  We have investigated the benefits and drawbacks of active EM surveying
(Controlled-Source EM or CSEM) for monitoring geothermal reservoirs in the
presence of strong industrial noise with an actual time-lapse survey over the
Reykjanes geothermal field in Iceland before and after the thermal stimulation
of the supercritical RN-15/IDDP-2 geothermal well. It showed that a high CSEM
survey repeatability can be achieved with electric field measurements (within a
few percent) but that time-lapse MT survey is a challenging task because of the
high level of cultural noise in this industrialized environment. To assess the
quality of our CSEM dataset, we inverted the data and confronted the resulting
resistivity model with the resistivity logged in the RN-15/IDDP-2 well. We
obtained a good match up to 2-3km depth, i.e. enough to image the caprock and
the liquid-dominated reservoir but not deep enough to image the reservoir in
supercritical conditions. To obtain such an image, we had to jointly invert
legacy MT data with our CSEM data. On the monitoring aspects, the analysis of
changes in electric fields did not allow to identify any CSEM signal related to
the thermal stimulation of the RN-15/IDDP-2 well. One possible explanation is
the weakness of the time-lapse CSEM signal compared the achieved CSEM survey
repeatability as a result of a limited resistivity change over a limited volume
within the reservoir.
