Win a free UXB desk study

Following our popular offer for a free day of utilities survey we would like to extend the offering to a UXB desk study. Over 2,000 UXB risk maps have been downloaded off our website this year alone illustrating the importance of this information to consultants, developers and local authorities. Zetica is best placed to provide a comprehensive desk study service based on the database we have compiled and practical knowledge of what constitutes a significant risk.

Click here to register to be in the draw which will be held on the 9^th January 2006. Click here to download a Sitesafe brochure.

Geophysical survey design – make your project a success

The first job of the geophysicist when faced with a particular engineering problem is to design a suitable survey based on the survey objectives and the nature of the target and its surroundings (both above and below ground). The survey proposed must also be cost effective and offer cost benefits compared with ‘traditional’ site investigation methods.

The successful design and implementation of a geophysical survey requires careful consideration of the following main factors:

• Target discrimination
  The nature and degree of contrast in physical properties between the target and its surroundings is a primary influence on a feasibility assessment of geophysics and choice of techniques that could be used. Information regarding the target and the expected surrounding materials may be limited or non-existent and in these cases the geophysicist should recommend a trial survey or the application of multiple techniques on the site. Keep in mind that what may work on one site may be unsuitable for the same target on another.
• Detection distance
  All geophysical methods are sensitive to the relationship between target size and detection distance in addition to the material composition of the target and its surrounding materials. In general, the greater the detection distance, the larger a target’s volumetric size and/or cross-sectional area needs to be to be detectable.
  

  Approximate minimum radius of cavity that will produce a 15µGal microgravity anomaly in host rock of density 2.3Mg/m3.

• Survey resolution
  The correct choice of sampling interval (frequency or spacing of sampling points) is critical to the success of the survey and its cost-effectiveness. For surveys conducted from the surface the sampling interval is dictated by the geophysical ‘footprint’ of the target that may be tens of centimetres for small diameter shallow pipes, a few metres for narrow fault zones or tens of metres for large diameter voids at depth. The optimum sampling interval will be one that samples the anomaly adequately (commensurate with the perceived risk of non-detection) to meet the survey objectives and with minimal loss of information.
• Site conditions
  The suitability of a site for collecting good quality geophysical data is often overlooked in the design of works. The issues affecting data quality that could be of concern relate to the method or methods being proposed. For example, in the case of electromagnetic and magnetic methods, signal degradation or geophysical ‘noise’ may be introduced by the presence of surface metallic structures and overhead power lines. For microgravity or seismic surveys noise may result from traffic movements or wind and waves. Where the level of noise exceeds the amplitude of the anomaly due to the target and where this cannot be successfully removed, the target will not be detectable. The best way to assess the likely influence of site conditions is to visit the site at the design stage and/or carry out a trial survey.

Click here to request more information on our geophysical mapping and site investigation services or visit our website www.zetica.com.

Cost benefit of geophysics – a customer perspective

This customer case history shows how a geophysical surveying can be used to avoid excessive costs / delays in relation to unknown obstructions within the ground and the use of piled foundations.

The image to the left clearly indicates a regular pattern of obstructions within the ground, which was obtained without disturbing the ground in anyway. The only requirement was that the existing ground had to be level to enable suitable, safe and accurate access over the area to be surveyed. From the initial results, Zetica produced the CAD drawing as shown above right to detail the actual obstructions within the ground. This clearly indicates a piled grid with pile caps and ground beams. It also highlighted two areas of existing floor slab that had not been previously removed (indicated as light blue).

This CAD drawing then allowed the piling sub contractor / structural engineer to design a new suitable layout around the existing ground obstructions. The complete process was carried out within two weeks of starting on site and did not delay the piling rig or overall programme. The client had £30,000 contingency to spend on unknown ground conditions, which we initially thought was insufficient taking into account what was thought to be in the ground. The electromagnetic survey process cost just over 10% of this and therefore enabled the client to save the remaining amount of his contingency. A good result for all parties concerned.

Thanks to Mansell Construction for permission to use this extract from an in-house technical bulletin.

Cutting the costs of site investigations – landfill example

A previous site investigation incorporating 4 boreholes, one located in each corner of the site, had determined that the depth to chalk bedrock, and the condition of the chalk was essentially similar in each borehole. The regulator was informed and permission given to construct the landfill cell. Not long after commencement, the earthworks contractor encountered large sinkhole features in the chalk and the works were halted. The regulator requested that a geophysical survey be carried out to clarify the extent of the problem.

A ground conductivity map was produced of the 1ha area (centre below) which highlighted numerous sinkhole features as ground conductivity highs (pink / red colours). These features were then further investigated using resistivity imaging and cone penetrometer testing (CPT) to determine the depth of the sinkholes.

A hypothetical exercise was carried out to determine how many boreholes would have been required to match the detail provided by the geophysical survey. The figures above left and above right show that for a 3 x 3 (9 no.) or even a 5 x 5 (25 no.) grid of boreholes the information on the extent of sinkhole development would have been patchy at best.

The most cost effective sequence of events would have been to carry out the ground conductivity survey first then drill 3 - 4 boreholes in targeted areas revealed by geophysics to be representative of background and anomalies.

Geophysics applied to agricultural projects – case history from South Africa

Zetica recently carried out an electromagnetic survey over approximately 1,000ha to select optimal areas for installing overhead irrigation (centre pivots) on previously unused agricultural lands in northern Kwazulu-Natal, South Africa. The criteria for locating pivots for cotton growing include well drained soils and low acidity values both of which can be indirectly mapped using ground conductivity. The blue – green colours in the image below left show optimal soils to a depth of 0.5-1.0m below ground level.

A real-time kinetic GPS system was integrated with the ground conductivity survey to produce accurate topography maps which also formed a critical part of the design of the irrigation infrastructure. An interpretation map was produced (below right) showing preferred soils overlain on topography to assist in the design of the irrigation infrastructure (example of pivot below centre).

Given the investment of over £2million in infrastructure on this project, where return on investment is a direct function of crop yield, the importance of a detailed geophysical survey to accurately map properties of the soils especially in heterogenous areas, cannot be overstated. Geophysics also plays a role in the ongoing monitoring of soil properties to facilitate targeted remediation as soils become depleted.

In the UK, resistivity is commonly used to delineate the boundaries of different soil types.

Images of the results from a resistivity survey carried out by the Australian Centre for Precision Agriculture on a wheat field, compared with actual wheat yield indicate a strong spatial correlation between each information layer (resistivity data below left, yield data below right).

Click here to request more information on Zetica’s geophysical services for precision agriculture.

Recent advances in borehole detection of unexploded bombs

As a product of a 3-year dedicated research programme with our partners at the University of Liverpool, Zetica now offers the following advances in analysing borehole magnetometer data for the location of unexploded bombs (UXB’s):

• Measuring truck characterised and effect removed from the raw data
• Number of probes required to locate UXB has been further minimised
• For close (<1m) probes only one borehole required to locate target
• For probes between 1m and 2m, only 2 boreholes now required
• Information on target type also utilised in target discrimination

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.

Everyone at Zetica thanks you for your valuable custom, and wishes you a Merry Christmas and a successful New Year!