Magnetotellurics Methods
Magnetotellurics (MT) is a geophysical method that measures naturally occurring electric and magnetic field variations with time to recover subsurface electrical properties. Passive energy sources generated from thunderstorms and interactions between the ionosphere and solar winds create the source field used by MT surveys. These magnetic variations induce an electrical current in the subsurface, known as telluric currents. The resulting orthogonal components of the magnetic and electric fields are measured to recover resistivity with depth. Audio-frequency magnetotellurics (AMT) uses only the audio band, which spans from 0.1 Hz to 20 kHz and falls in the MT spectrum’s higher frequency range. AMT recording lengths are shorter than a typical MT survey, and the method provides a higher resolution of the near-surface stratigraphy. The depth of investigation for an MT/AMT survey ranges from 10’s of meters to multiple kilometers. Magnetotellurics has a broad range of applications, the most popular being natural resource exploration for groundwater, minerals, and geothermal.
Schematic of MT theory, showing the magnetic source fields and geometry of the measured fields E (electric) and H (magnetic).
APPLICATIONS
- Groundwater exploration and mapping.
- Geothermal exploration and investigations.
- Mining exploration.
- Hydrocarbon exploration.
AMT/MT TECHNIQUE
The AMT/MT method uses a series of electric dipoles and magnetic field sensors on the ground surface to measure the earth’s electric and magnetic fields. After processing and inversion, AMT data are modeled as resistivity versus depth. Resistive or conductive variations in the subsurface are a function of how well earth materials conduct electrical currents. Properties such as water content, heat, grain size, mineral content, and infrastructure influence resistivity values enabling our ability to map differing subsurface features. The depth of investigation for AMT/MT surveys depends on the measured frequencies (a function of recording length), the subsurface’s resistivity, intrinsic signal levels, and the target dimensions. Higher frequencies translate to better resolution at shallower depths, while lower frequencies have deeper penetration depths. Since AMT is in the higher range of frequencies, this technique has a higher resolution at shallower depths (10s or 100s of meters); thus, it is useful for mapping shallow targets such as groundwater. In contrast, deeper targets, such as geothermal sources or hydrocarbons, use the conventional MT method for imaging multiple kilometers below the subsurface.
The survey set up for an MT survey includes four porous pot electrodes, two magnetic sensors, a power source, a field laptop, and a receiver console.
Data Acquisition
An AMT/MT survey station consists of burying three magnetic sensors in the X, Y, and vertical directions and four porous pot electrodes in four orthogonal directions (see MT set up illustration). At each station, the magnetic and electric fields are measured for a period of time (on the order of hours to days) to produce a time series measurement in one dimension. The data is then processed and combined with multiple stations producing a 2D or 3D cross-section of electrical resistivity data with depth. Resistivity is a measure of how well electrical currents flow through a medium, which provides essential insight into subsurface physical properties. Typically, hard and consolidated bedrock is more resistive, while porous and fractured rock is more conductive due to fluids filling the pore space. Thus, magnetotellurics is a useful tool to image geologic structure, lithology, and identify the water table. Conductivity will typically increase with heat, fluids, and metals, making it a useful parameter for identifying targets such as minerals, contaminants, hydrocarbons, or geothermal sources.
EXAMPLE
HGI conducted an AMT survey in California in 2020 to provide depth to bedrock, basin-fill thickness, and fill hydrogeological properties to support a potential drilling campaign for groundwater wells. The figure below highlights a clear contrast between resistive versus conductive material, indicating changes in physical properties. The AMT results were combined with gravity to provide a detailed image of the subsurface structure in the area. The two methods complemented one another to image fault structure, alluvial deposit thickness, and zones of saturation.
Click on the image for a larger view
ADVANTAGES of AMT/MT
- Rapid and cost-efficient data acquisition.
- Imaging depths on the order of 1,000’s of meters.
- Minimal acquisition footprint and environmental impact (typically do not require extensive permitting for surveys on BLM and State land).
- Data can be acquired in almost any terrain and across multiple scales, making surveys easily customizable to the client’s application and survey location.