Electromagnetic Induction
Electromagnetic induction (EMI) is an active geophysical sensor technique used within the Dutch AMZ cycle, mainly during the exploratory phase of a field survey. It is also used on, for example, paved areas where other methods do not work well. While EMI works faster than many other geophysical techniques, it produces relatively less detailed imagery. Even with this, EMI is an effective method for the mapping of walls, ditches, and the (palaeo)landscape.
What?
How does it work?
The EMI technique is a variation of the ordinary resistivity survey. In a resistivity survey, the electrical resistance of the soil is measured to determine how well it conducts electricity. Here, a current is introduced into the ground using electrodes and then measured. In contrast, an EMI survey instead uses an electromagnetic coil to transmit the current into the soil, while a second coil measures the soil’s resistance. This method does not require direct contact between the instrument and the ground (figure 1).
The distance and orientation between the transmitting and receiving coil determine the depth of measurement. The greater the distance between the coils, the deeper the survey reaches (figure 2). However, it is important to note that, like all geophysical techniques, the EMI method does not provide a point measurement but rather a volume measurement. This means that the data collected represents a volume of soil beneath the coils rather than a specific depth. As a result, the measured resistance is not recorded at a fixed depth but rather up to a certain depth—though even this is only an approximate boundary.
It is also important to note that EMI measurements are sensitive to metal objects or structures in their vicinity. This is because the current introduced into the ground induces an electromagnetic field. Magnetic fields are influenced by metal, meaning that nearby metal objects can distort the EMI measurement, making it unreliable. The extent of this interference depends on the ratio between the size of the metal object and the distance between the coils. With a short coil spacing of 50 cm, even a small metal object, such as a horseshoe, can significantly affect the measurement. However, with a coil spacing of 4 m, the same horseshoe would have little to no impact on the results.
Want to know more?In-depth explanation X
EMI instruments are typically marketed as devices for measuring soil conductivity. Since conductivity and resistivity are mathematically reciprocal, the terms can be compared using the following conversion formula:
Resistivity = 1 / Conductivity.
This also makes intuitive sense: where resistivity is high, conductivity is low, and where resistivity is low, conductivity is high.
When determining the measurement depth of an EMI survey, the orientation of the coils must also be considered. The coil orientation influences the depth at which the EMI measurement is most sensitive (figure 3):
- Vertical coil orientation: When both coils are positioned vertically, the EMI survey achieves the greatest measurement depth (solid line in figure 3). Sensitivity is relatively low just beneath the coils, meaning that variations in the uppermost soil layers have little influence on the results. The maximum sensitivity occurs at approximately half the coil spacing and gradually decreases below that depth.
- Horizontal coil orientation: When both coils are positioned horizontally, the EMI survey behaves slightly differently. In this case, the highest sensitivity is directly beneath the coils, but it declines rapidly with depth (dotted line in figure 3). As a general rule, the measurement depth in this orientation is roughly half that of the vertical coil configuration.
Besides coil orientation, the operating frequency is also important. The current passing through the transmitter coil determines the depth of measurement—the higher the frequency, the shallower the penetration. Most instruments operate at such low frequencies, typically in the range of a few dozen kHz, that the depth effect is negligible, as the variation in frequency is not significant enough to cause a noticeable difference in penetration depth.
What do you need?
At its core, an EMI sensor consists of a single transmitter coil and a single receiver coil, which are used in conjunction to measure to a specific measurement depth.
These transmitter and receiver coils must be precisely positioned relative to each other. The easiest way to achieve this is by mounting them in a fixed position within a tube or frame. For this reason, most EMI instruments consist of an elongated tube, with its length largely determining the measurement depth (figure 4):
- Instruments with a tube length of 1 to 1.5 m are used for measurements up to approximately 2 m deep.
- Instruments with a tube length of 4 to 5 m can reach depths of 6 to 8 m.
Both types are used in archaeology, depending on the required measurement depth. For even deeper measurements, fixed tubes become impractical, and instead, separate coils must be precisely oriented for each measurement. However, these instruments are not used in archaeology.
In addition to the coils, a data logger is often attached to the tube. This device handles system calibration, data storage, and the acquisition of positional information. Regular calibration is required when using an EMI instrument. In most cases, calibration is managed through the data logger and performed several times a day. In rare instances, calibration must be done manually or even by the manufacturer.
EMI surveys are typically conducted in combination with a Global Positioning System (GPS) receiver, allowing measurement data and location information to be recorded simultaneously. For sites where GPS cannot be used, alternative local measurement systems are sometimes available, but this depends on the specific data logger in use.
In-depth explanation X
Several manufacturers produce EMI sensors. Originally, Geonics from Canada was known for developing highly reliable and precise EMI instruments. Their EM38 and EM31 models have been in use since the 1980s. More recently, GF-Instruments from the Czech Republic has emerged with a range of equally reliable and accurate EMI instruments. Other notable manufacturers include GSSI from the United States and Dualem from Canada, both of which produce high-quality EMI measurement instruments. Each of these manufacturers designs EMI instruments tailored to specific measurement depths (see above).
Modern EMI instruments incorporate additional technical enhancements to extract more depth information from measurements. Some systems use multiple receiver coils alongside a single transmitter coil, allowing measurements at different depths, sometimes even simultaneously. Additionally, varying the measurement frequency can influence penetration depth, enabling the collection of more detailed depth information.
Can be used with…
The main advantage of EMI instruments over other geophysical prospecting methods is their high measurement speed and straightforward deployment. The instrument consists of a single tube containing all necessary components, with a logger that is typically connected via Bluetooth. This eliminates the risk of faulty cables or moisture-related connection issues. Additionally, no electrodes need to be inserted into the ground, making EMI surveys significantly faster compared to other methods.
EMI surveys are typically conducted using either a handheld or towed configuration. Towing can be done manually or with a motorised vehicle such as a quad or tractor (figure 5).
In-depth explanation X
Recently, experiments have been conducted with EMI mounted under a drone, allowing for aerial surveys. However, this research is still in a highly experimental stage (figure 6).
Archaeological Applications
Place in the Dutch archaeological heritage management process
EMI sensors are primarily used in the exploratory phase of the AMZ cycle.
Compared to other geophysical techniques, such as resistivity measurements, magnetometry, and ground-penetrating radar (GPR), EMI produces less sharply defined and contrasting results. Structures like walls do not appear in EMI data as clear, well-defined lines but rather as broad, less distinguishable zones. As a result, this method is less suitable for detecting small, sharply defined archaeological features such as walls or pits.
However, EMI can be useful in the exploratory phase because it quickly provides a general overview of a large area. This allows for the identification of potential research locations, which can then be further examined in the mapping phase using other geophysical methods for more detailed data. In this zoom-in approach, EMI proves to be a very helpful research technique.
Another area where EMI is highly beneficial is in mapping larger structures where smaller archaeological traces are not yet a focus. For example, EMI is a highly effective method for mapping buried (palaeo)landscapes. Structures like mounds—sand rises beneath clay or peat—are often tens of metres large and can be quickly and efficiently detected with EMI.
An additional unique application of EMI across all phases of the AMZ cycle is its use on hardened surfaces, such as beneath paving or floors (figure 7). In such cases, other methods can be problematic: resistivity meters cannot be used because the electrodes cannot make contact with the ground, and magnetometry is hindered by environmental noise. If clay is present, ground-penetrating radar is also not ideal. In these situations, EMI becomes one of the few remaining viable options for conducting geophysical research.
What types of archaeological materials/landscapes
Small archaeological structures such as walls, wells, graves, etc., are difficult to detect on an individual level using EMI. However, clusters of walls, graves, or wells can be distinguished from areas where these structures are absent. While individual structures cannot be mapped, areas of interest can be identified, allowing for further detailed research to zoom in on these areas.
On the other hand, larger structures such as ditches or patterns of trenches can be very effectively mapped with EMI. It is important that the ditches or trenches have had the opportunity to acquire a contrasting fill. For example, a ditch in a clay area that is quickly filled with clay after being dug will produce almost no measurable contrast. It is also not always the case that ditches and trenches have lower resistance. Depending on the fill and hydrology, a ditch or trench can sometimes have higher resistance than the surrounding area.
Figures 8.1–8.4 display the results of a geophysical survey conducted at Bakersbos Castle in Deil, Gelderland. These images show how the use of EMI technology during the exploratory phase helped identify the location of the castle. Detailed resistivity and magnetometry surveys later provided precise images of the castle’s structures. Thanks to this zoom-in approach, an area of 1.5 ha was surveyed in just one day, with EMI playing a crucial role in pinpointing the correct location. Notice the difference in detail: EMI measurements provide a general overview, while resistivity survey and magnetometry offer sharper and more detailed results.
Figure 8.1: Result of the EMI measurement to a depth of 1 m below ground level.
Figure 8.2: Result of the resistivity survey.
Figure 8.3: Result of the magnetometry survey.
Figure 8.4: Archaeological interpretation of the site based on the different survey results..
Very large structures such as dunes, riverbeds, and sand ridges can be effectively studied with EMI, even if they are buried deeper. This is due to the larger size of these structures and the fact that they often have a clearly contrasting geological fill, making them easily detectable.
The images below compare the results of an EMI survey (figure 9.1) with a borehole survey (figure 9.2) conducted at the same location. It is clearly visible that in the blue areas, there is a coinciding depression both in the resistivity and in the sand layer. In places where peaks of red-brown-purple are seen in the boreholes, elevated resistivity is also detected in the EMI measurements. The use of EMI surveying at this location significantly reduced the number of boreholes required.
Figure 9.1: Result of the EMI survey to a depth of 6 m below ground level.
Figure 9.2: Result of the borehole survey with sand depth in metres NAP.
Limitations/uncertainties
As mentioned earlier in this technique description, the biggest limitation is the presence of metal. Metal fences, enclosures, tree protectors, sheds, reinforcement in concrete floors, etc., cause interference in the measurements, so no conclusions can be drawn about the archaeological significance of the measurements in those areas. It is therefore important to carefully record these metal objects on a field sketch. These zones are often easily recognizable in the measurements due to the distinctive transitions from very low to very high resistance, which are atypical compared to the other measurements in the survey area. Moreover, a negative resistance is already a clear indication that something is wrong with the measurements, as negative resistance is physically impossible.
Want to know more?In-depth explanation X
The sensitivity to metal is quite significant. Especially with shallow EMI sensors, even a small amount of metal can severely disrupt the measurement (figure 10). For example, if the operator has metal rings on their shoes, this can interfere with the measurement to such an extent that no usable archaeological results are obtained. In this case, the EMI device functions more as an advanced pedometer, as the metal rings move in front of the coils with every step, thereby affecting the magnetic field and rendering the measurement unusable.
Casestudies
References/further reading
Van Doesburg, J., van der Heiden, M., Waagen, J., van Os, B., & van der Meer, W. (2022). Op zoek naar lijnen: De waarde van elektromagnetische inductie en optische en thermische infraroodbeelden in Siegerswoude (Friesland). Rapportage Archeologische Monumentenzorg 273.
Rensink, E., Theunissen, L., Feiken, R., Bourgeois, J., Deforce, K., van Doesburg, J., Emaus, R., van der Heiden, M., de Jong-Lambregts, N., Karagiannis, N., de Kort, J. W., Liagre, E., van Londen, H., Meylemans, E., Orbons, J., Stichelbaut, B., Terlouw, B., Timmermans, G., Waagen, J., & van Zijverden, W. (2022). Vanuit de lucht zie je meer. Remote sensing in de Nederlandse archeologie. Nederlandse Archeologische Rapporten (NAR) 80.