James W. Rector, III, Bruce P. Marion
Vertical seismic profiling (VSP) has been used over the past decade to provide seismic images near well bore. A new well bore seismic technique, the TOMEX^target Survey, uses the natural vibrations created by the drill bit during the drilling of a well as a downhole energy source. Since the TOMEX Survey requires no downhole instrumentation, it eliminates lost rig time and avoids the complications of open-hole wireline operations. These factors are important whenever rig time is expensive or the risk associated with open-hole operations are high. Furthermore, the ease with which surface receivers can be deployed makes multioffset surveys much more cost effective than conventional offset VSP.
Ideally, an explorationist wants the information obtained with a VSP to be available while the well is being drilled, since the ability to precisely locate the seismic target with respect to the current drill-bit position could enormously reduce drilling expenditures. However, prior to acquiring the VSP, the driller must trip out of the hole, and in many cases, a wiper run must be performed to condition the borehole walls. This drilling inactivity period can be rather long. Hence, to cut costs, VSPs are usually performed during the final logging run, later than information optimally may be needed. In contrast, TOMEX Survey data are acquired and processed as drilling proceeds. This measurement-while-drilling (MWD) characteristic of the TOMEX Survey provides time vs. depth curves and re lector images to the explorationist on site, in real time. These data can have a major impact on drilling since estimates of target depths and structural images of formations yet to be penetrated by the drill bit are available to the explorationist and the driller in time to make well deviation or drill-ahead decisions. The drill bit-generated signals are recorded with sensors attached to the top of the drillstring and at various surface geophone locations near the well. The sensor output at the top of the drillstring is used as a pilot signal for cross-correlation with the signals recorded at the surface-geophone positions. Cross-correlation is used to determine arrival times and to enhance the signal-to-noise ratio of drill bit-generated events. Deconvolution and time shifts are perfor ed to remove the effects of recording the pilot signal at a location which is distant from the location of the source of energy at the drill bit.
AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.