Magnetotelluric sounding of the geothermal zones enables constructing 3-D electric
resistivity model of the studied area, mapping the geothermal reservoir and
monitoring its macro-parameters. Spichak (1999, 2002) applied Bayesian inversion of
MT data in order to construct 3-D resistivity model of the Minamikayabe geothermal
zone (Hokkaido, Japan). It was found, in particular, that rather thin highly
conductive layers attributed to the clay cap (with resistivity values not exceeding
6 Ohm.m) cluster in the southern part of the studied area and their horizontal
dimensions at first increase with depth, reaching a maximum in the depth range from
about 200 to 800 m, and then decrease again.
Spichak (2001) has found the most suitable data transforms for adequate
interpretation of MT measurements carried out with the purpose of monitoring
variations in the geothermal reservoir resistivity with temperature. It was also
shown that the conductive channel (for instance, fluid filled fault) connecting the
reservoir with the surface strengthens the effect of the resistivity variation
inside the reservoir. Firstly, it increases the diameter of the zone for reliable
monitoring, and, secondly, reduces the period threshold sufficient for detection of
even small variations of the electrical resistivity.
Spichak and Popova (2000) developed an Artificial Neural Network (ANN) expert
system for detection of 3-D fault macro-parameters from MT data. Using this system
enables not only to get an idea about the fault location but also to monitor the
fluid circulation manifested in its electric resistivity variation.
Accurate interpretation of magnetotelluric data in the presence of near-surface
geological noise is still an unresolved problem. In order to overcome this
difficulty Spichak (2001) proposed a technique based on the upward analytical
continuation of the anomalous MT field to the reference plane located 200-500 m
higher than the top topographic point by means of the integral transformations.
This allows not only to filter the data that is necessary for proper imaging, but
also to reduce the data to a “common denominator” in the case of their collection
over the relief surface.
Finally, Spichak et al. (2006) have proposed the method of indirect temperature
estimations from MT measurements in the geothermal areas. Practical application of
this method enables, first, to decrease the temperature uncertainty range when the
amount of temperature logs available is insufficient; second, to perform more
precise temperature estimates in extrapolation mode; third, to monitor the well
temperature basing on surface observations of MT field and, at last, to carry out
remote temperature assessment in wells at depths with extreme conditions unsuitable
for available geothermometers.
The study was supported by RFBR grants (05-05-08013 and 07-05-00017) and INTAS
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