Human beings throughout history have always been curious about what lies beneath their feet. Digging holes to satisfy that curiosity was laborious and slow, but was the only option up until 1910 when German scientists Gotthelf Leimbach and Heinrich Löwy applied for a patent for a continuous wave borehole system that could be used to locate buried objects. The German patent was applied for only six years after the first radar patent was issued in 1904. The first acknowledged use of GPR, to image the subsurface came in 1929 when it was used to measure the depth of a glacier.
Radar technology and techniques advanced significantly during the 1960’s and 1970’s – particularly for use in sky and sea scanning radar equipment which has transformed the face of modern logistics and defence. Alongside this the hardware and software used for GPR surveys improved significantly during the 1970’s. However, there was limited interest in the technology until 1972 when NASA equipped the Apollo 17 moon mission with a bespoke GPR unit, which was able to achieve penetration of up to 1.3km (.807 mile) and was able to store its findings on film.
Following this, Ground Penetrating Radar quickly moved to the forefront of non-destructive subsurface investigative technologies. In the mid 1970’s, GPR was used in archaeological studies in Chaco Canyon, New Mexico, marking the first known successful use of GPR on an archaeological site. Initially, Ground Penetrating Radar technology was still very expensive and was mostly used for research by universities and government agencies. It wasn’t until the 1980s, when GPR units became relatively affordable, that GPR was adapted for widespread commercial and scientific use.
GPR uses high-frequency radio waves usually in the range of 10 MHz to 2.6GHz. The transmission equipment emits electromagnetic energy into the ground. When that energy meets with a buried object or a boundary between materials having different permittivities (the ability of a substance to store electrical energy in an electric field) it may be reflected or refracted or scattered back to the surface. An antennae receives the information then captures and records the variations in the signal. The principles involved in GPR are similar to seismology (the science that looks at earthquakes) except it utilises electromagnetic energy rather than acoustic energy.
The depths that GPR can reach depend obviously on the unit being used but also on the material being surveyed. GPR can be used to penetrate ice to a depth of several thousand metres at low frequencies. Dry sandy soils or massive dry materials such as granite, limestone or concrete tend to be resistant and the depth of penetration could be limited to only a few metres. In damp or clay soils and materials with high electrical conductivity, the ability to penetrate by GPR can be limited to as little as a few centimetres.
Today, GPR is considered a necessary tool for a wide range of commercial and academic projects that fall mainly into the following categories:
1. Archaeologists and forensic scientists – use GPR to identify buried structures, human remains, and artefacts safe in the knowledge that such investigations won’t cause any damage.
2. Defence – use GPR technology to identify landmines and IEDs in combat zones so they can be safely avoided and/or disposed of.
3. Engineering and construction – GPR is used for everything from initial construction site analysis (looking at the composition of the land) and locating existing services to locating and evaluating structural steel and voids in concrete walls and floors.
4. Geology – GPR is currently used in a wide range of projects from conducting geological surveys to measuring the depth of polar ice to locating accessible groundwater deposits and even fossilized dinosaurs and other paleontological remains.
The types of GPR units and scans, and the software that makes them useful as field instruments, have evolved as well. The first GPR results looked more like a medical readout and were limited to a single power setting and an incredibly bulky scanning apparatus. In addition, they didn’t have onboard storage, high-capacity hard drives, or 3D mapping capability. Today’s most commonly used GPR scanners weigh approximately 25kg, can fit in a backpack and can create real-time 3D maps of the subsurface being scanned. They also use alternate power settings to deliver high-resolution/low-penetration mapping and vice versa, giving a more comprehensive view of what lies under the surface. And, best of all for the construction industry and engineering applications, they’re fast, taking between 360-400+ scans per second.