P. Schmoldt, PhD - MTNet - DIAS

Sources for magnetotelluric recording
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For magnetotelluric (MT) investigations electromagnetic (EM) waves in the period range
10 −4 − 10 5 s (10 4 − 10 −5 Hz) are commonly used. The period range in an investigation is,
among others, dependent on the desired depth of investigation, subsurface characteristics,
and noise level. An overview about the MT source signals, their generation and characteristics,
is given in this Chapter, together with a description of assumptions regarding
characteristics of the source signals.
In principal, every EM wave incident on the Earth’s surface with known characteristics
can be used as a source for MT investigation, as long as its amplitude exceeds the noise
level of the instruments. Commonly used MT signal sources can be divided into two main
classes, namely Electric lightning discharge (Sec. 2.1) and electric currents in the magnetosphere
(Sec. 2.2). Both types are of natural origin and form the high and low end of the
spectrum used for MT investigation (Fig. 2.1), separated by the MT dead band. The MT
dead band is usually observed in a period range spanning from approximately 0.1 to 8 Hz
and is characterised by a poor signal-to-noise ratio (SNR). The low SNR in this period
range is due to low signal amplitudes in comparison with the present noise level from,
among others, the motion of trees, generating induced signals on the sensors. The used
spectra contains EM waves with periods that usually permit negligence of permittivity
and permeability effects (cf. Chapters 3.5 and 3.6).
Besides the above-mentioned natural sources, artificial sources have been developed
for the audio-magnetotelluric (AMT) period range in order to enhance the SNR, thereby
shorting the required recording time. Controlled source AMT (CSAMT) was first applied
by Goldstein and Strangway [1975] and descriptions of CSAMT applications are given in
various publications, e.g. Pellerin and Hohmann [1990]; Zonge and Hughes [1991]. For
the PICASSO Phase I project no controlled source recording is carried out, therefore the
focus of this Chapter is on natural sources and their implication on MT measurements.
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