--> ABSTRACT: Shear-Wave Velocity Variation in Jointed Rock: An Attempt to Measure Tide-Induced Variations, by Leigh I. Beem; #91040 (2010)

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Shear-Wave Velocity Variation in Jointed Rock: An Attempt to Measure Tide-Induced Variations

Leigh I. Beem

The research is aimed at using the perturbation of seismic wave velocities by solid earth tides as a possible method of exploration for fractured media. Velocity of compressional seismic waves in fractured homogeneous rock has been observed to vary through solid earth tide cycles by a significant 0.5-0.9%. This variation of seismic velocities may be attributed to the opening and closing of joints by tidal stresses. In an attempt to see if shear-wave velocities show a similar velocity variation, a pneumatic shear-wave generator was used for the source. The 5 receivers, 3-component, 2.0 Hz, moving-coil geophones, were connected to a GEOS digital recorder. The two receivers located 120 m and 110 m from the source showed large shear-to-compression amplitude ratio and a high s gnal-to-noise ratio.

A glaciated valley was chosen for the experiment site, since topography is flat and the granodiorite is jointed by a set of nearly orthogonal vertical joints, with superimposed horizontal sheeting joints. Detailed mapping of the joints and rock properties (scale 1:100) along the propagation path is used to quantify the jointed rock characteristics. Joint widths over a ray path normal to one joint set are summed and divided by the path length to estimate the ratio of void to solid space. A total of 32.10 mm of void space was measured, giving a void space-solid space ratio of 0.026%.

A slight velocity variation was noted in the first 200 consecutive firings; after which, the amplitude of the shear-wave begun to increase. This increase has been attributed to the compacting of the soil beneath the shear-wave generator (SWG). In the future, the soil will be compacted prior to placing the SWG or the SWG will be coupled directly to the rock to alleviate the amplitude fluctuation problem.

This research may have application in exploration for fracture permeability in the rock mass between existing wells, by measuring seismic velocities from well to well through the tidal cycle.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.