ABSTRACT
In this chapter the effect of structural anisotropy on shear wave splitting and polarization phenomena will be treated in some detail, due to its extreme importance in helping to characterize jointed or fractured reservoirs, and due to the improved insight it is giving into earthquake phenomena. The structural anisotropy may be stress aligned, and there are then logical ties to the principal permeability or drainage directions. Dominant jointing and natural fractures are of increasing interest to petroleum companies, both for production and for aiding stimulation, where matrix permeability is low but hydrocarbon storage high. This vitally important structural feature is notoriously poorly sampled by vertical core and well-bore scanning, since itself often sub-vertical or vertical. The ‘miracle’ of shear-wave splitting (with assistance from azimuthal AVO P-wave surveys) has provided the means of detecting the presence of these compliant, fluid-bearing fractures. Contrary to the classic wisdom of porous media fluid substitution theory, in situ fractures also seem capable of signalling to the shear-waves, whether they contain gas or brine, through subtle velocity reduction of the slow S-wave, due to the fluid-compressibility-altered fracture compliance magnitudes. Geophysicists utilise an unfortunately ambiguous way of describing fracture density: as number per unit volume times radius cubed. This ambiguity, meaning that millions of microcracks or a hand-full of fractures can give the same magnitude, nevertheless seems to have a remarkable proportionality to shear-wave anisotropy, but clearly this can be altered by changed compliances. The need to define a specific volume of fractures for reservoir understanding is urgent. Consequently, recent numerical dual-porosity poro-elastic modelling developments have become increasingly important for exploring the frequency-dependent velocity and attenuation resulting from the various potential scales of anisotropy. Case records both from seismology and petroleum engineering will be used to show recent trends in analysis of seismic survey data. Relevant rock mechanics experi-
rock joints will be referred to in several contexts, where deemed appropriate. There will also be a strong focus on the possible links between the joint or fracture shear and normal compliance used by geophysicists, and the macro-deformation, and inverted stiffnesses used for many years in rock mechanics models of jointed media. The need for compatible measures of volume-defined fracture densities and in situ values of compliance (as opposed to those obtained from hand-sized joint samples or roughened plates of Lucite) is necessary for further development.
