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

gravity signal is a rapid but Irregular 5 pgal increase
beginning on day 086. This increase in
gravity occurs over an 18 hour period which coincides
precisely with the duration of the only significant
rainfall In this 38 day data segment.
Analysis of rainfall effects in barometrically
adjusted gravity residuals is simplified by the
fact that there is a slight decrease in barometric
pressure directly associated with the release of
mass from the atmosphere. This decrease in
pressure equals the weight per unit area of the
released mass. If we let Ag represent the gravitational
attraction of a sheet of water of thickness
equal to the depth of rainfall, then it can
be shown that the barometric pressure correction
used to produce Figure Id contributes an amount
-Ag to the residual gravity signal starting at the
time of the rainfall. Thus the net result of rainfall
on barometrically adjusted gravity is simply
to produce an apparent change by an amount -Ag if
the sheet of water comes to rest above the gravimeter,
or +Ag if the sheet lies below the gravimeter.
In both cases, these changes vanish as
the water in question drains away from the vicinity
of the gravimeter. Considering the terrain
at The Geysers we would expect a zero change in
the present case, or, at most, a change of +2
ugal. Thus the 5 ygal observed change appears to
be a spurious effect, most likely due to tilt.
That tilt influences gravity measurements is
a direct consequence of the vector nature of the
gravitational field. If one assumes that a gravimeter
tilted by an angle 6 frora the local
vertical simply raeasures the component gcos9, the
theoretical tilt response to lowest order in 8 is
-1/2 pB^ or -4.6 x IO""* ugal/uradian^; i.e., the
gravimeter will read a maximum value when aligned
with the vertical. This, however, is not the
case for the cryogenic gravimeter. The magnetic
field geometry currently used to levitate the
test mass inside the instrument causes the tilt
response to have a larger magnitude and opposite
sign compared to the naive estimate. The actual
tilt response of The Geysers instrument is
+ 5.9 X 10-"* Mgal/pradian^. Thus the gravimeter
reads a minimum value when vertical and deviations
frora the vertical will tend to Increase the apparent
value of g. Tilts of 40 yradians (1 ugal)
would be observable and tilts of 100 yradians
( 5.9 ugal) would be sufficient to explain events
such as seen in Figure Id. The planned automatic
leveling system should limit tilt to less than
10 uradian (0.06 ugal).
In spite of the problens in Interpreting the
two rapid changes in gravity in this record, the
general trend of the gravity residual In Figure Id
shows a slow decrease of 4,5 ± 0.5 pgal over the
38 day segment. Extrapolating this trend yields a
rate of decrease In gravity of 43 ± 5 wgal per
year, which is in close numerical agreement with
the average rate of decrease of 46 + 7 ygal per
year Inferred from Isherwood's 1974 and 1977
gravity surveys (Isherwood, 1977). Isherwood
showed that this decrease could be explained by
the mass deficiency generated by steam production
over that two and one-half year period. The close
agreement of these two measurements is most likely
521
Olson, et al
fortuitous, considering that Isherwood's data represent
a yearly averaged effect, that the data
were collected during two drought years, and that
steam production has changed between 1977 and 1979.
The agreement, nevertheless, indicates that the
cryogenic gravimeter raay be capable of producing
results in one month that might take years to
accomplish with conventional gravimeters.
GEOTHERMAL IMPLICATIONS
Although the data presented here are insufficient
to yield new conclusions at this time regarding
reservoir dynamics, they do demonstrate
that the cryogenic gravimeter has both the sensitivity
and the stability required to produce new
results. The continuous nature of gravity observations
made feasible by this type of instrument
enormously expands the interpretative power of
gravity studies. With the addition of a tilt
stabilized platform, events which would otherwise
be obscured by long term averaging can be detected
and events whose contributions to the total gravity
change would otherwise be indistinguishable can
be separated and identified by their time signatures
and correlations with other events.
Even the short data segment presented here,
despite its tilt uncertainties, indicates that we
will be able to accurately observe the steady decrease
in gravity associated with continuous steam
production and thus provide the most direct available
raeasure of reservoir recharge. The accuracy
of these estimates will be further enhanced by our
ability to separate out those sudden effects which
appear to be unrelated to mass depletion. It is
not unreasonable to expect that meaningful estimates
of recharge may ultimately be obtained from
as little as 90 days of gravity observations,
thereby enabling study of seasonal fluctuations
as opposed to multiyear averages.
Furthermore, it may be possible to detect
short term mass redistributions within the reservoir
that could accompany changes In steam production
or changes in reinjection, thereby yielding
Information regarding the percolation-condensation
cycle of steam within the reservoir. In
addition, gravity events which are correlated
with seismicity could provide clues regarding
earthquake mechanisms at The Geysers and their
possible relation to reservoir exploitation.
ACKNOWLEDGEMENTS
We thank Richard Dondanville and the staff of
the Geothermal Division, Union Oil Company, for
their cooperation and assistance, especially
during the installation of the site, and Richard
Reineman for his technical expertise in fabricating
the gravimeter. This work is funded by the
United States Geological Survey through the Extramural
Geothermal Research Prograra under Grant
USDI-14-08-0001-G-297.
REFERENCES
Goodkind, J. M. and Prothero, W. A. Jr., 1968, A
Superconducting gravimeter. Rev. Sci. Instr.
v..39, "p. 1257.