75 2. Surface geophysical methods EM methods (AC) Electromagnetism methods (EM) are based on the study of electromagnetic field generated either by natural or controlled sources. With natural source (EM fields occurring naturally), the incident signal tends to behave like a plane wave at the air interface. The two components Ex and Ey of the electric field and the associated two components Hx, Hy and potentially Hz components of the magnetic field are measured. The apparent resistivity already defined in the previous section (in ohm.m) of the subsurface structure is usually estimated: ρ π π a xy x y f E H f Z , = = 1 2 1 2 2 2 2 (2.10) with Z, the impedance tensor, Ex and Ey the spectral horizontal components of the electric field expressed in V/m in the horizontal x and y directions, Hx and Hy the spectral horizontal components of the magnetic field expressed in A/m in the horizontal x and y directions, f the frequency in Hz. Inversion of apparent resistivity and phase of the impedance tensor, leads to earth resistivity model (2D and 3D). With controlled sources, manmade electric dipoles, current loops, …, the two components Ex and Ey of the electric field and the associated two components Hx, Hy of the magnetic field are measured. In some borehole applications, both vertical components: Ez and Hz can also be measured. The link between data and earth resistivity cannot be simplified anymore to the apparent resistivity concept, because the incident signal is not a plane wave anymore. Natural or manmade sources, 2D and 3D interpretation of EM data requires the derivations of the Maxwell’s equations (numerical modelling and inversion). At those frequencies, the EM signal is strongly attenuated through a diffusion process. Such attenuation is controlled by the so-called skin depth: δ≈ ρ 503 /f (2.11) Skin depth is defined as the distance along which the electromagnetic field has reduced to e–1 (or 37 percent) of its original amplitude value at the surface or source location. Signal penetration is therefore function of frequency and earth resistivity for natural source and also offset (distance in between source and receiver) for manmade source. Active audiomagnetotellurics (AAMT), transient electro-magnetic (TEM), time domain electromagnetic method (TDEM), controlled source electromagnetics (CSEM) and controlled source audiomagnetotellurics (CSAMT) methods fall in the category of active EM methods. Figure 2.16 is an example of receiver and dipole source for marine CSEM acquisition, the dipole source being towed behind the vessel and receivers dropped down on the sea floor. Magneto-telluric (MT) and audio-magneto-telluric (AMT) methods fall in the category of passive EM methods with natural source. The origin of the variations of earth’s magnetic fields, called magnetic micro-pulsation, is the ionospheric and
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