Magnetometry
Magnetometry is a non-invasive geophysical survey method that can be used in archaeology to detect and map variations in the earth’s magnetic field caused by archaeological objects or artefacts beneath the soil. Within the Dutch AMZ cycle, magnetometer surveys can be applied during the exploratory and mapping phases for both terrestrial and maritime contexts to locate (buried) archaeological structures.
What?
How does it work?
A magnetometer is an instrument used to map the direction and strength of the Earth’s magnetic field, allowing for the measurement of deviations in that field (figure 1). These anomalies are caused by local iron-rich (ferromagnetic) objects or sediments, even when they are covered by soil. By systematically recording readings with a magnetometer along transects, a magnetic image of the soil and subsurface can be generated, useful for investigating both land and underwater sites.
In-depth explanation X
Magnetometer prospection is based on the physical phenomenon of magnetism. A magnet always has a north and a south pole, and the area where magnetic forces are at work is known as the magnetic field. The Earth has its own magnetic field, which varies in strength and direction across the surface and changes over time (Breiner, 1999; Philip Kearey, 2002; van Hoek and Leo, 2018).
Anomalies are local variations in the Earth’s magnetic field caused by ferromagnetic objects. Examples include iron shipwrecks, unexploded ordnance, or even geological structures due to their mineral composition. In a magnetic anomaly, the total magnetic field vector changes, meaning the direction of the magnetic field shifts. These changes can be measured and analyzed to determine the location and nature of the anomaly (figure 2) (Marine Magnetics Corp, 2007; Philip Kearey, 2002; Reynolds, 2011).
The use of a magnetometer in archaeological fieldwork often follows a systematic approach. Geophysicists walk, drive, fly, or sail with a magnetometer according to a grid plan in a designated area, continuously collecting data. A magnetometer records various components of the magnetic field, including the total field strength, the horizontal component, and the vertical component (Carbon Trust, 2020). The resulting data is filtered, analyzed, and visualized to create a magnetic map of the subsurface. The magnetic values are measured in nanotesla (nT).
What do you need?
There is a wide range of suitable yet very different instruments which can be used to measure the magnetic field of the Earth. However, for the specific application of archaeological prospection, robust yet sensitive instruments are required that can be operated easily and quickly in the field (figure 3).
In addition to the magnetometer itself, the entire system consists of several components necessary for correctly applying this sensing technique. These are briefly described below:
- Positioning System: This is used to determine the location of the magnetometer. It can be a Global Navigation Satellite System (GNSS) or a Real-Time Kinematic (RTK) GPS.
- Positioning Software: This is essential for recording the magnetometer’s location. The results are linked to the data collected by the magnetometer itself.
- Software: Specialized software is required for processing, analyzing, and interpreting the data. Some examples include Oasis Montaj, Sensys Magneto, Geometrics MagPick, TerraSurveyor, Golden Surfer, and Geoplot.
Different types of magnetometers X
There are two types of magnetometers distinguished in archaeology, with the primary difference being the type of information they measure and provide about the Earth’s magnetic field. These are:
- Gradiometers / Vector Magnetometers: These measure the direction of the Earth’s magnetic field at a specific location. Gradiometers are widely applicable because they are relatively resistant to magnetic interference from modern metal structures. Although they are sensitive enough to map archaeological traces in most places worldwide, they may sometimes lack sufficient sensitivity in the Netherlands, where the contrast between natural soil and archaeology is generally quite low. These devices can be used with motorized vehicles (provided they are equipped with magnetic shielding), allowing for rapid surveys of large areas. Examples of this type of instrument include the Förster Ferex, Lea-MaxX Fluxgate, or Bartington gradiometer.
- Total Field Magnetometers: These measure the total strength or intensity of the Earth’s magnetic field at a specific location, but can also provide more information about the type or composition of archaeological objects. Total field magnetometers are known to be more sensitive, generally yielding better results in case studies in the Netherlands. However, this increased sensitivity makes these instruments more susceptible to magnetic interference from modern metal objects or structures, making it impractical for surveys near construction sites or highways. Another drawback of this type of magnetometer is that it can only be operated hand-held. Examples of this type of instrument include the caesium magnetometers from Scintrex or Geometrics.
The decision to use a particular magnetometer system must be carefully weighed against the advantages and disadvantages of each system, and it will largely depend on the situation and the specific goals of the research. In the Netherlands, it is essential to ensure a sensitivity of at least 0.1 nT and a probe distance of 50 cm, as this provides a resolution of 50 x 25 cm suitable for mapping most archaeological traces
Can be used with..
Terrestrial contexts
There are various platforms that can be used for surveys on land.
In general, magnetometers can be hand-held, though this limits the amount of area that can be surveyed in a single day. For larger areas, a mobile gradiometer setup pulled by a vehicle is often used (figure 4). It is important, however, to take measures to prevent potential interference when using this method.
Currently, research is also being conducted using drones in combination with magnetometers, though this is still in an experimental phase. Due to potential interference, the distance between the ground and the sensor, and the tilt of the probes within the sensor itself, there is some loss of sensitivity. Given the relatively low magnetic contrast between the subsurface and archaeology in the Netherlands, drone magnetometry can only be used under certain conditions.
Maritime contexts
For surveys on waterbeds, a magnetometer is towed behind a survey vessel, as close to the waterbed as possible (see Figure 5). The length of the towing cable should be at least five times the length of the vessel to prevent unwanted effects from the engines and the vessel itself. It is important to accurately track the position, height of the magnetometer relative to the seabed, the orientation of the survey lines, and the distance between the survey lines.
Archaeological Applications
Place in the Dutch archaeological heritage management process
Terrestrial contexts
In the Dutch archaeological heritage management workflow (Field survey, Protocol 4006 Expert Analysis), the magnetometer is used to map human-made structures beneath the surface.
Maritime contexts
In the Dutch archaeological heritage management workflow (Maritime Field Survey) the magnetometer is deployed from a survey vessel to detect anomalies on or just below the seabed (KNA Waterbeds protocol 4103, pp. 5-7). Beyond identifying individual ferromagnetic objects such as shipwrecks or cannons, the background signal can reveal changes in the seabed’s composition, making it possible to detect buried and infilled prehistoric river systems.
What types of archaeological materials/landscapes
Magnetometer surveys are suitable for a wide range of archaeological findings. Fire-related structures, such as kilns, are relatively easy to detect. Magnetic traces can also be found in organic materials due to the presence of bacteria, allowing features like pits, post holes, and ditches to be effectively mapped—though only if there is enough contrast between the soil and the archaeology. Depending on the types of stone used, building foundations may also be identified. However, graves, cavities, and some types of foundations are more challenging to detect with a magnetometer.
Limitations/uncertainties
In the Netherlands, it can be difficult to achieve clear results from magnetometer surveys on land due to the low contrast between archaeological remains and the natural soil. The relationship between soil type and magnetic susceptibility (the degree to which a material becomes magnetized) is quite complex and dependent on many factors such as climate, material composition, gravity, topography, water, flora, fauna, and anthropogenic activity. Right now, there’s no definitive answer on which landscapes or soil types are best suited for successful magnetometer surveys.
Want to learn more?In-depth explanation X
Magnetometer surveys are not affected by moisture, such as rain or wet soil, but they can be influenced by high groundwater levels, which over the years may wash away magnetic minerals. The magnetic contrast between archaeological features and the surrounding soil can be estimated using a kappameter, which measures magnetic susceptibility with a sensitivity of 1 x 10⁻⁶ SI or 1 x 10⁻⁷ SI. This method requires direct contact with the ground, making it unsuitable for drone-based magnetometer surveys.
A crucial factor for obtaining clear results from a survey is the type of archaeological features expected. Strong magnetic signals, like those from a fired kiln, can be detected in almost any soil. However, distinguishing ditches or pits filled with organic material—slightly magnetic due to bacterial activity—requires enough contrast with the surrounding soil. Typically, structures smaller than 25 cm are difficult to detect, while those larger than 50 cm are often clearly visible, provided there is enough contrast. Depth is also an important consideration; while magnetometers can theoretically measure several meters deep, the signal weakens and broadens as the depth increases. In the Netherlands, anomalies deeper than 2 meters are likely to be undetectable in the survey data.
Current land use in the research area is another significant factor. Agricultural land, and specifically the type of crops which are cultivated, can impact the quality of measurements. Traces from farming machinery may obscure archaeological features, as they often also appear as small anomalies of just a few nanotesla in the data. Experience has shown that measurements are more disrupted by maize farming than by grain cultivation, even after the crops have been harvested. Additionally, external magnetic interference from motors and high-voltage power lines can disrupt measurements. The process of filtering raw data into usable information is therefore a specialized task.
There are, however, conditions where magnetometer surveys can work effectively. For example, surveys in heathland areas often yield good results due to the lack of agricultural disturbances. To avoid poor outcomes, it is important to evaluate the expected archaeology, soil type, and landuse to estimate the likelihood of successfully detecting features with magnetometers. Despite these limitations, magnetometers remain a valuable tool for archaeological research, provided their constraints are understood and managed appropriately.
Casestudies
Curious to see how magnetometry has already been successfully used in archaeological field research? Click on the tiles below to explore case studies where this innovative sensing technique is applied!
References/further reading
Aspinall, A. Gaffney, C. Schmidt, A. (2008) Magnetometry for Archaeologists, Altamira Press 208 pp.
Breiner, S. (1999). Applications Manual for Portable Magnetometers. https://doi.org/10.6067/XCV8DJ5JD6
Fassbinder, J.W.E. (2015). Seeing beneath the farmland, steppe and desert soils: magnetic prospecting and soil magnetism. J. Archaeological Science, 56, p. 85-95.
Fassbinder, J.W.E. (2016). Magnetometry for Archaeology. In: Encyclopedia of Geoarchaeology, Springer Verlag, p. 499-514.
Jelsma, J., Verschoof-van der Vaart, W.B. (2021). Meten of vergeten: Een inhoudelijke evaluatie van de toepassing van geofysisch onderzoek tijdens archeologische prospectie van landbodems, Rijksdienst voor het Cultureel Erfgoed.
Kattenberg, A.E. (2008). The Application of Magnetic Methods for Dutch Archaeological Resource Management, Amsterdam Institute for Geo and Bioarchaeology.
Lambers, L.S.L., Fassbinder, J.W.E., Lambers, K. & Bourgeois, Q.P.J. (2017). The Iron-Age burial of Epe-Niersen, the Netherlands: results from magnetometry in the range of +/- 1 nT. Jennings B., Gaffney C., Sparrow T. & Gaffney S. (red.), 12th International Conference of Archaeological Prospection. International Conference on Archaeological Prospection 12 september 2017 – 16 september 2017. Oxford: Archaeopress. 132-134.
Lambers, L.S.L., Laan, W., De Smedt, P., Ullrich, B., Kniess, S., Zoellner, H., … van Wijk, I. (2021). A geophysical multi-method approach to investigate the archaeological landscape of Lanakerveld (NL). ARCHEOSCIENCES-REVUE D ARCHEOMETRIE, 45(1), 169–173. https://doi.org/10.4000/archeosciences.9358
Linford, N., Linford, P., Martin, L., & Payne, A. (2007). Recent results from the English Heritage caesium magnetometer system in comparison with recent fluxgate gradiometers. Archaeological Prospection, 14(3), 151–166. https://onlinelibrary.wiley.com/doi/abs/10.1002/arp.313
Rensink, E. (2019). Factsheets Archaeologische prospective: Methoden, technieken en strategieën van Inventariserend Veldonderzoek, Rijksdienst voor het Cultureel Erfgoed.
Schmidt, A., Linford, P., Linford, N., David, A., Gaffney, C.F., Sarris, A. and Fassbinder, J. (2015) EAC Guidelines for the use of Geophysics in Archaeology: Questions to Ask and Points to Consider. EAC Guidelines 2. Namur, Belgium: Europae Archaeologia Consilium (EAC), Association Internationale sans But Lucratif (AISBL). ISBN 978-963-9911-73-4. 135p. http://hdl.handle.net/10454/8129
Scollar, I., Tabbagh, A., Hesse, A., Herzog, I., (1990). Archaeological prospecting and remote sensing. 1990, 674pp
Stele, A. Kaub, L. Linck, Schikorra, M. and Fassbinder, J.W.E. (2023) Drone-based magnetometer prospection for archaeology, Journal of Archaeological Science, Vol. 158, https://doi.org/10.1016/j.jas.2023.105818
Van den Brenk, S. en R. van Lil, (2021). Inventariserend Veldonderzoek (opwaterfase) Magnetische lineaties en inventarisatie wrakresten IJsselmeer en Markermeer. Periplus Archeomare rapport 20A015-04
Van den Brenk, S., (2023). Geofysisch inventariserend veldonderzoek, verdronken stad Reimerswaal. Periplus Archeomare rapport 22A001-01
Van den Brenk, S., H. Huisman, N.W. Willemse, B. Smit, en B. J.H. van Os, (2023). Magnetometer mapping of drowned prehistoric landscapes for Archaeological Heritage Management in the Netherlands. Archaeological Prospection. DOI: http://dx.doi.org/10.1002/arp.1925.
Van den Brenk, S., Cassée, R., (in prep). Magnetometeronderzoek kavels Almere. Periplus Archeomare rapport 23A034-01