Seismic sources in open and cased boreholes

Abstract

Theoretical modeling, based on the equations of linear elasticity, is used to analyze the performance of downhole seismic sources, specifically, symmetrical radial sources which exert outward pressure on a length of the borehole wall and vertically driven sources clamped to the borehole wall. Torsional sources radiate pure S-waves whose form is independent of the existence of the borehole, and therefore requires no special discussion. The radial sources discussed are airguns, waterguns, and implosive sources. Such sources are in widespread use and development at present. From the calculations we find that radial sources emit almost all (>99 percent) their energy as tube‐waves which travel along the borehole and not out into the formation. Vertical sources radiate almost all their energy into the formation as P- and S-waves. However, if the casing bond slips, the vertical sources lose efficiency and radiate energy as extensional waves along the casing. The radial sources are little affected by the quality of cement bonding. Radial sources may be used at or near the surface to direct tube‐waves down the hole. Conversion of the tube‐waves to P-waves and S-waves either from the end of the hole or from a specially configured converter such as a constriction placed in the hole theoretically represents a viable downhole seismic source for reversed VSP (RVSP) and crosshole imaging. Little energy is lost in tube wave transmission and high acoustic power may be injected into a well.