The Audio Magnetotelluric method measures the frequency of electric currents induced into the ground by electromagnetic fields generated from distant lightning strikes (sheet lightning). Through analysing the frequency and phase of the currents electric and magnetic components the resistivity properties of the ground can be derived to depths far grater than Time Domain Electromagnetics can offer, from 1-5km depth. Resistivity measurements provide an insight to geological structures and can be interpret to reveal hydro-geologic structures, potential mineral deposits, geology for deep geotechnical work.
AMT (Audio Magnetotellurics)
Uses naturally occurring currents flowing in the Earth’s subsurface (natural fields). Audio Magnetotellurics (AMT) measure the frequency of electric currents induced into the ground by naturally electromagnetic fields. Through analysing the frequency (over a large frequency range) and phase of the currents (electric & magnetic components), the resistivity of the ground can be derived. AMT makes use of the distant lightning strikes which in most cases are several of hundred kilometers away from the station location. If more power is required an external transmitter (i.e. HSAMT) can supplement the AMT device by adding a low power transmission into the ground to fill in the weak signal band (AMT dead zone). Used widely in near surface geophysics this technique can record data from the shallow subsurface from a few meters to more than 1km in depth.
HSAMT (hybrid source AMT)
Exploits natural fields and enhances certain frequencies in the high-frequency dead zone where natural fields are weak using a portable, high-frequency, induction loop transmitter that runs on a standard 12 VDC car battery.
CSAMT (controlled source AMT)
Does not use natural fields but instead uses man-made signals from a controlled-source transmitter of 0.125 Hz to 8,192 Hz. CSAMT transmitters can put up to 30 Amps and 1,000 Volts into the ground, but they are expensive, labour intensive, require expertise, and large power supplies.