Publication 96
A new methodology for the acquisition and processing of audio-magnetotelluric (AMT) data in the AMT dead-band
Garcia, X. and A.G. Jones
Abstract
Distant lightning activity, the natural energy source
for the audio-magnetotelluric (AMT) method, has a signal
minimum between 1 and 5 kHz, the so-called AMT
dead band. The energy in this band exhibits both diurnal
and annual variation; magnetic-field amplitudes
during the daytime are often well below the noise levels
of existing sensors (coil magnetometers), thus reducing
the effectiveness of the method for quantitative
high-resolution studies of near-surface targets. To
overcome this deficiency, we propose a hybrid acquisition
and processing methodology based on combining
the telluric-telluric (T-T) and telluric-magnetotelluric
(T-MT) methods in this frequency range. Our method
records the telluric channels at several sites and at
base and remote reference stations during the day and
records the full magnetotelluric (MT) components at
the base and remote stations only during the night.
Applying a tensor multiplicative relationship between
these responses, we obtain the T-MT AMT transfer
functions for the sites; these transfer functions can represent
a reasonable approximation of the real AMT
impedance tensors. To test the approach, a T-MT experiment
was carried out in Sudbury, northern Ontario,
during summer 2000. We compare the processed daytime
data using the conventional MT approach to those
obtained from our T-MT approach. The results demonstrate
that our method can determine high-quality estimates
in the dead band, although the estimates can be
severely affected by noise.
Source
Geophysics, 70, 119-126.
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Alan G Jones / 21 April 2006 /
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