5.3 Electromagnetic methods and their applications

Electromagnetic Induction Principles

Electromagnetic (EM) methods exploit a simple but powerful idea: a changing magnetic field will induce electric currents in any nearby conductor. In geophysics, the "conductor" is the ground itself. By transmitting a controlled EM field and measuring how the subsurface responds, you can map variations in electrical conductivity at depth. These methods are workhorses for mineral exploration, groundwater mapping, and environmental site characterization.

Faraday's Law and Electromagnetic Induction

Electromagnetic induction is the process by which a changing magnetic field drives an electric current in a conductor. Faraday's law quantifies this: the induced electromotive force (emf) is proportional to the rate of change of magnetic flux through the conductor.

$\mathcal{E} = -\frac{d\Phi}{dt}$

where $\mathcal{E}$ is the induced emf and $\Phi$ is the magnetic flux. The negative sign reflects Lenz's law: the induced emf always acts to oppose the flux change that created it. This opposition is what generates the secondary fields you actually measure in a survey.

Electromagnetic Induction in Geophysical Surveys

In practice, a transmitter (typically a loop of wire or a long grounded wire) generates a primary EM field. When this time-varying field encounters conductive material underground, it induces circulating eddy currents in that material. Those eddy currents, in turn, produce their own secondary EM field.

A receiver measures the total field, which is the sum of the primary and secondary fields. The secondary field is typically much weaker than the primary, so sensitive instrumentation and careful signal processing are needed to extract it. The amplitude, phase, and frequency content of the secondary field all carry information about subsurface conductivity structure.

Depth of investigation depends on two factors:

• Frequency of the primary field: Lower frequencies penetrate deeper because they diffuse farther before attenuating.
• Subsurface conductivity: More resistive ground allows deeper penetration; highly conductive ground attenuates the signal quickly, limiting depth.

This frequency-conductivity tradeoff is governed by the skin depth concept, which describes how far an EM wave penetrates before its amplitude drops to $1/e$ (about 37%) of its surface value.

Frequency vs. Time-Domain Methods

The two main families of EM methods differ in how they transmit and record the signal. Each has distinct advantages depending on your target depth, resolution needs, and survey logistics.

Frequency-Domain Electromagnetic (FDEM) Methods

FDEM methods transmit a continuous sinusoidal primary field at one or more fixed frequencies. Common systems include ground conductivity meters (e.g., Geonics EM31, EM34) and airborne platforms (e.g., DIGHEM, RESOLVE). Operating frequencies typically range from a few hundred Hz to a few hundred kHz.

The receiver measures the secondary field in terms of two quantities relative to the primary:

• Amplitude: proportional to the conductivity of the subsurface.
• Phase shift: related to the ratio of conductive to resistive properties.

The secondary field is decomposed into two components:

• In-phase (real) component: influenced by both conductivity and magnetic susceptibility. This makes it useful for identifying magnetic materials but also means it can be harder to interpret for conductivity alone.
• Quadrature (imaginary) component: primarily sensitive to conductivity, making it the go-to component for conductivity mapping.

FDEM systems are generally compact, fast, and well-suited for shallow investigations (typically the upper tens of meters, depending on frequency and coil separation).

Time-Domain Electromagnetic (TDEM) Methods

TDEM methods take a different approach. Instead of a continuous sinusoid, the transmitter drives a current pulse through a loop, then abruptly shuts it off. Common systems include the TEM47, NanoTEM, and the airborne VTEM. The primary pulse is typically a square wave lasting milliseconds to seconds.

After the transmitter switches off, the receiver records the decay of the secondary field during the off-time. The decay rate directly reflects subsurface conductivity:

• Conductive materials sustain eddy currents longer, producing a slow decay.
• Resistive materials lose their eddy currents quickly, producing a fast decay.

Because you're measuring during the off-time (when the primary field is absent), TDEM avoids the challenge of separating a weak secondary field from a strong primary. This is a major practical advantage.

TDEM is also less affected by magnetic susceptibility than FDEM, since the off-time response is dominated by conductivity. Depth of investigation is generally greater than FDEM, ranging from a few meters to several hundred meters depending on the transmitter moment (current × loop area) and ground conductivity.

Electromagnetic Applications in Geoscience

Mineral Exploration

EM methods are among the most effective tools for detecting conductive ore bodies. The key targets include:

• Massive sulfide deposits (e.g., volcanogenic massive sulfides): highly conductive because interconnected sulfide minerals form continuous current paths.
• Graphite deposits: conductive due to the inherent electrical properties of graphite.
• Nickel-copper sulfide deposits (e.g., magmatic sulfides): conductive for the same reason as massive sulfides, with interconnected sulfide networks.

Airborne EM (AEM) surveys are the standard for large-scale reconnaissance. They cover vast areas quickly and cost-effectively, identifying anomalies that warrant follow-up. AEM data are typically integrated with magnetic and radiometric data to build a more complete geological picture.

Ground-based EM surveys provide higher resolution for detailed target characterization:

• Moving-loop methods (e.g., using SQUID sensors or SMARTem receivers): the transmitter and receiver move together along survey lines, producing detailed conductivity profiles.
• Fixed-loop methods (e.g., UTEM, DEEPEM): a large stationary transmitter loop energizes the ground while one or more receivers map the response. Fixed-loop setups generate stronger signals and are better for detecting deep or weakly conductive targets.

Environmental Studies

EM methods are valuable in environmental work because they're non-invasive and can cover large areas rapidly.

• Contaminant plume mapping: Conductive plumes from leaking underground storage tanks or landfill leachate show up clearly against a more resistive background.
• Buried object detection: Metal drums, pipes, and other buried waste produce strong EM anomalies.
• Geological characterization: Clay layers, aquitards, and other lithological boundaries can be identified from their conductivity contrasts, which is critical for understanding groundwater flow.

Borehole EM methods add vertical resolution to surface surveys:

• Downhole conductivity logging provides a detailed conductivity profile along a borehole, useful for identifying lithological boundaries and contaminated zones.
• Cross-hole tomography measures EM responses between boreholes to produce 2D or 3D images of the conductivity distribution between wells.

Interpreting Electromagnetic Data

Data Presentation and Interpretation

EM data are displayed in three main formats, each suited to different interpretation goals:

• Profiles: show the EM response along a survey line. Good for spotting anomalies and lateral conductivity changes.
• Maps: show the spatial distribution of the EM response across the survey area. Useful for identifying conductive zones and structural trends.
• Depth sections: show how conductivity varies with depth. These reveal the vertical distribution of conductive layers and targets.

Conductive targets appear as anomalies with higher amplitude and/or a more pronounced phase shift relative to the background. The anomaly shape tells you about the target geometry:

• Narrow, high-amplitude anomalies typically indicate shallow, compact conductors.
• Broad, low-amplitude anomalies suggest deeper or more laterally extensive targets.
• The orientation of an anomaly can reveal the strike and dip of the conductive body.

Advanced Interpretation Techniques

For FDEM data, the quadrature and in-phase components serve different roles. The quadrature component is your primary tool for mapping conductivity structure. The in-phase component helps identify magnetic materials and estimate magnetic susceptibility, but its sensitivity to both properties can complicate interpretation.

For TDEM data, the decay curve contains depth information:

• Early-time measurements (shortly after transmitter shutoff) are sensitive to shallow conductors.
• Late-time measurements sample progressively deeper, as the eddy current system diffuses downward through the subsurface over time.

Forward modeling and inversion are the standard tools for extracting quantitative conductivity models from EM data:

1. Forward modeling: You define a conductivity model, calculate the EM response it would produce given your survey geometry, and compare the predicted response to your measured data.
2. Inversion: The conductivity model is adjusted iteratively to minimize the misfit between calculated and observed data. The algorithm searches for the model that best explains your measurements.
3. Common inversion approaches include least-squares, regularized (which adds smoothness or other constraints to stabilize the solution), and smooth-model inversions.

Inversion is never unique. Multiple conductivity models can fit the same data, so geological constraints and independent information (e.g., from drilling) are essential for producing reliable interpretations.