61 2. Surface geophysical methods respect to grain volume (Sg) apparently does not depend on porosity. To remove the porosity effect on Vp/Vs and mimic a reflected ϕ vs. log(Sg) trend, they propose to use the following relationship between porosity ϕ, Vp/Vs and Sg: log(Sg m) = aϕ + b(Vp/Vs) + c (2.7) where it should be observed that Sg is multiplied by m to make Sg dimensionless. To establish equation (2.7), Fabricius et al. (2007) have looked at ultra-sonic data, porosity, and the permeability of 114 carbonate core plugs. 2.2 Geophysical methods Geophysical methods are currently used to build 2D or 3D models of the subsurface associated with variations of physical properties of rocks: • Gravity method with density variations, • Magnetic method with rock magnetization properties (magnetic susceptibility κ and remanence), • Low frequency Electrical and EM methods with resistivity variations, • High-frequency EM methods with permittivity variations (GPR), • Seismic methods with velocity and density variations. Figure 2.4 gives links between rocks and petrophysical parameters. Figure 2.4 Rocks and petrophysical parameters (density, magnetic susceptibility, velocity of elastic P-wave, resistivity) (after D. Chapellier, IFP School course, personal communication).
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