Page 105 - Geothermal Energy Renewable Energy and The Environment

6 Exploring for Geothermal

Systems

Geothermal systems that have the potential to provide economically useful heat are located
throughout the world in diverse geological settings. Finding them can be as easy as noting the pres-
ence of a geyser, a boiling mud pot, or a steaming pool. Indeed, the early development of geothermal
power installations commonly occurred in places that had obvious surface manifestations, such as
Larderello in Italy and The Geysers in California. However, there are many resources that have
little or no surface expression (so-called hidden resources). Over the course of the last few decades,
increasingly sophisticated geochemical, geophysical, and statistical techniques have been developed
to aid in the discovery, location, and evaluation of geothermal resources. These techniques assist
with identifying regions that possess a thermal resource and establishing its magnitude. They are
also needed to determine whether there is sufficient permeability to allow fluid flow, and whether
there is a high probability that sufficient fluid is present to transfer the heat to the surface. All of
these attributes are important for establishing the economic viability of a proposed application.
The general approach that is followed for exploration purposes is to first analyze available
geological information (such as geological maps, drill records, and surface features) to identify
environments likely to possess a resource. Once a target is identified, geochemical studies are under-
taken to determine the subsurface conditions. Usually this involves obtaining water samples from
springs, streams, and groundwater, or examining drillcore to see if alteration features consistent
with geothermal activity is present. If the results of such studies are promising, then more expen-
sive techniques are employed to further evaluate a potential target. These can include geophysical
surveys involving aeromagnetic or resistivity studies, or remote sensing surveys using infrared and
hyperspectral techniques. Finally, once a target is delineated, drilling programs are undertaken to
fine-tune concepts and models. This chapter describes many of the techniques currently employed
to address these issues.

FIeld GeoloGy and sUrFace manIFesTaTIons

volcanoes: acTive and dormanT
The most obvious geological features for identifying geothermal target areas are, of course, active
volcanoes. They invariably have associated with them geothermal circulation systems that are ener-
gized by magma chambers that reside within the upper crust at depths of a few to a few tens of
kilometers. The challenge such occurrences pose comes from the difficulty in identifying a drilling
target that can be reliably located to avoid the hazards associated with volcanic eruptions (e.g., ash
and lava flows, seismic activity, and fumarolic activity). Careful evaluation of how these challenges
are locally distributed has allowed successful development of geothermal resources in many areas,
including the Puna District on the Big Island of Hawaii and Iceland (Figure 6.1).
Volcanoes that have been active within the last few hundred thousand to few million years
can also be excellent targets for geothermal exploration. Most of these older systems no longer
retain the classic form of a volcano, since many of them have experienced a phase of catastrophic
eruption that blew apart the volcanic edifice, followed by extensive erosion. Such systems form a
caldera after the eruption. A caldera develops when the magma chamber empties and the overlying
ground settles into the resulting cavity. When this happens, a complex of faults pervades what is left

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