The audio-magnetotelluric (AMT) dead band
Han-Lun Hsu
Abstract
The frequencies of natural source for magnetotelluric (MT) method are about 10kHz to 1e-4 Hz. Those high frequencies energy source for electromagnetic waves are caused by distant lighting storms and which propagate 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. The signal minimum at frequencies in the 1 to 5 kHz is so-called the AMT dead band. In the first paper they try to analyze the time variations of high-frequency EM fields to assess the limitations of the efficient applicability of the AMT method. They demonstrate the magnetic-field amplitudes during the daytime are often well blow the noise levels of existing sensors. To overcome this deficiency, they propose a hybrid acquisition and processing method-ology based on combining the telluric-telluric (T-T) and telluric-magnetotelluric (T-MT) methods in this frequency range. Applying a tensor multiplicative relationship between these responses, they obtain the T-MT AMT transfer functions for several sites, these transfer functions can represent a reasonable approximation of the real AMT impedance tensors. In comparing the processed daytime data using the conventional MT approach to those obtained from the T-MT approach. The results demonstrate the method can determine high-quality estimates in the dead band, although the estimates can be severely affected by noise.
Reference
Xavier Garcia and Alan G. Jones, 2002, Atmospheric sources for audio-magneto-telluric (AMT) sounding, GEOPHYSICS, VOL. 67, NO. 2, P. 448¡V458.
Xavier Garcia and Alan G. Jones, 2005, A new methodology for the acquisition and processing of audio-magnetotelluric (AMT) data in the AMT dead band , GEOPHYSICS, VOL. 70, NO. 5, P. G119¡VG126.