Fig. 1. GPR uses radiowaves sent from and received by antennas in fixed array to detect subsurface materials of differing permittivity or polarizability.*
What is Ground-Penetrating Radar (GPR)?
Ground-penetrating radar (GPR) is a non-invasive, mobile geophysical technique that involves directing radiowaves from a transmitting antenna into the earth (Fig. 1). The radiowaves reflect off interfaces between subsurface materials of differing electrical permittivity. The permittivity of a substance describes how polarizable it is. Different subsurface materials have well-known, characteristic, and contrasting permittivities. The reflected radiowaves travel back to the surface where they are recorded by a receiving antenna. The depth and shape of the reflecting interface as well as information about the permittivity of materials on either side of that surface can be determined.
Fig. 2. (right) 3D GPR radargram of 50 x 40 ft area showing several utility lines (bright lines) on a horizontal slice at a depth of 20 inches. One utility line is interrupted at a filled rectangular trench that is slightly brighter than average background (edge at x=22 ft). (left) One GPR profile used to construct the 3D radargram that shows a flat reflection horizon at a depth of 5 ft that represents the trench floor and also several utilities (small bright white/black/white spots). **
A major advantage of GPR is its high spatial resolution. Resolution depends on the frequency of the transmitted radiowaves and their velocity in the subsurface (Fig 3). An antenna with a frequency of 400 MHz has a shorter wavelength, λ, than a 200 MHz antenna and, therefore, higher vertical resolution. The higher the frequency and shorter the wavelength of the radiowaves, the smaller is the depth of penetration.
Fig. 3. Schematic view of radiowave scans C and c from low- and high-frequency GPR antennas that differ in vertical resolution and penetration depth. Layer with thickness ∆D is recorded by the low-frequency antenna as a single reflection (scan C- bold). The high-frequency antenna records the upper and lower contacts of this layer because ∆D is >> wavelength of the high-frequency radiowaves (scan c- bold). Penetration depth varies inversely with the frequency of the radiowaves. ***
We specialize in using GPR to understand the shallow subsurface (geology, man-made materials – structures, ground ice) to depths of 30-60 ft. Geoscy uses antennas with frequencies from 900 to 200 MHz. Not only do we have extensive knowledge of GPR but excel at mapping GPR and integrating with other data.
Fig. 4. GPR profiling in Antarctica by continuously recording with a bi-static 100-MHz antenna. The antenna was dragged at walking speed.
* Annan, A. P., 2009, Electromagnetic principles of ground penetrating radar, in Jol, H. M., ed., Ground Penetrating Radar Theory and Applications Amsterdam, Elsevier, p. 3-40.
** Geophysical Survey Systems, Inc., 2015b, Concrete Handbook: GPR Inspection of Concrete. Salem, NH.
*** Buynevich, I. V., Jol, H. M., and Fitzgerald, D. M., 2009, Coastal Environments, in Jol, H. M., ed., Ground Penetrating Radar Theory and Applications Amsterdam, Elsevier, p. 299-322.