An em wave radar based drilling guidance system

Abstract

Described herein is a borehole radar system (100) for a drill string (200).The system (100) includes a radar transmitter (102) positioned within or adjacent to a housing of a bottom hole assembly (202) of the drill string (200).The radar transmitter (102) includes a transmission antenna (104) for coupling electromagnetic waves to the drill string (200) to guide the electromagnetic waves in one or more predefined modes, including TEM, TE11 and / or higher order modes, in a forward direction along an axis of the drill string (200) for transmission into a surrounding medium (302). A radar receiver (106) is positioned within or adjacent to the housing of the bottom hole assembly (202). The radar receiver (106) includes a receiver antenna (108) adapted to receive electromagnetic waves reflected or scattered from the surrounding medium (302). (Figure 1)

Description

AN EM WAVE RADAR BASED DRILLING GUIDANCE SYSTEMFIELD OF THE INVENTION

[0001] The present application relates to drilling systems and in particular to borehole drilling sensors.

[0002] Embodiments of the present invention are particularly adapted for a radar system for sensing a proximity to a surface of a surrounding medium. However, it will be appreciated that the invention is applicable in broader contexts and other applications.BACKGROUND

[0003] Damage to the tops of coal seams caused by incorrect blast stand-off distances is a serious issue within the Australian coal industry. It results in coal losses of up to 10-15%. . To date, there have been no effective and economically-sound techniques for addressing this problem.

[0004] PCT Patent Application Publication WO 2010 / 132927 entitled “Forward looking borehole radar to determine proximity of adjacent interface of different seams or layers" discloses a “feel-ahead” type borehole radar system that aims to detect a proximity of a drill bit to an interface by way of impedance changes near the drill bit. The system utilizes near-bit sensing of evanescent electromagnetic fields, which are typically only able to sense a distance of less than one metre from the drill bit. Further, the antennas used to drive the guided electromagnetic waves for sensing were remote to the drill string and had to be inductively coupled.

[0005] Some drilling systems have been developed to take measurements to look ahead of the drill bit. For example, US Patent 8,736,270 entitled “Look Ahead Logging System" assigned to Schlumberger Technology Corp (“Schlumberger”) discloses a drilling system with a radar for transmitting and receiving look ahead waves into a surrounding medium and measuring how that field is perturbed by the presence of a change in resistivity. One or more transmitter electrodes transmit an electric field into the surrounding medium and receiver electrodes measure the resultant electric field. In the presence of a boundary ahead of the drill bit, the field is altered, and the receiver detects a change in the total field structure and / or strength. To determine the distance to the interface, an inversion must be done, taking into account the spacings between all the electrodes which means the distance to the interface is rarely better than ±2-5 m. To obtain measurements accurate enough to be useful,Schlumberger uses electrode spacings up the drill string of 30 m (100 ft) or more. This is impractical for drilling blastholes, many of which are less than 30 m.

[0006] Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.SUMMARY OF THE INVENTION

[0007] In accordance with a first aspect of the present invention, there is provided a borehole radar system for a drill string, the system including: a radar transmitter positioned within or adjacent to a housing of a bottom hole assembly of the drill string, the radar transmitter including a transmission antenna for coupling electromagnetic waves to the drill string to guide the electromagnetic waves in one or more predefined modes, including TEM, TEn and / or higher order modes, in a forward direction along an axis of the drill string for transmission into a surrounding medium; and a radar receiver positioned within or adjacent to the housing of the bottom hole assembly, the radar receiver including a receiver antenna adapted to receive electromagnetic waves reflected or scattered from the surrounding medium.

[0008] Preferably, the system includes a processor for processing the received electromagnetic waves to determine the proximity of a drill string to an inhomogeneity of the surrounding medium. The processor is preferably located at or adjacent a top assembly of an associated drilling system. In some embodiments, the processor includes a plurality of processing components, which may be co-located or spatially separated.

[0009] In some embodiments, the transmission antenna and receiver antenna are colocated. In other embodiments, the transmission antenna and receiver antenna are positioned at separate locations.

[0010] In some embodiments, the transmission antenna and receiver antenna are embedded within a stabiliser of the bottom-hole assembly. In other embodiments, the transmission antenna and receiver antenna are embedded within a drillbit of the bottom-hole assembly. In further embodiments, the transmission antenna and receiver antenna are embedded in respective slots in the stabilizer or the drill string. Preferably, the slots are located on a flank region at the bottom part of the stabilizer or the drill-string.

[0011] In some embodiments, the radar transmitter and radar receiver are located in respective slots on opposing sides of the drill bit, stabiliser or a leading drill rod. The slots may be located on an outer surface of a housing of the drillbit, stabilizer or the bottom of a leading rod.

[0012] In some embodiments, the radar transmitter and radar receiver include respective control circuitry. The control circuitry may be located within an internal cavity of the drillbit, stabiliser or drill rod and connected to the radar transmitter and radar receiver by respective wires extending through apertures.

[0013] In some embodiments, the radar transmitter and radar receiver are embedded within a dedicated sub-unit of the bottom-hole assembly. In other embodiments, the radar transmitter and radar receiver are located at a bottom section of a leading rod. In further embodiments, the radar transmitter and radar receiver are attached to an outside of an element of the bottom-hole assembly or lower section of the drill-string. The radar transmitter and radar receiver may be removably attached to an element of the bottom-hole assembly or lower section of the drill string.

[0014] In some embodiments, the predefined modes include a propagating transverse electric (TE) mode. In some embodiments, the predefined modes include an evanescent, bound or leaky mode supported within the housing of the bottom hole assembly. These modes decay exponentially outside the housing of the bottom hole assembly. The predefined modes may include an evanescent TEM mode supported within the housing of the bottom hole assembly. These modes decay exponentially outside the housing of the bottom hole assembly.

[0015] In some embodiments, the transmission antenna is adapted to generate predefined modes in the form of a combination of a propagating TEn mode that is able to propagate into the surrounding medium and an evanescent TEM mode that is supported within the housing of the bottom hole assembly. Preferably the transmission antenna is controlled to selectively increase or decrease an amplitude of a propagating TEn mode that is able to propagate into the surrounding medium relative to an amplitude of an evanescent TEM mode that is supported within the housing of the bottom hole assembly.

[0016] In some embodiments, the transmission antenna is controlled such that the propagating TEn mode is configured to be reflected or scattered off inhomogeneities of the surrounding medium ahead of the bottom hole assembly along the axis of the drill string.

[0017] In some embodiments, the amplitude and frequency of the TEn mode is selected such that components of the TEn mode that are reflected or scattered from inhomogeneities within a range of >1 m of an end of the drill string are received by the receiver antenna.

[0018] In some embodiments, the transmission antenna and receiver antenna are controlled such that the evanescent TEM mode is configured to sense a resistivity of the surrounding medium within the radius of EM wavelength.

[0019] In some embodiments, the transmission antenna and receiver antenna include helical antennas. In some embodiments, the transmission antenna and receiver antenna include a quadrifilar helical antenna. In other embodiments, the transmission antenna and receiver antenna include two orthogonal co-wound half helices with opposite polarities. In some embodiments, the transmission antenna and receiver antenna is adapted to generate circularly polarized predefined modes.

[0020] In some embodiments, the transmission antenna and receiver antenna includes a Wu-King antenna. In one embodiment, the transmission antenna and receiver antenna includes a Wu-King monopole antenna. In another embodiment, the transmission antenna and receiver antenna includes a folded dipole antenna. In a further embodiment, the transmission antenna and receiver antenna includes a double monopole antenna.

[0021] In some embodiments, the transmission antenna is controlled to generate the predefined modes in a pulsed manner.

[0022] In accordance with a second aspect of the present invention, there is provided a borehole radar system for a drill string, the system including: a radar transmitter including a helical transmission antenna configured to generate electromagnetic waves for transmission into a surrounding medium; and a receiver including a helical receiver antenna adapted to receive electromagnetic waves reflected or scattered from the surrounding medium so as to extract information of inhomogeneities of the surrounding medium.BRIEF DESCRIPTION OF THE FIGURES

[0023] Example embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:Figure 1 is a schematic system diagram of an EM wave radar based drilling guidance system;Figure 2 is a side schematic of a drilling system having a drill string for drilling into a geological medium;Figure 3 is a side schematic of a first embodiment of a stabilizer of a drill string having borehole radar antennas embedded therein in slots around flank regions;Figure 4 is a side schematic of a second embodiment of a stabilizer of a drill string having borehole radar antennas embedded therein in slots around flank regions;Figure 5 is a side schematic of a third embodiment of a stabilizer of a drill string having borehole radar antennas embedded therein in slots around flank regions;Figure 6 is a side schematic of a fourth embodiment of a stabilizer of a drill string having borehole radar antennas disposed within a dedicated radar unit attached to a lower end of the drill string adjacent the stabilizer;Figure 7 is a sectional view of a top assembly of a drill rig, showing an example location of an uphole communications module;Figure 8 illustrates three side sectional views and a cross section of a borehole radar incorporating helical antenna elements;Figure 9 illustrates electric and magnetic field vector plots for a TEn electromagnetic mode;Figure 10 illustrates a simulation of a TEn mode propagating from a drill rod into a surrounding medium ahead of the drill bit using a folded Wu-King dipole antenna;Figure 1 1 illustrates electric and magnetic field vector plots for a TEM electromagnetic mode; andFigure 12 illustrates a feel ahead mode with near-total reflected TEM using a Wu-King double monopole antenna.DESCRIPTION OF THE INVENTIONSystem overview

[0024] Referring initially to Figure 1 , there is illustrated schematically a borehole radar system 100 for a drill string 200. The drill string 200 forms part of a drilling system 300, as illustrated in Figure 2, for drilling into a geological medium 302 to find and access natural resources such as coal, gas, iron ore and other minerals.

[0025] With reference to Figure 1 , the borehole radar system 100 includes a radar transmitter 102 positioned within or adjacent to a housing of a bottom hole assembly 202 of the drill string 200. The radar transmitter 102 includes a transmission antenna 104 for coupling electromagnetic waves to the drill string 200 and the bottom hole assembly 202 to guide the electromagnetic waves in one or more predefined modes in a forward direction along an axis of the drill string 200 and the bottom hole assembly 202 for transmission into the surrounding geological medium 302.

[0026] The borehole radar system 100 also includes a radar receiver 106 positioned within or adjacent to the housing of the bottom hole assembly 202. The radar receiver 106 includes a receiver antenna 108 adapted to receive electromagnetic waves reflected or scattered from the surrounding medium 302.

[0027] The borehole radar system 100 further includes radar electronics 110 for controlling various parameters of the radar transmitter 102 and radar receiver 106. The parameters to be controlled include output frequency, pulse duration and phase . The radar electronics 1 10 may be formed on one or more integrated circuits, microcontrollers or system-on-chip substrates. Separate electronics may be included for the radar transmitter 102 and radar receiver 106.

[0028] Although illustrated separately in the schematic of Figure 1 , in some embodiments, the transmission antenna 104 and receiver antenna 108 are co-located in a mono-static arrangement or may be slightly separated in a short offset bistatic transceiver arrangement. In other embodiments, the transmission antenna 104 and receiver antenna 108 are located on opposing sides of the bottom hole assembly 202. In some embodiments, transmission antenna 104 and receiver antenna 108 share the same antenna hardware.

[0029] Within the scope of this specification, the bottom hole assembly 202 collectively refers to the group of components of the drill string 200 that are located at or proximal to the drilling location. These include a drill bit and rod, drill collars, stabilizers, drill sub, the drillstring itself, drill housing, electronics, downhole motors, steering components and other associated components typically co-located with or in proximity to the drill bit. In some embodiments, the bottom hole assembly 202 may include a dedicated additional unit containing the borehole radar system 100 encased within its own a protective housing. This dedicated unit may be located above the conventional components of a bottom hole assembly 202 such that it is located higher on the drill string than those components.

[0030] The drill string 200 refers to the collection of components used for drilling in the drilling system 300. As illustrated in Figure 2, the bottom hole assembly 202 is a component of the drill string 200. Other components of the drill string include but are not limited to a drill pipe, transition pipe and drill collars (not shown).

[0031] As will be described below, the components of the borehole radar system 100 may be positioned at different locations within the bottom hole assembly 202 or drill string 200. However, in general, the antennas 104 and 108 must be positioned and configured to couple (and receive) the electromagnetic radiation to / from the structure of the bottom hole assembly 202 (which is primarily a metal conductor), which becomes an active emitter of the electromagnetic radiation into the surrounding geological medium 302 and acts to receive the returned signal from the geological medium 302. The transmission antenna 104 and receiver antenna 108 are attached to and form part of the bottom hole assembly 202. By way of example, transmission antenna 104 may directly couple the electromagnetic radiation into a stabiliser, drill bit and the drill string itself to provide a conductive medium for generating forward propagating waves. As a result, the bottom hole assembly 202 essentially becomes part of the antenna system for the borehole radar system 100 with antennas 104 and 108 being the drivers of this antenna system.

[0032] This direct coupling of electromagnetic radiation to the bottom hole assembly 202 and drill string 200 is essential for propagation of electromagnetic waves beyond short distances (e.g. greater than about 1 metre) of the drill string 200 into the geological medium 302. At the same time, the radar antennas 104 and 108 must be protected from the harsh environment of the bottom hole assembly 202 during a drilling process.

[0033] To achieve the direct coupling of electromagnetic radiation to the bottom hole assembly 202, the antennas 104 and 108 may be located in slots of the protective housing or in a separate antenna protective housing disposed outside the main protective housing of thebottom hole assembly 202 such that direct coupling to the bottom hole assembly 202 is possible.

[0034] Referring to Figures 3 to 5, in some embodiments, the transmission antenna 104 and receiver antenna 108 are both embedded within a stabiliser element 204 of the bottomhole assembly 202 that is adjacent a drill bit 206. In these embodiments, the transmission antenna 104 and receiver antenna 108 are embedded in respective slots 208 and 210 in flank regions of the stabilizer 204. In some embodiments, the flank regions may be in circumferentially opposing positions. The position of slots 208 and 210 are best seen in the cross section of Figure 3. The slots 208 and 210 are formed in outer flank regions of the housing 216 and extend along an axis of drilling.

[0035] The slots 208 and 210 are preferably shallow in depth so that antennas 104 and 108 are situated close to the surface of protective housing 216. The shallower the slots, the greater the coupling of electromagnetic radiation from antennas 104 and 108 to drill string 200. Although slots 208 and 210 are illustrated as being single slots extending along an axis of the stabilizer 204, it will be appreciated that slots 208 and 210 may comprise a plurality of axially separated slots. In the case of helical antennas (described below), the slots 208 and 210 may be helical such that they vary circumferentially around stabilizer 204 (or other component) as a function of axial position.

[0036] As shown in Figure 4, a pair of transmitter antennas 104 and a pair of receiver antennas 108 are electrically connected to radar electronics 110 via through-holes 212 and 214 in a protective housing 216 of the stabiliser 204. The electronics, including the radar electronics 1 10, battery 224 and bottom hole communications module 222 are disposed on or in an internal electronics module 21 1 . The internal electronics module 211 may be cylindrical in shape and have a central longitudinal opening that conforms with a central air cavity 213 of drill string 200. In this configuration, the radar electronics 1 10 are protected from the harsh drilling environment by the protective housing 216. In the embodiment of Figure 5, separate radar electronics are provided for the transmitter antennas 104 and receiver antennas 108.

[0037] Figure 5 illustrates a similar embodiment to that of Figures 3 and 4. However, in Figure 5, the radar electronics 1 10 for both antennas 104 and 108 is located in a single unit and disposed on an opposite side of stabiliser 204 to battery 224 and bottom hole communications module 222. It will be appreciated that the radar electronics 1 10 may be colocated with the antennas 104 and 108 in some embodiments.

[0038] In other embodiments, rather than being located in respective slots, the antennas 104 and 108 are located in respective antenna housings that are mounted to the protective housing 216 at an upper end or on the flank regions of the stabiliser 204. In some embodiments, a combination of slots and protective housings may be used to house the antennas 104 and 108. In further embodiments, antennas 104 and 108 are disposed on a surface of protective housing 216 but covered in a protective resistive material. Such materials may include synthetic polymers or aramid fibers like Kevlar™.

[0039] In further embodiments, the transmission antenna 104 and receiver antenna 108 are embedded within the drill bit of the bottom-hole assembly 202 in a similar manner to that of the stabiliser. The radar transmitter 104 and radar receiver 108 may also be located within other elements of the bottom hole assembly 202 such as the bottom of a drill rod or leading drill rod.

[0040] In each of these embodiments, the radar electronics 1 10 including control circuitry, is preferably located within an internal cavity of the drillbit, stabiliser, drill rod or other component and connected to the radar transmitter 104 and radar receiver 108 by respective wires extending through respective through-holes, as described above.

[0041] Referring now to Figure 6, the radar transmitter 104 and radar receiver 108 are embedded within a dedicated sub-unit of the bottom-hole assembly 202 in the form of a dedicated radar unit 220. The radar unit 220 includes radar electronics 1 10 and a communications unit 222 for communicating radar information external to the unit. The transmitter antenna 104 and receiver antenna 108 are located on a lower section of the drill string 200 adjacent to the radar unit 220. The embodiment illustrated in Figure 5 may be useful for incorporating the borehole radar system 100 to an existing drilling system.

[0042] Referring again to Figures 1 and 2, the drilling system 300 includes a drilling rig 304 that is configured to control and support the drill string 200. A top assembly 304 is located on an upper region of the drill string 200 and includes an uphole communications module 308 for communicating with a bottom hole communications module 222 located within the bottom hole assembly 202 as illustrated in Figure 1 .

[0043] As shown in Figure 7, the uphole communications module 308 may be disposed on a blast rig mast 320 of the top assembly 304 adjacent an air feed tube 322. The air feed tube feeds air through a drill drive head 324 to the internal air cavity 213 of the drill string 200.

[0044] As illustrated schematically in Figure 1 , the uphole communications module 308 and bottom hole communications module 222 are able to communicate data with each other wirelessly by transmitting GHz frequency electromagnetic (EM) waves through an internal air column of the drill string 200. In other embodiments, the uphole communications module 308 and bottom hole communications module 222 can communicate with each other via a communications cable (not shown) extending within drill string 200.

[0045] The bottom hole communications module 222 is adapted to receive radar control signals for controlling parameters of the borehole radar system 100 via the radar electronics 1 10 and also to transmit returned radar data received from the radar receiver 106 to the uphole communications module 308.

[0046] Drilling system 300 includes a surface processing and display unit 310 having a user interface with display 312 for receiving user input. As illustrated in Figure 2, this unit is typically located within a cabin of the drilling rig 304 but may be located elsewhere in other embodiments. The surface processing and display unit 310 also includes a processor 314 and a first network communications module 316 that is configured to communicate with a second communications module 318 within the top assembly 306. In this manner, a user is able to input control commands at interface / display 312 for controlling the data acquisition of the borehole radar system 100 via the various communications modules 222, 308, 316 and 318 and communicate with the drill rig 304 to obtain real-time drill-bit depth information. The system can also perform data calibration for each working site, automatic real-time data processing via processor 314, display and target tracking, and notifying a drill operator that they are approaching the recommended drilling target such as the coal seam distance from the drill bit.

[0047] In some embodiments, the top assembly 304 forms a wireless relay station to link the communication between the bottom hole communications module 222 and the surface processing and display unit 310. In other embodiments, drilling system 300 does not include the top assembly 304 and the bottom hole communications module 222 communicates directly with the first network communications module 316 within the surface processing and display unit 310.

[0048] Preferably, the primary processing of the received radar data is performed on the surface by processor 314. This includes processing the received electromagnetic waves by the radar receiver 106 to determine the proximity of the drill string to an inhomogeneity of thesurrounding geological medium 302. Given that the radar signal data must be transmitted to the surface either before or after processing so the driller has the information in real time, it is more efficient and requires less hardware design and manufacturing to put that processing at the surface. However, it will be appreciated that the processing of the received radar signals may be performed at least in part within the bottom hole assembly 202, top assembly 306 or externally to drilling system 300.

[0049] As illustrated in Figure 1 , the bottom hole assembly 202 and radar unit 100 may be powered at least in part by a local power supply 224 such as a battery. These components may be powered fully or in part by the drilling rig 304 via a power cable extending through the drill string 200 or by harvesting electricity from the drilling vibration / rotation or air-flow energies.Antenna design and mode generation

[0050] Various forms of radar antenna are possible in borehole radar system 100. The antenna design must be selected so as to be able to be attached to or integrated into the bottom hole assembly 202 and also to directly couple electromagnetic waves in the range of 10 MHz to 20 GHz to the structure of the bottom hole assembly 202 and drill string 200. The radar antennas must also be able to generate guided electromagnetic waves that propagate in one or more predefined modes in a forward direction along an axis of the drill string 200. This axis is the direction of drilling of the drilling system 300. In preferred embodiments, the transmission antenna 104 and receiver antenna 108 are less than 1 metre in length.

[0051] The modes must have properties such that at least some of the modes can penetrate into a surrounding medium for a distance of preferably greater than 1 metre. In some embodiments, the modes penetrate a distance in the range of 1 to 5 metres into the geological medium 302 beyond the drill string 200. In some embodiments, the modes penetrate a distance in the range of 1 to 10 metres into the geological medium 302 beyond the drill string 200. In some embodiments, the modes can penetrate a distance greater than 10 metres into the geological medium 302 beyond the drill string 200.

[0052] It will be appreciated that the distance that a mode can penetrate into a medium is dependent on the medium properties such as the medium’s conductivity and / or overall impedance to a particular electromagnetic frequency (or range of frequencies). A more highlyconductive medium will transmit electromagnetic waves over a shorter distance than a less conductive medium.

[0053] Referring to Figure 8, there is illustrated exemplary embodiments of borehole radar system 100 wherein the transmission antenna 104 and receiver antenna 108 are formed of helical antennas. Upper panel A of Figure 8 illustrates a single helical transmission antenna 104 wound around stabilizer 204 together with a single helical receiver antenna 108 such that there is a 90 degree phase difference between the position of the transmission and receiver antennas.

[0054] To achieve the desired transverse electromagnetic mode generation, each of the transmission antenna 104 and receiver antenna 108 include two antenna components. To achieve a TEM mode, the antenna components are driven with the same electrical polarity. To achieve a TEn mode (or higher order modes), the antenna components are driven with opposite electrical polarity. Middle panel B illustrates two transmission antenna components 104A and 104B, which together form transmission antenna 104. These antenna components 104A and 104B are wound around stabilizer 204 with a 180 degrees phase difference in their position. Although not illustrated in Panel B for simplicity, a similar arrangement is configured for the receiver antenna 108, which also includes two antenna components108A and 108B (see Panels C and D) wound around stabilizer 204 with a 180 degree phase difference. In this manner, both the transmission antenna 104 and receiver antenna 108 include two orthogonal co-wound helices with opposite polarities.

[0055] The resulting combination of 90 degree phase difference between transmission and receiver antennas with the 180 degree phase difference between antenna components is illustrated in Panel C and D of Figure 8. Panel C illustrates the four antenna components; two transmission antenna components with opposite electrical polarity and a 180 degrees phase difference and two receiver antenna components with opposite electrical polarity and a 180 degrees phase difference, wound around stabilizer 204. Panel D illustrates a cross-section of stabilizer 204 showing the four antenna components 104A, 104B, 108A and 108B, which are disposed at opposite circumferential locations at all times along stabilizer 204.

[0056] Although the transmission antenna 104 and receiver antenna 108 are illustrated as being co-wound around stabilizer 204, it will be appreciated that the antennas can be wound around any element of the bottom hole assembly 202 such as the drill bit or bottom section of the drill string (as in Figure 6) in a double-helix arrangement. In some embodiments, thetransmission antenna 104 and receiver antenna 108 can be arranged into a quadrifilar or higher order helical antenna arrangement.

[0057] This configuration of helical antenna with transmission antenna 104 and receiver antenna 108 including two components allows for the transmission antenna 104 and receiver antenna 108 to generate circularly polarized predefined modes of electromagnetic radiation (described below). Preferably, transmission antenna 104 and receiver antenna 108 are of the same or similar dimensions to each other but are mirror reflected vertically so they have opposite polarity. This opposing polarity reduces or minimises direct coupling of the transmitted signal to the received signal. When a circularly polarised wave is reflected, it will come back with opposite polarity so the receiver antenna must be reversed.

[0058] The helical antennas described above may be wound into slots of the bottom hole assembly component as described above. Alternatively, the helical antennas may be wound around the component and covered in a protective resistive coating such as resistive aramid fibers like Kevlar™.

[0059] Helical antennas are advantageous as they can offer a high signal-to-noise ratio compared to other types of antennas. However, it will be appreciated that other types of antennas may be used in borehole radar system 100. In other embodiments, the transmission antenna 104 and receiver antenna 108 may include Wu-King, Beverage or other antennas.

[0060] In other embodiments, the transmission antenna 104 and receiver antenna 108 include one or more of Wu-King monopole antennas, Wu-King folded dipole antennas and / or Wu-King double monopole antennas. These may be arranged in a double-barrelled transmitter antenna such that combinations of these antennas are used to couple the signal onto the drill string.

[0061] The Wu-King antenna was developed by RWP King and colleagues and is described in references 1 to 3 below. A Wu-King antenna is a non-resonant, wideband travelling wave antenna that is formed by disposing a plurality of spaced apart conductive elements separated by surface mounted resistors.

[0062] The transmission antenna 104 is controlled by radar electronics 1 10 to generate the predefined modes preferably in a pulsed manner. The amplitude and frequency of the modes may be controlled by a user at interface / display 312 via the various communications modules.The predefined modes may include modes that propagate forward and / or sideways from the drill bit 206 into surrounding geological medium 302.

[0063] The predefined modes generated by borehole radar system 100 preferably include a propagating transverse electric (TE) mode. By way of example, the TE mode may be a TEn mode. Vector fields of the electric and magnetic field lines for the TEn mode in a straight conductor are illustrated in Figure 9. The left panel illustrates the electric fields while the right panel illustrates the magnetic fields. The transmission antenna 104 is controlled by radar electronics 1 10 such that the propagating TEn mode (or other TE mode) is configured to be reflected or scattered off inhomogeneities of the surrounding medium ahead of the bottom hole assembly (generally along the axis of the drill string) and detected by receiver antenna 108. Modes that propagate a distance (greater than about 1 metre from the drill bit 206) into the surrounding geological medium 302 are referred to as “look ahead” modes.

[0064] For reflection or scattering of look ahead waves (e.g. a propagating TEn mode), inhomogeneities in the form of a change in conductivity or refractive index are required. Strong reflections occur where there is a sharp contrast in these parameters such as at an interface between two different materials. These look ahead waves are particularly useful in identifying an interface of a coal layer or other geologic boundary.

[0065] It is known that resistive rods support a complete spectrum of modes. The elements of the bottom hole assembly 202 are primarily formed of conductive metals such as steel for strength and generally represent resistive rods. Thus, excitation by the coupled radar transmitter 102 can excite electromagnetic modes which are supported in the structure of the bottom hole assembly 202. The fundamental TEM mode gets trapped on the drill string and near-totally internally reflects and that is what has been observed in computer modelling of drill string 200.

[0066] Referring to Figure 10, there is illustrated an image of a simulated look ahead mode of a drill bit with transmitted TEn using a folded Wu-King dipole antenna (i.e. two mono-pole antennas injected with opposite current flows). The drill string 200 and bottom hole assembly together are simulated as a cylindrical resistive rod in a resistive host medium. As illustrated, a pulse of the TEn mode propagates from the antennas within the drill string 200 or the bottom hole assembly 202 to the drill bit and then to the right of the drill bit (in front of the drill bit) into the surrounding medium.

[0067] The predefined modes may also include modes that are bound or confined to the housing of drill string 200. These may include an evanescent, bound or leaky mode supported within the housing of the bottom hole assembly wherein small amounts of energy leak into the surrounding geological medium 302. By way of example, the predefined modes generated by borehole radar system 100 preferably also include a TEM mode supported within the housing of the bottom hole assembly. Vector fields of the electric and magnetic field lines for the TEM mode in a straight conductor are illustrated in Figure 1 1 . The left panel illustrates the electric field while the right panel illustrates the magnetic field.

[0068] This TEM mode is generally bound within the housing of the bottom hole assembly 202 or drill string 200 and is evanescent outside of the excited drill string as its electric and magnetic field strength generally attenuates exponentially in the surrounding geological medium 302. As this TEM mode only extends a short distance (less than about 1 metre from drill bit 206) into the surrounding geological medium 302, these modes are referred to as “feel ahead” modes.

[0069] The degree of exponential decay of externally evanescent modes such as TEM modes is dependent on the resistivity of surrounding medium 302. Typically the signal strength of an EM mode will substantially approach zero within a distance from the bottom hole assembly 202 equal to about the radius of the wavelength of electromagnetic radiation. For electromagnetic waves in the MHz range, this is typically less than 1 metre even in a very resistive host geology.

[0070] The bottom hole assembly 202 and the drill string 200 are primarily formed of conductive metals such as steel for strength and form a continuous conductive string. Thus, excitation by the coupled radar transmitter 102 can excite electromagnetic modes which are supported in the structure of the bottom hole assembly 202 and drill string 200. This coupling allows for different placements of the antennas on or attached to the bottom hole assembly 202 or drill string 200.

[0071] Referring to Figure 12, there is illustrated a feel ahead effect with near-total reflected TEM mode using two Wu-King monopole antennas injected with same direction current flows. The drill string 200 and bottom hole assembly 202 together are simulated as a cylindrical conductive rod in a resistive host medium. As illustrated, an evanescent mode is present which extends from the antennas in a forward direction (to the right in Figure 12). When this hits the end of the rod, it inflates to an evanescent “balloon” which collapses and pumps backa near total reflection. This balloon is sensitive to changes in conductivity in front of and on either side of the end of a drill bit.

[0072] The transmission antenna 104 and receiver antenna 108 may be controlled by a control signal from radar electronics 1 10 such that the evanescent TEM mode is configured to sense a resistivity of the surrounding medium 302 within the radius of EM wavelength of the bottom hole assembly. E arrows rotate at the ends of the wire. A Huygens secondary source, formed on the abrupt lip at the end of the wire, drives a faint 3D near spherical energy bell, which opens to the right in an optical vortex, a “donut with a dark hole”. TEM fields are symmetric. Free space E fields are asymmetric, as are TEn fields. A monopole antenna, embedded in a slot cut in the flank of a drill rod, launches both the fundamental TEM and the first order TEn. Only the latter escapes through the drill bit. A folded dipole antenna can be used to preferentially launch asymmetric mode TEn.

[0073] Radar electronics 1 10 are able to control the transmission antenna 104 so as to generate predefined modes in the form of a combination of a propagating TEn mode that is able to propagate into the surrounding medium and an evanescent TEM mode that is primarily supported within the housing of the bottom hole assembly. In particular, radar electronics 110 can vary an antenna control signal such that the transmission antenna 104 is controlled to selectively increase or decrease an amplitude of a propagating TEn relative to an amplitude of an evanescent TEM mode. In this manner, a propagating TEn can be enhanced over a TEM mode or vice versa to essentially switch between look ahead and feel ahead modes.

[0074] The amplitude and frequency of the TEn mode (or other TE mode) may be selected using radar electronics 110 (e.g. by a user at interface 312) such that components of the TEn mode that are reflected or scattered from inhomogeneities or medium 302 within a range of 1 - 10 m of an end of the drill string 200 are received by the receiver antenna 108. The range of sensing is subject to parameters of the surrounding medium 302 such as the conductivity of the medium. As such, the radar parameters are designed to suit a predefined range or signal- to-noise ratio depending on the parameters of medium 302. Optimum coupling of the modes is obtained when the wavelength at the dominant frequency of the pulse is equal to the circumference of the steel components in the bottom hole assembly 202.System Operation

[0075] As described above, borehole radar system 100 can be integrated or retrofitted to a component of a bottom hole assembly 202 of a drill string 200 of a drill rig 300. The borehole radar system 100 is adapted to generate electromagnetic waves in the 10 MHz to 20 GHz frequency range that can couple to the structure of the surrounding bottom hole assembly 202 and / or or drill string 200 to generate look ahead (e.g. forward propagating TEn modes) and / or feel ahead (e.g. internally supported and externally decaying TEM modes) which can survey the geological medium 302 surrounding drill string 200.

[0076] Borehole radar system 100 is integrated within the bottom hole assembly 202 component such as a stabilizer 204 such that the antennas 104 and 108 and radar electronics 1 10 are protected from the forces of a drilling operation. Antennas such as helical antennas, Wu-King antennas and Beverage antennas, can be used and controlled such that they can generate combinations of the look ahead and feel ahead modes.

[0077] Electromagnetic radiation is transmitted into a surrounding geological medium 302 and reflected and scattered electromagnetic radiation is received by receiver antennas 108. Typically, radar transmitter 102 transmits the electromagnetic radiation in short pulses of I D- 20 cm in length ahead of the drill bit. The returned (reflected and scattered) signal is collected by radar receiver 106 and the associated signal information is transmitted from the bottom hole assembly 202 to a surface processing and display unit 310 via communications modules 222, 308, 318 and 316. A processor 314 processes the received information to predict the depth of the reflection off inhomogeneities in the geological medium 302.

[0078] Controlling the radar antennas such that the electromagnetic energy is primarily coupled into a TEn mode provides for sensing inhomogeneities in the geological medium 302 ahead of a drill bit such as a fault, bedding plane or a geological interface such as between overlying rock and a coal seam. These look ahead modes may sense inhomogeneities at distances greater than 1 metre ahead of a drill bit, providing ample time to stop drilling before the target geological interface is reached. The system is primarily directed to sensing ahead of the drill bit, but it is also capable of sensing inhomogeneities adjacent to the drill bit to sense where the drill bit has already pierced a layer.

[0079] Controlling the radar antennas such that the electromagnetic energy is primarily coupled into a TEM mode provides for sensing the conductivity and / or permittivity of thegeological medium 302 within a short distance (e.g. less than 1 metre) ahead and adjacent to the drill bit. These feel ahead modes can be useful for determining the rock type while drilling.

[0080] Coupling the radar waves on to the conductive drill string is the key for generating forward-looking TEn mode. But at the same time it will generate strong TEM mode which mainly reverberates within the drill string and will mask the look-ahead TEn waves. An important aspect of the present system is to tune the antennae design to enhance the look- ahead TEn mode and suppress the TEM mode or vice versa. However, TEM mode can be affected by the conductivity variation of the hole and can be used to “feel” or sense the conductivity of the strata. The ability to enhance or switch to the TEn or TEM mode makes this patent unique.

[0081] Embodiments of the present invention provide a real-time guidance system for blasthole drilling that will allow a predefined thickness of rock to be left above the target layer to minimise mixing of the overlying rock with the target layer during a blast. The system is designed to be robust enough to operate in-situ during a drilling operation. This system may be implemented on mine sites for routine use, improving blast-hole drilling control, reduced ore dilution and increased production and profitability in the open cut coal environment.

[0082] Embodiments of the present invention can be used to predict a coal seam top (or other geological layers) in real-time, while drilling blast-holes, by “seeing ahead” of the drill bit. This can improve blast hole drilling control, reduce damage to the resource and subsequent loss of product, as well as increase production and profits.

[0083] In contrast to resistivity measuring systems such as Schlumberger, the present invention transmits a short (10-20 cm) radar pulse ahead of the drill bit using a short (<1 m) antenna. It measures the time for a reflected pulse to return from an interface which does not require an inversion. This means that the system is not only much more compact and useful for shallow blastholes, but also more precise in measuring the distance to the interface (±0.1 - 0.2 m).

[0084] Embodiments of the present invention can also be used for other mine and civil engineering drilling environments.References1 . T. T. Wu and R. W. P. King, "The cylindrical antenna with nonreflecting resistive loading', IEEE Trans. Antennas Propagat., vol. 13, pp. 369-373, May 1965.2. L.-C. Shen and T. T. Wu, "Cylindrical antenna with tapered resistive loading", Radio Sci., vol. 2, pp. 191 -201 , Feb. 1967.3. Shen, Liang C. and Ronold W. P. King. “The cylindrical antenna with nonreflecting resistive loading.” IEEE Transactions on Antennas and Propagation 13 (1965): 369-373.Interpretation

[0085] Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "processing," "computing," "calculating," “determining”, analyzing” or the like, refer to the action and / or processes of a computer or computing system, or similar electronic computing device, that manipulate and / or transform data represented as physical, such as electronic, quantities into other data similarly represented as physical quantities.

[0086] In a similar manner, the term “controller” or "processor" may refer to any device or portion of a device that processes electronic data, e.g., from registers and / or memory to transform that electronic data into other electronic data that, e.g., may be stored in registers and / or memory. A “computer” or a “computing machine” or a "computing platform" may include one or more processors.

[0087] Reference throughout this specification to “one embodiment”, “some embodiments” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases “in one embodiment”, “in some embodiments” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0088] As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances oflike objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

[0089] In the claims below and the description herein, any one of the terms comprising, comprised of or which comprises is an open term that means including at least the elements / features that follow, but not excluding others. Thus, the term comprising, when used in the claims, should not be interpreted as being limitative to the means or elements or steps listed thereafter. For example, the scope of the expression a device comprising A and B should not be limited to devices consisting only of elements A and B. Any one of the terms including or which includes or that includes as used herein is also an open term that also means including at least the elements / features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.

[0090] It should be appreciated that in the above description of exemplary embodiments of the disclosure, various features of the disclosure are sometimes grouped together in a single embodiment, Fig., or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claims require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this disclosure.

[0091] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the disclosure, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0092] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the disclosure may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0093] Similarly, it is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limited to direct connections only. The terms "coupled" and "connected", along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Coupled" may mean that two or more elements are either in direct physical, electrical or optical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

[0094] Embodiments described herein are intended to cover any adaptations or variations of the present invention. Although the present invention has been described and explained in terms of particular exemplary embodiments, one skilled in the art will realize that additional embodiments can be readily envisioned that are within the scope of the present invention.

Claims

What is claimed is:1 . A borehole radar system for a drill string, the system including: a radar transmitter positioned within or adjacent to a housing of a bottom hole assembly of the drill string, the radar transmitter including a transmission antenna for coupling electromagnetic waves to the drill string to guide the electromagnetic waves in one or more predefined modes, including TEM, TEn and / or higher order modes, in a forward direction along an axis of the drill string for transmission into a surrounding medium; and a radar receiver positioned within or adjacent to the housing of the bottom hole assembly, the radar receiver including a receiver antenna adapted to receive electromagnetic waves reflected or scattered from the surrounding medium.

2. The system according to claim 1 including a processor for processing the received electromagnetic waves to determine the proximity of a drill string to an inhomogeneity of the surrounding medium.

3. The system according to claim 1 or claim 2 wherein the transmission antenna and receiver antenna are co-located.

4. The system according to any one of the preceding claims wherein the transmission antenna and receiver antenna are embedded within a stabiliser of the bottom-hole assembly.

5. The system according to any one of claims 1 to 3 wherein the transmission antenna and receiver antenna are embedded within a drillbit of the bottom-hole assembly.

6. The system according to claim 4 or claim 5 wherein the transmission antenna and receiver antenna are embedded in respective slots in a flank region at the bottom part of the stabilizer or the drill-string.

7. The system according to claim 6 wherein the radar transmitter and radar receiver are located in respective slots on opposing sides of the drill bit, stabiliser or a leading drill rod.

8. The system according to claim 6 or claim 7 wherein the slots are located on an outer surface of a housing of the drillbit, stabilizer or the bottom of a leading rod.

9. The system according to claim 7 wherein the radar transmitter and radar receiver include respective control circuitry and wherein the control circuitry is located within an internalcavity of the drillbit, stabiliser or drill rod and connected to the radar transmitter and radar receiver by respective wires extending through apertures.

10. The system according to any one of claims 1 to 3 wherein the radar transmitter and radar receiver are embedded within a dedicated sub-unit of the bottom-hole assembly.1 1 . The system according to any one of claims 1 to 3 wherein the radar transmitter and radar receiver are located at a bottom section of a leading rod.

12. The system according to any one of claims 1 to 3 wherein the radar transmitter and radar receiver are attached to an outside of an element of the bottom-hole assembly or lower section of the drill-string.

13. The system according to any one of the preceding claims wherein the predefined modes include a propagating transverse electric (TE) mode.

14. The system according to any one of the preceding claims wherein the predefined modes include an evanescent, bound or leaky mode supported within the housing of the bottom hole assembly.

15. The system according to claim 13 wherein the predefined modes include an evanescent TEM mode supported within the housing of the bottom hole assembly.

16. The system according to any one of the preceding claims wherein the transmission antenna is adapted to generate predefined modes in the form of a combination of a propagating TEn mode that is able to propagate into the surrounding medium and an evanescent TEM mode that is supported within the housing of the bottom hole assembly.

17. The system according to any one of claims 13 to 16 wherein the transmission antenna is controlled to selectively increase or decrease an amplitude of a propagating TEn mode that is able to propagate into the surrounding medium relative to an amplitude of an evanescent TEM mode that is supported within the housing of the bottom hole assembly.

18. The system according to claim 16 or claim 17 wherein the transmission antenna is controlled such that the propagating TEn mode is configured to be reflected or scattered off inhomogeneities of the surrounding medium ahead of the bottom hole assembly along the axis of the drill string.

19. The system according to claim 18 wherein the amplitude and frequency of the TEn mode is selected such that components of the TEn mode that are reflected or scattered frominhomogeneities within a range of greater than 1 m of an end of the drill string are received by the receiver antenna.

20. The system according to any one of claims 16 to 19 wherein the transmission antenna and receiver antenna are controlled such that the evanescent TEM mode is configured to sense a resistivity of the surrounding medium within the radius of EM wavelength.21 . The system according to any one of the preceding claims wherein the transmission antenna and receiver antenna include helical antennas.

22. The system according to claim 21 wherein the transmission antenna and receiver antenna include a quadrifilar helical antenna.

23. The system according to claim 22 wherein the transmission antenna and receiver antenna include two orthogonal co-wound half helices with opposite polarities.

24. The system according to claim 22 or claim 23 wherein the transmission antenna and receiver antenna is adapted to generate circularly polarized predefined modes.

25. The system according to any one of claims 1 to 20 wherein the transmission antenna and receiver antenna includes a Wu-King antenna.

26. The system according to claim 25 wherein the transmission antenna and receiver antenna includes a Wu-King monopole antenna.

27. The system according to claim 25 wherein the transmission antenna and receiver antenna includes a folded dipole antenna.

28. The system according to claim 25 wherein the transmission antenna and receiver antenna includes a double monopole antenna.

29. The system according to any one of the preceding claims wherein the transmission antenna is controlled to generate the predefined modes in a pulsed manner.

30. A borehole radar system for a drill string, the system including: a radar transmitter including a helical transmission antenna configured to generate electromagnetic waves for transmission into a surrounding medium; and a receiver including a helical receiver antenna adapted to receive electromagnetic waves reflected or scattered from the surrounding medium so as to extract information of inhomogeneities of the surrounding medium.

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