A swept-frequency borehole source for inverse VSP and cross-borehole surveying

Abstract

A swept-frequency borehole seismic source has been constructed and tested that consists of a portion of the borehole isolated from the remainder and driven to build up pressure oscillations by resonance.The length of the isolated portion is changed to vary the resonant frequency. The source radiates an approximately isotropic P-wave whose total energy is comparable in magnitude to that created by a surface vibrator truck. Strong shear waves are also generated.The source has been tested over a frequency range of 30 to 120 Hz, but the design can be operated from 15 to 500 Hz. Because the driven section of the borehole is isolated, strong tube waves are not generated. No damage to the casing-cement bond has been observed after prolonged operation of the source at a fixed depth.This source has the strength and uniform radiation pattern to suit it for both inverse VSP and cross-borehole surveying.