Atmospheric sources for audio-magnetotelluric (AMT) sounding
Xavier Garcia and Alan G. Jones
The energy sources for natural-source magnetotelluric (MT) frequencies above 1 Hz are
electromagnetic waves caused by distant lightning storms in the atmosphere propagating
within the Earth-ionosphere waveguide. The properties of this waveguide display diurnal,
seasonal and 11-year solar-cycle fluctuations, and these temporal fluctuations cause significant
signal amplitude attenuation variations especially at frequencies in the 1 kHz - 5 kHz so-called
AMT (audio-MT) dead band. In the northern hemisphere these variations exhibit an increase in
amplitude during the nighttime and for summer months, and a corresponding decrease during daytime
and winter months. Thus, one problem associated with applying the AMT method for shallow (<3 km)
exploration is the lack of signal in certain frequency bands during the desired acquisition interval.
In this paper we analyze the time variations of high frequency electromagnetic fields to assess the
limitations of the applicability of the AMT method, and demonstrate that magnetic field sensors need
to become two orders of magnitude more sensitive than they currently are in order to ensure acquisition
of adequate signal at all times. We present a proposal for improving AMT acquisition involving continuous
profiling of the telluric field only during the daytime, and MT acquisition at a few base stations
through the night.
Geophysics, 67, 448-458, 2001.
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Alan G Jones / 10 June 2004 /