Recognition of Borehole Radar Cable-Related ... - Semantic Scholar

Subsurface Sensing Technologies and Applications Vol. 2, No. 2, 2001

Recognition of Borehole Radar Cable-Related Effects Using Variable Offset Sounding Erich D. Guy* and Stanley J. Radzevicius Dept. of Geological Sciences, The Ohio State University, Columbus, Ohio 43210 Receiûed Sept. 25, 2000; Reûised Feb. 8, 2001 Conductive cables can influence borehole radar measurements and introduce artifacts into data and therefore must be considered during data analysis and interpretation. This study presents examples of some cable-related effects in data acquired with a radar system that relies on conductive cables for signal transmission. Data show that measurements can be affected when energy radiated from the transmitter antenna induces currents on conductive cables, which can function as an electromagnetic waveguide, allowing fields to propagate along cables and be detected by the receiver antenna. Additionally, periodic artifacts can result when currents traveling on cables reflect at system impedance mismatches. Variable offset soundings (VOS) are not typically conducted during borehole radar studies, but can be useful for recognizing cable-related effects on recorded data and studying propagation characteristics in a borehole. In addition to single-hole VOS measurements, VOS measurements made on the ground surface using E-Plane and H-Plane configurations are shown to have the potential for providing additional insight in regards to coupling mechanisms between borehole antennas and cables. Key Words. Borehole radar, radar.

1. Introduction In many cases the detectable depth range of ground penetrating radar (GPR) is restricted by either high wave attenuation in conductive materials, volume scattering, or the presence of strong reflecting interfaces [1–3]. Additionally, the acquisition of quality surface GPR measurements can be difficult in situations where topographic relief is extremely high or *To whom all correspondence should be addessed at 275 Mendenhall Lab., 125 South Oval Mall, Columbus, Ohio 43210. Email: [email protected] 127 1566-0184兾01兾0400-0127$19.50兾0  2001 Plenum Publishing Corporation

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where radio-frequency noise or obstructions exist at the surface. The utilization of boreholes allows radar measurements to be acquired when conditions are not favorable for surface GPR and permits the antennas to be located closer to the geology and potential anomalies of interest. Applications of borehole radar techniques include fracture detection [4,5], tunnel location [6], lithologic and structural interpretation [7,8], and soil water content determination [9]. The electronics and power source necessary to generate and record pulses radiated by borehole antennas can be located at the ground surface or adjacent to down-hole antennas within probe casings. Systems have been developed that are able to use non-conductive cables (fiber optical links) for signal transmission, by positioning the power source and electronics within thin probes that are suitable for use in almost all boreholes [5,10]. However, several commercial systems designed for shallow earth studies locate the power source and electronics at the ground surface so that a small probe diameter may be achieved, and therefore must rely on conductive cables for signal transmission. Previous studies have recognized that currents induced on borehole radar cables can sustain fields that can be recorded [11–14]. Accurate analysis and interpretation of borehole radar data depends upon the proper identification and measurement of recorded events. Difficulties in establishing system parameters, the improper setting of time zero, system drift, and deviations in positioning of the antennas are problems that can occur during data acquisition. Other factors that can affect measurements include the antenna radiation pattern, refractions generated by high velocity contrasts, wave guiding in layered sediments [15], and resonance effects [16]. Additionally, radar systems that use conductive cables can introduce cable-related artifacts into data that can complicate data analysis and interpretation. This paper discusses examples of cable-related effects that are shown to influence recorded data, and demonstrates that non-traditional variable offset soundings (VOS) conducted in both a borehole and on the ground surface (E-plane and H-plane configurations) can be useful for recognizing and studying these effects. 2. Borehole Radar Antennas and Measurement Configurations Borehole radar systems typically utilize dipole antennas that radiate electromagnetic fields with the electric field vector components predominantly oriented parallel to the long axis of the transmitter antenna. Borehole antennas are oriented axially along a borehole and usually measure only the co-polarized component, although recent research has demonstrated additional potential benefits of measuring the cross-polarized component [5,17]. The radiation pattern of a dipole antenna in a dry borehole with a

Borehole Radar Cable-Related Effects Using Variable Offset Sounding

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Figure 1. Single-hole reflection and VOS measurement configuration. Wave propagation ray paths are also shown.

homogenous surrounding medium is similar to that of a dipole in a whole space [13], however, changes in the surrounding medium and fluid filling the borehole can significantly alter the pattern. Antennas can be considered as directional (can provide azimuthal resolution relative to the borehole), or omnidirectional (cannot directly provide azimuthal resolution). Borehole radar measurements can be made in a single borehole (single-hole) or between boreholes (cross-hole). Single-hole measurements are typically made while raising or lowering the antennas with a constant separation (reflection mode). As shown in Figure 1, single-hole reflection measurements can detect energy scattered from electrical property discontinuities above, below, and away from the borehole. In addition to the traditional single-hole reflection configuration, VOS measurements can be made in a single borehole during the raising or lowering of one antenna while the second antenna remains stationary (Fig. 1). A VOS using borehole antennas is similar to a wide angle reflection and refraction (WARR) sounding commonly made with surface GPR antennas, in that data are recorded while varying the spacing between antennas to provide a means of measuring propagation characteristics of the radar signal.

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Cross-hole measurements between two boreholes can be made by raising or lowering both antennas at the same rate to acquire a constant offset profile (COP). A COP can provide information on bulk electrical properties between the boreholes by measuring variations in travel time and amplitude, and can also indicate the presence of discontinuities above, below, or between the holes by recording diffracted, reflected, and refracted energy. Measurements can also be made by keeping the transmitter antenna at a fixed position in one hole and raising the receiver antenna in another hole to acquire a multiple offset gather (MOG). Numerous MOG’s made with the transmitter antenna located at different depths are often merged and processed to create attenuation, dispersion, and velocity tomograms. 3. Examples of Cable-Related Effects Data were acquired using a commercial borehole radar system at a site characterized by highly varying unconsolidated glacial sediments. Borehole measurements were made in a permeable PVC cased well with the water table at a depth of 4.1 m. Omni-directional dipole antennas with a measured center frequency of 100 MHz (in air) were used. The radar system employed relied on 30.0 m long conductive cables approximately 10 mm in diameter with a dielectric coated surface for signal transmission between surfacelocated electronics and the antennas. In order to accurately position time zero and assure that there was no system-related drift during measurements, both the first and last trace of each record were acquired in air with a known separation between the antennas. Data processing consisted of dewow correction and high cut filtering at 120 MHz to reduce system noise, and all data presented in this paper are displayed with constant gain to preserve relative amplitude information. 3.1. Single-Hole VOS A single-hole VOS record is shown in Figure 2. Data were acquired with the transmitter antenna midpoint position fixed for the duration of the sounding at a depth of 24.0 m. The initial receiver midpoint was positioned at a depth of 22.0, and traces were recorded using a receiver step increment of 0.125 m (8 traces per m), with a final receiver midpoint at a depth of 13.5 m. The two linear events indicated as events A and B that intersect at roughly time zero (0 ns) in Figure 2, have equal but opposite (positive and negative) slopes (velocityG0.21 m兾ns). Both events are the result of the receiver antenna recording fields radiated by the pulse from the surface electronics traveling in the transmitter cable. Event A arrives earlier than time


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Figure 2. Single-hole VOS record. Event A results from the receiver antenna recording radiated fields from the transmitter cable by the initial pulse traveling from the surface electronics to the transmitter antenna. Event B results from an upward traveling pulse that was partially reflected from an impedance contrast at the cable and transmitter antenna connector. Event C is the direct coupling between the transmitter and receiver antennas (see text for discussion). Events D, E, and F are multiples of events A, B, and C respectively, with a period of 286 ns.

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zero and records the pulse on its way down the cable to the transmitter antenna (prior to radiation from the transmitter antenna). This event arrives earlier in time as the receiver antenna is stepped upwards. Event B has the same velocity as event A, and is an upward traveling pulse that resulted from a partial reflection of the initial downward traveling pulse at an impedance contrast at the cable and antenna connection. Event B, with lower amplitude than event A, arrives later in time as the receiver antenna is stepped upwards. The velocity of both events A and B remains constant with increasing antenna separation, despite changes in geologic porosity (and thus water content) that occur with depth as indicated by the geologic log for this well. The event indicated as C in Figure 2 is higher amplitude than events A and B, and is the direct coupling between the transmitter and receiver antennas. The direct coupling consists of the direct ground wave and also a wave radiated from the transmitter cable that travels through the ground, both of which have a similar travel path and timing. The wave radiated from the transmitter cable that is recorded by the receiver antenna is the result of induced currents propagating along the transmitter cable (which functions as an electromagnetic waveguide) above the transmitter antenna. Because the transmitter antenna radiates fields that are polarized parallel to the long axis of the antenna, currents are induced on the transmitter cable, which is also parallel to the long axis of the antenna. Detailed theory describing guided wave propagation along dielectric coated conductors is presented by Collin [18]. Comprehensive analysis of the direct arrival in single-hole radar data has been previously addressed [13–14], and is beyond the scope of this paper, as coupling between the antennas is influenced by numerous variables (antenna characteristics, borehole diameter and length, borehole fluid and surrounding material electrical properties, cable characteristics, and frequency). Further evidence that induced currents on the transmitter antenna cable contribute to the observed direct coupling in Figure 2 will be presented in following discussion. The velocity of event C indicates that a slight decrease in geologic porosity exists above a receiver midpoint depth of 19.7 m, and this is consistent with geologic information obtained from the drill log for this well. The amplitude of event C decreases as the receiver antenna is stepped upwards, and attenuation of this event appears gradual with increasing antenna separation. Antenna ring-down [19] occurs at close antenna offsets (between receiver midpoint depths of 22.0 m and 21.0 m, and between 100 and 270 ns time). Events D and E in Figure 2 are multiples of events A and B, with a period of 286 ns, and result from a partial reflection of the upward traveling pulse (event B) that occurs at an impedance contrast at the cable and system


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electronics junction on the surface. The velocity of both events D and E is also equal to 0.21 m兾ns, and the two-way travel distance of the pulse through the cables from the transmitter antenna to the surface electronics (60 m) agrees with the observed 286 ns period of these events. The partial reflection of the pulse at the system electronics on the surface causes the pulse to travel back down the cable and be re-radiated by the antenna, resulting in event F, which is a multiple of the initial direct coupling event C, also with a period of 286 ns. 3.2. E-Plane and H-Plane VOS For linearly polarized dipole antennas, the E-plane is defined as the plane containing the maximum electric field vector, while the H-plane is described as the plane containing the maximum magnetic field vector [20]. E-plane measurements are made in the plane parallel to the long axes of dipole antennas, with the antennas parallel to each other and parallel to the cables, while H-plane measurements are made in the plane orthogonal to the long axes of dipole antennas, with the antennas parallel to each other, and orthogonal to the cables. Both configurations can be used to acquire VOS measurements on the ground surface, although H-plane VOS data cannot be acquired during surveys in a borehole. E-plane and H-plane VOS measurements made using borehole antennas can be useful for studying possible induction effects on cables, and may be practically made on the ground surface, with data acquisition similar to that of a single-hole VOS (Fig. 2). Measurements are made with the position of the transmitter antenna remaining fixed while the receiver antenna is stepped at a constant interval away from the transmitter antenna. Figure 3 shows VOS antenna configurations used to make E-plane (Fig. 3a) and H-plane (Fig. 3b) measurements on the ground surface. Records acquired from VOS surveys with the antennas on the ground surface are shown in Figure 4, with measurements made in both the E-plane (Fig. 4a) and the H-plane (Fig. 4b). The E-plane and H-plane records in Figure 4 are displayed with the same constant gain to preserve relative amplitude information. Data were acquired in both configurations with an initial antenna separation of 2 m, using a receiver step increment of 0.125 m (8 traces per m), and had a final antenna separation of 10.5 m. In Figure 4a, event A results from the receiver antenna recording fields radiated from the transmitter cable by the initial pulse traveling from the surface pulser to the transmitter antenna. Event B results from interference of the direct air wave and the upward traveling cable pulse, and as a result is higher amplitude than event A (in Figure 2 the downward traveling pulse is higher amplitude than the upward traveling pulse when there is

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Figure 3. Configurations used to make VOS measurements on the ground surface: (a) E-plane, and (b) H-plane. E-plane measurements are made with the antennas parallel to each other and parallel to the cables, while H-plane measurements are made with the antennas parallel to each other, and orthogonal to the cables.

no air wave interference). Event C consists of the direct ground wave and a wave radiated from the transmitter cable that travels at a velocity intermediate between air and ground velocities. Events D and E are multiples of events B and C respectively. A multiple of event A is not clearly evident as the amplitude of this event is at the background noise level. In Figure 4b event F is the direct air wave, event G is the direct ground wave (with a velocity slower than event B), and event H is a multiple of event F. When the transmitter antenna is parallel to the cables (Figure 4a), radiated fields that are polarized parallel to the long axis of the transmitter antenna induce currents on the transmitter cable that radiate fields that are detected by the receiver antenna (event C). However, evidence for induced currents on the transmitter cable is not observed in H-plane measurements (Fig. 4b) when the antennas are aligned orthogonal to the cables. Currents are not induced on the cable in this configuration because a polarization mismatch [21] occurs when the polarization of the fields radiated by the transmitter antenna is orthogonal to the conductive cables. A comparison of these data demonstrates that the direct coupling in E-plane measurements is strongly affected by induced currents on the transmitter antenna cable


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Figure 4. VOS records acquired on the ground surface: (a) E-plane and (b) H-plane. Records are displayed with the same constant gain to preserve relative amplitude information. Event A results from the receiver antenna recording radiated fields from the transmitter cable by the initial downward traveling pulse. Event B results from interference of the direct air wave and the reflected upward traveling cable pulse. Event C consists of the direct ground wave and recorded fields radiated as a result of induced currents on the transmitter antenna cable. Events D and E are multiples of events B and C respectively. Event F is the direct air wave, and event G is the direct ground wave. Event H is a multiple of event F. The high amplitude event C is not evident in H-plane measurements made with the antennas orthogonal to the cables, indicating that induced currents on the transmitter cable strongly affect E-plane measurements.

(such as was the case during the acquisition of the single-hole VOS presented in Figure 2). 3.3. Single-Hole Reflection A single-hole reflection record acquired with the transmitter antenna beneath the receiver antenna, and with a constant separation of 2.0 m

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between the antennas is shown in Figure 5. The initial depth of the midpoint between the antennas was 7.7 m, and both antennas were raised by increments of 0.125 m (8 traces per m) to a final midpoint depth of 1.95 m. A low amplitude initial pulse indicated as event A, present prior to time zero is the result of the upward and downward traveling pulses (events A and B in Figure 2) that are radiated by the transmitter cable. The arrival times of the upward and downward traveling pulses are very close to each other and appear as one event in Figure 5 at an antenna offset of 2 m. The arrival time of event A remains constant despite changes in water content of the sediments that occur with depth (water table is at a depth of 4.1 m). Changes in the propagation velocity related to geologic media changes that occur with depth are evident in the slightly faster arrival time of the direct coupling (event B) above the water-saturated sediments. The direct coupling is slightly reduced in amplitude above the water table relative to that below the water table, and likely results from a combination of differences in antenna coupling related to the change in water content and increased attenuation associated with the increased presence of mineralogic clay-rich sediments above the water table as indicated by the drill log for this well. A multiple event of the direct coupling with a period of 286 ns is indicated as event C in Figure 5. Despite the velocity variations between the water-saturated and unsaturated sediments that occur with depth, the period of this multiple event remains constant. This consistency in period with depth (along with the evidence for multiple events discussed for the data shown in Figure 2) is further confirmation that this event results from re-radiation by the transmit antenna caused by partial internal cable reflections at system impedance mismatches, rather than scattering related to geologic contrasts. An extended time window would reveal additional events every 286 ns resulting from continued internal cable reflections, with the multiple events becoming progressively lower amplitude and lower frequency (due to attenuation and the loss of higher frequencies) with time. When the transmitter and receiver antennas are located below and above the depth of the water table respectively, the transmitted energy is reflected and refracted away from the receiver antenna resulting in a lower amplitude direct arrival. This apparent attenuation zone is evident in both the direct arrival and the multiple direct arrival, and is labeled as event D in Figure 5. An apparent attenuation zone associated with the position of the water table has also been observed in single-hole reflection data acquired by Dubois [8]. Although energy is likely scattered as a result of geologic and water content changes with depth, and when the antennas are near the soil兾air interface, no energy resulting from these impedance contrasts is evident in the record shown in Figure 5. This lack of observed scattered


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Figure 5. Single-hole fixed offset reflection record. A low amplitude event resulting from the upward and downward traveling pulses in the transmitter antenna is indicated as event A. Event B is the direct coupling between the antennas, and event C is a multiple of the direct coupling with a period of 286 ns. Also evident is a zone of apparent attenuation (event D) associated with the position of the water table.

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energy suggests that any scattering from these contrasts in this antenna configuration is very low amplitude. Data recorded using a time window in a medium with highly varying conductivity or propagation velocity that recorded only part of a multiple event could lead to misinterpretation of the event as late arrival scattered energy. Of additional importance is the fact that signal power is wasted because of the cable reflections that cause such multiple events. Because this periodic radiation occurred with this radar system the length of the transmitter cable limited the useful time window that could be recorded.

4. Conclusions Conductive cables employed by a borehole radar system for signal transmission can have a significant impact on recorded data. Data examples presented in this paper have shown that currents can be induced on conductive cables and be detected by the receiver antenna, and that periodic artifacts can be introduced into data as a result of partial reflections that can occur in cables at system impedance mismatches. Accurate analysis and interpretation of borehole radar data relies on the ability to properly identify and measure events, and it is therefore necessary to recognize possible cable-related effects. VOS measurements made in a borehole and on the ground surface (both E-Plane and H-Plane) offer a means for studying antenna coupling and propagation characteristics, and also the potential for recognizing possible effects of conductive cables on recorded data.

Acknowledgments The authors wish to thank Glen Frank for his assistance with data acquisition. Gratitude is extended to Dr. Motoyuki Sato for his valuable comments and suggestions. Additionally, manuscript reviews by Christina Chan and Jennifer Holt were appreciated.

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