Atmospheric sources for audio-magnetotelluric (AMT) sounding

The energy sources for natural-source magnetotelluric (MT) frequencies > 1 Hz are electromagnetic (EM) waves caused by distant lightning 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-especially at frequencies in the 1- to 5-kHz so-called audiomagnetotelluric (AMT) dead band. In the northern hemisphere these variations increase in amplitude during the nighttime and the summer months, and they correspondingly decrease during the daytime and the winter months. Thus, one problem associated with applying the AMT method for shallow (<3 km) exploration can be the lack of signal in certain frequency bands during the desired acquisition interval. In this paper we analyze the time variations of high-frequency EM fields to assess the limitations of the efficient applicability of the AMT method. We demonstrate that magnetic field sensors need to become two orders of magnitude more sensitive than they are currently to acquire an adequate signal at all times. We present a proposal for improving AMT acquisition involving continuous profiling of the telluric field only during the day-time and AMT acquisition at a few base stations through the night.