Robust processing of magnetotelluric data in the AMT dead-band using the Continuous Wavelet Transform
X. Garcia and A.G. Jones
The energy sources for magnetotellurics (MT) at frequencies above 8 Hz are electromagnetic
waves generated by distant lightning storms propagating globally within the Earthionosphere
waveguide. The nature of the sources and the properties of this waveguide
display diurnal and seasonal variations that can cause significant signal amplitude attenuation,
especially at 1 kHz - 5 kHz frequencies - the so-called AMT dead-band. This lack
of energy results in unreliable MT response estimates, and, given that in crystalline environments
ore bodies located at some 500 m - 1,000 m depth are initially sensed by the
AMT data within the dead-band, this leads to poor inherent geometrical resolution of target
structures. In this paper we propose a new time series processing technique that uses the
localization properties of the wavelet transform to select the most energetic events. Subsequently,
two coherence thresholds and finally a series of robust weights are implemented
in order to obtain the most reliable MT response estimates possible. Finally, errors are
estimated using a non-parametric Jackknife algorithm.
We present an application of our algorithm to AMT data collected in northern Canada.
These data were previously processed using traditional robust algorithms and also using a
telluric-telluric magnetotelluric (TT-MT) technique. The results show a significant improvement
of the estimates for the AMT dead band and permit their quantitative interpretation.
Geophysics, accepted, 26 May, 2008.
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Alan G Jones / 27 May 2008 /