Electrical Resistivity Survey
Electrical resistivity survey is one of the most commonly used geophysical techniques in the Netherlands. Within the AMZ cycle, this technique is primarily applied during the mapping phase of an archaeological field survey. Using electrical resistivity survey on terrestrial soils, underground archaeological features can be mapped to a depth of approximately 2 m.
What?
How does it work?
In an electrical resistivity survey, the electrical resistance of the top layer of soil is measured by placing electrodes (in the form of metal pins) into or directly onto the ground to inject current into the soil. This technique does not measure resistance at a specific depth but rather across the entire soil volume. By conducting systematic measurements along transects, zones with differing resistivity values can be identified, which may correlate with archaeological remains. Resistivity measurements are generally recorded in images where the registered values are visualized using a grayscale or color scale (figure 1).
The soil’s resistivity value is primarily influenced by moisture content, soil type (including pore size and grain size), and the presence of salts. For example, since water is highly conductive, moist clay has a lower resistivity value than dry sand. Organic material (such as humus-filled ditch deposits) generally retains a lot of moisture and therefore has a lower resistivity than the surrounding natural soil. In contrast, wall remains and foundations made of natural stone, brick, or concrete, as well as extraction trenches, rubble layers, and fill deposits, retain little moisture and thus have a higher resistivity value.
In-depth explanation X
Two key factors are essential in conducting an electrical resistivity survey: the survey depth and the measurement grid. The survey depth is determined by the distance between the electrodes (the probe separation). At a distance of 1 meter, resistance is measured to about 1 meter below the surface. The greater the distance between electrodes, the larger the soil volume influencing the measurement. Consequently, a larger distance results in lower resolution and a less detailed image. Additionally, all structures from the surface down to the specified depth are recorded, provided there are variations in resistivity. Typically, the electrode spacing (and thus survey depth) can range from 0.25 meters to 2 meters, depending on the expected depth of archaeological remains, the anticipated size of these remains, and the depth of natural deposits.
A single measurement point does not provide sufficient information, so multiple measurements are necessary. Therefore, measurements are taken in a regular grid. The spacing between grid points can be set beforehand, depending on the expected size of the archaeological features. For mapping the underground remnants of a castle, a 1 x 1 meter grid is usually sufficient, while smaller structures, such as ditches and burials, may require a denser 0.5 x 0.5 meter grid.
The survey speed depends heavily on the grid used. With a 1 x 1 meter grid, up to 0.5 hectares can be surveyed in a day using a frame, and up to 2 hectares with a hand cart. With a 0.5 x 0.5 meter grid, the number of measurements quadruples, allowing about 0.15 hectares per day with a frame and 0.5 hectares with a hand cart. Survey speed is also influenced by field obstacles, such as ditches, trees, etc., and by land use. For example, measuring a grassy field is easier than one with maize stubble, where cables may snag, potentially slowing down the survey.
What do you need?
Conducting an electrical resistivity survey requires specific equipment, including various electrodes (or probes) and a measuring device that both supplies electricity and records the resistivity values (figure 2). The measurements from an electrical resistivity survey consist of a list of data points (within a predefined grid) with the recorded resistivity values, expressed in Ohm-m. Several geophysical software packages, such as Geoplot, Snuffler, or Terrasurveyor, can process this data into a raster image.
Can be used with..
Electrical resistivity surveys can be conducted using different platforms. The most commonly used platform consists of a handheld frame with electrodes that are manually inserted into the ground (figure 3, left). Recently, resistivity measurements are also performed using carts, where electrodes mounted on wheels replace the handheld frame (figure 3, right). These carts can be pulled by a person or a vehicle. A significant advantage of these carts is their increased survey speed, allowing for a larger area to be covered per day. Additionally, other instruments, such as a magnetometer and/or a GNSS antenna, can be mounted on these carts and used simultaneously. A drawback of using a cart is less flexibility in setting the probe separation, as carts are typically limited to probe distances of 0.75 or 1.25 meters.
In addition to measuring ground surfaces in two dimensions, it is also possible to obtain vertical profiles and depth information by placing multiple electrodes in a line and measuring at different depths: Electrical Resistivity Tomography (ERT) survey (figure 4). The drawback of this survey type is its slower speed and the higher complexity of the measurements.
In-depth explanation X
In an electrical resistivity survey, there are several ways to position the electrodes, known as configurations. In Dutch archaeology, the Twin-Probe configuration is the most commonly used. In this setup, the measurement frame is connected via a long electrical cable to two additional electrodes positioned outside the area being surveyed. This configuration offers several advantages over others: measurements can be taken more quickly by walking in a zigzag pattern, the readings are less affected by geological variations in the subsurface, and anomalies manifest as a single peak rather than multiple peaks.
There are a few other configurations that are sometimes used in archaeology, including Wenner and Square:
– Wenner: In this configuration, all four probes are aligned in a row on the frame. This option is slower because the probes must remain in the same orientation. More importantly, the Wenner method yields much more complex results, as all pins traverse the archaeological features, resulting in multiple peaks for the same anomaly. This can be problematic on complex sites.
– Square: Here, the four electrodes are arranged in a square on the frame. An advantage of this setup is that there is no cable connecting the distant electrodes to the measurement frame. Additionally, different pairs of electrodes can be selected for measurement, which is beneficial for detecting linear anomalies (such as ditches).
In a standard resistivity survey, a single fixed measurement depth is set. However, with a device called a multiplexer, it is possible to simultaneously connect multiple pairs of electrodes with varying distances on the measurement frame. This allows for simultaneous measurements at different depths.
Archaeological Applications
Place in the Dutch archaeological heritage management process
Electrical resistivity surveying, like most other geophysical sensor methods, can be employed during the exploratory and mapping phases of an archaeological survey. The collected data provides an overview of areas with varying resistance values, accompanied by descriptions and interpretations. This information can be used to enhance our understanding of subsurface archaeological features and may inform future prospection or excavation efforts.
Want to know more?In-depth explanation X
Due to the slow nature of electrical resistivity surveying relative to other geophysical methods, this technique is generally employed in a targeted manner during the mapping phase. This can be achieved by combining resistivity surveys with other survey methods and techniques, such as faster geophysical techniques, borehole survey, test pits, or trial trenches. After a preliminary survey of a larger area—through surface mapping or aerial photo analysis—electrical resistivity surveying can focus on the mapping of specific locations, such as find or debris concentrations, or crop marks. Additionally, it can be used to investigate areas between excavation pits, for example, to trace the layout of foundations or ditches. However, it is always important that the results of electrical resistivity surveys are supplemented and validated in the field through corings or trial trenching.
What types of archaeological materials/landscapes
Electrical resistivity surveys can be employed in all situations where moisture differences between archaeological features and the soil are likely to be detectable. Whether these differences can be observed largely depends on the local conditions and the nature of the archaeological features (see table).
Electrical resistivity surveys provide sharp images under ideal conditions that are highly interpretable. Particularly, walls and foundations, deeply buried and/or rubble-filled pits, channels, (ring)ditches and even burial mounds typically appear as distinctly defined structures in the measurement results. In general, linear objects in the measurements are easier to identify than randomly distributed soil traces.
Limitations/uncertainties
There are three key limitations to electrical resistivity surveys: ground contact, moisture content, and time.
A significant limitation of electrical resistivity surveys is that the electrodes must be inserted into the ground to establish contact with the soil. Therefore, measurements cannot be taken on paved surfaces or hard ground. Additionally, raised or heavily disturbed topsoils can hinder resistivity measurements.
Since the resistivity values are largely determined by moisture content, electrical resistivity surveys should ideally be conducted when the moisture differences between the soil and archaeological remains are greatest. Thus, periods of (prolonged) drought, (heavy) rainfall, and frost are not suitable times for electrical resistivity surveys.
Finally, compared to other geophysical techniques electrical resistivity surveys are time-consuming.
Want to know more?In-depth explanation X
The negative impact on resistivity measurements caused by a raised or heavily disturbed topsoils is due to the high sensitivity of resistivity measurements at the surface level. In cases of hard surfaces or disturbed topsoil, electromagnetic induction (EMI) surveys may be a better choice, as this method is significantly less sensitive to structures close to the surface.
Drought is particularly problematic when conducting measurements on rapidly draining sandy soils (e.g., dune soils), but it can also affect other soil types during periods of extreme dryness. For instance, a clayey subsoil can become so hard that it is often impossible to establish contact with the electrodes. A sudden increase in soil moisture (e.g., due to heavy rain) can cause the topsoil to become extremely conductive, leading to a reduced measurement depth. Additionally, frost must be completely absent from the ground, not just from the surface but also from deeper layers, to avoid interference with the resistivity meter’s readings due to ice lenses.