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.
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1. There are basically three types of "gold": low concentration disseminated gold in ore, placer gold deposits and solid gold such as that associated with treasure. Magnetometers are used to find disseminated gold by its association with mineralized zones which also contain magnetite or other magnetic minerals. Magnetometers are often used in conjunction with airborne Electro-Magnetic surveys to find the conductive ore bodies. Placer gold is the type found in buried stream channels such as the gold which sparked the California gold-rush in 1849. Gold dust and magnetic minerals have been concentrated in river banks over thousands of years. Where there is gold there is often magnetite and therefore the magnetometer can be used to locate placer gold deposits. Gold treasure is a different story and being non-magnetic gold, silver, and other precious minerals are not directly detectable by the magnetometer.
2. The magnetometer can only detect ferrous (iron or steel) objects. If the gold is stored in an iron box or has iron materials next to the gold (such as colonial ship ballast stones in the marine environment), there is the possibility of detecting the iron material. This is true for land and marine (sunken galleon) gold bullion. The vast majority of target search surveys are performed on a grid in a "lawn mower" back and forth manner to cover the area of interest. Lane spacing is dependent on target size (magnetic mass).
3. At a sensor to target distance of 2 to 3 meters there will need to be at least 1-2 kilograms of iron. This can produce a 1-2 nT anomaly that is detectable in a magnetically clean environment. The ideal environment would be in a plowed farm field or the bottom of the ocean away from human activity i.e., away from a port or harbor. You will probably not be able to detect this small of an anomaly in a city or port location. The more iron mass there is, the better the detectability.
4. Training to use the magnetometer can take 1-2 days depending on experience with setting up computerized survey equipment and a GPS.
5. Processing the magnetic data requires several days of training and would require a geophysical background to interpret the final maps. We provide free software to make maps and estimate the target depth of burial (inversion). If you are unfamiliar with this procedure, we would recommend that you find a local geotechnical firm to look at the data to determine if there are anomalies that should be investigated further. Remembering that non-ferrous materials do not cause anomalies (gold, silver, copper, brass, aluminum, gems) you will be looking for anomalies either associated with the container OR associated with ground disturbance (i.e., gravesite). In this way some anomalies can be detected where there has been an excavation such as a gravesite.