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2.4 Geophysics 63
dramatic static shifts in MT data. There is no numerical method to correct for
the static shift and it is necessary to use information from other geophysical
methods such as the transient electromagnetic method (TEM or TDEM; see below)
that are not affected by static shift, using the vertical magnetic field component
data, or comparing all the survey responses with a priori geological or geophysical
information.
Even with the most beautiful interpretation of measured MT data, it has to be
kept in mind that MT models provide information on bulk resistivity alone, which
in terms of interpretation cannot be directly linked to any lithology, porosity of
the media, or hydraulic permeability without a priori hydrogeological information.
Resistivity measurements are affected simultaneously by lithology, the presence
of fluids, and structure of the pore spaces. Further research needs to address this
issue with the study of petrophysical relationships in order to quantitatively convert
resistivity into rock physical properties.
The single most significant disadvantage of the MT method is it provides slow
coverage of a prospect area and is therefore costly – but still cheap compared to
active seismic methods. While this limitation is owing to the underlying physics
and thus unlikely to change, the possibilities of the method usually outweigh its
shortcomings and make it the most applicable of all individual geophysical methods
for the exploration of deep geothermal reservoirs.

2.4.1.4 Active Electromagnetic Methods
Active EM methods are used mainly for shallow depth resistivity studies. One of
their main applications today is to support static shift corrections of MT data, for
which mainly TEM is used. TEM has become the standard among all active EM
measurements, as it is highly reliable and the most precise and cost effective of the
resistivity techniques. In the most common central-loop TEM method (Figure 2.9),
a loop of wire is laid on the ground which has a square shape, each side measuring
several hundred meters. A magnetic spool is placed at the center of the square and
serves as a receiver, after which DC current is applied to the loop. The current
builds up a magnetic field of known strength. The current is abruptly turned of,
leaving the magnetic field without its source, which induces an image of the source
loop on the surface. The current and the magnetic field decay and again induce
currents at greater depth. The spool at the loop’s center measures the magnetic
decay at the surface with time elapsed since the current was switched off. The decay
rate of the magnetic field with time is dependent on the current distribution that
in turn depends on the resistivity distribution. The induced voltage in the receiver
coil, measured as a function of time, can therefore be interpreted in terms of the
subsurface resistivity structure.
The depth of penetration is a limitation similar to most electrical methods.
However, the TEM method is less expensive and its interpretation is less time
consuming. It is more downward focused, has excellent resolution, and requires
significantly less area than other electric methods. Both two- and three-dimensional
modeling compiled from one-dimensional inversion of each TEM sounding are
routinely carried out. The method has been used extensively mostly in Iceland,