AU2004322187B2 - Method and apparatus for monostatic borehole radar - Google Patents

Description

WO 2006/015402 PCT/AU2004/001382 "Method and apparatus for monostatic borehole radar" Cross-Reference to Related Applications The present application claims priority from Provisional Patent Application No 2004904543 filed on 10 August 2004, the contents of which are incorporated herein by 5 reference. Field of the Tivention The present invention relates to an improved ground penetrating radar. In particular, it relates to a compact radar in which the transmitter and the receiver share 10 all or part of;the same antenia. Background .to the Invention For many years it has been desired to implement a radar system capable of imaging subterranean features and buried objects. Many systems have been developed 15 for a broad range of applications in the mining, geotechnical, environmental and safety areas. For .example, applications include the detection of underground pipes and cables, detection of buried landmines and bombs, the delineation of ore bodies, the detection of aquifers, road evaluation, and hazardous waste detection. Two types of ground penetrating radar exist. For deep applications, borehole radars are used. For 20 shallow applications, a surface ground penetrating radar is generally more suitable. In rocks that favour radar wave propagation, borehole radars are able to contribute to the safety of drillers and miners by allowing defects such as high-pressure gas-bearing dykes and paleo-stressed faults in the rock volume around the hole to be placed under surveillance, mapped, and monitored in order to alert miners to their 25 proximity and allow the dangerous ground either to be steered around and avoided, or to be broken into at a chosen instant. A borehole radar typically comprises a relatively powerful transmitter positioned in a borehole for generating electromagnetic pulses which excite an antenna to radiate energy into the surrounding rock or earth. Usually, the transmitted 30 electromagnetic pulses are characterized by short rise/fall times to obtain sufficient frequency spread and resolution in the eventual radar data, and by sufficiently high energy levels to overcome attenuation and spreading losses in the surrounding rock medium. Transmitted electromagnetic pulses propagate through the rock and/or reflect off geological features such as interfaces between rock media having differing 35 electromagnetic properties.

WO 2006/015402 PCT/AU2004/001382 The receiver must be sufficiently sensitive to detect signals which have suffered attenuation and/or reflection losses. Borehole radar systems usually employ a bi-static configuration, with the transmitter and receiver deployed as two completely separate units (probes). The physical distance between the two probes is increased until 5 adequate isolation between the receiver and the transmitted pulse is achieved. Signal synchronization is often achieved by use of an optic fibre between the two probes. The closest discernible target is determined by the duration of saturation (if present) in the receiver, and whether the resultant oblique signal path is within the radiation pattern of the transmitter and receiver antennas. Bi-static systems are awkward to deploy in 10 constrained spaces, for example mining stope faces. The optic fibre link is also susceptible to damage in mining and other industrial environments. Further, the bi static deployment necessitates use of a first antenna for transmitting and a second antenna for receiving, introducing the likelihood of mismatched characteristics between the first and second antennae, which may result in performance degradation. 15 Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim 20 of this application. Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 25 Summary ofthe Invention According to a first aspect, the present invention provides a ground penetrating radar comprising: a transmitter for generating electromagnetic transmissions for ground 30 penetration; a receiver for receiving reflected electromagnetic signals; and an antenna; wherein at least a portion of the antenna is used by the transmitter for transmitting and by the receiver for receiving. 35 According to a second aspect the present invention provides a method of constructing a ground penetrating radar comprising: WO 2006/015402 PCT/AU2004/001382 3 providing a transmitter for generating electromagnetic transmissions for ground penetration; providing a receiver for receiving reflected electromagnetic signals; and providing an antenna; 5 wherein at least a portion of the antenna is used by the transmitter for transmitting and by the receiver for receiving. According to a third aspect the present invention provides a method of ground imaging using ground penetrating radar, comprising: transmitting an electromagnetic ground penetrating signal using at least one 10 transmit antenna portion; and receiving reflected electromagnetic signals using at least one receive antenna portion; wherein at least one antenna portion is both a transmit antenna portion and a receive antenna portion, 15 Preferred embodiments of the invention may provide a ground penetrating radar in accordance with the first aspect of the invention and wherein the antenna is damped to minimise resonance between an antenna portion used exclusively by the transmitter and an antenna portion used exclusively by the receiver via the portion of the antenna used by the transmitter and by the receiver. Such damping is advantageous in 20 maintaining the broad bandwidth both of the signal launched by the transmitter and of the radar echoes sensed by the receiver, and in providing some level of shielding of the receiver from powerful transmitted signals or pulses. Further embodiments of the invention may provide a ground penetrating radar in accordance with the first aspect of the invention, wherein the antenna is a dipole 25 antenna having two transmitting elements. In further such embodiments, at least one of the transmitting elements may be used by both the transmitter and the receiver Further embodiments of the invention may provide a ground penetrating radar in accordance with the first aspect of the invention, wherein a counter electrode of the antenna is used by both the transmitter and the receiver. 30 The transmitter and receiver may share the whole antenna. In such embodiments, the ground penetrating radar preferably further comprises a transmit/receive switch for isolating the receiver from the antenna during transmissions by the transmitter, and for passing post-transmission received signals to the receiver. In preferred such embodiments, the radar may comprise a symmetrical dipole antenna 35 wholly shared by the transmitter and the receiver, and wherein the dipole antenna functions as a housing for both the transmitter and receiver. In such embodiments, first WO 2006/015402 PCT/AU2004/001382 4 and second elements of the dipole antenna may be connected by a low pass filter allowing DC power flow between the first and second elements and allowing radar frequency AC voltages to develop between the first and second elements. Such embodiments allow a DC power supply of the radar to be provided via the radar S housing. The low pass filter may comprise a RF choke. In alternate embodiments of the invention, the transmitter and receiver may share only a portion of the antenna. For example, the transmitter and receiver may share a common counter electrode. In such embodiments,.the transmitter may employ a transmit antenna element, and the receiver may employ a receive antenna element. 10 The transmit antenna element may be positioned remote from the receive antenna element in the radar, to provide some isolation of the receiver from transmitted signals. For example, in a longitudinal borehole radar, the transmit antenna element may extend from the radar in a first direction along a longitudinal axis of the radar, while the receive antenna element may extend from the radar in a second direction opposite to 15 the first direction along the longitudinal axis of the radar. Alternatively, at least one of the transmit antenna element and the receive antenna element may be resistively loaded to provide broadband antenna capability and to provide resistive isolation of the receiver, and in such embodiments the transmit antenna element may be positioned proximal to the receive anteina element. For example in a longitudinal borehole radar, 20 the transmit antenna element and the receive antenna element may each extend from the radar in a common direction parallel to the longitudinal axis of the radar. In particularly preferred embodiments of the invention, at least one portion of the antenna is interchangeable. For example, the transmit element and receive element may be interchangeable to allow site specific optimisation of antenna frequency and/or 25 resistive loading. By providing a ground penetrating radar in which at least. part of an antenna is used by both the transmitter and the receiver, embodiments of the present invention may provide a ground penetrating radar constructed as a single device. Such embodiments provide for easy deployment of a single device to a desired position, 30 without requiring separate deployment of a transmitter and receiver to desired positions. Such simplified deployment may further lead to cost savings, Preferably, the single device ground penetrating radar of such embodiments is provided in a housing suitable for insertion into mining drill holes, thus providing a borehole ground penetrating radar. For instance, the housing of such a ground penetrating radar may 35 have a diameter or largest cross-sectional dimension of no greater than substantially 32mm to enable deployment of such a borehole radar in 47mm drilI holes.

WO 2006/015402 PCT/AU2004/001382 5 Further, in embodiments of the present invention providing a single device ground penetrating radar, the need for an optical fibre for synchronisation of a receiver relative to a transmitter may be obviated. Additionally, signal processing requirements and associated costs may be mininised in embodiments of the present invention in 5 which the physical offset or spacing between the transmitter and receiver is small. If this offset is small compared to both the radiated principal wavelength and the distance to a typical target, the radar is known as a short offset system; if the distance becomes substantially zero, the radar is said to be "monostatic". The term 'monostatic' is used herein to refer to a radar in which transmission and receiving points are separated by 10 less than or substantially equal to a wavelength of a transmission centre frequency of the radar. For example the transmission and receiving portions of the ground penetrating radar may be separated by a half wavelength measured at substantially the centre of the frequency band transmitted by the radar. By sharing at least a portion of the antenna for use by both the transmitter and the receiver, embodiments of the present 15 invention may provide for such a monostatic borehole radar. By providing a ground penetrating radar in which at least a portion of the antenna is used by the transmitter for transmitting and by the receiver for receiving, embodiments of the present invention may provide for a monostatic borehole radar having a length significantly less than the length of a bi-static borehole radar. In 20 preferred embodiments, the length of such monostatic borehole radars is sufficiently small to enable synthetic aperture radar measurements to be obtained within a borehole drilled as a blast hole. Such blast holes are typically so shallow as to prevent synthetic aperture radar measurements by use of a relatively lengthy bi-static borehole radar. Still further, it has been realised that a large part of the total lifetime cost of a 25 ground penetrating radar is determined by the antenna(e) and by the mechanical skeleton or housing, Accordingly, embodiments of the present invention may enable reduced construction costs and improved durability in the field, by enabling the transmitter and receiver to share all or even a part of the same physical housing and some or all of an antenna. 30 In embodiments of the invention in which the radar is a borehole radar with a single housing accommodating both the transmitter and receiver, a part of the housing of the borehole radar may be conductive and may function as a counter electrode for both the transmitter and the receiver modules. Where the borehole radar is deployed inside a conductive drill string within a borehole, the borehole radar housing may 35 further be electrically connected to the conductive drill string, such that the drill string itself is employed as a counter electrode for both the transmitter and the receiver. In WO 2006/015402 PCT/AU2004/001382 6 such embodiments, the borehole radar should be configured to have a second antenna element operable to radiate electromagnetic energy into and/or receive energy from the medium surrounding the drill string. For example, the second antenna element may protrude from the drill string, for example the second antenna element may protrude 5 from a distal end of the drill string. Alternatively the second antenna element may be positioned within a portion of the drill string which is substantially electromagnetically transparent. The substantially electromagnetically transparent drill string portion may comprise an open slot in the drill string allowing EM propagation to and from the second antenna element, or may comprise a non-metallic substantially 10 electromagnetically transparent drill string portion enclosing the second antenna element. It will be appreciated that in accordance with embodiments of the present invention, the ground penetrating radar may be of the surface or borehole type. In preferred embodiments of the invention, transmission and reception do not occur 15 simultaneously. In preferred embodiments of the invention, a transmit/receive (T/R) switch is provided in order to further facilitate the transmitter and the receiver wholly or partially sharing a single antenna. Embodiments in which the transmitter and the receiver wholly share a single antenna may be advantageous in ensuring near-identical -20 transmitting and receiving beam shape, and may further avoid the cost and increased physical size associated with providing a partially shared antenna or two wholly separate antennas for transmission and reception. The T/R-switch may be used in conjunction with a parallel connection of the transmitter, receiver and antenna, such that the T/R-switch may in the transmit state disconnect the terminals of the receiver, or 25 in the receive state may disconnect the terminals of the transmitter from the parallel connection. Alternatively, in preferred embodiments of the invention, the T/R switch may operate to shunt transmitter current past the receiver into at least one element of the antenna during transmission, and may operate to shunt a received signal past the 30 transmitter into the receiver during the reception interval that follows transmission. Such embodiments of the invention may thus further comprise a transmit/receive (TfR) switch comprising: a first switch terminal for connection to a first terminal of the at least one -antenna; 35 a second switch terminal for connection to a first terminal of the transmitter; and switch receiver terminals for connection to the receiver; WO 2006/015402 PCT/AU2004/001382 7 wherein the T/R switch is operable to implement a transmit state by connecting transmit signals from the second switch terminal to the first switch terminal, and by isolating the switch receiver terminals; wherein the T/R switch is operable to implement a receive state by causing short 5 circuiting of'the transmitter connected to the second switch terminal such that signals from a second terminal of the antenna may be received at the second switch terminal via the short circuited transmitter; and wherein in the receive state the T/R switch is operable to pass signals received at the first switch terminal and the second switch terminal to the receiver terminals. 10 In further preferred embodiments, the switch receiver terminals comprise a first switch receiver terminal and a second switch receiver terminal. In such embodiments of the invention, switching between the transmit state and the receive state is preferably balanced switching, such that switching transients appearing at the first switch receiver terminal are substantially equal to switching transients appearing at the second switch 15 receiver terminal. Such embodiments enable common mode rejection of such switching transients in the receiver, thus providing for sensing of received signals to be achieved prior to settling of switching transients, and thus enabling short two way propagation time signals to be sensed. In preferred embodiments of the invention, a controllable connection between 20 the first switch terminal and the second switch terminal is provided by a first switch clement and a second switch element in series and having a ground connection between the first switch element and the second switch element. Where the first switch terminal and second switch terminal are matched, such embodiments provide for balanced switching of the controllable connection between the first switch terminal and the 25 second switch terminal. To further provide balanced switching, a controllable connection between the first switch terminal and the first switch receiver terminal is preferably provided by a third switch element, and a controllable connection between the second switch terminal and the second switch receiver terminal is preferably provided by a fourth switch 30 element, wherein the third switch element and the fourth switch element are matched. In preferred embodiments, common mode switching transients may be removed by use of a transformer, or by use of a differential amplifier. Preferably, the transmitter comprises an N-channel metal oxide semiconductor (NMOS) transistor, operable to produce transmit signals in the transmit state, and 35 presenting a. low impedance when the drain-source voltage is small, in the receive state.

WO 2006/015402 PCT/AU2004/001382 8 It has been realised that, contrary to conventional wisdom, platinum reefs such as the U02 platinum deposit in South Africa are transparent or translucent to ground penetrating radar. Accordingly, in a fourth aspect of the present invention, there is provided a 5 method of radar imaging in the vicinity of a platinum deposit, comprising: applying a ground penetrating electromagnetic signal; and receiving returned electromagnetic signals. Embodiments of the fourth aspect oF the invention may be particularly advantageous in determining a thickness or volume of a platinum deposit for assisting 10 in detennining the economic or practical viability of mining that deposit. Embodiments of the fourth aspect of the invention may further be advantageous in imaging the rock volume surrounding the platinum reef during mining, and particularly the rock volume on a distal side of the deposit from the radar imaging apparatus. For example, the UG2 platinum deposit is overlaid by chromatite layers known 15 as stringers having low tensile strength. Where those stringers are less than around 3m above the platinum deposit, there is a high risk of the stringers collapsing into the mine void during or following removal of the platinum, with attendant implications for employees and mining equipment in the mine. Even when the presence of stringers is determined and roof bolts are placed in order to prevent collapse, the length of such 20 bolts must be sufflicient that the bolts pass through the stringer layers and are secured into high strength rock above the stringer layers. Determining a suitable length for the roof bolts can be difficult and is time sensitive. Accordingly, embodiments of the fourth aspect of the present invention may be particularly advantageous in imaging the stringer layers in order to determine the proximity of the stringer layers to the roof of 25 the mine void, and in determining the thickness of the stringer layers and thus a suitable length for roof bolts. In preferred embodiments of the invention, the method of the fourth aspect of the invention is applied by use of a ground penetrating radar in accordance with the first aspect of the invention. In such embodiments, the single device borehole radar is 30 preferably adapted for insertion into blast holes drilled for breaking up the platinum reef, prior to the insertion of explosives into the blast holes. In particularly preferred such embodiments, where a drill array is used for drilling an array of blast holes, a plurality of single device borehole radars are preferably mounted for simultaneous insertion into at least a subset of the array of blast holes. 35 Brief Description of the Drawings WO 2006/015402 PCT/AU2004/001382 9 Examples of the invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a symmetric monostatic ground penetrating radar; Figure 2 illustrates an asymmetric monostatic ground penetrating radar: 5 Figure 3 illustrates an asymmetric monostatic ground penetrating radar housed in a wire-lining core barrel; Figure 4 illustrates a monostatic ground penetrating radar with parallel mounted resistively loaded transmit and receive antennas; Figures 5a and 5b are schematics of a borehole radar transceiver in accordance 10 with an embodiment of the invention; Figure 6 is a schematic of a borehole radar transceiver in accordance with a second embodiment of the invention; and Figure 7 is a circuit diagram of the borehole radar transceiver of Figure 6. 15 Detailed Description of the Preferred Embodiments Most surface ground penetrating radars (GPRs) illuminate only shallow surficial layers. Near-surface attenuation usually screens deeper targets; there is room enough on the surface to make separate transmitting and receiving antennas for standard GPR. Bistatic surface GPRs are therefore the norm. Borehole radars (BHRs), in contrast, 20 rarely work in the attenuating near-surface. Most BTR are run in relatively low loss hard rock, and targets in that rock can be far enough away for a broadband transmit receive switch to have had time to settle before echoes of interest arrive. FTI I shows an electrically symmetric form of a ground penetrating borehole radar 10 in accordance with a first embodiment of the invention. BJ{R 10 comprises 25 antehna elements II and 12 of a single dipole antenna used for both transmitting and receiving. The electrically symmetric form of radar 10 provides a radiation pattern which is symmetrical about a plane that lies normal to the axis of the borehole. A battery array 18 is mounted within a first half of housing 13, while transceiver electronics 14 are mounted in a second half of housing 13, with the two mechanically 30 similar halves of the housing 13 being separated by a ferrite bead RF choke 15, illustrated in greater detail in the exploded portion of Figure 1. The RF choke 15 allows DC power to flow from the battery array 18 to the transceiver electronics 14 , while resisting the flow of A C power between the battery and transceiver compartments of the housing 13. AC voltage develops across the 35 choke, thus enabling the antenna elements 11, 12 to be electrically excited to transmit and receive radar signals. BHR 10 further comprises a transmit/receive switch 16, for WO 2006/015402 PCT/AU2004/001382 10 example of the type set out in Figures 5 to 7, to isolate the receiver electronics from high power transmission signals during a transmit stage, and to pass signals received by antenna elements 11, 12 during a receive stage to the receiver electronics. FIG 2 shows an asymmetric borehole radar 20 in accordance with a second 5 embodiment of the invention. BHR 20 comprises an antenna element 21 for transmitting, and an antenna element 22 for receiving. BHR 20 further comprises an electrically conductive housing 23 which connects the antenna elements 21, 22 and serves as a counter electrode to each antenna element 21, 22. Transmitter electronics 24 are positioned proximal to transmit antenna element 10 21, while transmit/receive switch 26 and accompanying receiver electronics are located at an opposite end of conducting housing 23, proximal to receive antenna element 22. Housing 23 further houses batteries 28. By providing a BHR 20 in which only the counter electrode is used for both transmitting and receiving and having a transmit antenna element 21 distinct from 15 receive antenna element 22, and further by physically separating the transmitter electronics 24 and transmit/receive switch and receiver electronics 26, the BHR 20 reduces direct coupling between transmitter and receiver. Where the transmit electronics are spaced apart from the switch/receiver electronics by around 1.5m, coupling may be reduced by perhaps 10dB, depending on the physical and electrical 20 configuration of the electrical path between the transmitter electronics 24 and switch/receiver electronics 26 . Such a reduction in coupling may be particularly valuable in easing the isolation burden placed on a transmit/receive switch, and/or may enable the use of transmissions of a correspondingly greater magnitude (eg 10dB greater magnitude) to improve the signal-to-noise (SNR) ratio and/or range of the 25 borehole radar 20. The transmit/receive switch, suitably timed, further assists in isolating the: receiver during transmissions by transmitter 24. Asymmetric antenna 20 may provide: a radiation pattern having radiation lobes oriented in desired directions. FIG 3 shows an asymmetric borehole radar 30 in accordance with a third embodiment of the present invention, suitable for housing in a standard wire-lining core 30 barrel. In BHR 30, the transmitter, receiver and associated controller electronics are mounted within a conductive housing 33 co-located proximal to the end of a conducting tube 39, such as .a drill string. Again, BHR 30 includes a battery array 38, and antenna element 31, being resistively loaded for broadband performance and housed in a protective casing 32 which is substantially electromagnetically transparent. 35 Housing 33 and tube 39 together function as a counter electrode relative to antenna element 31. Radar signals are developed between the counter electrode, and WO 2006/015402 PCT/AU2004/001382 11 loaded, damped antenna 31 which protrudes from the end of the tube 39. Antenna element is used for both transmitting and receiving, with a transmit/receive switch The configuration of BHR 30 is advantageous in that the tube 39 provides physical protection for all of the BHR 30 apart from the antena 31. Antenna 31 may be 5 relatively inexpensive and adapted for regular replacement, and/or housed in a sturdy and substantially electromagnetically transparent protective casing. Figure 4 illustrates a BHR 40 in accordance with a fourth embodiment of the present invention. The receiver antenna element 41 and transmitter antenna element 42 lie side by side in close proximity, and share a counter electrode 43 which also serves 10 as a housing for the radar electronics. Each antenna element 41, 42 has distributed resistors along the length of the antenna element for damped broadband performance. Advantageously, such resistors also form a potential divider, which limits the power that can be delivered directly from the transmitter into the receiver. This eases the burden carried by the receiver, or by a transmit-receive switch at the receiver front end. 15 In the present embodiment, the resistors distributed in series along each antenna element 41, 42 arc selected to have increasing resistance with distance from the transmitter/receiver electronics, with the sum of all the resistors of each antenna coming to substantially 5k. In considering the amount of isolation of the receiver from transmitted pulses afforded by this configuration, we consider the circumstance in 20 which a lkV transmitted pulse is generated, and arcs from the transmit antenna element 42 to the receiver element 41 at the first resistor of each antenna element. Assuming the first resistor of each antenna element has a value of 50n, and that the receiver has a 502 input, then only about 330V is generated at the receiver, thus providing around 10dB isolation between transmitter and receiver in this circumstance. 25 Symmetric radars of the type shown in Figure 1 are simpler to analyse than asymmetric radars of the type shown in Figure 2, with which they may share the advantage o'f interchangeable antennas to match site-specific ranges and attenuation conditions, by screwing these different antennas onto the -1 meter long central section. Asymmetric BHR such as the BHR of Figure 2 reduce stress on the T/R switch, and 30 enable higher powered transmissions, with consequent rise in SNR. Monostatic radars are shorter than the conventional bistatic equivalents. The sort offset in a monostatic radar makes it possible to create viable synthetic apertures in blast holes, which might be only three or four radar wavelengths long. Robust monostatic radars are easier to build than their bistatic equivalents. 35 The present invention may incorporate a transmit/receive switch (T/R switch) of the type set out in Figures 5 to 7, for providing high isolation between the transmitter WO 2006/015402 PCT/AU2004/001382 12 and receiver, particularly in broadband applications where the delay between a transmitted signal and detection of a received signal is comparable to the duration of the transmitted signal. Figures Sa and 5b are schematics of a borehole radar transceiver 100 in 5 accordance with an embodiment of the invention. Transceiver 100 comprises an antenna 105, transmitter 110, transmit/receive (T/R) switch 120, and a receiver 130. A first antenna terminal 106 is connected to a first switch terminal 121, and a second antenna terminal 107 is connected to a second transmitter terminal 111. A first transmitter terminal 112 is connected to a second switch terminal 122. First switch 10 receiver terminal 123 and second switch receiver terminal 124 are connected to receiver 130. Figure Ga illustrates transceiver 100 in a transmit state. in which second switch terminal 122 is connected to first switch terminal 121 due to switching means 125 being closed. Switches 126 and 127 are open thus isolating switch receiver terminals 15 123 and 124, and thus isolating receiver 130 in the transmit state. As receiver 130 would typically comprise high gain pre-amplifiers. isolation is important during the production of high power signals by transmitter 110. The closing of switching means 125 during the transmit state allows signals produced by transmitter 110 to be transmitted by antenna 105. 20 Figure 5b illustrates transceiver 100 in a receive state, wherein electromagnetic signals detected by antenna 105 are passed to receiver 130. Transmitter 110 has been short circuited such that signals from second antenna terminal 107 are passed to second switch terminal 122. Opening of switching means 125 and closing of switching means 126 and switching means 127 permits received signals to pass from first switch 25 terminal 121 to first receiver terminal 123, and from second switch terminal 122 to second receiver terminal 124. This, in the receive state, transceiver 100 passes received signals to receiver 130. Figure 6 is a schematic of a borehole radar transceiver 200 in accordance with a second embodiment of the invention. The antenna 210 has a first terminal Al and a 30 second terminal A2, transmitter 220 has a first terminal TX1 and a second terminal TX2 and T/R-switch has two input terminals TRI and TR2. Al is connected to TRI, TR2 is connected to TX2, and TX1 is connected to A2. The T/R-switch is also. connected to the input temiinals RX1 and RX2 of the receiver 240. The T/R-switch has a pair of identical shunt switches P1 and P2 and a pair of identical series switches 35 Si and S2. Pl is connected between TRI and ground, whilc P2 is connected between WO 2006/015402 PCT/AU2004/001382 13 TR2 and ground. SI is connected between TRI and RXI, while S2 is connected between TR2 and RX2. Whenthe transceiver 200 is in a transmit state, the shunt switches P1 and P2 are on and thus in a low impedance state, and the series switches S1 and S2 are off, in a 5 high impedance state- This effectively connects TX2 to Al and disconnects RX1 and RX2 from TX2 and Al respectively. The transmitter 220 consequently drives the antenna 210 directly and the receiver 240 is isolated in two stages. In the receive state, shunt switches P1 and P2 are off, in a high impedance state, and the series switches S 1and S2 are on, in a low impedance state. The transmitter 220 10 is in a low RF impedance state so that A2 appears to be connected to TR2. Thus, RXI is effectively connected to Al and RX2 is effectively connected to A2. The antenna 210 consequently drives the receiver 240 directly with minimum loss in the transmitter 240 and T/R-switch. Switching the switches P1, P2, SI and S2 between low and high impedance 15 states introduces switching transients into the signal path. Such switching transients have sufficient amplitude to cause prolonged saturation of the input amplifiers of receiver 240fif applied directly to the receiver's input. Further, it is desirable for the T/R-switch to be able to switch the transceiver 200 from the transmit state to the receive state in a time comparable to the duration of the 20 signal transmitted and received on the antenna 210, in order for the receiver 240 to discern close-in targets. This causes the frequency spectrum of the switching transients to overlap that of the signal received on the antenna 210. The signal received on the antenna can consequently not be separated from the switching transient by frequency domain filtering. 25 Accardingly, in the present exemplary embodiment, shunt switch P1 is operated so that the transient it creates on TRI relative to ground is substantially identical to the transient created by shunt switch P2 on TR2 relative to ground. Similarly, series switch Si is operated so that the transient it creates between TR1 and RX1 is substantially identical to the transient created by series switch S2 between TR2 and RX2. Such 30 switching causes. the transient observed on RX1 relative to ground to be substantially identical to that observed on RX2 relative to ground. The large switching transient is a common mode event and consequently can not saturate the receiver 240 which detects the differential mode signal between RXI and RX2. Figure 7 is a circuit diagram of the borehole radar transceiver of Figure 2. The 35 transmitter 220 comprises an N-channel metal oxide semiconductor (NMOS) transistor QTX connected to a DC power supply VCC) through resistor RTX. A control voltage WO 2006/015402 PCT/AU2004/001382 14 is applied between the gate and source of QTX. The drain of QTX is at a voltage level close to that of VCCl in the steady state condition, where the transmitter's control voltage is zero. The gate voltage of QTX rises rapidly relative to the source voltage during 5 transmit mode. This causes QTX to create a sharp falling voltage transient between terminals TX1 and TX2 (QTX drain-source voltage). The equivalent drain-source impedance of QTX is then very low and remains in this state during the receive mode. which commences after the sharp falling transient has been generated. The transmitter 220 is allowed to recover to its steady state condition upon completion of the receive 10 state. The second terminal TR2 of T/R switch 230 is connected to TX2 of the transmitter, and the first input terminal TR1 of T/R switch 230 is connected to an antenna terminal Al. Transformer TF1 allows the T/R-switch 230 and transmitter 220 to have a common ground, even though the signal monitored by the T/R-switch 230 is 15 superimposed on the drain voltage of transistor QTX. The two shunt switches of Figure 6 are realized by two identical NMOS transistors Q1 and Q2, with their drains connected to a second, common DC power supply VCC2 through two identical resistors R1 and R2, respectively. The sources of Q I and Q2 are connected to the ground of the T/R-switch 230 and their drains are 20 connected to nodes TR1A and TR2A, respectively. The single control voltage TR CTRL of the T/R-switch 230 is applied to the gates of both QI and Q2. Two identical Schottky diodes Dl and D2 and two identical resistors R3 and R4 are used to implement the series switches SI and S2 of Figure 2. The anodes of D1 and D2 are connected to nodes TRIA and TR2A, respectively. The cathodes of D1 and D2 25 are in turn connected to a third, common DC power supply VCC3 through R3 and R4, respectively. The node at the cathode of DI is TRIB and at the cathode of D2 is TR2B. The voltage level of VCC3 is slightly higher than the anticipated voltage levels associated with leakage from, the transmitted pulse through to TRIA and TRIB, and is significantly lower than the voltage of VCC2. 30 A positive voltage higher than the gate-source threshold voltage of the NMOS transistors Ql and Q2 is applied to the control voltage input TR-CTRL of the T/R switch 230 during the transmit state. This causes Q1 and Q2 to enter a low drain source impedance state and the voltages on nodes TRlA and TR2A (being the drain voltages of Q1 and Q2) will drop substantially simultaneously to a value close to zero, 35 below VCC3. These voltage drops cause Dl and D2 to substantially simultaneously become reverse biased, and thus present a high impedance. This combination of the WO 2006/015402 PCT/AU2004/001382 15 low impedance between TRIA and TR2A, the high impedance between TR1A and TR IB and the high impedance between TR2A and TR2B isolates the receiver 240 from the transmitted pulse. The impedance seen between TR1 and TR2 is close to zero so that TX2 is in effect directly connected to Al. TXl is already hard wired to A2. The 5 transient generated by the transmitter 220 between TX1 and TX2 will therefore be radiated on the antenna 210. The control voltage TR-CTRL of the T/R-switch 230 is reduced to zero soon after the transmitter 220 radiates the voltage transient on the antenna 210, to initiate the receive state. This causes the drain-source impedance of Q1 and Q2 to rise and the 10 voltages on TRIA and TR2A to recover concurrently to a value slightly higher than VCC3. Dl and D2 simultaneously become conducting at this point and enter a low impedance state. This causes TRI A to rise further to a bias point determined by VCC2. VCC3, R1, R3 and the forward voltage of DI. TR2A rises to substantially the same. bias point, set by VCC2, VCC3, R2, R4 and the forward voltage of D2. The resultant 15 high impedance between TR1A and TR2A, the low impedance between TRIA and TR1B and the low impedance between TR2A and TR2B allows a differential signal to pass through with minimum loss from the input TR1, TR2 to the output RXl, RX2 of the T/R-switch 230. The low output impedance of the NMOS transistor QTX of the transmitter 220 20 during the receive state effectively connects TR2 to A2. TR1 is already hard wired to the Al. The signal received on the antenna is therefore passed through to the receiver with minimum loss. The synchronous switching action of Q1 and Q2 causes substantially identical waveforms on TRiA and TR2A relative to ground. DI is consequently activated in a 25 substantially identical manner as D2 and similar waveforms are generated between TRIA and TR1B and between TR2A and TR2B. The waveform observed on TRIB relative to the ground is therefore substantially identical to that observed on TR2B relative to the ground, and thus switching transients are controlled to be a common mode signalL As TF2 can only couple a differential signal from its primary winding to 30 its secondary winding, the common mode switching transients arc rejected whereas 1he differential signal applied between TRI and TR2 passes through TF2. The parasitic reactance of the devices used in this embodiment limits the bandwidth and phase response of the T/R-switch's differential path in receive mode. The main contributors are the drain-source capacitance of the NMOS transistors Q1 35 and Q2 and the leakage inductance of the transformers TF I and TF2. The inclusion of L1, C1 and C2 absorb the parasitic reactance of Q1, Q2 and TF2 in a second order, flat WO 2006/015402 PCT/AU2004/001382 16 phase, band pass filter, to ensure a controlled frequency response. The T/R-switch then combines with the receiver to create a path with fixed characteristic impedance, in the pass band. The value of RI and R2 and the value of R3 and R4 are compromises between 5 minimum loss in the differential path of the T/R-switch, quick recovery time of QI and Q2's drain voltages as well as sufficient bias current for D I and D2. Thus, the exemplary embodiments include a duplexer or transmit/receive switch (T/R-switch) that enables a transmitter and a receiver to use the same antenna. The T/R-switch has sufficient instantaneous bandwidth and linearity in its phase response to 10 function in a pulse system without compromising the pulse shape, range or resolution of the system. The T/R-switch implemented in such embodiments the invention provides adequate isolation between the transmitted pulse and the receiver during transmit-mode to prevent the receiver from saturating for prolonged periods and the loss introduced by the T/R-switch between the antenna and the receiver in receive 15 mode, is small enough to avoid substantial reduction in the signal to noise ratio, which would otherwise reduce the range of the system. Such a T/R-switch provides for a ground penetrating radar having the ability to switch from maximum isolation in the transmit state to minimum attenuation in the receive state in a time similar to the period of the transmitted and received signals, in 20 order to enable the receiver to record reflections from close-in targets. Notably, in the present embodiment the T/R-switch and transmitter are inserted in front of the receiver, in the receiver path. In a transmit state, the T/R-switch is responsible for isolating the receiver from the transmitted signal and all the energy from the transmitted pulse is sent from the transmitter to the antenna. In the receive 25 state, the transmitter itself presents a RF (radio frequency) short circuit, thus preventing echoed or reflected signals collected by the antenna from dissipating in the transmitter circuitry. Simultaneously the T/R-switch ensures that the received signals are delivered to the receiver with minimal dissipation. Thus, the T/R switch used in the preferred embodiment relates to transceivers in 30 which a transmit/receive switch connects an antenna to the transmitter while transmitting a signal and connects the same antenna to the receiver while receiving the reflected signal. The embodiment in accordance with the invention specifically relates to a T/R-switch with wide instantaneous bandwidth (such as a pulsed system) where the delay between the radiation of the transmitted signal and the detecting of the 35 reflected signal is comparable to the duration of the signal itself, the performance of WO 2006/015402 PCT/AU2004/001382 17 which is relatively unaffected by changes in the antenna impedance due to changing conditions in the surrounding medium. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific 5 embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims (20)

1. A ground penetrating radar comprising: a transmitter for generating electromagnetic transmissions for ground penetration; 5 a receiver for receiving reflected electromagnetic signals; an antenna; wherein at least a portion of the antenna is used by the transmitter for transmitting and by the receiver for receiving.

2. The ground penetrating radar of claim 1, comprising a transmit antenna portion 10 used by the transmitter and a receive antenna portion used by the receiver, and wherein the portion of the antenna used by the transmitter for transmitting and by the receiver for receiving comprises a counter electrode.

3. The ground penetrating radar of claim 2, wherein the antenna is damped to minimise resonance between the transmit antenna portion and the receive antenna 15 portion.

4. The ground penetrating radar of claim 3 comprising a plurality of damping resistors distributed in series along at least one of the transmit antenna portion and the receive antenna portion.

5. The ground penetrating radar of any one of claims 2 to 4, wherein the transmit 20 antenna portion is positioned remote from the receive antenna portion in the radar, to provide isolation of the receiver from transmitted signals.

6. The ground penetrating radar of claim 5, wherein the transmit antenna portion extends from the radar in a first direction along a longitudinal axis of the radar, and wherein the receive antenna portion extends from the radar in a second direction 25 opposite to the first direction along the longitudinal axis of the radar.

7. The ground penetrating radar of claim 3 or claim 4, wherein the transmit antenna portion is positioned proximal to the receive antenna portion in the radar

8. The ground penetrating radar of claim 7, wherein the transmit antenna portion and the receive antenna portion each extend from the radar in a direction parallel to a 30 longitudinal axis of the radar.

9. The ground penetrating radar of claim 1, wherein the transmitter and receiver wholly share the antenna.

10. The ground penetrating radar of claim 9, wherein the antenna is a dipole antenna having two dipole elements shared by the transmitter and the receiver. 35

11. The ground penetrating radar of claim 10, wherein the antenna is a symmetrical dipole antenna shared by the transmitter and the receiver. 19

12. The ground penetrating radar of claim 10 or claim 11, wherein the dipole antenna functions as a housing for both the transmitter and receiver.

13. The ground penetrating radar of any one of claims 10 to 12 wherein the two dipole elements of the dipole antenna are connected by a low pass filter allowing DC 5 power flow between the two dipole elements and allowing radar frequency AC voltages to develop between the two dipole elements.

14. The ground penetrating radar of claim 13 wherein a DC power supply of the radar is provided via the radar housing.

15. A method of constructing a ground penetrating radar comprising: 10 providing a transmitter for generating electromagnetic transmissions for ground penetration; providing a receiver for receiving reflected electromagnetic signals; and providing an antenna; wherein at least a portion of the antenna is used by the transmitter for 15 transmitting and by the receiver for receiving.

16. A method of ground imaging using ground penetrating radar, comprising: transmitting an electromagnetic ground penetrating signal using at least one transmit antenna portion; and receiving reflected electromagnetic signals using at least one receive antenna 20 portion; wherein at least one antenna portion is both a transmit antenna portion and a receive antenna portion.

17. A method of radar imaging in the vicinity of a platinum deposit, comprising: applying a ground penetrating electromagnetic signal; and 25 receiving returned electromagnetic signals.

18. A ground penetrating radar substantially as herein described with reference to the accompanying drawings.

19. A method of constructing a ground penetrating radar substantially as herein described with reference to the accompanying drawings. 30

20. A method of ground imaging using ground penetrating radar substantially as herein described with reference to the accompanying drawings.