Downhole instrument for real-time laser-induced breakdown spectroscopy (LIBS) analysis

Abstract

A downhole instrument for the real-time laser-induced breakdown spectroscopy (LIBS) analysis of a borehole wall. The instrument includes an elongate housing having a bottom end and a top end, with an optics head at its bottom end, a LIBS spectrometer at its top end and a laser therebetween. The optics head includes laser delivery optics and plasma emission collection optics, the spectrometer is fibre-coupled to the optics head, the laser is configured to transmit pulses directly to the laser delivery optics for focussing before passing through a side window of the optics head to a sample point on the borehole wall; and the optics head is configured to collect radiation emitted from laser induced plasma at the sample point and transfer the emitted radiation via the plasma emission collection optics to the spectrometer via the fibre coupling.

Description

DOWNHOLE INSTRUMENT FOR REAL-TIME LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS) ANALYSISRelated Application

[0001] This application claims convention priority from Australian provisional patent application 2023903389 filed on 24 October 2023, the content of which is incorporated herein by reference.Field of the Invention

[0002] The present invention relates to instruments and methods used to assay geological formations in-situ down boreholes during mineral exploration and mining. The invention finds particular use in drilling where a depth-accurate core or cuttings sample is not recovered from the borehole and so the analysis of recovered material using top-of-hole techniques is not readily available and / or is not depth accurate.Background of the Invention

[0003] Real-time downhole assay holds the promise of low-cost, automated, objective geochemistry that can be incorporated in a drilling workflow and inform “while- you-drill” decision making. Current technologies with the potential for near real-time assay come in the form of top-of-hole techniques and downhole techniques, the downhole techniques including for example prompt-gamma neutron activation analysis (PGNAA), which has the advantage of sensing large rock volumes but has calibration, sample collection and detection limit issues for certain minerals (such as gold).

[0004] An ideal real-time downhole instrument will be capable of delivering a bulk chemical composition for the geological sample being assayed, will not require any special preparation steps for the sample, and will be achievable through a miniaturised sensor. In this respect, physical borehole characteristics and complications, such as surface roughness, flowing water, drilling muds and other drilling aids, have proven to pose greater challenges to accurate analysis than the challenges of miniaturising the analysis hardware to allow borehole deployment.

[0005] There is a broad range of analytical techniques that can be used for chemical analysis of this type in a laboratory environment, including X-ray photoelectronspectroscopy, electron microscopy, various types of chromatography, UV and visible absorption spectroscopy, infra-red spectroscopy, fluorimetry and chemiluminescence, X-ray florescence spectrometry, atomic absorption and flame emission spectroscopy, atomic emission spectroscopy, nuclear magnetic resonance spectroscopy, mass spectrometry, labelling methods, potentiometric methods, voltametric and coulometric methods, secondary ion mass spectrometry, atom probe tomography, and many more.

[0006] However, most of these techniques require sample digestion or special sample preparation. For example, thermal analysis techniques require that the sample, contained in a suitable sample pan or crucible, is placed in a furnace and subjected to some desired temperature regime, and most spectrometry techniques require a digestion pre-step.

[0007] Whilst there are numerous techniques that can be used for chemical analyses in a laboratory environment (such as thermal analysis, wet chemistry, chromatography, various ICP-based techniques, etc.) there are only a limited number of techniques that could be adapted to a downhole tool and used for analysis of geological materials outside the laboratory space. Such techniques might include X- Ray Fluorescence (XRF) spectrometry, alpha particle X-Ray (APXS) spectrometry nuclear methods, including the prompt-gamma neutron activation analysis (PGNAA) mentioned above, proton-induced X-Ray emission (PIXE), proton induced gamma emission (PIGE), and switchable radioactive neutron sources (SRNS).

[0008] Additionally, while there are some commercially available downhole assay tools, they are limited to gamma and neutron-based instruments and are not multielement downhole tools. In this respect, gamma and neutron techniques require strict regulations around their usage in the field.

[0009] Before turning to a summary of the present invention, it is to be appreciated that various directional terms, such as upper, lower, upwardly, upright, top, bottom and the like, have been used throughout this specification to provide context and clarity for the invention with reference to the normal upright use of an instrument such as this within a borehole that is typically, but not always, a reasonably vertical borehole. These terms are not to be taken as limiting the invention to be used only in one particular orientation.

[0010] The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of this applicationSummary of the Invention

[0011] The present invention provides a downhole instrument for the real-time analysis of a borehole wall, the instrument utilising laser-induced breakdown spectroscopy (LIBS).

[0012] In particular, the present invention provides a downhole instrument for the real-time laser-induced breakdown spectroscopy (LIBS) analysis of a borehole wall, the instrument including an elongate housing having a bottom end and a top end, the housing including therewithin an optics head at its bottom end, a LIBS spectrometer at its top end and a laser therebetween, wherein: a) the optics head includes laser delivery optics and plasma emission collection optics; b) the spectrometer is fibre-coupled to the optics head; c) the laser is configured to transmit pulses directly to the laser delivery optics for focussing before passing through a side window of the optics head to a sample point on the borehole wall; and d) the optics head is configured to collect radiation emitted from laser induced plasma at the sample point and transfer the emitted radiation via the plasma emission collection optics to the spectrometer via the fibre coupling.

[0013] The elongate housing is preferably a one-piece housing with the bottom and top ends being removable to permit access to the interior of the housing and removal, as necessary, of the optics head, the laser and / or the spectrometer. Having said that, the housing may also be formed from multiple housing parts, or with removable access ports along its length, as required. The reference to the housing being ‘elongate’ is a reference to the length of the housing being greater than the diameter of the housing, usually by several factors, in order to maintain the diameter of the housing close to the diameter of a borehole while the housing with enough length to house the required components.

[0014] The elongate housing may also include therewithin a suitable cooling system, such as a circulating airflow system, operable in response to temperature readings taken about either or both of the laser and the spectrometers. Laser and spectrometer operation in such an instrument can be adversely affected by temperature, rendering the availability of such a cooling system quite useful in some circumstances. In this respect, the housing will ideally have internal pathways that permit the passage of air about its various components, the internal pathways being in fluid communication with a suitable fan or the like also situated within the housing.

[0015] The LIBS spectrometer may be any suitable type of optical spectrometer capable of receiving emitted radiation from laser-induced breakdown spectroscopy. LIBS has been found to offer an efficient and powerful method for the simultaneous multi-element analysis of materials. Elements that can be detected and theoretically quantified span the majority of the Periodic Table, including light elements such Li, Be, B, Na and Mg.

[0016] In principle, LIBS is a form of atomic emission spectroscopy, relying on characteristic spectra emitted from plasma generated by a series of high-energy laser pulses striking the surface of a sample. Each laser pulse results in high-intensity plasma being produced at the sample point on the borehole wall, and the resulting emission spectrum of the plasma contains atomic emission lines from the atomic species present in the plasma. Multiple spectra may be averaged and, with varying degrees of sensitivity, every element in the Periodic Table can be measured. LIBS data can also be collected in tandem with, and integrated with, datasets from other analytical techniques including hyperspectral, X-ray fluorescence and Raman spectroscopy. Analysis by LIBS also produces minimal damage to samples, consuming only nanograms of sample material per laser pulse.

[0017] In a preferred form, the LIBS spectrometer will be of sufficient resolution and wavelength range so that the resultant emission spectra produced by the laser pulse is well resolved. The type of spectrometer configuration could be Echelle or Czerny- Turner, but will preferably be a compact Czerny-Turner configuration due to its ease of miniaturisation.

[0018] In one form of the invention, the instrument may include two spectrometers, being a UV spectrometer and a visible to near-infrared (NIR) light spectrometer, ideally registered so that the finite viewing angle of the plasma emission collection optics observes the entire laser plasma and transmits this via the plasma emission collection optics and the respective fibre-couplings to the spectrometers. The ability to capture a very narrow time slice of the emitted radiation (the spectrometer integration time) at a defined time after the occurrence of the plasma (the spectrometer delay time) is useful for determining the temperature of the plasma and the resolution and intensity of the characteristic elemental emission lines. Accurate and precise characterisation of the plasma infers information about the laser-matter interaction and assists with calculation of quantitative elemental abundances.

[0019] In the present invention, the purpose of the laser is to ablate and thermally excite the sample such that a high temperature plasma is produced that emits light that can identify elements and their concentrations. In a preferred form, the laser will be able to control the energy of each pulse and will have consistent pulses. This consistency will include pulse length, wavelength and intensity profile.

[0020] In a preferred form, the laser will be a single- or double-pulsed laser with a laser excitation wavelength range between 200 and 1600 nm, with pulse lengths in the range of either nanoseconds or femtoseconds. Preferably, the laser will have a pulse length in the nanosecond range. For example, it is envisaged that a Q-Switched Nd- YAG laser could be utilised, typically being a useful source of high energy, q-switched pulses in the desired wavelength range.

[0021] As mentioned above, the optics head includes laser delivery optics and plasma emission collection optics and is configured to collect radiation emitted from laser induced plasma at the sample point, delivered to the sample point from the laser via the laser delivery optics, and then transfer that emitted radiation via the plasma emission collection optics to the spectrometer via the fibre coupling. In one form, the optics head includes an optics mount able to be housed within either an optics housing within the elongate housing or directly within the elongate housing itself, the optics mount having mounting sections able to receive and hold both the laser delivery optics and the plasma emission collection optics.

[0022] The optics mount may be formed from multiple parts as an assembly or may be formed as a one-piece member, such as being injection moulded or 3D-printed with appropriate mounting sections pre-formed and ready to receive the appropriate optics. Ideally, the optics mount will be configured to rigidly hold the laser delivery optics and the plasma collection optics to allow for precise adjustment and alignment of the optics. In this respect, and as will become apparent in the following description of the laser delivery optics and the plasma collection optics this adjustment and alignment may include, or be in relation to, beam alignment, back reflection protection, collimator positioning, laser deflecting mirror(s) and tilted optical windows(s).

[0023] In one form, the laser delivery optics includes a focussing lens for focussing the longitudinally directed (‘longitudinal’ in terms of the elongate housing) incoming collimated laser pulse and a laser deflecting mirror for redirecting the focussed pulse laterally from the laser through the side window to the sample point.

[0024] In a preferred form, the focussing lens will be a single- or multiple planoconvex lens arrangement with a damage threshold sufficient to withstand damage from the laser pulses. Corrective optics for laser alignment, astigmatism and other focal characteristics may also be present. Preferably, the focussing lens will be a glass plano-convex lens coated with a suitable anti-reflective coating for the laser excitation wavelength used.

[0025] The laser delivery optics may also include a beam expander to maximise the collimated laser beam diameter, thereby beneficially reducing the fluence on the optics and increasing the energy density at the focal point.

[0026] The laser deflecting mirror for the incoming laser pulse will preferably be a flat mirror with suitable reflective coating for the laser excitation wavelength, with a suitable damage threshold, and which will preferably be sized such that the dimension of the mirror is larger than the maximum size of the focussed beam. In this respect, the size of the incident beam on the laser deflecting mirror is dependent on the position of the focal point relative to the instrument housing, so when the sample is close to the instrument the beam size will be smallest and will define the damage threshold (maximum fluence), and when the sample is furthest from the instrument the beam size on the mirror will be largest and will define the mirror dimension.

[0027] Preferably, the laser delivery optics will also include an autofocussing arrangement (such as an autofocussing lens), which may include a distance sensor (such as a time-of-flight laser sensor) to measure the distance from the sensor to the sample on the surface, with the output of the sensor fed to the input of the autofocussing arrangement to assist with providing rapid and automatic focussing. The autofocussing arrangement may be positioned between the laser and the laser deflecting mirror.

[0028] With this in mind, the ordinarily rough surface of a borehole wall results in variable distance between the instrument and the wall as the instrument moves along the borehole and when the instrument relocates from one location to another. The abovementioned preferred arrangement of the laser delivery optics, including the adoption of the autofocussing arrangement and the distance sensor, assists in avoiding measurement and detection difficulties caused by the laser pulses not being at the same focus when they reach the borehole surface.

[0029] The autofocussing arrangement may simply be such as to provide a focussed beam with a consistent angle of convergence and therefore the laser energy density at the focal point. In this respect, the autofocussing arrangement may be a liquid lens, a gel lens, a deformable mirror lens, a galvano-mirror lens or a voice-coil activated lens, or the autofocussing arrangement may be simply achieved by physically moving a glass lens or a combination of lenses so as to move the focal point relative to the instrument.

[0030] Additionally, the distance sensor may be a time-of-flight sensor in the form of a single- or multi-zoned sensor with the emitter, receiver and processor on the same integrated circuit chip. The sensor will preferably utilise a laser diode outside the wavelength range of the spectrometers, such that the spectrum is not contaminated by other light sources. Preferably, a time-of-flight sensor will be a compact microprocessor-controlled sensor with a very low power requirement. Its resolution / accuracy is preferably less than the distance over which a plasma would form (namely, the waist region). For example, sensing might be + / - 2 mm, although the plasma will typically form within a 5 mm range.

[0031] The characteristics of the laser power adjustment may also necessitate the use of an adjustable (manual or remote) beam splitter to vary the pulse laser power tothe sample surface. This might be necessary because most lasers are optimised to produce greatest energy efficiency and pulse stability for a limited range of laser pulse repetition rate and energy. With a beam splitter, the laser can be operated at the optimised conditions, while the pulse energy at the sample surface is varied using the beam splitter.

[0032] In another form of the invention, a pulse energy measurement sensor may be adopted in order to measure the energy of the portion of the laser pulse that is reflected from the beam splitter. In doing so, it will be possible to characterise the transmitted pulse, and to determine changes in laser pulse energy, pulse length or optical mode structure.

[0033] Additionally, and given that the primary purpose of a beam splitter would be to reduce the laser energy that interacts with the sample, it may be desirable to include a suitably orientated quarter-waveplate (for example) after the beam splitter. In this form, any circularly polarised reflections from the focussing lens or the optical window can be prevented from propagating back to the laser.

[0034] In one form of the present invention, the plasma emission collection optics includes a deflecting mirror for concentrating radiation emitted at the sample point that passes back to the instrument during operation laterally through the side window and for deflecting that radiation through 90 degrees to direct it generally longitudinally towards a collimator. In this form, the collimator collects the emitted radiation and transforms it into a parallel (collimated) beam for transfer of the beam to the fibre coupling for transport directly to the spectrometers.

[0035] The position of the collimator with respect to the plasma emission deflecting mirror and the fibre coupling may be determined with reference to the position of the laser plasma and may need to be adjustable (if autofocussing is not adopted) to change the focussing requirements as the actual sample point moves way from an optimal focal point.

[0036] The plasma emission deflecting mirror for the emitted radiation will ideally be a flat or shaped mirror with a coating that preferably allows reflection of wavelengths between 180 and 950 nm. Preferably, the plasma emission deflecting mirror will have a parabolic shape that will deflect the plasma emission in a way that concentrates thelight at the optical collimator. In this respect, the parabolic mirror will preferably be an off-axis parabolic mirror, so the plasma emission is not reflected back at the focal point (namely, it is reflected at a 90° angle to the laser path). Additionally, the deflecting lens will have a hole of sufficient diameter to allow the passing of the excitation laser, but different optical layouts may make this unnecessary.

[0037] Additionally, the collimator will preferably be a fixed focus collimator with an anti-reflective coated lens and a coupling for the optical fibre connection. The collimator will preferably have a high transmission efficiency across the full wavelength range of the spectrometers. The collimator could be a single-lens or multi-lens system with either SMA905, FC / PC or FC / APC fibre coupling, but will preferably be a single-lens system with a SMA905 fibre coupling.

[0038] Further, the fibre coupling may be a SMA905, FC / PC or FC / APC system, but will preferably be a SMA905 system.

[0039] In one form of the present invention, the laser delivery optics includes an airjet that has two main functions, namely (a) keeping the laser exit window and the emission entry windows clean from debris due to the plasma and other dust that may be present in the borehole, and (b) flowing air over the sample surface to keep it free of dust and to aid drying if it is wet. Such an air jet system may be supplied with compressed air from an external top-of-borehole system or from an internal compressed air pump, but will preferably be an internal compressed air pump employing a filtration system. Such an air jet system may also be configured to first cool the internal components of the instrument.

[0040] In relation to the deployment of the instrument of the present invention, particularly with regard to the ability of the instrument to provide real-time measurement and analysis and thus potentially to provide continuous measurement and analysis, the harsh and varying conditions in a borehole, and the varying distance from the instrument to the borehole surface, mean that a readily controllable descent and ascent will be preferred. It is thus envisaged that the instrument of the present invention will ideally be interconnected with an above-ground winch that is able to receive control information provided directly or indirectly by a control system, using information from the LIBS spectrometers and the auto-focus system, cameras and additional sensors, ifpresent. Additional control of the lateral position and the axial rotation of the instrument within the borehole may also be provided, ideally by active or passive stabilisers affixed to the instrument.

[0041] The control information will preferably suggest position and speed information for the instrument, with the position and speed control of the instrument to be provided by a winch physically connected to the instrument from above, and thus supporting the instrument from above, by wires and / or cables or the like. In this respect, this physical connection between the winch and the instrument also provides an opportunity to provide the instrument with power and other communications, some of which may be enhanced or replaced by suitable wireless communication hardware and protocols (such as WiFi or 4G / 5G) if usable in the particular drilling environment.

[0042] In general terms, and without wishing to be bound by this technical theory, it will be understood that LIBS spectra are generated by the same processes of optical emission spectroscopy that underly other laboratory analytical methods, such as Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). In common with ICP-OES, LIBS creates plasma from a small sample of solid material and the atomic emission lines present in the light emitted by the plasma can be used to identify the elements present in a sample and their concentration. In this respect, light is emitted when electrons that have been excited to high energy levels by energy (delivered by the laser) fall back towards lower energy levels. The difference in energy between levels manifests as photons, which are collected and measured.

[0043] With this in mind, while the ICP approach creates a continuous stable plasma that can be analysed over timescales of seconds, LIBS plasma is not stable and typically has a life measured in microseconds. This introduces the complexity that the observed plasma will change in temperature and density over the observation period, and there can be substantial shot-to-shot variation in what is observed, even when the same material is analysed. Matrix effects arising from sample properties (such as sample composition, host mineral, mineral density and grain packing density and the like) also create plasma temperature and electron density variations such that the same combination of elements will not present at a common temperature and density. For these reasons, it is useful to understand how the appearance of spectra relates to temperature and density.

[0044] In relation to this complexity, at a moment in time, and after sufficient delay that continuum radiation has decayed, the optical signal consists of emission peaks at wavelengths characteristic of the emitting element for which the intensity of signal at a given wavelength in a LIBS plasma can be related to other parameters through this idealised relationship:

[0045] In this equation, intensity (I) at a given wavelength ( ) is related to element concentration (C). The terms that concentration is multiplied by consist of atomic constants (gk - upper level quad degeneracy), (Aki - Einstein coefficient), (Ek- upper level energy), plasma temperature (T), and the partition function U(T). This equation is solved together with another relationship that computes the equilibrium between different species in the plasma and is also constrained by conservation laws.

[0046] This is a strongly non-linear algebraic function of plasma temperature, which leads to the richness of LIBS spectra that makes LIBS spectra preferable for use in instruments and methods used to assay geological formations in-situ down boreholes during mineral exploration and mining. For example, this gives LIBS spectra a different character to hyperspectral spectra, which are inherently normalized to differences in excitation, and do not exhibit complexity between wavelength variations dependent on excitation. Furthermore, if temperature is fixed in the above relationship, then it can become a simple algebraic relationship. This algebraic function holds for all elements of the Periodic Table, enabling determination of element concentrations in-situ down boreholes. This is in contrast to other analysis techniques which cannot detect all elements (for example, XRF) or cannot measure in-situ (for example, ICP-OS).

[0047] This relationship is useful in the development of data analytics for the instrument of the present invention, being a part of a forward model that relates the observed spectrum to the composition of a material. In relation to the data analytics ideally adopted for use with the instrument of the present invention, the following are some preferred analytical tools: a) Evaluating fundamental LIBS equations given a partition function, and atomic parameters, to create simulated spectra, independently of other LIBSspectral models, to also create both elemental spectra and mineral species spectra for a selection of 204 mineral phases. b) Estimating plasma temperature and electron density through physics-based methods and through a method of fitting model-generated spectra to experimental spectra, and hence identifying relevant plasma parameters, in order to assist with the development of a calibration-free LIBS method. c) Applying these physical models in conjunction with an engineering simulation of the system that considers the real-world behaviour of the optical system, the spectrometers, and the laser. This leads to spectral models that are based on the engineered system, not on physically idealised models. d) Integration of modelling of real-world downhole environment including contaminants and moisture and statistical properties of drillhole mineralogy. e) Physical and engineering models may be integrated with machine learning to provide data-driven modelling. f) The data analytics may incorporate a feedback control system to optimise instrument parameters to the drillhole targets in real-time, based on available knowledge of what is being observed and the quality of spectra. A multi-pass system may be implemented where an initial coarse scan of the drillhole is used to optimise a second scan.

[0048] The purpose of the laser is to ablate and thermally excite the solid sample such that a high temperature plasma is produced that emits light that can identify elements and their concentrations. An example of engineering modelling is that a key parameter of the laser is the energy of each pulse and its consistency. If it is assumed that increasing pulse energy leads to increasing temperature, then the statistics of pulse energy will be useful in the forward modelling process mentioned above.

[0049] In this respect, useful properties have been shown to be that: a) The run-time of the laser tends not to lead to a pattern of increasing or decreasing energy; b) The pulse energy is approximately normally distributed around a mean pulse energy; andc) There are outliers from which spectra results that can be identified and removed due to their distinctive spectra. Outliers are more likely to be low energy pulses, not high energy pulses.

[0050] Knowing that laser energy can be characterised by a normal distribution means that normal statistics can be fed into equations that relate pulse energy to plasma temperature, and then into the LIBS equations to understand shot-to-shot variation in spectra caused by the laser. This assists with boundaries on errors in predictive modelling and the adoption of predictive modelling that is less sensitive to this source of variation. Indeed, laser energy stability tends to rely on the laser operating at a preferred power set-point, meaning that it may be preferred to avoid varying the laser power over a wide range in the context of feedback control.Brief Description of the Drawings

[0051] The present invention will now be described with reference to various figures, graphs and tables. However, the following description is not to limit the generality of the above description.

[0052] In the figures:

[0053] Figures 1 (a) and 1 (b) are schematic illustrations of an instrument in accordance with a preferred embodiment of the present invention;

[0054] Figure 2 is a section view of one selected configuration for the optics head of the instrument of Figure 1 ;

[0055] Figure 3 illustrates spectra intensities of emission lines for Fe- 256 nm, Ca- 393 nm and Na - 589 nm from a transect (dashed line) measured down six test blocks, when the test blocks were dry (black) and wet (grey);

[0056] Figure 4 is a schematic showing: (A) the impact of drillhole surface roughness and focal point; (B) the focal range where laser intensity is high enough to cause breakdown; (C) the experimental setup; (D) the direction of analysis compared to an angled surface; and (E) the experimentally-determined maximum intensity region of approximately 2 mm.Detailed Description of Preferred Embodiment

[0057] Illustrated in Figure 1 (a) is a downhole instrument 10 in accordance with a preferred embodiment of the present invention, the instrument 10 being for the realtime analysis of a borehole wall, and the instrument utilising laser-induced breakdown spectroscopy (LIBS). The instrument 10 includes an elongate housing having a bottom end 14 and a top end 16, the housing including therewithin an optics head 18 at its bottom end 14, in this embodiment two LIBS spectrometers 20 towards its top end 16 and a laser 22 therebetween.

[0058] The optics head 18 includes laser delivery optics and plasma emission collection optics (better illustrated in Figures 1 (b) and 2), with the spectrometers 20 being fibre-coupled to the optics head 18 via fibre coupling 28.

[0059] The laser 22 is configured to transmit pulses directly to the laser delivery optics for focussing before passing through a side window 30 (see Figure 1 (b)) of the optics head 18 to a sample point (not shown) on a borehole wall. The optics head 18 is configured to collect radiation emitted from laser induced plasma at the sample point and transfer the emitted radiation via the plasma emission collection optics to the spectrometers 20 via the fibre coupling 28 (see Figure 1(b)).

[0060] The housing may be a one-piece housing, however in this embodiment the housing is formed from multiple housing parts. The housing also includes therewithin a cooling system in the form of a circulating airflow system (not shown), operable in response to temperature readings taken about both of the laser 22 and the spectrometers 20. The housing includes internal pathways (not shown) that permit the passage of air about its various components, the internal pathways being in fluid communication with a fan (not shown) also situated within the housing.

[0061] The LIBS spectrometers 20 are of sufficient resolution and wavelength range so that the resultant emission spectra produced by the laser 22 is well resolved. The spectrometer configuration in this embodiment is two compact Czerny-Turner spectrometers due to their ease of miniaturisation, however the LIBS spectrometers 20 may consist of just one spectrometer.

[0062] The laser 22 ablates and thermally excites the sample surface such that a high temperature plasma 31 (see Figure 1 (b)) is produced that emits light that can identify elements and their concentrations. The laser 22 is also able to control the energy of each pulse and have consistent pulses. This consistency includes pulse length, wavelength and intensity profile.

[0063] The laser 22 in this embodiment is a single- or double-pulsed laser with a laser excitation wavelength range between 200 and 1600 nm, with pulse lengths in the range of nanoseconds, and specifically is a Q-Switched Nd-YAG laser.

[0064] Referring now to Figure 1(b), the laser delivery optics includes a focussing lens 50 for focussing the longitudinally directed incoming collimated laser pulse 52 (having passed through laser collimator 51 and wedge prisms 51a and 51b) and a laser deflecting mirror 54 for redirecting the focussed pulse laterally from the laser through the side window 30 to the sample point (not shown). The focussing lens 50 is a glass plano-convex lens coated with a suitable anti-reflective coating for the laser excitation wavelength used. The laser deflecting mirror 54 is sized such that the dimension of the mirror is larger than the maximum size of the focussed beam.

[0065] The laser delivery optics also includes an autofocussing lens 53 with a beam expander positioned between the laser 22 and the laser deflecting mirror 54.

[0066] The plasma emission collection optics in this embodiment includes a plasma emission deflecting mirror 60 for concentrating radiation emitted at the sample point that passes back to the instrument 10 during operation laterally through the side window 30, and for deflecting that radiation through 90 degrees to direct it generally longitudinally towards a collimator 62. The collimator 62 collects the emitted radiation and transforms it into a parallel (collimated) beam for transfer of the beam to the fibre coupling 65 for transport directly to the spectrometers 20 of Figure 1(a) via the optical fibre 28.

[0067] The collimator 62 is a fixed focus collimator with an anti-reflective coated lens and a coupling for the optical fibre connection. The collimator 62 has a high percentage transmission across the full wavelength range of the spectrometers 20. The collimator 62 is a single-lens system with a SMA905 fibre coupling.

[0068] The plasma emission deflecting mirror 60 is an off-axis parabolic mirror (with a through hole) that deflects the plasma emission in a way that concentrates the light at the collimator 62 and that allows reflection of wavelengths between 180 and 950nm

[0069] Referring now to Figure 2, the optics head 18 in this particular embodiment includes an optics mount 40 housed within the housing, the optics mount 40 having mounting sections 42,44,46 able to receive and hold both the laser delivery optics and the plasma emission collection optics. In this embodiment the optics mount 40 is a one-piece member, injection moulded or 3D-printed with the mounting sections 42,44,46 pre-formed and ready to receive the appropriate optics.

[0070] In this embodiment, the laser delivery optics again includes a focussing lens 50’ for focussing the longitudinally directed incoming collimated laser pulse 52’ and a laser deflecting mirror 54’ for redirecting the focussed pulse laterally from the laser through the side window 30’ to the sample point (not shown). The focussing lens 50’ is a glass plano-convex lens coated with a suitable anti-reflective coating for the laser excitation wavelength used. The laser deflecting mirror 54’ is again sized such that the dimension of the mirror is larger than the maximum size of the focussed beam.

[0071] The laser delivery optics in this embodiment will also include an autofocussing lens (not shown in Figure 2) positioned between the laser 22 of Figure 1 (a)) and the laser deflecting mirror 54’, in this embodiment including a distance sensor to measure the distance from the sensor to the sample on the surface, with the output of the sensor fed to the input of the autofocussing lens (not shown in Figure 2) to assist with providing rapid and automatic focussing. The autofocussing lens in this embodiment is an electrically tuneable liquid lens that has a rapid response time of greater than 10 Hz.

[0072] The plasma emission collection optics in this embodiment again includes a plasma emission deflecting mirror 60’ for concentrating radiation emitted at the sample point that passes back to the instrument 10 during operation laterally through the side window 30’, and for deflecting that radiation through 90 degrees to direct it generally longitudinally towards a collimator 62’. The collimator 62’ collects the emitted radiation and transforms it into a parallel (collimated) beam for transfer of the beam to the fibre coupling for transport directly to the spectrometers 20 of Figure 1 (a).

[0073] The collimator 62’ is again a fixed focus collimator with an anti-reflective coated lens and a fibre optic coupling 65’ for the optical fibre connection. The collimator 62’ has a high percentage transmission across the full wavelength range of the spectrometers 20. The collimator 62’ is again a single-lens system with a SMA905 fibre coupling.

[0074] The plasma emission deflecting mirror 60’ again has a parabolic shape that deflects the plasma emission in a way that concentrates the light at the collimator 62’ and that allows reflection of wavelengths between 180 and 950nm.

[0075] Initial testing was undertaken with an instrument 10 on a dry surface of a series of six test blocks (halved drill cores) which were stacked vertically and measured on their flat (cut) surface with instrument being lowered at a rate of 50 mm / min (Figure 3). These samples have clearly defined and visually distinct compositional differences between samples, as well as mineralogical variability on a cm scale, perpendicular to the drill direction allowing for validation of the raw data in the context of expected spatial distribution of the geochemistry visually, and without the need for complex data analytics.

[0076] These tests showed that major elements such as iron (Fe), sodium (Na) and calcium (Ca) could be detected and correlated with the expected spatial distribution in each test block, extracting the peak area of the strongest peak for a given element along the analysis transect.

[0077] Figure 3 clearly shows areas of high Ca and lower Fe and Na at the top of the transect, which corresponds to a Ca-rich plaster at the top of the first core, with the highest Fe signal observed when the measured transect intersects Fe-rich sulphide minerals in the second and third core. The raw intensity data presented here has not been processed in any way other than adjusting the x axes (depth) to align the separate wet and dry runs with the image.

[0078] To test the impact of a wet borehole on the LIBS spectra, the test blocks were sprayed with a solution of water and household detergent and immediately measured with the instrument 10. The results show that while the overall intensity of the signal is slightly attenuated for some elements, this is not consistent across all blocks and the expected downhole distribution of Fe, Ca and Na are still resolvable(Figure 3). An air jet of the type generally described above was used, but additional measures to enhance the signal, such as burn-off pulses to evaporate any surface water were not used, but would be expected to improve signal intensity. It should also be noted that the Fe, Ca and Na contents of these samples ranges from <100 ppm to >20 wt% by bulk lab assay.

[0079] In relation to Figure 4, and as mentioned above, a difficulty in any drillhole environment is the rough drillhole wall surface which results in variable distance between the instrument and the sample surface. The impact of the sample position relative to the instrument was investigated and the impact of focal distance was tested using an offcut from a test block, with a varying distance of 10 to 20 mm from the instrument to the sample surface. This experimental arrangement is shown in Figure 4, along with the measured maximum intensity region measuring approximately 2 mm.

[0080] T o test this further, a homogenous sample surface (terracotta clay) was tilted and moved in front of the instrument using a motorized stage. Terracotta was used as it is homogenous at the LIBS scale, the matrix and composition are similar to rock (compared with a metallic target), and the shape of the surface can be modified to simulate a rough surface within a borehole. The initial tests involved a smooth clay tile mounted at an angle such that, as the sample moved the surface moved through the focal range of the instrument. The sample was moved at a constant 50 mm / min with a 4 Hz laser repetition rate. This equated to a spacing of 0.2 mm spacing between analysis from one position to another position, with the sample in focus at the first position but out of focus at the final position.

[0081] As the sample moved out of focus from the first position to the final position, it was found that the plasma intensity decreased, that plasma was observed to form in the air in front of the sample surface, and that two spectrometers (present in this instrument) viewed the plasma (via a Y-shaped optical fibre connected to a single collimator) in a narrow envelope of overlapping circles. Thus, as the sample moved out of focus, the plasma moved out of the envelope viewed by both spectrometers, which was addressed by optimizing the position of the collimator. Alternatively, this could be addressed by including an auto-focus lens and an off-axis parabolic mirror, thus providing a co-aligned laser beam and plasma viewing optics.

[0082] In relation to the likely roughness of the sample surface, terracotta tiles simulating a rough surface were prepared by taking casts of graded gravel glued to a block, enabling duplicates of such topography to be made for analysis under different focus conditions. In this respect, the homogenous composition meant any variation in results would be due to the topography. The usefulness of a variable focus system was also confirmed.

[0083] Finally, it is to be understood that other variations and modifications of the matters described above may also fall within the scope of the present invention.

Claims

The claims defining the invention are as follows1. A downhole instrument for the real-time laser-induced breakdown spectroscopy (LIBS) analysis of a borehole wall, the instrument including an elongate housing having a bottom end and a top end, the housing including therewithin an optics head at its bottom end, a LIBS spectrometer at its top end and a laser therebetween, wherein: a) the optics head includes laser delivery optics and plasma emission collection optics; b) the spectrometer is fibre-coupled to the optics head; c) the laser is configured to transmit pulses directly to the laser delivery optics for focussing before passing through a side window of the optics head to a sample point on the borehole wall; and d) the optics head is configured to collect radiation emitted from laser induced plasma at the sample point and transfer the emitted radiation via the plasma emission collection optics to the at least one spectrometer via the fibre coupling.

2. An instrument according to claim 1 , wherein the elongate housing is a one-piece housing with the bottom and top ends being removable to permit access to the interior of the housing and to permit removal of the optics head, the laser and / or the spectrometer.

3. An instrument according to claim 1 , wherein the elongate housing is formed from multiple housing parts, or with removable access ports along its length.

4. An instrument according to any one of claims 1 to 3, wherein the elongate housing includes therewithin a circulating airflow system, operable in response to temperature readings taken about either or both of the laser and the spectrometers, the housing also including internal pathways that permit the passage of air about either or both of the laser and the spectrometers.

5. An instrument according to claim 4, wherein the circulating airflow system includes an air jet capable of flowing air over the laser exit window and the emission entry window, and also over the sample surface.

6. An instrument according to any one of claims 1 to 5, wherein the LIBS spectrometer is of sufficient resolution and wavelength range so that the emitted radiation is well resolved, the LIBS spectrometer preferably being a LIBS configured Echelle / Czerny-Turner spectrometer.

7. An instrument according to any one of claims 1 to 6, wherein the instrument includes two LIBS spectrometers, being a UV spectrometer and a visible-light spectrometer, preferably both Echelle / Czerny-Turner (optical) spectrometers, registered so as to observe the same sample point.

8. An instrument according to any one of claims 1 to 7, wherein the laser is a single- or double-pulsed laser with a laser excitation wavelength range between 200 and 1600 nm, and with pulse lengths in the range of either nanoseconds or femtoseconds, preferably in the nanosecond range.

9. An instrument according to claim 8, wherein the laser is a Q-Switched Nd-YAG laser.

10. An instrument according to any one of claims 1 to 9, wherein the optics head includes an optics mount able to be housed within either an optics housing within the elongate housing or directly within the elongate housing.

11. An instrument according to claim 10, wherein the optics mount includes mounting sections able to receive and hold both the laser delivery optics and the plasma emission collection optics.

12. An instrument according to any one of claims 1 to 11 , wherein the laser delivery optics includes a focussing lens for focussing longitudinally directed incoming parallel laser pulses and a laser deflecting mirror for redirecting the focussed pulses laterally from the laser through the side window to the sample point.

13. An instrument according to claim 12 wherein the focussing lens is a single- or multiple plano-convex lens arrangement with a damage threshold sufficient to withstand damage from the laser pulses.

14. An instrument according to claim 13, wherein the focussing lens is a glass planoconvex lens coated with an anti-reflective coating suitable for the laser excitation wavelength used.

15. An instrument according to any one of claims 12 to 14, wherein the laser deflecting mirror for the incoming laser pulse is a flat mirror with an anti- reflective coating suitable for the laser excitation wavelength used, with a damage threshold sufficient to withstand damage from the laser pulses, and sized such that the dimension of the mirror is larger than the maximum size of the focussed beam.

16. An instrument according to any one of claims 1 to 15, wherein the laser delivery optics includes an autofocussing lens positioned between the deflecting mirror and the side window, the autofocussing lens including a distance sensor to measure the distance from the sensor to the sample on the surface, with the output of the sensor fed to the input of the autofocussing lens.

17. An instrument according to claim 16, wherein the autofocussing lens is a liquid lens, a gel lens, a deformable mirror lens, a galvano-mirror lens or a voice-coil activated lens, or autofocussing is achieved by moving a glass lens, or a combination of lenses, so as to move the focal point relative to the instrument.

18. An instrument according to claim 16 or claim 17, wherein the distance sensor is a single- or multi-zoned sensor with the emitter, receiver and processor on an integrated circuit chip, wherein the sensor utilises a laser diode outside the wavelength range of the spectrometer, such that the spectrum is not contaminated by other light sources.

19. An instrument according to claim 18, wherein the distance sensor is a compact microprocessor-controlled sensor with a low power requirement.

20. An instrument according to any one of claims 1 to 19, including an adjustable beam splitter to vary the pulse laser power to the sample surface.

21. An instrument according to any one of claims 1 to 20, wherein the plasma emission collection optics includes a plasma emission deflecting mirror for concentrating radiation emitted at the sample point that passes back to theinstrument during operation laterally through the side window, and for deflecting that radiation through 90 degrees to direct it generally longitudinally towards a collimator.

22. An instrument according to claim 21, wherein the collimator collects the emitted radiation and transforms it into a parallel beam for transfer of the beam to the fibre coupling for transport directly to the LIBS spectrometers.

23. An instrument according to claim 21 or claim 22, wherein the plasma emission deflecting mirror for the emitted radiation is a flat or shaped mirror with a coating that allows reflection of wavelengths between 180 and 950nm.

24. An instrument according to claim 23, wherein the plasma emission deflecting mirror has a parabolic shape that deflects the plasma emission to concentrate the light at the collimator.

25. An instrument according to any one of claims 21 to 24, wherein the collimator is a fixed focus collimator with an anti-reflective coated lens and a coupling for the optical fibre connection.

26. An instrument according to claim 25, wherein the collimator has a high percentage transmission across the full wavelength range of the spectrometer.

27. An instrument according to any one of claims 1 to 26, wherein the instrument is interconnected with an above-ground winch that is able to receive control information provided directly or indirectly by a control system, using information from at least the LIBS spectrometer.

28. An instrument according to claim 27, wherein the control information includes position and speed information for the instrument, with the position and speed control of the instrument to be provided by the winch from above the instrument.

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