496 ENGINEERING GEOLOGY/Geophysics munitions-factory sites, military bases, military training grounds, and wartime target areas Apart from low-metal landmines, most ordnance is composed of metal casing, which can be targeted by geophysical methods (depending on the size and depth of burial) In London, UK, which was heavily bombed during the Second World War, there may be a risk of the presence of unexploded bombs to depths of 10 m or more, depending on the size of bomb dropped and soil conditions Deep bombs such as these are undetectable using surface geophysical methods but can be detected using sensors lowered down boreholes or mounted in push rods An example of how geophysics can be used to mitigate the risk posed by unexploded ordnance is shown in Figure 16 A sitewide gradient magnetometer survey was carried out on a close grid spacing to detect small ferrous ordnance items These were then flagged in the field for identification and disposal Following removal of identified targets a second survey was carried out to confirm that all located ordnance had been removed Non-Destructive Testing Engineering geophysics forms a significant component of the field of non-destructive testing, which encompasses all methods used to detect and evaluate the integrity of materials Non-destructive testing is used for in-service inspection and condition monitoring of structures and plant including the measurement of physical properties such as hardness and internal stress Examples of geophysical methods used in nondestructive testing are tabulated in Table Containment Structures As well as playing a role in the assessment of ground conditions prior to construction of a containment structure such as a lagoon or dam, geophysics can also be used to effect in the assessment and monitoring of the integrity of a structure prior to and during use and in the investigation of breaches in containment during its lifetime In the latter case, where the storage of hazardous materials such as chemical and nuclear wastes is involved, a geophysical survey is often a cost-effective means of investigating the problem as it avoids the need to remove the stored material The most common requirement for geophysics at the pre-commissioning stage is in the assessment of the integrity of lined structures such as domestic landfills and leachate lagoons using the electrical leak location method This technique originated in the USA during the mid-1980s and has since been adopted widely It now forms a recommended part of the Construction Quality Assurance testing of new Figure 16 (A) The results of scanning an area using gradient magnetic profiling to detect ferrous ordnance items (B) The lo cated items were removed by trained EOC (Explosive Ordnance Clearance) personnel, and (C) the area was rescanned to prove that all items had been removed landfill cells in the UK The technique comprises a novel form of resistivity profiling, which takes advantage of the insulating properties of geomembrane materials, such as HDPE (high density polyethylene) and PVC (polyvinyl chloride), in order to pinpoint holes or tears as small as 0.5 mm2 Variants of the basic method have been developed for testing soil-covered liners, cell side slopes (with no cover material), and water-filled lagoons and ponds