Page 127 - Geothermal Energy Renewable Energy and The Environment

Exploring for Geothermal Systems 113

6.16). The form and location of the concentrations are suggestive of a localized source of fluid dis-
charge. This type of occurrence could be a useful indicator of geothermal fluids flowing into the
surface or near-surface.
Although use of these techniques currently is suitable only for regions where there is low rainfall
and sufficient development of evaporite deposits to support such an analysis, it is likely that addi-
tional strategies will be developed for use of such remotely collected data. The advantages of such
an approach are the specificity it allows for diagnostic mineral identification and the ability to survey
large areas relatively quickly.

case sTUdy: Fallon, neVada
Exploration for geothermal resources, as discussed above, involves evaluation and integration of a
variety of data. A study conducted by Combs et al. (1995) in western Nevada provides a good exam-
ple of such an effort. It includes a review of earlier geological studies (such as Monastero et al. 1989),
an analysis of the seismic activity (and anomalies associated with that), geochemistry, geophysics,
and exploratory drilling. In the end, a sound basis was developed for concluding that a significant
resource was available, accessible, and could be economically developed.
The area studied was in the volcanically active Basin and Range Province. The city of Fallon is
located in the Carson Desert, a valley of the Basin and Range Province where episodes of volcanic
activity have occurred between 43 million years ago and 20,000 years ago (Stewart and Carlson
1976). Numerous hot springs occur throughout the area.
Previous geological studies had documented that the valley is enclosed by ranges that are
bounded by normal faults, which is typical of the Basin and Range Province. The valley floor
and subsurface geology are composed mainly of relatively recent sandstones, siltstones, clays,
and other sedimentary rocks. This combination of a fault-bounded valley filled with porous sedi-
ments is a favorable situation for runoff to recharge aquifers by infiltration along faults. Many of
the faults are likely to extend to considerable depth, providing access of infiltrating water to the
deeper hydrothermal system, if it is present. The presence of basin fill composed of porous sedi-
ments could provide sufficient permeability to accommodate flow rates sufficient to allow fluid
extraction for power generation. Such a geological framework is hypothetically ideal for siting a
geothermal power facility. The caveat, of course, is whether or not the resource exists at acces-
sible depths.
The record of recent volcanic activity and the location of the Carson Desert within the
active portion of the Basin and Range would suggest that some seismicity would be expected.
However, there is little in the way of recorded seismic events. Generally, as Combs et al. (1995)
point out, geothermal sites have microseismic activity (magnitudes in the range of −2 to 3.0).
In this instance, they argue that the absence of recorded seismicity is more likely a reflection
of the absence of appropriately placed seismometers than a real absence of seismicity, since the
magnitude of these earthquakes make it unlikely that they could be detected at any significant
distance from the center of rupture. This approach to data analysis will be discussed further at
the end of this section.
In 1980, an analysis was conducted on a local hot spring that had 70°C water flowing from
depth (Bruce 1980). A version of the Na–K–Ca geothermometer gave a temperature for the res-
ervoir of 204°C. On the basis of that work, several temperature gradient holes were drilled to
determine the subsurface temperature distribution. The measured temperature gradients ranged
between 97°C/km and 237°C/km. Fluids collected from the drilling effort were chemically ana-
lyzed and geothermometer calculations using the Na–K–Ca model also gave 204°C as the source
region temperature, which matched the results for the artesian well (Combs et al. 1995). These
geothermal gradients are high for continental crust, even within the Basin and Range Province,
thus suggesting there exists at depth a significant thermal source. The geothermometer results are
consistent with this conclusion.