- SPECIAL REPORTS
- THE MAGAZINE
In an episode of CSI: NY, a detective uses ground penetrating radar (GPR) to search for human remains under the turf of Giants Stadium in East Rutherford, N.J. This is an obvious work of fiction by Jerry Bruckheimer and the folks at CBS, but the application is real: Human remains have been searched for and found with GPR in real-world forensic applications. Could the same technology have practical use in land surveying and its search for existent land corners that have been deemed lost or obliterated?
To investigate, I contacted John Kessinger, regional director of Ground Penetrating Radar Systems Inc. (GPRS) in Sylvania, Ohio. Kessinger agreed to meet with me and perform a demonstration of GPR equipment. According to Kessinger, 75 percent of GPRS’ work nationwide is dealing with concrete and finding associated objects such as rebar spacing, buried conduit and post-tensioning cables. Another 15 percent deals with environmental issues like buried tanks and dumpsites, and the remaining 10 percent is split between forensics (1 percent) and miscellaneous applications such as geological and archaeological explorations (9 percent). While Kessinger was in our office, he ran his equipment in our conference room. He showed us with precision and accuracy where the heating ducts were, horizontally and vertically, and the thickness of the concrete slab. He could even tell us that the duct material was metallic. This information was interesting, but I wanted to see whether the GPR could locate a culvert pipe or, more importantly, a lost survey stone from the mid-1800s.
Locating AnomaliesI set up a mock scenario to test the GPR equipment. During a road construction project in June 2008, I had exposed a cornerstone on one of our county roads. I had solid reference ties and GPS coordinates to aid in the stone’s recovery, as shown in Figure 1. After excavation, I noted the top of the stone to be 18 inches below the top of the existing pavement. Later in the construction season, the road was reclaimed and resurfaced, and I remeasured the elevation. The stone was still 18 inches deep.
Kessinger set up his GPR equipment--an SIR-3000 with a 400 MHz antenna by Geophysical Survey Systems Inc. (GSSI) of Salem, N.H. Since Kessinger was unfamiliar with survey monuments, I explained the approximate dimensions of a typical stone and let him search. He ran a 1-foot grid back and forth over the search area. Sure enough, he had a hit appear on his screen that was unmistakably a large object under the road (Figure 2). The grid determined that it was an isolated object and not a long one. Of course, it was the stone. He hit the horizontal location precisely, and his depth was within an inch or so of my previously measured 18 inches. The equipment seemed fairly simple to use, and the results appeared easy to analyze with some experience.
At our office, we have to recover many old drain tiles across fields, and for my next test, I wanted to see how well GPR would work in this application. So Kessinger’s next demonstration was to test the equipment on a new crossover with a 12-inch PVC pipe. We knew where the pipe was, of course, but I wanted to see what the screen looked like when the radar found an anomaly. Kessinger was easily able to locate the pipe and even the position of the stone backfill where the crossover was installed. The anomaly showed up on the screen with its horizontal and vertical position, just like the stone. Once again, the vertical distance from the road surface to the top of the pipe was accurate to within an inch or so. Figure 3 shows a screen shot of the 12-inch PVC pipe crossover at a depth of approximately 30 inches. The technology performed flawlessly.
How the Technology WorksGPR operates much like a sophisticated and superexpensive fish finder. But while a fish finder uses sonar (sound navigation and ranging based on acoustic waves), GPR uses radar (radio detection and ranging), which is based on electromagnetic waves. GPR sends radar into the ground at varying frequencies as limited by the Federal Communications Commission (FCC). Unlike sonar and LiDAR, the FCC states that “GPRs operate only when in contact with or within close proximity of the ground for the purpose of detecting or obtaining the images of buried objects.” The FCC also stipulates, “The energy from the GPR is intentionally directed down into the ground for this purpose. Operation is restricted to law enforcement, fire and rescue organizations, to scientific research institutions, to commercial mining companies, and to construction companies.”
GPR is harmless and environmentally friendly. Unlike radiography (X-ray technology), there is no radiation involved. In fact, GSSI states that “it is extremely safe and emits roughly 1 percent of the power of a cellular phone signal.” GSSI further explains in its literature that “GPR works by sending a tiny pulse of energy into a material via an antenna. An integrated computer records the strength and time required for the return of any reflected signals. Subsurface variations will create reflections that are picked up by the system and stored on digital media. These reflections are produced by a variety of materials such as geological structure differences and man-made objects like pipes and wire.” (It is important to note that since GPR operates by transmitting and receiving electromagnetic [EM] energy over a very wide frequency band, cellular phones, two-way radios and pagers should be turned off as they also transmit EM energy and may interfere with a survey. If these devices must be used, they should be kept at least 10 to 20 feet away from the antenna.)
Accuracy, Limitations and Associated CostsThe GPR unit from GSSI has a clicker on the wheel much like a clicker on a typical survey measuring wheel. The unit maps the data by creating a series of baselines and plotting the location of the anomaly over the map. The dimensions along the baseline are tracked by the clicks on the wheel. The baselines or the objects themselves can be located either with GPS (which can be plugged right into the GPR unit) or with a total station. The horizontal locations are accurate to within the size of the antenna. The 400 MHz unit we tested was approximately 1 foot square, but various antenna sizes can be used to perform different tasks. For example, a much larger 200 MHz antenna might be used for mapping objects at greater depths, while a much smaller 2600 MHz antenna would probably be used for detailed concrete work.
The vertical aspect of GPR greatly depends on the soil type and density. In general, GPR works better in areas with sandy soil than in areas with dense, heavy clay. In our experiment with the embedded cornerstone, the soil was a fine, sandy loam. The 1-inch discrepancy might have been because the equipment was picking up the road base on top of the stone itself. “Typically the ratio in which GPR will pick up objects is 1 inch diameter for every 1 foot of depth,” Kessinger says. Therefore, the stone we located (being about 6 inches in diameter) would have disappeared from the GPR screen at a depth of approximately six feet.
The United States Department of Agriculture (USDA) did a study on the effects of soil composition on GPR equipment. According to the study, “A common concern of GPR service providers is whether or not GPR will be able to achieve the desired depth of penetration in the soils of an assignment area. In many soils, high rates of signal attenuation severely restrict penetration depths and limit the suitability of GPR for a large number of applications.” These maps were produced using data from the State Soil Geographic and Soil Survey Geographic databases. Figure 4 shows the GPR soil suitability map of the conterminous United States as published by the USDA and the Natural Resources Conservation Service.
The study goes on to state that “the penetration depth of GPR is determined by antenna frequency and the electrical conductivity of the earthen materials being profiled. Soils having high electrical conductivity rapidly attenuate radar energy, restrict penetration depths, and severely limit the effectiveness of GPR. The electrical conductivity of soils increases with increases in water, clay and soluble salt contents.” In other words, a dry, sandy site yields the greatest depth, precision and accuracy.
Yet another consideration with GPR is cost. GPR units carry a price tag of around $17,000 to $26,000 depending on the desired components and software (e.g., a $17,000 model might include only the basic GPR equipment, while a $26,000 system might be packaged with the antenna and software). Companies such as GSSI also provide contract services at an hourly rate. As with any technology, an increase in demand will lead to lower prices, which will likely drive additional applications for GPR in the future.
The Future of GPRI believe that GPR applications in land surveying will grow exponentially in the next few years. GPR equipment may not be in every private surveyor’s truck, but I do see it showing up at the city, county, state and federal government levels. We will be able to find that hidden field tile, buried oil drum or abandoned pipeline. We will be able to measure the thickness of roads and depth to bedrock, determine rebar spacing in concrete and identify the post-tensioned cables. We can even go on forensic searches or simply look for hidden treasure--or “lost” survey corners.
Incidentally, this technology should make any surveyor think twice before monumenting a corner based on a single or double proportionate measurement. According to Walt Robillard, et al, “When a corner is positioned in accordance with the lost corner theory of proportioning, the surveyor must be convinced that no future surveyor or court will be able to locate the original corner in a different place. Proportionate measurement is always a rule of last resort.”
GPR is one more tool that can now be used to hold this theory true and help us all sleep better at night.
1. “New Public Safety Applications and Broadband Internet Access Among Uses Envisioned by FCC Authorization of Ultra-Wideband Technology,” Federal Communications Commission, Feb. 14, 2002, www.fcc.gov.
2. Frequently Asked Questions about GPR, www.geophysical.com/gssifaqs.htm.
3. Natural Resources Conservation Service, soils.usda.gov/survey/geography/maps/GPR/index.html.
4. Robillard, Walter G., and Wilson, Donald A., “Brown’s Boundary Control and Legal Principles,” 4th Ed., John Wiley and Sons, 1995, p. 218.