## Publication 30

##
Static Shift of Magnetotelluric Data
and its Removal in a Sedimentary Basin Environment

###
Alan G. Jones

### Abstract

Previous modeling investigations of the static shift of
magnetotelluric (MT) apparent resistivity curves have limited appeal in that the
electric fields used were point measurements, whereas field observations
are of voltage differences.
Thus, inhomogeneities of dimension of the order of the electrode line length
could not be investigated.
In this paper, by using a modeling algorithm that derives point voltages,
rather than point electric fields, I consider the effect on the MT
responses of local near-surface distorting structures, which are both
outside of, and inside, the telluric electrode array
I show that static shift effects are of larger spatial size but of less
magnitude than would be expected from conventional modeling.
Also, the field observation that static shift affects only the apparent
resistivity curve but not the phase response can be replicated by the voltage
difference modeling.
If there exists within the earth a layer whose variation in
electrical resistivity along the profile can be treated in a parametric
fashion, then static shift of the apparent resistivity curves can be
corrected.
Deriving the modal value from a sufficient number of
observations for the layer resistivity is the most useful approach.

### Source

*Geophysics*, **53**, 967-978, 1988.

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Alan G Jones / 10 June 2004 /
alan-at-cp.dias.ie