ABSTRACT: Feasibility study of Seismic-While-Drilling using hammer drilling technology

Seismic-While-Drilling (SWD) has been shown to work with roller-cone bits but not PDC bits.This study investigates the suitability of hammer drilling technology for SWD applications. It has been completed using numerical modelling of the drilling system. Hammer drilling is a high performance drilling technique, particularly in a hard rock formation.

Two key issues of hammer drilling as a seismic source are to overcome the limitations of the narrow frequency band signal of the drill-bit operation and to determine the zero-time signal at the source. The solution of the first implies some control of the source characteristics in order to extract a wide bandwidth seismic signal. The source signal was numerically modelled and evaluated using a Linear Sweep Frequency (LSF), which is used in Vibroseis surveys and a Space Distributed Frequency (SDF). The evaluation used the autocorrelation of the modelled source signal and the crosscorrelation function using the SWD simulation assuming a continuous drilling operation. This accounted for the drill-string transfer function and the re-radiation at the drill-bit by the multiple reflection of the source signal in the drill-string. The result showed a high potential for a "check-shot" survey capability. Both LSF and SDF patterns of the source signature leads to similar results with less than an octave of frequency bandwidth. The method of control of the frequency of hammer drilling is discussed and the pattern of the sweep is suggested. The second issue dealing with the estimation of the drill-string travel time was investigated using Cepstrum analysis. This analysis shows that travel-time information can be obtained to the accuracy required. This work was carried out as part of the APCRC Abnormal Geopressure Program.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia