Ground Penetrating Radar
Written by Sara Gale   

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Forensic investigations are making use of advanced technology, especially in investigations seeking buried evidence. Among the tools being used more and more is ground penetrating radar (GPR), which can be used to locate victims and hidden caches of weapons or evidence, and even find the remains of soldiers and civilians killed in war zones. In addition to finding positive evidence, GPR is a useful tool for narrowing down or limiting areas of interest to reduce costly and disruptive excavation.

Comparing commonly used crime scene investigation tools

Forensic scientists and crime scene specialists use a variety of technologies and tools to look for buried evidence and clandestine burials. Such tools include soil probes, cadaver dogs, and metal detectors, along with expert geophysical methods such as ground penetrating radar (GPR) and electromagnetic induction (EMI).

Soil probing is extremely cost-effective in terms of initial investment, requiring only a simple probe and a sheet of paper to record soil compaction results. However, the process may require covering a large area at a high sample density, which takes a great deal of time—both time to collect data and to gain the skills required to consistently record compaction levels.

Metal detectors are relatively affordable and easy to use. They are well suited for finding buried evidence and shallow burials that contain ferrous metal. However, they have a limited penetration depth and are unsuitable for investigating non-ferrous (metal) objects—for example, an older burial. Despite these drawbacks, metal detectors should be a part of any crime scene investigator’s toolkit.

Cadaver dogs are considered a valuable tool by some forensic experts. The level of accuracy and consistency is highly variable. Dogs can only be used for buried evidence with a human component. Some dogs are trained to detect only those still living or recently deceased, while others are trained to detect older burials, and a few are trained in both.

Electromagnetic induction is a geophysical method that looks for disturbed soils by finding changes in soil conductivity. Disturbed soils vary in conductivity from surrounding soils. Best used in open fields or wooded areas, EMI can be used quickly but does not provide depth estimates.

Ground penetrating radar is a key tool used by law enforcement and crime scene investigators for locating evidence behind brick or concrete walls, under wooden floors, or in hidden compartments, including clandestine burials and buried objects. It can be used to identify disruptions in soils, so even if the evidence itself is difficult to resolve, simply identifying that a hole was dug can point investigators in the right direction. It can also rule out suspect areas, which limits excavation, saving time and money.

For forensic investigations, GPR is typically used with a 400 MHz antenna (or one with a similar frequency), which has a depth range of up to 15 feet in ideal soil conditions. This makes it perfect for buried remains, whether formally or expediently (hurriedly) buried. The resolution of the 400 MHz antenna is high enough to visualize evidence as small as a loaf of bread in good soils. Lower-frequency antennas reach greater depths with less resolution, so they can sometimes be used if the buried evidence or remains are deeply buried.

Rise in interest in cold cases spurs use of GPR

The rising interest in cold case investigation has brought with it an increased use of advanced technology, including geophysical methods.

For example, Steve Shiner of the Henry C. Lee Institute of Forensic Science, affiliated with the University of New Haven in West Haven Connecticut, says GPR is being used for a variety of cold cases. Investigators occasionally obtain information on the location of a clandestine gravesite and have to go out to locate it. If the burial is old enough, there may not be much more than a stain or a hole left to identify. Other cases may involve following up on the possibility of buried evidence, for example, a weapon, a shipping container – and in one case, artwork.

The Institute received a federal grant for a variety of new technologies and a mobile crime scene van, which it makes available to Connecticut law enforcement agencies. Equipment with the van includes GPR systems, a variety of imagers, X-ray equipment, lasers, cameras, crime scene processing equipment, and alternative light sources.

The Institute has used GPR on sites as consultants to the police department, usually for cases looking for buried bodies, weapons, explosives, or various types of other evidence. In one case, it was used to look for artwork, jewelry, and other fenced stolen items. They use a 400 MHz antenna, which Shiner says works well in the 0 to 12 feet range. “Soils in the Northeast, Florida, and the Northwest are very good for the use of GPR. Once you get to the central part of the country you get a lot of mixed suitability. There were several cases in Connecticut where GPR was used to look for buried bodies.”

The Institute also provides forensics training to University of New Haven students, officers and forensics scientists from around the world and graduate students from across the United States. Students may participate in visiting scholar programs, 6-month residencies, as well as a variety of one-day, weeklong, or specialized courses.

GPR training is one of the central features of the Institute’s courses. To provide a real-life hands-on training experience the Institute plants graves with laboratory skeletons and salts a 3-acre field with evidence, like cartridge casings, knives, and clothing. The evidence is planted in the soil several months before the class is scheduled so the soil can settle naturally and the evidence can react with the elements.

“Using heavy excavation equipment takes a lot of time and expense and you may be ripping up a lot of ground for nothing in return, so using GPR helps screen areas before bringing in the excavators,” says Shiner. “We teach the students how to use GPR to narrow down a large area of interest to a smaller area. Once the students identify where they think the burials and buried evidence are located, they excavate it.”

The classes focus on showing students how to operate the equipment and interpret the data while in the field. Learning how to move the antenna over the topography and in different survey conditions while also interpreting the data in real time is a skill that takes both training and experience. Students without access to GPR equipment are interested in receiving hands-on instruction, which they can take back to their own agencies and countries.

Students are instructed in ground-penetrating radar theory and operation. Photo courtesy of Henry C. Lee Institute of Forensic Science.

Georgia agency uses GPR for felony investigations

In addition to its use in training the next generation of forensic scientists, GPR is also employed by law enforcement agencies in active investigations around the country. One example is the Georgia Bureau of Investigation (GBI), an independent, statewide agency that assists the state’s criminal justice system with felony investigations, forensic laboratory services, and computerized criminal justice information.

Eric Schwalls, a crime scene specialist in GBI’s investigative division, explains that the division obtained GPR a few years ago after a local technical college closed and its equipment (unopened and still in boxes) was donated to state agencies. Schwalls jumped at the chance to add the equipment to his toolkit. Included in the equipment cache was a system from Geophysical Survey Systems, Inc. (GSSI) with a 400 MHz antenna. Training on the equipment at GSSI’s New Hampshire headquarters was also part of the deal.

Since acquiring the system, only two of the 30 crime scene specialists have been trained on using the GPR, which they have used to search for clandestine graves, buried vehicles, and Conex boxes (shipping containers). “We had one case where information came in about a meth lab underground in a Conex box buried in a field,” said Schwalls. “There was no visual above-ground evidence of it being buried so we used the radar to see if they had buried something. Nothing was found there, so we could confirm the lack of evidence rather than having to dig out the entire field.”

Schwalls confirms one of the key benefits of GPR use that was noted earlier: using GPR can speed up an investigation by reducing or eliminating the digging portion. “If we can tell there is nothing underground there is no reason to dig.” Scanning a larger area tells investigators where any anomalies may exist, narrowing the dig site. Finding a lack of evidence (negative data) is just as important as finding positive data and the GPR equipment is an important tool for this purpose.

In one case Schwalls used the GPR to search a vacant lot next to a residence in North Georgia. The local department had information that a body may have been buried next door to the residence or under a concrete slab on the property. The GPR displayed an anomaly at a depth of 4 to 5 feet but nothing was noted under the concrete slab. The area where the anomaly was located was dug up and the local agency found buried asphalt ­—but no body. The scan saved the local agency time and money; without the use of GPR, the entire lot, along with the concrete slab, would have been excavated.

GPR has also been used in several cases involving graveyards. In one case, a church requested GBI’s assistance in determining whether an open field next to the church had previously been a cemetery. “We used the radar and discovered there were graves in the field. There were no headstones but we determined it was actually a graveyard for that church at one time. That church goes back to about the 1800s and different buildings have been built on and around the property but by the time of the request nobody knew that the field had been a graveyard.”

Schwalls also notes that GPR data can be incorporated with data from the GBI’s Leica scanner, a measuring tool that creates a crime scene or structure in a 3D digital environment. This is an exciting new development in which subsurface radar data is exported out of the GPR software and goes through AutoCad into the Leica software to tie it in with the Leica 3D model. Creative use of data like this is becoming an important part of forensic investigations.

Still, Schwalls acknowledges that GPR technology is expensive relative to its niche uses. With only a dozen cases over the past five years, he does not think the GPR technology is as cost effective as it could be, especially given the cost of training and equipment upgrades. As local agencies discover the advantages of using the GPR, they will avoid the unnecessary expense of using heavy equipment to excavate huge swaths of land. Whenever GBI gets a crime scene request mentioning a search for buried bodies, they recommend the use of the GPR rather than rushing in to dig with a back hoe before they arrive.

At present, the biggest hurdle to incorporating GPR into an investigation is having only two trained GPR operators so leaving the region for a case means the region is unoccupied. “We have had requests where someone wanted us to come on a particular day, due to search warrant and time constraints. Both trained operators were busy with other job duties, so the agency had to dig up an entire back yard for no reason.” Luckily, GBI now has approval to get two more investigators trained and is hoping to add funds to the budget in the future for additional equipment.

Eric Schwalls searches for buried cars and car parts using the GPR. Photo courtesy of Georgia Bureau of Investigation.

Lessons learned from the GBI and Henry C. Lee Institute

A few common themes emerge in the discussions with the GBI and Henry C. Lee Institute. First of all, it is clear that integrating GPR with crime scene investigation requires a certain amount of expertise, which takes time to develop. The ability to run a GPR system, interpret the data, and provide meaningful feedback for a crime scene takes training and experience. Developing expertise with any geophysical tool may seem daunting if the user does not know where to find training or support.

Training is another important theme, and Shiner and Schwalls offer two different examples of paths that can be taken. GPR manufacturers offer training and support, but don’t always have someone in-house familiar with forensic applications. Manufacturer classes can range from free, with the purchase of a GPR, to several thousand dollars depending on location and number of days. Non-manufacturer-specific classes, like those taught at the Henry C. Lee Institute of Forensic Science, offer a forensic-focused approach to learning the geophysical methods. The cost for these varies highly based on location and length of the class.

Both training and in-field use are necessary to becoming an expert in GPR and a crime scene specialist should be able to participate in both, as well as have time in the field. There is not currently one class or certification that a person can earn to validate their knowledge of GPR for use in crime scene investigation.

Another challenge is educating the novice on how GPR and other geophysical methods fit into the forensics toolkit. An agency may not solicit a geophysical investigation if they do not understand how the method works, or how it would benefit their case.

Take the example Schwalls presented of clearing areas of suspected burials using GPR not to make a positive identification but to save time and money by not having to excavate a large area. An important part of integrating geophysical methods into crime scene investigation is educating the non-users on which cases, or circumstances, may benefit from these tools. A more concerted effort should be made within the industry to educate or guide people through the process of deciding whether use of geophysical methods is warranted on a case by case basis.

Finally, the last hurdle to integrating geophysical methods, and sometimes the biggest, is funding. A GPR suitable for forensic investigations will cost between $10,000-25,000, making it an investment. This system typically includes a control unit that powers the system and provides a screen for viewing the data in the field for real-time identification, a 400 MHz, or similar frequency antenna that provides a high enough resolution image for most clandestine burials, and a cart or survey wheel for tracking distance.

Schwalls explained that their GPR system is used sporadically over the course of the year, even though they cover an entire region and not just one city or county. More education within the region could increase GPR use, but it can be challenging to support the purchase of a system if it will only get used a couple of times a year. Schwalls has been called upon to use the GPR in different states and outside of his region. Collaboration with other agencies or universities is a good way to not only share expertise, but also share funding.

The use of GPR in forensic investigations shows great promise, but tends to remain challenging for a local municipality or county due to budgetary concerns. More success is reported if there is a regional focus or in densely populated areas, like New York City, or Miami-Dade in Florida. As more case studies are developed and shared that show the benefits of using geophysical methods in crime scene investigation, finding funding will become less of a hurdle. When agencies learn about the benefits of GPR in finding evidence and saving money on large area excavations, its use is sure to grow.

About the Author

Sara Gale is an Application Specialist and Technical Trainer for Geophysical Survey Systems, Inc (GSSI), the world leader in ground penetrating radar. Sara has 15 years of experience with ground penetrating radar in the archaeology and forensic science fields, and her Masters in Anthropology from the University of Denver focused on the application of geophysical methods prior to excavation. Sara actively participates in a number of professional archaeology and forensics groups and volunteer associations.

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