in Kansas City. “In this instance we reach for the newer 350 MHz HS antenna because the digital antenna offers a clearer image than a traditional GPR antenna and a 50 percent improvement in depth pen- etration. It also works with the newer control unit we use (the GSSI SIR 4000), which has more power and flexibility to image these more difficult to image targets.” Along with locating sewer lines and drain pipes, Echo GPR also uses the 350MHz HS antenna to locate underground storage tanks. Sinkholes or underground caves When working for a geotechnical company at a property where there are sinkholes or underground caves, Echo GPR uses a 100 MHz bi- static antenna, one of the few GPR firms to do so. This enables the company to scan more than 20+ feet down, while mapping out soil layers and deeper anomalies. Field conditions can make antenna selection more challenging DeSchepper warns that selecting the correct antenna for achieving the depth penetration required can sometimes be tricky. He explains this happens with more frequency than might be expected – mainly because actual conditions in the field turn out to be different than those given to the GPR crew by a contractor. He offers this example: “Recently, a contractor asked us to scan a 20 foot by 200 foot area because they needed to cut out a hole and install a new piece of equip- ment. I grabbed the 2600 MHz antenna, my usual go-to for concrete cutting. Now, when I teach GPR for those getting the industry certifi- cate through the Concrete Sawing & Drilling Association (CSDA), the first thing I tell students is that the most important thing when scanning concrete is to find the bottom of the concrete – for both slab on grade and suspended slab applications. This lets you know you are seeing what you need to assist the cutting/coring contractor. When I scanned with the 2600 MHz antenna I could not see the bottom of the concrete. This was a definite red flag, since the contractor said the concrete was 8 inches thick and the 2600 MHz was set up to look up to 12 inches deep. I quickly realized this was not the right antenna. I switched out to the 1600 MHz – and lo and behold, the concrete they thought was 8 inches thick was actually 18 inches thick. DeSchepper was able to call the concrete cutting company before they left the shop; they came out with the right saw and right size blade to cut through the concrete. Antenna selection should be based on a thorough understanding of what the customer wants to accomplish, as well as comfort and conve- nience when working on a ladder or overhead.”
Echo GPR scanned the area and identified 16 pieces of rebar reinforcement.
Successful drill locations as identified in GPR scan.
dren’s hospital. The hospital was building nine stories of research labs on top of an existing parking deck. Contractors needed to thread new vertical rebar into form savers that were buried in concrete. The form savers have a profile of ¾ to 1 inch in diameter and are capped with a piece of plastic. Contractors could not locate the form savers and called in Echo GPR to help locate them. Using the 2 GHz Palm antenna, Echo located the plastic-capped rebar thread couplers in the Z axis. Says DeSchepper, “It was so accurate that contractors could hammer drill and find 16-30 rebar couplers in each column. We had scanned 800-900 of these rebar couplers and the contractor later told us we had never missed a piece of post tension cable and only missed 2 form savers in the entire project. This was gratifying, because the vertical and plastic capped profile is such a hard thing to image.” Locating sewer lines and drain pipes According to DeSchepper, the best antenna choice for customers who want to install new drain lines but do not know where the existing sewer is would be a 400 MHz antenna or the 350 MHz digital antenna with HyperStacking (HS) technology, which allows users to see deeper targets. The resolution may be less than other antenna options, but it allows them to look for larger targets 6 to 8 feet below ground. Another challenge Echo GPR faces is when they’re asked to look for a clay tile pipe drain line that is in the clay soil that is so prevalent
JAMI HARMON is the Marketing Operations Manager for Geophysical Survey Systems, Inc (GSSI), the world leader in ground penetrating radar. Jami has been with GSSI for twelve years and has experience with ground penetrating radar in the utility locating, concrete inspection, transportation infrastructure, archaeology, and geophysics markets. Jami is an active Board Member and Committee Chair for the Concrete Sawing and Drilling Association (CSDA) and participates in several industry associations.
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csengineermag.com march 2020
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