[0001 ] The present invention relates generally to a system and method for monitoring earth composition while drilling boreholes and more particularly, to the prediction of changes in the composition of earth and earth layers such as lithological layers.

[0002] The invention has been developed primarily for use to predict the location of a coal seam when drilling boreholes for blasting, thereby allowing the drilling operator to terminate drilling in advance of reaching the coal seam layer. However, while the invention is described with particular reference to applications involving the identification of coal seams during drilling, the method and system may also be applied to many other drilling applications where identification of changes of earth composition is desired.

Background of the Invention

[0003] The following discussion of the prior art is intended to facilitate an understanding of the invention and to enable the advantages of it to be more fully understood. It should be appreciated, however, that any reference to prior art throughout the specification should not be construed as an express or implied admission that such prior art is widely known or forms part of common general knowledge in the field.

[0004] In drilling blast holes for mining the earth, it is often desirable to drill a blast hole to within a defined proximity of a known geological lithological layer. An example of this requirement is found in open cut coal mining blast hole drilling. An "overburden" layer of earth above a coal seam is drilled to create a pattern of blast holes. The holes are used for the placement of blasting charges to break up the overburden layer so it may be removed, exposing the coal seam underneath. Terminating the drill some distance, typically between 20cm to two metres above the underlying coal seam, so as to not penetrate the coal seam, is considered to be desirable. It is generally agreed that coal dilution resulting from the subsequent blasting will be minimised where the boreholes have been consistently terminated in such a manner. The process of terminating a borehole in proximity to a detected interface may be referred to as "geo-stopping", a term which can also be used to describe the above practice. [0005] The current state of the art in the coal mining industry usually involves the blast hole rig first drilling four blast holes, with a borehole in each corner of the blast pad, which is typically 90m wide and may be up to several 100m long. These first four blast holes necessarily drill through the coal seam. The depth of the coal seam in each of these corner blast holes is inferred from the change in colour of the cuttings and the increase in instantaneous penetration rate of the drill as it transitions into the easier to drill coal seam. The target depth of the coal seam in all other last holes in the blasting pattern is then interpolated between the corner locations. However this process cannot account for geological non-linearity in the seam profile and is prone to inaccuracies, often resulting in blastholes being terminated at sub optimal depths. This leads to inefficient blasting outcomes and dilution of the orebody with overburden.

[0006] More advanced technologies for geo-stopping exist in other industries, and in particular the oil and gas industry. One such technology for geophysical sensing ahead of the bit known in the oil and gas industry is used on Schlumberger's RAB (resistivity at the bit) tool. This tool is capable of detecting rock interfaces to within 1 m of the advancing drill bit.

[0007] Applying similar RAB techniques to the blasthole drilling in the coal industry has previously been proposed and investigated. Resistivity varies with earth composition and particularly may change between various rock types. When the transitions are sudden, such as those between strata layers, resistivity has been shown to be an effective indicator of layers. Coal is also known to have a relatively high resistivity compared to the adjacent strata.

[0008] Resistivity measurement ahead of the drill is possible by injecting a known electric current into the formation ahead of the bit. The electric field generated ahead of the bit by this current flow is distorted depending on the resistivity of the formation earth composition surrounding the bit but also including earth composition ahead of the bit. A sensor, usually located in the drill collar immediately behind the bit, monitors the variation in current emanating from the bit and resolves a resistivity value.

[0009] However existing technology involves robust and ruggedized, complex electronics including current generators and sensors to be placed in a specially modified drill collar which sits immediately behind the bit. Significant differences in cost drivers and the frequency of application make its implementation generally incompatible with comparatively cost sensitive industries such as coal mining. To realise a commercially viable technical solution for the blasthole application, the high developmental and capital cost, coupled with much tighter physical constraints regarding battery life and size, an alternative solution is desirable.

[0010] It is an object of the present invention to overcome or substantially ameliorate one or more of the deficiencies of the prior art, or at least to provide a useful alternative.

Summary of the Invention

[001 1 ] Accordingly, in a first aspect the invention provides a method of monitoring earth composition surrounding a drill bit mounted on a drill-shaft during drilling of a borehole, including monitoring the electrical resistance through the earth between an earth contacting down-hole first electrode at or adjacent the distal end of the drill-shaft and a second electrode located out of the bore hole.

[0012] In a second aspect the invention provides a method of monitoring earth composition surrounding a drill bit mounted on a drill-shaft during drilling operations to drill a borehole from a drill head, the method including:

providing an earth contacting down-hole first electrode at or adjacent the distal end of the drill-shaft;

providing an electrical pathway, electrically insulated from the borehole wall and surrounding earth through the drill-shaft, to the first electrode from the drill head;

providing an earth contacting second electrode remote from the first electrode; and

monitoring electrical resistance through the earth between the first and second electrodes, including the steps of:

generating and conveying an electrical signal to one of the electrodes;

monitoring the electrical signal at the other electrode thereby to determine ground resistivity data between the first and second electrodes; and

analysing the ground resistivity data to determine changes in composition of the earth proximal to the bit.

[0013] In a third aspect the invention provides a method of drilling a blasting bore hole including:

monitoring earth composition surrounding a drill bit mounted on a drill-shaft during drilling by means of: providing an earth contacting down-hole first electrode at or adjacent the distal end of the drill-shaft ;

providing an electrical pathway, electrically insulated from the borehole wall and surrounding earth through the drill-shaft , to the first electrode from the drill head;

providing an earth contacting second electrode remote from the first electrode;

drilling a borehole by cutting with the drill bit to advance the borehole;

monitoring electrical resistance through the earth between the first and second electrodes, including the steps of:

generating and conveying an electrical signal to one of the electrodes;

monitoring the electrical signal at the other electrode thereby to determine ground resistivity data between the first and second electrodes; and

analysing the ground resistivity data to determine changes in composition of the earth proximal to the bit; and

terminating advancement of the borehole when ground resistivity data reflect a pattern indicative of a predetermined target in advance of the drill bit.

[0014] Preferably, the step of monitoring the electrical signal includes measuring the change in electrical current of the signal at different points of drill advancement.

[0015] Preferably, the step of monitoring the electrical signal includes calculating change in ground resistivity.

[0016] Preferably, the method includes analysing ground resistivity data to predict changes in composition of the earth in advance of the drill bit.

[0017] Preferably, the method includes correlating ground resistivity data with other data to predict changes in composition of the earth in advance of the drill bit, the other data including any one or more of: the drill depth; the drill rate of advancement; and known geological features.

[0018] Preferably, the method includes analysing ground resistivity data to predict proximity to a geological target including a lithological layer such a coal seam layer. [0019] Preferably, measuring the change in current of the signal includes monitoring the signal and compensating for the effects of noise and/or electrical artefacts.

[0020] Preferably, the method includes the step cutting with the drill bit to advance the borehole.

[0021 ] Preferably, the step of monitoring electrical resistance through the earth is performed substantially concurrently with the step of cutting with the drill bit. However alternatively, the steps of cutting with the drill bit and monitoring electrical resistance through the earth are performed substantially discretely.

[0022] Preferably, step of monitoring the electrical signal includes synchronous demodulation of the signal.

[0023] In fourth aspect the invention provides a system for monitoring earth composition surrounding a drill bit mounted on a drill-shaft during drilling of a borehole from a surface position, the system including:

an earth contacting down-hole first electrode disposed at or adjacent the distal end of the drill-shaft ;

an electrical pathway to the first electrode from the drill head, the electrical pathway being electrically insulated from the borehole wall and surrounding earth through the drill-shaft ;

an earth contacting second electrode remote from the first electrode;

an electrical signal generator electrically connected to one of the electrodes, for generating an electrical signal;

signal monitoring means electrically connected to the other electrode for monitoring the electrical signal between the first and second electrodes; and

means for analysing the monitored electrical signal to provide an indication of changes in resistivity between the electrodes to thereby determine changes in composition of the earth surrounding the bit.

[0024] Preferably, the signal is conveyed to the first electrode and monitored at the second electrode. More preferably, the drill bit is used as the first electrode.

[0025] Preferably, the drill-shaft forms the electrical pathway, the drill-shaft being electrically insulated from the borehole wall and the drill rig and the drill-shaft includes an electrically insulating sheath. Alternatively the electrical pathway is electrically insulated from the drill rig and the drill-shaft and includes an electrical cable positioned in an internal cavity within the drill-shaft.

[0026] Preferably, the drill-shaft is formed of discrete elongate coupleable sections, each section including an insulated electrical pathway between inter-engageable electrical connectors disposed at opposite ends for establishing an electrical connection between an adjacent section. However alternatively the drill-shaft may comprise a substantially continuous shaft or tube such as coiled tubing.

[0027] Preferably, the drill bit is insulated from substantially the remainder of the drill- shaft.

[0028] Preferably, the second electrode is located adjacent the drill head at a surface location. Alternatively, the second electrode is located in a borehole such as a borehole in a target lithological layer.

[0029] Preferably, the drill rig includes a mobile drilling platform including a drill mast and a drive head movable along the mast, for driving the drill shaft.

[0030] Preferably, signal monitoring means includes a synchronous demodulator for monitoring the signal.

[0031 ] In a fifth aspect the invention provides a drill rig including the system of the fourth aspect.

[0032] In another aspect the invention provides a drill rod for a drill string formed of discrete elongate coupleable like drill rods, each drill rod including a respective electrical conduit between inter-engageable electrical connectors disposed at opposite ends for establishing an electrical connection between an adjacent section.

[0033] Preferably, the drill rod includes a continuous elongate passageway and an outer sleeve, said electrical conduit disposed within said passageway. [0034] Preferably, the drill rod includes an insert mounted at each end for securing the electrical conduit therebetween and the conduit is an electrically conducting cable insulated from the section wall by the insert.

[0035] The term "surface" as used herein to define the general position from which the bore hole is drilled. While that is mostly commonly from a position on above ground, it is not intended to exclude cases where drilling is commenced from a position underground, such as from an underground mine site. As such, the term "surface" is used here in to indicate out-of-hole rather than down-hole positions.

[0036] The term "drill-shaft" as used herein to define a drill shaft arrangement to drive and/or position a drilling head down-hole. While the invention is described largely with reference to a segmented rigid pipe drill-string, it will be appreciated that the invention is not limited to such application and may be modified for use with other types of drill-shafts, drill strings and drilling systems such as coiled tubing drilling systems and cable drilling systems.

[0037] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are intended to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Brief Description of the Drawings

[0038] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0039] Figure 1 is a schematic view of a drill rig equipped with a system for monitoring earth composition surrounding a drill bit, in accordance with the invention;

[0040] Figure 2 is a partial section view of a drill rod coupling in accordance with one embodiment of the invention;

[0041 ] Figure 3 is a partial section view of a drill rod coupling in accordance with another embodiment of the invention; [0042] Figure 4A is a partial section view of a drill rod coupling including end caps for modifying a drill rod to include a electrical conduit;

[0043] Figures 4B is are perspective view of complementary end caps of Figure 4A;

[0044] Figure 5 is a partial section view of a rotary drive head of a drill rig including a rotary swivel enabling connection to an electoral pathway in the drill string; and

[0045] Figure 6 is a circuit schematic of a synchronous demodulator for monitoring the signal in accordance with one aspect of the invention.

Preferred Embodiments of the Invention

[0046] Referring to Figure 1 , the invention is shown and described as fitted to a typical mobile drilling rig 1 , comprising tracked platform 2 having a vertical drilling mast 3. The mast supports a drive head 4, movable up and down the mast, for driving a drill shaft or drill-string 5 having an earth boring drill bit 6 disposed on the distal end of the shaft, for cutting a borehole 7 from a respective bore-head. While the system will be described with reference to the mobile drilling rig comprising a segmented drill-string as shown, it may be installed onto other types of drilling systems such as coiled tube drilling rigs or the like, in a variety of different scales and configurations depending on drilling requirements.

[0047] Referring again to the figures, the invention provides a system and method for monitoring earth composition surrounding the drill bit 6 during down-hole drilling of a borehole 7 and in particular for estimating changes in earth composition in advance of the drill bit. An electrical circuit is joined through the earth by passing an electrical signal between an earth contacting, down-hole first electrode 8 disposed at or adjacent the distal end of the drill string 5, and a second electrode 9.

[0048] Suitably, the second electrode is located at the surface preferably at or adjacent the bore head or drilling rig. However the second electrode may be located elsewhere, either another surface position spaced from the bore-head or in a sub-surface location such as in a down-hole location of an adjacent bore-hole. In one embodiment the second electrode is positioned in or adjacent a target lithological layer. Furthermore, it will be appreciated that the term "surface" includes the surface with respect to the drilling rig/equipment or drill head and may include an underground "surface" position. [0049] The electrical circuit is isolated from the earth between the down-hole first electrode 8 and the second electrode 9. This allows the electrical resistance through the earth between the first and second electrodes to be monitored which in turn provides an indication of changes in earth composition proximal to the drill bit.

[0050] The system and method for monitoring the earth composition may be activated while the drill bit is cutting to advance the borehole. Alternatively, drill bit cutting operations may be periodically suspended and the system and method for monitoring the earth composition be activated while the bit is down hole.

[0051 ] The circuit includes the provision of an electrical pathway 10 from the bore-head at the surface to the down-hole first electrode 8 thereby allowing for the transmission of an electrical signal from a surface electrical signal generator 1 1 rather than relying upon a down-hole signal generator. The pathway 10 is incorporated into the drill string 5 and allows the signal generator 1 1 , along with signal monitoring equipment 12, and any other sensitive electrical equipment to be positioned remote from the harsh down-hole environment. An electrical connection is provided between the end of the electrical pathway at the bore head and the signal generator.

[0052] The electrical signal is preferably passed to the earth in a controlled fashion so that the signal is altered or transformed by interaction with the surrounding earth and/or rock layers. In the case of drilling operations, where the intention is to monitor the earth being penetrated by the drill in real time and/or attempt to predict upcoming layer transitions in advance of the drill bit, it is preferable to maximise the interaction of the signal with the earth in advance of the drill, while minimising interaction with earth behind the drill.

[0053] One method of maximising the signal interaction with layers and earth in advance of the drill is to pass the signal to the earth close to the leading part of the drill, meaning, at or adjacent the distal end of the drill shaft or drill bit, while at the same time preventing the signal passing to the earth from position remote of the drill. Other methods may also be applied to maximise interaction of the signal with the layers such as electronic signal focussing, however these too, normally require the electrodes and/or electronics near the distal end of the drill. The predominant remainder of the electrical pathway and optionally the drill string, and/or the signal carrying parts of the drill shaft are electrically insulated from the borehole wall, so as to prevent the signal passing into the earth prematurely, thereby causing a "short circuit" back to the second electrode 9. [0054] Likewise, the signal generator must be connected to the electrical pathway in such a way that the signal does not pass to the earth at a point out of the borehole, at the surface.

[0055] Figure 1 shows this concept schematically. The signal / ^' from the down-hole first electrode 8 will pass through and/or be affected by geological composition and/or lithological layers surrounding the drill, and including those layers immediately in advance of the drill. For instance, the electrical signal / ^' which is represented by dotted lines emanating forwardly (downwardly) from the end of the drill, passes through or is transformed by coal seam C and coal seam transition layers before returning to the second electrode 9 on the surface.

[0056] While the down-hole first electrode 8 may be positioned at or adjacent the distal end of the drill shaft, in this embodiment electrode sections of the drill bit 6 function as the down-hole first electrode 8 such that the signal is passed to earth through the drill bit being in cutting contact with the earth. The electrode sections may include the cutting teeth.

[0057] In view of the above, and with reference to detail in Figure 1 , the drill bit/down- hole first electrode 6, 8 and electrical pathway 10 are electrically isolated from any item of the drill string above, that that may contact the borehole wall and "leak" signal into the earth. For instance, a remote length of drill pipe or stabiliser sub that due to flexing or vibration contacts the borehole wall. One option is for any stabiliser sub or drill bit saver sub 13 to include an insulating section 14 to electrically isolate the drill bit. Alternatively, the isolating location could optionally be contained in the lowermost drill pipe or the bit saver sub.

[0058] There are a number of alternative solutions for providing the electrical pathway down the drill string from the surface.

[0059] One type of drill string/rod system is shown in Figure 2, where the electrical pathway is provided by the drill shaft or drill string itself. This approach makes sense because drill strings are normally formed of an electrically conductive metal material well suited to carrying an electrical signal, commonly configured as a hollow sleeve or pipe with an internal passageway. Commonly, drill strings are extended by joining together elongate segments or rods 20 by threaded torque conveying couplings. Figure 2 displays the coupling portions of a pair of drill rods 21 , 22 axially aligned for connection. It will be appreciated that only the respective end portions of each rod are displayed. The upper rod 21 in the figure shows the box end 23 incorporating a female threaded coupling, while the lower rod 22 displays the pin end 24 incorporating a complementary male threaded coupling. These couplings are usual metal to metal and thus able to transmit current between drill rods. In the case of coiled tubing, the tubing is essentially continuous metal and able to carry an electrical current.

[0060] As noted however, where the drill string itself is used to transmit the signal, the current carrying drill string must be isolated from the borehole wall so that the signal is not passed into the earth prematurely. One method of avoiding such a short circuit is to prevent contact of the drill string with the wall by the use of insulated spacers maintaining the drill sting in the center of the bore hole and spaced from the wall. Another method is to provide the drill string with an insulating sheath 25 of non-electrically conductive material such as a plastic material or ceramic covering over the metallic sleeve wall.

[0061 ] Alternatively the drill string rods may provide an electrical conduit for transmitting the electrical signal divorced from the load carrying wall of the drill string. A number of off the shelf or readily adaptable solutions are known which are used in other drilling applications such as underground in-seam drilling. Whilst many of these are designed for communication and carriage of data to and/or from down-hole electronics, they may be adapted for use in delivering a simple electrical signal to the bit.

[0062] Figure 3, displays the coupling portions of a pair of drill rods 27, 28 housing one internal conduit running coaxially through the internal passageway of the rod. The upper rod 27 in the figure shows the box end 29 incorporating a female threaded coupling, while the lower rod 28 displays the pin end 30 incorporating a complementary male threaded coupling. A central co-axial electrical cable 31 within the drill rod segments has an insulating cover 32 in case of contact with an internal wall of the drill rod.

[0063] The cable 31 is held in place by means of coaxial end caps 33 which not only anchor the cable but also provide a mounting point for complementary electrical connectors 34, 35. Each electrical connector is configured to automatically inter-engage with a corresponding connector on an adjoining rod as the drill rods are joined by otherwise conventional coupling techniques. In one form, the electrical connection comprises a female pin type connector 34 which engages within a corresponding male type connector 35 on an adjacent rod. Referring to Figure 3, it can be seen that the box end 29 includes a female electrical connector 34 while the pin end 30 below includes a corresponding male 35 electrical connector. Of course the position of the male and female connectors could be reversed on the box and pin ends respectively. It will be appreciated that in engaging the coupling ends of the rods, the rods are drawn together along the drill axis so that the male and female electrical connectors are automatically pushed into engagement thereby providing an electrical connection between rods insulated from the outer structural wall of the rod. The electrical connectors may include a sealing shroud which seals the connectors from dirt, debris and fluids.

[0064] One characteristic of the rods shown in Figure 3 and particularly with the communication type drill rods known in the art is that the end caps and connector partially obstruct the internal passageway 36. This limits the ability for transmission of fluids to the drill bit moving through the center of the rod which can cause problems while drilling. The issue is exacerbated for smaller diameter drill rods because the electrical connectors will obstruct a greater proportion of the available cross sectional flow area provided by the passageway. Another issue is that existing rods used for communication purposes, while available, are a specialty type of drilling rod and being specialist equipment, and considerably more expensive than standard drilling rods.

[0065] Accordingly, in further aspects, the invention proposes alternative drill rod and electrical connectors which reduce the obstruction of the drill rod passageway and/or allow the modification of an existing drill rod to provide an insulated electrical pathway and connection means.

[0066] As can be seen with reference to Figures 4A, 4B and 4C, the drill rod includes end caps 40a, 40b for anchoring a coaxial electrical conduit or cable 41 in the center of the rod at the box and pin ends respectively. Each end cap includes an outer sleeve 42a, 42b and a central cable anchor position 43 in the form of a coaxial bore. The anchor position

43, is connected to the sleeve by means of circumferentially spaced, radially extending fins

44. In this embodiment there are three equally circumferentially spaced fins 44 however the number and configuration of fins may vary. Each fin is disposed generally parallel with the longitudinal axis of the drill rod to minimise its frontal area with respect to the internal passageway 45 of the rod.

[0067] The passageway is otherwise kept devoid of material so as to allow the transmission of fluids through the rod for usual drilling practises such as circulation of cuttings. In the case of drilling blasting holes, the borehole is usually dry and the fluid is simply air, however, it also allows transmission of other gasses and liquids from the surface while drilling in subterranean environments. This can be particularly advantageous when using smaller diameter drills which provide restricted flow along the borehole.

[0068] Alternatively, the fins may be co-operatively inclined and configured to induce a vortex flow in the fluid as it passes through the passageway.

[0069] Referring to Fig. 4B, end cap 40a, is configured to be inserted into the end of the box end of the rod. It includes a circumferential electrical connector 46a extending axially from or adjacent the sleeve 42a, and into the box end cavity. The electrical connector 46a includes biasing means in the form of a helical spring 47, which in this embodiment provides both the connector 46a as well as bias means.

[0070] The complementary end cap 40b for the pin end of the rod is shown in Fig. 4C. This end cap is configured to be inserted into the passageway of the pin end of the rod and includes a corresponding electrical connector 46b in the form of an annular contact surface 48.

[0071 ] With reference to Fig 4a, when respective box and pin ends of adjacent rods are engaged by means of conventional threaded coupling, the ends will draw together compressing the helical spring 47 against the annular contact surface 48 to provide an electrical connection.

[0072] The spring biased connector system advantageously allows for any variation in dimensions of the rod ends due to inaccurate machining, wear or misalignment during rod coupling to ensure a positive contact and an electrical connection. Furthermore the spring not only helps establish a connection, but also helps the connection be maintained in the case of vibrations encountered while drilling. The spring connectors may also provide a degree of self cleaning in otherwise dirty conditions due to a sweeping action between the spring and the annular contact surface. Furthermore, the circumferential positioning of the connector places it out of the main airflow so as not to obstruct the rod passageway.

[0073] It is also necessary to provide an electrical pathway between the respective connectors and the cable 41 . In this embodiment this is provided by the end caps which are made from a conductive metal thereby allowing any electrical signal to pass directly from the cable 41 and anchor point, through the radial fins 44, to the respective connector 46a, 46b. However to prevent the signal shorting to the exterior structural wall of the rod, each end cap is provided with an insulating barrier. In this embodiment the end caps are each fitted inside a surrounding insulating insert 49a, 49b made from a plastics material to electrically isolate the current carrying end cap from the rod wall. The insert provides an insulating sheath which closely conforms to the dimensions of the end cap.

[0074] In other embodiments however the end cap may be made of an insulating material itself thereby eliminating the requirement for an insulating insert. However in such embodiments, because the end cap cannot directly carry the signal between the connectors and the cable, an alternate electoral pathway must be provided. This may be accomplished for instance with a short lead wire.

[0075] It will now be apparent that the end caps 40a, 40b allow retro-fitment inside the end of a conventional drill rod to provide an isolated electrical pathway between the ends of the rod as well as complementary automating electrical connectors between adjacent like rods. Preferably, the outer diameter of the end cap, or insert where fitted, is marginally smaller that the inner diameter of the drill rod passageway so that the end cap is held axially in place within the rod. In this embodiment each end cap 40a, 40b also includes a radially extending circumferential locating rim to engage an abutment shoulder in the respective rod end so that when inserted, the end cap is stopped at a position adjacent the end of the rod. Furthermore, in this embodiment the rim also serves as the annular contact surface 48 on the end cap 40b.

[0076] The coaxial bore of the cable anchor position 43 allows for connection of the cable 41 between the end caps located at respective ends of the rod. The cable may be inserted through the bore and secured. For instance the cable may secured by a wire clamp such as a ring clamp, compression or tension clamp. It may also be secured with a nut or push nut. Tensioning in the cable is used to hold the end caps cooperatively in place against respective abutment shoulders ensuring they are maintained within the respective ends of the rod.

[0077] Clearly the end caps allow the fitment of an electrical conduit to conventional drill rods thereby providing a potentially economical solution when compared to the alternative of special equipment used for communication with down-hole sensors. [0078] Retrofitting a drill rod involves the following steps:

1 . Thread the electoral cable through the central passageway within the rod;

2. Push a first end cap (and insert where used) into one end of the rod;

3. Anchor the cable to the anchor position of the first end cap;

4. Push a second end cap (and insert where used) into the other end of the rod;

5. Tension the cable and anchor it to the coaxial bore of the anchor position on the second end cap to cooperatively hold the inserts in place.

[0079] Referring back to Figure 1 , the system also includes a connection 50 from the electrical pathway 10 in the drill pipe in the proximity of the rotary drive head 4. In one embodiment, more clearly shown in Figure 5, the drive head 4 has an air drive hose 51 repositioned to enter the drive head laterally via a tee piece 52. The drive head includes a standard box end coupling 53 refitted with a coaxial electrical connector 54 to mate with the pin end coupling and corresponding electrical connector of a drill rod 55.

[0080] An insulated electrical cable runs 56 through the drive head, drive motor 57 and tee piece 52 to a rotary swivel cap 58 combined with a slip ring 59 thereby enabling continuous electrical connection to the electrical pathway within the drill string during operation of the drill. Of course the connection is broken each time a new drill rod section is added to the drill string however, drill advancement is also suspended.

[0081 ] It should be noted that if the electrical pathway is provided by drill rods having a insulating coating as seen in Figure 2, it is necessary to at some point to insulate the drill string from the drill rig to create a circuit. One logical place to do this is with the use of gap sub with a slip ring positioned at the rotary drive head. As such the rotary drive head shown in Fig 5 may then be used and the electoral pathway connected via cable 56. Alternatively the entire drive head and/or drill rig may be insulated.

[0082] In a preferred embodiment, for the purpose of abating potential electrical "noise" generated by drilling equipment and drill rig, the drill string is isolated from the rest of the drilling equipment despite not being directly used to carry the electrical signal from the signal generator, as with drill strings formed by means of the rods shown in Figure 3 and 4, This may be achieved by the use of a second insulating gap sub positioned directly under the drive head in addition the gap sub adjacent the earth contacting first electrode. As such the structural wall of the drill string is electrically isolated and any electrical "noise" generated by the electrical systems of the drill rig will not be passed down-hole to interfered with the signal passed to the earth.

[0083] Returning again to Figure 1 and the overall system, in this embodiment, as can be seen the electrical connection at the drive head 4 is connected to an insulated wiring loom 60 on the drill rig 1 . The loom 60 runs along with or in a similar manner to the umbilical of hydraulic and/or air hoses connected from the rotary drive head 4 to the mast 3 thereby along unrestricted movement of the drive head 4 up and down the mast 3. The loom 60 is connected to the signal generator and/or monitoring equipment 1 1 , 12 positioned on the drill rig 1 , preferably within the operator's cabin 70.

[0084] In alternative embodiments the signal generator may be located at or adjacent the drive head and connect directly to the electrical pathway. In still further embodiments, the entire rig and drill pipe is isolated from the ground.

[0085] The circuit is completed with an electrical connection between the signal generator and/or monitoring equipment 10, 1 1 and the second electrode 9 thereby enabling the change in current in the circuit and more particularly, between the first and second electrodes, to be monitored.

[0086] On a mobile drill rig, the second electrode 9 may be incorporated into the ground engaging vehicle tracks. However in this embodiment, the rig includes a dedicated ground contacting electrode 9 connected by electrical cable 61 . The electrode 9 may also be driven into the surface for instance an earth stake, and/or placed in a remote location, or be positioned in an adjacent borehole and tethered to the monitoring equipment using an insulated wire. In one embodiment the second electrode is located within or in close proximity to a target lithological layer, suitably a borehole drilled into the layer.

[0087] In alternative embodiments the signal may be applied to the second electrode and monitored by the down-hole first electrode.

[0088] As noted, the signal generator and monitoring equipment 1 1 , 12 shown in Figure 1 are preferably collocated in the driver's cab of the drilling rig, and may in fact be the same device performing both functions. However the functions of signal generating and signal monitoring are separate and separate device could be used and positioned together or at distinct locations. [0089] In any event, the monitoring equipment 12 monitors the change in ground resistivity of the current between the electrodes. Generally the means includes a current monitoring sensor which applies Ohm's law to determine the resistance of the circuit between the first and second electrodes. Changes in ground resistivity between the down- hole and surface electrode and may be analysed to provide an indication of changes in composition of the earth surrounding the bit.

[0090] There are many circuit designs, techniques for monitoring the signal and compensating for the effects of noise, error and other electrical artefacts encountered during resistivity measurement. For example, one signal processing system used to extract the signal from the monitored current is shown in Fig 6 and is known as a synchronous demodulator. This type of demodulator may be enacted by either software running on a microprocessor and/or dedicated hardware.

[0091 ] The reference signal from the signal generator 1 1 is developed by the reference signal generator 70. This signal is used to drive both the decoding hardware or software and the excitation of the bridge 71 .

[0092] Then both the reference signal and the input signal resulting from the bridge 71 are fed to either software or dedicated hardware as indicated in Fig. 6.

[0093] The input signal is filtered 72 to reject aliased signals and digitised 73 (for software) and then fed into in phase and quadrature multipliers 74a ,74b. The result is further low pass filtered 75a, 75b for both streams resulting in an in phase (I) and quadrature phase (Q) representation of the signal typically known as I and Q. The phase and magnitude are derived by the equations below. This method can extract a signal in as much as 140Db of noise.

Magnitude = J I ² + Q ² Phase = tan ^_1 y

[0094] Once the current and resistivity have been determined, the system employs software processing algorithms to determine significant changes based on expected values for the top of coal. This data may be combined with other parameters monitored by drill systems such as the estimated depth of the target layer, the current drill depth, rate of penetration, existence of other known layers to improve prediction of top of coal.

[0095] It will be appreciated that the present invention provides a system and method of detecting changes in earth composition and particularly the identification of rock interfaces immediately ahead of the drill bit while the drill is in operation. Advantageously, the system and method provides forewarning of imminent drilling conditions thereby allowing the operator to take action before encountering such conditions.

[0096] As previously seen, this ability is particularly advantageous when drilling blast holes for coal mining. In this application terminating the drill some distance, typically between 20cm to two metres above the underlying coal seam, so as to not penetrate the coal seam, is considered to be desirable for efficient blasting of over burden. Advantageously, the present invention provides the drilling operator with sufficient forewarning to terminate drilling in advance of a coal seam so that blast holes are drilled to an optimal depth. Moreover the invention enables this determination to be made while drilling thereby providing for efficient drilling operations.

[0097] Moreover while other RAB products exist, the invention provides a solution which removes complex electronics from the high vibration and harsh downhole environment, which in turn reduces the cost of components and increasing reliability. Rather, the invention utilises established system components, each of which are generally reliable and proven. For instance the processing software; as out of hole data capture and processing can be performed on a regular personal computer using software such as labview, without the need for specialist microprocessors in the electronics. The installation time and training required to use the system is reduced when compared with alternatives that use complex sensing electronics at the bit.

[0098] As has been seen the invention, in one aspect provides for modification of an existing, conventional drill string rather than the requirement for purpose built equipment and or downhole electronics packages.

[0099] The drill string is electrically isolated and even if it contacts the side of the borehole either continuously or intermittently it plays a limited role in the circuit compared to conventional at bit sensing systems. This is an advantage because a low noise environment is essential for detecting adequate signals. [0100] In summary, the invention minimises product development cost, capital cost and operating cost, whilst maximising product reliability functional performance. For these reasons, the invention provides a significant advantage making it commercially viable for coal production and other lower cost model applications.

[0101 ] It will be appreciated that no such in these and other respects, the invention represents a practical and commercially significant improvement over the prior art.

[0102] 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", analysing" or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing component, that manipulate and/or transform data represented as physical, such as electronic, quantities into other data similarly represented as physical quantities.

[0103] In a similar manner, the term "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.

[0104] The methodologies described herein are, in one embodiment, performable by one or more processors that accept computer-readable (also called machine-readable) code containing a set of instructions that when executed by one or more of the processors carry out at least one of the methods described herein. Any processor capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken are included. Thus, one example is a typical processing system that includes one or more processors. Each processor may include one or more of a CPU, a graphics processing unit, and a

programmable DSP unit. The processing system further may include a memory subsystem including main RAM and/or a static RAM, and/or ROM. A bus subsystem may be included for communicating between the components. The processing system further may be a distributed processing system with processors coupled by a network. If the processing system requires a display, such a display may be included, e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT) display. If manual data entry is required, the processing system also includes an input device such as one or more of an alphanumeric input unit such as a keyboard, a pointing control device such as a mouse, and so forth. The term memory unit as used herein, if clear from the context and unless explicitly stated otherwise, also encompasses a storage system such as a disk drive unit. The processing system in some configurations may include a sound output device, and a network interface device. The memory subsystem thus includes a computer-readable carrier medium that carries computer-readable code (e.g., software) including a set of instructions to cause performing, when executed by one or more processors, one of more of the methods described herein. Note that when the method includes several elements, e.g., several steps, no ordering of such elements is implied, unless specifically stated. The software may reside in the hard disk, or may also reside, completely or at least partially, within the RAM and/or within the processor during execution thereof by the computer system. Thus, the memory and the processor also constitute computer-readable carrier medium carrying computer-readable code.

[0105] Furthermore, a computer-readable carrier medium may form, or be included in a computer program product.

[0106] In alternative embodiments, the one or more processors operate as a standalone device or may be connected, e.g., networked to other processor(s), in a networked deployment, the one or more processors may operate in the capacity of a server or a user machine in server-user network environment, or as a peer machine in a peer-to-peer or distributed network environment. The one or more processors may form a personal computer (PC), a tablet PC, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine.

[0107] Note that while some diagrams only show a single processor and a single memory that carries the computer-readable code, those in the art will understand that many of the components described above are included, but not explicitly shown or described in order not to obscure the inventive aspect. For example, while only a single machine is illustrated, the term "machine" shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein.

[0108] Thus, one embodiment of each of the methods described herein is in the form of a computer-readable carrier medium carrying a set of instructions, e.g., a computer program that is for execution on one or more processors, e.g., one or more processors that are part of web server arrangement. Thus, as will be appreciated by those skilled in the art, embodiments of the present invention may be embodied as a method, an apparatus such as a special purpose apparatus, an apparatus such as a data processing system, or a computer-readable carrier medium, e.g., a computer program product. The computer- readable carrier medium carries computer readable code including a set of instructions that when executed on one or more processors cause the processor or processors to implement a method. Accordingly, aspects of the present invention may take the form of a method, an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present invention may take the form of carrier medium (e.g., a computer program product on a computer-readable storage medium) carrying computer-readable program code embodied in the medium.

[0109] The software may further be transmitted or received over a network via a network interface device. While the carrier medium is shown in an exemplary embodiment to be a single medium, the term "carrier medium" should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term "carrier medium" shall also be taken to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by one or more of the processors and that cause the one or more processors to perform any one or more of the methodologies of the present invention. A carrier medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical, magnetic disks, and magneto-optical disks. Volatile media includes dynamic memory, such as main memory. Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise a bus subsystem. Transmission media also may also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications. For example, the term "carrier medium" shall accordingly be taken to included, but not be limited to, solid-state memories, a computer product embodied in optical and magnetic media; a medium bearing a propagated signal detectable by at least one processor of one or more processors and representing a set of instructions that, when executed, implement a method; a carrier wave bearing a propagated signal detectable by at least one processor of the one or more processors and representing the set of instructions a propagated signal and representing the set of instructions; and a transmission medium in a network bearing a propagated signal detectable by at least one processor of the one or more processors and representing the set of instructions. [01 10] It will be understood that the steps of methods discussed are performed in one embodiment by an appropriate processor (or processors) of a processing (i.e., computer) system executing instructions (computer-readable code) stored in storage. It will also be understood that the invention is not limited to any particular implementation or programming technique and that the invention may be implemented using any appropriate techniques for implementing the functionality described herein. The invention is not limited to any particular programming language or operating system.

[01 1 1 ] Reference throughout this specification to "one embodiment" 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 invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. 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.

[01 12] Similarly it should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention 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 claimed invention requires 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 invention.

[01 13] 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 invention, 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.

[01 14] Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.

[01 15] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention 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.

[01 16] 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 or electrical 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.

[01 17] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as falling within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.