Understanding the limits of detection of UXO

There are currently no recognised standards in the UK governing the advice provided or service offered to mitigate ordnance risk. As a result anyone can profess to be an expert which has led to clients receiving ambiguous advice on safety critical issues such as the limits of detectability of buried UXO. Exaggerated claims of UXO detection depths for surface surveys or detection radii for probe or drilling based bomb detection methods will have tragic consequences if left unchallenged.

Zetica has published a technical note spotlighting detectability issues and recommending ways of improving survey integrity and quantifying detection assurance levels.

UXO detection and clearance contractors are paid to certify that an area which has been scanned is clear of UXO before construction activities can commence. It is essential therefore to quantify a detection assurance level ie the distance from a sensor less than which UXO of a certain size can be detected with 100% confidence. This measure will vary from site to site and within a site depending on the variable composition of made ground or geology an the specific target size.

For example, on relatively ‘clean’ sites a 10kg High Explosive (HE) shell buried at 3m may be detectable from surface but the same target may only be detectable to 1.5m on more ‘noisy’ sites.

It is recommended that companies requiring UXO detection surveys specify that contractors clarify survey design parameters in terms of target size, depth of burial and anticipated or quantified site conditions Companies should also specify that equipment / processing capabilities should be certified by a 3^rd party.

Click here to access copy of technical note.

Statistical basis for assessing the risk of UXB to site works

To date, a formalised statistical approach for assessing the risk of unexploded bombs (UXB) to land developments has been lacking. A technical note published by Zetica in Ground Engineering in May 2006 introduces a statistically rigorous method for quantifying these risks so that the potential impact of a few boreholes on a site or a dense grid of clustered piles can be measured.

The recommended steps for dealing with the threat of UXB are:

1. Determine the overall risk of UXB existing in an area by downloading public domain regional risk maps (www.zetica.com/uxb_downloads.htm)
2. If a risk is indicated, conduct or commission a desktop study of available site specific evidence including records of known enemy air-raids, the MOD’s official register of abandoned UXB, contemporary aerial photographs, the presence of strategic targets such as munitions facilities and firing ranges, to name but a few.
3. If the potential for UXB is highlighted by the desk study, then the threat of detonating any UXB should be formally assessed. This involves a review of development plans including site investigation methods (eg boreholes, CPT’s), foundation designs (eg raft or piled), records of post-war land use, composition of made ground, and near-surface geology. To date sites have been qualitatively ranked as low, medium or high risk in terms of the likelihood of detonating UXB. The qualitative and often subjective nature of this ranking process has been brought into question.
4. If a significant threat exists then the process moves towards appropriate risk mitigation. A new CIRIA steering group has recently been set up to standardise UXB risk mitigation methodologies.

Zetica’s approach (UXB Probability Calculator ‘UXB-PC’) computes the probability of detecting multiple ellipsoidal UXB by means of either random (boreholes or CPT for SI) or regular grid (pile layout) sampling. In the case of uniform simple random sampling, the probability of striking at least one of multiple UXB can be computed both analytically in closed form using probability theory, as well as numerically using Monte Carlo simulation, and results are almost identical.

Probability of detonation for regular grid of different spacing and 1UXB/ha, with and without a 2m zone of influence.

The method provides a quantitative and validated measure of the risk of striking a UXB which can be combined with other land development and construction risks.

Click here to request more information.

Multichannel GPR survey of ballast to measure cross fall

A commission for West Coast Engineering in the UK has been completed using a trolley-based multichannel GPR system to scan ballast cross fall post-renewal for the blockade between Crewe and Wilmslow.

A combination of 400MHz, and 900MHz antennae were deployed to meet the objectives. An example of the customised output produced is shown below.

Graphic showing Trackside assets, 2 no. 400MHz (cess and 6ft) radargrams and 1 no. 900MHz (4ft) radargram, thickness plots and calculated gradient with accepted limits (crossfall).

Technical Manager, Jon Gascoyne:
“This survey has shown the power of dedicated processing software to accurately pick layer thickness from 3 variable frequency antennae with variable ride height over ballast. The survey was carried out on schedule and to budget.”

Click here to request more information.

Highways Agency research subcontract with Atkins

Atkins has recently been awarded a Highways Agency contract to further investigate the use of routine data to assess pavement foundation performance. Atkins’ geotechnical and pavement engineers assessed readily available HRM ride quality data (obtained to HD29/94) in conjunction with geotechnical as built records to review pavement foundation performance of one section of road.

At present HRM data is only used to assess pavement surfacing performance. By developing a technique to use the data to assess foundation performance it is possible to improve the understanding of overall pavement performance on existing roads, and draw upon the findings to improve industry good practice in pavement foundation construction and maximise the benefits of non-traditional pavement design.

Zetica was originally contracted by Atkins to collect a range of GPR data (200MHz to 1.2GHz) in a first phase which proved successful (see example below).

Graphic showing 900MHz (top) and 1,200MHz (middle) with layer thickness plots and calibration core overlaid – M4, UK

Zetica has also been invited to participate in the second phase of the research which will involve a full trial including TRACS data capture + GPR (high and low frequency) on a custom-built trolley.

GPR data is acquired on active carriageways with Zetica’s high-speed GPR pavement assessment system (HiPAS) in accordance with the general requirements laid out in Volume 7 of the Design Manual for Roads & Bridges (DMRB) HD29/94.

The HiPAS system combines multi-channel GPR, differential GPS and digital video data to provide an extremely high level of positional accuracy (<0.5m laterally) that helps ensure accurate location of the GPR data with respect to calibration core information. Developed in 2003, this system enabled Zetica to obtain one of the highest scores in the layer thickness accuracy test during the Second GPR Performance Trials organised by the UK’s Transport Research Laboratory (TRL).

Click here to request more information on Zetica’s HiPAS service for pavement investigations.

Research & Publications

Zetica is ideally placed in the UK to undertake important applied geophysics research through a combination of relevant skills and experience and close links to the University of Liverpool. Zetica provides a state-of-the-art test site in Enstone, Oxfordshire with unique facilities for testing surface and borehole geophysical equipment and methodologies. Zetica's test site is the UK's only civilian proving ground for UXO detection technologies.

• Particular areas of research expertise include:
• Unexploded ordnance detection methods
• Utilities detection using GPR
• Buried hazard mapping using integrated surface geophysical methods
• Trackbed investigation using high speed GPR
• Engineering geophysics applied to geotechnical design

Click here to access list of publications.

Click here if you are interested in partnering to research and develop new non-invasive investigation methods.

Technical note - ReMi

Commonly used techniques of measuring shear velocities and deriving Gmax (value of shear modulus at very small strains) such as cross-hole and down-hole seismics are relatively costly to execute and the cost-benefit can be further degraded in noisy urban settings, in heterogeneous geological settings such as variable boulder clay or in settings where the coupling between borehole and the geological medium is poor.

The refraction microtremor (ReMi) surface wave method overcomes these problems by using standard P-wave recording equipment and ambient noise to produce average one-dimensional shear-wave profiles down to 50m depths. The combination of simple recording with no source, a wavefield transformation data processing technique, and an interactive Rayleigh-wave dispersion modeling tool, exploits the most effective aspects of the microtremor, spectral analysis of surface wave (SASW), and multichannel analysis of surface wave (MASW) techniques.

The method has been very effective for cheaply carrying out liquefaction analysis, soil profile determination, mapping the subsurface and estimating the strength of subsurface materials.

A comparison of the shear wave velocity derived from surface wave measurements, cross-hole and down-hole on our test site in Oxfordshire is shown in the figure below.

The average surface wave velocities compare well to the cross-hole and down-hole results to about 15 - 20m. The lateral averaging of velocities and the decrease in resolution with depth inherent in surface wave methods explains the differences you see. Surface wave methods are not likely to resolve thinner layers (<5m) at depths greater than 15m.

The above figure also compares the 1D shear wave velocity profile with depth derived using a shorter array (16m) limiting averaging over the whole array length with a profile derived over the whole array (46m). Averaging effects are evident in the latter.

In summary, whilst the ability to resolve thin layers is greater with a cross-hole survey, the surface wave method gives you a more expanded view and shear wave velocities which are similar to 15 - 20m and broadly equivalent to greater depths. If the client is interested in average properties then ReMi can be a cost-effective solution.

Click here to request more information.

Lunchtime Seminars

Zetica offer a popular Geological Society registered seminar, with CPD points, on the uses and abuses of geophysics. Engineers are brought up to date on the latest geophysical methods available in the market place and interesting areas of research and development. The presentation normally lasts 30 – 40 minutes and is case history-based with a 15 minute discussion session following.

Click here to email Ellen Stevens to discuss your requirements and arrange a seminar at your offices.