During the World Geothermal Congress 2005 (WGC2005) held last April in Antalya
(Turkey) the papers presented concerning geothermal geophysical surveys were as follows:
20 papers concerning resistivity survey, of which:
- 5 papers concerned Schlumberger resistivity or similar (two on 1-D inversion
concerning Egypt and Japan; and three on 3-D inversion concerning Mexico and Indonesia);
- 16 papers concerned MT (six on 1-D inversion concerning Ukraine, Japan,
Philippines, Iceland; five on 2-D inversion concerning Ukraine, Japan, Mexico, Iran,
Turkey; three on 3-D inversion concerning Russia, Japan, Indonesia, Korea, USA; one
on 1D-inversion CSMT);
- 1 paper concerned resistivity measured from cores (Iceland). 10 papers concerned
heat flow surveys, of which:
- 2 papers concerned flow manifestations measurements (Japan, New Zealand);
- 4 papers concerned borehole temperature surveys (Ukraine, Russia, USA);
- 2 papers concerned aerial gamma ray/magnetic (Egypt);
- 2 papers concerned earthquake surveying (Japan, Iceland);
- 2 papers concerned gravity measurements (Japan, Indonesia);
- 2 papers concerned reflection seismics (by ENEL, Italy);
- 2 papers concerned porosity measurements (Russia, Iceland).
Reservoir monitoring during exploitation
5 papers concerned micro-gravity transients (Philippines, Japan, Indonesia) and 4
papers micro-seismic activity evolution (Russia, Indonesia, Japan). These methods
seem to be the main geophysical surveys monitoring geothermal production. In
addition, one paper each on SP transients, changes in magnetic field and changes in
borehole temperature (Japan) were presented.
It is worth focusing on the case of Philippines, as they are the second geothermal
power in the world with approximately 2,000MWe of installed geothermal power and the
highest growth rate. There, before 1995 the key method to geothermal exploration was
the Schumberger Resistivity. This was used to delineate geothermal resources, well
sitting and estimation of the plant size. They have been successful with
approximately 1,000MWe installed at that time. After 1995 they shifted to MT with 1-D
inversion, due to its simplicity, easy to use, and its ability to provide reliable
earth imaging at greater depths, down to 2km.
Imaging quality of MT method for depths deeper than 2-3km was in question, due to
intense noise at frequencies below 2Hz from surface sources. The result was to
identify deeper hydrothermal resources and at greater aerial extent, which resulted
in another 1,000MWe to be installed within the next 10 years. Schumberger resistivity
was only able to identify shallow smectite zones of high alteration, which form the
cap of the deeper geothermal reservoir. On the other hand, MT method was able to
directly locate the actual deeper drilling target of intermediate resistivity, as
well as several enhanced geothermal zones of high resistivity, which at present are
not exploration targets but may become in the future.
Controlled source MT have been used in low enthalpy groundwater systems, based on US
experience for water surveys.
Although with only two papers, both on reflection seismics, they are worth to be
mentioned, as ENEL has successfully applied reflection seismics in order to locate
deep fault zones as drilling targets at depths 3-4km. Faults tend to bring fluid from
depth and create high permeability.
MT, 3-D inversion Papers here concerned mainly basic research. 3-D inverted MT data
are theoretically more accurate than 1-D and 3-D inversion can provide a better
starting point to the earth scientist to define the resistivity substructure.
However, its use in the field has been limited by its complexity and the need for
sophisticated computing equipment. More research is needed in inversion algorithms,
elimination of noise which hinders its accuracy at lower frequencies, and especially
correlation with field data from deep wells.