Experimentally achieving borehole antenna radar directivity in the time domain in the presence of strong mutual coupling

Abstract

It is difficult to achieve significant directivity in the radial direction of a borehole radar antenna, because the spacing of antenna elements is typically constrained by the borehole diameter to be considerably less than a wavelength. Previously published borehole radar antennas have achieved directivity by post processing data received in the frequency domain, or by constructing an aperture antenna, where borehole dimensions allowed this. In this paper, a time-domain technique is investigated for determining the radial direction of reflectors detected in borehole radar images. The antenna itself is an array of four elements and the delay in arrival of the signal between elements is used to determine its direction. We show here that a relatively slow sampling rate is adequate to resolve the small time intervals between signals received on different antenna elements. Mutual coupling between the antenna elements does affect the relative timing, but does not prevent the extraction of usable directional data. Experimental data from a test tank confirms that estimates of reflector direction can be made to within avout +15 degrees /-5 degrees of the true direction for antenna elements 20 mm apart in water, excited with a pulse that has a centre frequency of 250 MHz