Geophysical Prospecting, 1995, 43, 805-829 Elastic anisotropy due to alignedcracksin porous rockl LeonThomsen2 Abstract All theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way, for example to the case of stiff pore fluids, or of the absence of equant porosity, or of a moderately high frequency band. Because of these restrictions, previous theory is not suitable for application to the upper crust, where the pore fluid is brine (1(, = KJ2O), the equant porosity is often substantial (do > 0.t;, and the frequency band is sonic to seismic. This work removes these particular restrictions, recognizing in the process an important mechanisrn of dispersion. A notable feature of these more general expressions is their insensitivity, at low frequency, to the aspect ratio of the cracks; only the crack density is critical. An important conclusion of this more general model is that many insights previously achieved, concerning the shear-wave splitting due to vertical aligned saturated cracks, are sustained. However, conclusions on crack orientation or crack aspect ratio, which were derived from P-wave data or from shear-wave 'critical angles', ffiay need to be reconsidered. Further, the non-linear coupling between pores and cracks, due to pressure equalization effects, means that the (linear) Schoenberg-Muir calculus may not be applied to such systems. The theory receives strong support from recent data by Rathore et al. on artificial samples with controlled crack geometry. lntroduction This work concerns the theory of the effect of a set of aligned circular cracks upon the elasticity of the resulting composite material. Of course, the primary effect is a reduction in certain of the elastic moduli, so that the resulting composite material is elastically anisotropic. Assuming that the matrix material is homogeneous and iso- tropic, the composite material is clearly transversely isotropic, with its symmetry axis lying perpendicular to the flat facesofthe cracks. At issue is the dependenceof the anisotropy upon angle, upon crack density, upon crack shape, upon stiffness of I Paperpresented at the 53rd EAEG meeting,May 1991, Florence,Italy. Received June 1993, revision accepted January 1995. 2 Amoco Production Company,4502 East4lst Street,Tulsa, OK, 74102-3385, U.S.A. @ 1995EuropeanAssociation of Geoscientists & Engineers 805