INTRODUCING ANNIE - A SIMPLE 3-PARAMETER ANISOTROPIC VELOCITY MODEL FOR SHALES

A simple three-parameter transversely isotropic (TI) model is presente d as a reasonable first approximation for the elastic behavior of a wi de variety of shales. The model has the property that the zero-offset normal-moveout velocity is equal to the vertical velocity, a condition often approached by many shales. This constraint reduces the number o f independent parameters to four. Under this assumption, a simple accu rate approximation to the P-phase slowness surface may be found from w hich all oblique P-wave slownesses are predictable from just two speed s, the zero-offset normal-moveout velocity and the horizontal velocity . When seismic exploration data is confined to compressional waves rec orded in a rather narrow aperture, such a Ti medium will 'appear isotr opic'. A further constraint on the model parameters, reducing the numb er of independent parameters to three, connects the behavior in the ho rizontal plane to the behavior in the vertical plane. It allows for a generalization of the Lame parameters to the case where the TI anisotr opy is due solely to the existence of two shear moduli, one vertical, the other horizontal. These simple models for shales behave as a TI me dium equivalent to an isotropic medium stiffened by the inclusion of h orizontally bedded thin rigid plates.