March 2005 Volume 2, Number 2

In this issue

 

 

 

 

 

 

 

 

 

Useful Links:

Solution matrix for road or motorway applications
The US Federal Highways Dept has published a very useful matrix of geophysical and NDT methods for solving engineering problems during SI, construction and maintenance of road infrastructure.

WW2 bomb risk maps
Download risk maps for your region (28 regions across the UK)

UXO frequently asked questions
Download a list of FAQ demystifying various aspects of unexploded ordnance

 

 

 

 

 

 

 

 

 

Standards and Guidelines for geophysical work:

ASTM D5777-00
Standard guide for using the seismic refraction method for subsurface investigation

BS 7022:1988
Geophysical logging of boreholes for hydrological purposes

ASTM D4428M-00
Standard test methods for crosshole seismic testing

BS 98/564053
Code of practice for investigation of potentially contaminated sites

BS CP 1021:1973
Code of practice for cathodic protection

Geophysics for civil engineers – An introduction
Briefing note by the Institute of Civil Engineers

Geophysics in engineering investigations
CIRIA, 2002
ISBN 0 86017 562 6

 

 

 

 

 

 

 

 

 

Contact Us:


www.zetica.com
info@zetica.com
Tel: 01993-706767

Chairman
asger@zetica.com

Managing Director
mike@zetica.com

Technical Manager
jon@zetica.com

Copyright © 2005

Facts about unexploded ordnance – detection radius

As we’ve discussed in previous newsletters, many companies have received ambiguous guidance concerning various issues affecting UXO risk assessment. One of the most contentious and potentially dangerous issues is the detection radius claimed by different operators for probe or drilling based bomb detection methods.

Zetica will shortly publish a unique guide on the detection radius of unexploded bombs. This result of peer-reviewed research is the culmination of 3 years of collaboration with the University of Liverpool. For the first time unequivocal evidence of practical magnetometry detection radii for ferrous bombs in different orientations and in different burial settings, is available.

An extract from our guide showing detection radii for a 50kg bomb in typical high to low noise environments is shown below.

Bomb Type

50kg

Ambient noise level

Low

Medium

High

High confidence of detection (m)

<1.5

<1

<0.5

Low confidence of detection (m)

1.5 to 2

1 to 1.5

0.5 to 1

No confidence (m)

>1.5

>1.5

>1

The results of our research are backed up by control experiments on Zetica’s own test site in Oxfordshire where a range of bomb sizes were lowered into a custom-built shaft in different orientations and their actual magnetic ‘fingerprint’ measured.

An example of the research carried out is discussed below. For each bomb size and orientation, 24 test positions were modelled (from the grid as shown) using proprietary software to produce a theoretical anomaly (verified by site data), which was also superimposed with actual site noise (ranging from high to low noise magnetic background).

An inversion function was applied to estimate the distance, as well as the depth and moment parameters. An example of the output of the programme is shown below for a 50kg bomb in a north-south orientation on a site with low noise.


Modelled anomaly (dotted line) and data (solid) for a 50kg bomb at various detection distances in a low noise setting.

The key point to note in this example is that in theory a 50kg bomb can be found at 2m distance but in practice this would be impossible with the level of noise as shown here.

The output of the modelling and inversion permutations was combined to produce so-called distance bias plots. Based upon these plots, a detection radius table was obtained with threshold settings at cut-off points where the estimation becomes statistically ambiguous. The high confidence region is defined as the overlapping part when both distance bias and depth bias are relatively small (In bias plots shown below for a 50kg bomb in a north-south orientation, ”blue” indicates a small bias value). Usually, beyond this region, the inversion of distance can still achieve reasonable results in some areas, which are defined as region with ambiguity (low confidence). Outside this ambiguity region, no anomaly can be detected in the data hence it does not contain any information (no confidence).


Depth (left) and distance (right) bias plots for a 50kg bomb in a low noise setting

So the moral of the story is? A radius of detection can only be objectively estimated based on actual site conditions. 3m for a 250kg bomb on one site may only be 2m on another and vice versa. This can have a significant impact on the costs of mitigating the risk of UXB on your site. Those operators who ignore these facts are either putting your operations at risk or charging you more than you need to spend.

Zetica is dedicated to providing the highest standards of ordnance and explosive contamination advice to customers in the UK and Europe.

Click here to request a copy of our guide to detection radii.

Inner city shear wave seismic survey results

We reported last time that Zetica had won a prestigious contract to detect intrusive dykes and map depth to rock head using shear wave reflection seismics as part of the ground investigation for a tunnelling scheme in Northern Ireland.

Having successfully proved the method during a 3 day trial, Zetica were commissioned to complete the entire survey totalling over 2850 line metres of profiling data.

The figure below is an example of the final product. From the top: seismic section converted to depth using interval velocities gained from the processing; interval velocity panel showing picks; plan of line location and associated borehole locations; schematic interpretation of seismic data, with nearby borehole information for correlation, showing location of two possible dykes.

In total 15 possible dykes were identified across the survey area. These dyke locations were then plotted on a plan and extrapolated along their expected strike angle to the intersection with the proposed tunnel route. This has provided engineers with important information for the tunnel construction and will help reduce the risk of unforeseen delays.


Open Day - the UK's first brownfield SI test site

Zetica, Lankelma and Norwest Holst will jointly host an open day in June 2005 for developers, engineering consultants, and contractors at our test site at the Enstone airfield near Chipping Norton, Oxfordshire.

Have you ever questioned how cone penetrometer tests are carried out and how results relate to measurements from boreholes and the surface? Are you interested in the capabilities of geophysics to map buried hazards such as unexploded ordnance, buried tanks or voids? Would you like to see wireline and crosshole geophysics being demonstrated to characterise near-surface geology?

Visitors will be able to experience a variety of surface and borehole geophysical methods and geotechnical methods first hand, as well as participate in interactive discussion sessions. Invitations will be sent out closer to the date. This is an event not to be missed.

Click here to email Ellen Stevens to register your interest in attending our open day.

New website launched

Zetica is pleased to formally announce the launch of our new website which we believe is more user-friendly, more easily navigable and more content rich in important areas such as case histories and guidance documentation. If the number of hits in recent weeks is anything to go by it has gone down well with customers! Newsletters, upcoming events and recent publications can all be found on the News page which we urge you to bookmark to stay abreast of latest developments.

Visit www.zetica.com and let us have your feedback.

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.