What are the Differences Between EMF, EMI, and EM Surveys?

This article is about the differences between the three types of surveys involving electromagnetic radiation and the distinct purposes of each. These three types of surveys are:

1. Electromagnetic Field Radiation Surveys, EMF, or EMR;

2. Electromagnetic Interference Surveys, EMI, and

3. Electromagnetic Surveys, EM. Some of the tools used for each type will be discussed. 

Electromagnetic Field Radiation Survey (EMF Radiation Survey)

These surveys assess the electromagnetic radiation in the environment at a specified location.

  • The primary purpose of an EMF Survey is to assess human or equipment exposure to electromagnetic field radiation from power lines, cell towers, Wi-Fi, microwave ovens, etc.

  • EMF Surveys are used for occupational safety, regulatory compliance, health impact studies, and electronic equipment shielding. This shielding is usually for either radio frequency or AC magnetic field radiation from a source such as a transformer, electrical switch gear, conduit, or electrical closets.

  • Generally, EMF Surveys are conducted using portable meters that measure the strength of existing EM fields across various frequency bands. A good assessment covers the entire frequency range of the standard applied or a project’s requirements.

  • The tools often used for EMF Surveys are listed below.

Electromagnetic Interference (EMI) Survey

An electromagnetic interference survey aims to measure and assess the unintended or harmful electromagnetic emissions that can interfere with the functioning of electronic devices or systems. (Please note: Elexana prefers the term EMI survey to determine the harm or damage electromagnetic radiation causes to human beings' electrical systems. Unlike the tools others use for an EMF survey, discussed below, we often use tools for a classical EMI survey, such as spectrum analyzers, EMI analyzers, sensor probes, oscilloscopes, etc. This is because we believe there is more to assessing the intensity of electromagnetic radiation’s effect than solely measuring the power density amplitude.)

An EMI survey aims to identify the sources of interference, both controlled and uncontrolled, both from the internal circuitry of the equipment and from what is in the environment that would couple onto the equipment’s circuitry, and to assess whether they exceed acceptable EMI limits, which can disrupt equipment or communication systems.

The essential measurements needed for an EMI assessment are:

1. Conducted emissions are harmonic transients, signal noise, traveling along cables, traces, power lines, etc.)

2. Radiated emissions are electromagnetic energies traveling through the air.

Subsets of these emissions are:

A. Inductive Coupling is magnetic field energy generated by current, and the transients are coupled with the magnetic field’s fundamental. In the case of an AC magnetic field in the USA, this fundamental is 60 Hertz. In Europe, the fundamental is 50 Hertz.

B. Capacitive coupling is electric field energy generated by voltage potential, where harmonic transients cosignal with the electric field’s fundamentals. An AC electric field in the USA has a fundamental frequency of 60 Hertz. In Europe, the fundamental is 50 Hertz.

Common reasons why someone would want an EMI survey are:

  1. Aerospace and defense systems (shielding effectiveness).

  2. Medical devices (ensuring hospital equipment is restored to normal function).

  3. Industrial or communication sites (preventing signal loss or distortion).

Electromagnetic Field (EMF) Radiation Survey

An EMF survey measures the strength and presence of various electromagnetic field radiation in an environment, typically to assess human exposure levels. In the US, guideline levels for human exposure are found in Bulletin OET-65.

An EMF survey aims to ensure compliance with health and safety regulations related to EMF exposure, not necessarily to address equipment interference.

Essential measurements and their units are:

  1. Electric fields (V/m: Voltage per meter).

  2. Magnetic fields (A/m, µT, or mG: Amperes per meter, microTeslas, or milliGauss).

  3. Power density (W/m², W/cm²: Watts per square meter or square centimeter across various frequency bands).

The general applications for an EMF survey are:

  1. Assessing radiation from power lines, cellular towers, Wi-Fi routers, and electrical wiring and equipment.

  2. Workplace or residential safety evaluations.

  3. Environmental impact assessments.

Electromagnetic Survey (EM Survey)

EM surveys are used in geophysics and subsurface exploration.

  1. The reason for conducting an EM survey is to detect variations in the ground’s electrical conductivity and magnetic permeability.

  2. These surveys are used for mineral exploration, groundwater mapping, environmental site assessment, and locating buried objects.

  3. Instruments send or inject electromagnetic signals into the ground; the response (secondary fields) is measured to infer what’s underground.

  4. Time-domain EM systems and frequency-domain EM instruments are used for EM surveys.

The main difference among the three types of surveys is:

  1. EMF surveys focus on whether electromagnetic fields affect human health and safety.

  2. EMI surveys focus on whether electromagnetic emissions may disrupt equipment and systems.

  3. EM surveys focus on locating what is below Earth’s surface.

The Tools Used for Different Surveys

While electromagnetic interference (EMI) surveys, electromagnetic field (EMF) radiation surveys, and electromagnetic surveys (EM) deal with electromagnetic phenomena, they have different goals and use other tools.

Electromagnetic Field (EMF) Radiation Survey

  1. The aim is to measure electromagnetic field levels to assess human exposure or environmental impact, often for health, safety, or regulatory compliance (e.g., near cell towers, power lines, or industrial equipment).

  2. The standard equipment used for measuring EMF is:

  3. Broadband EMF meters — handheld devices that measure field strength across a wide frequency range, usually giving total field strength (V/m, A/m, W/m²).

  4. Gaussmeters or magnetometers — to measure static or low-frequency magnetic fields.

  5. Electric field meters — to measure static or low-frequency electric fields.

  6. Isotropic probes — sensors that capture field strength from all directions, 360º. These are essential for assessing human exposure.

  7. Personal EMF dosimeters are generally wearable devices for logging individual exposure over time.

  8. These tools are adept at measuring field strength levels, time-averaged exposure, and safety compliance with limits set by agencies like the FCC, ICNIRP, IEEE, or national health guidelines.

Electromagnetic Interference (EMI) Survey

  1. The aim is to identify, locate, and quantify sources of electromagnetic noise that interfere with the operation of electronic devices or systems (e.g., computers, radios, avionics, medical equipment). (Please note: At Elexana, we take this one more step to troubleshoot for optimal solutions that further suit our clients’ needs and budget. Often, the survey will include the solutions.

  2. Typical Tools:

    • Spectrum analyzers — measure signal levels over various frequency bands and help detect interference sources.

    • EMI receivers are specialized equipment that measure conducted and radiated emissions. They are compliant with standards (like CISPR and MIL-STD).

    • Near-field probes — handheld probes (magnetic and electric) for local “sniffing” near circuit boards or cables.

    • Current clamps are used to measure noise on power or signal lines.

    • Anechoic chambers are shielded rooms designed to prevent outside EM signals and reflections.

    • Portable TEM cells are impedance-balanced transverse electromagnetic enclosures, usually within a Faraday cage, that serve a similar purpose to an anechoic chamber.

  3. An EMI survey focuses on specific frequency bands, transient signals, narrowband or broadband interference, and compliance with regulatory limits (e.g., FCC, CISPR).

Electromagnetic (EM) Survey

  1. Time-domain electromagnetic (TDEM) Systems- Inject pulsed EM fields into the ground and measure transient responses over time.

    Specific tools: Geonics PROTEM, Zonge GDP-32, ABEM WalkTEM

  2. Frequency-Domain Electromagnetic (FDEM) Systems - Transmit continuous EM waves at multiple frequencies, measure ground conductivity.

    Tools: Geonics EM31, EM34, EM38, DualEM-421, GEM-2 by Geophex

  3. Controlled-Source Audio Magnetotellurics (CSAMT) - Use an artificial EM source to probe deeper structures, combining electric and magnetic field data.

    Tools: Zonge CSAMT, Phoenix Geophysics systems

  4. Magnetotelluric (MT) Systems—These passive systems use natural EM signals from lightning and solar activity to probe the deep earth.

    Tools: Phoenix MTU-5A, Quantec Spartan MT

  5. Ground Conductivity Meters: Handheld or portable devices for shallow surveys, often for environmental or agricultural work.

    Tools: Geonics EM38, EM31; CMD Explorer

  6. Loop and Coil Sensors (Transmitter/Receiver Coils) - Transmit and receive EM signals; vary in size for shallow or deep penetration.

    Tools: Custom loop setups with induction coils.

  7. Resistivity and Induced Polarization (IP) Systems. - While not strictly EM, it is often combined to measure resistivity and chargeability alongside EM surveys,

    Tools: IRIS Syscal Pro, ABEM Terrameter

  8. Data Logging and Processing Software: - Specialized software to model subsurface conductivity from collected EM data.

    Tools: EMIGMA, Maxwell, Res2DInv, Aarhus Workbench.

Elexana LLC is recognized globally as a top-tier Electromagnetic Interference (EMI) site survey provider due to its comprehensive approach, technical expertise, and commitment to client-centric solutions. Here's an overview of the factors contributing to its esteemed reputation.