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Near Surface Geophysics


- Archaeology

- Forensics

- Unexploded ordnance detection (UXO)

Two magnetic prospecting methods exist – magnetic susceptometry and magnetic gradiometry – to help archaeologists. Both rely on soil disturbance or enhancement of the soil's magnetic properties. Many phenomena will cause a contrast in the magnetic susceptibility of a particular location in comparison with the surrounding soil – either enhancement or decrease in susceptibility. Examples include the digging and refilling of trenches, compaction of the ground, the presence of habitation (bricks, rocks, etc.) and fire. 

Magnetic susceptometry enables 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. 

Once a location with increased susceptibility has been indicated, more detailed imaging can be obtained using the magnetic field gradiometer. This instrument will provide a precise image of ditches, houses, kilns, paths, roads, etc. 

Unexploded Ordnance (UXO)

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 magnetic field 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 magnetic field gradiometer lowered into the borehole to check the magnetic 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 magnetic field gradiometer to detect the location of unexploded ordnance. 

Pipe and Cable Location 

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 Smagall magnetic field 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. Smagall has also designed bespoke sensors for integration into small pipe and cable location equipment. 

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