Page 124 - Geothermal Energy Renewable Energy and The Environment

110 Geothermal Energy: Renewable Energy and the Environment

release and receiving a seismic signal the “two-way travel time” can be determined. That travel time
provides the means to determine the depth to a reflector, using the relationship

t = 2 × (D/V),

where t is the two-way travel time, D is the distance traveled, and V is the velocity of the seismic
wave in the medium. For a geological system more complex than that depicted, additional terms
would be required to account for different velocities of different rock units traversed by the ray and
the effects they would have on the ray path.
Recent advances in computer-assisted signal processing techniques have allowed much more
information to be extracted from received seismic waves in seismic reflection studies, than was
previously possible. Estimates can now be made of the density and elastic properties of a reflector,
and in some cases the porosity of the material can be approximately inferred. An example of a
relatively recently completed seismic study in Larderello, Italy provides an example of what can be
achieved (Cappetti et al. 2005). Shown in Figure 6.15 is the result of sophisticated signal processing
techniques applied in that seismic reflection study. The region marked as the “Geothermal target”
has high reflectivity, in part due to the fact that it is a zone of high fracture density, as confirmed by
the drilling that intersected that horizon about 400 meters to the left of the target. The high fracture
density resulted in a higher impedance contrast and, hence, greater reflectivity.

TemperaTure measuremenTs
Once geological and geophysical techniques have identified potential geothermal resources, it is
imperative that measurements be made in the subsurface of the temperature gradient. Such measure-
ments are fundamentally important because it is only through them that the presence of a thermal
resource can be established. However, drilling programs that are used to acquire these data are
expensive, and hence undertaken only after there is good evidence from other sources that a geother-
mal resource is available at depth.
The principle means for accomplishing this is to identify a target region and then conduct a drill-
ing program in which numerous small diameter boreholes are drilled to depth. Since drilling is one
of the most expensive components of an exploration and development project, it has become routine
to use slim hole drilling technology rather than large diameter rotary drilling methods. Slim holes
(diameters less than about 15 cm) are drilled with drilling rigs that are smaller than large diameter
rotary drilling equipment, require less materials for drilling, and are easier to complete. Slim holes
have been drilled to depths of 2000 m, thus making them suitable for exploration programs in a
variety of settings.
Heat flow and temperature gradient measurements can be made using downhole equipment that
is lowered on a wire line. The equipment commonly is a small diameter (ca. 6.25 cm), and allows
relatively rapid measurement of temperatures at many levels in a borehole. Such data allow con-
struction of temperature profiles (such as those shown in Figure 4.11, page 63). Once samples are
collected from cores taken during drilling, and the thermal conductivity of the material is measured
in the laboratory, the heat flow can be calculated using Equation 2.1. Such data also allow the cal-
culation of geothermal gradients.
Interpretation of heat flow values and geothermal gradients requires knowledge of the regions
baseline, or average, heat flow in order to reasonably identify shallow level thermal anomalies.
Rarely are such data available in sufficient coverage of a region to allow detailed thermal anomaly
maps to be drawn. However, high heat flow values, on the order of 100 mW/m , are reasonable
2
indicators that elevated temperatures can be reached at modest depths.
However, regions in which geothermal reservoirs exist are notorious for possessing complex
thermal structure in the subsurface. As the Long Valley caldera case study in Chapter 4 demonstrated,
linear geothermal gradients are often inadequate to describe the subsurface thermal regime. Indeed,