Shales are regarded as anisotropic during low-strain dynamic elasticity behavior. This means that, in seismic analysis, we must assume that the commonly used seismic properties like modulus, velocity, and impedance are anisotropic in shales. The electromagnetic property resistivity, important in recent controlled-source electromagnetic surveying, is also assumed to be anisotropic. Due to the nature of shales, the seismic and electromagnetic properties are assumed to behave as a vertical transverse isotropic (VTI) medium. They are then isotropic in the plane orthogonal to the symmetry axis, normally being the vertical direction. Seismic amplitudes created by lithology contrasts involving at least one shale layer are then controlled by this anisotropy, especially at larger incidence angles (or offsets). Seismic waves in shales travel with different velocities at zero and nonzero incidence angles, making anisotropy a controlling factor also in seismic processing. The overwhelming presence of shales in many basins stresses the importance of understanding how the seismic and electromagnetic waves behave in this rock type.

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