Page 89 - Geothermal Energy Systems Exploration, Development, and Utilization

2.4 Geophysics 65
yx). This correction showed that in many cases the two polarizations were shifted
very differently. Thus, it does not suffice to determine static shift in one place and
polarization direction alone but for every MT data point acquired.
Another active EM method that is used routinely in exploration is CSAMT,
which is described in more detail, for example, by Zonge (1992). It is similar to
MT, the main difference being that it uses an artificial source, and it is a method of
choice, if noise is a particular problem for MT surveys. The source provides a stable
signal, allowing higher precision and faster measurements than those acquired
with natural-source measurements in the same spectral band. An electric dipole
with a length of 1 or 2 km grounded at a distance of 4–10 km from the receiver
stations in the area to be measured serves as the source. Measurements are usually
made with continuous stations along a line or with individual stations in a grid to
determine 2Dor3Dbehaviorofthesubsurface.
The resolution with depth is governed by the same equations (Equation 2.11) as
for MT: the depth of exploration or investigation is related to the square root of
ground resistivity and the inverse square root of signal frequency. These equations
do not define a depth limit for the resolution; however, the maximum depth
for practical use is usually between 2 and 3 km. The limiting factor on depth of
exploration with all of the data in the far field is usually signal level. Both E-
and H fields vary as a function of frequency and earth resistivity and decrease
3
as 1/r ,where r is the separation between the transmitter and receiver, so signal
strength decreases rapidly with depth. As a general rule, when sounding over a
relatively homogeneous territory, transmitter and receiver should be about five
times the depth of exploration apart, so for an investigation depth of 1 km a
receiver–transmitter separation of about 5 km is recommended. If the distance
between transmitter and receiver is less than three times the depth of interest, the
far-field condition is no longer applicable and the change of resistivity with depth
no longer obeys the rules summarized in Equation (2.11), and calculation of the
subsurface properties becomes more complicated. Therefore, surveys are usually
carried out with receiver–transmitter separations between 5 and 15 km.
Lateral resolution depends mainly on the length of the electric dipole serving as
a source. Theoretically, the dipole can be reduced as much as necessary to get the
desired lateral resolution, but a reduction in dipole length also reduces the strength
of the signal. Received signal strength is directly proportional to the length of the
dipole, such that half the dipole length results in half the signal strength.
CSAMT is often used in environments where the background noise is more than
10 times the signal level, and MT measurements are of limited use. An example
of such an application is the survey for the potential EGS site near Skierniewice in
Poland, which was performed within the I-GET project (Bujakowski et al., 2010).
The original MT measurements yielded highly noisy data, making an interpretation
of the reservoir properties at 4 km depth nearly impossible, despite good quality
remote reference data. Additional CSAMT measurements helped to determine
the resistivity patterns of the uppermost kilometer and to put constraints on the
interpretation of the MT data for the rocks below.