68 Geophysics in Geothermal Exploration 2.2.2 Magnetic method Magnetic properties describe the behavior of any substance under the influence of a magnetic field. All minerals are affected in some way by a magnetizing field. The capacity of a mineral to acquire magnetism by induction is described by its magnetic susceptibility. The strength of the induced magnetization M (magnetic dipole moment per unit volume) is directly related to the strength of the applied magnetic field H: M = κ H (2.10) with κ the magnetic susceptibility. Rock types vary in magnetic susceptibility (Figure 2.10a). The most important fact in magnetic exploration for petroleum is that sedimentary rocks are nearly nonmagnetic, that is; have very small susceptibility compared to basement rocks. The susceptibilities of non-sedimentary rock types are larger than those of sedimentary rocks by a factor of 10 to 1000 times. The magnetic fields measured in practice are flux densities. The unit is called Tesla T. For most geophysical purposes the tesla is too large as a unit and flux densities are more conveniently expressed in nanotesla (nT = 10–9 T). (a) (b) Figure 2.10 Magnetic method. a: Magnetic susceptibility versus rock type (after D. Chapellier, IFP School course, personal communication), b: Magnetic anomaly observed in Martinique (after Girard, 2017).
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