Geomagnetic Methods
3.9 Applications and case histories
3.9.7 UneXploded Ordnance (UXO)
Location of short wavelength, high amplitude anomaly.
Location of short wavelength, low amplitude anomaly.
Location of long wavelength, high amplitude anomaly.
Location of long wavelength, low amplitude anomaly.
Drum grave
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Figure 3.68 (A) Magnetic anomaly map (bottom sensor) from the magnetic gradiometer and (B) the corresponding map of interpreted zones of influence of steel drums and drum graves. From Reynolds (2002), by permission. [C]
In addition to the magnetic survey, a seismic refraction investi- gation was also undertaken (see Chapter 5, Section 5.5.5). Where both magnetometry and seismic refraction data were obtained over the same transects, it was possible to integrate the analysis. From this it was possible to identify the locations of the drums and to see where within the tar they may be located. One such integrated profile is shown in Figure 3.69. It was clear that the drums tended to be located in the tar that had the lightest viscosity (as evidenced by the lowest seismic P-wave velocity values).
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Figure 3.69 (Top) Magnetic profile and model across the acid tar lagoon with modelled responses, and (bottom) a seismic velocity section with the locations of the four magnetic targets indicated. From Reynolds (2002), by permission. [C]
software has determined to be the x,y location above the target.
Other anomalies are present on the magnetic field data but have not been selected by the software. UXO-automated picking soft- ware may not be as reliable as manual selection by an experienced operator. In one survey over a former Ministry of Defence munitions site, the military UXO system detected 85% of targets identified by a manual operator. It also failed to deselect targets associated with known non-ordnance features such as a large manhole cover! It is essential that the known locations of such features are identified so that corresponding magnetic anomalies can be deselected. It is also helpful to use the analytical signal to help clarify features that may represent anomalies associated with targets of interest. To the eye, there is less clutter on the analytical signal data shown in Figure 3.71A than on the total field magnetic data in Figure 3.71B. Spe- cific correlations between the anomalies picked by the automated UXO-detection software can be seen in the screen capture shown in Figure 3.72. The target location is indicated by the position of the crosshair and the target identification number, target depth and target weight are displayed on the map and also indicated in tab- ular form and as separate graphical displays. The target weight is an effective weight based upon an assumption of the magnetic sus- ceptibility of the metal in the targets. More sophisticated database
correlations can give a more direct indication as to the type of mu- nitions that best fits the parameters determined from the magnetic anomaly. On sites used as artillery or bombing ranges, the types of munitions used should be well documented and thus the degree of uncertainty can be constrained. However, in areas of former mil- itary conflict, this information may not be so easily obtained. It should also be stressed that magnetic methods are not particularly useful in detecting anti-personnel mines that have been specifically designed to have minimal metal content to avoid detection.
Another aspect of importance is being able to identify whether a magnetic anomaly is being caused by a single piece of ordnance or a cluster. While on small-scale sites the identification of individual items of UXO is important, on much larger sites, wide-area assess- ments to map clusters rather than individual items may be sufficient (Gamey, 2008). The ratio of the target separation to sensor height (separation/height ratio – SHR) determines the amount of response overlap between targets. If the target separation is more than 1.5 times the sensor height (i.e. SHR>1.5), then the overlap is suffi- ciently small to indicate that the targets must be treated as discrete objects. If the SHR is between 0.5 and 1.5 there is some overlap, but it is insufficient to increase significantly the response amplitudes.
Individual peaks may still be recognisable, but it is difficult to
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Figure 3.70 Typical magnetic intensity amplitudes plotted as a function of depth below a magnetic sensor for a range of UXO types. From Stanley and Clark (2003), by permission.
determine the magnetic parameters of any constituent objects.
When the magnetic anomalies of objects overlap significantly, then the usual UXO performance metrics start to fail. It is also important to try to resolve individual targets but when their respective mag- netic anomalies overlap, this can mask diagnostic anomaly shapes.
To differentiate between targets, high-pass, two-dimensional filter- ing can be undertaken (Ren´e et al., 2008). This filtering process can
provide more accurate estimates of magnetic dipole parameters, and help to improve detection, discrimination and characterisation of identified magnetic anomalies and reduce false alarms.
Modelling of magnetic data to identify UXOs is complicated by the fact that the normal inversion assumes that the target is a spheroid of arbitrary size, shape, orientation and location. There are an infinite number of spheroids that can generate exactly the same
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Figure 3.71 (A) Analytical signal and (B) total magnetic field showing an anomaly (circled) picked by the automatic anomaly identification software (marked by the crosshair). [C]
Figure 3.72 Extract of screen view using automated anomaly-picking software showing (A) the selected anomaly with the target
identification number (204), target depth (1.94 m), and effective target weight (237.92 kg), with (B) a view of the associated database. [C]
magnetic dipole moment to satisfy the inversion of the observed magnetic field. If electromagnetic induction (EMI) techniques are used, such as with a Geonics EM63 TEM instrument, the resulting data can be inverted to help constrain the orientation of the target, which can then be used to refine the analysis of the magnetic data (Pasion et al., 2008). The cooperative analysis using both TEM and magnetic inversion can help to reduce the number of false alarms by being able to identify more clearly the targets of interest
Figure 3.73 Helicopter-mounted VG-16 high-resolution magnetic gradient system. Courtesy of Batelle-Oak Ridge Operations, USA, by permission. [C]
from clutter. This process has been taken further by developing a joint EMI-magnetometer instrument using a Geonics EM73 and Geometrics G823A caesium vapour magnetometer in one hand- held UXO-detection device (Wright et al., 2008).
To cover large areas of ground most efficiently it is possible to deploy multisensor magnetic gradiometers from a small heli- copter that is flown with a minimal ground clearance, typically 0.5–1.5 m (Figure 3.73). The system illustrated is the Batelle VG-16,
which has been designed to produce better production rates on a wider swath on wide-area assessment surveys, whereas the VG-22 (see Figure 10.22B) was designed for high-resolution detection of small ordnance under good field conditions (Doll et al., 2008a,b).
Depending upon the sensors deployed and their geometry relative to the aircraft being used, it may be necessary to compensate for the effects of the aircraft’s rotor blades as they pass across the sen- sor array (Billings and Wright, 2009) to improve data quality. To demonstrate the effectiveness of a low-altitude helicopter multi- sensor magnetic system, comparison surveys using a Hammerhead array, where the forward sensors are mounted on a rigid boom shaped like a T, and a TEM system (see also Figure 10.22A), took place over a test site at the Badlands Bombing Range, South Dakota, USA, where different types of munitions were buried along eight southwest to northeast trending rows (Figure 3.74; Beard et al., 2008). The measured results for the magnetic and TEM trial sur- veys are shown in Figure 3.75A and B, respectively. Both systems were able to detect the larger ordnance (bombs and most artillery rounds), but the Hammerhead system failed to detect many of the medium ordnance items, such as mortar rounds, that the TEM was able to detect. Some non-ordnance items, such as iron pipes, nails and rebar rods, produced large magnetic responses but small or no TEM responses. The average flying height for both surveys was about 1 m above ground level. The Hammerhead system was an early boom-mounted system and its sensitivity has been surpassed by later total field and vertical gradiometer systems. A compari- son of the results using the VG-22 magnetic system and a TEM-8 helicopter-mounted EM system is also illustrated in Figure 11.79 and discussed in Chapter 11, Section 11.3.3.6.
Airborne magnetic data can in some circumstances compare favourably with data acquired using ground-based systems; the
Figure 3.74 Indicative map of items buried along eight southwest to northeast trending rows at Badlands Bombing Range, USA. From Beard et al. (2008), by permission.
same cannot be said of airborne EM. The decay of the EM sig- nal with altitude is too great for low-altitude airborne EM data to compete successfully with ground-based EM systems, although sig- nificant development is ongoing in both airborne EM and magnetic UXO-detection technologies.
In ground-based surveys, it is strongly recommended that the background geophysical response of a site is determined as the first stage in a UXO-detection survey, in order to help refine the investi- gation technique to ensure that the targets being sought lie within the detection limits of the equipment. Depending upon the ambient noise levels, different survey specifications might be necessary, such as profile line spacing, sensor elevation and along-line sampling interval. It is also essential that the claims made by any Explosive Ordnance Clearance (EOC) contractor as to detectability with their equipment are verified by a third party, as there have been occasions when equipment to be used clearly and demonstrably had insuffi- cient detection range to identify the targets being sought. Had this not been checked, the EOC company might have given a site clear- ance certificate, when in fact they had not investigated the full depth of interest and a significant risk of UXO remained. Surface-based UXO systems have a limited depth of investigation, and if there is a demonstrable probability of large unexploded bombs (UXB) existing on a site, where the depth of burial may be up to 15 m in unconsolidated sediments, the only way to detect such devices is though borehole- or CPT-delivered magnetic sensors. In these cases a slim-line magnetometer is deployed either at vertical in- crements as a borehole is constructed, or via a CPT tool pushed into the ground. In both cases, the principle is that the magnetic sensor is looking a short distance ahead of the tool and to a radius around the hole or axis of the CPT profile, typically to around 1–2 m from the hole/CPT probe. Even in these cases, the maximum radius
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Figure 3.75 Anomaly maps from the Badlands Bombing Range, USA. (A) Total magnetic field anomalies using a Hammerhead total-field magnetic system, and (B) ORAGS-TEM response 230 ms after end of transmitter turnoff ramp for a large loop configuration. From Beard et al. (2008), by permission. [C]
of detection may vary considerably depending upon site conditions.
It is essential that the detection assurance level (i.e. 100% confidence that the target-size UXO can be detected) is indicated. It must not be assumed that because a detection range of 3 m was achieved on one site, that it will be the same on another; it may be significantly shorter. Detection ranges for even a 500 kg bomb may be as small as only 1 m. It is also important that EOC firms do not hide the technical limitations of their equipment using ‘military secrecy’ or
‘commercially proprietary technology’ as grounds to refuse to dis- close limits of detectability. If they do not provide this information, do not use them. Any reputable company should be prepared to provide such information. To exaggerate claims of UXO detection depths for surface UXO surveys or detection radii for CPT probe or drilling UXB methods could result in tragic consequences. It is crucial that any intending client has an independent qualified third-party check over any such claims thoroughly.
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