Development of an active pulsed radar receiver for a mono-static borehole-radar tool

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DEVELOPMENT OF AN ACTIVE PULSED RADAR

RECEIVER FOR A MONO-STATIC BOREHOLE-RADAR

TOOL

Brian Keith Woods

(Thesis presented In partial fulfilment of the degree of Master of Englnearlng al the University of Slellenbosch)

March 2003

Study leaders

Prof. K.D.Palmer and Prof J.H. Cloete

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Declaration

I, the undersigned, hereby declare that the work contained in this thesis is my own original work (unless otherwise indicated) and that I have not previously in its entirety or in part submitted it at any university

for a degree.

Brian Woods Date

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Abstract

Borehole rsdar applications are fsr resching in vsrious fields, none more so than in the mining industry, where sccurste, high resolution mapping of hidden geologiesl structures snd events is invaluable for mine development snd plsnning. Borehole radar technology snd methods have been under development for a number of yems, and experimental surveys hsve delivered exciting and useful results. The tools used in existing borehole radar systems are generally bi-ststic, pulsed radar systems. A brief study of an existing, operstional borehole radar system is performed. Two sub-systems sre identified for consideration in this report, namely the receiver snd the rsdiating structure. The aim is to develop these sub-systems of the borehole radar system so that they csn be employed in a mono-ststic borehole radar tool.

Computational methods sre introduced and verified for analysis of radisting structures. Time domain methods and definitions sre suggested to enable direct comparison. Existing radisting structures arc critically analysed snd a new symmetrical antenns form is suggested for mono-ststic spplication. Frequency and time domsin characterization of the suggested antenna topology. with vsrious resistive loading profiles in a simplified environment is performed. A psrametric lumped element representation of the typicsl sntenna impedance is extrscted. The operational borehole radar environment is defined, and the effects of different environmental factors on the sntenna structure are considered. A final antenna structure is suggested for mono-static application. A prototype antenna and probe structure is designed for field-testing purposes.

The operstional requirements of s borehole radar receiver sre considered, and receivers used in existing bi-ststic systems are chsrscterized by measurement. The limiting factors of existing receivers for mono-static spplication are identified A new receiver chain is suggested, and the identified limitstions are addressed by the implementation of vsriable gain in the amplifier chsin. The realized receiver circuit is characterized by measurement and compsratively verified during experimental field-testing. The operstion of an integrated mono-static system is presented and verified by bench messurement. Data gathered during field-testing of the different mono-ststic sub-systems is also presented.

The antenna Investigation presented suggests that the use of different structures in different operational environments may be necessary to facilitste scceptable and stable isolstion. The variable gain receiver is able to operate in the mono-static configuration. Noise figure and sweep transient effects must still receive attention, however.

.J.

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