Page 104 - Geothermal Energy Systems Exploration, Development, and Utilization

80 2 Exploration Methods
of basin structure has an important economic application, especially in oil and
gas exploration, but could also be applied for geothermal reservoirs and potential
EGS systems. For the most part, basin fill typically has a much lower susceptibility
than the crystalline basement. Thus, it is commonly possible to estimate the depth
to basement and, under favorable circumstances, quantitatively map basement
structures, such as faults and horst blocks (Prieto and Morton, 2003).
The magnetic method has thus expanded from its initial use solely as a tool for
finding iron ore to a common tool used in exploration for minerals, HCs, ground
water, and geothermal resources. The speed with which the measurements can be
made and the relatively low cost for campaigns have made the method very popular
during the last 30 years. Restrictions are the resolution with depth; the complexity
of the interpretation, which makes it most reliable only for structures with simple
geometric shapes; and the insensitivity to the actual presence of water. With these
restrictions in mind, the method is not more or less useful for EGS or conventional
geothermal systems. Its value is mainly the potential to determine heat at depth, a
characterization of the regional tectonics and the outline of a potential heat source.

2.4.4
Data Integration

The most important objective of applying geophysical methods is to obtain quan-
titative information over the subsurface model space. The transformation from
raw data to an estimated geophysical model is usually achieved using numerical
forward modeling and inversion procedures, to provide a description of the sub-
surface fitting to the observed data. Joint inversion of different geophysical sets is
used to constrain the possible subsurface models with multiple independent data
sources, using either a deterministic approach or a probabilistic approach such as
stochastic inversion methods
To perform the integration of geophysical measurements with hydrogeological
and hydrothermal measurements, the scale problem, as well as the nonuniqueness
and uncertainty of the geophysical and geochemical models, and which specific
petrophysical relationship is most appropriate for each case study have to be consid-
ered. Thereafter, integration and estimation approaches that focus on defining the
spatial distribution and the magnitude on the geothermal system can be applied.
The first step is to obtain reliable geophysical models with which to translate
geophysical properties into (thermo)hydraulic parameters. The second step is the
quantitative conversion of the geophysical and geochemical property to hydroge-
ological and hydrothermal properties that may be obtained (i) via direct mapping
using a petrophysical relationship, the so-called deterministic approach, or (ii)
by applying stochastic methods such as geostatistics or Bayesian techniques, the
so-called probabilistic approach.
The most general way to integrate a priori information and data for nonlinear
problems is to apply stochastic inversion methods where the resulting model
parameters are given by a probability distribution. The probabilistic weight of each
element is considered in the iterative posterior inversions to improve the models.