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.