WO2020208419A1 - Method for the compositional characterization of materials of the wall of a hole in the ground - Google Patents

Method for the compositional characterization of materials of the wall of a hole in the ground Download PDF

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Publication number
WO2020208419A1
WO2020208419A1 PCT/IB2020/000263 IB2020000263W WO2020208419A1 WO 2020208419 A1 WO2020208419 A1 WO 2020208419A1 IB 2020000263 W IB2020000263 W IB 2020000263W WO 2020208419 A1 WO2020208419 A1 WO 2020208419A1
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WO
WIPO (PCT)
Prior art keywords
libs
raman
excitation
probe
ground
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Application number
PCT/IB2020/000263
Other languages
French (fr)
Inventor
Salvatore Siano
Andrea Azelio Mencaglia
Juri AGRESTI
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Consiglio Nazionale Delle Ricerche
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Publication of WO2020208419A1 publication Critical patent/WO2020208419A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/65—Raman scattering
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0202—Mechanical elements; Supports for optical elements
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G01J3/0208—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using focussing or collimating elements, e.g. lenses or mirrors; performing aberration correction
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G01J3/021—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using plane or convex mirrors, parallel phase plates, or particular reflectors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G01J3/0218—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using optical fibers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G01J3/0248—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using a sighting port, e.g. camera or human eye
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0294—Multi-channel spectroscopy
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/44—Raman spectrometry; Scattering spectrometry ; Fluorescence spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/71—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light thermally excited
    • G01N21/718—Laser microanalysis, i.e. with formation of sample plasma
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/85—Investigating moving fluids or granular solids
    • G01N21/8507—Probe photometers, i.e. with optical measuring part dipped into fluid sample

Definitions

  • the present invention relates to a method for the compositional characterization of the materials of the wall of a hole made in the ground in any direction whatsoever, for ex ample downwards or upwards (in the case of working inside a cavity), in particular of a borehole for mining exploration, precision agriculture, environmental control.
  • LIBS Laser Induced Breakdown Spectroscopy
  • the LIBS technique is widely used in various applications since it is highly sensi tive and does not require sample preparation.
  • Object of the present invention is, therefore, to provide a method for the composi tional characterization of the materials of the walls of a hole made in the ground, in partic ular a mining borehole.
  • FIG. 1 is a side view of the probe used in a method according to the invention
  • FIG. 2 is a time chart showing the acquisition times of a LIBS signal and a Raman sig nal
  • FIG. 3 is a block diagram of the steps of a method for the compositional characteriza tion of the material of the wall of a hole in the ground according to the present invention.
  • the probe used in a method according to this invention comprises a series of components which make it possible to overcome the difficulties when using traditional LIBS and Raman instruments for the measurement of profiles and compositional maps of materials in difficult operational contexts.
  • the probe is arranged to be introduced into a hole made in the ground in order to carry out a compositional characterization of the materials of the wall of said hole.
  • the probe comprises, in a way known per se, an external metal pipe which con tains the components described hereinafter, and a quartz optical window which emits a excitation laser beam such as to strike the material of the wall of the hole and which col lects the resulting LIBS and Raman signals generated by the material, in order to carry out the material characterization, as described here below.
  • the probe is based on the induced plasma mass spectroscopy (LIBS), on the wall of a hole subject to investigation, by a first pulsed high intensity laser beam (for example 10 8 -10 12 W/cm 2 for pulses in the order of 1 -100ns), and on Raman spectroscopy carried out by means of excitation of a second laser beam, continuous or pulsed, in a wide inten sity range (10 2 -10 6 W/cm 2 , where the upper limit, as above, refers to laser pulses in the order of 1 -100ns).
  • LIBS induced plasma mass spectroscopy
  • Figure 1 illustrates a side view of a probe 1.
  • the probe 1 comprises two optical fibers 2 and 4 which are coupled with a Raman excitation laser source 2a and a LIBS excitation laser source 4a respectively, an optical assembly 6 with associated roto-translational assembly 8, a spectrometer assembly 10 (comprising LIBS and Raman spectrometers), and known USB electro-optic communica tion devices.
  • the Raman excitation laser 2a is placed physically inside the probe 1
  • the LIBS excitation laser 4a is placed outside the probe 1 , and for this reason illustrated with a broken line.
  • both the excitation lasers 2a, 4a are placed inside the probe 1 or, in a further variation, both the excitation lasers 2a, 4a are placed outside the probe 1 .
  • the optical assembly 6 is arranged so as to direct said laser beams towards the wall of the hole under examination and, at the same time, collect LIBS and Raman signals generated by said wall and send them to the spectrometer assembly 10 by means of re spective optical fibers.
  • the probe 1 is arranged to be mechanically connected to a handling and fluxing unit, and both are electrically and optically connected to a calculation unit for the trans mission, collection and processing of the analytical data, not illustrated in the figure.
  • the probe 1 complete with the excitation laser sources 2a, 4a, the handling and fluxing unit and the calculation unit together constitute an analytical scanning system for the compositional characterization of the material of the wall of a hole made in the ground.
  • the excitation lasers 2a, 4a coupled in the fibers 2, 4 are a first pulsed laser and a second CW type laser, for example a Nd:YAG QS laser at 1064nm and a CW diode laser at 638 nm, 785nm, or other wavelengths, it being known that Raman emission can be easily excited in the entire Vis-NIR region.
  • the optical assembly 6 comprises a rotating parabolic mirror 12, five optical lines 14 each comprising an optical fiber, collimation lenses of the laser excitation beams 19 and focusing lenses in fiber of the signals 16, two filters 18 (one on the Raman excitation line, to block the Raman signal from fiber 2, and one on the acquisition line of the Raman signal from the material of the wall, to block the wavelength from the Raman laser 2a), and an opto-electronic control line.
  • Excitation of the materials of the walls of the hole to be analyzed by means of the laser excitation beams (in other words, the transfer of the excitation beams towards the material), and the collection of the LIBS and Raman optical signals coming from said ma terial, are carried out using the off-axis parabolic mirror 12 (in other words a mirror whose optical axis does not coincide with the mechanical axis), which is arranged for focusing the excitation beams and for collimating the LIBS and Raman signals coming from the material of the wall under examination.
  • the parabolic mirror 12 is able to handle at least two laser beams, preferably for focusing a beam (an optical line) or for focusing and collecting (two optical lines).
  • the parabolic mirror 12 handles six laser beams, also considering the beam coming from a beam sensor 20 described hereinafter.
  • the roto-translational assembly 8 gives the optical assembly 6 three degrees of freedom; it can rotate on itself (as indicated by the arrow R in figure 1 ) and translate along two directions A and B, defined longitudinal and transverse respectively with respect to the axis of the hole.
  • the rotation device 8c is arranged to subject the parabolic mirror 12, and all its components forming integral part, to a rotating movement with variable speed, amplitude and direction, in particular an oscillating movement, of a given amplitude, to a limit of up to ⁇ 180 degrees. Said oscillating movement produces a shift in the focus of the rotation plane of the optical axis.
  • the optical window is cylindrical in shape
  • the parabolic mirror 12 is not connected to the mechanical supports of the other optics and performs a continual rotation movement of 360 degrees.
  • the parabolic mirror 12 has a focusing angle of between 30-120° (preferably 90°) with respect to the collimated beams.
  • the optimal position of the parabolic mirror 12 with respect to the material under examination is determined by feedback transmitted by an exit signal S originating from a laser sensor 20, which monitors the beam emitted (not the one reflected) by the material resulting from the excitation caused by the LIBS laser source 4a and produces said exit signal S representing the radiation emitted by the material.
  • two optical lines 14 are arranged to transmit the excitation beams, and three optical lines 14 are arranged to collect the LIBS and Raman signals.
  • the excitation beams are collimated by the collimation lenses 19, while the LIBS and Raman signals, which are also laser beams, are collimated by the mirror 12 and fo cused by the focusing lenses 16 to be coupled in a fiber.
  • the two excitation channels can, alternatively, be reduced to one (so freeing an optical line 14 which can be used to collect the signal), by coupling the two lasers 2a, 4a into one optical fiber by means of known mixing and filtering techniques.
  • only one excitation laser is present, for example Nd:YAG QS at 1064nm or at 532nm which, similarly, makes it possible to reduce the number of strictly necessary optical lines.
  • emission of the LIBS signal coming from the irradiated wall to be analyzed is continuously monitored by a beam sensor 20, preferably a video-camera (which also makes it possible to observe the area under examination), a photodiode, a thermal sensor, or the said LIBS spectrometers.
  • a beam sensor 20 preferably a video-camera (which also makes it possible to observe the area under examination), a photodiode, a thermal sensor, or the said LIBS spectrometers.
  • the beam sensor 20 produces the exit signal S which determines the above- mentioned feedback on the roto-translation assembly 8, and enables tracking of the best position for measurement taking.
  • the exit signal S is acquired by a calculation unit 20’ where it is processed and sent to the roto-translational assembly 8 in a way known per se, so as to check its opera tion.
  • the method according to this invention integrates in an innovative way two known techniques (LIBS and Raman) into a hybrid system and therefore makes it possible to measure profiles and compositional maps (ele mental/atomic, molecular and crystalline recognition), superficial (in close proximity to the surface) or in depth (depth profile) of the materials to be analyzed. It makes a LIBS- Raman type acquisition (coincident/confocal) at every point and therefore makes it possi ble to obtain a compositional profile while slowly introducing the probe 1 into the hole. Said profile can become a compositional map should the movements activated by the ro to-translational assembly 8 be used either independently or combined with said longitudi nal movement.
  • LIBS and Raman LIBS- Raman type acquisition
  • the probe 1 makes it possible to obtain the“superficial” geochemical and mineralogical characterization of the materials outcropping on the walls of the hole de pending on the depth level, by analyzing the compositional profiles measured.
  • the probe 1 makes it possible to carry out a process to track the maximum signal, in other words tracking the focal position where the best quality signal of the beam col lected by the beam sensor 20 is obtained. Seeking of said position is carried out by gen erating feedback on the roto-translational assembly 8 determined by the exit signal S of the beam sensor 20.
  • the feedback on the roto-translational assembly 8 is determined by suitably integrating the beam collected by the group of spectrometers 10.
  • the signal that is used to activate the roto-translational assembly 8 is no longer the exit signal S but rather the one detected by the LIBS spectrometers.
  • the roto- translational assembly 8 In any case (whichever signal is used to determine the feedback), the roto- translational assembly 8, and in particular its movement along direction B, is driven in such a way that the exit signal S from the beam sensor 20, and therefore the one detected by the LIBS spectrometers, is maintained around its maximum value.
  • the roto-translational assembly 8 determines a reversal of the movement along the direc tion B.
  • the ro to-translational assembly 8 determines a further shift in the same direction B until it goes beyond the position where the maximum signal is obtained, so reaching the case and, consequently, the direction movement will be reversed.
  • the roto-translational assembly 8 seeks to maintain the exit signal from the beam sensor 20 at around its maximum value, passing alternatively from the nearest to the most distant positions but always very close to the optimum value.
  • This optimization method by means of movement along the focal axis B is com bined with other two available movements (according to the translational direction indicat ed with the arrow A and the rotation R in figure 1 ) in the case of operation in scansion mode.
  • the maximum LIBS signal is obtained by positioning the focus of the excita tion laser beam slightly below the surface of the material (a few hundred microns inside) but, alternatively, depending on the physical properties of the irradiated material, the max imum signal occurs with the focus of the excitation laser beam positioned on the surface or slightly outside.
  • the parabolic mirror 12 can also be subjected to an oscillating movement (scansion mode).
  • the rotation movement can also be utilized to accelerate recovery of the maximum LIBS signal condition during acquisition for profile scansion and mapping, wherever the translation B, alone, has limited efficacy due to pronounced irregularity of the wall (marked profile modulation).
  • said rotation movement is controlled by the feedback sig nal of the beam sensor 20.
  • the translational and rotation device 8 is furthermore arranged to subject the par abolic mirror 12 and all its integral optical parts to a translational movement along the di rection A, longitudinal to the probe 1 and orthogonal to the movement along the direction B.
  • This permits a slight movement of the probe 1 which can be utilized in combination with said oscillating movement (carried out by the rotation device 8c), as well as the transla- tional movement, to seek the maximum signal, in order to obtain longitudinal composition al profiles and two-dimensional or three-dimensional compositional maps (the third di mension is given by the cavity produced by the LIBS laser ablation), on areas of a few square centimeters which can be extended if necessary by moving the entire probe 1 .
  • the probe also includes a channel 22 for the passage of gas (air, argon or other), together with a channel 24 for the passage of water. Said channels 22, 24 are connected to the above-described handling and fluxing unit.
  • the channel 22 for the passage of gas is used to increase the analytical signal.
  • an air bubble or of other gas for example argon
  • the presence of air or gas improves signal acquisition.
  • the channel for the passage of water 24 is useful in those cases where it is nec essary to perform cleaning operations of the optical window of the probe.
  • the flow of air and water in the respective channels 22, 24 is controlled by a feed back signal, in particular by the sequence of images collected by the sensor 20 or by the intensity of the signal registered by it.
  • a probe 1 as the one described above is arranged wherein the spectrometers of the spectrometer assembly 10 have complementary spectra parts com prising Raman and LIBS signals, and is introduced into a hole made in the ground, posi tioning the optical window at a point on the wall of the hole subject to analysis.
  • a second step 102 the LIBS and Raman spectra are simultaneously acquired and converted into elemental, molecular and crystalline composition components following interpretation and processing of the acquired spectra in a way known per se.
  • the method according to the invention makes it possible to maximize efficiency and minimize unwant ed interaction between the two spectra acquisitions.
  • the Raman excitation laser 2a is always on, the LIBS excitation laser 4a emits a first pulse which triggers the plasma that starts the acquisition of the consequent LIBS spectrum to which an acquisition time is assigned equal at most to a few hundred micro seconds (typical duration of the LIBS signal is limited to a few tens of microseconds), at the end of which a trigger is issued (starting signal or impulse) which starts the acquisition of the Raman spectrum in cascade, and to which an integration time slightly lower than the interval time that separates the two successive LIBS excitation laser pulses is as signed (100 ms in the example in Fig. 2).
  • step 104 the acquired spectra are processed as described here below.
  • the single LIBS spectrum is acquired and processed, the Raman spectrum is given by the sum of a certain number of acquisitions according to the above- described method, in order to accumulate a Raman signal in characteristic times of a few seconds.
  • the final Raman signal is then processed with methods known in the art.
  • LIBS spectra are acquired at a given point and, thereafter, with LIBS excitation laser in standby, a Raman spectrum with typical acquisi tion times of a few seconds.
  • step 106 characterization of the material is carried out with techniques known in the art.
  • the probe 1 enables various operational modes already known in the art.
  • one-, two- or three-dimensional scansion of the wall of the hole can be carried out in a given area by making use of the rotation movement R and translation along the direction A (oscillation and translation) of the roto-translational assembly 8.
  • the previous measuring steps are then repeated at various depths by moving the probe 1 in ways known per se using its mechanical support.

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  • General Physics & Mathematics (AREA)
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Abstract

Procedure for the compositional characterization of the material of the wall of a hole in the ground including the steps of: - arranging (100) a probe comprising: two excitation laser sources (2a, 4a) arranged to emit respective excitation beams directed to the material to be analyzed, an optical assembly (6) arranged to direct the excitation beams on to the wall of a hole made in the ground and to collect the signals generated by the material excited by the excitation beams, a roto-translational assembly (8) connected to the optical assembly (6) and arranged to rotate and translate said optical assembly (6); and a Raman spectrometer (10) and a LIBS spectrometer (10) for the acquisition of a Raman signal and a LIBS signal respectively, coming from the excited material to be analyzed according to any one of the claims 1 to 7, and introduce it into a hole made in the ground; - acquiring (102) LIBS and Raman spectra of the material wherein the Raman excitation laser source (2a) is always on, the LIBS excitation laser source (4a) emits a first pulse which starts the acquisition of the LIBS spectrum and to which a predetermined integration time is assigned, at the end of which a trigger is issued which starts the acquisition of the Raman spectrum in cascade, and to which an integration time lower than the interval time that separates two successive laser pulses of LIBS excitation is assigned; - processing (104) the spectra acquired in the previous step; - characterizing (106) the material.

Description

Method for the compositional characterization of materials of the wall of a hole in the around
The present invention relates to a method for the compositional characterization of the materials of the wall of a hole made in the ground in any direction whatsoever, for ex ample downwards or upwards (in the case of working inside a cavity), in particular of a borehole for mining exploration, precision agriculture, environmental control.
LIBS (Laser Induced Breakdown Spectroscopy) is a technique used to detect the presence of various elements in a sample, directing the emission of an instantaneous high powered beam coming from a pulsed laser towards the sample so as to stimulate plasma formation. The plasma then undergoes spectral analysis so as to determine the composi tion of the sample.
The LIBS technique is widely used in various applications since it is highly sensi tive and does not require sample preparation.
In situ material characterization by means of molecular and elemental maps and profiles and crystalline recognition using an analytical probe, with associated hardware and software, represents an important problem in many application contexts with no satis factory solution in the known art.
For example, the problem of in situ compositional logging of the walls of a mining borehole, which represents a crucial step for the evaluation of the mining interest of a site, is substantially unresolved today and the same can be said for subsoil evaluation in preci sion agriculture or for chemical assessment in environmental safety.
In said fields, systems able to“simultaneously” provide the elemental and miner- alogical composition of the walls of boreholes are unavailable due to the complex opera tional conditions where said analyses are to be carried out, which are in contrast with the criticality of said techniques (high surface irregularity, presence of water, presence of dirt on the optical window, etc.).
Despite the fact that borehole logging techniques have been hypothesized in prior literature based, independently, on the LIBS technique or on Raman spectroscopy, they have not yet actually been applied to the mining, precision agriculture or environmental sectors due to the above-mentioned unfavorable conditions, while the hypothesis of hybrid instruments has not even been formulated.
Some hybrid Raman-LIBS instruments do in fact exist per se but they cannot be used dynamically in analytical scanning of holes in the ground and, moreover, they involve cumbersome components and setup which are not compatible with the compaction re quired in the above-mentioned borehole logging application.
Therefore, the problem of compositional characterization of the materials present in the walls of a borehole in the ground, of pipelines or other cavities with scarce accessi bility (wells, material walls which can only be reached with a suitably movable compact analytical probe, etc.) exists.
Object of the present invention is, therefore, to provide a method for the composi tional characterization of the materials of the walls of a hole made in the ground, in partic ular a mining borehole.
Said object is reached by means of a method for the compositional characteriza tion of the materials of the wall of said hole made in the ground as described in claim 1.
Preferred embodiments according to the invention form part of the dependent claims, which are to be considered integral part of the present description.
Further features and advantages of the present invention will become more appar ent from the following detailed description of an exemplary but non-limiting embodiment thereof, as illustrated in the accompanying drawings, in which:
- Figure 1 is a side view of the probe used in a method according to the invention,
- Figure 2 is a time chart showing the acquisition times of a LIBS signal and a Raman sig nal, and
- Figure 3 is a block diagram of the steps of a method for the compositional characteriza tion of the material of the wall of a hole in the ground according to the present invention.
The probe used in a method according to this invention comprises a series of components which make it possible to overcome the difficulties when using traditional LIBS and Raman instruments for the measurement of profiles and compositional maps of materials in difficult operational contexts.
The probe is arranged to be introduced into a hole made in the ground in order to carry out a compositional characterization of the materials of the wall of said hole.
The probe comprises, in a way known per se, an external metal pipe which con tains the components described hereinafter, and a quartz optical window which emits a excitation laser beam such as to strike the material of the wall of the hole and which col lects the resulting LIBS and Raman signals generated by the material, in order to carry out the material characterization, as described here below.
The probe is based on the induced plasma mass spectroscopy (LIBS), on the wall of a hole subject to investigation, by a first pulsed high intensity laser beam (for example 108-1012 W/cm2 for pulses in the order of 1 -100ns), and on Raman spectroscopy carried out by means of excitation of a second laser beam, continuous or pulsed, in a wide inten sity range (102-106 W/cm2, where the upper limit, as above, refers to laser pulses in the order of 1 -100ns).
Figure 1 illustrates a side view of a probe 1.
The probe 1 comprises two optical fibers 2 and 4 which are coupled with a Raman excitation laser source 2a and a LIBS excitation laser source 4a respectively, an optical assembly 6 with associated roto-translational assembly 8, a spectrometer assembly 10 (comprising LIBS and Raman spectrometers), and known USB electro-optic communica tion devices. In Fig. 1 , the Raman excitation laser 2a is placed physically inside the probe 1 , the LIBS excitation laser 4a is placed outside the probe 1 , and for this reason illustrated with a broken line.
In one variation, both the excitation lasers 2a, 4a are placed inside the probe 1 or, in a further variation, both the excitation lasers 2a, 4a are placed outside the probe 1 .
The optical assembly 6 is arranged so as to direct said laser beams towards the wall of the hole under examination and, at the same time, collect LIBS and Raman signals generated by said wall and send them to the spectrometer assembly 10 by means of re spective optical fibers.
The probe 1 is arranged to be mechanically connected to a handling and fluxing unit, and both are electrically and optically connected to a calculation unit for the trans mission, collection and processing of the analytical data, not illustrated in the figure.
The probe 1 , complete with the excitation laser sources 2a, 4a, the handling and fluxing unit and the calculation unit together constitute an analytical scanning system for the compositional characterization of the material of the wall of a hole made in the ground.
Advantageously, the excitation lasers 2a, 4a coupled in the fibers 2, 4 are a first pulsed laser and a second CW type laser, for example a Nd:YAG QS laser at 1064nm and a CW diode laser at 638 nm, 785nm, or other wavelengths, it being known that Raman emission can be easily excited in the entire Vis-NIR region.
The optical assembly 6 comprises a rotating parabolic mirror 12, five optical lines 14 each comprising an optical fiber, collimation lenses of the laser excitation beams 19 and focusing lenses in fiber of the signals 16, two filters 18 (one on the Raman excitation line, to block the Raman signal from fiber 2, and one on the acquisition line of the Raman signal from the material of the wall, to block the wavelength from the Raman laser 2a), and an opto-electronic control line.
Excitation of the materials of the walls of the hole to be analyzed by means of the laser excitation beams (in other words, the transfer of the excitation beams towards the material), and the collection of the LIBS and Raman optical signals coming from said ma terial, are carried out using the off-axis parabolic mirror 12 (in other words a mirror whose optical axis does not coincide with the mechanical axis), which is arranged for focusing the excitation beams and for collimating the LIBS and Raman signals coming from the material of the wall under examination.
The parabolic mirror 12 is able to handle at least two laser beams, preferably for focusing a beam (an optical line) or for focusing and collecting (two optical lines).
In particular, in the present invention, the parabolic mirror 12 handles six laser beams, also considering the beam coming from a beam sensor 20 described hereinafter.
The roto-translational assembly 8 gives the optical assembly 6 three degrees of freedom; it can rotate on itself (as indicated by the arrow R in figure 1 ) and translate along two directions A and B, defined longitudinal and transverse respectively with respect to the axis of the hole.
A translation device 8a known per se connected to the optical assembly 6 per forms a transverse translation, a translation device 8b performs a longitudinal translation, while a rotation device 8c rotates the parabolic mirror 12 and the other optical components forming integral part.
In particular, the rotation device 8c is arranged to subject the parabolic mirror 12, and all its components forming integral part, to a rotating movement with variable speed, amplitude and direction, in particular an oscillating movement, of a given amplitude, to a limit of up to ±180 degrees. Said oscillating movement produces a shift in the focus of the rotation plane of the optical axis.
In a variation of the probe 1 the optical window is cylindrical in shape, the parabolic mirror 12 is not connected to the mechanical supports of the other optics and performs a continual rotation movement of 360 degrees.
The parabolic mirror 12 has a focusing angle of between 30-120° (preferably 90°) with respect to the collimated beams.
The optimal position of the parabolic mirror 12 with respect to the material under examination is determined by feedback transmitted by an exit signal S originating from a laser sensor 20, which monitors the beam emitted (not the one reflected) by the material resulting from the excitation caused by the LIBS laser source 4a and produces said exit signal S representing the radiation emitted by the material.
In the probe 1 , two optical lines 14 are arranged to transmit the excitation beams, and three optical lines 14 are arranged to collect the LIBS and Raman signals.
The excitation beams are collimated by the collimation lenses 19, while the LIBS and Raman signals, which are also laser beams, are collimated by the mirror 12 and fo cused by the focusing lenses 16 to be coupled in a fiber.
The two excitation channels can, alternatively, be reduced to one (so freeing an optical line 14 which can be used to collect the signal), by coupling the two lasers 2a, 4a into one optical fiber by means of known mixing and filtering techniques.
Alternatively, only one excitation laser is present, for example Nd:YAG QS at 1064nm or at 532nm which, similarly, makes it possible to reduce the number of strictly necessary optical lines.
As said above, emission of the LIBS signal coming from the irradiated wall to be analyzed is continuously monitored by a beam sensor 20, preferably a video-camera (which also makes it possible to observe the area under examination), a photodiode, a thermal sensor, or the said LIBS spectrometers.
The beam sensor 20 produces the exit signal S which determines the above- mentioned feedback on the roto-translation assembly 8, and enables tracking of the best position for measurement taking.
The exit signal S is acquired by a calculation unit 20’ where it is processed and sent to the roto-translational assembly 8 in a way known per se, so as to check its opera tion.
The method according to this invention, described in detail hereinafter, integrates in an innovative way two known techniques (LIBS and Raman) into a hybrid system and therefore makes it possible to measure profiles and compositional maps (ele mental/atomic, molecular and crystalline recognition), superficial (in close proximity to the surface) or in depth (depth profile) of the materials to be analyzed. It makes a LIBS- Raman type acquisition (coincident/confocal) at every point and therefore makes it possi ble to obtain a compositional profile while slowly introducing the probe 1 into the hole. Said profile can become a compositional map should the movements activated by the ro to-translational assembly 8 be used either independently or combined with said longitudi nal movement.
In the above-mentioned case of borehole logging in the mining field, the probe 1 according to this invention makes it possible to obtain the“superficial” geochemical and mineralogical characterization of the materials outcropping on the walls of the hole de pending on the depth level, by analyzing the compositional profiles measured.
Even if the main beneficiary of this invention is considered the mining field, it can easily be adapted to a wide variety of material characterization problems, for example in matters of the environment, precision agriculture, food safety, archeometry, process con- trol, etc., wherein the probe according to this invention can guarantee easy measurement reproduction and the possibility of carrying out compositional maps and profiles.
The probe 1 makes it possible to carry out a process to track the maximum signal, in other words tracking the focal position where the best quality signal of the beam col lected by the beam sensor 20 is obtained. Seeking of said position is carried out by gen erating feedback on the roto-translational assembly 8 determined by the exit signal S of the beam sensor 20.
Alternatively, the feedback on the roto-translational assembly 8 is determined by suitably integrating the beam collected by the group of spectrometers 10.
In this case, the signal that is used to activate the roto-translational assembly 8 is no longer the exit signal S but rather the one detected by the LIBS spectrometers.
In any case (whichever signal is used to determine the feedback), the roto- translational assembly 8, and in particular its movement along direction B, is driven in such a way that the exit signal S from the beam sensor 20, and therefore the one detected by the LIBS spectrometers, is maintained around its maximum value.
If a small shift in positioning produces a decrease in said exit signal
Figure imgf000009_0001
case), then the roto-translational assembly 8 determines a reversal of the movement along the direc tion B. Vice versa, if a small shift produces an increase in the exit signal (“+” case), the ro to-translational assembly 8 determines a further shift in the same direction B until it goes beyond the position where the maximum signal is obtained, so reaching the case and, consequently, the direction movement will be reversed.
With suitable and preventive adjustment of the parameters which determine the feedback amplitude, the roto-translational assembly 8 seeks to maintain the exit signal from the beam sensor 20 at around its maximum value, passing alternatively from the nearest to the most distant positions but always very close to the optimum value.
This optimization method by means of movement along the focal axis B is com bined with other two available movements (according to the translational direction indicat ed with the arrow A and the rotation R in figure 1 ) in the case of operation in scansion mode. By maximizing the LIBS signal collected by the parabolic mirror 12, to which max imum values of the Raman signal also correspond, it is possible to obtain high efficacy.
In fact, a shift in the LIBS excitation laser focus in the order of about a hundred mi crons leads to enormous variations in the corresponding detected LIBS signal when using, as for this probe, a short focus.
Vice versa, for the Raman spectroscopy, said shift, if contained within ±500 pm with respect to the surface, produces small variations in the acquired signal (within 90% of the maximum).
Often, the maximum LIBS signal is obtained by positioning the focus of the excita tion laser beam slightly below the surface of the material (a few hundred microns inside) but, alternatively, depending on the physical properties of the irradiated material, the max imum signal occurs with the focus of the excitation laser beam positioned on the surface or slightly outside.
As said above, the parabolic mirror 12 can also be subjected to an oscillating movement (scansion mode).
In the face of uneven earthy, stony and mineral materials, if intending to measure their average local as well as point composition, said movement makes it possible to ob tain averages by acquiring many measurements along a line and so increasing the repre sentativeness of the LIBS-Raman measurements.
The rotation movement can also be utilized to accelerate recovery of the maximum LIBS signal condition during acquisition for profile scansion and mapping, wherever the translation B, alone, has limited efficacy due to pronounced irregularity of the wall (marked profile modulation). In this case, said rotation movement is controlled by the feedback sig nal of the beam sensor 20.
The translational and rotation device 8 is furthermore arranged to subject the par abolic mirror 12 and all its integral optical parts to a translational movement along the di rection A, longitudinal to the probe 1 and orthogonal to the movement along the direction B. This permits a slight movement of the probe 1 which can be utilized in combination with said oscillating movement (carried out by the rotation device 8c), as well as the transla- tional movement, to seek the maximum signal, in order to obtain longitudinal composition al profiles and two-dimensional or three-dimensional compositional maps (the third di mension is given by the cavity produced by the LIBS laser ablation), on areas of a few square centimeters which can be extended if necessary by moving the entire probe 1 .
The probe also includes a channel 22 for the passage of gas (air, argon or other), together with a channel 24 for the passage of water. Said channels 22, 24 are connected to the above-described handling and fluxing unit.
The channel 22 for the passage of gas is used to increase the analytical signal. For example, for mining boreholes in water it is necessary to create an air bubble or of other gas (for example argon) in proximity of the area to be analyzed, which can guaran tee sufficient emissivity and detection of the plasma induced by the laser beam on the sur face of the material, and to remove the interference of water on the Raman spectra. In a general case, the presence of air or gas improves signal acquisition.
The channel for the passage of water 24 is useful in those cases where it is nec essary to perform cleaning operations of the optical window of the probe.
The flow of air and water in the respective channels 22, 24 is controlled by a feed back signal, in particular by the sequence of images collected by the sensor 20 or by the intensity of the signal registered by it.
Hereinafter a method for the compositional characterization of the material of a wall of a hole made in the ground will be described with reference to figure 3, preferably a mining borehole, according to the present invention.
In a first step 100, a probe 1 as the one described above is arranged wherein the spectrometers of the spectrometer assembly 10 have complementary spectra parts com prising Raman and LIBS signals, and is introduced into a hole made in the ground, posi tioning the optical window at a point on the wall of the hole subject to analysis.
In a second step 102 the LIBS and Raman spectra are simultaneously acquired and converted into elemental, molecular and crystalline composition components following interpretation and processing of the acquired spectra in a way known per se.
Having positioned the probe and started acquisition of the exit signal of the beam sensor 20, as far as timing of the LIBS and Raman acquisition is concerned, the method according to the invention makes it possible to maximize efficiency and minimize unwant ed interaction between the two spectra acquisitions.
Acquisition of the LIBS and Raman spectra takes place as described here below with reference to figure 2, which illustrates a time acquisition diagram of a LIBS signal and a Raman signal.
The Raman excitation laser 2a is always on, the LIBS excitation laser 4a emits a first pulse which triggers the plasma that starts the acquisition of the consequent LIBS spectrum to which an acquisition time is assigned equal at most to a few hundred micro seconds (typical duration of the LIBS signal is limited to a few tens of microseconds), at the end of which a trigger is issued (starting signal or impulse) which starts the acquisition of the Raman spectrum in cascade, and to which an integration time slightly lower than the interval time that separates the two successive LIBS excitation laser pulses is as signed (100 ms in the example in Fig. 2).
In step 104, the acquired spectra are processed as described here below.
In particular, the single LIBS spectrum is acquired and processed, the Raman spectrum is given by the sum of a certain number of acquisitions according to the above- described method, in order to accumulate a Raman signal in characteristic times of a few seconds. The final Raman signal is then processed with methods known in the art.
Alternatively, a certain number of LIBS spectra are acquired at a given point and, thereafter, with LIBS excitation laser in standby, a Raman spectrum with typical acquisi tion times of a few seconds.
Finally, in step 106, characterization of the material is carried out with techniques known in the art.
In particular, considering the compositional mapping of the wall of the hole under examination, the probe 1 enables various operational modes already known in the art. By utilizing one of the defined time methods, one-, two- or three-dimensional scansion of the wall of the hole can be carried out in a given area by making use of the rotation movement R and translation along the direction A (oscillation and translation) of the roto-translational assembly 8.
Finally the measurements are defined in terms of average composition, profile and compositional maps.
The previous measuring steps are then repeated at various depths by moving the probe 1 in ways known per se using its mechanical support.
Naturally, without prejudice to the principles of the present invention, all the em bodiments and implementation details can be widely varied with respect to that which has been described and illustrated for purely exemplary and non-limiting purposes without de parting from the scope of protection of the present invention as defined in the appended claims.

Claims

1. Method for the compositional characterization of the materials of a wall of a hole made in the ground comprising the steps of:
- arranging (100) a probe comprising:
two excitation laser sources (2a, 4a) arranged to emit respective excitation beams directed to the material to be analyzed,
an optical assembly (6) arranged to direct the excitation beams on the wall of a hole made in the ground and to collect the signals generated by the material excited by the excitation beams,
a roto-translational assembly (8) connected to the optical assembly (6) and ar ranged to rotate and translate said optical assembly (6); and
a Raman spectrometer (10) and a LIBS spectrometer (10) for the acquisition of a Raman signal and a LIBS signal respectively, coming from the excited material to be ana lyzed, and introduce it into a hole made in the ground;
- acquiring (102) LIBS and Raman spectra of the material wherein the Raman excitation laser source (2a) is always on, the LIBS excitation laser source (4a) emits a first pulse which starts the acquisition of the LIBS spectrum and to which a predetermined integra tion time is assigned, at the end of which a trigger is issued which starts the acquisition of the Raman spectrum in cascade, and to which an integration time lower than the interval time that separates two successive laser pulses of LIBS excitation is assigned;
- processing (104) the spectra acquired in the previous step;
- characterizing (106) the material.
2. Method according to claim 1 , comprising the operation of transferring, by means of a rotating parabolic mirror (12) of the optical assembly (6), the excitation beams towards the material to be analyzed.
3. Method according to claims 1 or 2, comprising giving the optical assembly (6), by means of the roto-translational assembly (8), three degrees of freedom, rotating said opti cal assembly (6) on itself (R) and translating along two directions (A, B) longitudinal and transverse with respect to the axis of the hole.
4. Method according to claim 1 , comprising the arrangement, in the optical assembly (6), of five optical lines (14) each comprising an optical fiber, collimation and focusing lenses (16) and two filters (18).
5. System for the compositional characterization of the material of the wall of a hole in the ground comprising a probe according to any one of the preceding claims, a handling and fluxing unit connected to the probe and arranged to supply water and gas to the probe, and a calculation unit for the collection and processing of the data acquired by the probe arranged to perform compositional characterization of the material.
PCT/IB2020/000263 2019-04-08 2020-04-07 Method for the compositional characterization of materials of the wall of a hole in the ground WO2020208419A1 (en)

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