The Interpretation Model - Models and Methods of Magnetotellurics - page 396

Fig. 11.25 Apparent-
resistivity curves filtered by
the North-West technique

time t. One-dimensional modeling shows that in the absence of sharp resistivity
contrasts the transient apparent-resistivity curves TEM (5 . 15t)and the undistorted

magnetotelluric apparent-resistivity curves MT ( T ) are almost similar. As long as

we avoid comparing the steep parts of the MT -curves, the transient

apparent-resistivity curves offer rather reliable reference for correcting magnetotel-
luric apparent-resistivity curves distorted by the static shift. A practical example
of such a correction is given in Fig. 11.26. Here the transient

TEM - and

TEM -curve offers
a resistivity-depth profile that is in line with well-log data. At the same time the
magnetotelluric TEM -curve, and

its inversion differs severely from well-log data. Correcting this distortion, the

MT -curve is displaced to the left from the transient

MT -

curve is shifted to the right so that its high-frequency branch fits the TEM -curve. Now

the inversion of the static-shift corrected

MT -curve is in reasonably good agreement

with the well-log resistivities.
Another shift-correction technique involving TEM-soundings has been
suggested by Pellerin and Hohmann (1990). To improve the accuracy in com-
paring MT- and TEM-data, the apparent-resistivity

TEM -curve is converted to

the apparent-resistivity MT -curve. The correction scheme is simple. The one-

dimensional TEM-inversion gives a resistivity-depth profile, from which a reference

MT -curve is calculated. Then the distorted MT -curve is matched with the reference

MT -curve. Here, contrary to Sternberg et al. (1988), we compare data of the same
kind and do not place restrictions on the resistivity contrasts. The technique is