A key matter for the exploration of geothermal systems is the geophysical detection
and monitoring of reservoirs at several kilometers depth. In the past decade there
has been a tremendous increase in time-lapse reservoir monitoring and development of
seismic methods like repeated 3D surface seismic, surface-to-borehole vertical
seismic profiling and boreholeto- borehole cross well seismic. At the same time
electromagnetic methods, and particularly magnetotelluric (MT) sounding have been
extensively used to detect fluid circulation at depth, since resistivity is very
sensitive to the presence of brines. Thanks to improved methodology and software, MT
is now very affordable in terms of costs and field logistic, and has gained a lot in
popularity.
Seismic imaging, whilst being a powerful geological mapping tool, has not always
resulted in
a significantly improved understanding of the nature and composition of the deep
structure of geothermal systems. In order to make advances and reduce the cost of
geothermal exploration and monitoring, resistivity needs to be included in the
interpretation. An integrate approach may help to go beyond the limits of seismic and
MT data. Various examples are provided regarding limits of both methodologies and
improvement in the interpretation when the two are integrated.

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