RU2191413C1 - Process of spectrometric gamma-ray logging and gear to implement it - Google Patents

Abstract

FIELD: quantitative determination of radioactive element in rocks. SUBSTANCE: process consist in measurement of intensity of gamma radiation filtered by screen made of metal with low atomic number, under that of titanium, in recording of gamma radiation by scintillation detector, in numbering of recorded signals, in their storage in the form of amplitude-time spectra and in their transmission to ground surface. Gamma radiation is additionally passed through screen made of metal with high atomic number, not less than that of lead, spectrum having characteristic form in region of 0.02-0.3 meV is recorded and stored as reference spectrum, measurements are conducted in hole and each obtained spectrum is brought to correspondence with spectrum having characteristic form in region of 0.02-0.3 me V according to least squares technique, for example. Gear to implement process has guarding enclosure made of titanium which houses gamma-radiation detector connected to photoelectron multiplier fed from high-voltage power supply unit that has output connected to input of analog-to-code converter which second input is connected to output of secondary voltage converter and which outputs are connected to central processor: one directly and the other one through storage circuit of amplitude- time spectra. Output of central processor is connected to high-voltage power supply unit which input is connected to output of secondary voltage converter. Output of the latter is connected to storage circuit of amplitude-time spectra which input is connected to output of switch fitted with connector and duct connector and which output is connected to input of central processor. Detector is surrounded by screen made of lead. EFFECT: stabilization of energy scale of spectrometer. 2 cl, 3 dwg

Description

 The invention relates to nuclear geophysics, and more particularly to the field of spectrometric measurements of gamma radiation used to quantify the content of radioactive elements in rocks.

 A known method of stabilizing the energy scale of a spectrometric device [1], in which the measurement of the intensities of scattered gamma radiation is performed by covering not more than half of the detector’s working area with a screen that cuts off the soft (up to 200 keV) part of the spectrum. In this case, the shape of the recorded spectrum weakly depends on the absorbing properties (effective atomic number) of the medium under study. The ratio of intensities in two energy regions is used as a reference value for stabilizing the scale. Moreover, one energy interval is selected in the region of 240-280 keV, where the maximum of the scattered gamma radiation formed as a result of partial filtering is located, and the second in the higher energy region in the falling part of the spectrum.

 Closest to the claimed method is a method of stabilization of a scintillation gamma-ray spectrometer [2], in which the ratio of two spectral intensities selected on a steeply falling section of the recorded spectrum of scattered gamma radiation in such a region where the shape of the recorded spectrum remains constant independently is used as a reference value from changes in the properties of the scattering medium and the energy of the sources of primary gamma radiation.

 The disadvantage of this known method is the loss of a certain part of the statistics due to the use of only part of the gamma quanta that fall into the windows of spectral intensities by which the ratio is calculated.

 Closest to the claimed device is a downhole spectrometer [3], containing a ground-based computer connected by a cable to the downhole tool, which is a metal (steel or titanium) casing, in which there is an electrical circuit with a gamma radiation detector.

 A disadvantage of the known device is that to stabilize the energy scale, a reference source is used, interspersed in an additional detector, optically connected to the main detector. For the backgroundless extraction of the reference spectrum, a coincidence scheme is used, which registers the fact of registration by the main gamma-ray detector and the alpha-particle stabilization detector. This complicates the device and transfers it to the category of devices containing radioactive isotopes.

 New in relation to the prototype is that in the method of spectrometric gamma-ray logging, which consists in measuring the intensities of gamma radiation filtered by a screen made of a metal with a small atomic number, for example titanium, registering gamma radiation with a scintillation detector, digitizing the recorded signals, their accumulation in the form of amplitude spectra, transmission to the surface, gamma radiation is additionally passed through a screen made of lead, a reference spectrum having a characteristic shape is recorded y in the region of 0.02-0.3 meV, the spectrum is stored, similarly measurements are taken in the well and the energy scale of each obtained spectrum is brought into line with the energy scale of the reference spectrum, which has a characteristic shape in the region of 0.02-0.3 meV, for example , by the method of least squares.

 Studies of patent technical information showed that there are no known cases when gamma radiation is additionally passed through a screen made of lead, a spectrum having a characteristic shape in the region of 0.02-0.3 meV is recorded, the spectrum is stored, measurements are taken in the well, and each the resulting spectrum is brought into correspondence with a spectrum having a characteristic shape in the region of 0.02-0.3 meV, for example, by the least squares method. The proposed solution meets the criterion of "Novelty."

 New in relation to the prototype in a device for spectrometric logging is that in a protective casing made of titanium, a gamma radiation detector is placed optically connected to a photoelectronic multiplier provided by a high voltage power supply and having an analog output to the input of the conversion unit - a code whose second input is connected to the output of the secondary voltage conversion unit, and the outputs are connected to the central processor unit and to the amplitude spectral storage unit, which is connected by a bi-directional communication line with the central processor unit, the output of the central processor unit is connected to a high voltage power supply unit, the input of which is connected to the output of the secondary voltage conversion unit, the output of which is connected to the amplitude spectral storage unit, the input of the secondary voltage converter is connected to the output of the switching unit, connected to the downhole tool head connector and the feedthrough connector, and the output of the switching unit is connected to the input of the central processor unit, the cent The input processor is connected by its input to the output of the secondary voltage converter, and the detector is placed in a screen made of lead.

 Studies of patent technical information showed that there are no known cases when the detector is placed in a screen made of lead, and to increase the output of the formation of x-ray radiation of lead, the protective casing of the downhole tool is made of a material with a low atomic number, for example, titanium. Due to this, in a practically unchanged form of the spectrum of scattered gamma radiation, a stable additional line of gamma radiation appears, resulting from the characteristic radiation of lead. The proposed solution meets the criterion of "Novelty."

 It is known that the shape of the spectrum of gamma radiation due to multiple scattering of gamma radiation at energies below 0.2-0.5 MeV is determined, first of all, by the effective atomic number of the studied rocks. This effect underlies the methods for determining the effective atomic number of a medium by measuring the intensity of gamma radiation in the soft region [3]. However, if the scintillation gamma-ray detector is surrounded by a filter made of a material having a high atomic number (for example, lead), then the shape of the spectrum of scattered gamma-radiation in the energy range 0.02-0.20 meV remains unchanged for almost all rocks actually encountered in crossed wells [1, 2]. Choosing differential windows in this spectrum by one technique or another, one can stabilize the energy scale of the spectrometer by their ratio.

 Figure 1 shows a block diagram of a downhole tool.

 In FIG. 2 shows a functional block diagram of an analog-to-code converter.

 In FIG. 3 shows a functional block diagram of the accumulation of amplitude spectra.

 A device for spectrometric logging consists of a protective casing 1 made of titanium, a gamma radiation detector 2, optically connected to a photoelectronic multiplier 3, which is powered by a high voltage power supply 4. The photoelectric multiplier 3 has an output to the second input of the conversion unit analog - code 5, which is designed to digitize input pulses from the system "photomultiplier tube 3 + detector 2". The first input of the analogue conversion unit - code 5 is connected to the first output of the secondary voltage conversion unit 6. The first output of the analogue conversion unit - code 5 is connected to the first input of the central processor unit 7, and the second output - to the second input of the amplitude spectrum storage unit 8. First input the accumulation unit of the amplitude spectra 8 by a bi-directional communication line is connected to the first output of the central processor unit 7, the second output of the central processor unit 7 is connected to the first input of the high voltage power supply 4, the second input of which is connected to the second output of the secondary voltage conversion unit 6, the third output of which is connected to the third input of the amplitude spectral storage unit 8. The input of the secondary voltage converter 6 is connected to the fourth output of the switching unit 9, the first, second and third outputs of which are connected to the first, the second and third contacts of the head connector 10. The first, second and third outputs of the switching unit 9 are connected to the first, second and third contacts of the feedthrough connector 11, and the fifth output of the switching unit 9 is connected to the second input of the Central processor unit 7, to the third input of which is connected the fourth output of the secondary voltage conversion unit 6. The detector 2 is placed in a screen 12 made of lead.

 The analogue-to-code converter 5 consists of a current-voltage converter 13, to which a signal is supplied from the anode of the photoelectronic multiplier 3. The voltage pulse from the current-voltage converter 13 is transmitted to the “quality” determinant of the pulse 14, the pulse maximum determiner 15, and the digital-to-analog converter 16.

 The accumulation unit of the amplitude spectra 8 consists of a processor of the memory unit 17, an address decoder 18, input registers 19 and 20, and the actual memory 21.

The device contains:
- protective casing 1 (serves to protect the electronic components of the downhole tool from external pressure and is an additional converter of gamma radiation);
- scintillation detector 2 (designed to detect gamma radiation and convert it into light pulses);
- photoelectronic multiplier 3 (designed to convert light pulses from a scintillation detector into electrical pulses);
- high voltage power supply 4 (designed to power the PMT high voltage, for example, negative relative to the housing);
- analog-to-code 5 conversion unit (designed to convert analog pulses to the corresponding digital code)
- secondary voltage conversion unit 6 (designed to obtain the required secondary voltages inside the downhole tool, for example +5 V, -5 V, +12 V, -12 V, +24 V);
- the central processor unit 7 (serves to communicate the downhole tool with the on-board computer and at the same time buffers data for cable transmission, controls the operation of the software blocks of the downhole tool electronics);
- block accumulation of amplitude spectra 8 (produces the accumulation of recorded spectra);
- switching unit 9 (designed to connect or disconnect the electronic components of the downhole tool from the cable cores);
- connector of the head of the downhole tool 10 (three wires and armor of the logging geophysical cable are connected to it);
- straight through connector 11 (subsequent modules are connected to it);
- lead shield 12.

 A device for conducting spectrometric logging works as follows.

A spectrometric logging device placed in a guard casing 1 made of titanium is connected to a logging station through a geophysical logging cable articulated with the head of the downhole tool 10. The number of wire logs does not affect the logging method, for example, a three-wire version is considered . The 1st and 2nd veins are the power supply for the downhole tool and at the same time the information transmission / reception line, the 3rd core is the control one and serves for switching electronic components of the downhole tool directly to the 1st and 2nd veins. When applying to the third core relative to the armor of the geophysical logging cable of positive potential (25-30 V), the switching unit 9 connects the 1st and 2nd veins of the geophysical logging cable to the electronic units of the downhole tool. Further, the 1st and 2nd veins are supplied with power relative to the armor of the logging cable (depending on the modification of the device, this can be either direct or alternating voltage). In this case, the secondary voltage conversion unit 6 and the high voltage power supply unit 4 begin to work. When secondary voltages appear inside the downhole tool, the central processor unit 7 resets the amplitude spectral storage unit 8 to the default state, the analog conversion unit is code 5, and the high power supply unit voltage 4. The high voltage power supply 4 is made programmable, i.e. its output voltage, which feeds the photomultiplier tube 3, can be controlled by commands from a ground computer, thereby changing the gain of the information signal. When set to “default”, the voltage of the photoelectronic multiplier 3 is set by the high voltage power supply unit 4 to the value obtained from the settings of the device. Usually at a temperature of 20 ^o With the state of "default" provides the position of the energy scale of the device in a given working area.

 As a result of the interaction of gamma rays with the phosphor of the scintillation detector 2, the latter converts the energy of gamma radiation into light flashes - scintillation. In this case, the total energy of the emitted photons is proportional to the energy left by the gamma quantum in the detector 2. The gamma radiation scintillation detector 2 is surrounded by a screen 12 made of a material with a high atomic number (for example, lead), as a result of which the shape of the spectrum of scattered gamma radiation remains unchanged for almost all rocks that are actually found in sections crossed by the well.

 Next, the photomultiplier 3 converts the light pulse into an electric pulse. The charge collected from the output of the photomultiplier 3, ceteris paribus, is proportional to the total scintillation energy of the phosphor of the detector 2 and, therefore, the energy left by the gamma quantum in the detector 2. In traditional switching schemes, the photomultiplier 3 is a current source, the output of which is connected current-voltage transducer 13. Due to the finite value of the scintillator flashing time and the flight of electrons between the electrodes of the photoelectron multiplier 3, the presence of stray capacitors in uktsii photomultiplier 3 and the input stages of the converter current - voltage 13, voltage pulse produced from the system "detector photomultiplier 2 + 3 + converter current - voltage 13" can be described by some function, e.g., to a first approximation, gaussoidoy. The amplitude of this pulse, while maintaining the invariability of the above parameters, will be proportional, ultimately, to the energy of the detected gamma ray.

 The current pulse from the anode of the photomultiplier tube 3 is fed to the input of the current-voltage converter 13, the voltage pulse is output from the output to the digital-to-analog converter 16, pulse maximum determiner 15, pulse quality determinant 14. Pulse 14 quality determiner is used to select pulses not complicated by any interference and other signals. If this condition is met, it gives permission to operate the digital-to-analog converter 16. The pulse maximum determinant 15 gives a command to sample and store for the digital-to-analog converter 16 and, accordingly, starts it to work. As a result of the conversion, an analog - code 5 transformation block outputs a digital code proportional to the energy left by the gamma ray in the scintillation detector 2. The digital signal "conversion end" confirms the presence of stable conversion data at the output of block 5.

From the output of the analog-to-code 5 conversion unit, the data is fed to the input of the amplitude spectrum accumulation unit 8. The operation mode of the spectrum accumulation unit 8 is determined by the processor of the memory unit 17. That, in turn, receives commands via the serial interface (communication lines T • D and R • D) from the block of the central processor 7 and along the control line CSb allows operation of the address decoder 18 DA. The signal "end of conversion" is a parallel code, the result of the conversion in the analog-to-code 5 conversion unit is latched in the input registers 19 RG1, 20 RG2 and in the address decoder 18 DA. Moreover, 7 low bits of the conversion result are sent to register 19 RG1, 7 high bits to register 20 RG2, and three high bits to decoder 18 DA. The DA decoder 18 is designed in such a way that if there is at least one logical unit in the conversion code in the three high order bits (AD7-9), register 20 RG2 is connected to the address bus AD0-7, otherwise, register 19 RG1. At the same time, the signal “end of conversion” arrives at the processor of the memory block 17 and initiates the increment procedure of the selected memory cell. Thus, the instrument accumulates amplitude spectra. Moreover, the entire spectrum occupies 256 memory cells - 128 for the "soft" region (each of the first 128 channels of the 1024-channel spectrum) and 128 for the entire spectrum (7 high-order bits of the conversion). Thus, selective resolution of the recorded spectrum is carried out. So, for signals, the code whose amplitudes are in the range of 0-127 bits of the 10-bit spectrum, addressing to the required memory cell occurs through register 20 RG1, respectively, with 10-bit resolution. For signals whose code the amplitudes are in the range of 128-1024 bits of a 10-bit spectrum, addressing to the required memory cell occurs through register 20 RG2, respectively, with 7-bit resolution. As a result, the registered 256-channel (8-bit) spectrum consists of two areas:
- A full range of all input signals from 128 channels (7-bit);
- The "soft" part of the spectrum of 128 channels, but digitized with 10-bit resolution.

 The transmission of the accumulated spectra is carried out according to the instructions from the central processor unit 7, arriving at the accumulation unit of the amplitude spectra 8 via the line of the R • D serial interface. By this command, the processor of the memory block disconnects the 19 RG1 and 20 RG2 registers from the A0-A9 bus and, sorting the addresses from 00H to FFH, reads the contents of the memory and sends 256 16-bit words to the telemetry board on the telemetry board, while writing them to the read-out the cells are zeroes, after which it is again switched on to the spectrum accumulation mode.

 The communication of the downhole tool with the on-board computer is supported by the block of the central processor 7, made in the traditional way.

по зависимости Δu= F(a) определяют корректировочное значение высокого напряжения на фотоэлектронном умножителе, проводят корректировку (при необходимости) высокого напряжения с целью стабилизации энергетической шкалы спектрометра.The next accumulated spectrum S (n), where n is the channel number, is compared with the reference spectrum C (n), find, for example, by the method of least squares the conversion coefficient of the energy scale and the current spectrum:

according to the dependence Δu = F (a), the correction value of the high voltage on the photoelectronic multiplier is determined, the high voltage is adjusted (if necessary) in order to stabilize the energy scale of the spectrometer.

Sources of information
1. A.S. USSR 1343380, IPC G 01 V 5/00, 1987, BI 37.

 2. A.S. USSR 393706, IPC G 01 T 1/36, 1974, BI 43 [prototype].

 3. US patent 4717825, NKI 250/256, IPC G 01 V 5/00, 01/05/08 [prototype].

Claims (2)

 1. The method of spectrometric gamma-ray logging, which consists in measuring the intensities of gamma radiation filtered by a screen made of metal with a small atomic number, for example, no more than that of titanium, registering gamma radiation with a scintillation detector, digitizing the recorded signals, accumulating them in in the form of amplitude spectra, transmission to a surface, characterized in that gamma radiation is additionally passed through a screen made of metal with a large atomic number, for example, not less than that of lead, reg striate a spectrum having a characteristic shape in the region of 0.02-0.3 meV, store it as a reference spectrum, similarly carry out measurements in the well and the energy scale of each obtained spectrum is brought into line with the energy scale of the reference spectrum having a characteristic shape in region 0, 02-0.3 meV, for example, by the method of least squares.  2. A device for conducting spectrometric logging containing a guard made of metal with a small atomic number, for example, no more than titanium, which houses a gamma-ray detector optically coupled to a photoelectronic multiplier, a central processor unit, the first output of which connected by a bi-directional line of communication with the first input of the block of accumulation of amplitude spectra, and the first input is connected with the first output of the conversion unit analog - code, the first input of which is connected to the first output of the block n secondary voltage conversion, the second input is connected to the output of the photomultiplier tube, and the second output is connected to the second input of the accumulation unit of amplitude spectra, characterized in that the second output of the central processor unit is connected to the first input of the high voltage power supply, the output of which is connected to the input of the photomultiplier, and the second input is connected to the second output of the secondary voltage conversion unit, the third output of which is connected to the third input of the amplitude spectral storage unit, the input the secondary voltage conversion lock is connected to the fourth output of the switching unit, the first, second and third inputs of which are connected to the first, second and third contacts of the head connector, the first, second and third outputs of the switching unit are connected to the first, second and third contacts of the passage connector and the fifth output the switching unit is connected to the second input of the central processor unit, to the third input of which is connected the fourth output of the secondary voltage conversion unit, and the detector is placed in a screen made of metal a high atomic number, for example, no less than that of lead.

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