Unmarked graves, such as those in which murder victims are often callously concealed, may soon be easier to locate, thanks to a technique demonstrated by US researchers.
They have found that LIDAR, a tool proving increasingly useful in archaeology, can also be applied to forensics.
They have shown that the shallow graves these bodies are sometimes buried in are characterised by an elevation change in the ground, and that this elevation change can be positively identified using LIDAR.
"Unmarked human graves can be difficult to locate because their surfaces are often camouflaged through natural processes and/or deliberate concealment," the researchers wrote in their paper.
"Natural processes include new vegetation growth, dry leaf litter, or other debris accumulation. Deliberate concealment can involve perpetrators attempting to hide a body by mimicking these natural processes."
According to the research team, when a person is buried in a shallow grave, the landscape always undergoes a few changes.
First, there's always disturbed soil. And, because there's now additional mass in the ground, the elevation rises. As the body decomposes, the ground over the grave then sinks in a little, resembling a depression.
To determine if LIDAR is capable of detecting these changes, the team spent two years, over 2013 and 2014, conducting an experiment.
They obtained several corpses donated to forensic science, and buried them in unmarked graves at an outdoor forensic research facility in Tennessee.
The first grave contained a single body, the second grave contained three bodies, and the third grave contained six bodies - all without coffins or body bags.
A fourth grave was dug and filled in without a body, to serve as a control. The original grass sod was replaced on top of the graves once they had been filled in.
They used a tripod-mounted, ground-based LIDAR to survey the area both before the bodies were buried, and at several points after. This was so they could compare the undisturbed ground to the elevation changes made by the burials, and track those elevation changes over time.
Even when the graves were covered with leaves and debris, the LIDAR was able to detect elevation changes.
"This study established that localised elevation change at human burial and non-burial disturbance surfaces is measurable and separable from changes at undisturbed surroundings," the researchers wrote in their paper.
"Three observable - and potentially overlapping - phases of elevation change are noted from our limited collections: (1) localised elevation gain following initial burial, (2) localised elevation loss during soil settling and decomposition, and (3) stasis, characterised as uniform elevation change across all surfaces. Measuring elevation change with terrestrial LIDAR may prove useful in identifying a disturbance signature for narrowing down unmarked graves."
They also found that the technology can be used to monitor ongoing disturbance activity, since it picked up a backfill pile that was not able to be immediately returned to the grave.
Given that current methods of locating unmarked graves are arduous and time-consuming - such as surveying the land by foot, conducting exploratory excavation, and even using ground-penetrating radar - this technique could help narrow down where to look.
"We acknowledge that signs of disturbance are not necessarily signs of burial activity, but the aim for this study is to support the narrowing down of possible unmarked grave locations," the researchers said.
"An approach that combines LIDAR data with other established methods may improve overall results."
Additional research is required to apply this technique to different localised environments, and improve the database of what these elevation changes look like under LIDAR surveys.
As of 2017, there were 651,226 missing persons recorded in the US National Crime Information Center, of which 88,089 were active. There are millions more missing persons around the world.
If this technique can help more families find lost loved ones, it's a great leap forward.
The team has published their research in the journal Forensic Science International.