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Heating with geothermal systems 181

benefitted from healing baths, the warmth of the earth, and tools fashioned from the
volcanic rocks. Although weapons were made from hard, sharp obsidian rocks and
used to attack enemies, geothermal places were usually declared neutral and often sa-
cred sites where all people, friend and foe alike, could gather socially and recuperate.
While there are many places marked by obvious surface geothermal activity, there are
many more that are not, so-called “blind resources” that can only be discovered with sen-
sitive instrumentation, painstakingly deployed, whose signals can be interpreted to
reveal structures and activity deep below the surface beyond the human senses.
It was once thought that geothermal activity was solely associated with the move-
ment of the gigantic tectonic plates that comprise the cracked crust of the earth. Sub-
duction zones are created where one plate slides beneath an adjacent one, giving rise to
deep-seated melting of the crust and the rising of magma, melting its way upward to-
ward the surface. Volcanoes are thus brought into life - an event witnessed by ancient
people time and again. Legends and myths reveal that they were in awe of the destruc-
tive power of volcanoes. Residual heat left in the near-surface crust radiated and con-
ducted its way through the soils and gave rise to the aforementioned manifestations
and peaceful uses of this natural energy.

5.2.1.1 Geological observations

An understanding of geology and the history of geologic events is the starting point for
exploration. Age-dating of rocks tells how young an area might be in geologic time.
Places with no current geothermal surface activity may yet show traces of extinct
ancient behavior that hint at anomalous formations now hidden. Even with no surface
evidence at all, there may still be commercial geothermal resources lurking beneath the
surface. Several techniques are now available to help uncover them.

5.2.1.2 Geochemical techniques

If any fluids are present in the area, they can be subjected to chemical analysis to deter-
mine their origins. Warm or hot springs are obvious examples that are suitable for
study. The presence of these may be associated with faults or fractures that serve as
conduits for geothermal fluids. It is possible to estimate the temperature of the deep
fluids from an analysis of surface waters. The concentration of various minerals,
such as silica, dissolved in the fluids can be correlated to deep temperatures using
the solubility-temperature characteristics of the fluids. The pH of the fluids plays an
important role in this assessment.

5.2.1.3 Geophysical assessment

The spatial subsurface distribution of physical properties reveals discontinuities that
indicate the presence of faults. The density of the rocks affects the local gravitational
acceleration, and extremely sensitive instrumentation may be used to detect these
slight variations. Thus, gravity highs and gravity lows can be discovered which may
indicate zones with different porosity or permeability, which are very important in re-
gard to storage and transport of geothermal fluids. The electrical conductivity is