Resistively Loaded Insulated Antennas for Narrow Borehole Radar Daniel M Claassen ARCOLab, Dept Engineering Science, Oxford University Parks Road, Oxford, OX1 3PJ England tel/fax $44-1865-273085/010; email: zyxwvu daniel.claassen@ox.ac.uk Rajeev Bansal Electrical and Systems Engineering, University of Connecticut 260 Glenbrook Road U-157, Storrs, CT 06269 tel .+1-203-486 2878 Iain M Mason, Jonathan E Hargreaves ARCOLab, Dept Engineering Science, Oxford University Abstract zyxwvutsrqp - The transmission-line model for an insulated symmetric dipole in a lossy medium has been extended for the case of an insulated asymmetric dipole with a con- ductive arm and a uniformly resistive arm. The model is used to improve the understanding of the characterrstics of a narrow borehole impulse radar recently developed. It predicts that sufficient loading of the resistive arm will dampen the antenna suitably for pulse transmission, de- spite the presence of the conductive arm. The model is partly verified by laboratory experiments. Examples of pulses received zyxwvutsrqpo zn sztu in marble by the radar are pre- sented. INTRODUCTION Borehole radar has been used in a number of applica- tions such as ore, tunnel and cavity detection and geo- logical surveys, eg. [1], [2]. Recently a new multi-channel narrow borehole impulse radar has been developed by the ARCOLab, Oxford University. It has been tested success- fully both in basalt and marble/limestone environments. Efficient pulse transmission and reception, needeld for achieving adequate radar imaging resolution, require broadband antennas. The antenna design for the AR- COLab radar is, however, constrained by the minimum borehole diameter of 47mm, as well as a need for a simple inexpensive design so that the downhole probes would not be too costly to lose. Furthermore, the radar has to oper- ate in a variety of geological environments. This compli- cates any attempts at broadband antenna matching since the electromagnetic properties of the rock influence the antenna input impedance. Resistively loaded antennas are used in a number of borehole impulse radar designs El], [2]. Some employ the non-uniform resistivity profile described by Wu and King [3] to achieve a high bandwidth. For simplicity, the ARCOLab radar employs a resistively loaded monopole with a Wu-King resistivity profile that is fed directly against the conductive housing of the batteries and cir- cuitry. Thus the structure is an asymmetric linear dipole, with a resistively loaded arm and a conductive arm of different lengths and diameters. The dipole is insulated from the host medium by a 33" diameter PVC casing and the borehole (air), and is about 2m long. It is thus assumed to be electrically thin. TRANSMISSION-LINE MODEL Symmetric resistively loaded dipole antennas zy in air have been analyzed extensively both experimentally and numerically, eg. [4]. A numerical analysis of a bore- hole radar with a symmetric conductive dipole has been reported [5] for a case where it was felt that the sur- rounding lossy medium would dampen the resonant be- haviour of a conductive dipole sufficiently to enable ac- ceptable pulse transmission. The analysis uses the lossy transmission-line (TL) model for an insulated dipole in a lossy medium [ti]. In this paper, the TL model has been extended to the case of an asymmetric dipole of which one arm is resistively loaded. Initially the loading is ap- proximated as being uniform. Fig. 1 shows an asymmetric dipole in a borehole mod- eled as a co-axial transmission-line, based on the usage of [6]. The real effective values of the permittivity, zyx E,, and conductivity, zyxwv ce, are used. The permeability, zy po, is assumed to be that of air. The axial distance is z and UA, aB and b are radii. With the TL model assumed, the axial current, I(z), on the antenna is derived from the standard TL equations, (d2/dz2+kiA)I(z) = 0 and (d2/dZ2$kiB)I(Z) = 0, (1) for side A and B respectively. For the conductive arm kLA, the complex wavenumber, is given by equation (8.3.15) of [6]. The series impedance per unit length ZLB and shunt impedance YLB is used to calculate kLB for the resistive arm. Equation (1.2.10) in [6] gives YLB. The equation ZLB = 2 : + + zy ze is used with 21 the value of the uniform resistive loading of the resistive conductor. zi and ze are given in [6]. The applicability of the TL model depends on the ac- curacy of the approximation for the wavenumber kL for 0-7803-2567-2195 $4.00 zyxwvutsr 0 1995 IEEE 1711