Page 122 - Geothermal Energy Renewable Energy and The Environment

108 Geothermal Energy: Renewable Energy and the Environment

become routine instruments to employ in field surveys. As a result, it is possible to map variations
in rock density in the subsurface in remarkable detail.
Consider, for example, what would be detected using such an instrument if the subsurface struc-
ture described previously in the New Zealand aeromagnetic survey and the model that was derived
from it (Figure 6.12) were subjected to a gravity survey. The density of the Nukuhau volcanic rock
will be approximately 10 to 30% greater than that of the hydrothermally altered rocks. Since they
are both surrounded, for the most part, by the same sedimentary and volcanic rocks, one would
expect to detect a stronger gravitational field over the Nukuhau location than over the area above
the hydrothermally altered rock. Given that they are at approximately the same depth but differ in
thickness by at least a factor of two, one would predict that the gravitational signal would vary by a
few parts per hundred across this region, a variation that is well within the sensitivity of most high
precision gravimeters.
However, interpreting the results of gravity surveys can be difficult. Rock density can vary by
50% in the near surface, and the thickness of individual rock units, such as lava flows or sedimen-
tary layers that occur in a place like Wairakei, for example, can vary by a factor of ten or more over
short distances. Given that these kind of rocks can often overlap each other, there are many possible
ways in which rock sequences can be combined to produce models that result in identical gravity
anomaly patterns.
Consider, too, the processes that result in the development of a geothermal field can change rock
densities through fluid–rock interaction, as well as through affects on the density of the fluid that
fills fractures and rock pores. Rock densities can be increased by replacement of clay minerals and
carbonates by silica, or they can be decreased by replacement of feldspars by clays and zeolites. In
addition, fracturing will decrease rock density, as will replacement of pore fluids by steam. Hence,
the ability to identify a potential geothermal site using gravimetry depends to a great extent on
additional information available about the processes that have been operating at the site of inter-
est. Consider, for example, the contrasting signatures of the geothermal sites at Heber, California,
and The Geysers, California. The Heber site, which was originally explored as a potential oil and
gas field, was identified, in part, by the coincidence of a positive gravity anomaly with a thermal
anomaly (Salveson and Cooper 1979). The positive gravity anomaly was related to an increase in
the local rock density due to geochemical processes that altered the country rock, resulting in the
replacement of porosity and lower density minerals with higher density minerals. The Geysers, on
the other hand, fall within a regional gravity low, related both to the local geology, and the fact that
the geothermal field is charged with low density dry steam (Stanley and Blakely 1995).
Even so, gravity surveys, when combined with other geophysical techniques can greatly dimin-
ish the ambiguity associated with proposed subsurface models, since the models must satisfy the
constraints of each technique. It is precisely for this reason that a combination of techniques is
always employed when exploring for geothermal resources. In addition, recent advances in signal
processing technology have improved the ability to extract information and develop model struc-
tures, enhancing the usefulness of combined surveys.

seismiciTy and reflecTion seismoloGy
The rocks of the Earth are very good transmitters of low frequency energy. Such energy can be prop-
agated for thousands of miles and detected at remote locations. Earthquakes are the classic example
of generators of low frequency energy—seismic waves generated by large earthquakes can travel
around the Earth several times and are readily detected by sensitive seismometers. Techniques that
utilize this behavior for exploring and characterizing the subsurface have been developed for the oil
and gas industry and are being refined and adapted to the needs of the geothermal community.
Seismic energy is transmitted in several different modes. Body waves propagate through the
interior of a body and are of two types. Pressure waves, or P-waves compress and expand materials
in the direction in which the wave is traveling. These waves are the fastest seismic waves, traveling