Solutions for a complex world: Audio Magnetotellurics
Audio Magnetotellurics for Subsurface Characterization
Article by: Hannah Peterson | hydroGEOPHYSICS
As populations worldwide increase in size and scope, so does our need for resources to maintain these populations. Many of these needed resources are found deep within the subsurface of the earth and provide the building blocks to create the products and services we consume. Geophysics plays a significant role in subsurface exploration, serving to locate and map minerals, precious metals, groundwater, and energy resources. Magnetotellurics (MT) and Audio Magnetotellurics (AMT) are two geophysical methods offering robust techniques to discover and characterize these resources.
Magnetotellurics (MT) is a geophysical method that measures the earth’s naturally occurring electric and magnetic fields with time to understand subsurface electrical properties with depth.
Magnetotellurics (MT) is a geophysical method that measures naturally occurring electric and magnetic field variations with time to recover subsurface electrical properties with depth. The source field for MT is generated by naturally occurring passive energy sources emitted from thunderstorms and interactions between the ionosphere and solar winds. These magnetic variations induce an electrical current in the subsurface, known as telluric currents, and components of the magnetic and electric fields are recorded to recover resistivity data. Audio magnetotellurics (AMT) uses only the audio band, which spans from 0.1 Hz to 20 kHz, falling in the higher frequency range of the MT spectrum. AMT recording lengths are shorter than a typical MT survey, and the method provides a higher resolution of the near-surface stratigraphy.
Properties, such as water content, heat, grain size, mineral content, and infrastructure, influence resistivity values enabling our ability to map differing subsurface features.
The AMT method uses a series of electric dipoles and magnetic field sensors to measure the earth’s electric and magnetic fields with time, and resulting 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, the resistivity of the subsurface, intrinsic signal levels, and the target dimensions. Higher frequencies translate to better resolution at shallower depths, while lower frequencies can see deeper. Since AMT is in the higher range of frequencies for this method, the use of this technique has a higher resolution at shallower depths (10s or 100s of meters). In contrast, the conventional MT method is used for imaging multiple kilometers deep.
Unsworth, M. 2007, Magnetotellurics: Encyclopedia of Geomagnetism and Paleomagnetism, edited by D. Gubbins and E. Herrero-Bervera., Springer, Dordrecht, The Netherlands.
The basic theory for MT describes energy being emitted from the magnetosphere and lightning strikes, creating a magnetic field and inducing electric currents. This diagram shows the energy propagation and the orthogonal components of the fields being measured from an MT station on the surface. E represents the electric field, and H denotes the magnetic field.
Reasons to include AMT into your next investigation may be:
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Rapid and cost-effective data acquisition.
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Minimal acquisition footprint and environmental impact.
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Stations can be collected in almost any terrain and across multiple scales, making surveys easily customizable to your application and location.
The 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 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.
The result of an AMT survey that was conducted for the purpose of measuring depth to bedrock, the thickness of basin fill, and identifying potential fracture zones for groundwater exploration in California. The image shows a 2D cross-section of an AMT survey result. Highly resistive areas are indicated by the yellow to orange hues, and conductive areas are indicated by olive to blue hues. The AMT station locations are shown on the surface.
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About the Author: Hannah Peterson | hydroGEOPHYSICS
Hannah Peterson is a Staff Engineer and geophysicist for hydroGEOPHYSICS with a background in engineering, coding, and data analysis. She has broad field experience involving geophysical data processing, surveying, and geological interpretation in both land and marine settings. Her professional focus is on electromagnetics, electrical methods, data processing, and inversion.
Hannah holds a BS in Geophysical Engineering from the Colorado School of Mines and an MS in Earth Science from the University of California San Diego.
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