Triaxial seismic measurement while drilling and estimation of subsurface structure: H. Asanuma & H. Niitsuma, Geothermal Science & Technology, 5(1–2), 1995, pp 31–51

Abstract

A reverse Vertical Seismic Profile (VSP), which utilizes a drill-bit as a seismic source, is promising for evaluating subsurface structure and for on-line determination of a drilling target, such as geothermal reservoir. A seismic measurement was made in a measurement well 140m from the drilling well and the associated seismic signal at bit depths from 313 m to 354 m was continuously recorded. We have found that the hodogram of the three component signal is polarized along the direction of the drill-bit and that the seismic signal shows spectral whiteness. The direction of the drill-bit was successfully estimated by a spectral matrix analysis of the 3-C seismic signal. A reverse VSP technique (TAD-VSP; Triaxial Drill-bit VSP) based on a 3-D hodogram analysis of a continuous seismic signal by a single-point observation was introduced. In TAD-VSP, a spatial-temporal cross-correlation analysis of the particle motion was made to detect reflected waves. Delay and propagating directions of reflected waves were discriminated from the extreme values of the 2-D cross-correlation coefficients. We also devised a method to project the 2-D cross-correlation coefficients into the object plane, which represents the distribution of the reflection coefficients. Distribution of reflection coefficients estimated by TAD-VSP agreed satisfactorily with the core observation and sonic log in the field. The TAD-VSP survey has potential to be effectively used for on-line evaluation of deeper subsurface structure.