Page 103 - Geothermal Energy Systems Exploration, Development, and Utilization

2.4 Geophysics 79
However, today, high-resolution aeromagnetic (HRAM) surveys have a resolution
in the subnanotesla scale, such that magnetic surveys are no longer restricted to
magmatic rocks but can also be used to map intrasedimentary faults, as long as there
are some layers containing elevated magnetite concentrations that generate small
anomalies (>10 nT at 150 m elevation, Nabighian et al., 2005). Such HRAM surveys
are considered industry standard, and they are often used in HC exploration, but
flight specifications for a high-resolution survey vary from one country to another.
Typical exploration HRAM surveys have flight heights of 80–150 m and line
spacings of 250–500 m (Millegan, 1998).
Hydrothermal activity influences the susceptibility of rocks. In the conventional
volcanic environment, circulation of hydrothermal fluids causes alterations in
the rock, which in turn cause a reduction in susceptibility. This reduction is a
consequence of the destruction of the magnetite contained in the rocks. That
way, units of volcanic rocks and lava flows can easily be distinguished from
hydrothermally altered rock units, which makes geomagnetic surveys a useful tool
for geothermal prospecting at high enthalpy volcanic reservoirs. Alterations are
usually caused by high temperature fluids that may be related to a geothermal
reservoir and structures such as faults or dykes, which allow fluid circulation.
Areas where this method was used to outline such features for exploration are, for
example, in New Zealand, Japan, Kenya, Iceland, or the western United States, to
name but a few.
An additional potential for the geomagnetic method is its ability to detect the
depth at which the Curie temperature is reached. Various ferromagnetic minerals
have differing Curie temperatures,but for the two most strongly magnetic minerals,
◦
magnetite and pyrrhotine, the temperatures are 580 and 320 C, respectively. For
magnetite, the temperatures can vary with titanium content, adding a degree of
uncertainty to depth estimates using the degree of magnetization. Nonetheless,
keeping in mind the mentioned uncertainties, the deepest level of detectable
magnetization provides a useful estimate for the temperature at the depth and thus
of the temperature gradient and the heat content. For magnetic field observations
made at or above the surface of the earth, the magnetization at the top of the
magnetic part of the crust is characterized by relatively short spatial wavelengths,
while the magnetic field from the demagnetization at the Curie point in depth will
be characterized by longer wavelength and lower amplitude magnetic anomalies.
This difference in frequency characteristics between the magnetic effects from
the top and bottom of the magnetized layer in the crust can be used to separate
magnetic effects at the two depths and to determine the Curie point depth.
This approach, using the creation of a Curie point depth map as an integral
part of the exploration, has been adopted for many attempts to discover new
geothermal prospects (Yellowstone National Park, Cascade Range of Oregon,
Japanese Islands, Northern Red Sea, Trans-Mexican Volcanic Belt, parts of Greece,
etc.).
For regional exploration, magnetic measurements can be important for under-
standing the tectonic setting, for example, in Iceland or at Dixie Valley, Nevada,
USA (Smith, Grauch, and Blackwell, 2002). With the HRAM surveys, the study