IT201900005388A1 - Probe, system and procedure for the compositional characterization of the wall materials of a hole made in the ground - Google Patents

Description

DESCRIPTION of the industrial invention entitled: "Probe, system and procedure for the compositional characterization of the materials of the wall of a hole made in the ground"

The present invention relates to a probe, a system and a procedure for the compositional characterization of the materials of the wall of a hole made in a ground according to any direction, for example downwards or upwards (if one operates inside a a cavity), in particular a borehole for mining exploration, precision agriculture, environmental control.

Laser Induced Breakdown Spectroscopy (LIBS) is a technique for detecting the presence of various elements in a sample by directing the emission of an instantaneous high-power beam from a pulsed laser towards the sample in a to stimulate the formation of a plasma. The plasma is then spectroscopically analyzed to determine the composition of the sample.

The LIBS technique is widely used in various application fields as it is very sensitive and does not require sample preparation.

The characterization of materials in situ through elemental, molecular and crystalline recognition profiles and maps using an analytical probe, and an associated hardware and software, represents an important problem in many application contexts, and does not find a satisfactory solution in the prior art.

For example, the problem of compositional borehole logging, which represents a crucial phase for the evaluation of the mining interest of a site, is currently substantially unsolved. say for subsoil investigations useful in precision agriculture or for chemical tests of environmental safety.

In these areas, no systems have been proposed capable of providing "simultaneously" the elemental and mineralogical composition of the wall of these boreholes, due to the complex operating conditions in which such analyzes should be performed, which contrast with the criticality of these techniques (strong surface irregularities, presence of water, presence of dirt on the optical window, etc.).

Although borehole logging techniques based, independently, on the LIBS technique or on Raman spectroscopy have been hypothesized in the literature, they have not yet been concretely applied in the mining, precision agriculture or environmental sectors due to the aforementioned unfavorable conditions, while the hypothesis of hybrid instruments has not even been formulated.

There are in themselves some hybrid Raman-LIBS instruments, which however are not dynamically used in analytical scans of holes in the ground, and also exploit bulky setups and components that are ill-suited to the compaction required in the aforementioned application of borehole logging.

There is therefore the problem of the compositional characterization of the materials present in the wall of a borehole in the ground, of pipes or other cavities with poor accessibility (wells, material walls that can only be reached with a suitably moved compact analytical probe, etc.).

The object of the present invention is therefore to propose a probe, a system and a procedure for the compositional characterization of the materials of the wall of a hole made in the ground, in particular of a mining borehole.

This object is achieved with a probe whose characteristics are defined in claim 1, with a system which integrates said probe for the compositional characterization of the material of the wall of a hole made in the ground as defined in claim 8, and with a procedure for the characterization compositional of the wall materials of said hole made in the ground as defined in claim 9.

Particular embodiments form the subject of the dependent claims, the content of which is to be understood as an integral part of the present description.

Further characteristics and advantages of the invention will appear from the following detailed description, carried out purely by way of non-limiting example, with reference to the attached drawings, in which:

Figure 1 is a side view of a probe according to the present invention,

Figure 2 is a timing diagram of acquisition of a LIBS signal and a Raman signal; And

- Figure 3 is a block diagram of the steps of a procedure for the compositional characterization of the wall material of a hole in the ground.

The probe of the present invention includes a series of components which allow to overcome the difficulties of using traditional LIBS and Raman instruments for the measurement of profiles and compositional maps of materials in difficult operating contexts.

The probe of the present invention is suitable for insertion inside a hole made in the ground to perform a compositional characterization of the materials of the wall of said hole.

The probe of the present invention comprises, in a per se known manner, an external metal tube that encloses the components described below, and a quartz optical window from which an excitation laser radiation capable of striking the material of the wall of the hole comes out and from which the consequent LIBS and Raman signals coming from the material enter, to perform the material characterization, as described below.

The probe of the present invention is based on plasma spectroscopy induced (LIBS), on the wall of a hole under investigation, by a first pulsed high intensity laser beam (for example 10 <8> -10 <12> W / cm <2> for pulses of the order of 1-100ns), and on Raman spectroscopy that can be performed with excitation of a second laser beam, in continuous or pulsed, in a wide range of intensity (10 <2> -10 <6> W / cm <2>, where the upper limit refers, as above, to laser pulses of the order of 1-100ns).

Figure 1 shows a side view of a probe 1 according to the present invention.

The probe 1 comprises two optical fibers 2 and 4 which couple, respectively, a Raman excitation laser source 2a and a LIBS excitation laser source 4a, an optical group 6 with associated roto-translational group 8, a group of spectrometers 10 (comprising LIBS and Raman spectrometers), and known USB electro-optical communication devices. In the setup of Fig. 1, the Raman excitation laser 2a is physically placed inside the probe 1, the LIBS excitation laser 4a is placed outside the probe 1, and therefore shown with a dashed line.

In a variant of the invention, both excitation lasers 2a, 4a are placed inside the probe 1 or, in a further variant, both excitation lasers 2a, 4a are placed outside the probe 1.

The optical group 6 is able to direct said laser beams towards the wall of a hole under examination and, at the same time, to collect LIBS and Raman signals from the latter and send them to the group of spectrometers 10 through respective optical fibers.

Probe 1 is designed to be mechanically connected to a handling and flushing unit, and both are electrically and optically connected to a computing unit for driving, collecting and processing analytical data, not shown in the figure.

The probe 1, complete with excitation laser sources 2a, 4a, movement and flushing unit and calculation unit, constitute as a whole an analytical scanning system for the compositional characterization of the wall material 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 1064nm Nd: YAG QS laser and a 638nm, 785nm CW diode laser, or other wavelengths, it being known that the Raman emission can be easily excited in the whole Vis-NIR region.

The optical group 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 fiber focusing lenses of the signals 16, two filters 18 (one on the Raman excitation line , to block the Raman signal of the fiber 2, and one on the Raman signal collection line of the wall materials, to block the wavelength of the Raman laser 2a), and an optoelectronic control line.

The excitation of the materials of the hole wall to be analyzed by means of the excitation laser beams (that is, the transfer of the excitation beams towards the material), and the collection of the LIBS and Raman optical signals coming from said materials, are carried out using the mirror off-axis parabolic 12 (that is, a mirror whose optical axis does not coincide with the mechanical axis), which serves for focusing the excitation beams and for collimating the LIBS and Raman signals coming from the wall material under examination.

The parabolic mirror 12 is able to manage at least two radiation beams, preferably to focus a beam (one optical line) or to focus and collect (two optical lines).

In particular, in the present invention, the parabolic mirror 12 manages six radiation beams, also considering the radiation coming from a radiation sensor 20 described below.

The roto-translational group 8 gives the optical group 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, respectively, longitudinal and transversal with respect to an axis of the hole.

A per se known translation device 8a connected to the optical unit 6 carries out the transverse translation, a translation device 8b the longitudinal one, while a rotation device 8c rotates the parabolic mirror 12, and of the other optical components integral with it.

In particular, the rotation device 8c is adapted to subject the parabolic mirror 12 and all the components integral with it to a rotation movement with variable speed, amplitude and direction, in particular an oscillatory motion, of a given amplitude, up to the limit up to to ± 180 degrees. This oscillatory motion produces a displacement of the focus in the plane of rotation of the optical axis.

In a variant of the probe 1 the optical window is cylindrical in shape, the parabolic mirror 12 is released from the mechanical supports of the other optics and performs a continuous rotation motion of 360 degrees.

The parabolic mirror 12 has a focusing angle with respect to the collimated beams of between 30-120 ° (preferably 90 °).

The optimal position of the parabolic mirror 12 with respect to the material under examination is determined by a feedback driven by an output signal S originating from a radiation sensor 20, which monitors the radiation emitted (not the reflected one) by the material as a consequence of the excitation by the LIBS laser source 4a and produces said output signal S representative of the radiation emitted by the material.

In the probe 1 of the present invention, two optical lines 14 are used to send the excitation beams, and three optical lines 14 are used for collecting the LIBS and Raman signals.

The excitation beams are collimated by the collimating lenses 19 and the LIBS and Raman signals, which are also radiation beams, are collimated by the mirror 12 and focused by the focusing lenses 16 to be coupled in fiber.

The two excitation channels can, alternatively, be reduced to one (freeing an optical line 14 which can be used for signal collection), by coupling the two lasers 2a, 4a in the same optical fiber by means of mixing and filtering techniques per se. Note.

Alternatively, there is only one excitation laser, for example Nd: YAG QS at 1064nm or at 532nm which, similarly, allows to reduce the strictly necessary optical lines.

As mentioned above, the emission of the LIBS signal coming from the irradiated wall to be analyzed is monitored continuously through the radiation sensor 20, which is preferably a video camera (which also allows the area under examination to be observed), a photodiode, a sensor thermal, or the LIBS spectrometers themselves.

The radiation sensor 20 produces the output signal S which determines the aforementioned feedback on the roto-translation group 8, and which allows the tracking of the optimal position for carrying out the measurement.

The output signal S is acquired by a calculation unit 20 'which processes it and sends it to the roto-translational group 8 in a per se known way, in order to check its operation.

The probe 1 of the present invention integrates in an original way two known techniques (LIBS and Raman) in a hybrid system, and therefore allows the measurement of profiles and compositional maps (elemental / atomic, molecular and crystalline recognition), superficial (near the surface) or depth profile of the materials to be analyzed. At each point it performs a LIBS-Raman type acquisition (coincident / confocal) and therefore allows to obtain a compositional profile by advancing the probe 1 in the hole. This profile can become a compositional map if the movements operated by the roto-translational unit 8 are used independently or combined with said longitudinal displacement.

In the aforementioned case of borehole logging in the mining sector, the probe 1 of the present invention allows to obtain the "superficial" geochemical and mineralogical characterization of the materials emerging on the wall of the hole as a function of the depth level, through the analysis of the measured compositional profiles .

Although the mining sector is the main recipient of the probe of the present invention, it is easily adaptable to a wide variety of material characterization problems, for example in environmental contexts, precision agriculture, food safety, archaeometry, control of processes, etc .., within which the probe of the present invention can guarantee easy reproducibility of the measurement and the possibility of executing compositional profiles and maps.

The probe 1 allows to carry out a process of tracking the maximum signal, that is, a tracking of the focal position in which the best quality of the radiation signal acquired by the radiation sensor 20 is obtained. The search for this position is performed by generating a feedback on the group of roto-translation 8 determined by the output signal S of the radiation sensor 20.

Alternatively, the feedback on the roto-translation group 8 is determined by suitably integrating the radiation acquired by the spectrometer group 10.

In this case, the signal used to drive the roto-translational group 8 is no longer the output signal S but the one detected by the LIBS spectrometers.

In any case (whatever the signal used to determine the feedback), the roto-translation unit 8, and in particular the movement along the direction B, is driven in such a way that the output signal S of the radiation sensor 20, and therefore that revealed by the LIBS spectrometers, is maintained around the maximum value.

If a small displacement in the positioning produces a decrease (case "-") of this output signal, then the roto-translation group 8 determines an inversion of the direction of motion along the direction B. Conversely, if a small displacement produces an increase of the output signal (case "+"), the roto-tralsational group 8 determines a further displacement in the same direction B until the position for which the maximum signal is obtained is exceeded, so that one finds oneself in the case "-" and consequently the sense of motion will be reversed.

With an appropriate and preventive adjustment of the parameters that determine the amplitude of the feedback, the roto-translation unit 8 attempts to keep the output signal from the radiation sensor 20 around its maximum value, passing alternately from positions closer to positions more distant but always very close to the optimal one.

This optimization mode by moving along the focal axis B is combined with the other two available movements (according to the direction of translation indicated by the arrow A and the rotation R in figure 1) when operating in scanning mode.

Great efficacy is obtained by maximizing the LIBS signal collected by the parabolic mirror 12, to which also maximum values of the Raman signal correspond.

In fact, a shift of the focus of the LIBS excitation laser of the order of a hundred microns involves enormous variations in the corresponding LIBS signal detected when, as for the present probe, short focal lengths are used.

Conversely, for Raman spectroscopy, such a displacement, if contained within ± 500 µm 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 excitation 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 maximum signal it manifests itself with the focus of the excitation laser beam positioned on the surface or slightly outside.

As stated above, the parabolic mirror 12 can also be subjected to an oscillatory motion (scanning mode).

In the face of inhomogeneous earthy, stone and mineral materials, in the case of measuring their local average composition as well as punctual, this movement allows to perform averages by acquiring many measurements along a line and thus increasing the representativeness of the LIBS-Raman measurements.

The rotation motion can also be exploited to accelerate the restoration of the condition of maximum LIBS signal during the acquisition by scanning of profiles and maps, where translation B, alone, was not very effective due to pronounced irregularity (marked profile modulation ) of the wall. In this case, said rotational motion is controlled through the feedback signal of the radiation sensor 20.

The translation and rotation device 8 is also adapted to subject the parabolic mirror 12 and all the optics integral with it to a translation movement along the direction A, longitudinal to the probe 1 and orthogonal to the movement along the direction B. This allows a movement end of probe 1, which can be exploited in combination with the aforementioned oscillatory motion (performed by the rotation device 8c), as well as with translation, to search for the maximum signal, to obtain longitudinal compositional profiles and two-dimensional or three-dimensional compositional maps ( the third dimension is given by the excavation produced by the laser ablation involved in the LIBS), on areas of a few square centimeters, possibly extendable by moving the entire probe 1.

The probe of the present invention also includes a channel 22 for the passage of gas (air, argon or other), together with a channel 24 for the passage of water. These channels 22, 24 are connected to the handling and flushing unit described above.

The gas passage channel 22 serves to increase the analytical signal. For example, for mining boreholes in water it is necessary to create an air or other gas bubble (such as argon) near the area to be analyzed, which guarantees sufficient emissivity and detectability of the plasma induced by the laser pulse on the surface of the material, and to remove water interference on the Raman spectrum. In a general case, the presence of air or gas improves signal acquisition.

The channel for the passage of water 24 is useful, on the other hand, in cases where cleaning operations of the optical window of the probe are necessary.

The flows of air and water in the respective channels 22, 24 are controlled by a feedback signal, in particular by the sequence of images collected by the sensor 20 or by the intensity of the signal it records.

With reference to Figure 3, a method will now be described for the compositional characterization of the wall material of a hole made in the ground, preferably of a mining borehole, according to the present invention.

In a first step 100 a probe 1 like the one described above is prepared in which the spectrometers of the spectrometer group 10 detect complementary parts of the spectrum comprising the LIBS and Raman signals, and it is introduced into a hole made in the ground by positioning the optical window in correspondence to a point on the wall of the hole of analytical interest.

We then proceed to step 102 with the acquisition of LIBS and Raman spectra which are translated into elemental, molecular and crystalline composition components after interpretation and processing of the acquired spectra in a per se known manner.

Once the probe has been positioned and the acquisition of the output signal of the radiation sensor 20 has begun, as regards the timing of the LIBS and Raman acquisitions, the procedure according to the present invention allows to maximize efficiency and minimize unwanted interactions between the two acquisitions. ghostly.

The acquisition of the LIBS and Raman spectra takes place as described below with reference to Figure 2, which shows a timing diagram for the acquisition of a LIBS signal and a Raman signal.

The Raman 2a excitation laser is always on, the LIBS 4a excitation laser emits a first pulse that triggers the plasma and triggers the acquisition of the consequent LIBS spectrum which is assigned an acquisition time of a maximum of a few hundred microseconds (the typical duration of the LIBS signal is limited to a few tens of microseconds), at the end of which a trigger is emitted (start signal or pulse) which cascades the acquisition of the Raman spectrum, which is assigned a time of integration slightly less than the time interval that separates two successive LIBS excitation laser pulses (100 ms in the example of Fig. 2).

At step 104 the acquired spectra are processed as indicated 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 procedure just described, in order to accumulate a Raman signal over characteristic times of a few seconds. The final Raman spectrum is then processed according to known methods.

Alternatively, a certain number of LIBS spectra will be acquired at a given point and, subsequently, with the LIBS excitation laser in standby, a Raman spectrum with a typical acquisition time of a few seconds.

Finally, in step 106, a characterization of the material is carried out with techniques known per se.

In particular, considering the compositional mapping of the wall of the hole under examination, the probe 1 allows different operating modes attributable to known methodologies. Using one of the defined temporal modalities, it will be possible to perform the uni- or three-dimensional scanning of the wall of the hole in a given area by exploiting the movements of rotation R and translation along the direction A (oscillation and translation) of the roto-translational group 8.

The measurements are ultimately translated in terms of average composition, profile and compositional map.

The previous measurement steps are then repeated at different depths by moving the probe 1 in a per se known way using one of its supporting mechanics.

Naturally, the principle of the invention remaining the same, the embodiments and construction details may be widely varied with respect to what has been described and illustrated purely by way of non-limiting example, without thereby departing from the scope of protection of the present document. invention defined by the appended claims.

Claims (9)

CLAIMS 1. Probe for the compositional characterization of a material of the wall of a hole made in a ground comprising: - two excitation laser sources (2a, 4a) able to emit respective excitation beams directed to the material to be analyzed, - an optical unit (6) adapted 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 unit (8) connected to the optical unit (6) and able to rotate and translate said optical unit (6); And - a Raman spectrometer (10) and a LIBS spectrometer (10) for the acquisition of a Raman signal and a LIBS signal respectively from the excited material to be analyzed. Probe according to claim 1, wherein the optical assembly (6) comprises a rotating parabolic mirror (12) adapted to transfer the excitation beams towards the material to be analyzed and to collect the LIBS and Raman optical signals coming from said material. 3. Probe according to claim 2, wherein the parabolic mirror (12) is an off-axis mirror 12 adapted to focus the excitation beams and to collimate the LIBS and Raman signals coming from the material. 4. Probe according to any one of the preceding claims in which the roto-translational unit (8) confers three degrees of freedom to the optical unit (6), said optical unit (6) being able to rotate on itself (R) and translate along two directions (A, B) longitudinal and transverse to the axis of the hole. 5. Probe according to claim 2, wherein the roto-translational unit (8) comprises a translation device (8a) suitable for carrying out a transversal translation, a translation device (8b) suitable for carrying out a longitudinal translation, and a device rotation (8c) adapted to effect a rotation of the parabolic mirror (12). Probe according to claim 1, wherein the laser sources (2a, 4a) comprise a first pulsed laser and a second CW type laser, preferably a 1064nm Nd: YAG QS laser and a 638nm CW diode. Probe according to claim 2, wherein the optical assembly (6) further comprises five optical lines (14) each comprising an optical fiber, collimating and focusing lenses (16) and two filters (18). 8. System for the compositional characterization of the wall material of a soil hole comprising a probe according to any one of the preceding claims, a handling and flushing unit connected to the probe and suitable for supplying water and gas to the probe, and a unit calculation for the collection and processing of the data acquired by the probe to perform compositional characterization of the material. 9. A process for the compositional characterization of the wall material of a hole in the ground comprising the operations of: - preparing (100) a probe according to any one of claims 1 to 7 and introducing it into a hole made in the ground; - acquire (102) LIBS and Raman spectra of the material in which 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 it is assigned a predetermined integration time, at the end of which a trigger is emitted that starts the acquisition of the Raman spectrum in cascade, and which is assigned an integration time shorter than the time interval that separates two successive LIBS excitation laser pulses; - processing (104) the spectra acquired in the previous step; - characterize (106) the material.