Subsurface Sensing Technologies and Applications Vol. 3, No. 4, October 2002 (2002)
GPRHistory, Trends, and Future DevelopmentsA. P. Annan* Sensors & Software Inc., 1091 Brevik Place, Mississauga, Ontario L4W 3R7, Canada Receied Noember 23, 2001; reised May 22, 2002Ground penetrating radar (GPR) is a relatively new geophysical technique. The last decade has seen major advances and there is an overall sense of the technology reaching a level of maturity. The history of GPR is intertwined with the diverse applications of the technique. GPR has the most extensive set of applications of any geophysical technique. As a result, the spatial scales of applications and the diversity of instrument congurations are extensive. Both the value and the limitations of the method are better understood in the global user community. The goal of this paper is to provide a brief history of the method, a discussion of current trends and give a sense of future developments.Key Words. GPR.
1. Introduction Before delving into the history, GPR needs denition. GPR uses electromagnetic elds to probe lossy dielectric materials to detect structures and changes in material properties within the materials. Reection and transmission measurements, as depicted in Figure 1, are employed. Most applications to date have been in natural geologic materials, but widespread use in man-made composites such as concrete, asphalt, and other construction materials also occurs. In such lossy dielectric materials, electromagnetic elds will penetrate to some depth before being absorbed. With GPR, the electromagnetic elds propagate as essentially nondispersive waves. The signal emitted travels through the material, is scattered and or reected by changes in impedance giving rise to events similar to the emitted signal. In other words, signal recognition is simple because*To whom all correspondence should be addressed. Phone: 905-624-8409; fax: 905-624-9365; e-mail: email@example.com 2531566-0184 02 1000-0253 0 2002 Plenum Publishing Corporation
Figure 1. Ground penetrating radar uses radio waves to probe the subsurface of lossy dielectric materials. Two modes of measurement are common. In the rst, detection of reected or scattered energy is used. In the second signal, variation after transmission through the material is used to probe a structure.
the return signal looks like the emitted signal. Figure 2 depicts the general character of EM eld phase velocity and attenuation in a lossy dielectric material vs. frequency illustrating the GPR plateau. One has to contrast GPR measurements with electromagnetic induction sounding methods where the elds are diffusive and dispersive in character (Annan (1996)). GPR eld behavior occurs over a nite frequency range generally referred to as the GPR plateau where velocity and attenuation are frequency independent. The GPR plateau usually occurs in the 1 MHz to 1000 MHz frequency range. At lower frequencies the elds become diffusive in character and pulses are dispersed. At higher frequencies several factors increase signal absorption such that penetration is extremely limited.
2. History The following is necessarily brief and intended to give high lights. References lead to other perspectives for those interested in a more extensive understanding of GPR. It is interesting to note that accounts of some activities are published many years later and sometimes not all.
19001950 During this time a great deal of research on radio wave propagation above and along the surface of the Earth occurred. Although several hints
GPRHistory, Trends, and Future Developments
Figure 2. General character of EM eld phase velocity and attenuation with frequency illustrating the GPR plateau. (a) Shows the ground typical character whereas (b) and (c) show the detailed behavior for a simple material and the relationship to relative permittivity K and electrical conductivity . Z0 is the free space impedance, 377 Ohms.
at the possibility of using radio waves to probe the subsurface are mentioned, there are no reports of successfully making this type of measurement. A vast number of papers appeared on the subject of communications, direction nding, and radar. 19501955 In this time frame, the rst reported attempt at measuring subsurface features with radio wave signals was reported. El Said (1956) attempted to
use the interference between direct air transmitted signals and signals reected from the water table to image the water table depth. 19551960 The next reported observation of radio frequency sounding of geological materials came about when the USAF reported altimeter errors when attempting to land aircraft on the Greenland ice sheet (Waite and Schmidt (1961)). This was the rst time that repeatable indications of penetration into the subsurface through a naturally occurring material were reported. This spawned the era of researchers focused on developing radio echo sounding in ice. 19601965 The majority of activity during this interval involved the radio echo sounding in ice. Groups, such as the Scott Polar Research Institute at Cambridge, Bailey et al. (1964) and the Geophysical and Polar Research Center at the University of Wisconsin, Bentley (1964), Walford (1964) were active in polar regions and also on glaciers. 19651970 During this time the ice radio echo sounding activity continued. In addition, applications in other favorable geologic materials started to be explored. Cook (1973) explored the use in coal mines since coal can be a low loss dielectric material in some instances. Similarly, Holser et al. (1972), Unterberger (1978), and Thierbach (1973) initiated evaluations in underground salt deposits for similar reasons. This period was the start of lunar science mission planning for the Apollo program. Several experiments were devised to examine the lunar subsurface which was believed to have electrical character similar to that of ice. The work of Annan (1973) reports on some of these developments. Key discoveries were the understanding of wave elds about antennas on the ground surface and modied antenna directivity as indicated in Figure 3. 19701975 This period saw numerous advances. The Apollo 17 lunar exploration program involved the surface electrical properties experiment (Fig. 4) which used interferometry concepts similar to the work carried out by El Said (1956) while the lunar orbitor carried a pulsed radar sounder similar
GPRHistory, Trends, and Future Developments
Spherical air wave Evanescent wave Head or lateral wave
Air GroundSpherical ground wave
TE Antenna Pattern90120 60
(d)Figure 3. (a) Wave fronts and antenna directivity for a small electric dipole on the surface of a dielectric. (b) and (c) Show the TE and TM antenna directivity when the source is placed on the surface of a dielectric. (d) Shows the variation of the TE pattern as relative permittivity is changed.
Figure 4. The surface electrical properties experiment carried out on Apollo 17 used a 3 component vector receiver mounted on the lunar rover and a dual axis multi-frequency dipolar antenna laid out on the surface to sound the subsurface.
to the ice sounders which made measurements from orbit over the lunar surface (Simmons et al. (1973) and Ward et al. (1973)). During the same period Morey and others formed Geophysical Survey Systems Inc. which has been manufacturing and selling ground penetrating radar since that time (Morey (1974)). In addition a better understanding of electrical properties of geologic materials at radio frequencies started to become available. Work such as that by Olhoeft (1975, 1987) led to a much better understanding of the electrical character of natural occurring geological materials and the relationship between electrical conductivity and dielectric polarization of these materials. 19751980 During this period, applications started to grow because of the availability of technology and a better understanding of geology. The Geological Survey of Canada explored a number of applications, the primary one being
GPRHistory, Trends, and Future Developments
Figure 5. GPR system being used to survey potential pipeline routes in the Canadian Arctic (1975).
a better understanding of permafrost terrain in the Canadian Arctic. A GPR system in operation is shown in Figure 5. Proposals for pipelines out of the Arctic to carry oil and gas to southern markets drove a great deal of interest in engineering in frozen soil environments. GPR was a tool which offered great promise and some of the initial results are reported by Annan and Davis (1976). During this period the effect of scattering on radio echo sounding in temperate glaciers became better understood. The impact of scattering and the need for lower frequency radars was reported by Watts and England (1976). Experiments with GPR were reported by the Stanford Research Institute where measurements were made by Dolphin et al. (1978) for archeological applications. Other work carried out in this period which paralleled the Geological Survey of Canada permafrost efforts was lead by Olhoeft at the United States Geological Survey who worked on the Alaska pipeline routes. Extensive work was carried out in potash mines in western Canada. This led to a whole series of ever improving GPR measurements and work in this geological setting by the Geological Survey of Canada. These results were reported by Annan et al. (1988). Further coal mine developments were reported by Coon et al. (1981).