Chapter 16 Fracture Detection and Characterization from Hydrophone Vertical Seismic Profiling Data

Abstract

Fractures intersecting a borehole can be detected and characterized by tube waves generated by a fluid pulse injected into the borehole when a seismic wave impinges on and compresses the fracture. A vertical seismic profiling (VSP) survey, using a hydrophone array, provides the ideal data for tube wave recording and analysis. In this chapter the theory of tube wave generation in a borehole by a seismic wave is reviewed. A technique is developed to calculate the dip of a fracture from a single-offset hydrophone VSP survey from P-, SV-, and tube-wave amplitudes. In addition, the contribution to tube wave generation from particle motion parallel to the fracture plane is evaluated. On the basis of an analysis of the fluid flow regime in the fracture, it is shown that this type of particle motion causes fluid to be injected into the borehole which is several orders of magnitude less than that caused by compression of the fracture. Therefore, it is concluded that parallel-plane particle motion is negligible in terms of generating tube waves. A technique is then developed whereby the amplitude ratio of the SV-generated tube wave to the P-generated tube wave can be used to determine the dip of the fracture plane. The complete characterization of a fracture (dip and strike of fracture plane and hydraulic conductivity) can be achieved if hydrophone VSP data are acquired from several surface source locations. This method is used to estimate fracture properties from a VSP experiment conducted at Kent Cliffs, New York and Hamilton, Massachusetts. The results are compared to fracture orientation estimates from borehole televiewer measurements and core data.