Page 100 - Geothermal Energy Systems Exploration, Development, and Utilization

76 2 Exploration Methods
polarization direction. The determination of other parameters such as aspect ratio
of cracks and fluid-fill content, which can give information about the state of the
fluid, cannot easily be accomplished and requires high-quality data sets. But even
then, results may be non-unique, as the effect of cracks saturated with vapor on
shear wave polarizations is similar to that of water-filled cracks with high aspect
ratios (MacBeth, 1999). Clearly, these data acquisition requirements also need
adequate computing power.

2.4.3
Potential Methods

2.4.3.1 Gravity
Gravity measurements are used to determine differences in density and their lateral
extent in the subsurface. These differences are usually very small and require
highly sensitive equipment to determine relative gravity anomalies. Measured
data need time-dependent (e.g., drift and tidal effects) and static (e.g., elevation
and topography) corrections for local and regional conditions and are then used
to construct a contour map of Bouguer anomaly with lines of equal gravity
anomaly. These lines are called isogals – gal in memory of Galileo Galilei. Positive
gravity anomalies (compared to their surroundings) correspond with higher density
subsurface. They can be of interest for geothermal exploration, as they are associated
with mafic to intermediate intrusions, and geologically young intrusions (<∼1Ma)
can provide a potential heat source. Such structures would also commonly be
detectable by a positive magnetic anomaly. Positive density anomalies can also be
caused by deposition of silicates from hydrothermal activities.
Negative gravity anomalies can have several causes, some of which also have
promising implications for geothermal exploration. For example, lower densities
can be caused by felsic intrusions such as granites, magma bodies, higher porosities,
or by highly fractured parts of a rock. Highly porous or highly fractured rocks would
provide potentially interesting zones of higher fluid content and/or permeability.
Alteration minerals produced by circulation of hot water can also cause a negative
density anomaly.
Faults can also be traced by gravimetric tools, as they usually display a distinct
change in density across a more or less well-defined linear zone. These faults
that might have no surficial impressions can accommodate the upwelling of hot
water. Gravity anomaly maps can show the extent of the sedimentary cover in
basins as negative anomalies and be used to estimate the depth of the underlying
basement. Such maps can provide useful first information about heat and volume
of a potential geothermal reservoir. For example, gravity surveys were performed
at the classic Italian site of Larderello (Fiordelisi and Bertani, 2006; Orlando, 2005),
where 23 000 stations were acquired, corresponding to 1 station per kilometer,
to provide subsurface structural information. 2D/3D modeling in conjunction
with experimentally determined density data pointed out deep low density bodies
(6 I-GET report WP2) related to molten intrusions: the potential heat source of the
system.