For years, police have been using Ground Penetrating Radar (GPR) — the same technology used in construction projects to find underground water, power, and sewer lines — to dig up forensic evidence including buried drugs, weapons, and even bodies.
Rather than sending radar waves through the air to pinpoint airplanes and speeders, GPR sends radar energy into the ground. Some of that energy then bounces back to a receiving antenna when it hits something in the soil, and “pings.”It can be key to cracking cold cases. GPR was used to unearth the remains of King Richard III, who died in the 15th century — and much more recently in the investigation of the 1997 disappearance of college student Kristin Smart.
According to the The Blade, GPR technology was also used to search for the remains of Natalee Holloway in Aruba after she disappeared in 2008.
But the technology has its limits, and factors including soil condition and the experience of the technician using it can make or break a case.GPR can’t find skulls or bones.
Instead, scientists say that the technology finds breaks and disturbances in the soil’s natural horizontal layers, such as a blanket or coffin that the body could be buried in.
GPS can detect irregularities, but often can’t be more specific about whether they are tree roots, rocks, or other objects that could confuse the case.
GRS can detect irregularities in the soil, but can’t identify the source.
Jim Doolittle, a soil scientist with the USDA-Natural Resources Conservation Service (NRCS) Doolittle said in an interview, reported by the Soil Science Society of America, that he was once asked by law enforcement to locate crushed cars that could contain a murder victim’s remains. He found irregularities in the soil, he said, but was stressed because unearthing them would involve digging in a “backyard was right out of Better Homes and Gardens magazine,” he says, “with lush plantings and elegant walkways and fountains.” In the end, the police dug up the yard and found the cars — but not the victim’s remains.
In the Kristin Smart case, a contractor named Gary Mann performed a GPR search in the backyard of the home owned by suspect Paul Flores’s mother Susan Flores in Arroyo Grande, and The Daily Beast reported that the results were “inconclusive.”
But they can still provide key facts. Mann “said he felt there was an older man-made type excavation below the second step on the east side of the house” that “did not not look like a natural geographic feature and is about six feet long and five and one half feet deep,” according to a transcript of the FBI search warrant submitted by FBI Agent Jack Schaefer in 2000, provided by KristinSmart.com.
He later performed additional testing on the house next door, and said he had concluded that the excavation did not continue under the fence to the adjacent yard.
Experts say that most operators can learn the basics of the technology in just a few days — but experience matters. In the Smart case, Mann explained that he was “less sure of his findings because he did not have the right equipment for GPR testing for bodies” and he “lacked experience in that this was the first time he conducted a GPR search for a grave site.”
When it comes to the condition of the soil, it’s all about the (pH) numbers.
Soil scientists now know that under certain conditions the radar energy gets absorbed by the chemical properties of the soil rather than reflecting back, according to Mary Collins, a retired University of Florida professor and GPR expert.
GPR can’t “see” anything, in other words, under high pH, high salt, or high clay conditions. In a 2012 report for the Department of Justice, PhD John J. Schultz concluded that “dry soil or low soil moisture resulted in reduced demarkation of the grave.”
Since there are so many factors involved, it may make sense to revisit the scene of the crime. Like DNA or any other technology, GPR equipment gets better every year — so in some cold cases, it may make sense to dig again.
Main image: Using GPR. [The Official CTBTO Photostream via Wikimedia Commons]