New techniques in borehole seismic*

Vertical seismic profiles (VSP) have been used extensively throughout Australia to provide improved seismic information in the vicinity of the borehole. In its simplest form, VSP can be regarded as the extension of a check shot survey. The positioning of a wall lock geophone within the borehole allows the simultaneous recording of upgoing and downgoing reflected energy. When correctly processed, a normal vertical raypath VSP provides improved time versus depth correlation of seismic events. Careful analysis of VSP data in conjunction with well logs and synthetic seismograms yields good information on seismic events close to the well. The simple concepts of VSP can be extended to non-vertical raypath or offset VSP where the energy source is positioned at one or more non negligible offsets from the well. By recording multigeophone levels in the well with varying source offsets a detailed high resolution seismic picture can be built up around the borehole to laterally extend VSP seismic coverage away from the well. The different methods for offset VSP and recommendations for offset VSP shooting are presented. Processing methods are explained and several examples from Australia are discussed.