Modern portable gravimeters can routinely achieve a
5 ugal uncertainty with careful measurement
procedures involving multiple station occupations in
the same day, and stacking of readings over at least
15 minutes during each occupation. Although further
improvements in gravimeter accuracy are feasible,
other practical factors relating to repeat surveys of
geothermal fields make such improvements of
limited value. The two most important factors are
benchmark elevation variations (3 ugal/cm) and
groundwater level fluctuations (5-10 ugal/m). Dual
frequency GPS receivers can give elevations to about
2 cm after 30 minutes of recording, and for reducing
groundwater uncertainties, repeating the surveys
during the same season coupled with checks of
groundwater monitor wells is advisable.
Simple models are presented for the gravity effects of
evolving steam zones at Wairakei field, Dixie Valley
field, and The Geysers field. Changes during the
main pressure-drawdown phase of development
probably caused gravity decreases of 200 °© 1000
ugal, but subsequent changes would have been much
smaller in amplitude. Gravity monitoring is able to
discriminate between steam zone dry-out and steam
zone resaturation. However the amplitude of gravity
increases associated with individual injection wells is
often small and may not be resolvable with annual
surveys. Once fields have passed their initial
pressure-drawdown phase and the rate of gravity
change decreases, the frequency of gravity surveys
should be decreased.

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