Ground Penetrating Radar (GPR)

Basic Concept

Ground penetrating radar (GPR) is an electromagnetic geophysical method that transmits radio wave pulses at select center frequencies into the ground to study the subsurface. GPR capitalizes on the effects that the electrical properties of matter (i.e., dielectric permittivity, electrical conductivity, and magnetic permeability) have on electromagnetic (EM) energy propagation. If a wave pulse encounters a material interface of sufficiently different electromagnetic properties, some of the energy is reflected back while the remainder continues to propagate.

The amplitude variations and wave velocity-dependent travel time of the returning signal are measured and recorded by the GPR receiver. This information can be used to determine the depth to- and geometry and magnitude of subsurface reflectors. The result of a GPR survey resembles a cross-section, which can depict the subsurface electrical structure and/or reveal features of interest. The GPR method is ideal for subsurface investigations requiring an understanding of geologic structure or the location of discrete objects.

The possible configurations and applications of GPR are numerous. However, the most common deployment includes a transmitter and receiver tied together at a fixed (or common) offset. With such configurations, data are gathered at or near walking speeds, which allows for the high-resolution imaging of near-subsurface interfacial geometry. GPR is a popular method owing to its rapid data collection, high spatial resolution (e.g., sub-meter), and because qualitative/semi-quantitative interpretations are possible in real time during collection.

Conceptual diagram of ground penetrating radar with subsurface layers of different electrical properties — Schematic diagram of a GPR survey. Reflected waves penetrating the subsurface travel at different velocities based on the dielectric permittivity (ε) of the media in which they encounter. All subsurface anomalies, such as the draining pipe, are observed by the GPR. The progression of the survey is represented by dashed lines. (source Brett Trottier, USGS)

Theory

The ground-penetrating radar method relies on the principles of electromagnetic theory. GPR uses high-frequency pulsed radio waves that are generated by and spherically spread out from a transmitter antenna. The portion of the transmitted wave field that penetrates and propagates through the subsurface is the radar (i.e., Radio Detection And Ranging) signal used to image the subsurface. The receiver antenna measures the strength (i.e., amplitude) of the reflections off subsurface features and time required to detect them.

Radar reflection occurs at interfaces with sufficient wave impedance contrast, and the amplitude of the reflected signal is dependent on the contrast magnitude. Radar waves respond to these contrasts according to Snell’s law and Fresnel’s equations, which describe how EM waves are refracted, reflected, and transmitted. Thus, if a portion of energy is reflected to the surface, the remaining part of the wave continues traveling through the subsurface and may reach another reflector or dissipate completely.

While wave impedance can be depicted as a relation between electrical conductivity, dielectric permittivity, and magnetic permeability, conductivity does not significantly influence reflection. Furthermore, the magnetic permeability is often not high enough to impact the interpretation of GPR data and can be considered negligible. However, under certain conditions (e.g., in the presence of magnetite), magnetic permeability can strongly influence radar attenuation and velocity, and such effects can be considered (Van Dam and others, 2013).

Thus, in most subsurface environments, the contrast in dielectric permittivity (ε) is the primary factor that controls GPR signal return. Dielectric permittivity is the capacity of a material to polarize under the influence of an electric field. Note that relative permittivity (ε_R) is often used to describe dielectric properties and has a value relative to the permittivity of a vacuum (εo) such that ε_R = κ = ε / ε_o. At the frequencies employed by GPR, dielectric permittivity is strongly sensitive to variations in water content.

The GPR method uses the two-way travel time (TWT) and velocity (v) of the wave to estimate the depths of reflectors below land surface. Through space, radar waves travel at the speed of light (c) , and their velocity decreases with dielectric permittivity (ε) such that v ≈ c ∕ √ε . Typically, however, GPR analysis uses an average velocity that is estimated using a reference reflector at a known depth, the common midpoint survey method, or literature values (Musset and Khan, 2000).

Initially, the GPR record assumes that reflected radar signals are vertically linear and, thus, the location of a signal-producing reflector is directly below the unit. However, radar waves are conical and can reflect off spatially offset objects. Horizontal surface reflectors are imaged realistically, but discrete objects or spherical reflectors are shown as hyperbolas. The center of a hyperbola-producing reflector is located at the inflection point, and the tails occur because of the invalid vertical assumption.

As a GPR unit is moved toward or away from a point reflector (e.g., pipe/utilities, vertical/dipping structures), the waves continue to reflect off it from different incident angles. Because the TWT increases with distance, the GPR system images it at a deeper location positioned beneath the unit when the signal is received. With adequate velocity information, a migration processing step can be performed that will focus discrete reflectors and correct the slope of dipping reflectors.

Radar waves lose amplitude as they travel through, reflect off, and interact with subsurface materials. Radar energy is lost through the antennas themselves, ground coupling, geometric signal spreading, scattering, and intrinsic attenuation. Intrinsic attenuation (α ) is the loss of radar wave energy in the subsurface through absorption and is typically the principal dictator of GPR depth of investigation (DOI). Smith and Jol (1995) reported a DOI up to 50 meters in dry sand/gravel, and Arcone and others (1995) observed 100 meters in ice.

In theory, intrinsic attenuation is affected by dielectric permittivity, electrical conductivity, and magnetic permeability, where each is a frequency-dependent complex quantity. Practically speaking, however, signal attenuation is primarily controlled by electrical conductivity (σ), and GPR is most useful in low electrical loss (i.e., less conductive) materials. Because of the prevalence of electrically conductive groundwater and clay minerals with high cation exchange capacity, ideal GPR conditions are rare in nature, and DOI is limited.

Applications

Modern GPR surveys typically involve the common-offset configuration where an antenna pair (i.e., a bistatic antenna) is mounted on a pushcart. The unit is moved along a transect while the system transmits radar pulses that last a few nanoseconds and are within the frequency range of 25 to 1,500 MHz. The returning signal amplitude is measured over time, and signal traces are displayed in cross-section on a digital handheld recorder and stored for future analysis.

Separable transmitter and receiver antennas can also be employed by the GPR method. For example, a common-midpoint survey involves symmetrically moving the two antennas away from a point and allows for detailed analysis of the vertical velocity structure. Separate antennas can also be used for transmission surveys, wherein the primary focus is on the transmission of waves through materials (e.g., subsurface materials between boreholes, trees, buildings, etc.).

Because of the spherical nature of the transmitted EM fields, a portion of the field is transmitted above ground. These air waves can reflect off surface objects or electrical utilities and produce artificial reflectors in the subsurface record. Additionally, direct ground waves (i.e., those that travel directly from one antenna to another) and ground coupling can generate strong signals that mask information at the near surface ( ≈ 1.5 times the wavelength).

An additional complication is the ambiguity of data interpretation. Though the GPR record may reveal very detailed stratigraphy, there is no absolute correlation between signals and geologic or hydrologic properties (Wightman and others, 2003). Thus, target calibration may prove beneficial. Site selection is also key to successfully collect GPR data. In environments with surficial layers of saturated clays or saline water, the DOI may be limited to centimeter scale.

One of the most attractive assets of the GPR method is its high-resolution imaging capabilities. However, there is a tradeoff between resolution, which improves with increased signal frequency, and depth of penetration. This is due to the relation between attenuation and signal frequency (i.e., lower frequencies attenuate slower and penetrate deeper than higher frequencies and with lower resolution). Thus, a proper operating frequency should be determined prior to field work based on site parameters and survey objectives.

Despite some pitfalls, the GPR method has been successfully used as an investigative tool applied to hydrogeological, environmental, engineering, and archaeological studies to aid the following:

Mapping bedrock configuration and depth
Identifying karst features, voids, and burrows
Locating pipes, tanks, and other utilities
Delineating soil horizons and soil stratigraphy
Determining ice thickness
Locating the groundwater surface
Determining water depth in lakes
Mapping fractures, changes of rock type, and rock fabrics
Identifying and mapping the extent of groundwater contamination
Detecting unexploded ordinance and unmarked graves
Studying moisture/gas dynamics in soils
Locating lava tubes
Monitoring tree root distribution and health

Monostatic GPR antenna shown collecting data — Monostatic GPR unit steadily being towed by rope. GPR unit contains transmitter and receiver. Distance surveyed measured by survey wheel. Acquisition and data recording performed by field laptop. (USGS stock image)

Examples/Case studies

Bristow, C.S., Augustinus, P.C., Wallis, I.C., Jol, H.M., and Rhodes, E.J., 2010, Investigation of the age and migration of reversing dunes in Antarctica using GPR and OSL, with implications for GPR on Mars: Earth and Planetary Science Letters, v. 289, no. 1-2, p. 30-42, doi:10.1016/j.epsl.2009.10.026.

Abstract: GPR provides high resolution images of aeolian strata in frozen sand in the McMurdo Dry Valleys of Antarctica. The results have positive implications for potential GPR surveys of aeolian strata on Mars. Within the Lower Victoria Valley, seasonal changes in climate and a topographically-constrained wind regime result in significant wind reversals. As a consequence, dunes show reversing crest-lines and flattened dune crests. Ground-penetrating radar (GPR) surveys of the dunes reveal sets of cross-strata and low-angle bounding surfaces produced by reversing winds. Summer sand transport appears to be dominant and this is attributed to the seasonal increase in solar radiation. Solar radiation which heats the valley floor melts ice cements making sand available for transport. At the same time, solar heating of the valley floor generates easterly winds that transport the sand, contributing to the resultant westward dune migration. The location of the dune field along the northern edge of the Lower Victoria Valley provides some shelter from the powerful föehn and katabatic winds that sweep down the valley. Topographic steering of the winds along the valley and drag against the valley wall has probably aided the formation, migration and preservation of the dune field. Optically-stimulated luminescence (OSL) ages from dune deposits range from 0 to 1.3 kyr showing that the dune field has been present for at least 1000 yr. The OSL ages are used to calculate end-point migration rates of 0.05 to 1.3 m/yr, which are lower than migration rates reported from recent surveys of the Packard dunes and lower than similar-sized dunes in low-latitude deserts. The relatively low rates of migration are attributed to a combination of dune crest reversal under a bimodal wind regime and ice cement that reduces dune deflation and restricts sand entrainment.

Gase, A.C., Brand, B.D., and Bradford, J.H, 2017, Evidence of erosional self‐channelization of pyroclastic density currents revealed by ground‐penetrating radar imaging at Mount St. Helens, Washington (USA): Geophysical Research Letters, v. 44, no. 5, p. 2220-2228, doi:10.1002/2016GL072178.

Abstract: The causes and effects of erosion are among the least understood aspects of pyroclastic density current (PDC) dynamics. Evidence is especially limited for erosional self‐channelization, a process whereby PDCs erode a channel that confines the body of the eroding flow or subsequent flows. We use ground‐penetrating radar imaging to trace a large PDC scour and fill from outcrop to its point of inception and discover a second, larger PDC scour and fill. The scours are among the largest PDC erosional features on record, at >200 m wide and at least 500 m long; estimated eroded volumes are on the order of 106 m3. The scours are morphologically similar to incipient channels carved by turbidity currents. Erosion may be promoted by a moderate slope (5–15°), substrate pore pressure retention, and pulses of increased flow energy. These findings are the first direct evidence of erosional self‐channelization by PDCs, a phenomenon that may increase flow velocity and runout distance through confinement and substrate erosion.

Lester, J. and Bernhold, L.E., 2007, Innovative process to characterize buried utilities using Ground Penetrating Radar: Automation in Construction, v. 16, no. 4, p. 546-555, doi:10.1016/j.autcon.2006.09.004.

Abstract: Today's non-invasive technologies for locating buried utilities can be considered as ancient. However, Ground Penetrating Radar (GPR) has recently received significant attention from the scientific community since it showed great promise in detecting landmines. Yet, the complexities of the underground, especially in inhabited areas, makes “seeing-through-the-earth” to find buried utilities extremely difficult. This paper presents the results of a data processing method, called Translation Invariant Wavelet Packet Detection (TIWPD), applied to filtering GPR data collected on a university campus. It first provides a brief introduction into the working principles of scanning the ground with electromagnetic radar waves that are being refracted, scattered, and reflected by buried objects of all sizes and materials. In its main section, the paper presents the results of experimental deployment of the system during a construction project that involved the extensive excavation trenches to lay chilled water pipes. The significance of this paper lies in its use of real-world GPR data to demonstrate the performance characteristics of the filtering process and its validation with the actual condition found during excavation. The encouraging results of this work should provide the basis for developing a near-real time utility detection system that can be used by laborers in the field.

Rodriguez, V., Gutiérrez, F., Green, A.G., Carbonel, D., Horstmeyer, H., and Schmelzbach, C., 2014, Characterizing Sagging and Collapse Sinkholes in a Mantled Karst by Means of Ground Penetrating Radar (GPR): Environmental and Engineering Geoscience, v. 20, no. 2, p. 109-132, doi:10.2113/gseegeosci.20.2.109.

Abstract: Sinkhole development created by diverse subsidence mechanisms (suffosion, collapse, sagging) constitutes the main hazard in most karst terrains. Precisely mapping the limits of sinkholes and inferring the subsidence mechanisms are critically important for the effective mitigation of sinkhole risk. Gathering this information commonly requires the application of subsurface investigation methods, such as geophysics or trenching. Here, we analyze the potential of ground-penetrating radar (GPR) for sinkhole characterization in covered karsts. Extensive GPR surveys have been conducted across two buried active sinkholes of contrasting genetic types (collapse vs. sagging) in the mantled evaporite karst of Zaragoza city, NE Spain. Data were acquired with 100 MHz and 50 MHz unshielded antennas and ∼180 MHz shielded antennas. Interpretation of the processed GPR profiles allowed us to map reliably the boundaries of the sinkholes, characterize their internal geometries (deformation style), infer the subsidence mechanisms, and estimate subsidence magnitudes. The suitability and limitations of the GPR technique in covered karsts are illustrated considering different sinkhole types and sizes, as well as data acquired with different antennas (shielded versus unshielded and various frequencies).

Sonkamble, S., Satishkumar, V., Amarender, B., and Sethurama, S., 2013, Combined ground-penetrating radar (GPR) and electrical resistivity applications exploring groundwater potential zones in granitic terrain: Arabian Journal of Geosciences, v. 7, p. 3109-3117, doi:10.1007/s12517-013-0998-y.

Abstract: Frequent failures of monsoons have forced to opt the groundwater as the only source of irrigation in non-command areas. Groundwater exploration in granitic terrain of dry land agriculture has been a major concern for farmers and water resource authorities. The hydrogeological complexities and lack of understanding of the aquifer systems have resulted in the failure of a majority of the borehole drillings in India. Hence, a combination of geophysical tools comprising ground-penetrating radar (GPR), multielectrode resistivity imaging (MERI), and vertical electrical sounding (VES) has been employed for pinpointing the groundwater potential zones in dry land agricultural of granitic terrain in India. Results obtained and verified with each other led to the detection of a saturated fracture within the environs. In GPR scanning, a 40-MHz antenna is used with specifications of 5 dielectric constant, 600 scans/nS, and 40 m depth. The anomalies acquired on GPR scans at various depths are confirmed with low-resistivity ranges of 27–50 Ω m at 23 and 27 m depths obtained from the MERI. Further, drilling with a down-the-hole hammer was carried out at two recommended sites down to 50–70 m depth, which were complimentary of VES results. The integrated geophysical anomalies have good agreement with the drilling lithologs validating the MERI and GPR data. The yields of these bore wells varied from 83 to 130 l/min. This approach is possible and can be replicated by water resource authorities in thrust areas of dry land environs of hard rock terrain around the world.

Terry, N. and Slater, L., 2017, Gas bubble size estimation in peat soils from EM wave scattering observed with ground penetrating radar: Water Resources Research, v. 53, no. 4, p. 2755-2769, doi:10.1002/2016WR019783.

Abstract: The size of biogenic gas bubbles in peatlands is believed to regulate ebullition of carbon gases to the atmosphere. The measurement of electromagnetic (EM) wave travel times using ground penetrating radar (GPR) is a proven field‐scale method for indirect estimation of volumetric gas content. However, there is also the possibility that information on the size of the gas bubbles can be determined from the analysis of the spectral content of GPR signals as scattering attenuation possesses a frequency dependence for bubbles smaller than the EM wavelength (Rayleigh‐type scattering). Theoretical modeling shows that GPR data acquired with typical antenna frequencies are likely to be affected by bubble size in peat soils. Analysis of GPR data from two recent studies on peat monoliths where biogenic gas production was documented produced results consistent with the model predictions. Using the approach, zero offset cross‐borehole GPR data in a northern peatland suggest that large bubble clusters (i.e., 0.05 m radius) occur in peat. These findings broaden the utility of GPR for providing information on biogenic gas dynamics in peatlands.

References

Annan, A.P., 2009, Electromagnetic Principles of Ground Penetrating Radar, in Jol, H.M., ed., Ground Penetrating Radar Theory and Applications: Amsterdam, The Netherlands, Elsevier Science, p. 3-40.

Arcone, S.A., Lawson, D.E. and Delaney, A.J., 1995, Short-pulse radar wavelet recovery

and resolution of dielectric contrasts within englacial and basal ice of Matanuska Glacier, Alaska, U.S.A.: Journal of Glaciology, v. 41, no. 137, p. 68-86, doi:10.3189/S0022143000017779.

Davis, J.L. and Annan, A.P., 1989, GROUND‐PENETRATING RADAR FOR HIGH‐RESOLUTION MAPPING OF SOIL AND ROCK STRATIGRAPHY: Geophysical Prospecting, v. 37, no. 5, p. 531-551, doi: 10.1111/j.1365-2478.1989.tb02221.x.

Mussett, A.E. and Khan, M.A., 2000, Electromagnetic Methods, in Looking into The Earth: An Introduction to Geological Geophysics: New York, Cambridge University Press, p. 210-232.

Smith D.G. and Jol, H.M., 1995, Ground penetrating radar: antenna frequencies and maximum probable depths of penetration in quaternary sediments: Journal of Applied Geophysics, v. 33, no. 1-3, p. 93-100, doi:10.1016/0926-9851(95)90032-2.

Sonkamble, S., Satishkumar, V., Amarender, B., and Sethurama, S., 2013, Combined ground penetrating radar (GPR) and electrical resistivity applications exploring groundwater potential zones in granitic terrain: Arabian Journal of Geosciences, v. 7, p. 3109-3117, doi:10.1007/s12517-013-0998-y.

Van Dam, R., Hendrickx, J., Cassidy, N., North, R., Dogan, M., and Borchers, B., 2013, Effects of magnetite on high-frequency ground-penetrating radar: Geophysics, v. 78, no. 5, p. H1 H11, doi:10.1190/geo2012-0266.1.

Wightman, W.E., Jalinoos, F., Sirles, P., and Hanna, K., 2003, Application of Geophysical Methods to Highway Related Problems: Lakewood, CO, Federal Highway Administration, Office of Bridge Technology, 742 p.