Modelling of a monostatic borehole radar antenna

[摘要] The successful operation of a pulsed monostatic borehole radar system requires that cur-rent on the antenna due to transmitter pulses subsides quickly. Deterioration of the radarantenna feed-point current settling times when deployed in water-¯lled boreholes showedthat the radar system's performance is highly environmentally sensitive. Electromagneticmodels are used to investigate this e®ect, since measuring the feed-point and radiativecharacteristics of an insulated antenna deployed in a borehole is practically impossible atpresent.A transmission line model for insulated antennas is utilized to model the borehole radarantenna in electrically dense media. Predicted input impedance values however do notcorrespond well to those from numerical ¯eld simulation software and the model is shownto be inadequate for modelling insulated antennas in environments of low conductivity.Radiated ¯eld results are however found to be accurate.A study of the feed-point and radiative characteristics of the borehole radar antennain a range of di®erent borehole environment is performed using electromagnetic ¯eldsimulation software. Results con¯rm that the borehole radar antenna has longer feed-point current settling times and degraded radiated pulse waveforms when deployed inwater-¯lled boreholes.Simple lumped element networks with driving-port impedances approximately equalto antenna input impedances are synthesized from simulated input impedance resultsfor a range of borehole environments. This allows diagnostics on the radar system tobe performed in the laboratory, with the antenna load appearing as if the system weredeployed in a borehole.The use of an antenna with distributed resistive and capacitive loading is proposedas a modi¯cation that would result in improved feed-point characteristics in water-¯lledboreholes. Results from simulations and experiments are presented that con¯rm thatthe new antenna design substantially reduces feed-point current settling times after thetransmitter ¯res.

[发布机构] Stellenbosch University