Synthetic Shear VSP’s in Azimuthally Anisotropic Media

The azimuthal anisotropy due to a system of aligned vertical fractures, even when very weak, is sufficient to enable the strike of the fractures to be determined from shear VSP data. Synthetic VSP data were computed for several offsets using two perpendicular horizontal point forces into pairs of perpendicular horizontal receivers. The medium consisted of an anisotropic layer of orthorhombic symmetry (with the degree of azimuthal anisotropy due to the fractures an order of magnitude less than the degree of non-azimuthal transverse isotropy due to horizontal layering) over an isotropic halfspace. The line of sources from the well was at a fixed angle to the strike of the fractures. Source and receiver pairs were rotated numerically in small increments from the in-line to cross-line direction. When sources and receivers were aligned with the directions normal to, and parallel to, the fractures, the downgoing signal from a source to the receivers orthogonal to it decayed rapidly with depth. At zero offset this signal vanished identically at all depths.