly different from those of the surrounding rocks. Therefore, electromagnetic methods can be viable tools to help identify target sites suitable for exploration, and to monitor reservoirs during energy production or CO 2 injection. Among the wide variety of electromagnetic methods available, controlled-source magnetotelluric (CSMT) may be particularly suitable because of (i) its ability to resolve both electrically resistive and conductive structures, (ii) controlled sources offering noise control and thus facilitating surveys in populated regions, and (iii) the potential of penetration throughout the depth range accessible by drilling. Nevertheless, CSMT has not yet been widely employed because of logistical challenges of field operations and the requirement of complex and highly computer-intensive data processing. With these difficulties gradually being mitigated by recent technological developments, CSMT may now be reconsidered as an exploration tool. Here, we investigate by 1D and 3D numerical simulations the feasibility of detecting gas shales and identifying sites eligible for geothermal exploration or CO 2 sequestration from CSMT data. We consider surface-to-surface, borehole-to-surface, and cross-hole configurations of the sources and receivers. Results and conclusions on the detectability of the targets of interest are presented for various exploration and monitoring scenarios, which are roughly representative of the geological setting of the North German Basin.

Approximate calculation of low‐frequency three‐dimensional magnetotelluric responses using a multiple plate model

The numerical methods now used to calculate the magnetotelluric (MT) response of three‐dimensional (3-D) conductive inhomogeneities are still expensive in terms of computing cost. This is a more severe problem in many actual field conditions where multiple conductors exist. There is then a clear need for the development of faster algorithms for data interpretation. We can rapidly compute the 3-D MT response of a multiple‐conductor structure embedded in a layered earth at low frequencies using an approximate algorithm where each conductor is simulated by a set of plates. There are two limitations to the technique. The first low‐frequency limitation is a requirement that the induction number of any plate in the model be small compared with unity. The second condition specifies that the range from an observation point on the surface of the earth to a typical plate must be small compared with the skin depth in the layered background medium. These conditions allow us to ignore local self‐induction within the plates and mutual induction between the plates and the host medium. The secondary anomalous fields generated by any plate are assumed to be due to a distribution of current dipoles in the plane of the plate. The strengths of the distributed dipoles are found by solving a Fredholm integral equation of the second kind with the layered earth MT electric field at depth as the driving function. MT responses obtained with our approximate algorithm for three prismatic models compare favorably with previously published data obtained with complete algorithms for the same models. The use of the technique to model multiple conductors is illustrated with controlled source MT data collected at the Cavendish Test Site.