Partly funded by the European Commission, ORFEUS is a three year project that began in November 2006, with the overall aim of improving the accuracy and reliability of GPRs for locating buried infrastructure. Such technology improvements will lead to the reduction in the need for excavation of highways, minimise direct and indirect costs, reduce the incidence of pollution and enhance safety. Figure 1 demonstrates a typical tangle of pipes and cables that can be encountered under the streets of most cities and damaging them can result in expensive third party claims.

One of the project’s prototypes is an innovative GPR-based real-time obstacle detection system for mounting on steerable bore-heads of horizontal directional drilling pipe and cable laying systems.

The project group face the challenges of designing an antenna to fit within the constraint on shape and dimensions imposed by the limited size of the drilling head, and of providing robust packaging for the electronics to protect against the high levels of shock, vibration and heat generated during drilling.

Requirements for HDD GPR

The ground penetration required to fulfil detection objectives is dictated by the physical parameters of the drilling process and the time and distance required by the operator to decide upon and take avoiding action.

Objects that lie in the path of the pilot bore and those that may be present in the volume of a cylinder surrounding the pilot bore must be detected. This means that the radar must possess look-ahead and radial detection capabilities. Targets include pipes and cables that are both parallel to and crossing the direction of the pilot bore; other man-made artefacts such as old building foundations, inspection holes, etc; and, naturally occurring objects such as stones and boulders or changes in ground consistency or geological situation.

In addition to fulfilling its primary detection role, the radar will also provide information on the presence of objects that may cause problems for the drilling and subsequent pipe-laying operations. This will include the angular position and distance of objects, and whether there is one or more present. Figure 2 depicts an operative situation for the bore-head radar.

A comprehensive set of requirements was developed by the end-user group at the start of the project, constituting the input to the project design phase. These requirements included definitions of:

• Detection distance
• Minimum detectable object size
• Resolution
• Axial and radial accuracy
• Target detection percentage
• False target generation percentage
• Surveying rate capability
• Form of data display

Several critical issues need to be solved during the system’s design phase. The most important of these being the limited space available on the bore-head and on the drill rod dictating a need for small antennas immersed in a metallic housing.

It is also important that the system can detect very small diameter (< 10 mm) pipes with a high detection rate of above 95 per cent and low false alarm generation. In addition, the system needs a greater than 50 cm detection range – both in the bore-sight and azimuth directions – in the presence of drilling fluids that attenuate radar waves.

The limited cross-range resolution of the small antennas makes extraction of useful information from the radar data difficult, however the system has to integrate a transmitting and receiving GPR antenna housed in the drilling head to look-ahead; a GPR antenna hosted in the drill rod to look sideways; a multi-channel timing circuit to control the antennas; an A/D conversion board and interface to the control computer; and, a power supply module for the electronics. Bore-head design The bore-head should be designed to assure the best drilling performance of the machine, while taking into account the need to accommodate the drilling fluid flow.

An additional requirement is the need to find a material that is transparent to electromagnetic radiation for protecting the antennas but strong enough to survive in a harsh environment. Such a material has been identified and is currently undergoing rigorous environmental testing to confirm fitness for purpose. Figure 3 shows a schematic diagram of the bore-head GPR while figure 4 illustrates the antennas mounted in the bore-head. Testing the solution The initial prototype is planned to be tested at an artificial test site. Whilst the tests must be realistic, they must also provide results that can be analysed and interpreted in a straightforward manner. Phase 1 of the testing process is planned for late 2008.

Issues to be addressed during the testing include: the types of targets to be detected, their sizes and construction details, their geometric configuration in relation to the pilot bore direction and to each other, and the characteristics of the soil in which they are located.

The test site has sufficient flexibility to be re-configured to produce a range of pipe and cable configurations and burial materials.

The second phase of the program, scheduled for early 2009, will test the radar in representative field conditions to extend the evaluation of its performance. Because the system is new with no pre-existing experience, the testing regime will be conservative. In the first instance, the emphasis will be on the safety of the public and the personnel conducting the trials, and then on producing results that can be reliably interpreted. The objective of the program is to allow the users to evaluate the radar’s performance and compare it with the requirements.

This article is a summary of a paper written by ORFEUS project manager Howard Scott. The paper, titled The ORFEUS Project: Design of a bore-head GPR for Horizontal Directional Drilling (HDD) equipment, was presented at the 12th International Conference on Ground Penetrating Radar, held at the University of Birmingham in June.