Detection of open fractures with vertical seismic profiling

Abstract

In Vertical Seismic Profiling surveys tube waves are generated by compressional waves impinging on subsurface fractures or permeable zones. The problem of generation of these waves by a non-normal incident P wave for an inclined borehole intersecting a tilted parallel wall fracture is formulated theoretically. The amplitude of tube waves depends on the permeability. the length of the fracture, and on the frequency. The relative effects of these parameters are studied individually. The problem is also formulated for a thin oblate ellipsoidal (penny-shaped) fracture. The results for the two fracture models are compared and contrasted.Field data from Tyngsboro, Massachusetts are shown for open fractures in granite. From tube wave amplitudes normalized to P wave amplitudes, calculated permeabilities are on the order of one hundred millidarcys.