Page 101 - Geothermal Energy Systems Exploration, Development, and Utilization

2.4 Geophysics 77
In a volcanic environment, the gravimetric differences depend strongly on the
chemistry of the rocks and on their porosity. Generally, solid magmatic bodies
are much denser than layers of pyroclastic rocks, where densities are usually low.
In highly porous rocks such as rhyolitic tuffs (where porosities can be higher
than 40%), densities are also strongly influenced by the fluid content. A dry rock
will be much lighter than a fluid saturated rock; steam and liquid water would
also be clearly distinguishable. Examples of volcanic geothermal reservoirs, where
this observation was used to delineate the zones of hydrothermal alteration, are
Broadlands and Ohakuri in the Taupo geothermal area of New Zealand (Hunt
et al., 1990).
The primary use of gravimetric measurements today is to help constrain the
structural context of an area, outline trends of faults, and determine the depth to
basement. Gravity surveys using surface, airborne, or even satellite data are used
more and more for this purpose as one of the first steps to characterize a region
of interest and to help constrain areas for further inspection. Even though they
can help constrain the extent of the resource, interpretation of the results is rarely
unequivocal to make economical decisions. For example, mafic to intermediate
composition intrusions, commonly associated with positive anomalies, can be
negative anomalies, if they ascend through a dense metamorphic basement (for
example, the Denver Basin, Colorado, USA). Another example of positive anomalies
would be the occurrence of hydrothermal alterations in sedimentary environments.
They are usually thought to cause a densification, as observed at Salton Sea,
California, USA. But, hydrothermal alterations are also used to explain local gravity
lows at The Geysers steam field in California. In the case of potential EGS, the
differences in gravity may be even less meaningful, as porosities and permeabilities
affected by hydrothermal activities can be much smaller than those in conventional
systems and with higher temperatures.
Another common use of gravity monitoring surveys in geothermal areas is to
define the change in groundwater level and for subsidence monitoring. Fluid
extraction from the ground which is not replaced within an adequate time span
causes an increase of pore pressure and hence of density. This effect may induce
subsidence at surface, whose rate depends on the recharge rate of fluid in the
extraction area and the rocks affected by compaction. Repeated gravity monitoring
in conjunction with weather monitoring may define the relationship between
gravity and precipitation that produces the shallow ground water level change.
When gravity is corrected by this effect, gravity changes show how much natural
inflow replaced water mass discharged to the atmosphere. The underground
hydrological monitoring of a gravity survey is an important indication of the fluid
recharge in geothermal systems and the need for reinjection.
The advantages of gravimetric methods over other geophysical methods are that
they are comparatively easy to use and fairly economical. They do provide a good
estimate of the extent of bodies with certain density contrasts and can thus help
constrain the location and extent of reservoirs. The resolution and quality of data,
however, decrease considerably with depth. Gravimetric studies therefore provide
a useful tool for shallow (<∼2 km) reservoirs in conventional systems and, given