Page 88 - Geothermal Energy Systems Exploration, Development, and Utilization

64 2 Exploration Methods

Transmitter Receiver
Secondary
magnetic

Induced current

Time

Time

Figure 2.9 TEM sounding setup: transmitted current and measured transient voltage.
mostly to add additional information to existing geological, geochemical, and MT
data or even instead of MT, as it is cheaper and has a much higher resolution
at lower depths. For deep prospection in high temperature fields, however, it is
mainly used to correct MT data for the static shift. Without the correction, apparent
resistivities obtained by MT, which were up to a factor of 10 too low, have been
observed in volcanic areas of Iceland. If the shift is not corrected for, interpretation
would give 10 times too low resistivity values and about three times to shallow
depths to resistivity contrasts. For this purpose, MT surveys in such areas are
´
now routinely complemented by TEM measurements (Arnason, Eysteinsson, and
Hersir, 2010).
For a joint inversion of MT and TEM, data should be collected from nearly
identical places. To determine the static shift, a joint 1D inversion of MT and TEM
soundings is performed. The misfit between the resistivities calculated for the two
methods determines the static shift. This misfit can vary significantly within an
area and between polarization directions. This was shown by a full 3D inversion of
MT soundings acquired around the Hengill volcano in SW Iceland, which required
correction of the static shifts separately for the two polarization directions (xy and