Figure I Generalized map of the Wilbur Mining ... - University of Utah

Geothennal Resources Council, TRANSACTIONS, Vol. 3 Septetrher 1979
CONTINUOUS GRAVITY OBSERVATIONS AT THE GEYSERS:. A PRELIMINARY REPORT
ABSTRACT
A cryogenic gravimeter has been installed at
The Geysers to continuously monitor gravity variations
at the Mgal level. A 38-day record is presented
to illustrate the type of information that
can be obtained from such an instrument. In
addition to information directly related to mass
transport within the reservoir, the data reveal a
sudden 6 ygal decrease in gravity prior to a local
earthquake. We also observe a 5 ugal increase in
gravity during a heavy rainfall; however, interpretation
of gravity variations at this level is
limited by uncertainty in tilting of the gravimeter
pier. The potential Irapact of continuous
gravity observations on the study of reservoir
characteristics is discussed.
THE CRYOGENIC GRAVIMETER
Jeffrey J. Olson and Richard J. Warburton
University of California at San Diego
La Jolla CA 92093
Although cryogenic gravimeters have been in
existence for several years (Goodkind and Prothero^
1968, and Warburton and Goodkind, 1978), this is
the first use of such an instrument in a geothermal
application. The instrument differs from conventional
gravimeters in that mechanical springs
and levers are replaced by magnetic fields generated
from persistent currents in colls of superconducting
wire. These fields support a one-inch •
dlamecer superconducting sphere (the graviroeter's
only moving part) with a force that does not
significantly diminish in time, due to the persistence
of superconducting currents. Thus the
cryogenic gravimeter does not exhibit the instrumentally
produced signal drift which is characteristic
of conventional gravimeters. Several years
of gravity observations at Pinon Flat in southern
California indicate that the total Instrumental
drift is 0 ± 5 ygal/yr. A planned side-by-side
test of two cryogenic gravimeters at Pinon Flat
should determine whether this residual variation
is of instrumental or geophysical origin.
The short term precision of the cryogenic
gravimeter appears to be limited only by the uncertainty
with which the contributions frora known
sources such as earth and ocean tides and atmospheric
density variations can be subtracted. The
data presented below indicate that this can be
accomplished empirically to a precision of ±1
ligal. Spurious tilting of the gravimeter platform
can degrade this performance, but tilt will soon
be controlled by the addition of an automatic
519
leveling system.
OBSERVATIONS AT THE GEYSERS
A cryogenic gravimeter was installed in
February of 1979 in The Geysers steam field at
latitude 38° 48' 25" and longitude 122° 48' 50".
This site is on a spur of ridge which extends
downward from Well Sulphur Bank 19 toward Units 3
and 4. McLaughlin (1974) maps this general area
as a quaternary landslide, however, the spur
appears to be an outcrop of relatively unfractured
Franciscan graywacke. Weathered graywacke
slightly uphill of the outcrop was excavated to a
depth of 2 - 3 m to expose bedrock, upon which a
3 m high reinforced concrete pier was erected.
The gravimeter platform rests on this pier, supported
on three points two of which are heavy
duty micrometer heads to allow alignraent of the
gravimeter with the vertical. The active element
of the gravimeter is immersed in liquid helium in
a Dewar which is fixed to the platform.
The output signal from the gravimeter is in
the form of an analog voltage which is filtered
electronically for sampling at a variety of rates
from 20 seconds to 15 minutes. Barometric
pressure as measured by a temperature regulated
aneroid barometer is also filtered and monitored
at 15 minute intervals. These signals are recorded
in the field on digital tape cassettes by a
microcomputer-controlled data system, which can
also transmit stored data by telephone to our laboratory
for inspection.
A sample of raw data together with the results
of various stages of its reductiori are shown
in Figure 1. All four graphs are plotted as
functions of time at 15 minute Intervals beginning
at 067:00:00:00 UTC and ending 38 days later at
105:00:00:00 UTC. The observed gravity signal,
Figure la, is dominated by smooth tidal variations
(the coarseness of this graph is an artifact of
the digital plotter: the point density is nearly
1000 points per inch on this scale). The principal
component of this signal is the direct gravitational
attraction of the moon and sun together
with the effects of deformation of the earth's surface
due to tidal forces and loading from ocean
tides. These tidal effects contain little information
relevant to the geothermal reservoir and
must be removed before local effects can be observed.
This can be accomplished by subtraction