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SUMMARY
Geophysical methods used in
geothermal exploration
by Axel Björnsson,
Orkustofnun — Geolhermal Division
Grensásvegur 9, Reykjavík, Iccland
Several different geophysical methods
have been applied to exploration of Icelan-
dic geothermal fields in the last four
decades. Measurements of the resistivity of
the subsurface layers have been the most
powerful tool in finding and delineating
both high-temperature and low-
temperature geothermal fields (Figs. 4 and
5). The resistivity depends strongly on water
content and temperature and all major
geothermal fields in Iceland are charac-
terized by significantly low values of res-
istivity. The resistivity anomalies are often
of the order of one or two magnitudes. The
Schlumberger DC-sounding is the most
commonly entployed exploration method
(Figs. 2 and 3), but equatorial dipole-dipole
soundings and some magnetotelluric
measurements have also been found useful
for deeper investigations (Fig. 6) in recent
years. Temperature gradient measurements
in boreholes are performed in order to map
thc regional heat flow pattern of the country
as a whole, or large parts thereof. They are
also used in confined geothermal areas in
order to estimate the temperature of in-
dividual geothermal systems and their depth
(Fig. 1). Another important exploration
technique is magnetic ntapping. Many
high-temperature geothermal fields are
associated with negative magnetic
anomalies, which are caused by high tem-
perature and the alteration of magnetic
ntinerals. Aeromagnetic maps often give a
good estimate of the extent of the geother-
mal fields (Fig. 10). In low-temperature
areas the flow of the thermal water is often
controlled by faults and dikes, which can be
traced by magnetic surveys on the ground
(Figs. 8 and 9). Gravity surveys, seismic
measurements and microearthquake studies
have been succsessfully used to map major
geological structures, such as intrusive com-
plexes and magnta chambers, which are
associated with geothermal systems.
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