76 Geophysics in Geothermal Exploration magnetospheric currents caused by plasma (solar winds) emitted from the sun and interfering with the earth’s magnetic field. The micro pulsations induce eddy currents in the ground, called telluric currents, and their density and distribution depend on the local conductive structure of the ground. The natural EM-field has a very wide spectrum, low frequencies, from 0.0001 to 10 Hz are used in investigations for depths of several tens to hundreds of kilometers (actual MT method), while higher frequencies mostly due to lightning around the world, from 10 to 1000 Hz are used for shallower targets (audio-magneto-telluric method – AMT). MT, in association with gravimetric (Figure 2.7) and magnetic (Figure 2.10b) surveys, has been successfully used for geothermal exploration in Martinique (Girard, 2017). (a) (b) Figure 2.16 Receiver (a, courtesy of Cripps Institution of Oceanography) and dipole source (b, courtesy of EMGS) for marine CSEM acquisition. Figure 2.17 is an example of mCSEM (marine CSEM) from the Hoop area of the Barents Sea. The area in question covers a significant oil discovery in the Hoop Fault Complex on the Bjarmeland Platform in the Barents Sea, Norway (Alvarez et al., 2017). A densely sampled dataset consisting of six lines of 2D seismic and towed streamer CSEM data were acquired concurrently in 2015 by PGS. The survey area lies in water depths of approximately 400 m. Two public domain wells in the area provide calibration for the integrated analysis. Some mCSEM data acquired along line 5001, in the form of source gathers at 1 Hz are shown. A significant response to the accumulation encountered at Wisting Central can be clearly seen in the CSEM data, particularly in the phase response (Figure 2.17a, lower panel around 611 km Easting). This is observed across a wide band of frequencies. The mCSEM data for six frequencies (0.2 Hz, 0.8 Hz, 1 Hz, 1.4 Hz, 2.2 Hz, 2.6 Hz) were inverted using an Occam approach (Constable et al., 1987; Key, 2016) to derive anisotropic resistivity models. The inversion was performed in stages. Firstly, an unconstrained inversion was run to examine the resistivity structure obtained in the absence of any a priori information. However unconstrained inversions in general have poor resolution. Resolution can be improved by including structural
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