The Move to Multi-Frequency: How More Gpr Antennae Can Help Your Field Projects

The popularity of ground penetrating radar in the damage prevention industry has soared in the past decade. Costs have come down, user interfaces have become more approachable, and the recognition that GPR can identify targets that traditional locators cannot have all contributed to this increase in usage. An emerging trend in GPR technology that has caught on in the last few years has been the push towards multi-frequency systems. GPR uses an antenna to produce electromagnetic waves that travel through the subsurface to detect buried targets of interest. Each antenna produces a signal at a central frequency, and that frequency is one of the major determinants of signal depth and resolution. The higher the frequency, the shallower the signal travels, but the finer the resolution. Alternatively, the lower the frequency, the deeper the signal travels, but the coarser the resolution.

It was recognized early on that different antenna frequencies would have greater or worse success depending on the types of utilities being investigated since buried utilities vary dramatically in diameter and depth. GPR manufacturers responded to this need by developing more efficient systems. These multi-frequency systems collect data from two or more antennae at the same time. This integration helps solve several problems on utility locate projects.
One student in my training program that utilizes a low frequency antenna to locate buried lot because the soil consisted of porous fill containing air. This allowed the signal to travel deep enough to reflect off the pipe. The pipe ran across the parking lot, under some landscaping, and into a commercial building. Once the soil conditions
transitioned from fill to clay (which was below the landscaping), the pipe could no longer be detected with the mid-frequency antenna. With a multi-frequency system however, a lower antenna frequency may have allowed us to continue tracing the pipe after the transition to different soil conditions without having to transfer antennae in
and out.

While multi-frequency systems can be cumbersome, antenna frequencies are often preset, and multiple antennae may interfere with overall services related that this system had been effective in detecting large, deep piping such as sewer and storm water and has on occasion been able to identify smaller, shallower utilities such as gas. This individual primarily works on transportation and highway projects and often must identify small targets buried just below thin layers of asphalt. These have been the most difficult for him to detect. Unfortunately, he regularly
gets both deep pipes and shallow utilities on the same site. Some systems allow the user to switch antennae and perform the survey again with a different frequency but this can be time consuming and outside of project limits. However, multi-frequency systems allow a single pass to detect both types of services in half the time.

Another emergent benefit of multi-frequency systems is the ability to trace services in a single survey even when ground conditions change. During a recent workshop, students and I were conducting basic locate and marking of a water pipe buried below a parking lot with a mid-range frequency. With this frequency, the pipe was visible below the parking data quality; the benefits outweigh these potential drawbacks under most circumstances. At least one
system allows users to choose from several different antennae to create a unique setup based on current site conditions and expected target depths and diameters. This suggests that innovation in GPR technology continues to remain exciting and we all have a lot to look forward to in the future of damage prevention.

Dan Bigman is founder of LearnGPR.com and is an expert in non-invasive subsurface mapping and 3D imaging. He can be reached at dbigman@bigmangeophysical.com.

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