Page 86 - Geothermal Energy Systems Exploration, Development, and Utilization

62 2 Exploration Methods
imaging of the reservoir difficult. The clay cap can cause further confusion when a
(paleo-) geothermal reservoir is exhausted. Many of these problems are discussed
in detail in Pellerin, Johnston, and Hohmann (1996).
In terms of resolving power with respect to targets of interest, attention has to
be given to a priori geological assumptions, mesh size, and data dimensionality.
In general, a good resolution demands dense site spacing, dense meshes in the
models, and use of appropriate sensors for the period range. Undersampling is
often the cause for the lack of adequate resolution of the targets, because the
measurement sites are located too far apart in a heterogeneous medium. The
apparent target size increases with depth due to increasing recording frequency,
while target resolution decreases. As seen in the skin depth section, ground
resistivity can change the investigation depth and consequently the resolution of
the retrieved information for the same frequency range.
Data distortion is produced by the presence of three-dimensional local scale
structures, located in the shallow subsurface, producing an anomalous charge
distribution over its surface area. This presents a problem often encountered in the
MT method, and all resistivity methods that are based on measuring the electric
field on the surface, and is usually referred to as telluric or static shift.In practice,
static shift is a vertical displacement of the apparent resistivity curves, where the
phase angle curve is not affected. This phenomenon is caused by inhomogeneities
of the resistivity close to the electric dipoles, which often occur in areas with a
heterogeneous distribution of rocks near the surface, as is usually the case in
regions shaped by volcanic activity. In areas where the near-surface rocks are
homogeneous, such as sedimentary layers, and with little resistivity variation at
shallow depth, static shift is usually not a problem.
There are basically two phenomena that produce static shifts: (i) voltage distortion
(dependence of the electric field on the resistivity where the voltage is measured)
and (ii) current distortion (current channeling). Voltage distortion occurs as a
variation of the voltage in the surface when a constant current density flows
through domains of different resistivity. For example, when the resistivity near the
dipoles is lower than that in the rocks a little further away, the electric field (or the
voltage difference over a given length) is lower in the low resistivity domain. This
lowering of the electric field is independent of the frequency of the current. If the
dipole is closer to the higher resistivity rocks, the electric field would be higher
than a little further away. Current distortion occurs when current is flowing in the
ground and encounters a resistivity anomaly. If the anomaly is of lower resistivity
than the surroundings, the current is deflected (channeled) into the anomaly and if
the resistivity is higher, the current is deflected out of the anomaly. If the anomaly
is close to the surface, this will affect the current density at the surface and hence
the electric field. As for the voltage distortion, this effect is independent of the
frequency of the current density.
The problem here is that the electric field at the surface is scaled by an
unknown factor (shifted on log scale) by anomalies in the vicinity of the measuring
dipole. Sternberg, Washburne, and Pellerin (2006) have published results of model
calculations showing that the voltage distortion and current channeling can produce