Page 78 - Geothermal Energy Systems Exploration, Development, and Utilization

54 2 Exploration Methods
If surface conductivity contributes significantly to the overall conductivity, as is
the case in geologic formations containing clay minerals, Archie’s equation is not
applicable (Klein and Sill, 1982). The degree of resistivity change with clay content
depends on clay mineralogy: The strongest effect is observed for montmorillonite
– which can lower the resistivity by two orders of magnitude (Nishikawa, 1992) –
and for sericite, whereas it is not as pronounced for kaolinite, alunite, and chlorite.
The clay effect is strongest when the salinity of the fluid is low, while it becomes
negligible for salinities of 0.1 mol l −1 KCl and more.
Methods to measure resistivity of the subsurface can basically be divided into
two general groups:
• those that measure the difference in electrical potential (DC, i.e., direct current);
• those that measure electromagnetic fields, natural or artificially created.
There is one main difference between electrical potential and electromagnetic
(inductive) techniques. The latter usually provide information on conductivity–
thickness products of conductive layers, and, generally, only thickness information
on resistive layers. In contrast, resistivity techniques usually provide information
on resistivity–thickness products for resistive layers and conductivity–thickness
products for conductive layers. Because in most cases the exploration target is
conductive, EMs are more suitable.
Complications with electrical methods in geothermal exploration arise, if the
surrounding rocks are hydrothermally altered and also display low resistivities.
Such alterations, often indicative of previous hydrothermal activity, can make even
dry rocks look like a promising reservoir.

2.4.1.1 Direct Current (DC) Methods
A common method for studying the electrical resistivity in the subsurface is to
apply an electric potential to two electrodes driven into the ground separated
some distance from each other. The potential field, built up between this pair of
electrodes, is recorded by means of a sensitive voltmeter connected to another
pair of electrodes. The depth of penetration is given by the geometry of the
array used and apparent resistivities (in ohm meters ( m)) can be calculated for
resistivity depth soundings and/or resistivity mapping. Resistivity distribution in
the subsurface can then be obtained either by forward or inverse modeling.
Standard potential methods use DC depth soundings with varying electrode
configurations such as the Schlumberger or the Wenner arrays. These meth-
ods are simple to use, but relatively slow in progress. The application of these
soundings is limited in many geothermal areas where the lateral extent of the
anomalous resistivities is small compared to the required spread between the
electrodes. They are therefore often used for mapping and delineation of shallow
resources.
Other DC methods are developed around dipole sources. In the dipole–dipole
arrangement, two similar pairs of closely spaced electrodes are moved along
a profile; all electrodes are kept in one line. This procedure is repeated with
varying electrode spacings, thus yielding the so-called pseudosections of apparent