What Is The Electrical Magnetic Method?

The electrical magnetic method, also known as electromagnetic surveying, is a geophysical technique used to detect electrical conductivity variations in the subsurface. This allows for identifying changes in subsurface geology and locating metallic mineral deposits, unexploded ordnances, underground utilities, and archaeological artifacts.

It works by inducing electrical currents in the ground using transmitter coils and measuring the resulting magnetic fields using receiver coils. Variations in the subsurface electrical conductivity will cause changes in the induced currents, which are reflected in the secondary magnetic fields. These magnetic field measurements are then used to infer information about the subsurface.

The electrical magnetic method is commonly used in mineral exploration, environmental surveys, forensic investigations, infrastructure mapping, and archaeological surveys. Its key advantages are its sensitivity to subsurface features and ability to survey large areas rapidly without needing to take physical samples.

History of the Electrical Magnetic Method

The electrical magnetic method was first developed in the 1920s by American geophysicists Frank Wenner and Paul D. Davis as a technique for prospecting minerals. At the time, it was known as electrical resistivity surveying. Wenner conducted extensive research on the method at the US Bureau of Standards, publishing a key paper on it in 1915.

In the 1930s, the introduction of more sensitive electronics and automated data recording helped expand the use of the electrical magnetic method in mineral exploration. Swedish geophysicist Gunnar Enderlin is credited with further refining the techniques in the 1940s-50s.

As digital computing advanced in the 1960s, the electrical magnetic method could process larger datasets and generate more detailed subsurface images. This enabled wider adoption for groundwater, environmental and engineering applications. Newly developed time-domain EM instruments allowed detection of deeper conductors in the 1970s-80s.

Today, electrical magnetic surveying takes advantage of faster computers, GPS, advanced software modeling, 3D imaging and other modern technologies. However, the core principles of inducing currents, measuring responses and mapping subsurface electrical conductivity remain the same.

Principles

The electrical magnetic method is based on the principles of electromagnetics and Maxwell’s equations. Maxwell’s equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents. Specifically, the induced polarization method relies on Maxwell-Ampere’s law, which relates magnetic fields and electric current density:

∇ × B = μJ

Where ∇ × B is the curl of the magnetic field intensity, μ is the magnetic permeability, and J is the current density.

This equation shows that a changing electric current will produce a magnetic field. In electromagnetics surveys, an electric current is injected into the ground. This current produces a primary magnetic field. As the current interacts with conductive bodies in the subsurface, it induces eddy currents. These eddy currents generate a secondary magnetic field that can be measured by sensors on the surface.

By analyzing the primary and secondary magnetic fields, the location, size and conductivity of subsurface bodies can be determined. This allows geologists to identify features like mineral deposits, groundwater resources and buried artifacts.

Equipment Used

The key equipment used in the electrical magnetic method includes:

Magnetometers – These measure spatial variations in the earth’s magnetic field. Different types of magnetometers used include proton precession magnetometers, alkali vapor magnetometers, and fluxgate magnetometers. These are sensitive instruments capable of detecting tiny variations in the magnetic field.

Inductive coils – These can measure high frequency variations in the magnetic field. Inductive coils are used alongside magnetometers.

Cables – Insulated multi-core cables are used to connect the magnetometers and transmitter. Low noise cables are preferred to reduce interference.

Transmitters – Electric currents can be transmitted into the ground through grounded wires. Transmitters generate these currents at desired frequencies and waveforms.

Recorders – Data recorders are used to collect and store the magnetic field measurements from the magnetometers and inductive coils. Analog and digital recorders are available.

Computers – Computers are used for controlling the equipment, data recording and processing. Specialized software is used for tasks like modeling, inversion and interpretation.

Data Collection

Data collection for the electrical magnetic method involves surveying the subsurface using specialized equipment. Measurements are taken by technicians at predetermined stations along survey lines. Spacing between stations depends on the survey objectives and desired resolution, but are typically between 2 to 10 meters apart along each line. Perpendicular survey lines are also spaced in a grid pattern with spacing ranging from 10 to 100 meters.

Prior to surveying, the survey parameters such as line spacing, station spacing, and area coverage are determined based on the geological target and desired resolution. The spacing between measurement stations impacts the resolution, with closer spacing providing higher resolution data. However, closer spacing requires more time in the field. Electromagnetic surveys are often done as reconnaissance with wide line spacing to identify conductive anomalies, followed by infill surveys with tighter station spacing to better define targets.

At each station, the technician places the EM instrument on the ground and triggers it to take a measurement. The instrument records the secondary electromagnetic field response resulting from eddy currents induced in the subsurface by the transmitted primary field. This secondary field data provides information about subsurface electrical conductivity. For frequency domain systems, multiple measurements are taken at different frequencies to assess variation of conductivity with depth.

Data Processing

Data processing is a crucial step in the electromagnetic survey method. It involves filtering, transforming, and modeling the raw field data collected during the survey to generate useful information and interpretations.

Filtering refers to removing unwanted noise from the data. This can involve frequency filtering to remove specific noise frequencies or spatial filtering to smooth the data. Filters help isolate the relevant signal and minimize artifacts and errors.

Common transformations applied to EM data include conversions between measured components, normalization, and dimensionality reduction. Transformations simplify data relationships and prepare the data for modeling and interpretation.

Forward modeling generates synthetic model responses for comparison with measured data. Inversion modeling adjusts a starting model iteratively to find the subsurface model that best fits the actual measurements. Models estimate the subsurface distribution of physical properties like conductivity.

Careful data processing streamlines interpretation and ensures accurate results from the survey. It is key to extracting meaningful information from the raw electromagnetic field measurements collected in the field.

Interpretation

Interpreting electromagnetic survey data involves converting the measurements into geologic models that estimate the subsurface structure. Geophysicists analyze the survey data to identify anomalies, which are deviations from the norm that may indicate the presence of certain geological features.

Variations in conductivity are mapped to create images of the subsurface. Conductive zones may represent mineral deposits, geological contacts, water saturation levels, and other important targets. Anomalies are evaluated to determine if they represent noise or significant geological features.

Modeling is conducted to test different geological scenarios against the survey data. This iteratively refines interpretations until a model is found that best explains the observed measurements. Advanced processing can even generate 3D renderings of the subsurface architecture.

Careful interpretation requires understanding how factors like soil types, water content, and mineralization impact EM readings. Experience is key for accurate analysis. The end result is converting abstract geophysical data into actionable insights on subsurface structures.

Applications

The electromagnetic method has several important applications across a range of fields:

Mineral Exploration

One of the most common uses of the electromagnetic method is in mineral exploration. It is particularly useful for detecting conductive mineral deposits such as sulfide ores. The conductivity contrast between the ores and the host rocks allows geophysicists to map potential drill targets. Electromagnetic surveys can cover large areas quickly and are often used for initial exploration scanning.

Environmental

The electromagnetic method can also be used in environmental assessments and engineering projects. It is able to map things like groundwater resources, soil salinity, and subsurface contamination. Electromagnetic surveys provide continuous coverage to delineate environmental targets. The data can also be used to monitor changes over time.

Engineering

In geotechnical engineering, electromagnetic surveys help locate potential hazards and geologic features before construction begins. This allows for more optimized project planning. The method can also assess engineering properties of the subsurface and detect objects like buried pipes. Overall, it provides engineers and planners with critical information about the subsurface.

Advantages

The electrical magnetic method offers several key advantages over other geophysical survey techniques:

High Resolution

Electrical and electromagnetic surveys can detect small objects and subtle changes in the subsurface, providing higher resolution than some other methods like gravity or magnetics surveys. This makes the EM method well-suited for detailed mapping of the shallow subsurface.

Cost-Effective

Electrical and EM equipment is relatively inexpensive compared to other geophysical tools. Smaller EM instruments can be operated by one person, keeping labor costs low. The surveys are also efficient and can cover large areas quickly.

Versatile

The electrical method can be adapted to suit different applications by adjusting parameters like electrode spacing or frequency. Arrays like Schlumberger or dipole-dipole provide flexibility. EM methods can also detect both conductive and resistive targets.

Non-Invasive

Since electrical and EM surveys are conducted at the surface, they provide subsurface information without any drilling or excavation. This makes them well-suited for environmentally sensitive sites.

Limitations

The electrical magnetic method has some limitations that must be considered when using this geophysical technique:

Depth penetration – EM methods have limited depth penetration, usually only up to a few hundred meters. The depth depends on the conductivity of the subsurface. In conductive geologic settings, the depth of penetration will be shallower.

Cultural interference – Man-made metallic objects like pipelines, power lines, fences, railroads, etc. can create noise and distortions in the EM data. Careful survey design is required to avoid or minimize these effects.

Sensitivity to conductive bodies – The EM method is most sensitive to mapping conductive bodies, like mineral deposits or groundwater. Resistive bodies, like oil and gas reservoirs, produce weaker or ambiguous responses.

Lateral resolution – The lateral resolution depends on the coil/sensor separation distance. Small, closely spaced sensors provide better resolution of small or narrow targets. Large coils generate deeper signals but have lower resolution.

Complex datasets – The EM method collects large, complex datasets that require specialized software and experienced interpreters to process and analyze correctly.

Equivocal results – The geologic meaning of EM anomalies may be ambiguous and require corroborating information from other techniques to confirm the source.