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Is that ‘anomaly’ really a UXB?

Introduction

Zetica UXO was recently asked to help a regular client solve a conundrum. One of their contractors working on site had employed a UXO specialist to undertake a downhole magnetometer survey (MagDrill) during the installation of boreholes as part of a ground investigation. The UXO specialist had detected geophysical ‘anomalies’ in two of the boreholes at depths of approximately 5 m and 8 m. They suggested that these were Unexploded Bombs (UXB).

No detailed UXO risk assessment had been undertaken for the site. Our client commissioned us to undertake such an assessment to determine whether there was even the potential for a UXB to be present on the site. Our research concluded that the site was located in an area with a very low bombing density during World War One (WWI) and World War Two (WWII). There were no records to indicate that the site had been bombed. With no evidence of any military activity having taken place, we concluded that the site had a low UXO hazard level.

Worcester 3

Given the site’s historical use as a sewage treatment works, there were numerous potential explanations for the presence of magnetic anomalies on the site. Despite this, the contractor’s UXO specialist was adamant that the anomalies were potential UXB and that to mitigate the risk they needed to be intrusively investigated.  Given that the water table was shallower than 2 m, the prospect of excavating down to the anomalies provided a major engineering headache, not to mention engendering significant cost implications.

Investigation

Whilst our client was content that the anomalies were unlikely to be UXB based on our risk assessment, they felt compelled to undertake further investigation because of the emotive pressure being applied by the contractor’s UXO specialist. Unfortunately, the UXO specialist had not collected geophysical data when undertaking the MagDrill survey. As such, Zetica was unable to analyse the data at the anomaly locations to determine whether they had the potential to be UXB-related. Consequently, we undertook our own survey.

Using a cone penetration testing based method (MagCone), Zetica undertook additional magnetometer surveys at and surrounding the locations of the suspected anomalies. Geophysical data was collected for back-office analysis by our experienced geophysicists. We were able to demonstrate that no geophysical anomalies consistent with potential UXB were present at the borehole locations. The anomalies likely related to small ferrous metal objects, or geophysical noise associated with the background geology. This gave our client and, more importantly, the contractor additional comfort that construction works could proceed without further UXO risk mitigation.

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Figure – This plot shows the results of the MagCone survey at the borehole location where a ‘UXB’ was anticipated at 5 m depth.   An anomaly indicative of scrap metal was detected at approximately 3 m (see arrow) but there was no geophysical response indicative of a bomb at 5 m.

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Figure – This plot shows the same as the figure above, albeit with the anticipated response of a UXB at 5 m depth superimposed.

Conclusion

The project provides a case study for how industry good practice was not followed, specifically:

  • Undertaking UXO risk mitigation prior to a detailed risk assessment being completed.
  • Not collecting geophysical data for analysis during UXO detection surveys.
  • Scaremongering by suggesting that an ‘anomaly’ is a bomb without any positive evidence.

We see this far too often in the industry, where corners are cut and unrealistic conclusions derived from scant evidence.

Fortunately, on this occasion the ultimate client was able to call on Zetica to go back to basics and employ industry good practice. This minimised the expense and programme delays that would have been avoided entirely had such practice been followed from the beginning.