Borehole-Radar Reflection Logging

Basic Concept

Borehole-radar reflection logging is an electromagnetic (EM) wireline technique that uses radio wave pulses at select frequencies to image the formation intersected by and around boreholes. The principles of borehole-radar reflection are comparable to those of the ground penetrating radar method. Borehole-radar reflection systems transmit impulsive radar (i.e., Radio Detection and Ranging) waves and measure their response to subsurface materials.

Analysis of the returning signal amplitude and travel time provides high resolution information of the location, orientation, and lateral extent of electromagnetic discontinuities (i.e., reflectors). Borehole-radar reflection logs can be interpreted qualitatively/semi-quantitatively in real time and provide structural, petrophysical, and hydrological information about the formation in the vicinity of the well (Haeni and others, 2002).

Theory

Borehole-radar reflection systems consist of a fixed antenna pair (i.e., transmitting and receiving) that is moved at specific intervals throughout open or polyvinyl chloride (PVC) cased boreholes. The transmitting antenna generates radar pulses that propagate as waves through the materials surrounding the borehole. Radar waves interact with the materials and can be redirected to the receiving antenna. Wave interaction depends on the wave impedance of the material, which is a function of electrical conductivity (σ ), dielectric permittivity ( ε ), and magnetic permeability ( μ ).

Radar waves that encounter interfaces with a wave impedance contrast are reflected, refracted, or scattered. Given a relatively significant contrast, a portion of radar energy is reflected back to a receiving antenna. The reflected signal amplitude, which depends on the wave impedance contrast magnitude, is measured and recorded. The remaining portion of energy continues traveling through the subsurface until it reaches another reflector or dissipates completely.

The reflected signal has a wave velocity-dependent two-way travel time (TWT) that is used to determine the distance to and geometry of reflectors. Thus, the wave velocity ( v ) is needed to accurately deduce the subsurface structure and can be determined using common-midpoint analysis (see ground penetrating radar for more details). Dielectric permittivity (ε ) is the primary factor that controls radar velocity through an earth material. Through space, radar waves travel at the speed of light ( c ), and their velocity through a material decreases with its ε such that

v ≈ c / √ε

Dielectric permittivity (ε ) is capacity of a material to polarize under the influence of an electric field and it is the primary factor that controls radar reflection in most subsurface environments. Relative permittivity ( ε_R ) is often used to describe dielectric properties and is relative to the permittivity of a vacuum ( ε_o ) such that ε_R = ε /εo . The ε_R of common minerals ranges from 4 to 9, whereas freshwater has a ε_R of 81. Thus, ε_R is sensitive to and often indicates water content variations (Haeni and others, 2002).

Applications

Single-hole radar-reflection logging can be conducted with omnidirectional or directional receiving antennas. Omnidirectional antennas are more common and able to identify the location, dip angle, and lateral continuity of planar reflectors. However, directional antennas are required to determine the strike of a planar reflector or azimuth to a point reflector. Additionally, borehole deviation logs are critical for accurate orientation information, and can be used to support radar logs collected with both antenna types (Haeni and others, 2002).

Radar-reflection tools image a larger volume than most other borehole methods and are able to detect features in the formation that do not intersect the borehole (Johnson and Joesten, 2005).

Thus, radar-reflection logging can be conducted using two or more boreholes to image the materials between them. Cross-hole radar logs often employ antennae at the same elevation or use tomographic methods that have a stationary transmitter in one borehole and a trolling receiver in another. See Lane and others (2004).

Generally, the attenuation of radar waves controls the radius of investigation (ROI) of the tool and is greatly controlled by the electrical conductivities of the formation and fluid. The ROI may reach 40 meters in resistive materials and less than 1 m in highly conductive environments. Thus, radar-reflection methods are most useful in low electrical loss materials, and prior knowledge of conductivity-influencing parameters (e.g., lithology, porosity, saturation, clay content, conductivity) is beneficial (Singha and others, 2000).

Radar-reflection logs can theoretically resolve features larger than ¼ of the signal wavelength, which is equal to the ratio of formation velocity to signal frequency. As such, resolution is also related to velocity (or dielectric permittivity) and frequency, and, thus, resolution may be improved by increasing signal frequency. However, attenuation is also frequency dependent (i.e., higher frequencies typically penetrate less than lower frequencies), and there exists a trade-off between resolution and ROI.

Borehole-radar logs have supported numerous environmental studies. Because measurements combine matrix and fluid properties, borehole radar can be used to identify fractures or layers containing conductive contaminants (Dorn and others, 2011). Tracer pathways can be deduced using transient or differencing logs that remove background measurements from saline tracer measurements (Lane and others, 1996; Lane and others, 2004). Additionally, even prior to any conversion, radar-reflection data may reveal key features (e.g., water-bearing fractures, lithologic contacts, cavities/voids) and can aid the following applications:

Detection and characterization of fracture zones
Characterization of rock or stratigraphic layers behind PVC casing
Imaging of bedding planes, lithologic contacts, fractures, cavities
Investigations of water supply
Mapping saline tracers or remediation efforts (with differencing/transient logs)
Mapping the extent of groundwater contamination characterized by conductive fluids

Examples/Case studies

Dorn, C., Linde, N., Le Borgne, T., Bour, O., and Baron, L., 2011, Single‐hole GPR reflection imaging of solute transport in a granitic aquifer: Geophysical Research Letters, v. 38, no. 8, 5 p., doi:10.1029/2011GL047152.

Abstract: Identifying transport pathways in fractured rock is extremely challenging as flow is often organized in a few fractures that occupy a very small portion of the rock volume. We demonstrate that saline tracer experiments combined with single‐hole ground penetrating radar (GPR) reflection imaging can be used to monitor saline tracer movement within mm‐aperture fractures. A dipole tracer test was performed in a granitic aquifer by injecting a saline solution in a known fracture, while repeatedly acquiring single‐hole GPR sections in the pumping borehole located 6 m away. The final depth‐migrated difference sections make it possible to identify consistent temporal changes over a 30 m depth interval at locations corresponding to fractures previously imaged in GPR sections acquired under natural flow and tracer‐free conditions. The experiment allows determining the dominant flow paths of the injected tracer and the velocity (0.4–0.7 m/min) of the tracer front.

Greenwood, A., Caspari, E., Egli, D., Baron, L., and Holliger, K., 2018, Characterization of a Hydrothermal Fracture Network Embedded in Crystalline Rock Utilizing Borehole Radar and Geophysics, in Proceedings, 24th European Meeting of Environmental and Engineering Geophysics: Porto, Portugal, European Meeting of Environmental and Engineering Geophysics, p. 1-5, doi:10.3997/2214-4609.201802588.

Abstract: A near-vertical hydrothermally active fault zone, embedded in sheared and fractured crystalline rocks of the Central Swiss Alps, has been drilled and geophysically explored in view of its potential similarities with planned petrothermal reservoirs in the Northern Alpine foreland. The GDP1 Grimsel Pass borehole acutely intersects the core of Grimsel Breccia Fault and is situated entirely within its surrounding deformation zone. Pervasive brittle deformation, which overprints Alpine shear zones is characterized by fractures of varying aperture and is the dominant response in all of the borehole data. In this study, we utilize borehole radar data to image fluid-filled fractures outside of the immediate vicinity of the borehole. This is possible due to their strong permittivity contrast with respect to the granitic host rock. In combination with optical televiewer data analysis, the borehole radar image indicates a complicated network of intersecting fractures. Additionally, to shed more light on the characteristics of the system, we use tube wave data from a hydrophone VSP survey and self-potential data. The former is responsive to mechanically compliant and hydraulically transmissive areas, whereas the latter may indicate zones of in- and outflow.

Liu, S. and Sato, M., 2006, Subsurface Water-filled Fracture Detection by Borehole Radar: A Case History: Journal of Environmental and Engineering Geophysics, v. 11, no. 2, p. 67-160, doi:10.2113/JEEG11.2.95.

Abstract: Operating radar equipment in boreholes offers the possibility of greater depth penetration and higher resolution than is achievable with surface-based ground-penetrating radar systems. We have acquired single-hole radar reflection data in a series of vertical boreholes situated within a granite body west of Beijing, China. Geological logs demonstrate that numerous fractures intersect the boreholes. After processing the single-hole reflection data, linear reflections from many of these fractures are observed. In addition, a large number of linear features that do not intersect the boreholes are interpreted as fracture reflections; they can be traced to distances of 20–30m from the boreholes. Information from single-hole radar reflection data allows the minimum lengths, dips and distances of planar fractures from boreholes to be estimated, but not the azimuths. To determine the azimuths, information from two or more boreholes is required. For our survey site, this was achieved for three of the fractures.

Orlowsky, D., Holst, C., and Lehmann, B., 2018, 3D-Borehole Radar - A Routine Tool for The Detailed Imaging of Salt Structures, in Proceedings, 24th European Meeting of Environmental and Engineering Geophysics: Porto, Portugal, European Meeting of Environmental and Engineering Geophysics, p. 1-5, doi:10.3997/2214-4609.201802544.

Abstract: During the last decade, the 3D-borehole radar technique has demonstrated to be a valuable tool for the localization of the edges and for the investigation of the internal structure of salt domes. This methodology evolved to an integral component of the standard investigation program for new cavern development projects at many salt dome sites in Europe. A 3D-borehole radar probe, which runs within a vertical drill-hole through the salt formation, emits omni-directional radar signals. To map the 3D spatial coordinates of radar wave reflectors (geological interfaces) within the salt, the directions of the incoming radar signal reflections are determined apart from the distance of the probe to the reflector. A so called cross loop antenna as receiver unit in the probe allows for the recording of three separate signal shares. Together with the knowledge of the probe’s orientation the incoming angle of each signal can be clearly set and thus the coordinates of reflectors in the 3D space can be determined.

Serzu, M.H., Kozak, E.T., Lodha, G.S., Everitt, R.A., Woodcock, D.R., 2004, Use of borehole radar techniques to characterize fractured granitic bedrock at AECL's Underground Research Laboratory: Journal of Applied Geophysics, v. 55, no. 1–2, p. 137-150, doi:10.1016/j.jappgeo.2003.06.012.

Abstract: Single-hole radar reflection and cross-hole radar tomography surveys have been used to assist in characterizing a 105-m3 block of granite rock at AECL's Underground Research Laboratory (URL) in southeast Manitoba, Canada. The surveys were conducted in a series of seven boreholes drilled in moderately fractured granite rock from the URL 240 meter level. The RAMAC borehole radar system with rated dipole antenna frequencies of 22 and 60 MHz was used for these surveys. Results of single-hole radar reflection surveys revealed several linear reflectors and hyperbolic diffractions events. Some of the linear reflectors were interpreted to be reflections from fracture planes; others were from boreholes near or within the survey area. The hyperbolic diffractions are from point reflectors related to discrete vertical fractures or inhomogeneities in the rock. The 60-MHz surveys provided high-resolution reflection records and detected reflectors up to 50 m away from the boreholes. Compared to 60-MHz surveys, the 22-MHz reflection data showed marked decrease in resolution but considerable increase in probing-range (∼100 m). Both the 22- and 60-MHz surveys were able to detect water-saturated discrete fractures and fracture zones a few centimeters thick. Reflections from the HQ size (96-mm diameter) boreholes were also detected in both the 22- and 60-MHz reflection surveys. The radar velocities in the Moderately Fractured Rock (MFR) study block varied from 105 to 125 m/μs, which translates to a total velocity variation of 8–10% in the URL granite (with average velocity 120 m/μs). Results from borehole radar surveys were compared with core log data and hydraulic test results from the boreholes. The single-hole reflection data correlate well with fractures and fracture zones observed in the core logs. Combined interpretation identified low dipping fracture zones (with 10–30° dip) and two sets of subvertical fractures trending northeast and southwest. In addition, the radar velocity images from tomographic surveys show good correlation with the geologic model reconstructed from core log data. Above-average radar velocities correlate with more competent rock and lower velocities with more fractured rock. The tomography interpretations are also consistent with transmissivity values from hydraulic tests in the boreholes. The regions of low radar velocity anomalies correspond to transmissivity values of 1×10−6–1×10−8 m2/s in the boreholes, and high radar velocities to transmissivity values of 10−12–10−13 m2/s. In addition, the lower radar velocities correlate with increase in permeability as observed from groundwater flow measurements (e.g. 22 l/min in borehole MF12) and higher radar velocities corresponding to lower groundwater flow rates (e.g. 0.5–0.8 l/min in borehole MF6).

Spillmann, T., Maurer, H., Willenberg, H., Evans, K.F., Heincke, B., and Green, A.G., 2007,

Characterization of an unstable rock mass based on borehole logs and diverse borehole radar data: Journal of Applied Geophysics, v. 61, no. 1, p. 16-38, doi:10.1016/j.jappgeo.2006.04.006.

Abstract: Unstable rocky slopes are major hazards to the growing number of people that live and travel though mountainous regions. To construct effective barriers to falling rock, it is necessary to know the positions, dimensions and shapes of structures along which failure may occur. To investigate an unstable mountain slope distinguished by numerous open fracture zones, we have taken advantage of three moderately deep (51.0–120.8 m) boreholes to acquire geophysical logs and record single-hole radar, vertical radar profiling (VRP) and crosshole radar data. We observed spallation zones, displacements and borehole radar velocity and amplitude anomalies at 16 of the 46 discontinuities identified in the borehole optical televiewer images. The results of the VRP and crosshole experiments were disappointing at our study site; the source of only one VRP reflection was determined and the crosshole velocity and amplitude tomograms were remarkably featureless. In contrast, much useful structural information was provided by the single-hole radar experiments. Radar reflections were recorded from many surface and borehole fracture zones, demonstrating that the strong electrical property contrasts of these features extended some distance into the adjacent rock mass. The single-hole radar data suggested possible connections between 6 surface and 4 borehole fractures and led to the discovery of 5 additional near-surface fracture zones. Of particular importance, they supplied key details on the subsurface geometries and minimum subsurface lengths of 8 of the 10 previously known surface fracture zones and all of the newly discovered ones. The vast majority of surface fracture zones extended at least 40–60 m into the subsurface, demonstrating that their depth and surface dimensions are comparable.

References

Haeni, F.P., Halleux, L., Johnson, C.D., and Lane Jr., J.W., 2002, Detection and Mapping of Fractures and Cavitites using Borehole Radar, in Proceedings, Fractured Rock: Denver, CO, National Ground Water Association, 4 p.

Johnson, C.D., and Joesten, P.K., 2005, Analysis of borehole-radar reflection data from Machiasport, Maine, December 2003: U.S. Geological Survey Scientific Investigations Report 2005-5087, 44 p., doi:10.3133/sir20055087.

Lane Jr., J.W., Haeni, F.P., Placzek, G., and Wright, D.L., 1996, Use of borehole-radar methods to detect a saline tracer in fractured crystalline bedrock at Mirror Lake, Grafton County, New Hampshire, USA, in Proceedings, Sixth International Conference on Ground-Penetrating Radar (GPR'96): Sendai, Japan, Tohoku University Department of Geoscience and Technology, p. 185-190.

Lane Jr., J.W., Day-Lewis, F.D., Versteeg, R.J., Casey, C.C., and Joesten, P.K., 2004, Application of cross-borehole radar to monitor field-scale vegetable oil injection experiments for biostimulation, in Proceedings, Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP): Denver, Colorado, Environmental and Engineering Geophysical Society, p. 429-448, doi:10.4133/1.2923356.

Singha, K., Kimball, K., and Lane Jr., J.W., 2000, Borehole-radar methods: Tools for characterization of fractured rock: U.S. Geological Survey Fact Sheet 054-00, 4 p., doi:10.3133/fs05400.

Spillmann, T., Maurer, H., Willenberg, H., Evans, K.F., Heincke, B., and Green, A.G., 2007, Characterization of an unstable rock mass based on borehole logs and diverse borehole radar data: Journal of Applied Geophysics, v. 61, no. 1, p. 16-38, doi:10.1016/j.jappgeo.2006.04.006.