By Kelsey Lowe
Archaeological geophysics techniques are tools to map, locate and produce images of buried cultural materials and features. There are several different methods that can be used depending upon the presence of different physical properties below the Earth’s surface. Differences between buried cultural materials, such as a pit, wall or burial, and the background soil will result in an ‘anomaly’. For archaeologists, it is these anomalies that we are interested in finding.
What is ‘Geophysical Survey’?
Geophysical survey uses non-destructive techniques to ‘look’ below the surface. It involves no disturbance of the ground and causes no damage to objects or features and therefore is a great reconnaissance tool. Data is collected by either dragging or carrying an instrument across the ground using tightly spaced survey transects. Maps are then produced using the collected data to show patterns or trends (‘anomalies’) that can be used to suggest future excavation locations. Only excavating these features can reveal their true form to determine whether they’re cultural (produced by people) or not, which is when we turn to standard archaeological excavation techniques to further our investigations (such as described in our blog post about Wondai Gumbal).
How are we using it?
For the Native Mounted Police project, we are using two instruments: a ground-penetrating radar (referred to as GPR) and a magnetic gradiometer. GPR works by pulsing radar waves into the ground. When the radar energy encounters (for example) walls or floors, part of the energy is reflected back to the surface and is measured by the GPR’s receiving antenna. We chose GPR as one of our key techniques because it provides 3D images of material below the surface. In particular, we are using it to located buried floors, walls and paths associated with the NMP camps.
In contrast, magnetic gradiometers do not emit anything into the ground. Instead, they measure the strength or changes of the Earth’s magnetic field across an area. Differences in this local field (anomalies) are caused by iron (metal) or fire, such as kilns and pits. We chose the gradiometer for our project because it can be used to cover large open areas very quickly. In addition, we are interested in locating iron-rich material, such as burnt features, buried metal and dumps, so it is perfect for our needs.
On one of the NMP camps we found that GPR was particularly useful for mapping buried floors.
In this case, the GPR data produced a rectangular-shaped image (what we call a ‘high reflection’ [this shows up as the red feature in the left image below]). This means that when the GPR wave hit the buried feature, the signal was very strong and produced a high reading (reflection). We interpreted this high reflection feature to be some kind of floor based on its shape and size and its location next to wooden posts.
To ‘ground-truth’ our interpretation, we excavated a small section of the GPR anomaly. As you can see on the image on the right, the ‘floor’ turned out to be a very hard and compact surface mixed with stones and gravel. It was clearly visible at around 5 cm below the surface, continuing to 10 cm below the surface. Made from crushed ant mounds and mixed with gravels, this floor created the anomaly or high reflection that we could see in the GPR data. Such ant bed floors were quite commonplace in colonial Australia, where residents were quite inventive in their use of locally available resources (see Lewis nd).
By excavating the GPR anomaly, we confirmed our interpretations of the geophysical data we had collected. This gives us confidence in interpreting other similar features in this (and potentially future) datasets that we perhaps weren’t or won’t be able to excavate.
Lewis, M. nd Australian Building: A Cultural Investigation. Section Three: Earth and Stone. E-resource.