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 wideband 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 kilohertz 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.

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