Near Surface Geophysics: Applications
Two magnetic prospecting methods exist – magnetic susceptometry and magnetic gradiometry – to help archaeologists. Both rely on soil disturbance or enhancement of the soil magnetic properties.
Many phenomena will lead to a contrast in the magnetic susceptibility of a particular location in comparison with the surrounding soil – either enhancement or decrease in susceptibility. Examples include digging and refilling of trenches, compaction of the ground, presence of habitation (bricks, rocks, etc.) and fire.
Magnetic susceptometry will enable users to map a large site very quickly, then determine in which part of the site magnetic susceptibility enhancement can be observed, and thus in which location archaeological features are most likely to be found.
This mapping can be performed using the Bartington MS2/MS3 Meter in combination with the MS2D or MS2F probe.
Susceptibility mapping can also be used to reconstruct the palaeomorphology of a site by tracking the location of horizons of cultural importance (increased magnetic susceptibility). Use the MS2/MS3 Meter with MS2H for downhole measurement or MS2K for a trench.
[Photograph courtesy of PACEA, CNRS & University of Bordeaux]
Once a location with increased susceptibility is indicated, more detailed imaging is obtained with the Grad601-1 or Grad601-2 Gradiometer. This instrument will provide a precise image of ditches, houses, kilns, paths, roads, etc. The individual sensors contained within the Grad601 – the Grad-01-1000L – can also be mounted on a multi-sensors system for increased ground coverage in cultural profiling or mapping activities.
Before undertaking construction in a heavily shelled region such as Northern Europe, it is best practice to carry out surveys to check for the presence of significant underground features, whether UXO or archaeological monuments. Initial horizontal surveys can be performed with the Bartington Grad601 Gradiometer.
When building foundations such as concrete piles, the absence of important or dangerous elements underneath the drilling location should first be confirmed. The suggested methodology is to conduct a survey at surface level, and once the first few metres are seen to be clear, drilling can commence down to a few meters. At this stage, the drill head should be removed and the Grad-03 Gradiometer or Mag-03MSS lowered into the borehole to check the signatures. If nothing is detected, drilling can continue for a further few metres. This procedure is then repeated until the required drilling depth is achieved. Should an anomaly be detected indicating UXO, the area will need to be secured and careful digging carried out to remove the ordnance.
This process can also take place at sea, where for example a pipeline or telecom cabling is being laid down. In this case, a horizontal survey can be performed using the Grad-03 Gradiometer to detect the location of unexploded ordnance. Customers have also utilised the Grad-01-1000L sensor that they have made waterproof and assembled in their own system.
Similar to UXO, where digging is undertaken it is important to confirm the absence of underground structures such as pipes and cables that could be damaged. The Bartington Grad601 Gradiometer or Grad-03 Gradiometer can be employed to detect the presence of these installations. Pipes or cables will create a fairly strong and continuous anomaly that can be mapped to determine their location. Bartington Instruments has also designed bespoke sensors for integration into small pipe and cable location equipment.