In this issue
Basic explanation of various surface geophysical methods
Comprehensive round-up of borehole geophysical methods including discussion on quality control
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)
Standards and Guidelines for geophysical work:
Standard guide for using the seismic refraction method for subsurface investigation
Geophysical logging of boreholes for hydrological purposes
Standard test methods for crosshole seismic testing
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
We hope you have a Merry Christmas and a Successful New Year!!
Copyright © 2004
Commissioning geophysical surveys
Some prospective users of geophysics are put off by the perceived lack of guidance available to them on how to procure geophysical surveys. Rather than get involved with something new they simply rely on tried and tested tools.
A good reference document for engineers requiring a geophysical survey is the Geological Society Engineering Geology Special Publication 19, “Geophysics in Engineering Investigations” published in 2002. This is the result of collaboration between CIRIA, a Working Party of the Engineering Group of the Geological Society, the BGS and BRE, which does a decent job of discussing the procurement of geophysical services including the role of a Geophysical Adviser.
It is also relatively easy to stay abreast of applications of engineering geophysical methods by building a relationship with a trusted service provider with a solid reputation for delivery and a track record for innovation. Regular lunchtime seminars with guest speakers are a useful way of keeping abreast with new developments and can also count towards CPD.
When to use geophysics
Engineering geophysics can be applied throughout the life cycle of an engineered structure, from design, through construction to the ongoing monitoring of the integrity of the structure during its lifetime.
At the design stage geophysics can play an important role in the initial ground investigation. Typical applications might be the determination of bedrock depth, the in-situ determination of engineering properties of the bedrock strata (such as small strain shear strength) and the location of potential hazards such as voids, faults and even unexploded ordnance (UXO).
During the various stages of construction, non-destructive testing methods including time-domain reflectometry (TDR), sonic logging and seismic testing can be used to QC works such as piling and consolidation grouting. These methods and others can also be used to monitor the ongoing integrity of the structure and help to determine when to schedule essential maintenance tasks. Examples include pavement or ballast renewal on roads and railways.
Working closely with the engineer, a Geophysical Advisor can provide clarity on the likely value that can be obtained using geophysics at each of these life-cycle stages.
Detection of mineshafts
Why does geophysics have such a chequered history when it comes to detecting buried mineshafts? The answer lies in the fact that geophysical techniques are all too often indiscriminately applied. A geophysicist must design a suitable survey based on knowledge of the target and its surroundings (both above and below ground). Knowledge of the material properties likely to be associated with a mineshaft or adit target is therefore essential to choosing the correct method(s) and interpreting the results obtained.
If a geophysicist isn’t asking all of the following questions, be skeptical about the advice provided:
- What are the expected construction parameters of the mineshaft (material used, diameter, capping, and depth extent)?
- What is the expected burial depth?
- What materials are believed to overly the mineshaft?
- Are the mineshafts suspected to be back-filled with spoil materials, water filled or air voids?
Depending on the answers to these the geophysical solution may range from a £1,000/ha gradient magnetic survey to detect a shallow reinforced concrete capped shaft to a £20,000/ha detailed microgravity survey to detect 5m deep water filled, uncapped and concrete-lined shafts.
Zetica has in the past year won several large-scale mineshaft detection contracts ranging from £30,000 to £250,000 in value. Click here to request a quotation to detect a mineshaft.
FAQ – about unexploded ordnance
In the past our clients have received ambiguous guidance as to issues affecting a UXO risk assessment. Strangely, there are no recognised standards governing the advice provided or services offered to mitigate ordnance risk. Anybody can set himself or herself up as an “expert” in UXO issues and many have.
To assist in demystifying some of the grey areas that have arisen, Zetica has published a Frequently Asked Questions flyer in collaboration with UXB International Inc. This document provides a realistic view of the potential UXO risks that can apply to construction and engineering projects in the UK and has been well received by recipients.
Zetica is dedicated to improving the consistency and standards of ordnance and explosive contamination advice offered to customers in the UK and Europe. Click here to request a FAQ flyer.
Inner city shear wave seismic survey
Zetica has recently 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.
Zetica researched the available vibratory shear wave sources across Europe and the United States and finally settled on Bay Geophysical’s proprietary micro vibrator (MicroVib). The shear wave reflection technique was selected in collaboration with John Arthur of Top Hole Site Studies Ltd. and was chosen in preference to a more conventional P-wave survey due to the shorter signal wavelengths and commensurate increase in survey resolution offered by shear waves. The MicroVib source offers additional benefits including a higher immunity to traffic noise.
Challenges include running profiles over a mixture of hard surfaces (asphalt, concrete, brick) and grass with some lines crossing road junctions. A trial survey will be undertaken first to ensure that the method is suitable under the given site conditions and to optimise survey parameters such as source and receiver spacing. The outcome of this survey will be reported in a future newsletter.
Zetica is proud to announce that it has attained ISO 9001:2000 certification for all services allied to engineering geophysics and ordnance risk mitigation. “Achieving ISO 9001:2000 certification provides both external and internal benefits to Zetica," said Mike Sainsbury, Director. "Our customer base views our certification as evidence that we operate to a world-class quality standard, and as evidence of a methodical approach to continuously improving operational execution, and product quality."
Free Advice on Feasibility of Geophysics
The success of all geophysical methods relies on there being a measurable contrast between the physical properties of the target and the surrounding medium. The design of a survey can be aided by the use of powerful 2D and 3D forward modelling geophysical software. With information on the expected target size, depth and composition, a geophysicist can evaluate the feasibility of a particular method including likely error bars on modelled size or depth and can determine optimum survey design parameters.
Zetica offers free geophysical advice to our customers. Just call and discuss your project with one of our experienced geophysicists.
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.