Proceedings of the International Conference "Nuclear Geophysics ´97", Kraków, 20-23 October 1997

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The Henryk Niewodniczański Institute of Nuclear Physics, the Faculty of Geology, Geophysics and Environmental Protection and the Faculty of Physics and Nuclear Techniques at the University of Mining and Metallurgy, Kraków, the Institute for Geophysics and Radiation Technologies at the International Higher Education Academy of Sciences, Moscow, organised the International Conference NUCLEAR GEOPHYSICS '97.The Conference was held in Kraków on 20-23 October 1997.

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Jan Czubek was the greatest European scientist in nuclear geophysics. His scientific results and activity have won him world recognition. He has encyclopedic erudition and an extremely broad range of scientific interests. First and foremost, he developed theoretical basics for all methods of nuclear geophysics as applied to exploration and production of oil, coal, uranium, and ore minerals.

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Recent years have brought continued developments in nuclear geophysical techniques. Technological advances have made possible new measurements that were not previously practical. A number of the developments that have been made, and how they have resulted in new measurements systems, are described. Not surprisingly, key advances come from the basic components in any nuclear geophysics measurement: sources, detectors, electronics, and computer capabilities. These advances have a significant impact on not only the measurement systems themselves, but they also have an impact on the applications to which the data can be put, especially in the important area of real-time analysis. Furthermore, advances have also been made by applying previously established measurements to new environments. A more advance has been the application of data from established measurement techniques in new ways by the developments that have been made in inorganic geochemistry and other areas. Some examples of these approaches will be presented, not only to show they have been achieved, but also to illustrate the method by which data can be used in new ways to solve new problems. Finally, this author's views on possible areas that will see new developments in the next few years will be presented. These areas will provide a significant drive for novel advances in hardware capabilities.

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The treatment of complicated neutron transport problems may be simplified by use of the diffusion approximation. However, this may lead to errors. First in this paper a simple derivation of the diffusion equation is done, and the assumptions underlying it are discussed. Some improvements in the accuracy may be obtained by the choice of suitable boundary conditions, both for finite and infinite systems. Problems arising when taking the energy dependence into account are then discussed. A weakness of the diffusion approximation is its difficulty to treat inhomogeneous media accurately. Some suggestions are given for handling localized absorbers. Alsothe possibility to study a material with stochastic distribution of its absorption properties is taken up.

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During last ten years important change occurred in methods used for fitting curves to data, optimication of processes, estimation of solutions and stochastic functions or mapping from one space to other frequently of different dimensionality. From purely statistical methods we are frequently passing to the use of artificial intelligence methods based on biological models namely:
      - artificial neural networks based on brain functions, or
      - genetic algorithms relying on the biological systems evolution.
   Those methods start to be implemented in gamma well logging. The neural networks are shortly described and their applications are presented: for identification of well test model in a pressure transient test, lithology recognition from geophysical well logs and rock permeability estimation from well-logging data and others.

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By its 100-year jubilee (1996) the methods of nuclear geophysics have formed the information core of the modern set of logging methods applied to the solution of a wide set of problems in petroleum geology. Interpretational, algorithmical and  metrological systems of nuclear logging methods provide a wealth of information when applied in combination with other logging methods for the solution of such problems as lithological stratification, correlation of deposits, prediction of reservoir production capacity, lithologico-genetic and georhytmological analyses.

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The Earth Sciences have always been intensive and extensive users of Information Technology. The requirements to quantify as much and as precisely as possible environments and phenomena which consist basically of ill-posed problems strain even today the capacity of available computing systems.
   There are three aspects of data processing and IT in the earth sciences in general and geophysics in particular that need to be considered.
   Firstly, the requirement to describe and quantify physical phenomena and models, to explain measurements taken and derive properties of scientific and economic interest by some complex inversion and interpretation process.
   Secondly, the requirement to visualise results clearly and relate them as precisely and reliably as possible to a specific location in space and time, or statically and dynamically.
   Thirdly, the requirement to be able to forecast future states and events, so that particular actions can be taken.
   These aspects are discussed with examples taken from the areas of seismic processing, wireline logging and reservoir simulation, in order to show the necessity of integrating data from different sources, within a geological model, with the goal of deriving a reliable model of the subsurface for technical and economic decisions.
   The interplay between complexity of models and computer system performance is briefly investigated, from the two points of view of the use of high performance computer systems and desktop workstations, and the increasing importance of efficient and safe data storage is addressed.

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The Scientific Engineering Department AO "Tatneftgeofizika" has developed and tested the instrumentation-software complex for pulsed neutron-neutron logging (AINK-90NG) based on an essentially new generator monoblock ING-06 manufactured by specialists of the VNIIAvtomatika Institute. Preprocessing of signals is conducted in the downhole tool. Next, digitized data are transmitted to the surface and processed with a PC. High stability of the neutron yield and two-exponential data interpretation model allows one to assess parameter Σ_a with an accuracy of 5%. The instrumentation-software complex AINK-90NG can be applied at the oil-and-gas fields for solving the following problems:
      - lithology analysing of pay reservoirs and assessing their flow properties;
      - tracing the location of OWC and GWC in operating formations; 
      - determining the residue oil saturation in completely water-flooded reservoirs which were originally saturated with oil;
      - detecting hydrodynamic communication between perforated pay zones for the above/and below-lying water beds.
   The AINK-90NG complex is particularly useful when applied it in old well stock for detecting productive reservoirs missed because of an abnormally low formation pressure. To provide the most effective solution to this problem, it is advisable to combine the PNL method with the wide-band acoustic and spectral gamma-ray logs. Tests of the AINK-90NG equipment conducted in the oil fields of Tatarstan, both in terrigenous and carbonate deposits, have demonstrated its high effectiveness.

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Nuclear methods are now available to analyse dynamic processes, such as transport of water or oil in media, such as porous rocks. Notably, dynamic neutron radiography can be used to visualise these processes with good contrast and high spatial resolution. Both qualitative and quantitative information can be obtained.
   Dynamic neutron radiography of a porous sandstone from Sweden was carried out using a neutron beam generated from a research nuclear reactor. During the experiment, water was forced into a dry rock, under various pressures, as was oil into a water saturated rock. The dynamic images clearly show increasing flow rate with increasing pressure in accordance with Darcy's Law. These images also highlight the effect of small inhomogeneities in the flow structure.
   It is found that the neutron intensity versus distance from the flow source occurs in three distinct zones, and can be described quantatively. Quantitative analysis can indicate changing water concentration through a rock, and an approximation of porosity can also be made. Solutions, or approximations of solutions, of the non-linear flow equations in a porous medium can be developed with the use of these images. A tractable solution, involving a simple tanh(x) function, to the non-linear 1D fluid flow equation has been found by analysing the neutron radiography image of water flowing in the porous sandstone sample.

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Methods of mass analysis for geological samples under laboratory conditions using devices with laser-plasma ion sources are considered. Among them are laser mass spectrometry and pulse neutron flux analysis. The former is based on laser beam interaction with a sample in the vacuum chamber of a mass spectrometer. It permits one to carry out quantitative analysis in the mass range of masses 1-500 a.m.u. with the detection limit up to 10^-12 g. The range of chemical element concentration to carry out measurements is 10^-6 - 10 atomic percent, and mass resolution is (1-5)·10³. The latter method uses a powerful stable pulse flux of neutrons, generated by a compact accelerator with a laser-plasma deutron source. The neutron yield (T+D or D+D - nuclear reactions) of the device can reach 10⁹ n/pulse withe pulse duration within the range of 10^-8-10^-6 s, and the repetition rate of up to 100 Hz. Such a neutron source permits one to carry out sensitive instant activation analysis, for example, of a substance containing short-lived isotopes.

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Since the late eighties the semi-empirical method of the porosity neutron tool calibration has been developed by Professor Jan Andrzej Czubek. Now this method is in common use being treated as a main part of calibration procedure in Geofizyka Kraków. The full set of calibration charts can be obtained for the wide range of the rock parameters and the borehole conditions as a final result of the semi-empirical method. The ability of the smoothing interpolation and reliable extrapolation of calibration charts beyond the range of experimental data is the most important feature of this method. The method can be employed for all kinds of neutron tools such as single or double detectors, measuring either thermal or epithermal neutrons. A base for the application of the semi-empirical method was prepared directly by Professor Czubek or under his direct supervision during last months of his life.
   Presented paper concerns the works performed within 1996 and 1997 in Geofizyka Kraków towards the practical application of the semi-empirical method to the interpretation of PKNN3 neutron tool readings. A full calibration procedure is shown in the presented paper starting from the results of calibration measurement in the Zielona Góra Calibration Station, through the general calibration curve and through some correction charts, up to field example. Some comparisons to Halliburton tools are also presented.

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Detector response, i.e., registered count rate of pulses, is an interpretable quanatity received immediately from nuclear geophysics measurements. From count rate we pass on to physical, and next to petrophysical and geological parameters of studied rocks. 
   There is considerable interest to investigate dependence of ³He detector response on composition and properties of surrounding media, and in particular on the contents of anomalous neutron absorbers in rocks. The analytical expressions for count rate are obtained with efficiency of ³He detector in the whole energy interval of registration, depression of neutron field in the vicinity of the detector, and its physical and geometrical characteristics in view. The expressions used for spectra of thermal and slowed-down neutrons have enabled us to consider absorbing properties of rocks in a wide interval of their change. Total count rate of neutrons by bare ³He detector may be represented as the sum of two components corresponding to thermal and epithermal neutrons. The relative contirbution of the epithermal component to the total count rate depends on absorbing and scattering rock properties and detector characteristics.
   Experimental investigations of the response of the SNM-17 ³He detector depending on the content of chlorine and boron in water and water-bearing sand are performed. The measurements were carried out for both bare and filtered detectors. The ratio of count rates of epicadmium and slow (thermal + epithermal) neutrons considerably increases with growth of absorber concentration. Theoretical and experimental data agree quite well.
   Our studies may be used for estimating responses of gas-filled detectors (³He, BF₃) in boreholes, including investigations of rocks with anomalous neutron absorbers, and also for developing and improving techniques of neutron log interpretation.

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The paper presents results of Monte-Carlo calculations performed with the aid of the MCNP code for a neutron dual-counter probe used in formation porosity measurements. The Monte-Carlo modelling of the probe responses allows to extend the results of probe calibration obtained with the aid of calibration pits onto a wider range of such measurement parameters as borehole temperature and/or pressure, borehole-fluid salinity and trace concentrations of strong neutron absorbers. Some problems encountered when using the MCNP modelling tool are also discussed.

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The scaling procedure to establish the calibration curve (for the dual detector neutron porosity tool as well as for a single detector) was elaborated by Prof. Jan A. Czubek in 1992-1995. The general calibration curve was obtained for the whole range of porosity at any kind of borehole diameter, formation lithology (including variable absorption cross section and density of rock matrix), borehole and formation salinity, tool sand-off, and drilling fluid physical parameters. The phenomenon of the neutron transport was approximated by the energy two-group diffusion approach in a two-layer cylindrical region, i.e., the borehole and the geological formation. The calibration method has been successfully applied for the Polish ODSN-102 tool equipped with an Am-Be neutron source at the calibration facility at Zielona Góra.
   The paper presents the next step of a development of the theoretical base which gives a possibility to calculate the apparent neutron slowing down and migration lengths in the three-layer cylindrical system which represents the borehole, the intermediate zone (e.g. mud cake at the borehole walls), and the geological formation. A solution of the neutron diffusion equation is given for the three layer cylindrical coaxial geometry. The calculation is performed in energy two-group approach which distinguish the thermal and epithermal neutron fluxes in the media irradiated by the fast neutron point source. A comparison to the solution for the two-layer cylindrical geometry (which does not include a presence of the intermediate zone) is given in the paper. The influence of the intermediate zone is discussed.

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A four-section gas detector is designed for a multispaced neutron logging tool. The detector has a cylindrical structure filled with a gas agent common to all sections. It consists of four galvanically isolated cathode sections which have a common axisymmetric anode line. Depending on gas composition and high voltage the detector may operate in two modes: proportional and corona. The detector can be connected in two ways. In the case when signals are taken from the cathodes of the sections and transmitted to separate amplifier and data-processing circuits, the detector may operate both in the corona and proportional modes. If the signal is taken from the common anode, the detector may operate in the proportional mode only. In that case, data belonging to different sections are separated using a low-voltage bias of signals produced by different cathode sections. This method provides a reliable resolution of peaks of the resulting differential spectrum. In that case, a common amplifying and data-processing channel is applied.
   The detector proposed here provides enhanced measurement accuracy thanks to the elimination of noncorrelated variations in the responses of different sections.

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Two-spacing pulsed neutron-neutron log (2INNK) is presently one of the most effective technologies of logging oil and gas wells, including the cased ones. Basic geophysical parameters measured by the 2INNK log are the macroscopic neutron absorption cross-section (Σ_a) and bulk hydrogen content (W) or neutron porosity (limestone porosity Ø_por). Values of these parameters are used for solving a number of exploration and production problems in prospecting, exploration and development of oil and gas fields. Structural-functional flow-diagram of the 2INNK information-measuring system is discussed. Software designed for recording, processing, and interpretation of 2INNK logging data and assessing the logging data quality is presented here. Analysis results for the 2INNK error model, methods and tools for metrological verification, and a software system for metrological certification of the 2INNK log are discussed. Ways for improving the 2INNK logging tools and the measurement technique are considered.

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A high correlation exists between the time decay constant of thermal neutrons in the fixed sample-moderator system and the absorption rate of the sample. The correlation allowed to elaborate a fast method of determination of the thermal neutron macroscopic absorption cross section of the rock matrix. Numerous experimental results obtained by Czubek's method, i.e., the time decay constants measured in the two-region cylindrical sample-moderator systems and the absorption rates determined for the samples, were sufficient to be used as the data to calculate the calibration lines utilising the existing correlation. Every line has been prepared for a fixed size of sthe moderator. The regression analysis made for the linear model shows that a fit of the straight line to the experimental data gives very good results.
   An idea of the correlation method is as follows. For the investigated composed sample (i.e., the crushed rock matrix saturated with an absorbing fluid) only one time decay constant has to be measured in a sample-moderator system for a fixed size of the moderator. The absorption rate of the sample is determined from the respective calibration line. Next, a simple calculation is performed (the same as in Czubek's method) to obtain the mass absorption cross section of the investigated rock matrix itself.
   For every calibration line there are boundary values of the decay constant and of the absorption rate within which most calibrating points are situated. These boundaries indicate preferred intervals for applying a given calibration line and they help to choose the best one (that means, the size of the moderator) to measure with a high precision the absorption rate of the sample. 
   The results obtained by the correlation method for natural rock samples of different lithology from Polish deposits in the Carpathian Mountains are presented in the paper.

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Methods of nuclear magnetic resonance (NMR) constitute one of the most promising lines in the nuclear geophysics. These methods have a number of modifications: downhole NMR logging, laboratory NMR method (NMR in the Earth field, NMR in a high magnetic field, high-resolution NMR based on the Fourier transforms, etc.).
   The NMR methods have unique capabilities allowing to examine fluidal systems and fluid-saturated porous media both at molecular and macro levels. Among specific features of the NMR methods are their immunity to the magnetic and electric properties of rocks, polymictic composition of rocks, mineralization of fluids, etc.
   The NMR methods whose response highly correlates with the amount of movable fluid inside pores have been verified in various complicated geological, geophysical, and thermal conditions of the West Siberia oil and gas fields for the purpose of prompt identifying of reservoir formations, substantiating effective thickness of oil-and-gas saturated zones, and determining the saturation type.
   The laboratory NMR methods were applied to examine non-extracted large-size core samples, standard samples, cuttings, and drilling mud recovered from pay zones and promising intervals immediately while drilling. Data produced have allowed to assess the porosity, saturation type, and contents of free and interstitial waters, permeability and viscosity of fluids. 
   Results produced by the NMR methods were used as grounds for estimating a number of parametrers when assessing the oil and gas reserves of the West Siberia fields (Urengoi, Yamburg, Kruzenstern, Kharasavey, Talin, etc.).
   The advantages of the NMR - NMR Logging complex over other well logging methods and increased efficiency of NMR-based well logging are demonstrated.

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The absorption cross-section Σ_a for thermal neutrons is an important rock parameter used in evaluation of readings of neutron compensated well-logging tools in order to obtain the porosity of surrounding rock. As well known the rock matrix porosity in determining the hydrocarbon content and therefore the size of the oil or gas pool. Σ_a is difficult to measure even on samples. It depends mainly on microcomponents of the rock, e.g., the rare earth elements concentrations. We have earlier noticed correlations which exist between Σ_a and K, U, Th concentrations. The neural network backpropagation algorithm permitted to find a dependence (embedded in neural network weights) of Σ_a = f(K, U, Th, lithology). We will show neural network analysis based on a larger data sample from western Carpathian region of Poland.

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It has become clear that the knowledge of the macroscopic thermal neutron absorption cross section (Σ_a) and hydrogen/water content of the solid phase of the geological formation can improve the interpretation of neutron porosity logs significantly. The view is generally taken that a reliable value of Σ_a can be obtained only by a direct measurement. Therefore, many laboratories launched measurements of Σ_a for rock samples. On the other hand, the question of hydrogen content determination seems to be still underestimated. Hydrogen inherent in the solid phase of rocks is usually associated with clay minerals and, therefore, its estmation is mostly based on the petrographic  analysis.

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The paper gives an overview of the present possibilities of the Laboratory for the thermal neutron research dedicated to geophysics.
   Two measurement methods of the thermal neutron macroscopic absorption cross section are used it the Neutron Transport Physics Laboratory at the Henryk Niewodniczański Institute of Nuclear Physics. The pulsed neutron generator is used in both methods, the properties of the time-dependent thermal neutron field in small bounded systems are utilised in the interpretation.
   The zero dynamic material buckling method (Czubek's method) is a pulsed method in which the fundamental decay constant of the thermal neutron field is considered analytically as a function of the neutron parameters and geometry of the two-region system. The absorption cross section of the medium of interest can be determined when a set of decay constants for a given sample-moderator system is measured on the experimental set-up at the pulsed neutron generator. A special methodology is elaborated to measure the absorption cross section for geological samples.
   The correlation method elaborated in the Laboratory is based on a correlation between the thermal neutron absorption rate of the sample and the time decay constant measured in the two-region cylindrical system in a fixed geometry. The method is prepared to measure the absorption cross section of geological materials, especially of rock samples from boreholes cores.
   The experimental set-up at the pulsed neutron generator and procedures of preparation of the rock samples for the absorption measurement are described in the paper.
   Methods of calculation of the neutron parameters from the elemental composition of the material have been elaborated with computer codes. Code SLOWN calculates the neutron slowing-down parameters, and codes SIGSA and NEROTH determine the thermal neutron diffusion parameters. SIGSA calculates the parameters for homogeneous mixtures of compounds, including hydrogenous materials. NEROTH is directly oriented on rock samples and uses as the input data the elemental composition of the rock matrix, its mineralogical density, porosity, water saturation, brine salinity, and the density of hydrocarbons.

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Pulsed neutron logging (PNL) is an efficient method for determining water saturation and neutron porosity of formation through casing and tubing.
   Information is presented about new Russian apparatuses and technologies for pulsed neutron logging. The apparatuses use vacuum or gas-filled neutron acceleration tubes and ³He or NaJ detectors. Neutron tubes usew the T(d,n)³He reaction. In the vacuum tubes deuterons are produced from electrode spots of the arc discharge in vacuum. In gas-filled tubes deuterons and tritons are produced from a gas discharge in a crossed electromagnetic field.
   New efficient algorithms and software systems designed for analysing PNL data are proposed. These algorithms allow one to calculate thermal neutron capture cross sections of the borehole and formation. Perspectives of the PNL system optimization are considered.

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The complex of geophysical methods applied in the prospecting, exploration, and operating wells of West Siberia includes such nuclear logs as neutron, gamma-ray, and gamma-gamma (formation density) logs. Spectral gamma-ray logging is applied as a complementary method. For the purpose of quantitative geological interpretation of well logs of primary importance is an adequate petrophysical system.
   The ZapSibNIIGeofizika Institute in cooperation with the VNIIYaGG Institute has created a complex of nuclear-physical units designed for the core analysis: neutron and gamma-ray sounding and gamma-ray spectrometric units. Overall more than 10,000 core samples recovered from basic pay horizons of oil and gas fields of the West Siberia were analyzed by now. As a result we established basic petrophysical correlations between the total radioactivity and content of shale material in the Tyumen suite, between the summary radioactivity of uranium and thorium and shaliness of Lower Cretaceous deposits, between the hydrogen index of solid phase and total radioactivity in the deposits of the Bazhen suite. Produced are the reference distributions of total hydrogen index, total radioactivity, and content of natural radioactive elements. These distributions are applied for rating the response of corresponding nuclear logs. Diagrams showing the trend of  above-mentioned parameters with depth and diagrams showing their variation throughout the field area are built.
   Based on the nuclear physical analysis of core samples and analysis of nuclear logs we devised techniques for determining the porosity and shaliness indices, contents of organic matter and basic clay minerals. When combined with the electric and acoustic logs these techniques provide the more reliabe lithological stratification of a section, isolation of reservoirs and determining their flowing properties.

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Scintillation gamma-ray detectors are widely applied in the nuclear geophysics research. To decide upon the scintillation material and design of a detector for a specific application one has to look for a compromise between the desired and achievable parameters and take into account such requirements as the measurement geometry and detector dimensions, gamma-ray or X-ray energy range of measurement, measurement regime (total counting, spectrometry), operating temperature range, admissible shock and vibration limits, cost, etc.
   Over the last years a number of new scintillation materials were making their appearance in some applications. However latter are only of a limited use in geophysical tools where classical crystals made of NaI(T1), CsI(Na), BGO and few others are mostly applied until now.
   Special ruggedized, high-temperature assemblies for well-logging tools were designed by various producers. Some common design principles and characteristics of those detectors are discussed and the best Western and Russian designs are compared. Also the present state-of-the-art and capabilities of the Russian scintillation detectors in other field and laboratory geophysical applications are presented.

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Pulsed neutron logging (PNL) consists in irradiation of rocks surrounding the well by a pulsed periodic flux of fast neutrons generated by a portable accelerating tube. Detection of thermal neutrons is carried during pauses between the neutron pulses. Measured is the time distribution of thermal neutrons - the PNL response. At present the PNL response is considered as the sum of two exponentially decaying components, one of them related to the well and the other one - to formation.
   A complex structure of the "borehole" (tool - borehole fluid - casing) manifests itself in the shapew of PNL response being not a single borehole component but a superposition of several components, each of them related to specific radial zones. Proposed is the algorithm for estimating the number of exponentially decaying components of the PNL response. Basing on this algorithm it was established, for arbitrary variations of geometrical and neutron parameters of the radial zones, that the PNL response in oil-and-gas wells is consisting of two components.
   Two classes of algorithms for decomposing the PNL response into two exponentially decaying components are described. The immunity of decomposition results to statistical variations is ensured by a regularization procedure.|
   Procedures for estimating the thermal neutron capture cross-section of formation and the formation porosity are presented.

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Thin-layered formations can severely disturb neutron and density log responses, producing apparent anomalies being the result of smoothing caused by interaction of neutrons or photons passing from the source, through beds boundaries, to the detector. Ranges of the bed-thickness influence depends on the source - detector distance. It varies from 15 to 40 cm for density tools and from 30 to 70 cm for neutron tools. Classic calibration including measurements in various infinite and homogeneous media must be extended on measurements in thin layered models for determination of the vertical responsed function (VRF) - if a tool is the two-detector type, each of detectors must be analysed separately.