Modelling of a monostatic borehole radar antenna
Date
2006-03
Authors
Gouws, Marcel
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
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 radar
antenna feed-point current settling times when deployed in water-¯lled boreholes showed
that the radar system's performance is highly environmentally sensitive. Electromagnetic
models are used to investigate this e®ect, since measuring the feed-point and radiative
characteristics of an insulated antenna deployed in a borehole is practically impossible at
present.
A transmission line model for insulated antennas is utilized to model the borehole radar
antenna in electrically dense media. Predicted input impedance values however do not
correspond well to those from numerical ¯eld simulation software and the model is shown
to 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 antenna
in a range of di®erent borehole environment is performed using electromagnetic ¯eld
simulation software. Results con¯rm that the borehole radar antenna has longer feed-
point current settling times and degraded radiated pulse waveforms when deployed in
water-¯lled boreholes.
Simple lumped element networks with driving-port impedances approximately equal
to antenna input impedances are synthesized from simulated input impedance results
for a range of borehole environments. This allows diagnostics on the radar system to
be performed in the laboratory, with the antenna load appearing as if the system were
deployed in a borehole.
The use of an antenna with distributed resistive and capacitive loading is proposed
as a modi¯cation that would result in improved feed-point characteristics in water-¯lled
boreholes. Results from simulations and experiments are presented that con¯rm that
the new antenna design substantially reduces feed-point current settling times after the
transmitter ¯res.
Description
Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006.
Keywords
Dissertations -- Electrical and electronic engineering, Theses -- Electrical and electronic engineering, Ground penetrating radar, Radar -- Antennas