Magnetotellurics (MT) refers to a technique in which electrical resistivity is determined by making measurements of electric and magnetic fields related to naturally occurring currents (“tellurics”, caused mostly by lightning strikes) flowing in the ground. Typical MT frequencies are from 0.0005 Hz to 1,000 Hz. The ratio of the amplitudes of the electric and magnetic fields is used to calculate the electrical resistivity of the ground at a depth determined by the ground resistivity and the frequency of the measured signal. Higher ground resistivity and lower frequencies allow greater depth of investigation. For traditional low-frequency MT, typical depth of investigation is up to 20 km or greater, but generally targets within the first 100 meters cannot be resolved.
Audio magnetotellurics (AMT) is similar to standard MT in that it uses naturally-occurring currents, but the frequency band is limited to the audio range, generally from 0.1 Hz to 8,000 Hz. Depth of investigation for AMT is typically from 30m to 2 km. Geometrics’ AMT instrument (Geode EM3D AMT) is designed to investigate this depth range, operating in the frequency band of 0.1 to 10,000 Hz.
Controlled-source audio magnetotellurics (CSAMT), in its most common variation, does not use naturally-occurring currents, but instead only uses a man-made transmitter generating currents in the frequency range of from 1 Hz to 10 kHz. Geometrics CSAMT’ instrument (Geode EM3D CSAMT) uses a controlled-source transmitter operating in the frequency band of 0.1 Hz to 10,000 Hz. Depth of investigation ranges from about 20 m to 2 km.
Geometrics’ Hybrid-Source AMT (HSAMT) instrument (Stratagem EH4) uses the natural field signals from 0.1 Hz to 100,000 Hz, but also uses a controlled-source transmitter to supplement the natural-field low frequencies for a depth of investigation of 5m to 2 km. The Geometrics hybrid source transmitter provides 15 separate frequencies ranging from 800 Hz to 70,000 Hz.
Common applications for AMT (Geode EM3D AMT) and HSAMT (Stratagem EH4):
Minerals and ground water exploration to 1,500m depth.
Deep engineering site characterization.
Considerations and Limitations for AMT and HSAMT:
Data quality for AMT and the low-frequency bands of HSAMT depend on the availability of natural field sources. Natural AMT signal availability depends on the season, time of day, and weather.
Contamination by 50 Hz or 60 Hz power sources such as power lines, industrial machinery, or urban settings negatively affect data quality.
Advantages of AMT and HSAMT over similar techniques:
AMT acquisition is faster than traditional MT. Acquisition for low-frequency MT data requires up to 12 hours on a single station. Collection of high-frequency AMT data at 10 Hz and above can be done in less than 15 minutes.
HSAMT transmitter setup is much faster and easier. A traditional grounded dipole CSAMT transmitter can take several hours to set up. A dual-loop induction transmitter as a high-frequency source can be set up in less than 10 minutes.
HSAMT can resolve shallow targets. HSAMT up to 100 kHz can image targets as shallow as 5 meters. Traditional low-frequency MT cannot resolve targets in the upper 100m. AMT to 10 kHz can resolve targets as shallow as 20 meters in conductive earth.
AMT and HSAMT sensor setup is easier. Traditional low-frequency MT surveys require the magnetic sensors to be buried at least 20cm in the ground, which can take considerable time and may be impossible in frozen or otherwise hard ground.
High-frequency AMT or HSAMT magnetic sensors can often be used unburied. MT electric sensors use non-polarizing porous pot electrodes which must be buried in moist ground. The AMT electrodes can be metal stakes that are simply hammered into the ground.
Deliverables for AMT and HSAMT:
MT processing is used for MT, AMT, and HSAMT measurements. The processing generates impedance, phase, coherency, and other parameters of the earth’s response. 1-D and 2-D transformation and inversion software are used to generate 1-D soundings and 2-D depth sections of depth and true resistivity. 3-D inversion software is under development in several academic settings . An example of a 2-D section showing a conductive brine zone (red) is shown below.