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).
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.