P. Schmoldt, PhD - MTNet - DIAS

6.2. Processing of magnetotelluric data
in nearly all cases, it is profitable to apply some type of processing to time-series data
in order to enhance the estimate quality. The individual steps along with the different
methods of data processing are presented in the following sections.
For the MT method, variation of the magnetic field and the related electric response of
the subsurface are used, which are obtained through measurements at the Earth’s surface
as described in Section 6.1. It should be noted that the electric field is derived from
potential difference measurements along an electric dipole of finite length:
e(t) = dV(t)/L (6.1)
with e: electric field in the time domain (indicated by the character t), dV: potential
difference, and L: length of the electric dipole. The circumstance that the electric field
is obtained from a potential difference between a finite distance, rather than from a point
measurement, can result in a bias of the derived valued (cf. Sec. 4), but measuring along
finite distance is inevitable for logistic reasons.
6.2.1. Pre-processing of the time-series data
Initially, the time-series of electric and magnetic fields are corrected for mistakes such
as layout errors and bad records within the datasets. The former usually involves only
simple mathematical modifications or re-assignment of the dataset vectors:
• For the case of reverse connection of an electric dipole, e.g. connecting the southern
electrode to the channel for the northern electrode and vice versa, the correction can
be accomplished by simply inverting the sign for all entries in the related dataset.
• Accidentally swapping two dipoles or swapping the coil sensors during the connection
of the recording channels, can be taken into account for by re-arranging the
dataset vectors.
• The reverse installation of a magnetic coil sensor or a misalignment of the fluxgate
sensor by an exact multiple of 90 degrees can be corrected for by swapping
the datasets and altering the nominal layout accordingly. E.g. for the case of a
reverse coil sensor measuring the magnetic field in the north-south direction (Hx)
the correction involves interchanging the datasets for two horizontal magnetic fields
and an increase of the angle between magnetic north and the Hx orientation by 90
degrees.
An elementary visualisation of the re-assignment can be found, for example, in the user
manual by Phoenix Geophysics [2005].
Even though such layout errors are supposed to be avoided, they do occasionally happen
(apparently often enough for Phoenix Geophysics to implement an automatic layouterror
correction in their processing software), but for the cases described above correction
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