Page 64 - Geothermal Energy Systems Exploration, Development, and Utilization

40 2 Exploration Methods
reservoir and are commonly controlled by structures such as fractures and faults.
A low permeable but highly porous reservoir can be enhanced into a productive
geothermal system if the naturally missing fractures are artificially induced by
stimulation. From this perspective, it is obvious that a successful development of
a geothermal field strongly depends on the understanding of the geologic setting
at different scales from microscopic pore space to regional fault patterns. Thus,
any assessment of geothermal potential requires a comprehensive analysis of the
geologic setting that aims to elucidate critical issues beyond temperature, such as
porosity, permeability, spatial extent of the reservoir body, structural framework,
and surficial features indicative of geothermal activity.
The first step in the assessment of the geological and geodynamic setting of
hidden geothermal resources is the compilation of all available data for a region
from all possible sources: the national or regional surveys, land owners, the oil and
gas industry, downhole temperature data, published academic data, and remote
sensing data. Such a data compilation is usually an inexpensive but potentially
time-consuming part of the exploration process.
Remote sensing has turned out to become a more commonly used and immensely
useful tool. It is possible to get detailed surface information even for areas with
limited access prior to any on-site activity. Even though such methods have existed
for quite some time, new technology using sensors to detect different wavelengths
of light can now distinguish between different types of rock. The analysis of
rock forming minerals associated with geothermal activity can be used to get a
first idea about the orientation of geological structures that may control pathways
of geothermal fluids, before an area is further explored with ground-based field
work. With the introduction of hyperspectral surveys, a new, albeit expensive, tool
with high spatial resolution from airborne instruments is now available. It allows
detailed mapping of mineral distribution, which could allow the reconstruction of
the geological history and potential changes in mineralogy caused by geothermal
activity. That way, prospective zones without obvious surface expressions may be
delineated and specifically targeted for ground-based exploration.
The next step in the exploration of a potential geothermal prospect is the evalu-
ation of the regional temperature field. In this context, information from existing
wells is of preeminent importance. Data on the subsurface temperature distribution
is widely available from previous activities, although information becomes scarce
and less reliable with increasing depth. If the data availability is not sufficient,
there are several standard approaches to measure subsurface temperatures, which
help in the evaluation of a geothermal prospect. Most commonly, shallow (1–3 m
deep) holes are drilled or a hollow tube is hammered into the ground and the tem-
perature is measured. These frequently applied methods are inexpensive and give
a general idea about the distribution of a subsurface heat source. More important
are temperature gradient holes (TGHs) that are drilled to get a better idea about
the temperatures at depth at a given location. These TGHs can be a few hundred
meters or even more than a kilometer in depth depending on the target depth. This
method can yield much more reliable information than the shallower holes but