The depressurization of coal seam gas formations causes in situ fluids to migrate through pores and fractures in the earth. The removal or discharge of large volumes of water from coal measures reduces in situ fluid pressure allowing natural gas to be released from the coal matrix. This process results in a time-dependent resistivity variation in the subsurface. Increasing the connectivity of in situ fluids may lead to a reduction in resistivity of the targeted lithologies. A correct assessment of such resistivity variations is of significant interest not only for the industry to optimize production and extraction well locations but also for the regulatory bodies, in which a desire for a reliable method for monitoring changes in subsurface fluid distribution allows sound risk assessment of potential environmental hazards. From an industrial field study conducted in Queensland, Australia, we have found that the magnetotelluric (MT) method in the bandwidth of 100 Hz to 100 s could be used to monitor changes in the bulk resistivity of depressurized lithologies. Results from our study indicate the orientation of fluid flow resulting from depressurization, which can be mapped and directly attributed to spatial and temporal variations in permeability. The MT method is introduced as a low-cost, low-impact technology that can be used for short- and long-term environmental monitoring.
Abstract We present two‐dimensional electrical resistivity models of two 40 km magnetotelluric (MT) profiles across the Frome Embayment to the east of the northern Flinders Ranges, South Australia. The lower crust shows low resistivity of 10 Ω m at around 30 km depth. The middle crust is dominated by resistive (>1000 Ω m) basement rocks underlying the Flinders Ranges. Adjacent to the ranges, conductive lower crust is connected to vertical zones of higher conductivity extending to just below the brittle‐ductile transition at ∼10 km depth. The conductive zones narrow in the brittle upper crust and dip at roughly 45° beneath the surface. Zones of enhanced conductivity coincide with higher strain due to topographic loading and sparse seismicity. We propose that fluids are generated through neotectonic metamorphic devolatilization. Low‐resistivity zones display areas of fluid pathways along either preexisting faults or an effect of crustal compression leading to metamorphic fluid generation. The lower crustal conductors are responding to long‐wavelength flexure‐induced strain, while the upper crustal conductors are responding to short wavelength faulting in the brittle regime. MT is a useful tool for imaging crustal strain in response to far‐field stresses in an intraplate setting and provides important constraints for geodynamic modeling and crustal rheology.
Landfill sites and saline disposal basins are just two of the numerous situations in which escape of fluids from the system can contaminate soils and groundwater. Techniques to map fluid flow, delineate the extent of contamination and monitor changes during remediation are needed for environmental monitoring applications. Theoretical and laboratory-based testing of Electrical Resistance Tomography (ERT) was undertaken to examine the capabilities and limitations of this method.The performance of a series of two-dimensional numerical models led to the conclusions that a contaminant plume can be imaged, that downhole current transmission sites with both downhole and surface potential measurement sites is preferable and that smaller scale fingering effects associated with free convection are not readily resolved. It also highlighted the importance of electrode configuration for successful ERT surveying. An ERT survey conducted in a glass tank to monitor the development of a dense, saline plume was also conducted. Visual comparison of photographs with ERT images agreed with theoretical results.
The resistivity structure of the crust is broadly expected to be homogeneous, with highly resistive lower crustal rock overlain by more conductive rock in the upper crust. However, observed data shows that although the upper crust is typically resistive, the lower crust can be much more conductive. The presence of such high electrical conductivity in the lower crust is remarkable and suggests a substantial highly connected material, melt or fluid. Has the low resistivity structure been present since inception, or is it the result of a later overprinting event. The secondary objective to establish how such a low resistivity region can be preserved over such an extended time scale.Data has been collated from magnetotelluric (MT), and geomagnetic depth sounding (GDS) surveys collected over the last thirty years. Three different methods have been used to model thousands of data points. A thin-sheet inversion of thousands of GDS data has been used to place constraints on the regional scale electrical conductance. Inversions of the MT data in both 2D and 3D have provided more detailed models of how the Moho is connected to the upper crust.Strong correlations were observed between major tectonic domains (such as the Gawler Craton) and regions of high resistivity within the crust. The 2D profiles show broad regions of low resistivity at the boundary between the upper and lower crust (10-15 km depth), with low resistivity zones extending for tens of km. Above the boundary, the low resistivity regions transform in to narrow pathways penetrating through the resistive upper crust and the areas with the lowest resistivity were found to have a strong correlation with known major mineral provinces. This leads to the suggestion that crustal low resistivity anomalies are likely a product of fluxes of fluid and possibly melt from the upper mantle and lower crust.
We have developed a petrophysical joint inversion between magnetotellurics (MT) and gravity. It utilises Archie?s Law and the porosity-density relationship. Through these equations it can be shown that both conductivity and density are dependant on porosity. Porosity then forms the crucial link between the two techniques used by the joint inversion. The approach was tested using synthetic models. The results demonstrate the joint inversion produces a better representation of the subsurface, as compared to individual MT or gravity inversions. It shows sharper boundaries and more accurate parameter values. Our joint inversion is viable in real Earth applications and this was demonstrated through a case study of the Renmark Trough, South Australia.
The clean energy transition will require a vast increase in metal supply, yet new mineral deposit discoveries are declining, due in part to challenges associated with exploring under sedimentary and volcanic cover. Recently, several case studies have demonstrated links between lithospheric electrical conductors imaged using magnetotelluric (MT) data and mineral deposits, notably Iron Oxide Copper Gold (IOCG). Adoption of MT methods for exploration is therefore growing but the general applicability and relationship with many other deposit types remains untested. Here, we compile a global inventory of MT resistivity models from Australia, North and South America, and China and undertake the first quantitative assessment of the spatial association between conductors and three mineral deposit types commonly formed in convergent margin settings. We find that deposits formed early in an orogenic cycle such as volcanic hosted massive sulfide (VHMS) and copper porphyry deposits show weak to moderate correlations with conductors in the upper mantle. In contrast, deposits formed later in an orogenic cycle, such as orogenic gold, show strong correlations with mid-crustal conductors. These variations in resistivity response likely reflect mineralogical differences in the metal source regions of these mineral systems and suggest a metamorphic-fluid source for orogenic gold is significant. Our results indicate the resistivity structure of mineralized convergent margins strongly reflects late-stage processes and can be preserved for hundreds of millions of years. Discerning use of MT is therefore a powerful tool for mineral exploration.
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