Controlled Source Audio-frequency Magnetotellurics

Controlled Source Audio-frequency Magnetotellurics (CSAMT) is a low-impact, nonintrusive, ground geophysical survey method used extensively in minerals, geothermal, and groundwater exploration since 1978 when Zonge introduced a commercial data-collection equipment system for CSAMT to the industry.
CSAMT is a geophysical investigation method for obtaining information about subsurface resistivity. Resistivity values calculated from the CSAMT data relate to geology. Primary factors affecting resistivity include rock or sediment porosity, pore fluids, and the presence of certain mineral assemblages.
For mineral, hydrological, mining and some petroleum applications, CSAMT data can provide critical information about geologic structure, lithology, water-table trends, pore fluid salinity, and contaminant concentrations.
Although resistivity information can also be obtained in IP surveys, CSAMT and IP methods each have separate, distinct advantages. CSAMT provides better resolution and much deeper information with more efficient use of field-work time. The polarization data from IP surveys, however, can add valuable information for interpretation and for prioritizing target areas. Thus CSAMT is often used for reconnaisance of larger geographic areas to determine depth to bedrock and to locate faults and resistive anomalies for further, selective IP investigation.
A CSAMT survey involves transmitting a controlled signal at a suite of frequencies into the ground from one location (the transmitter site) and measuring the received electric and magnetic fields in the area of interest (the receiver site). The ratios of orthogonal, horizontal electric and magnetic field magnitudes (e.g. Ex and Hy) are used to calculate the resistivity structure of the earth.

Natural-source Audio-frequency Magnetotellurics

Natural-source Audio-frequency Magnetotellurics (AMT) is an electromagnetic survey technique that uses naturally-occurring ionospheric currents and lightning storms — passive energy sources — to electrically map geologic structure to depths of 500 meters or more.
Natural-source electromagnetic (EM) signals are generated in the atmosphere and magnetosphere. The time-varying electric and magnetic fields induce currents into the earth and oceans, which produce magnetotelluric (MT) signals, which are measured by AMT and MT data acquisition systems.
Low-frequency magnetotelluric EM signals (< 1 Hz) are generated by the interaction
between the earth’s magnetosphere and solar wind. High-frequency sources in the audio range (> 1 Hz) are generated by thunderstorms worldwide.
The AMT and MT geophysical methods combine measurements made of sitespecific electric and magnetic fields using grounded dipoles and magnetic field antennas over a wide band of frequencies. Low frequencies sample deep into the earth and high frequencies (AMT) correspond to shallow samples.
Ground resistivity values are calculated from the magnitude and ratios of these components and then mapped using Zonge inversion and modeling software. Ground resistivity relates to the geology.
Advantages of AMT
• No need for power source or highvoltage electrodes.
• Minimal environmental impact.
• Stations can be acquired almost anywhere and can be placed any distance apart.
• Zonge’s backpack portable system allows for use in difficult terrain,
• Relatively easy field logistics for large-scale regional reconnaissance exploration and for detailed surveys of local geology. Fast data collection.
AMT Exploration Applications
• Mineral, groundwater and geothermal exploration
• Excellent for imaging moderately deep geologic structure and nearsurface geology to depths of about 500 meters in detail
• Onshore oil and gas exploration where seismic is not feasible or is cost prohibitive
• Mining and water resource management

 MT (Magnetotellurics)

The MT (Magnetotellurics) process is an electromagnetic survey and imaging technique that uses naturally occurring ionospheric currents and lightning storms, both passive energy sources, to electrically map structure/geology to depths of 5000 meters or more. Sources for these natural source electromagnetic (EM) signals range from high frequency EM sources in the audio range (> 1 Hz) generated by worldwide thunderstorms and lighting to lower frequency Magnetotelluric (MT) EM signals (< 1 Hz) generated by the interaction between the earth’s magnetosphere and solar wind.

These time-varying electric and magnetic fields induce currents into the earth and oceans. Electric currents within the earth produce magnetotelluric signals that are measured by the Zonge MT System. The fields are measured over a range of frequencies using grounded dipoles and magnetic field antennas. Ground resistivity values are calculated by measuring the magnitude of the electric and magnetic field.

Advantages of MT for Exploration

  • Map large-scale regional areas or detail local geology.
  • See deep geologic structure and near-surface geology.
  • Use in difficult terrain with portable backpack equipment.
  • Simplified logistics.
  • Collect data without high-voltage current electrodes or requiring a signal source.
  • Acquire MT as individual stations or along extended profiles.
  • Operate with minimal environmental impact.
  • Useful for deep mineral, groundwater, geothermal development, hydrocarbon exploration, and water resource management applications.
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