THE EFFECTS ON SEISMIC WAVES OF INTERCONNECTED NEARLY ALIGNED CRACKS

Abstract

Transfer of fluid between connected cracks may occur during the passage of seismic waves. Such fluid flow can be modelled using an extension of effective medium theory (Hudson et al. 1996) and is effected via non-compliant pores. The flow is governed by a parameter τ representing the relaxation time of pressure equalization between cracks. However, if the cracks are fully aligned and have the same aspect ratio, the theory produces the unexpected result that, at low frequencies, the cracks are effectively isolated and at high frequencies they are fully drained. The artificial restriction of the model to perfectly aligned cracks of identical aspect ratio is seen to be the cause of this result. By reworking the model to allow the crack orientation and aspect ratios to vary, we see that a more realistic model has the usual properties in which the cracks are isolated at high frequencies and undrained at low frequencies. We have chosen the distributions of aspect ratios to be in agreement with observation (Hay et al. 1988). Thomsen's parameters (Thomsen 1986) and the attenuation coefficients are seen to be frequency-dependent via the non-dimensional parameter Ωτ.