Page 93 - Geothermal Energy Systems Exploration, Development, and Utilization

2.4 Geophysics 69
Reflected wave
S G
x
S Shot point
G Geophone
D D Depth
L/2
x Distance from S to G
L Ray path

Figure 2.12 Schematic diagram of reflected wave.

system. v s is not as sensitive to saturation, such that the ratio v p /v s is a very helpful
indicator. In contrast, the attenuation of v p is relatively sensitive to the presence of
vapor and can therefore be indicative of zones containing steam.
Both seismic reflection and seismic refraction surveys have been used in geother-
mal exploration. Refraction surveys are limited to some extent because of the
amount of effort required to obtain refraction profiles giving information at depths
for more than a few kilometers and the difficulties caused by the generally complex
geological structures in areas likely to host geothermal systems. Seismic refraction
is normally restricted to cases where the densities of the rocks and thus seismic
velocities increase with depth. In addition, geophone arrays for refraction mea-
surements need profile length of at least four to five times (sometimes even eight
times) the sampling depth because of the very nature of refraction. These distances
require higher shot energy (i.e., more explosives) and limit the applicability of re-
fraction methods in exploration to shallower targets or to large-scale investigations
of the earth’s crust and upper mantle with very energetic sources. Most of the time
and also within reflection surveys it is used to get a first approximation about the
velocity distribution at depth.
In general, reflection seismic methods are more commonly used in geophysical
exploration, as they require much shorter profiles and therefore less shot energy and
have a much higher lateral resolution. However, reflection signals are much more
complex to detect and to analyze than refraction signals as they never arrive first,
which implies time and labor intensive filtering and detection from a multitude
of overlapping data. Moreover, the specific setup for reflection measurements
requires more logistic preparation and personnel, which makes it generally a lot
more expensive than refraction methods. It is nonetheless the method of choice in
HC exploration, as it can resolve structural details of a reservoir.
In seismic reflection, the two way travel time is measured, which is the time
it takes for a wave from its source to the reflector (some sort of mechanical
discontinuity) and back to the receiver (Figure 2.12). Unless the rocks above the
reflector and their seismic velocities are known, the depth of the reflector and the
velocity can be determined by the use of many seismic stations and many different
shot points.