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GPR 6

Ground-Penetrating Radar

Ground-penetrating radar (GPR) is a widely used geophysical technique in the Netherlands. Within the AMZ cycle, it is primarily applied during the mapping phase of field surveys. Applying GPR in terrestrial contexts enables mapping of subsurface archaeological features down to approximately 4 meters, supporting the detection and documentation of buried traces.

What?

How does it work?

In a ground-penetrating radar (GPR) survey, an antenna sends electromagnetic waves into the ground. When these waves encounter boundaries (reflectors), such as the transition between different soil layers or between a soil layer and an object, a portion of the waves is reflected back. The remaining waves continue deeper into the ground and reflect off other boundaries. By measuring the time between the emission and reception of the waves, the depth of each reflection can be calculated. This process allows for the detection of subsurface structures based on differences in material properties.

By moving the antenna along a survey line, vertical cross-sections of the subsurface (GPR profiles) can be obtained. By systematically measuring such profiles and then digitally combining them, a three-dimensional image of the ground can be generated, allowing for the visualization of horizontal planes at different depths (so-called time slices) (figure 1).

Figure 1: An example of the results of a GPR survey (source: ArcheoPro).
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In-depth explanation X

In a GPR survey, the frequency of the antenna (ranging from 50 to 1000 MHz) is crucial. This frequency determines both the maximum measurement depth and the resolution (table 1). A low frequency provides a greater measurement depth but lower resolution, while a high frequency offers limited depth but higher resolution. The optimal antenna(or combination of antennas) for the best results depends on the research question as well as the size and depth of the archaeological remains.

Table 1: The relationship between the measurement frequency, depth limit, and resolution scale of GPR results (source: Geofysisch onderzoek – grondradaronderzoek | Rijksdienst voor het Cultureel Erfgoed).

In addition to the frequency, the depth range also depends on the number of reflectors (such as cables, pipes, or debris layers) in the (direct) subsurface. Each reflector sends a portion of the emitted waves back, which causes the emitted signal to weaken progressively. Certain types of soil and groundwater also lead to significant attenuation of the signal (see below).

During a GPR survey, measurements are typically obtained in a systematic, grid-like pattern. The distance between measurement lines (the so-called crossline distance) and the distance between measurements on each line (the so-called inline distance) can be adjusted depending on the size of the expected archaeological features or objects. For large research areas, a grid pattern is typically used, with measurement lines spaced 1 meter apart. Within each line, a measurement is taken every 5 cm.

What do you need?

A GPR survey requires specific equipment, including an antenna (containing both a transmitter and receiver), a data logger, and a control unit (figure 2).

A data logger is a device that stores all the measurement data collected by the antenna. During a GPR survey, the electromagnetic waves emitted by the antenna and reflected by different soil layers are recorded as digital data. The data logger continuously registers this data and stores it for later processing and analysis.

The control unit is the interface through which the user operates the GPR system. With this unit, the user can adjust the measurement settings. In some modern systems, the control unit can also display the stored data from the data logger in real-time via a live feed.

The measurements from a ground radar survey consist of GPR profiles. There are several geophysical software packages, such as EKKO_Project, Geoplot, Surfer, or Terrasurveyor, that can process this data into time slices and raster images.

Figure 2: Example of a complete GPR system in use during a survey (source: Ferry van den Oever, Saricon).

Can be used with..

The most commonly used platforms consist of a (hand) cart with wheels or a sled for moving the antenna. Using a sled has the advantage of maintaining better ground contact on uneven terrain (see also below).

Figure 3: Example of a GPR system mounted on a sled, pulled by an ATV  (source: BNN Vara © photographer: Rob Buiter).

The cart or sled can be pulled either by hand or by a vehicle, such as an ATV (Figure 3, above). During a survey, the antenna can be moved across the terrain at a speed of approximately 3 to 20 km per hour. Nowadays, measurements are often recorded using GPS or a local measurement system. The latter is used, for example, when conducting surveys inside a building (Figure 4, below).

Figure 4:  Example of a GPR survey conducted inside of a building (source: Ferry van den Oever, Saricon).
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In-depth explanation X

Nowadays, GPR systems are available that measure with multiple antennas (and at multiple frequencies) simultaneously. This allows for the detailed mapping of both shallow and deeper remains in a single pass.

Archaelogical Applications

Place in the Dutch archaeological heritage management process

Like most other geophysical methods, GPR surveys can be applied during the exploratory and mapping phases of an archaeological inventory. The collected data provides an overview of areas with anomalies, along with descriptions and interpretations. These results can be used to better understand subsurface archaeological features and to inform further prospecting or excavation research.

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In-depth explanation X

A major advantage of GPR survey is the ability to measure on a variety of surfaces, even if these are hardened or located within a building. This makes GPR particularly useful for research in urban areas where other geophysical techniques are often ineffective due to surface paving (concrete, asphalt) or interference from above-ground metal objects. GPR can, in principle, also be used on open water, peat, and snow, although there are certain limitations.

Given these advantages and the relative speed of GPR surveys, this technique is well-suited for providing an initial indication of anomalies (and archaeology) in the research area. Specific zones with potential archaeological remains can then be further investigated using other prospecting methods and techniques, such as augering, test pits, or trial trenches.

For large areas, it is recommended to conduct a pilot survey to assess the suitability of GPR. Additionally, it is always important to complement and validate GPR results in the field using augering or trial trenches. (Supplementary) augering is necessary to calibrate the depth readings of the GPR.

What types of archaeological materials/landscapes

GPR surveys can be used in any situation where it is likely that there is a contrast between the physical composition of the objects or features and the (natural) soil in which they are embedded. Whether these differences can be detected depends greatly on local conditions and the reflective properties of the objects themselves.

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In-depth explanation X

Under ideal conditions, GPR surveys produce highly detailed images, with generally higher resolution than that of other geophysical techniques. GPR is suitable for detecting natural and brick walls, foundations, trench fills, hollow spaces (such as cellars), as well as geological layers and stratigraphy. Pits, ditches, and moats can be identified if there is a physical contrast between these features and the surrounding soil. In forensic archaeology, GPR is also used to locate individual graves. However, line- or layer-shaped features are generally easier to identify in the data than randomly distributed features.

Limitations/uncertainties

There are three main limitations to ground radar surveys: ground contact, signal attenuation due to soil type and moisture, and depth determination.

One limitation of ground radar surveys is that the antenna must stay in contact with the ground during measurements. Therefore, terrains with obstacles, such as stumps, bushes, or – when indoors – thresholds, are less suitable for GPR surveys. Strong above-ground reflectors, such as large vehicles, metal fences, or walls, in or near the survey area can also interfere with measurements. However, these disturbances are generally easy to recognize in the data.

Additionally, GPR is less suitable for areas with high clay content or a high groundwater table, as these factors significantly affect the depth which can be reached by the radar waves. Clay and moisture in the soil strongly influence wave attenuation: clay, for example, causes rapid attenuation, resulting in a limited depth range. Groundwater acts as a strong reflector, generally preventing measurements below the water table. Reflectors near the surface, such as debris layers or numerous cables and pipes, also limit the depth range.

Finally, determining the precise depth of a reflection is complex and depends on various soil properties. It is essential that the method used to determine reflection depths is clearly explained in the survey report.

Casestudies

Curious to see how Ground Penetrating Radar (GPR) has already been successfully used in archaeological field research? Click on the tiles below to explore case studies where this innovative sensor technology is applied!

References/further reading

Conyers, LB. 2004: Ground Penetrating Radar for Archaeology, Walnut Creek.

Conyers, LB. 2012: Interpreting ground-penetrating radar for Archaeology: An introduction for Archaeologists, Walnut Creek.

Gaffney, C. & J. Gater 2003: Revealing the buried past: Geophysics for archaeologists, Stroud.

“Grondradar voor onder het oppervlak,” Vroege Vogels, BNNVARA,  02-02-2018. Retrieved from https://www.bnnvara.nl/vroegevogels/artikelen/grondradar.

Jelsma, J. & W.B. Verschoof-van der Vaart (red.) 2021: Meten of Vergeten. Een inhoudelijke evaluatie van de toepassing van geofysisch onderzoek tijdens archeologische prospectie van landbodems (Rapportage Archeologische Monumentenzorg 266).

Schmidt, A., P. Linford, N. Linford, A. David, C. Gaffney, A. Sarris & J. Fassbinder 2016: EAC Guidelines for the use of Geophysics in Archaeology. Questions to Ask and Points to Consider (EAC Guidelines 2), https://europae-archaeologiae-consilium.org/eac-guidlines.