Geophysical methods play a key role in geothermal exploration since many objectives of geothermal exploration can be achieved by these methods. The geophysical surveys are directed at obtaining indirectly, from the surface or from shallow depth, the physical parameters of the geothermal systems. A geothermal system is made up of four main elements: a heat source, a reservoir, a fluid, which is the carrier that transfers the heat, and a recharge area. The heat source is generally a shallow magmatic body, usually cooling and often still partially molten. The volume of rocks from which heat can be extracted is called the geothermal reservoir, which contains hot fluids, a summary term describing hot water, vapour and gases. A geothermal reservoir is usually surrounded by colder rocks that are hydraulically connected with the reservoir. Hence water may move from colder rocks outside the reservoir (recharge) towards the reservoir, where hot fluids move under the influence of buoyancy forces towards a discharge area.
The first aspect of defining a geothermal system is the practical one of how much power can be produced. In most cases, the principal reason for developing geothermal energy is to produce electric power, although as an alternative the geothermal heat could be used in process applications or space heating. The typical geothermal system used for electric power generation must yield approximately 10 kg of steam to produce one unit (kWh) of electricity. Production of large quantities of electricity, at rates of hundreds of megawatts, requires the production of great volumes of fluid. Thus, one aspect of a geothermal system is that it must contain great volumes of fluid at high temperatures or a reservoir that can be recharged with fluids that are heated by contact with the rock. A geothermal reservoir should lie at depths that can be reached by drilling. It is unreasonable to expect to find a hidden geothermal reservoir at depths of less than 1 km; at the present time it is not feasible to search for geothermal reservoirs that lie at depths of more than 3 or 4 km. Experience has shown that each well drilled in a geothermal field must be capable of supporting 5 MW of electrical production; this corresponds to a steam production of 10 tonnes/h. To accomplish this, a well must penetrate permeable zones, usually fractures, that can support a high rate of flow. In many geothermal fields the wells are spaced so as to produce 25 – 30 MW km-2. At a few locations, where the reservoir consists of a highly fractured and shattered rock and there is little interference between wells, production rates may reach several hundred megawatts per square kilometre over small areas.
The geological setting in which a geothermal reservoir is to be found can vary widely. The largest geothermal fields currently under exploitation occur in rocks that range from limestone to shale, volcanic rock and granite. Volcanic rocks are probably the most common single rock type in which reservoirs occur. Rather than being identified with a specific lithology, geothermal reservoirs are more closely associated with heat flow systems. Many of the developed geothermal reservoirs around the world occur in convection systems in which hot water rises from deep within the earth and is trapped in reservoirs whose cap rock has been formed by silicification, or precipitation of other mineral elements. As far as geology is concerned, therefore, the important factors in identifying a geotherma1 reservoir are not rock units, but rather the existence of tectonic elements such as fracturing, and the presence of high heat flow. The high heat flow conditions that give rise to geothermal systems commonly occur in rift zones, subduction zones and mantle plumes, where, for unknown reasons, large quantities of heat are transported from the mantle to the crust of the earth.
Geothermal energy can also occur in areas where thick blankets of thermally insulating sediment cover basement rock that has a relatively normal heat flow. Geothermal systems based on the thermal blanket model are generally of lower grade than those of volcanic origin. An important element in any model of a geothermal system in a vo1canic area is the source for the system: in other words, the existence of an intruded hot rock mass beneath the area where the shallower reservoirs are expected to occur. The source of a geothermal system could be either a pluton or a complex of dykes, depending upon the rock type injected (see Fig. 1).
Figure 1. Diagrammatic cross sections of hypothetical geothermal systems. The system on
the left is not closely associated to an intrusion but results from higher than normal thermal
gradients through a sequence of thermally resistant rocks. The system on the right has an
intrusion as the heat source.
A geothermal exploration programme is based on a number of phenomena associated with the intrusive model for a geothermal system. The model system that we hypothesise needs a source, in the form of intrusions that have been emplaced no more than half a million to one million years ago, so that excess heat still exists. The region above the source of the geothermal system must be fractured by tectonic activity, fluids must be available for circulation in a convection cell, and the precipitation of a cap rock must have taken place. All these elements represent targets for the application of geological, geophysical and geochemical prospecting techniques. Because of the high temperatures involved, both in the geothermal reservoir and in the source of the geothermal system, we can expect major changes to have taken place in the physical, chemical and geological characteristics of the rock, all of which can be used in the exploration project. Heat is not easily confined in small volumes of rock. Rather, heat diffuses readily, and a large volume of a rock around a geothermal system will have its properties altered. The rock volume in which anomalies in properties are to be expected will, therefore, generally be large. Exploration techniques need not offer a high level of resolution. Indeed, in geothermal exploration we prefer an approach that is capable of providing a high level of confidence that geothermal fluids will be recovered on drilling.
A geothermal assessment programme on a regional basis will begin with a review and oordination of the existing data. All heat flow data acquired previously will have to be reevaluated,
re-gridded, smoothed, averaged and plotted out in a variety of forms in an
attempt to identify areas with higher than normal average heat flow. Similarly, the volumes
of volcanic products with ages younger than 106 years should be tabulated in a similar way
to provide a longer-range estimate of anomalous heat flow from the crust. Because
fracturing is important, levels of seismicity should be analysed, averaged and presented in a
uniform format. All information on thermal springs and warm springs should be quantified
in some form and plotted in the same format. Comparison of these four sets of data, which
relate directly to the characteristics of the basic geothermal model described above, will
produce a pattern that will indicate whether the area possesses the conditions favourable for
the occurrence of specific geothermal reservoirs. These areas should then be tested further
by applying some or all of the many geophysical, geological and geochemical techniques
designed to locate specific reservoirs from which fluids can be produced.
The next stage of exploration consists of techniques for detecting the presence of a
geothermal system. A geothermal system generally causes inhomogeneities in the physical
properties of the subsurface, which can be observed to varying degrees as anomalies
measurable from the surface.
These physical parameters include temperature (thermal survey), electrical conductivity
(electrical and electromagetic methods), elastic properties influencing the propagation
velocity of elastic waves (seismic survey), density (gravity survey) and magnetic
susceptibility (magnetic survey). Most of these methods can provide valuable information
on the shape, size, and depth of the deep geological structures constituting a geothermal
reservoir, and sometimes of the heat source. An increase of silica amount in volcanic rocks
decreases the density. Thermal surveys can delimit the areas of enhanced thermal gradient,
which is a basic requirement for high-enthalpy geothermal systems, and define temperature
distribution. Information on the existence of geothermal fluids in the geological structures
can be obtained with the electrical and electromagnetic prospectings, which are more
sensitive than other surveys to the presence of these fluids, especially if salty, and to
variations in temperature. Moreover resistivity are strongly affected by porosity and
saturation. Resistivity decreases with increasing porosity and increasing saturation.
The same parameters measured indirectly from the surface can also be obtained from
wells, by the method known as geophysical logging.
The depth at which rocks become conductive because of thermal excitation can be
determined with relatively good reliability by means of the magnetotelluric method.
Experience around the world has shown a remarkably good corre1ation between the depth
to a thermally excited conductor and regional heat flow, as indicated in Fig. 2. If thermally
excited rocks occur at depths as shallow as 10 to 20 km within the crust, it is almost certain
that a partially molten intrusive is present; the normal depth for thermally excited
conductive rocks ranges from fifty to several hundred kilometres.
Fig. 2. Observed correlation between the depth to thermally excited conductive zone in the
crust or mantle (based on magnetotelluric soundings) and heat flow.
The Curie point method has the potential for providing confirmation of the existence of
a hot rock mass in the crust. When rocks are heated above temperatures of a few hundred
degrees Centigrade, they lose their ferromagnetism. Under favourable circumstances, the
depth to this demagnetisation level can be determined with reasonable accuracy.
Further confirmation can be obtained by p-wave delay and shear wave shadow studies.
(When an anomalously hot mass of rock is present in the ground, the compressional (p)
waves from earthquakes are delayed in transit, while the shear (s) waves are reduced in
amplitude). To detect such an effect, an array of seismograph stations is set up in the
vicinity of an anomaly. The seismograph stations are operated over a sufficiently long time
to record a few tens of teleseisms. The wave speeds for various ray paths through the
suspected anomalous zone are then computed; if the rock is partially molten, the p-wave
velocities will be from 20 to 30 per cent lower than their normal values. An increase of
silica amount in volcanic rocks decreases the p-wave velocity. Moreover seismic velocity is
strongly affected by porosity and saturation. Wave velocity is reduced by increasing
porosity but shows different behaviour for different saturation, with an inverse relationship
when saturation is high (100/85%) and a direct relationship when saturation is low, being
constant for saturation of 15-85%.
A group of prospect areas should be defined, with reference to regional data and
reconnaisance surveys. These areas may range in size from a few hundred to a thousand
km2. In rare cases, such as extensive thermal systems, they may be even larger. With the
lack of resolution characteristic of the reconnaissance studies, it is unlikely that a prospect
can be localised to an area of less than 100 km2. Detailed geophysical, geological and
geochemical studies will be needed in order to identify drilling locations once a prospect
area has been defined from reconnaissance.
The objective of the more detailed studies is to identify the existence of a productive
reservoir at attractive temperatures and depths. Geochemical surveys provide the most
reliable indications of reservoir temperatures if the thermal fluids emerge at the surface. In
any event, all springs and other sources of groundwater should be sampled and various
geothermometer calculations carried out. Some prospect areas will probably show much
more positive geochemical indicators than others. This could merely reflect the difference
in the amount of leakage from subsurface reservoirs, but it does provide a basis for setting
priorities for further testing; the geothermal reservoirs that show the most positive
indications from geochemical thermometry should be the ones that are investigated first by
other geophysical techniques.
The sequence in which geophysical methods are applied depends to a considerable
extent on the specific characteristics of each prospect. It is not wise to define a particular
sequence of geophysical surveys as being applicable to all potential reservoirs. In some
cases, for example where we expect to find a subsurface convection system, various types
of electrical survey could be highly effective in delineating the boundaries of the
convecting system. Where large clay masses are present in the prospect area, on the other
hand, electrical resistivity surveys can be deceptive. The particular type of electrical
resistivity survey used at this stage is a matter of personal preference. Schlumberger
sounding, dipole – dipole surveys, dipole mapping surveys and electromagnetic soundings
can all be used to good effect. To some extent the choice of method here depends upon
accessibility. The dipole–dipole traversing method and the Schlumberger sounding method
are much more demanding in terms of access across the surface. The dipole mapping
method and the electromagnetic sounding method can be applied in much more rugged
terrain.
The objective when carrying out electrical surveys is to outline an area of anomalously
low resistivity associated with a subsurface geothermal reservoir. When such an area has
been identified, it is still necessary to confirm that the resistivity anomaly is the result of
temperature and to locate areas within the anomaly where fracture permeability is likely to
be high. Confirmation of subsurface temperatures is best done at this stage by drilling one
or more heat flow boreholes. These heat flow holes need be only a few hundred metres
deep if the area is one in which surface groundwater circulation is minimal. However, in
volcanic areas where groundwater circulation occurs down to great depths, reliable heat
flow data can be obtained only by drilling to one or two kilometres depth, in which case the
heat flow hole becomes a reservoir test hole.
The number of heat flow holes that need to be drilled in a given prospect can vary
widely; a single highly positive heat flow hole may be adequate in some cases while in
others we may need several tens of heat flow holes to present convincing evidence for the
presence of a geothermal reservoir at greater depth.
Once the probable existence of a geothermal reservoir has been established by a
combination of resistivity studies and heat flow determinations, it is advisable to search for
zones of fracture permeability in the reservoir before selecting a site for a test hole.
Microseismic surveys are a widely used tool for studying activity in fracture zones in a
prospect area. Surveys may require many weeks of observation in a given area. The
accuracy with which active faults can be located using microearthquakes is often not good
enough for the control of drill holes, although in some cases it is adequate. A potentially
valuable by-product of a microearthquake survey is the determination of Poisson’s ratio and
related rock properties along various transmission paths through the potential geothermal
system. Poisson’s ratio and attenuation of seismic waves can be strongly affected by
fracturing. The identification of anomalous areas of Poisson’s ratio and p-wave attenuation
can provide encouraging evidence for high permeability zones in the reservoir.
The most effective technique for studying a potential reservoir before drilling takes place is
the seismic reflection method. Where there is a bedded structure to the subsurface this
technique can be applied to detect faults through the disruption to the continuity of the
bedding. The seismic reflection technique is extremely expensive, and a survey over a
geothermal prospect may cost a significant fraction of the cost of a test well, but the results
obtained with the seismic reflection method are usually much more conclusive than the
results obtained with any other geophysical technique.
All of these geophysical surveys, targetted at defining the main characteristics of the
geothermal reservoir, can also be supplemented with other types of geophysical surveys
that assist us in understanding the regional geology and the local geological structure in a
geothermal prospect. The self-potential survey is useful for our understanding of
groundwater movement in an area. The gravity survey can be used to study the depth of fill
in intermontaine valleys, and to locate intrusive masses of rock. Magnetic surveys can be
used to identify the boundaries to flows in volcanic areas. Once all these detailed
geophysical surveys have been carried out, a convincing set of data should be available
before the decision is taken to locate a drill hole. There must be evidence for heat, there
must be evidence for permeability, and the conditions for drilling must be established. Once
we have achieved these objectives, we can tackle the problem of whether to drill a deep test
well or not.
It is surprisingly difficult to record a true bottomhole temperature during the course of
drilling a well. Mud is circulated through the well and removes much of the excess
temperature as drilling progresses. In a closely controlled drilling programme, the
temperature and volume of mud supplied to the well and recovered from the drilling
operation should be monitored closely. Differences in the temperature between the mud
going in and the mud coming back to the surface can be used to estimate bottomhole
temperatures in a crude fashion. With the development of a mathematical model for the loss
of heat from the rock to the drilling mud, it is conceivable that an even more precise
temperature estimate will be attainable. The best temperature estimates during drilling are
obtained by lowering maximum-reading thermometers to wellbottom whenever the bit has
to be removed from the well for some reason. Several thermometers should be inserted at
the same time in case one breaks or provides a false reading. They should be lowered to
wellbottom on a heavy weight so as to be positioned as close as possible to the undisturbed
rock at the bottom face of the borehole.
The following sections give a detailed review of the various geophysical techniques used
in geothermal exploration, with particular emphasis on the requirements for data
acquisition, handling, processing and interpretation.
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