Page 79 - Geothermal Energy Systems Exploration, Development, and Utilization

2.4 Geophysics 55
resistivity. These sometimes show a good correlation with contour maps of the
subsurface resistivity distribution. In the so-called bipole–dipole arrangement (or
roving dipole), one pair of electrodes is kept fixed while the other pair, usually more
closely spaced, is moved around, which allows the determination of anisotropies in
the resistivity of the subsurface.
These methods allow reasonable resolution down to depths of 2000 m and were
used quite regularly for geothermal prospecting in the past. Another application
in DC electrical prospecting for geothermal anomalies is the self-potential method
which was also quite common for measurements in geothermal areas where it
revealed anomalous regions associated with near-surface thermal zones and faults
that are thought to be fluid conduits.
More commonly applied is DC Tomography and E-Scan, which is a proprietary
method. The other methods mentioned make use of electromagnetic fields.

2.4.1.2 Electromagnetic Methods
The principle behind EMs is governed by Maxwell’s equations that describe the
coupled set of electric and magnetic fields’ change with time: changing electric
currents create magnetic fields that in turn induce electric fields that drive new
currents. Most EM techniques (controlled source audio magnetotellurics (CSAMT),
TDEM, FDEM, GPR, and NMR) use a controlled artificial electromagnetic source
as a primary field that induces a secondary magnetic field, while MT methods use
the earth’s natural electromagnetic field as source signal.
EM methods can be used for exploration and monitoring of circulating fluids in
reservoirs or faults and thus provide important information about their activity and
fluid content. As the phase change of pore fluid (boiling/condensing) in fractured
rocks can result in resistivity changes that are more than one order of magnitude
greater than those measured in intact rocks, EM methods can provide information
of primary economic significance. In addition, production-induced changes in
resistivity provide valuable insights into the evolution of the host rock and resident
fluids and thus into the sustainability of a reservoir.

2.4.1.3 The Magnetotelluric Method
In the MT method, the earth’s impedance to the natural EM wave field is measured
to extract information about variations in the resistivity of the subsurface. The
method has been used for about 30 years now and has improved continuously
in both equipment and interpretation, and, despite its numerous pitfalls, it has
become the standard method in geothermal exploration. The main advantage
to all other electrical methods is its ability to probe depths of several tens of
kilometers
In the MT method natural EM waves, generated by thunderstorm activity, provide
signals with frequencies higher than 1 Hz, while frequencies lower than 1 Hz are
caused by large-scale ionospheric currents created by the interaction between the
solar wind and the magnetosphere. At large distances from the source, the resulting
electromagnetic field is a plane wave of variable frequency (from about 10-5 Hz
up to audio range at least). The subsurface structure can be studied by making