Page 90 - Geothermal Energy Systems Exploration, Development, and Utilization

66 2 Exploration Methods
Advantages of EM Geophysical Methods Among the geophysical methods that
sense bulk electrical and effective properties of the subsurface, EM have a greater
depth capability and provide better resolution than DC electrical measurements.
In terms of resolution, only reflection seismics has the potential to yield better
results. EM methods are cost effective, relatively easy to operate in the field, and
a variety of data processing options are available, ranging from the construction
of apparent resistivity curves or pseudosections for fast subsurface evaluations to
1D and 2D forward and inverse modeling. 3D inverse modeling is not yet fully
developed although research is moving forward rapidly in this field, where new
codes are being tested. However, the main concerns in all EM methods are cultural
noise sources such as power lines, pipelines, and DC trains among others that
screen and disturb the geophysical signal.
Electromagnetic induction methods are the most widely used and versatile
geophysical methods in geothermal exploration and investigation at different
scale ranges. A diverse set of techniques and instruments available provides the
possibility of conducting cross-scale investigations. Selection of the appropriate
technique depends strongly on the objectives of the study, time, financial aspects,
and computational facilities.
2.4.2
Seismic Methods

The reason for the widespread application of seismic methods in many exploration
tasks is that they provide the most detailed structural information at depth. They are
standard exploration methods in HC exploration and therefore highly developed
in every aspect: data acquisition, logistics, and interpretation. In geothermal
exploration, the focus are fluid-filled rock volumes that are not necessarily linked to
specific structures but the structural setting itself (e.g., faults, dykes, and grabens)
and the parameters of possible resource regions as well as underground conditions
(e.g., stress, strain, and pore pressures) are also in focus of the investigations.
Body waves travel through the interior of the earth (Figure 2.10). They follow ray
paths bent by the varying density and modulus (stiffness) of the earth’s interior.
The density and modulus, in turn, vary according to temperature, composition, and
phase. Two basic types of seismic waves are of interest in exploration of subsurface
resources: P-waves and S-waves. P-waves are longitudinal (compressional) waves
and they are the fastest of the elastic waves (P-waves = primary waves). S-waves,
also called shear waves or secondary waves, are transverse waves that travel more
slowly than P-waves and thus appear later than P-waves on a seismogram. Particle
motion of S-waves is perpendicular to the direction of wave propagation and they
do not exist in fluids as water or in gases (air).
Seismic methods can be divided into two main subclasses:
• active seismic methods, which make use of waves created by artificial sources
• passive seismic methods, for which the sources are natural earthquakes or
rupture processes, induced by for example, injection or extraction officials (e.g.,
hydraulic fracturing).