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104 Geothermal Energy: Renewable Energy and the Environment

8.00

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Rc/Ra 4.00

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238 240 242 244 246 248
Longitude (east)
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FIGUre 6.11 Variation in He isotope ratios across the western United States. The ratio Rc/Ra is the He/ He
ratio, corrected for crustal values. The open squares are samples that have a purely crustal signature while the
filled circles have some component of mantle-derived He. The curve that encloses the points is drawn simply
as an envelope to delineate the pattern. (From Kennedy, B. M. and van Soest, M. C., Science, 318, 1433–36,
2007.)
isotopes identify good targets for further exploration for geothermal resources. Importantly, helium
isotope surveys such as this can provide data useful for identifying sites for Enhanced Geothermal
Systems (EGS), an important topic that is explored in detail in Chapter 14.

GeophysIcs as an exploraTIon Tool

aeromaGneTic surveys
Although the dominant magnetic field that is sensed by a handheld compass is the Earth’s magnetic
dipole field (the geodynamic field), which establishes the magnetic north and south poles, there exist
other magnetic fields that can also be detected and measured. One of these fields is that which is
imposed on the Earth by the solar wind, which interacts with the geodynamic field. Less significant,
but of use for geophysical surveys, is the local magnetic field resulting from the magnetic charac-
teristics of the local geology.
Rocks are magnetic to varying degrees, depending upon their mineralogy, history, and tem-
perature and pressure conditions. A few iron-bearing minerals, particularly magnetite (Fe O ) and
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3
maghemite (Fe O ), are themselves magnetic, while a variety of other minerals are responsive to
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3
magnetic fields by developing an induced magnetic field of their own when placed within an exter-
nal magnetic field (such as the Earth’s magnetic field), a phenomenon called magnetic susceptibility.
Rocks vary in their content of magnetic minerals and their magnetic susceptibility, and thus possess
weak magnetic fields of their own.
If one were to carry a sensitive magnetometer across the land surface, it would be possible to
detect the local magnetic field, which would be a combination of the intrinsic field due to the rocks
in the region plus that of the solar and geodynamic magnetic fields. By knowing the magnitude and
direction of the nonlocal magnetic fields, their effects can be subtracted from measurements made
at a particular location, and the resulting magnetic field (direction and magnitude) can be deter-
mined. The challenge then becomes how to interpret the magnetic pattern that one has obtained.