Most utility locators have a good understanding of electromagnetic field (EMF) locators. While there are similarities between EMF and ground penetrating radar(GPR) technologies, both have their downsides and using GPR alone can be dangerous.
Similarities between EMF and GPR locators start with frequencies – single, dual and multi array. Like an EMF locator, GPRs with more than one frequency help in most situations.
GPR frequencies can range from 200-800MHz and the lower the frequency, the longer the wave, which means deeper penetration.
The higher the frequency, the shorter the wave, highlighting smaller services at shallow depths.
The most common low frequencies are around 250MHz. In ideal conditions these can penetrate up to 6m, but due to the composition of Australian soils, it may be difficult to reach anywhere near that.
Another downside of lower frequencies is that they struggle finding smaller, shallower targets in great detail.
The medium frequency range – 400-600MHz – is a good all-rounder. These frequencies don’t penetrate as deep as 250MHz, but find smaller targets in mid-to-shallow depths and locate 3m-4m deep. This is a good frequency range when using a single frequency radar.
Next is the 700-800MHz range – high frequencies. These generally don’t travel more than 1-2m in depth, but are great at locating small, shallow targets which may be missed by lower frequency GPRs.
A dual frequency GPR is the best option for locator professionals, varying between 200-450MHz in the deep-to-medium range, and 700-800MHz in the shallow range.
Another function of GPR locators is gain. Similar to gain on EMF’s, the more gain used, the more sensitive the antennas become. But similarly having too much gain creates noise interference and can make it difficult to locate targets.
There is also a difference between gain and contrast. While contrast adjusts the image on the screen by winding up variances in the image, gain adjusts the antenna sensitivity, highlighting targets that may require more sensitivity.
The popularity of multi-array antennas is starting to increase. Mainly used for mapping medium-to-large areas, some units can be towed at road speeds which can fasten the process of locating utilities and remove the need for traffic control.
Data collected is then processed through dedicated software to show a 3D image or tomographic map of the area.
Normally, these must have a GPS/RTK antenna or total stations for position accuracy. Multi-array units also have the advantage of locating the target with several antennas along the path of the service, allowing differentiation between a rock and a pipe or cable.
GPRs can assist in improving a Quality Level C, according to the updated AS5488 Standard, while EMFs can get to Quality Level B as the locator is physically attached to the utility.
In terms of data quality and accuracy of GPRs, the standard is quite good, but soil conditions mean that accurate location cannot be guaranteed when using single, dual and multi-array antennas.
Users are also unable to accurately identify what is on the screen as they aren’t physically connected to anything as a reference.
It is good to think of GPRs as an excellent tool for blind searching as a whole area can be scanned using a grid or simply running across the area multiple times and marking when you detect a target – this is where multi-array antennas can speed up the process.
It can also complement EMF locating and locate services that traditional EMF locators are unable to, such as plastic gas and water pipes.
While it is good to understand the differences in the phase and reflections of a GPR image, it doesn’t help in determining the type of service below.
To get the most out of GPR locators, training with the manufacturers is the best solution as they have the knowledge, courses and skills to help you get the most out of the machine.
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