Page 102 - Geothermal Energy Systems Exploration, Development, and Utilization

78 2 Exploration Methods
their often ambiguous results, in combination with other geophysical methods.
For potential reservoirs in EGS, this method may be even more limited than for
conventional systems.

2.4.3.2 Geomagnetics and Airborne Magnetic
Magnetic surveys measure changes of the earth’s magnetic field over time and
space. The latter are associated with the composition and structure of the rock
formations at the subsurface. The parameter of interest here is the magnetic
susceptibility, which is a material property and can be determined and calibrated
using rocksamples inthe laboratory. The magneticsusceptibilityof rocks influences
the natural magnetic field, which is measured in nanotesla (1 gamma = 1 nT
(nanotesla)). Measurements are performed either at the surface or airborne, if
the objective is regional mapping. Various instruments can be used. The most
commonly used instruments are high precision proton precession magnetometers
and cesium–vapor magnetometers, which nowadays are operated together with
differential GPS to record time and location accurately.
Rock magnetism is acquired when the rock forms, and it reflects the orientation
of the magnetic field at the time of formation. But, rock magnetism can also change
with time: if the rock is subjected to temperatures above a certain point, called the
Curie temperature, it loses its magnetic properties. It is remagnetized once it cools
down, now induced by the magnetic field present at that time.
The shape and magnitude of a magnetic anomaly depends primarily on two
factors:

• The shape and orientation/position of the magnetic structure in the subsurface.
• The latitude of the location. This factor is important because of the dipolarity
of the earth’s magnetic field. The inducing magnetic field has a dip angle that
varies from place to place over the surface of the earth: at the magnetic North
Pole, it is vertical, and the pattern of magnetic anomalies is symmetrical, while
the patterns of anomalies that are recorded become more complex away from the
pole.

The magnetic susceptibility of a rock and the temperature at which it disappears
depend strongly on the rock components, the more or less magnetic minerals.
Minerals with high magnetic susceptibility are, for example, magnetite, ilmenite,
hematite, and pyrrhotite. Silicate minerals, rock salt (halite), and limestones
(calcite) have very low magnetic susceptibilities and are therefore not useful for
magnetic measurements. Consequently, sedimentary rocks usually have much
lower magnetic susceptibilities than igneous or metamorphic rocks. Thus the
magnetic method has been traditionally used for identifying and locating masses
of igneous rocks that have relatively high concentrations of magnetite, which is
the most common of the magnetic minerals. Strongly magnetic rocks include
basalt and gabbro, while rocks such as granite, granodiorite, and rhyolite have only
moderately high magnetic susceptibilities. Since hydrothermal activities are often
associated with plutonism, magnetic interpretation can be the first step in finding
areas favorable for the existence of a potential geothermal reservoir.