Page 119 - Geothermal Energy Renewable Energy and The Environment

Exploring for Geothermal Systems 105

Magnetic surveys are now routinely done using airplane- or helicopter-mounted magnetometers,
allowing large areas to be surveyed and magnetically mapped within a matter of days. The resolu-
tion of the survey results will depend on the spacing of the flight lines that are flown, their elevation,
and the sampling rate of the magnetometer, relative to the speed of the aircraft. High-resolution
surveys allow surface features as small as a few meters in diameter to be resolved. However, most
features of interest in geothermal surveys are many meters in the subsurface, and the resolution
obtainable for such magnetic features will be significantly poorer than that.
Magnetic anomalies of interest for geothermal exploration result from the fact that magnetic prop-
erties of rock are sensitive to hydrothermal alteration. When a hot, flowing aqueous fluid migrates
through a rock, alteration of the original mineralogy will occur. This usually includes transforma-
tion of magnetic minerals such as magnetite and maghemite to hydrous oxide minerals that are not
magnetic and have a low magnetic susceptibility, thus reducing the overall magnetic susceptibility of
the rock. Clay minerals often end up replacing other minerals that have significant magnetic suscep-
tibilities, as well, the net consequence being that low magnetic anomalies can indicate hydrothermal
alteration that can be associated with geothermal systems.
To rigorously evaluate the significance of the mapped anomaly patterns, it is important to know
the magnetic susceptibility of the rocks in the region. Accomplishing that requires conducting a field
sampling effort in which representative samples are collected and their magnetic susceptibilities are
measured in the laboratory. Using the surface geology as a constraint for how one distributes the
known rock types in the subsurface, models are then constructed of how the rocks might be distrib-
uted underground, in an attempt to reproduce the observed magnetic field pattern. This approach
cannot provide a unique answer to what is in the subsurface, since there will be a very large number
of ways to distribute the known rock types and generate the observed anomaly pattern. However,
when combined with the local known geology and geological history, and other measurement con-
straints, only a few plausible configurations are likely to emerge.
Shown in Figure 6.12 is an example of the data that can be collected on a flight line, and a model
that was constructed to fit the data (adapted from Hunt et al. 2009). The unit used for magnetic
surveys is the Tesla (T) or nanoTesla (nT), which is a measure of the magnetic flux density. The
units in the lower figure are referenced to the Tesla, but represent intensity of magnetization. Several
features of this model are worth noting. One point that is important to observe is that the low point
in the magnetic pattern does not fall directly over the hydrothermally altered zone in the model.
This results from the combination of rocks and their respective intensity of magnetization. The
buried volcanic rock with the very high intensity of magnetization (4.0 A/m) essentially masks the
low magnetization of the hydrothermally altered zone. To quantitatively account for these effects,
numerical simulations, such as the one used to generate the lower figure, are required.
The presence of a low magnetic intensity region in the subsurface at a depth of 500–1000 meters
below the ground surface is a potentially attractive target. It may indicate the presence of rocks
that have interacted with hot fluids, resulting in the formation of clays and the alteration of mag-
netic minerals. However, given the nonuniqueness of models such as this, it would be important to
develop additional data that could assist in evaluating the probability that such a system actually
existed.

resisTiviTy and maGneToTelluric surveys
Complementary to aeromagnetic surveys are studies that evaluate the electrical and induced mag-
netic responses of rocks. Electrical resistance is a function of material properties. Geological mate-
rials are generally poor electrical conductors and therefore have a high resistivity, which is measured
in units of ohm-meters. The presence in pores and fractures of fluids, especially fluids that have an
elevated concentration of dissolved species that are electrically charged, increases the conductiv-
ity of the rock substantially and correspondingly reduces the resistance. These basic concepts have
been known for decades, but they weren’t applied to geothermal exploration until the mid-1960s and