Electromagnetic (EM) Mapping Surveys

Electromagnetic Mapping (EM) is a geophysical technique that uses electromagnetic induction to map features in the subsurface.

It's often used in exploration and environmental applications across both greenfield and brownfield sites. EM mapping is a great tool for exploring potential resources or hazards and can be used to map a variety of features, including:

Voids, mineworking, cavities and dissolution features.
Buried obstructions such as foundations, USTs and buried services.
Contaminants in the subsurface, such as leachate mapping and landfill delineation.

How Electromagnetic Mapping Works

The basic principle of EM survey is simple: if you expose the subsurface to a changing primary field, an electrical current will be induced in the ground. This arriving primary current will create its own secondary electromagnetic field, which can be measured from the surface using sensors. The resultant electric field magnitude is influenced by the subsurface material's geometry and electrical properties.

Variations in the subsurface conductivity (which is related to the composition of the ground) will induce different amounts of current. This, in turn, will create variations in the magnetic field that can be mapped.
The electrical conductivity of some materials on earth (such as metals) is very high, while others (such as most rocks) are poor conductors. The receiver responds to the arriving primary and secondary fields showing differences in phase, amplitude, and direction.

Electromagnetic surveys can be used to map both natural and man-made features. For example, variations in the subsurface geology can be imaged using EM, as can buried objects such as pipelines, drums, and UXO.

There are two main types of near-surface systems used for EM mapping: Frequency-Domain (FDEM) and Time-Domain (TDEM).

Frequency-Domain Electromagnetic Mapping (FDEM) Systems

FDEM surveys uses a transmitter coil to emit a primary electromagnetic field across a range of frequencies in the ground. As this propagates through the subsurface, it interacts with materials and objects, producing eddy currents which emit secondary fields that are in part measured by the receiver at the surface.

The returning secondary field is measured in two components; the EM energy which is in-phase with the transmitted electromagnetic field and the energy which is 90 degrees "out of phase" – this is called the quadrature component.

The in-phase and quadrature components can then be subsequently inverted into magnetic susceptibility (the measure of how magnetic something is), and the apparent conductivity respectively.

FDEM surveys are an excellent reconnaissance technique and are typically best deployed on greenfield sites. Its main applications include the mapping of voids, shallow geological variations and contamination plumes.

Time-Domain Electromagnetic Mapping

TDEM surveys uses a transmitter loop to emit a transient pulse of electric currents during a "time on" phase of recording. As this propagates through the subsurface it interacts with materials and objects producing eddy currents which emit secondary fields. During the "time off" stage, the receivers measure the decay of the secondary magnetic field as a millivolt response. The strength of the response along with the time observed can be an indication ferrous content and depth.

Providing rapid data acquisition, TDEM surveys are also a well placed method for reconnaissance surveys but typically best deployed on brownfield sites. This is because the instruments are generally less susceptible to sensor saturation by above-ground sources of metal.

This technique's main applications include buried obstruction and foundation mapping, as well as Unexploded Ordnance (UXO) and Underground Storage Tank (UST) detection.

EM Survey Applications

EM mapping is a versatile tool that can be used in a variety of applications. Here are some examples:

Exploring for buried objects: EM mapping can be used to detect buried objects, such as pipelines, storage tanks, and UXO (unexploded ordnance).

Mapping geologic features: Variations in subsurface composition can be mapped using EM methods. This is especially useful in mapping superficial deposit variations, faults and quarrying resources such as gravel deposits.

Environmental applications: EM methods can be used to map groundwater contamination, leachates from landfill sites, and other environmental hazards.

Surveying for archaeological relics: EM mapping can be used to locate buried archaeological artefacts.

EM methods are often used in conjunction with other reconnaissance geophysical techniques, such as Ground Penetrating Radar (GPR) to provide an indicative map of subsurface features/obstructions. It is also complimented well by techniques such as electrical resistivity and seismic surveys to provide further anomaly classification and depth constraint.

These combined approaches help to give a more complete picture of the subsurface.

Advantages of Electromagnetic Mapping

There are many advantages to using EM techniques for mapping:

EM surveys are non-destructive and non-invasive: This means that they can be used in sensitive areas without causing any damage.

EM surveys are relatively inexpensive: When compared to other geophysical techniques, EM methods are relatively low-cost. This makes them a great option for large-scale surveys.

EM surveys are versatile: They can be used to map a variety of features, both man-made and natural.

EM surveys can be used in difficult terrain: Other geophysical techniques, such as seismic methods, can be difficult to use in areas with rough terrain. EM methods, on the other hand, can be easily adapted to work in these types of conditions.

Electromagnetic Mapping FAQs

What equipment is needed for an EM survey?

EM mapping typically uses sensors that are placed on the ground surface. There are many varieties of EM equipment available, each suited to a specific application.

These include:

Conductivity Meters
Frequency Domain Systems
Time Domain Systems
Metal Detectors
Transient Electromagnetics (TEM)

How accurate are electromagnetic surveys?

The accuracy of an EM survey depends on a number of factors, including the type of equipment used, careful survey design, and the nature of the subsurface. In general, however, EM surveys provide accurate data and can be used to obtain detailed information about the subsurface.

How long does an electromagnetic survey take?

The length of an EM survey depends on the size and complexity of the area being surveyed. A small survey might take a few hours, while a large EM survey could take several days or even weeks.

How much does an electromagnetic survey cost?

The cost of an EM survey depends on a number of factors, including the size and complexity of the area being surveyed. In general, however, EM surveys are relatively low-cost when compared to intrusive investigation techniques.

Get in touch

At SEP Geophysical, we offer a wide range of geophysical services, including EM mapping. We have a team of experienced professionals who are ready to help you with your next project.
Contact us today to learn more about our services and how we can help you achieve your goals.

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