Mineral detection setup and method for detecting and characterizing minerals in a borehole

Abstract

Mineral detection arrangement for borehole eigenfield resonance spectroscopy comprising a housing (1) suitable for insertion into a borehole (2), and external electronics (3a) located outside the borehole (2) and electronically connected to internal electronics (3i) in the housing (1), wherein a probe (11) is located inside the housing (1), characterized in that • the probe (11) is an intrinsic field resonance probe (11) which consists of probe electronics (111) and an antenna (112) and • an oscillating magnetic field of suitable frequency can be emitted by the antenna (112), which in the radial direction, in the plane perpendicular to the longitudinal axis of the borehole (2) has a low radial location dependence close to the theoretical optimum, which in a region of the vicinity of the antenna (112), which is directly adjacent to the surface of the antenna (112), has a proportionality between the magnetic field amplitude and the inverse of the radial distance B1 ~ 1 / r.

Description

[0001] The invention relates to a mineral detection arrangement and a method for detecting and characterizing minerals in a borehole, in particular for use in mining.

[0002] The subterranean exploration and characterization of ore deposits using drilling is of great importance for mining, as it enables reliable analysis of the Earth's surface even at great depths of more than 1 km. Previously, this approach primarily involved taking drill cores and analyzing these cores in the laboratory.

[0003] In addition, on-site analysis at / in the borehole is also possible using various physical methods, which can be divided into geoelectrical, magnetic, acoustic, radar, and radioactivity methods. For this purpose, a special probe is lowered into the borehole, which analyzes the immediate vicinity of the borehole using one or more of the aforementioned methods and creates a depth profile of the parameters under investigation (so-called "well logging").

[0004] An important method in this regard is nuclear magnetic resonance (NMR), as it enables the non-invasive identification and characterization of various substances (including minerals) relevant to the mining industry.

[0005] NMR is a physical phenomenon in which the spins of atomic nuclei are polarized in the direction of the field by a strong static magnetic field B0. When exposed to an oscillating magnetic field B1 near the resonance or Larmor frequency f0, they are deflected into the plane perpendicular to B0 and respond with electromagnetic emission at the same frequency. The amplitude of the oscillating magnetic field B1 is typically several orders of magnitude weaker than the static magnetic field B0.

[0006] The resonance frequency scales with the static magnetic field B 0,eff at the nucleus, which is usually slightly influenced by the chemical environment and further depends on the gyromagnetic ratio γ of the isotope involved ( 1 H, 13 C, 65 Cu, ....) depends: f0=γ2π⋅B0,eff

[0007] The amplitude A of the reaction is described by: A∝sin(γ⋅B1p⋅τ)⋅e−τ⋅(1T1+1T2), with B 1p the portion of B1 perpendicular to B0, τ the B1 pulse duration and T1 or T2 the longitudinal or transverse relaxation time of the substance to be investigated.

[0008] The signal-to-noise ratio (SNR) of an NMR measurement scales according to SNR∝N∫VA(r→)⋅c(r→)⋅f02(r→)⋅B1p(r→) / ld3r→, with N being the number of repetitions of the measurement used to calculate the mean value, V being the resonance volume, A being the frequency of the magnetic field B which is determined according to equation (2) by the generally existing spatial dependence of the magnetic field 1p conditional location-dependent reaction of the nuclear spins, c the location-dependent concentration of the substance to be examined, f0 the location-dependent resonance frequency determined according to equation (1) by the generally existing location dependence of the magnetic field B0, B1 p / I of the location-dependent sensitivity of the B1 coil or antenna, given by the ratio of the location-dependent magnetic field B 1pto coil current I. The method is only economically viable if a specified minimum detection limit for the substance under investigation can be guaranteed within a given maximum measurement time. Consequently, the resonance volume and the resonance frequency f0 must be maximized for economic viability. According to equation (1), maximizing the resonance frequency f0 is equivalent to maximizing the static magnetic field B0.

[0009] The resonance volume is the volume in which the spatially dependent resonance frequency f0 nearly coincides with the frequency of the oscillating magnetic field B1. The permissible frequency deviation is determined by the natural linewidth of the NMR peak under consideration and the frequency uncertainty of the B1 pulse such that these frequency ranges must at least partially overlap. Consequently, the resonance volume is characterized by the static magnetic field B0 meeting a certain minimum requirement for spatial homogeneity, which is determined, among other things, by the substance under investigation.

[0010] Analogous to the resonance volume, an effective measurement volume can be defined. This is characterized by the part of the resonance volume in which the product amplitude(r→)⋅B1p(r→) / l makes a significant contribution to the integral of equation (3).

[0011] The challenge in implementing an NMR measuring device for use in boreholes lies in generating the most suitable magnetic fields B0 and B1 while simultaneously adhering to the geometric restrictions. While in conventional laboratory-based NMR, the sample is placed in a setup that can be optimized almost arbitrarily with regard to magnetic field generation, in well logging, the substances to be analyzed are located within the borehole wall and thus outside the NMR setup. For physical reasons, generating homogeneous magnetic fields outside the NMR setup is difficult, so the magnetic fields B0 and B1 within the borehole wall can only be realized in the form of relatively inhomogeneous stray fields, which meet the requirements for spatial homogeneity only within a very limited resonance volume.

[0012] The application of NMR measurement methods within a borehole has so far been limited essentially to the measurement of the 1 H-nucleus for the purpose of formation fluid characterization, which allows differentiation between free and bound water, as well as gas and oil. (See Freedman R and Heaton N 2004 Fluid Characterization using Nuclear Magnetic Resonance Logging PETROPHYSICS 45 241-50). Some patents in this application area are presented below.

[0013] EP 0 459 064 A1 discloses an apparatus for investigating a characteristic of an earth formation penetrated by a borehole, the apparatus comprising: • an elongated body which is longitudinally movable in the borehole; • magnetic means carried by the body, arranged to generate a static and substantially homogeneous magnetic field in a volume of the formation on one side of the body, which field has a field direction extending substantially perpendicular to the longitudinal axis of the body; • Antenna means carried by the body operable as antennas for generating an oscillating magnetic field in the volume and for detecting a signal representative of nuclear magnetic precession of a population of particles in the volume; • wherein the antenna means are offset from the longitudinal axis of the body to said one side of the body and are not enclosed by the body, and wherein the body is designed to press said one side of the body against the borehole wall.EP 0 649 035 A1 discloses an apparatus for investigating a characteristic of an earth formation penetrated by a borehole, which comprises an apparatus with: • an elongated body designed to be longitudinally movable in the borehole; • magnetic means carried by the body, arranged to generate a static magnetic field in a volume of the formation; • antenna means carried by the body and operable to generate an oscillating magnetic field in the volume and to detect a signal representative of nuclear magnetic precession of a population of earth formation particles in the volume; • wherein the elongated body comprises a metallic housing which provides structural strength to the body, and the antenna means are not enclosed by the metallic housing.

[0014] The metallic housing is non-magnetic, with the magnet means mounted within the metallic housing and the antenna means offset relative to one side of the body. The body is configured to bring the antenna means into contact with the borehole wall, with the magnet means configured to generate a substantially homogeneous static magnetic field within the volume, and the antenna means configured to generate the oscillating magnetic field with a field direction that is substantially perpendicular to the field direction of the static magnetic field.

[0015] EP 0 581 666 A2 discloses a device for logging during drilling, comprising i) Drilling equipment with: (a) a drilling bit for drilling a borehole in a formation; b) an outer surface and a depression in the outer surface; and c) a housing means; (ii) a pulsed nuclear magnetic resonance (NMR) means housed within the casing for performing pulsed NMR measurements in a region of the formation surrounding the borehole as the borehole is drilled, comprising pulsed NMR means: a) two tubular magnets, each magnet having poles and arranged so that a pair of like magnetic poles are opposite each other; b) an antenna mounted between the two tubular magnets and in the depression of the outer surface of the drilling means; and c) driver circuit means for driving the antenna, which circuit is mounted in the housing means; and (iii) Conduit means for conducting borehole fluid through the drilling means, which conduit means is surrounded by the two tubular magnets and the antenna and which comprises a barrel arranged to isolate the two tubular magnets from the borehole fluid, wherein the pulsed NMR comprises means for suppressing the component of the pulsed NMR measurements due to the borehole fluid by applying a pulsed magnetic field which is strong in the borehole fluid and weak in said region of the formation.

[0016] Furthermore, EP 0 581 666 A2 discloses a method for investigating a magnetic characteristic of a population of particles in an earth formation penetrated by a borehole using an apparatus comprising an elongated body which is configured to be longitudinally movable in the borehole, said method comprising: • Generating a static magnetic field with radially directed and substantially homogeneous field lines in a volume of the formation on one side of the body; • Irradiating the volume with an oscillating magnetic field; and • detecting NMR signals at the Larmor frequency of the particles under the influence of the static and substantially homogeneous magnetic field within the volume;wherein the irradiation and detection are carried out with antenna means offset from the longitudinal axis of the body to said one side thereof, while said one side of the body is pressed against the borehole wall.

[0017] Furthermore, EP 0 581 666 A2 discloses a method for logging a borehole in an earth formation while the borehole is being drilled, which method comprises: • Using a section of a logging-while-drilling tool to drill the borehole; • Using another section of the tool to perform pulsed nuclear magnetic resonance (NMR) measurements in a region of the formation surrounding the borehole as the borehole is drilled, by generating a rotationally symmetric magnetic field about the longitudinal axis of the borehole with two tubular magnets, each having magnetic poles, arranged so that a pair of like magnetic poles are opposed to each other, and • Using an antenna mounted between the two tubular magnets to excite and acquire the NMR measurements; • Passing borehole fluid through both sections of the tool with a barrel defining a channel and isolating the two tubular magnets from the borehole fluid; and • Suppressing the component of the NMR measurements attributable to the borehole fluid by applying a pulsed magnetic field that is strong in the borehole fluid but weak in the said region of the formations.

[0018] The disadvantage of all these technical solutions is that they require the use of magnetic means, which are necessary to generate a static magnetic field within a volume of the formation. Furthermore, the SNR significantly depends on the strength and spatial homogeneity of the generated static magnetic field or is limited by them, so that these solutions cannot be transferred to other economically interesting application areas, or only to a very limited extent. For example, a radial reduction in the size of the aforementioned structures, which would be necessary for use in boreholes with smaller diameters, would lead to a corresponding reduction in the static magnetic field B0 and consequently to a deterioration of the signal-to-noise ratio. The same applies if the aforementioned methods are to be applied to minerals. The gyromagnetic ratios of any relevant isotopes are significantly lower than for 1H-based NMR, e.g., γ( 65 Cu) = 0.29 γ( 1 H), which according to equations (1) and (2) significantly reduces the resonance frequency and thus the SNR.

[0019] An effect physically related to NMR spectroscopy is nuclear quadrupole resonance (NQR). In NQR, the electric quadrupole moment of the nucleus couples to the electric field gradient influenced by the chemical environment. Despite the physical differences, samples in NMR and NQR spectroscopy behave almost identically when viewed externally, with the significant difference that NQR spectroscopy does not require a static magnetic field B0. Using NQR instead of NMR in the borehole therefore avoids any problems associated with generating the static magnetic field.

[0020] The special case of an unnecessary externally applied static magnetic field can also occur in NMR if the substance under investigation itself generates a magnetic field. This effect has been observed for various minerals, including chalcopyrite, hematite, cubanite, and pyrrhotite. (See: Bennett D, Miljak D, Schwitter B and Khachan J 2010 Zero Field NMR and NQR measurements of natural copper minerals Proc. - 34TH Annu. Condens. MATTER Mater. Meet. - WAIHEKE ISLAND, AUCKLAND, NZ 1, Anderson DH 1966 Nuclear Magnetic Resonance of Fe57 in Single-Crystal Hematite Phys. Rev. 151 247-57; Gavrilenko AN, Pogoreltsev AI, Matukhin VL, Korzun BV, Schmidt EV and Sevastianov IG 2016 Low-Temperature Studies of CuFe2S3 and CuFeS2 by 63.65Cu NMR in the Internal Magnetic Field J. Low Temp. Phys. 185 618-26 and Bastow TJ and Hill AJ 2018 57Fe NMR characterization of pyrrhotite J. Magn. Magn. Mater. 447 58-60)

[0021] In the following, both phenomena, NMR in the substance's own magnetic field and NQR, are summarized under the term self-field resonance.

[0022] US 2008 / 0224696 A1 discloses a device for detecting various minerals using NMR in an earth formation penetrated by a borehole. The key advantage of the design described therein is that it does not require a magnetic means to generate a static magnetic field B0. This results, as also described in US 2008 / 0224696 A1, in a larger effective measurement volume compared to the NMR methods previously used in boreholes, which in this case is limited solely by the range of the oscillating magnetic field B1. The latter is generated by an elongated coil with several turns wound around a cylinder of highly permeable core material. The coil thus acts like an oscillating two-dimensional magnetic dipole and generates B1 magnetic field amplitudes for radial distances r that are significantly smaller than the longitudinal extent of the coil, which are proportional to the inverse square of the distance (1 / r 2) decay. The sensitivity of the coil arrangement B 1p / I is proportional to the coil diameter.

[0023] This approach has two major disadvantages: Firstly, the described scaling of sensitivity with coil diameter has a negative effect on the signal-to-noise ratio when used in small diameter boreholes.

[0024] Second, the radial dependence of the B1 magnetic field does not correspond to the theoretically achievable optimum, which corresponds to the inverse of the distance (1 / r) near the field-generating array. As a result, the effective measurement volume and the signal-to-noise ratio are not optimal.

[0025] US 2016 / 0 238 734 A1 discloses a permanent monitoring instrument for a wellbore. The measurements are taken at various times by continuously monitoring an area of ​​a formation penetrated by the wellbore. From the measurements, one or more properties of the formation are derived at one or more investigation depths within the region. Any changes in one or more derived formation properties are determined, and one or more reservoir management decisions are made based on the determined changes.

[0026] Based on the changes detected by this method and monitoring instrument, statements can be made about CO2 storage, the geological storage of subsurface water, the monitoring of water content, the monitoring of environmental pollution in this water or the long-term monitoring of water- or oil-bearing materials.

[0027] The disadvantage of this technical solution is that only gaseous and liquid substances in geological formations are detected, whereas solids in rock, such as minerals, cannot be detected.

[0028] US 2008 / 0 224 696 A1 discloses an instrument for in-situ investigation of the properties of formation fluids in an earth formation comprising a body which houses a nuclear quadrupole resonance (NQR) probe, wherein the probe essentially consists of at least one coil and an electronic coupling, wherein the body is suitable for introduction into a borehole within the earth formation in order to determine the identity of a mineral, such as chorite, smectite, kaolinite, muscovite or feldspar or a mineral which carries at least one of the elements hydrogen, carbon, oxygen, nitrogen, arsenic, copper, boron, aluminum, chlorine, sodium, manganese, magnesium, potassium, silicon and calcium, in formation fluids of a formation.

[0029] The disadvantage of this technical solution is that only formation fluids can be examined and no direct mineral detection can be performed on solids in a geological formation.

[0030] The publication Balzano, Quirino; Garay, Oscar; Siwiak, Kazimierz: The near field of dipole antennas, Part 1: Theory. In IEEE, Transactions on Vehicular Technology, Vol. VT-30, 1981, No. 4, pp. 161-174, ISSN 0018-9545 discloses a comprehensive theory of antenna near fields in the form of a general conceptual framework for the more detailed spectral theory, through a general spatial description of the electromagnetic field in the antenna forefield based on an asymptotic interpretation of the Wilcox expansion. For this purpose, a far-field radiation pattern is constructed by deriving the Wilcox expansion from the multipole expansion, which allows for the analysis of the energy exchange processes between the different regions in the antenna environment and the analytical representation of the effect and contribution of each mode.

[0031] The disadvantage of this publication is that it provides computational models for the analysis of energy exchange processes between different areas in an antenna environment and the effect and contribution of each mode, but no detection method for the detection of solids in the rock of geological formations, such as minerals.

[0032] US 2018 / 0 074 221 A1 discloses an apparatus and method for transmitting a common signal into a geological formation using a first transceiver antenna and receiving, in response to the transmission at the first transceiver antenna, a first corresponding nuclear magnetic field from a first volume of the formation as an NMR resonance signal.

[0033] The disadvantage of this technical solution is that no solids in the rock of geological formations, such as minerals, can be detected.

[0034] DE 199 52 786 A1 discloses a method for generating a pulse sequence using a tool for creating a drilling report. For this purpose, a pulse programmer is provided for the adaptive control of the generation and application of a sequence of RF magnetic field pulses to a ground formation. To control the RF pulses, a section of a memory device is divided into several tables in which the control information is stored. The pulse programmer generates the pulse sequence using the information found in the memory device and the operating conditions of the tool. The structure of the divided memory device allows the pulse programmer to autonomously adapt and modify the timing of the pulse sequences in the borehole.

[0035] The disadvantage of this technical solution is that no solids in the rock of geological formations, such as minerals, can be detected.

[0036] US 2014 / 0 167 759 A1 discloses resonance spectroscopy without an externally applied static magnetic field in connection with deep borehole logging. For this purpose, a device comprises a radio-frequency coil and a magnetic field detection device. The device is configured to be positioned in a first magnetic field. The first magnetic field polarizes nuclear spins of species in a detection area near the device. The device generates a second magnetic field with the RF coil for a specific period of time to excite the nuclear spins of the species in the detection area. The device then uses a magnetic field sensor to measure the electromagnetic radiation generated by the particles through the excitation of the nuclear spins.

[0037] The disadvantage of this technical solution is that no solids in the rock of geological formations, such as minerals, can be detected.

[0038] The object of this invention is to avoid the disadvantages of the prior art and to enable the detection of certain minerals in an earth formation penetrated by a borehole, even for boreholes of small diameter, with a good signal-to-noise ratio.

[0039] This object is achieved by an arrangement according to the first patent claim and a method according to the seventh patent claim. Advantageous embodiments of the invention are specified in the subordinate claims.

[0040] This problem is solved by proposing an arrangement and a method which enable the quantitative identification of substances directly in a small-diameter borehole by analyzing them in their naturally occurring eigenfield using a device for measuring NMR or NQR spectra (so-called borehole eigenfield resonance spectroscopy).

[0041] The arrangement according to the invention for carrying out borehole self-field resonance spectroscopy comprises a housing which is suitable for being introduced into a borehole, as well as external electronics which are located outside the borehole and are electronically connected to the devices in the housing.

[0042] Inside the housing is the self-field resonance probe, consisting of probe electronics and an antenna.

[0043] The latter is characterized by the fact that it can emit an oscillating magnetic field of suitable frequency, which has in the radial direction, ie in the plane perpendicular to the longitudinal axis of the borehole, the smallest possible radial spatial dependence close to the theoretical optimum.

[0044] The latter is characterized near the antenna by a proportionality between the magnetic field amplitude and the inverse of the distance (B1 ~ 1 / r). Near the antenna means: in the area surrounding the antenna that directly adjoins the antenna surface.

[0045] The generation of a static magnetic field B0 is not necessary, since only substances suitable for self-field resonance spectroscopy are to be analyzed.

[0046] This means that the resonance condition is fulfilled in a quasi-unlimited volume and the effective measurement volume is only limited by the spatial dependence of the B1 field.

[0047] The low radial spatial dependence of the B1 field provided by the antenna thus enables a large measurement volume close to the theoretical optimum, which increases the signal-to-noise ratio and makes the detection of certain substances possible from a practical point of view.

[0048] The method according to the invention includes the selection of a housing suitable for the borehole (i.e. the housing must be designed with regard to the borehole diameter so that it can be moved freely in the borehole), the division of the electronic components required for the measurement into a group that is accommodated inside the housing (probe electronics) and a group that is placed outside the borehole (external electronics), the selection of an antenna suitable for the substance to be examined or the frequency range to be examined, the introduction of the housing into the borehole, the depth-dependent measurement of the self-field resonance of the substances surrounding the borehole by emitting an oscillating magnetic field B1 adapted to the respective measurement task, the reception of the response signal of these substances by the antenna and the evaluation of the measurement data obtained.

[0049] The invention is explained in more detail below with reference to a preferred embodiment and the figure. It shows: Fig. 1: a schematic overview of an embodiment of the mineral detection arrangement according to the invention.

[0050] The Fig. The mineral detection arrangement shown in Figure 1 comprises, among other things, an antenna (112). The following should be noted: In conventional borehole NMR, which generates a static magnetic field B0 by means of magnetic means, B1 must be as perpendicular to B0 as possible in the measurement volume.

[0051] In the case of the invention, this restriction does not apply since the directions of the eigenfields are randomly distributed and consequently no preferred direction for B1 exists.

[0052] The dipole antenna (112) or variations of this antenna shape are suitable for generating a magnetic field dependence B1 ~ 1 / r near the array. Near the antenna (112) means: in the area surrounding the antenna (112) that directly adjoins the surface of the antenna (112).

[0053] For a dipole antenna (112) and also for a shortened dipole antenna, the radial dependence is approximately valid under the constraint r << L, |z| << L: B1(r,z)=μ0⋅μr(r,z)⋅l2πr with r the radial distance to the antenna (112), z the coordinate in the direction of the longitudinal axis of the borehole (2) relative to the center of the antenna (112), L the length of the antenna (112), µ0 = 4π · 10 -7 N / A 2 the vacuum permeability, µ r the relative permeability of the material at the location (r,z) and I the electric current through the center of the antenna (112).

[0054] The magnetic field rotates azimuthally around the longitudinal axis of the borehole (2) and is thus invariant with respect to a rotation of the antenna (112) around the longitudinal axis of the borehole (2).

[0055] It is therefore possible to integrate the structure into a drill and to apply the method according to the invention during drilling without the measurement results being affected by the rotation of the structure.

[0056] The antenna (112) is fed as in Fig. 1, at the antenna end. However, it can also be done at any other point on the antenna (112).

[0057] The optimal antenna in terms of sensitivity B1 / I is the λ / 2 dipole (112), whose length is half the wavelength λ. Alternatively, shortened variants (so-called shortened dipoles) can also be considered. Possibilities for shortening include embedding the antenna rod in a highly permeable material and using so-called extension inductors.

[0058] The advantage of a (shortened) dipole antenna (112) is that the alternating magnetic field it generates depends solely on the current through the antenna (112), but not on its radius.

[0059] This makes this antenna shape particularly suitable for small-diameter boreholes (2). The minimum radius of the antenna (112) is practically limited only by the ohmic losses, which scale with the inverse of the antenna radius for a given antenna current.

[0060] To operate the antenna (112), its resonant frequency must be adjusted to the resonant frequency of the substance under investigation, and its input impedance must be matched to the impedance of the antenna feed line. This is achieved by an automatic matching network in the form of an antenna tuner (8), which also compensates for potential fluctuations in these quantities resulting from the interaction of the emitted magnetic field with the substances in the wall of the borehole (2).

[0061] The Fig. The mineral detection arrangement shown in Figure 1 also includes, among other things, external electronics (3a) and internal electronics (3i).

[0062] A typical arrangement suitable for self-field resonance spectroscopy in the borehole (2) comprises several electronic components which, depending on the measuring task, can be located inside the housing (1) [so-called internal electronics (3i) in the form of probe electronics (111) of the self-field resonance probe (11)] or outside the borehole (2) [so-called external electronics (3a)].

[0063] A computer (4) with appropriately equipped software serves as an interface to the user, for controlling and monitoring the measurement and its parameters as well as for evaluating and displaying the measurement results.

[0064] This computer (4) is connected via a digital interface to a programmable pulse generation unit (5). The latter either receives the parameters of the B1 field oscillation to be generated, such as the amplitude and length of the pulse, or receives the specific pulse shape in sampled form.

[0065] The pulse generation unit (5) generates an analog voltage signal, which is sent to a power amplifier, the so-called transmitter (6).

[0066] The decisive parameter for the transmitter (6) is the maximum pulse output power P0 to which the pulse can be amplified. This is because the B1 amplitude scales proportionally to the square root of P0. The pulse amplified by the transmitter (6) is passed to the duplexer (7), which forwards the pulse to the antenna tuner (8) and the antenna (112). There, it is converted into an electromagnetic emission that interacts with the substance under investigation (see equation 2).

[0067] This interaction also creates an electromagnetic response signal, which lasts the length of the pulse and decays approximately exponentially after the end of the pulse.

[0068] The response signal is received by the antenna (112) and fed back to the duplexer (7) via the antenna tuner (8). This duplexer then forwards the response signal to a preamplifier (10), which amplifies the signal sufficiently for it to be further processed by the downstream electronics without significantly affecting the signal-to-noise ratio.

[0069] The amplified response signal is then digitized by a data acquisition unit (9), transferred to the computer (4) and evaluated by it.

[0070] The Fig. The mineral detection arrangement shown in Figure 1 also includes a housing (1).

[0071] This housing (1) is designed to fulfill the following four functions, if necessary. It protects its contents from the environment in the borehole (2), it displaces any mud that may be present in the borehole (2), it electrically isolates the antenna (112) from the environment in the borehole (2), and it enables connection to the external electronics (3a) and other measuring devices used on or in the borehole (2).

[0072] Boreholes (2) drilled through the ground by the mining and oil industries are often filled with mud, which may contain the substances to be examined.

[0073] Since this mud is a mobile medium, the detection of a substance in the mud at a certain depth can lead to a misinterpretation of the measurement, since it cannot be ruled out that the detected particles originally originate from a different depth.

[0074] By displacing the sludge through the housing (1), the interior of the housing (1) is protected on the one hand, and on the other hand, any response signals that may occur from the sludge are reduced.

[0075] The housing (1) also protects its contents from other harmful influences, such as high pressure, vibrations and general mechanical effects.

[0076] The arrangement according to the invention can be operated together with other measuring devices. In this case, mechanical and electronic interfaces must be provided on the housing (1).

[0077] The housing (1) is designed in such a way that the emission and reception of electromagnetic signals in the frequency range relevant for the measuring task are possible without impairment.

[0078] In the immediate vicinity of the antenna (112), the housing (1) consists of materials of negligible electrical conductivity.

[0079] This mineral detection arrangement is used in a method for detecting and characterizing minerals in a borehole (2).

[0080] The implementation of this procedure requires first the selection of a suitable housing (1) and the division of the electronic components of the setup according to the requirements of the measurement task.

[0081] The diameter of the housing (1) must be small enough to allow for easy insertion into the borehole (2). On the other hand, the diameter of the housing (1) should be as large as possible, taking the aforementioned constraint into account, as this minimizes oscillation of the assembly within the borehole (2) and maximizes the displacement of any mud potentially present in the borehole (2). A large diameter of the housing (1) also simplifies the implementation of the assembly, particularly the internal electronics (3i).

[0082] The distribution of the electronic components depends primarily on the depth and diameter of the borehole (2). Some electronic components, such as the transmitter (6), require a certain minimum diameter of the housing (1). The smaller the diameter of the borehole to be examined (2), the more electronic components must be placed outside the borehole (2) as external electronics (3a).

[0083] An analogous limitation arises from the depth of the borehole (2) and the associated exponential attenuation of the transmitted high-frequency signals with increasing transmission line length. For example, a typical attenuation of 2 decibels per 100 meters of line length for a line length of one kilometer results in a loss of 99% of the transmitted power. Consequently, with increasing depth of the borehole to be investigated (2), a larger proportion of the electronic components must be placed within the housing (1) as internal electronics (3i).

[0084] From the aforementioned trends, it can be deduced that boreholes (2) that fall below a certain limiting diameter and simultaneously exceed a certain limiting depth cannot be examined using the method according to the invention. In contrast, if the diameter or depth of the borehole (2) is considered alone, no explicit restrictions can be specified.

[0085] The division of the electronic components into external electronics (3a) and internal electronics (3i) will be illustrated below using three examples, without being limited to these.

[0086] In the case of a 1 km deep borehole (2) with a diameter of 15 cm or larger, all electronic components are implemented as probe electronics (111). However, to allow user monitoring of the measurement, at least the computer (4) is left outside the borehole (2).

[0087] In the case of a 200 m deep borehole (2) with a diameter of 7 cm, the transmitter (6) with sufficient pulse output power P0 is placed outside the borehole (2) together with the computer (4). Assuming an attenuation of -2 dB per 100 m of cable length, a reasonable 40% of the generated pulse power reaches the probe electronics (111) of the self-field resonance probe (11).

[0088] In this case, the external electronics (3a) therefore consists of the computer (4), the programmable pulse generation unit (5) and the transmitter (6).

[0089] The remaining components [duplexer (7), antenna tuner (8), preamplifier (10) and data acquisition unit (9)] then correspond to the internal electronics (3i) and in this case are part of the probe electronics (111) of the self-field resonance probe (11).

[0090] In the case of a 3 m deep borehole (2) with a diameter of 1 cm, the current state of the art in electronic components does not allow any of the aforementioned electronic components to be miniaturized to such an extent that they can be inserted into the borehole (2) within the housing (1). In this case, the external electronics (3a) comprises all of the aforementioned electronic components (4 to 9). Furthermore, in this special case, the use of a housing (1) enclosing the antenna (112) may be dispensed with, so that only the antenna (112) is inserted into the borehole (2).

[0091] In the method according to the invention, the design of the B1 pulses must be adapted to the measurement task, in particular to the substance to be examined, and the existing measurement conditions.

[0092] In principle, the pulses should be designed to maximize the signal-to-noise ratio, assuming that the substance to be examined is evenly distributed in the wall of the borehole (2). Any limitations that may arise due to the particular implementation of the inventive arrangement, such as the maximum pulse power or the maximum duty cycle of the transmitter (6) used, must be taken into account.

[0093] The pulses generated by the pulse generation unit can be described by at least the following parameters: the frequency f, the voltage amplitude V, the pulse duration τ and the time interval between two consecutive pulses ΔT.

[0094] The frequency f of the pulse must be tuned to the resonance frequency of the substance to be examined.

[0095] The choice of the remaining parameters can be derived theoretically from equations (2) and (3) or determined by preliminary tests. The following specifications are suitable as rough guidelines: τ = T2, ΔT = 3T1, and adjust the voltage amplitude V so that the B1 field near the surface of the housing is: γ · B 1p · τ = π, where B 1p ≈ 0.7 · B1.

[0096] The condition γ · B 1p · τ ≈ π, according to equation (2), ensures that the response signal from materials close to the wall of the casing (1) is suppressed. It should therefore be used if parasitic signals from the immediate vicinity of the wall of the casing (1) are to be expected, e.g., if the borehole (2) is filled with mud containing the substances to be investigated.

[0097] In principle, the application of more complex pulse sequences is possible to obtain further information, such as determining relaxation times, or for imaging purposes.

[0098] However, such pulse sequences are not necessary for the identification of the substance to be examined, since this is done based on the resonance frequency.

[0099] The amplitude of the measured response signal is directly proportional to the average concentration of the substance to be examined in the measuring volume.

[0100] The measurement of the borehole (2) is usually automated.

[0101] The measurement value recording begins as soon as the pulse generation unit (5) is programmed, the resonance frequency of the antenna (112) is set to the Larmor frequency of the substance to be examined using the antenna tuner (8) and the housing (1) with the measuring arrangement has been inserted into the borehole (2).

[0102] The depth profile as a result of the measurement is obtained by plotting the amplitude of the response signal over the immersion depth of the antenna (112) into the borehole (2).

[0103] Typically, the immersion depth of the arrangement is continuously increased.

[0104] However, it is also possible to increase the immersion depths gradually, e.g. because the measurement is to be repeated for each discrete depth value under identical conditions.

[0105] The depth profile P(T) obtained by the measurement corresponds to a mathematical convolution operation given by: P(T)=∫c(T−z)⋅g(z)dz with T being the immersion depth of the antenna (112) into the borehole (2), c(T) being the mean concentration of the substance to be examined at the depth T, g(z) being the device function and z being the coordinate along the longitudinal axis of the borehole (2) relative to the center of the antenna (112).

[0106] The device function g(z) describes the fact that the antenna (112) is sensitive over a certain depth range due to its extension in the z-direction, which “smears” the depth resolution over this range.

[0107] The depth resolution according to equation (5) corresponds approximately to the length of the antenna (112).

[0108] If the device function is known, e.g. by theoretical derivation or experimental measurement, the depth-dependent substance concentration c(T) can be calculated from P(T) using a deconvolution operation in a final processing step.

[0109] With the mineral detection arrangement described above and the method using this arrangement, it is possible to enable the detection of certain minerals in an earth formation penetrated by a borehole, even for boreholes of small diameter, with a good signal-to-noise ratio, thus avoiding the disadvantages of the prior art.

[0110] The advantage of borehole self-field resonance spectroscopy is that it is carried out directly on site in a borehole, without the need for NMR means to generate B0.

[0111] Another major advantage is that borehole self-field resonance spectroscopy enables direct spectral identification of minerals based on the occurrence of a specific spectral pattern for individual minerals and is thus insensitive to and unaffected by vein material.

[0112] The present invention (the arrangement and method for borehole self-field resonance spectroscopy) is directed at all mining companies and other companies that are active in mineral exploration and in particular in the field of borehole methods and therefore have considerable economic expenditures, particularly in deep exploration or the "exploration under cover" scenario.

[0113] However, the present invention is neither limited to the field of mineralogy, nor is there a fundamental limit to the borehole diameter. Therefore, other applications could also be affected, such as the characterization of drugs or explosives by injecting a thin needle into a test substance.

[0114] All features presented in the description, the embodiments and the following claims can be essential to the invention both individually and in any combination with one another. Reference symbol 1 housing 2 boreholes 3a external electronics 3i internal electronics 11 Self-field resonance probe 111 Probe electronics 112 Antenna 4 computers 5 Pulse generation unit 6 transmitters 7 duplexers 8 antenna tuners 9 Data acquisition unit 10 preamplifiers

Claims

[1] Mineral detection arrangement for borehole self-field resonance spectroscopy comprising a housing (1) which is suitable for being introduced into a borehole (2), and an external electronics (3a) which is located outside the borehole (2) and is electronically connected to devices of an internal electronics (3i) in the housing (1), wherein a probe (11) is located inside the housing (1), characterized by , that • the probe (11) is a self-field resonance probe (11) which consists of a probe electronics (111) and an antenna (112) and • an oscillating magnetic field of suitable frequency can be emitted by the antenna (112), which has a low radial spatial dependence close to the theoretical optimum in the radial direction, in the plane perpendicular to the longitudinal axis of the borehole (2), which has a proportionality between the magnetic field amplitude and the inverse of the radial distance B1 ~ 1 / r in an area of ​​the vicinity of the antenna (112) which directly adjoins the surface of the antenna (112). [2] Mineral detection arrangement according to claim 1, characterized by that the antenna (112) is a dipole antenna which is designed as a λ / 2 dipole antenna or shortened dipole antenna. [3] Mineral detection arrangement according to claim 1 or 2, characterized by that for the antenna (112) under the constraint r << L, |z| << L the radial dependence: B1(r,z)=μ0⋅μτ(r,z)⋅l2πr with r being the radial distance to the antenna (112), where: • z of the coordinate in the direction of the longitudinal axis of the borehole (2) relative to the antenna center, • L is the length of the antenna, • µ0 = 4π · 10 -7 N / A 2 the vacuum permeability, • µ r the relative permeability of the material at the location (r, z) and • I corresponds to the electric current through the center of the antenna. [4] Mineral detection arrangement according to one or more of claims 1 to 3, characterized by that the external electronics (3a) and the internal electronics (3i) are connected to a computer (4), wherein the computer (4) is connected to a programmable pulse generation unit (5) via a digital interface. [5] Mineral detection arrangement according to claim 4, characterized bythat the pulse generation unit (5) receives either the parameters of the bi-field oscillation to be generated in the form of the amplitude and length of the pulse or the concrete shape of the pulse in sampled form, wherein the pulse generation unit (5) generates therefrom an analog voltage signal which is passed to a power amplifier in the form of a transmitter (6). [6] Mineral detection arrangement according to claim 5, characterized by that the pulse amplified by the transmitter (6) is passed to a duplexer (7), which forwards the pulse to an antenna tuner (8) and the antenna (112). [7] A method for borehole mineral eigenfield resonance spectroscopy using the mineral detection arrangement according to one or more of claims 1 to 6, comprising the following steps: • the selection of a housing (1) suitable for the borehole (2), which is designed with regard to the diameter of the borehole (2) so that it can move freely in the borehole (2), • the division of the electronic components required for the measurement into a group that is housed inside the housing (probe electronics) and a group that is placed outside the borehole (2) (external electronics), • the selection of an antenna suitable for the substance to be examined or the frequency range to be examined (112), • inserting the housing (1) into the borehole (2), • the depth-dependent measurement of the self-field resonance of the substances surrounding the borehole (2) by emitting an oscillating magnetic field B1 adapted to the respective measuring task and • receiving the response signal of these substances by the antenna (112) and evaluating the measurement data obtained. [8] Method according to claim 7, characterized by that for the operation of the antenna (112) its resonance frequency is tuned to the resonance frequency of the substance to be examined and its input impedance is tuned to the impedance of the antenna feed line. [9] Method according to claim 8, characterized by that the tuning is carried out by an automatic adaptation network in the form of an antenna tuner (8), which also compensates for potential fluctuations in the resonance frequency of the substance to be examined, the input impedance of the substance to be examined and the impedance of the antenna feed line, which arise due to the interaction of the emitted magnetic field with the substances located in the borehole wall. [10] Method according to claim 9, characterized bythat the electromagnetic emission emitted by the antenna tuner (8) and the antenna (112) interacts with the substance to be examined in the borehole (2), this interaction giving rise to a response signal which is also electromagnetic and which outlasts the length of the pulse and decays approximately exponentially after the end of the pulse, the response signal being received by the antenna (112) and fed back to the duplexer (7) via the antenna tuner (8), the duplexer (7) feeding the response signal to the preamplifier (10), which amplifies the signal to such an extent that it can be further processed by the downstream electronics (3a and i) without significantly impairing the signal-to-noise ratio, and finally the amplified response signal is passed on in digital form by a data acquisition unit (9) to the computer (4) and evaluated by the computer.

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