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Pulsing with possibilities

SITE INVESTIGATION: Ground penetrating radar is one of the most adaptable geophysical tools for geotechnical engineers, says Steve Openshaw.

Shallow geophysical methods offer an important source of data to geotechnical engineers. Perhaps the most adaptable is ground penetrating radar (GPR). This non-intrusive method of sub-surface imaging has an almost infinite number of potential uses.

In engineering the most common uses are examination of concrete structures and pavements, location of utilities, culverts, foundations and shallow geological phenomena.

With the recent publication of guidance on piling on contaminated land, these non-intrusive methods will undoubtedly provide further useful means to pre-evaluate development sites (2001, Guidance on piling on contaminated land, Westcott et al, Environment Agency, National Groundwater and Contaminated Land Centre report NC/99/73).

Ground radar systems can be set up to examine very shallow depths (less than 1m), such as for investigation of concrete detail and sub-slab voiding. At the opposite extreme a radar system can image the ground to depths of 10m or more, for example when locating buried tanks. Its use in boreholes opens up the prospect of surveys at even greater depths, for example when installing deep piles in Karstic areas.

Ground radar uses pulsed electromagnetic energy in the frequency range 10MHz to 2,500MHz.Energy is radiated into the ground from an emitter and partially reflected where a change in the sub-surface dielectric properties exists, usually at material interfaces.Data can be recorded at several hundred scans per second, allowing a near continuous profile to be built up as the system traverses the surface.

Recorded data represents the amplitude of the reflected electromagnetic energy and therefore the degree of contrast between materials. A radar image shows the distribution of contrasts in a vertical profile along the line of survey. In a similar fashion to seismic surveys, the travel time of the electromagnetic energy can be used to determine true depth.

A typical radar system consists of a battery, display/processing unit and an antenna unit. All are lightweight and portable and can be run by one or two skilled operators or pulled behind a vehicle.A survey wheel can be attached for placing distance markers over long traverse lines.

In the field, radar data is digitally recorded in real-time with minimal processing.In the office, it can be downloaded to PC for processing with commercial or in-house software, allowing clarification of targets of interest and preparation for reporting. This is where operator experience becomes vital.

Radar systems are specialised geophysical tools and should be operated by highly trained and experienced geophysical contractors.This means the client can be told the likelihood of success for any particular survey.

As with any survey, planning is critical. Likely types of ground material to be found at a particular site need to be identified, as there are 'good' and 'bad' material types depending on the technique used.

Radar energy is drastically attenuated by material with high electrical conductivity such as steel and wet clay. Of course it is possible for radar to detect the presence of highly conductive materials within low conductivity materials, an ability which can provide further site information.

The second stage of planning involves the types of target.The likelihood of detection, particularly for man-made objects, is related to their depth, construction material and diameter. Detection of small diameter utilities at depth is difficult. However, it may be possible to detect their presence using contrasts of the fill material around them.

The next step is to choose the frequency of the radar antenna for the survey.A high frequency antenna of about 1GHz will penetrate to between 250mm and 500mm and will image objects at high resolution. Concrete detail surveys are most often carried out using high frequency antennae since small voids, reinforcement and fractures can be imaged.

Lower frequency antennas will propagate more electromagnetic energy to greater depths.An examination of a former landfill site or a survey over a proposed pipeline route would require antennae anywhere between 50MHz and 400MHz.There are a number of calculations that can be performed to assess the type of antenna to use as well as the operator's own experience.

The survey orientation and layout are also important considerations.On a site with targets of known orientation, survey lines are usually performed perpendicular to the target.This ensures the target is crossed a number of times allowing, accurate positioning.On a site where the orientation and size of targets are unknown, a survey must try to image sufficient ground so as not to miss targets.The spacing of the survey lines, their orientation and whether the survey lines are laid out on a grid are all considerations at this stage.

Applied correctly, many geophysical methods can provide near continuous sub-surface information from a site using non-intrusive methods. Data procurement is relatively rapid and large areas can be covered at high resolution. This allows pre-development planning, enables targeted intrusive investigation and above all can reduce the risk of the unknown.

The ability to use geophysical data to target only zones of interest for intrusive investigations is a great advantage and can be very important on sites due for regeneration and when used to monitor manmade structures and contaminated ground.

Non-intrusive methods such as ground radar and other geophysical methods have a bright future in geotechnical engineering as requirements for site and structural investigation increase.

Confidence will continue to grow in radar as an engineering tool, provided the industry can maintain its quality standards and educate the engineering community as to the correct application of the methods.

Dr Steve Openshaw is head of the geophysics division of foundation, structural and materials testing firm Testconsult.

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