Comparison of high‐resolution wax‐embedded and pneumatically coupled borehole seismic detectors

Abstract

High‐quality seismic measurements at frequencies up to about 2000 Hz are needed if projected resolution limits on the order of 1 m in spatial dimension are to be realized in reservoir structure delineation, cross‐well sonic logging, and shallow reverse VSP applications. While sources and detectors are critical to this goal, we have investigated detector requirements in an objective way to demonstrate a successful design philosophy capable of achieving unprecedented wide‐band frequency response and data quality in three‐component shallow‐borehole sensors. Two prototype detectors were developed: a nearly ideal responding wax‐embedded “reference” detector and a pneumatically coupled detector exhibiting closely comparable performance. Our approach uses a three‐axis accelerometer sensor assembly installed in a borehole drilled through the weathered surface to a depth at which the ground is competent enough to support practical kilo‐hertz wave propagation. The wax‐coupled detector is planted using a meltable wax embedment to achieve a rigid, stress‐free, conformal coupling at the bottom of the hole. Experimental test results show this wax‐embedded detector to have excellent broadband three‐component response at frequencies up to 2500 Hz; a range heretofore unexplored for seismic applications. The pneumatically coupled detector, although limited by modal resonance distortion effects in the highest range of frequencies, demonstrated useful three‐component response at frequencies up to 1500 Hz. Tests of the two coupling techniques under identical conditions illustrate their high‐quality responses and their differences. Field tests of the prototype pneumatically coupled detector in shallow reverse vertical seismic profiling (VSP) measurements demonstrate the practical effectiveness of the basic high‐resolution probe design concepts.