C+S March 2020 Vol. 6 Issue 3 (web)

office retail spaces, and has performed GPR scans at all the hospitals in Kansas City. They also work for geotechnical and environmental firms to locate underground storage tanks. DeSchepper explains that grocery stores cannot afford to accidentally cut power to their cash registers. Also, they may have underground re- frigeration pipes that run to their coolers. The last thing they want to do is cut into these refrigeration pipes, which would result in downtime, potential environmental issues, and even store evacuation. Here are a few real world examples of antenna selection, taken from the nearly 8,000 GPR scans Echo has performed across the United States: Simple concrete cutting or core drilling For simple concrete cutting or core drilling, Echo GPR opts for one of the higher frequency antennas, like the 1600 MHz or 2600 MHz antenna. “I usually reach for the 2600 MHz antenna first. This is the highest frequency and gives the highest resolution. But higher resolu- tion gives less depth of investigation. If the project is in a thicker con- crete environment, we will switch out to a 1600 MHz antenna, which is a very sharp resolution antenna but can penetrate a bit deeper into the ground” DeSchepper says. Working upside down and on a ladder Currently, Echo GPR is working on a basement remodeling job with a steel erection company locating rebar prior to anchor-drilling steel plates for support mounts. “The engineer elected to hire us because they

size and depth of the target. A lower frequency antenna will provide deeper penetration, but the tradeoff is that the targets must be larger to be detected. When looking only 1-2 feet beneath the surface, for example, when im- aging in concrete and looking for very small reinforcing and conduits, use a higher frequency GPR antenna, which offers greater detail. For deeper targets or characteristics, the primary and secondary choices are lower frequency antennas. These allow users to see deeper but one trades off the ability to resolve smaller or very shallow features and targets. When scanning utilities, the mid-range antennas provide sufficient resolution to find 4-5 inch pipes at four to five deep. The general rule of thumb for utility locating is a 1:1 ratio; for every foot down, one inch in diameter is required to return a strong hyperbolic reflection of a potential target. To scan deeper, for example, mapping geological features 20-50 feet deep, low frequency antennas will work, because very high resolution is not required. Antenna selection based on a solid understanding of cus- tomer goals According to DeSchepper, antenna selection should be based on a pre- job conversation that establishes an understanding of the depth of the investigation and what the customer wants to accomplish. Comfort, convenience, and safety when working on a ladder or overhead is an- other consideration. Echo works extensively with electricians and plumbers who are re- modeling existing buildings. “We do everything from a local grocery store that wants to install new plumbing in the floor and needs to lo- cate existing utilities and underground conduits – all the way up to nuclear power plants,” says DeSchepper. In addition, the Echo GPR team works in commercial remodels of strip malls, office spaces, and Range of available antenna frequencies, their depth of penetration and applications for which each is most suited.

know that once we have marked out all the steel they can establish where the pre-manufactured supports are and will be able to drill into the ceil- ing without hitting steel. This speeds up their pro- cess.” For this purpose, DeSchepper selected a 2 GHz Palm antenna, which gave him high resolution data with a good depth of penetration, while being extremely lightweight. “I am working upside down and off a ladder, with both hands above my head as I am scanning, so my an- tenna choice in this case is based on comfort and convenience.” Locating targets in the Z axis The Palm antenna was also key to the success of a project at a local chil-

Using a 2 GHz Palm antenna, the contractor located the plastic-capped rebar thread couplers in the Z axis.


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