From utility detection to geological investigations, ground penetrating radar (GPR) provides solutions

The versatility and ability of GPR to detect both metallic and non-metallic services and structure, is testament to its success. The use of GPR equipment can make companies more cost efficient, increasing both profitability and safety. GPR provides solutions!

Utility Detection

Ground Penetrating Radar, or GPR as it is more commonly known, is fast becoming an ever more popular choice of tool for locating professionals and survey teams throughout the world. When GPR was first introduced to the utility detection market back in the early 1990’s to improve pipe locating techniques, it was considered too expensive, cumbersome and complicated to use. However, with recent improvements in technology, the GPR equipment available today is more compact, ergonomic and much easier to use; improved user interfaces allow for simple operation with minimal set-up, clear data presentation and an overall performance that better addresses the needs of the locating industry. All this and a much more competitive price tag. GPR equipment can now be considered a standard tool alongside traditional EM locators.

Utility Surveys with GPR

The Construction and Maintenance team at Norfolk County Council in the United Kingdom is successfully using GPR alongside traditional EM pipe and cable locating equipment to carry out utility surveys throughout the county. Rob Rogers, who heads up two new survey teams, takes up the story.

“Our teams are responsible for surveys throughout Norfolk, with particular emphasis on surveying land prior to the building of new houses. The utility companies provide us with detailed plans of their buried pipes and cables but, unfortunately, these plans don’t always show where these utilities branch off to be connected to individual properties. As many of the pipes are UPVC or plastic, they cannot be detected with conventional EM pipe and cable locators. We were, therefore, looking for a system that could detect these hazards. GPR was found to be the ideal solution. Not only is it able to detect the underground utilities but also its ease of use meant that in one training session our existing personnel were up and running.”

Two survey teams are now kept busy throughout the area. No longer do they have to dig holes “on spec” to find these buried utilities. “There has been a significant reduction in the number of damaged utilities.” Rob continues. “Using the new equipment and setting up the survey teams has saved the Council both time and money. Money due to less being spent on repairs and, in some cases, compensation and time due to the general hassle of reporting the damage and then getting it repaired. We also have complete control of all projects and don’t have to subcontract. I would say that the teams will have paid for themselves within 6-months. A great investment in some great products.”

This story is not unique and many operators working in the utility detection and survey industry are now realizing the benefits of using GPR alongside the more traditional EM locators, giving them the best chance to solve subsurface detection problems that were previously either very difficult, time consuming, costly, or a combination thereof.

As commented by Rob, the GPR equipment in use by the Norfolk County Council teams can be operated with the minimum of training. Following switch-on, the user needs only adjust a few simple settings before the ground can be scanned for buried utilities at full walking speed. The data obtained provides the user with precise horizontal positioning and depth to target. This particular system also supports two different frequency antennas that are easily interchangeable and enable the detection of these buried services at various depths. Having a choice of antennas operating with different centre frequencies allows the user to pick the one that best suits the application in hand and the prevailing site conditions, providing optimum depth penetration and resolution accordingly. With a dedicated, single-button controlled user interface that is housed in a field rugged monitor, rated to IP67, the GPR being used by the Norfolk County Council teams has been designed by users for users. An optional Transreflective LCD screen provides for maximum visibility outdoors, without the need for a visor or sunshade, even on the brightest of days.

Geological Investigations

The use of low frequency GPR systems can be both a cost-effective and efficient way to obtain geological knowledge of subsurface conditions. Recent innovations in antenna design have made these types of investigations noticeably easier, allowing equipment to be used by a single operator and therefore cutting costs.

The need for such reliable ground investigation techniques reflects the increased need for precious resources and raw materials. GPR measurements have been used for a long time to investigate the ground conditions for soil mapping, detection of cavities, bedrock surface mapping, peat thickness mapping, lake and riverbed sediment mapping, groundwater resources contamination mapping, etc., and have often provided adequate and reliable results. A typical geological survey is carried out with low frequencies in the range of 10 to 50 MHz, maybe up to 100 MHz, giving relatively detailed information down to soil depths of 10 - 50 m in non-conductive environments. Existing systems, however, most often demand at least two operators to carry out the field measurements, due to the use of sensitive fibre-optic cables and long antenna elements which are often a problem to handle and position in rough or non-cleared (dense) terrain and environments.

When conducting geological surveys and investigations, low frequency GPR systems are needed in order for the signal to penetrate and to reach greater depths. These low frequency antennas (in the range of 10 to 50 MHz) are physically quite large and have long antenna elements (up to 4 m). These antennas are also of the unshielded design, which means they consist of separate transmitting and receiving antenna elements. This again demands quite an effort during fieldwork as both antenna elements must typically be moved together as a pair, and often with up to 4 m separation between the two, hence the need for additional manpower.

The fieldwork with two separate unshielded antenna elements can be carried out in several different ways. Most common is the use of the two antenna elements to measure the radar profiles in broadside configuration where the transmitter and receiver dipole antennas are held perpendicular to the measurement line with a fixed distance in between them, in a so called co-polarized manner. If the radar antennas are used in this normal configuration, quite wide paths have to be prepared in the investigation area and at least two people have to be involved in the data collection. To increase the measurement capacity, make fieldwork easier, and only be dependent on one single operator, a new antenna has been developed as an in-line parallel configuration, a so called Rough Terrain Antenna (RTA).

A Unique Concept

The RTA concept changes the face of low-frequency GPR surveying. The unique in-line, all-in-one, antenna design provides improved performance for deeper penetration. The flexible “snake” like design allows the antenna to be maneuvered easily and efficiently through the densest or most uneven of terrain without affecting ground contact, providing optimum results in the most difficult of environments’. The single most important benefit being that the operator doesn’t have to clear an access path or route prior to the profile or survey. Even in relatively easy environments the RTA out performs the traditional broadside configuration, with proven results showing a significant reduction in survey time and manpower costs. Existing users of the 50 MHz variant report reductions in survey time and manpower requirements to a third of those experienced when using traditional unshielded broadside antennas.

Clearings of measurement lines are most often unnecessary; where the operator can walk, the antenna will follow. This design concept also allows it to be attached behind a vehicle or boat, depending on the investigation type, to further improve the efficiency of the survey.

Conclusion

The information presented above demonstrates that GPR is a versatile, accurate and reliable technique that can save significant time and money for survey teams, councils, utilities and geologists alike and the improvements in this technology can provide instant results which until recently, may have been difficult and/ or costly to obtain using conventional or traditionally configured systems.
However, it is worth noting in conclusion that whilst GPR is an effective locating tool, as clearly demonstrated; unfortunately, it doesn’t work in all areas! GPR is not a “magic wand” and has its limitations like any tool. GPR works best in soils or medias that are non-conductive, so it does not work equally well in all soils (soils can range from very favorable to unfeasible). As a general rule of thumb, soils with high mineralogical clay content inhibit the performance of the GPR signal, whilst sandy or gravely soils are much more suited to the method.