70 Geophysics in Geothermal Exploration Oil & Gas and Mining Industry, Geothermal and Near surface studies (civil engineering, groundwater monitoring and environmental purposes). First known success occurred with mineral exploration on highly conductive sulfide metal ores bodies. A behavior of an EM field is controlled by 3 main parameters: • Electrical conductivity/resistivity • Dielectric permittivity • Magnetic susceptibility The Electrical conductivity or resistivity is the most important for DC and low frequency methods (i.e. below 1 kHz for MT, AMT, mCSEM, AEM) whereas the Electric permittivity is the most important for high frequency methods (i.e. above 1 MHz for GPR). The chart (Figure 2.11) shows that materials which are part of E&P investigations are distributed over a massive range of orders of magnitudes regarding electric resistivity. Resistivity variations in sediments are controlled by variations of porosity, permeability, pore connectivity geometry and the fluids contained by the pores. As standard approximations, the industry often takes 0.3 Ω·m for seawater, 1.5 to 3 Ω·m for sediments saturated with brine and up to 100 Ω·m for hydrocarbon bearing reservoirs. Almost two orders of magnitude between sediments containing brine and those containing HC. On the other side, elastic waves could not even pretend to be 1 order of magnitude of difference. Figure 2.11 Electric resistivity versus rock type.
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