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Exploring for deeply buried ore deposits
Published in Natalia Yakovleva, Edmund Nickless, Routledge Handbook of the Extractive Industries and Sustainable Development, 2022
Raymond J. Durrheim, Musa S.D. Manzi, Glen T. Nwaila, Susan J. Webb
The propagation of electromagnetic (EM) fields is governed by the conductivity and dielectric properties of rocks and the frequency of the signal. The EM field attenuates with depth because currents induced in the subsurface lose energy by joule heating of the rock; the more conductive the rock, the greater the absorption. The magnetotelluric (MT) method is a passive geophysical technique that determines the resistivity structure of the Earth using natural fluctuations of the Earth’s magnetic field, which originate in the magnetosphere (the region around the Earth that includes the atmosphere and the ionosphere) and from lightning activity. The fluctuations induce a flow of electric current within conductive zones in the Earth’s crust and mantle. An MT system simultaneously measures the electrical field using grounded electrodes and the three components of the magnetic field using high-frequency induction-coil magnetometers. In its simplest form, the output of an MT survey can be compared to an electrical log of a borehole that measures the variation of resistivity with depth, which can be interpreted in terms of rock type and pore fluids.
Geophysical methods for geothermal resources exploration
Published in D. Chandrasekharam, Jochen Bundschuh, Low-Enthalpy Geothermal Resources for Power Generation, 2008
D. Chandrasekharam, Jochen Bundschuh
The magnetotelluric (MT) method utilizes the earth's naturally occurring magnetic field and current. The magnetic field is measured over the surface of the earth. Basically this method measures the lateral and vertical electrical conductivity variation within the crust caused by the presence of ions or conductive solids (Simpson and Bahr 2005). Thermally excited solids (rocks) become highly conductive. Ions are present in the circulating geothermal fluids in the upper crust, or in the partial melts in the lower crust or in the mantle. This method can be applied in locating geothermal reservoirs within the crust. Basically this method images the earth's electrical resistivity structure at depths greater than 100 m. Low frequency method is used for locating large reservoirs located at greater depths, while audio-frequency is used for locating shallow reservoirs, and those on a regional scale. Since high heat flow values indicate the presence of subsurface geothermal reservoirs, heat flow variation in conjunction with MT surveys form an excellent tool in geothermal exploration. Based on the MT data, subsurface models can be developed using appropriate computer software. A schematic cross section of a typical geothermal system and the modeled MT survey results are shown in Figure 7.3. The low resistivity anomalous zones (Fig. 7.3a) lie above two geothermal reservoirs (Fig. 7.3b) with varying temperatures and different depths. Thus, MT surveys, in conjunction with field and other geophysical investigations described above, provide valuable information not only on the 3-D geometry of the geothermal reservoir, but also on the temperature variation of the reservoir with depth (Harinarayana et al. 2006).
SQUID Magnetometers
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
A technique known as magnetotellurics (MT) [11] can be used to determine the electric conductivity distribution of the Earth’s crust by measuring the Earth’s electric and magnetic field. Because the Earth is a good electrical conductor compared to the air, the electric field generated in the ionosphere (due to the solar wind) is reflected at the Earth’s surface, with components of both the electric and magnetic field decaying as they penetrate into the Earth. The decay length or skin depth (Δ) can be expressed as
Applied geophysics for cover thickness mapping in the southern Thomson Orogen
Published in Australian Journal of Earth Sciences, 2018
I. C. Roach, C. B. Folkes, J. Goodwin, J. Holzschuh, W. Jiang, A. A. McPherson, A. J. Meixner
The magnetotelluric (MT) method measures the natural magnetic and electrical (telluric) fields of the Earth, and uses their relationship to characterise the electrical structure of the Earth. When applied using appropriate data inversion and interpretation, the method provides a means to investigate the resistivity structure of the subsurface from depths of a few tens of metres to hundreds of kilometres. The AMT method (Chave & Jones, 2012; Dobrin & Savit, 1988; Vozoff, 1991) provides higher-resolution information within the upper part of the Earth’s crust at depths typically <2 km, when compared with BBMT, by acquiring data at higher frequencies between 20 kHz and ∼1 Hz.