Underground utility survey equipment generally falls into two categories. Those are electromagnetic location and ground penetrating radar.
When these tools are used together to detect underground utility lines, the data sets can complement one another to verify the positional accuracy of the required service as they locate targets using different electromagnetic methodologies.
It is important to remember that no equipment specifically locates underground utilities. It is the responsibility of the surveyor to use there training and experience to ensure they are interpreting the information they are receiving correctly.
Identifying underground utilities with electromagnetic location (EM)
There are a number of manufacturers and models of electromagnetic underground utility survey equipment.
The principles of the systems from the different locating equipment manufacturers are the same. EM location equipment detects electromagnetic fields, not services specifically. An electromagnetic field can be visualised as a circular magnetic field surrounding a conductor, such as an electric cable. These pieces of equipment locate either passive or active electromagnetic fields.
Utility locators: Passive location
A passive electromagnetic field is one which is generated naturally without the user having to induct their own signal into the line.
For a power cable, the electric current produces the magnetic field we trace and this would be located with the Power mode on a locator.
For other conductors detectable with EM location, such as steel pipes, the field would be generated by the absorption of UHF radio waves from the atmosphere. These passive signals can be located with the Radio setting of a locator.
There are limitations with passive location. Cables carrying very low current or pot ended cables, although live, cannot be detected on radio or power. In the case of a pot ended high voltage electricity cable, this could have serious safety repercussions.
Utility locators: Active location
The other EM location method is one where an electromagnetic field has been actively inducted to a utility line or cables by the surveyor. This is achieved by using a signal generator.
Inducting an active field is the preferred method of location and can be achieved by either direct connection, induction or with a accessories such as a cable clamp.
With both active and passive location methods, accurate location can become problematic where there are several conductors adjacent to each other. In these instances the circular fields combine creating an oval effect, which returns inaccurate data.
A trained and competent surveyor will be able to undertake on site checks to determine the validity of the field being generated.
Electromagnetic locating equipment
As previously stated there are two principal pieces of equipment in an EM location kit. These are the signal generator and receiver.
The underground utility survey equipment used by professionals undertaking a utility location survey falls into the higher end of the products available on the market.
The leader in the EM location field is Radiodetection and the flagship RD8200 precision cable locator would be the tool you would expect a utility surveyor to be using. The signal generator which accompanies this would be from the TX range. These are currently the TX5 and TX10. The number is relating to the output wattage the generator is capable of.
All of the high end equipment have a number of different transmit and locate frequencies, can locate on either a peak or null setting and can estimate depths accurately.
Top of the range devices have a significant amount of options over and above the standard transmit and locate functionality. These include Bluetooth, gps and data logging. The data logging is significant as it provides a GPR survey company with the ability to audit the results and usage of the equipment by surveyors.
For users who are not utility locating professionals, the CAT and Genny series 4 are the standard radio detection products. These have significantly less functionality than the RD which is why they are unsuitable for underground utility survey equipment. However, they are perfect for operatives involved in excavation works, provided they are trained and competent in their use.
In addition to Radiodetection, other manufacturers provide similar products. For underground utility survey equipment, as opposed to more general use, the biggest of these names would be Vivax.
Their vLoc series of receivers are hugely popular and the integration of survey-grade GPS allows accurate positioning technology not found in any other product.
Ground penetrating radar (GPR)
The other principal piece of underground utility survey equipment is ground penetrating radar.
A ground penetrating radar (GPR) system has the ability to locate targets which are not metallic in nature, something the EM location equipment is unable to achieve without access to a pipe for a sonde or tracer wire. Therefore provided the underlying conditions are suitable, a GPR can locate pvc pipes, fibre optic cables etc in addition to the conductors which can also be identified with EM location.
A Ground Penetrating Radar works by transmitting electromagnetic pulses into the ground which are reflected back to a receiver by objects within the soil.
These objects can be buried obstacles such as foundations or even individual items such as rocks. In the same manner as EM location, GPR does not directly locate services. It is up to the training, skill and experience of the utility survey company to determine the difference between targets which have been acquired and interpreting these as underground services.
The transmitters and receivers are located within an antenna and are available at different frequencies for varying applications.
An antenna with a frequency of over 1000mhz would be used for shallow, high-resolution surveys such as reinforcement location. A low-frequency antenna, less than 250mhz, would be used for archaeology or geotechnical work. The mid-range antennas are the ones which would be generally used for utility survey applications.
The quality of data from a GPR survey can be greatly affected by a number of external factors.
Ground penetrating radar antenna frequencies and limitations
Ground penetrating radar is not without its limitations. For instance, the theoretical penetration depth of a mid-frequency antenna, such as a 450mhz general-purpose utility locator would be up to approximately 4m. The data returned is often greatly reduced by the presence of clay soils, moisture content and surface materials like reinforced concrete slabs.
It is not uncommon for effective penetration to be limited to 1.0m or even less in normal working environments.
These same factors also affect the quality of the data. Dispersion of the signal and interference are caused by these external factors which can make the identification of ground penetrating radar targets problematic.
In addition to the single-frequency antenna GPR units, there are multi-frequency units and high-density arrays available to use for underground utility survey equipment.
A dual or multi-frequency unit would have complementary frequencies installed. In the IDS Opera Duo the frequencies are 250mhz and 750mhz. This range provides a contrast of resolution and penetration ability for various disciplines.
For multi-antenna arrays, like the IDS Stream series, the antennas are closely spaced together and because they are dual polarised, they allow the collection of data in longitudinal and transverse directions simultaneously, with a negligible gap in the data. This system, often vehicle mounted allows data to be collected 30-50 times faster than a traditional single antenna system.
Data processing and interpretation for identifying underground utilities
Once data is collected there are two options for data interpretation. These are having a surveyor interpret the information in real-time and marking locate targets on the floor and post-processing data in an office environment with specialist software.
There are pros and cons for each approach. Directly marking on-site can be faster especially for uncomplicated locations. However, you are relying on operator skill and experience often leaving no audit trail for quality control. These two factors are a major drawback.
When moving into areas with congested utility infrastructure, using post-processing is really a must.
Using the more complicated ground penetrating radar hardware, such as the IDS Stream, allows a specialist geophysics team to make the judgment calls in collaboration with the survey team and provides an audit trail for data quality.
These factors combined provide the end-user with a win-win situation, giving them the best possible GPR survey for their environment.
The underground utility survey industry is constantly evolving. Survey specifications evolve to provide greater benefits and equipment is always evolving as methodology and technology improves. A good utility survey company evolve with the hardware to always provide their clients with the most up to date technology and beneficial when undertaking a survey.
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