Crossed-dipole antennas can be used to reduce clutter and improve the signal-to-noise ratio of ground penetrating radar (GPR) surveys, depending upon field ...
Proceedings of the Symposium on the Application of Geophysics to Environmental and Engineering Problems, 2000, The Environmental and Engineering Geophysical Society, pp. 407-413.
SIGNIFICANCE OF CROSSED-DIPOLE ANTENNAS FOR HIGH NOISE ENVIRONMENTS Stanley J. Radzevicius1, Jeffrey J. Daniels1, Erich D. Guy1, and Mark A. Vendl2 Department of Geological Sciences, The Ohio State University, Columbus, Ohio 43210 2 U.S. EPA, Region V, Chicago, Illinois 1
ABSTRACT Crossed-dipole antennas can be used to reduce clutter and improve the signal-to-noise ratio of ground penetrating radar (GPR) surveys, depending upon field conditions and the targets of interest. The crossed-dipole antenna consists of transmit and receive antennas oriented orthogonal to each other, and is sensitive to field components oriented parallel to the long axis of the receive antenna. These cross-polarized components can be introduced by scattering from subsurface targets or may be composed of scattered cross-polarized components present in the field incident on the target. The physical shape and composition of targets will influence the polarization of the scattered field, and this enables cross-pole and co-pole antenna configurations to discriminate between different classes of targets for clutter removal. The crossed-dipole antenna configuration also improves isolation of the receive antenna from the direct arrival of the transmit antenna. The improved isolation and ability to discriminate between different targets can therefore result in an improved signal-to-noise ratio. INTRODUCTION Co-pole antennas are commonly used for environmental and engineering GPR investigations (Davis and Annan, 1989; and Daniels et al., 1997). Crossed-dipole antennas are not commonly used in GPR surveys because of their lack of sensitivity to stratigraphy. Crosseddipole antennas can be used to improve the signal-to-noise ratio in environments characterized by antenna ring-down and when the objects of interest include linear targets (pipes and rebar), or objects that produce a strong scattered cross-component (rough surfaces and small targets). Physical model examples are used to demonstrate which targets can be imaged with crosseddipole antennas. Field examples from a site characterized by antenna ring-down are also presented to illustrate the potential effectiveness of using crossed-dipole antennas. ANTENNAS AND ANTENNA RING-DOWN Dipoles and bow-tie antennas are widely used in impulse GPR, because they are relatively easy to design, are wide-band, are non-dispersive, and are linearly polarized (de Jongh et al., 1998). Most GPR antennas are located on or just above the ground to couple electromagnetic energy into the ground efficiently. The close proximity of the antenna to the ground causes the current distribution and antenna impedance to be strongly influenced by the ground. The antenna impedance changes with different soil types, moisture contents, and surface roughness. Most GPR antennas are designed for use over ground having a specific impedance. An impedance mismatch between the antenna and feed cable occurs when a GPR survey is conducted over ground having an impedance other than what the antenna was designed for. This causes the currents to bounce back and forth between the antenna feed and the ends of the
407
antenna. The currents decay and radiate over extended time intervals, and thus the radiated or received pulse is not a clean single pulse. The resulting antenna ring-down reduces resolution, is a source of noise, and wastes signal power. Antenna ring-down is often reduced by loading the dipole or bow-tie antenna with discrete resistors on the ends of the element or by placing tapered resistors along the antenna element (Clarke and Rodriguez-Tellez, 1998; and Shlager et al., 1994). This resistive loading absorbs some of the energy that has not been radiated. Resistive loading also has the additional benefit of increased bandwidth. However, the price for resistive loading is the loss of efficiency and sensitivity. An alternative to resistive loading is to elevate the feed point and form a horn-fed bow-tie with the horn’s cavity filled with a dielectric (Chen, 1997). The elevated feed reduces the ground’s effect on antenna impedance. CROSSED-DIPOLE ANTENNAS AND TARGET DETECTION The electromagnetic field radiated by antennas has both a magnitude and a direction. Polarization describes the magnitude and direction of this field as a function of time and space. The scattering properties of objects are polarization dependent. The polarization and orientation of the transmit antenna is thus important to ensure sufficient energy is scattered from subsurface targets to allow measurement by the receive antenna. The ability of the receive antenna to measure these scattered fields is determined by the power of the scattered fields and the polarization match between the scattered field and receive antenna. Most of the energy radiated from dipole or bow-tie antennas is linearly polarized and oriented predominantly along the long axis of the antenna (Annan 1973; Annan et al. 1975; Engheta et al. 1982; and Smith 1984). A complete polarization mismatch using dipole antennas results when the scattered field and polarization of the receive antenna are both linearly polarized and oriented at right angles to each other. For example, rotating ideal dipole antennas orthogonal to each other (crossed-dipoles) results in a complete polarization mismatch (Figure 1).
T
R1
R2
Figure 1. Antenna orientations for acquiring co-pole (T & R1) and cross-pole data (T & R2). T denotes the transmit antenna and R denotes the receive antennas. A specular reflection from a smooth plane does not introduce field components (crosscomponents) that were not originally present in the incident field. A rough plane or targets that are small compared to the wavelength of the incident field scatter cross-components. Figure 2 shows data recorded with a 500 MHz centerband Geophysical Survey Systems Inc. (GSSI)
408
antenna over a square dielectric block 2.4 m on a side, 0.6 m thick, and buried at a depth of 1.07 m in a test pit filled with sand. The top and bottom of the foam block are easily observed with the co-pole configuration (Figure 2a). The block is much fainter on the cross-pole configuration (Figure 2b) and would be invisible if not for the non-ideal nature of the dipole antennas that radiate a small cross-polarized component. A small disk 12 cm in diameter and 2 cm thick was buried near the surface. The presence of this disk was unknown at the time the survey was conducted, but was later verified by excavation. Unlike the plane, the disk introduced a strong scattered cross-polarized component easily visible with cross-pole antennas (Figure 2b).
0
Distance (m) 1.2
Dielectric Block Top
10
Time (ns)
2.4
20
Bottom 30
40
a) co-pole
b) cross-pole
Figure 2. GPR data recorded over foam block 2.4 m on a side, 0.61 m thick, and buried at a depth of 1.07 m: a) top and bottom of the block are clearly visible at 19 ns and 23 ns respectively in the co-pole data, and b) reflections from the block are nearly absent in the cross-pole data, since no cross-components are introduced upon reflection. Objects can also be observed with cross-pole antennas when their geometry results in scattered field components oriented parallel to the long axis of the dipole receive antenna. A good example of this geometric effect is linear targets (pipes) that have their maximum response when the cross-dipole antennas are oriented at a 45 degree angle to the pipe (Daniels et al., 1988). Figure 3 shows GPR data acquired in the same test pit and using the same antenna as in Figure 2, but over a copper pipe buried at a depth of 0.46 m, having a diameter of 0.64 cm, and a length of 3.05 m. In Figures 3a and 3b, the GPR survey was conducted orthogonal to the long axis of the pipe and in Figures 3c and 3d, the survey was conducted at a 45 degree angle to the long axis of the pipe. In Figure 3a both dipole axes were aligned with the long axis of pipe, as opposed to Figure 3b where the transmit antenna was aligned with the long axis of the pipe and the receive antenna was oriented orthogonal to the long axis of the pipe. The maximum cross-pole response occurs when the dipoles are oriented 45 degrees relative to the pipe (Figure 3d).
409
Time (ns)
0
Distance (m) 1.5 3
10
20 30
o
a) co-pole 0
o
b) cross-pole 0
o
c) co-pole 45
o
d) cross-pole 45
Figure 3. GPR data from surveys orthogonal and at a 45 degree angle to the long axis of a copper pipe buried at a depth of 0.46 m, having a diameter of 0.64 cm, and a length of 3.05 m: a) co-pole antennas oriented parallel to the long axis of the pipe with orthogonal survey, b) cross-pole antennas with orthogonal survey, c) co-pole antennas with survey oriented 45 degrees to orthogonal, and d) cross-pole antennas with survey oriented 45 degrees to orthogonal. FIELD EXAMPLES GPR surveys were run at an abandoned industrial site characterized by clay-rich glacial soils with abundant standing water. The wet, heterogeneous soils, and rough surface conditions produced a strong impedance mismatch between the antenna and feed cables. This impedance mismatch resulted in antenna ring-down that limited resolution and served as a source of noise. Data were acquired using coincident antennas (closely spaced common-offset) with both co-pole and cross-pole antenna configurations. The polarization match of the co-pole antennas caused the receive antenna to be sensitive to the large amplitude direct wave radiated by the transmit antenna. Cross-pole receive antennas are not sensitive to the direct wave radiated by the transmit antenna due to the polarization mismatch. Antenna ring-down caused the transmit antenna to transmit several pulses that decay with time, rather than a single clean pulse. The strong direct wave and subsequent antenna ringdown masked subsurface reflections in the co-pole data. In Figure 4 the GPR survey traversed standing water centered at a distance of 15 m along the survey line. The standing water bottom is visible at 7 ns on cross-pole data because of the rough interface that scattered a cross-polarized component. Although more energy is scattered on the co-pole component, the antenna ring-down above 7 ns is also 20 dB greater on the co-pole component. This high energy antenna ring-down on the co-pole data makes the water-soil interface appear less sharp on the co-pole data. Rebar within a concrete foundation is also clearly visible on the cross-pole data, but not clearly visible in the co-pole data (Figure 5). Rocks and other heterogeneous features in a gravel road produced scattered cross-polarized components that are clearly visible on the cross-
410
0
Time (ns)
0
7.5
Distance (m) 15 22.5
30
Distance (m) 15 22.5
30
10 20 30 a) co-pole
Time (ns)
00
7.5
10 20 30 b) cross-pole
Figure 4. Co-pole (a) and cross-pole (b) GPR 2D profiles across an area with standing surface water.
Time (ns)
0
0
7.5
Distance (m) 15 22.5
30
Distance (m) 15 22.5
30
10 20 30 a) co-pole
Time (ns)
0
0
7.5
10 20 30 b) cross-pole
Figure 5. Co-pole (a) and cross-pole (b) GPR 2D profiles across a foundation with rebar and a gravel road.
411
pole data. The boundary between the gravel road and surrounding soil is not as clearly defined on the co-pole data. CONCLUSIONS Resistive loading alone may not be sufficient to reduce antenna ring in environments having an impedance significantly different than that for which the antenna was designed. The antenna ring-down on co-pole components can mask scattering from subsurface targets of interest. Crossed-dipole antennas, with their reduced crosscoupling, can produce cleaner images under appropriate conditions and depending on the targets of interest. Cross-pole antennas are useful for imaging linear targets, and small or rough targets that scatter cross-polarized components. Stratigraphy is better imaged using co-pole antennas. The best choice of antenna configurations will be determined by the targets of interest, the antennas used for the survey, and the field conditions. REFERENCES Annan, A.P., 1973, Radio interferometery depth sounding: Part I- Theoretical discussion, Geophysics, Vol. 38, No. 3, 557-580. Annan, A.P., Waller, W.M., Strangway, D.W., Rossiter, J.R., Redman, J.D., and Watts, R.D., 1975, The electromagnetic response of a low-loss, 2-layer, dielectric earth for horizontal electric dipole excitation. Geophysics, Vol. 40, No. 2, 285-298. Chen, C., 1997, A New Ground Penetrating Radar Antenna Design- The horn-fed bowtie (HFB). Proceedings of the Nineteenth Meeting and Symposium of Antenna Measurement Techniques Association, Boston, Massachusetts, 67-74. Clarke, R.W., and Rodrizuez-Tellez, J., 1998, A comparison of sheet metal and thick film cavity backed bow-tie antennas. Proceedings of the Seventh International Conference on Ground Penetrating Radar, Lawrence, Kansas, Vol. 1, 59-62. Daniels, J.J., Grumman, D.L., and Vendl, M.A., 1997, Coincident antenna three-dimensional GPR. JEEG, Vol. 2, 1-9. Daniels, D.J., Gunton, D.J., and Scott, H.F., 1988, Introduction to subsurface radar. IEE Proc. F, Vol. 135, No. 4, 278-321. Davis, J.L, and Annan, A.P., 1989, Ground-penetrating radar for high-resolution mapping of soil and rock stratigraphy. Geophysical Prospecting, Vol. 37, 531-551. de Jongh, R.V., Yarovoy, A.G., Ligthart, L.P., Kaploun, I.V., and Schukin, A.D., 1998, Design and analysis of new GPR antenna concepts. Proceedings of the Seventh International Conference on Ground Penetrating Radar, Lawrence, Kansas, Vol. 1, 81-86. Engheta, N., Papas, C.H., and Elachi, C., 1982, Radiation patterns of interfacial dipole antennas. Radio Science, Vol. 17, 1557-1566. Shlager, K.L, Smith, G.S., and Maloney, J.G., 1994, Optimization of bow-tie antennas for pulse radiation. IEEE Transactions on Antennas and Propagation, Vol. 42, No. 7, 975-982.
412
Smith, G.S., 1984, Directive properties of antennas for transmission into a material half-space. IEEE Transactions on Antennas and Propagation, Vol. 32, 232-246.
413
