Introduction.
1 Theoretical foundations of GPR.
1.1 Physical foundations for the construction and operation of subsurface sounding radars.
1.2 Restoration of the characteristics of the studied medium by georadiolocation methods.
1.3 Calculation of the radiation pattern of electric and magnetic dipoles at the boundary of two media.
1.4 Estimation of the maximum range (probing depth) of the radar and its resolution.
1.5 Influence of medium characteristics on the structure of the reflected signal.
2 Radar data processing.
2.1 The shape of the probing pulse.
2.2 The shape of the reflected signals.
2.3 Methods for detecting and estimating the parameters of reflected signals.
2.4 Methodology for developing a program for collecting, processing and visualizing GPR data.
3 Methods of ground sounding of natural environments and their practical application.
3.2 Methodology for selecting GPR characteristics for solving the task.
3.3 Technique of recognition of objects on the radargram.
33.4 Peculiarities of radio sounding of freshwater reservoirs from the surface of water and ice.
3.5 Experimental verification of the method of sounding water bodies.
3.6 Methodology for the use of georadar in field archeology.
3.7 Features of the use of GPR in construction.
4 Features of conducting experiments and processing data of subsurface probing of the soil of planets from the spacecraft.
4.1 Planning an experiment to probe the surface of Phobos.
4.2 Results of processing data from the Marsis orbital radar in the Mars Express mission.
Introduction to the thesis (part of the abstract) on the topic "Methods for conducting experiments on radar subsurface sounding of the Earth and terrestrial planets"
The relevance of the work
Subsurface radar is, apparently, the most productive and technological among other geophysical methods used to solve engineering-geological, hydrogeological, environmental, geotechnical and planetary problems. The areas of application of georadar are currently constantly expanding. With the improvement of equipment production technologies and the emergence of more productive algorithms and means for processing radar data, the georadar is becoming one of the most important non-destructive testing instruments.
In space research, at the moment, subsurface radio sounding is practically the only available means for determining the internal structure of space bodies. The main problem lies in the specifics of ultra-wideband (UWB) signals and ultrashort pulses used in the practice of subsurface sounding, which require the development of special methods for their reception and processing. In addition, the frequency ranges used in GPR do not allow the use of highly directional antennas due to restrictions on the optimal weight of the radar. Therefore, when developing equipment, one has to take into account many conflicting factors: the need to reduce weight and power consumption while increasing the potential capabilities of the radar; increase in the depth of sounding with a decrease in transmitter power; increasing the resolution without increasing the upper limit of the frequency range, etc. Thus, the development of ground penetrating radars and the related planning and conduct of experiments on subsurface radio sounding are an urgent task, both in the applied and purely scientific aspects of studying the environment, both on Earth and on other planets and satellites.
The theoretical foundations for the use of radio waves for the study of geological structures were laid by G. Lovy and G. Leimbach in 1910, and in 1912 they substantiated the possibility of searching for ores and groundwater by radio interference methods. The first georadar was created in Austria in 1929 to determine the thickness of the glacier. The radar got its name - RADAR in 1934, by abbreviation RAdio Detection And Ranging .
Reflections of electromagnetic wave impulses from subsurface inhomogeneities were accidentally discovered by A. Waite in 1957 at an ice airfield in Antarctica. Further experiments showed that, using a pulsed radar, it is possible to determine the thickness of the ice both from its surface and from the air. This led to research that showed the ability to see the lower boundary of not only ice, but also water, as well as to conduct subsurface sounding of soils.
In 1960, John C. Cook attempts to use radar to investigate subsurface reflections. Instead of the usual radar pulse with high-frequency filling, in radars he proposed the use of a carrierless pulse, which is sometimes called a monopulse or a video pulse. In this method, which is called the "impact antenna excitation method", a voltage drop is applied to the transmitting antenna, which forms a video pulse. This method of forming a video pulse is still used in georadar. Moffatt and Puskar were able to use the system they developed to solve several problems: detecting underground tunnels, mines, as well as detecting other changes in the ground, such as changes in humidity, detecting rock and rock formations under the ground surface.
7In 1967, a georadar system similar to Stern's ice radar was created, which was used in the exploration of the Moon on Apollo 17.
In 1976, Moffatt and Puskar presented one of the variants of the basic theory of GPR and its signal processing, which was developed and supplemented by other scientists who proposed more efficient methods for processing and analyzing GPR data.
According to foreign sources, georadar began to be widely used in the 70s of the XX century, primarily for military tasks, in particular, to search for local underground structures and tunnels in Vietnam. Soon, various companies became interested in applying GPR methods for their practical needs, such as mapping underground utilities and pipes under city streets, detecting voids and caves. Until recently, some types of ground penetrating radar have been used from aircraft carriers to map subsurface reflections under jungle and forest cover. Air-based radar has been used by archaeologists to explore large areas covered with jungle, such as research on and/or Yucatan in Central America.
In our country, the first experimental work with electrical prospecting methods using radio frequency signals was started in 1925 by A.A. Petrovsky. After a significant break, research on subsurface location was resumed in 1968 at the Radar Department of the Riga Red Banner Institute of Civil Aviation Engineers (RKIIGA) under the guidance of prof. M. Finkelstein. In 1989 The company Radar Systems, Inc. appeared, which was engaged in the development and production of ground-penetrating radars of the Zond series. Subsequently, they were quite successfully competed by the Oko GPRs of the LOGIS company and other Russian-made GPRs.
8In 1990, specialists from the Institute of Terrestrial Magnetism of the Ionosphere and Radio Wave Propagation (Troitsk) began developing the onboard radar of the rover in the Mars-94 space program. It was supposed to conduct research under the surface of Mars and search for water in a frozen form. In the future, the development experience was successfully used for the design and production of ground penetrating radars of the Trot" and "Loza" series.
At the Moscow Institute of Physics and Technology, work on subsurface sounding began with a study of the electrophysical properties of soils by a group of employees led by Professor Leshchansky. Subsequently, the staff of this group proposed a number of interesting solutions in terms of the development and creation of subsurface sounding radars.
At the Department of Seismometry and Geoacoustics, Faculty of Geology, Moscow State University, GPR has become a logical continuation of seismic studies due to the similarity of methods used for data processing. For the first time in our country, the department began to widely use GPRs of the Zond series for studying geological sections, gained extensive experience in complex geophysical surveys using GPRs, and developed multifunctional software for processing GPR data.
At the Moscow Aviation Institute, the main direction in relation to subsurface radar has been the use of methods for solving inverse problems and improving methods for recognizing images of hidden objects.
Since the mid-80s, in the remote sensing laboratory of the Moscow State Technical University. N.E. Bauman, a cycle of work was begun on the use of subsurface radars to obtain images of objects lying in the ground at a shallow depth. As a result of the performed research cycle, a method for multi-frequency sounding of condensed media was developed using subsurface radars with continuous signal emission. On the basis of the developed method, the RASCAN radar was designed and put into pilot production, designed for high-resolution sounding of building structures.
In the IRE RAS and SKB IRE RAS, Fryazino, work on the development of the theory of subsurface radar, the development of equipment and experimental research in this area began in 1980 in connection with the Mars-84 - Phobos project. The work was attended by specialists from RKIIGA together with specialists from the IRE RAS. To obtain depth profiles of the Martian satellite Phobos, the RLC-84 radar complex was created, which operated in the frequency ranges 300200, 150-90 and 4.7-5.3 MHz. Using one of the samples of the RLK-84 device, the employees of the IRE and SKB IRE carried out successful experiments to assess the thickness of the tuff in Armenia, detect a gas pipeline and measure the depth of seasonal thawing of frozen soils in Western Siberia. The first experiment was carried out with the installation of devices on a car, the last two - from a helicopter. These works continued as part of the Mars-96 project. For the Mars-96 mission, SKB IRE, together with IRE RAS, developed and installed on board the spacecraft a new radar RLK-M for subsurface radar of Mars.
In 1998, IRE RAS and SKB IRE RAS began joint development of the two-channel georadar "Gerad-2" commissioned by the Stavropol archaeological expedition. This georadar and the author's software developed for it served as a prototype for a series of georadar developed at IRE and SKB IRE.
A feature of the use of ground-based georadar is the predominantly contact arrangement of antennas with respect to the surface of the probed medium. This ensures a reduction in energy losses due to radiation: firstly, the air-ground reflection characteristic of remote sensing is excluded, secondly, the antenna radiation pattern is formed directly in the medium under study (the antenna directivity increases), predominantly in the near zone of the antenna (the spherical divergence of the wave can be ignored).
However, these same features are responsible for the shortcomings of GPRs. Firstly, since the environment and the objects included in it are in close proximity to the antenna system, they affect its parameters, respectively, the characteristics of the emitted signal are not determined (they change during measurements). Secondly, the calculations of the electromagnetic field in the near field of the antenna are quite complex, which makes modeling difficult and practically excludes the use of quantitative characteristics in the analysis of the received signal. In this regard, the issues of improving the algorithms for processing and displaying the received radar data and developing research methods in order to increase the practical impact of the method are of particular relevance.
Unlike ground penetrating radars, space-based radars explore dehydrated or frozen media - the surfaces of space bodies (with low radio signal absorption). Therefore, even with limited power consumption, their potential is sufficient to probe surfaces from large distances. Such large distances are often due to the peculiarities of setting up experiments in space: the orbit of the spacecraft, the impossibility of approaching the object under study less than a given distance due to flight safety, etc. Sounding from long distances requires the use of radars with a high signal-to-noise ratio. This is achieved by using signals with a long pulse duration - an improvement in the signal-to-noise ratio is achieved due to the large signal accumulation time during hardware processing. Basically, for these purposes, radars are used either with chirp modulation or with a step-by-step change in the carrier frequency, as in RLC-M.
If the range of distances during sounding varies from a few meters to tens (or hundreds) of kilometers, the use of pulses with a long duration becomes impossible, especially in predominantly single-antenna space-based radars (to reduce the weight of a space-based radar, one antenna is usually used, alternately working to transmit or receive a signal ). One of the engineering solutions to the "large range" problem was the use of radar systems. In such a complex, for each range of distances, a radar was used with optimal signal characteristics for it and the method of its formation. In fact, this led to the placement of several radars on board the spacecraft, which did not have the best effect on its weight characteristics. In the long-wave planetary radar DPR, another method is proposed: at short ranges, a radio signal with a pulse duration is used that provides the resolution necessary to solve the problem. At long distances, a complex signal is used, given by a combination of the above pulses. In this case, the resolution practically does not change (does not deteriorate), and the accumulation is carried out by increasing the number of simple subpulses in the emitted (and, accordingly, received) complex signal.
The purpose of the dissertation work is to solve the problem of setting up and conducting experiments on subsurface radar sounding of natural environments, developing methods for radio sounding the Earth's surface and the nearest celestial bodies of the Solar System, developing methods for collecting and processing measurement data when setting up specific experiments and interpreting the results obtained.
Realization of the goal is achieved on the basis of solving the following tasks:
1. Study of the dependence of dipole radiation patterns on the parameters of the medium and the size of the gap between the antenna and the surface of the medium. Analysis of the influence of medium characteristics on the amplitude and shape of the signal reflected from the subsurface.
2. Development of algorithms and creation of a software package for collecting and processing GPR data, taking into account the specifics of the formation and propagation of ultra-wideband signals.
3. Development of methods for sensing various natural media: terrestrial soils, water media from the surface of a reservoir and from the surface of ice, remote sensing of the soil of space bodies from spacecraft.
4. Conducting experiments on the detection and recognition of hidden subsurface objects (based on the developed methods and the created library of radar images of the most characteristic types of subsurface objects).
5. Development of a methodology for preparing an experiment on radar sounding of the surface of Phobos in the Phobos-Grunt mission, substantiation of the choice of a sounding signal, modeling of the reflected signal processing process taking into account noise. Approbation of the developed algorithms on experimental data obtained by the Marsis radar in the European interplanetary mission Mars-Express.
Provisions submitted for defense: 1. The created universal package of programs for processing radar data of subsurface sounding allows to increase the potential capabilities of the radar and improve the quality of interpretation of the results obtained.
2. The developed methods for preparing, conducting and analyzing the results of experiments on radar sounding of various natural media are applicable to solving applied problems in archeology, construction, engineering geophysics and other areas.
3. The developed methods make it possible to optimally plan and simulate the stages of preparation, implementation and analysis of the results obtained in experiments on radar sounding of the cryolithosphere of Mars and the surface of Phobos.
The reliability of the results of the work is ensured by the following statements:
1. The results obtained are consistent with theoretical calculations and, in particular cases, with the results described in the literature.
2. The developed methods have been tested in practice.
3. The results of the interpretation of experimental data in experiments on the detection of hidden subsurface objects are confirmed by excavation, drilling, or direct observation (extrapolation and interpolation from known positions of the object under study).
Scientific novelty and practical significance
A universal program for collecting and visualizing georadar data from a two-channel georadar in real time has been developed. With the direct participation of the author, for the first time a two-channel georadar for archaeological research was manufactured and tested.
Based on the analysis of the calculated radiation patterns of GPR antennas when radiating into light soil and water, it is shown that when carrying out GPR measurements, one should avoid detaching the antenna from the surface of the medium under study by more than OD of the radiation wavelength.
To interpret the results of subsurface sounding, a library of radar images was created for the most typical types of objects hidden in the ground.
For the first time, a method for probing the subsurface structure of the soil of Phobos with an organized phase-code-manipulated signal emitted by a radar located on board a spacecraft (SC) from the waiting orbit and from the landing trajectory is proposed.
Based on the analysis of ballistic and navigation data, an optimal scheme for the operation of the DPR radar in the Phobos-Grunt interplanetary space mission has been developed.
The results of the dissertation work can be used to develop and modernize programs for the collection and mathematical processing of radar data, to prepare and conduct experiments on subsurface sounding of low-absorbing terrestrial media and soil of terrestrial cosmic bodies, as well as to analyze and interpret the obtained measurement data.
The work was carried out at the V.A. Kotelnikov Institute of Radio Engineering and Electronics RAS (Fryazinsky branch) in the period from 1991 to 2008. The results presented in the dissertation are part of the radiophysical research conducted at the V.A. Kotelnikov FIRE RAS. The general management of these studies was carried out by Doctor of Technical Sciences, Professor N.A. Armand, Doctor of Physical and Mathematical Sciences. V.A. Andrianov and d.p.m.s. V.M. Smirnov.
The studies performed within the framework of this work correspond to the specialty 01.04.03 "Radiophysics", section 5 "Development of scientific foundations and principles of active and passive remote diagnostics of the environment, based on modern methods for solving inverse problems. Creation of remote monitoring systems for reo-, hydrosphere, ionosphere, magnetosphere and atmosphere. Radio astronomical studies of near and far outer space.
The results of the dissertation (software developed, including the Hilbert transform, inverse filtering, output of results at the processing stage) were used and are being used in the implementation of the ISTC project No. decimeter (UNB) ranges for surface and subsurface sounding of soils and vegetation. The project leaders involved the dissertation student to conduct experiments to study the possibility of using a georadar in studying fluctuations in the permittivity of soil under forest cover. The results obtained in these experiments are presented at all-Russian and international conferences. The obtained results testify to the prospects of using GPR for such purposes.
Publications on the topic of dissertation. The dissertation materials were published in full in 26 papers, including peer-reviewed journals included in the HAC list (3), other journals (2), collections of scientific papers (1), abstracts of reports at Russian and international conferences (20).
Structure and scope of work. The dissertation consists of an introduction, four chapters, a conclusion and a list of references. The work contains 163 pages, 69 figures and 4 tables.
Similar theses in the specialty "Radiophysics", 01.04.03 VAK code
Assessment of diagnostic capabilities of GPR systems of natural and natural-anthropogenic objects 2002, Candidate of Technical Sciences Yankovsky, Konstantin Petrovich
Improving the advanced control of a rock mass by radars during the sinking of underground mine workings 2002, candidate of technical sciences Izyumov, Sergey Viktorovich
Development and research of signal processing methods in problems of subsurface sounding by electromagnetic pulses 1998, candidate of technical sciences Tereshenkov, Dmitry Alexandrovich
Digital signal and image processing in a pulsed subsurface sounding radar 2004, candidate of technical sciences Tolmazov, Boris Borisovich
Pulsed ultrawideband forest tomography 2009, Candidate of Physical and Mathematical Sciences Klokov, Andrey Vladimirovich
Dissertation conclusion on the topic "Radiophysics", Marchuk, Vasily Nikolaevich
Conclusion
In this dissertation, the following results are obtained:
1. Calculations of the radiation pattern of antennas located at the interface between two media have been carried out. It is shown that the directivity pattern of radiation in air has a much lower directivity than in the medium (soil). It is shown that when the antennas are separated from the ground, the directivity increases, and the integral power of the signal radiated into the ground decreases.
2. Modeling of GPR signal propagation in a layered medium was carried out. The effect of the radiophysical characteristics of a layered medium on the amplitude and shape of the reflected pulse is studied. Based on the results of numerical modeling and experimental measurement data, a catalog of radargrams for the most common sounding objects was created.
3. A technique has been developed for creating algorithms for collecting and processing GPR data. Based on this technique, a software package for GPRs of the Gerad, DAO, GIR series has been developed.
4. Methods have been developed for preparing, conducting and analyzing the results of experiments on radar sounding of freshwater reservoirs from the surface of water and ice, as well as various natural media in relation to various applied problems: archeology, construction, engineering geophysics. The techniques have been verified experimentally.
5. A technique has been developed for preparing an experiment on radar sounding of the surface of Phobos in the Phobos-Grunt mission. The choice of the probing signal is substantiated. The simulation of the reflected signal processing is carried out taking into account the noise. Based on the analysis of ballistic and navigation data, an optimal scheme for conducting experiments for the DPR radar was developed. 6. The developed methodology for processing data from the DPR instrument was verified by processing experimental data obtained by the Marsis radar in the European interplanetary mission Mars-Express.
In conclusion, the author expresses his deep gratitude to his supervisor V.M. Smirnov for his help in carrying out this work, as well as sincere gratitude to the co-authors of the papers for a fruitful discussion of the results obtained. The author is grateful to the staff of the Federal State Unitary Enterprise SKB IRE for the development of equipment and assistance in conducting experiments on subsurface sounding.
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Subsurface sounding radar equipment is specially designed for customs inspection of a special category of objects: bulk and liquid cargoes located in railway cars, on platforms, in bunkers, tanks, containers, ship holds (for example, grain, crushed stone, ore, sand, vegetable raw materials, mineral fertilizers, timber).
WBC items can be hidden in the amount of the declared cargo in the expectation that it will not be unloaded and inspected by the customs service. Caches and hidden investments in objects of customs control that are large in length and volume cannot be detected by X-ray scanning. And the use of optical-mechanical means in these cases is ineffective.
That is why it was decided to create special technical means based on principles of radiolocation.
Location - a method of determining the location of an object using sound or electromagnetic waves. Radar uses radio waves(that is, electromagnetic radiation of the meter or decimeter wavelength ranges).
In technology, there are a large number of different modifications of the radar method. For the purposes of customs inspection of objects, the most suitable is the so-called active radar. Let's take a look at its principles.
The location is based on the following properties of radio waves:
The constancy of the speed of propagation;
Straightness of the propagation path;
Focusing of radio waves by antennas;
Reflection of radio waves from inhomogeneities of the medium encountered on their way.
A directed beam of radio waves is radiated into the examined medium. If an object with properties different from the medium is encountered on its path, then radio waves can be reflected at its boundary. Then part of their energy forms a reflected signal, which will be directed towards the radiation source. The presence of a reflected signal indicates the detection of an object in the environment. The reflected signal is recorded and the distance to the detected object is calculated from its delay time (with respect to the emitted signal).. During the delay time, the radio waves travel
distance to the detected object and back. If the speed of propagation of radio waves in the examined medium is known, then depth of the detected object H can be calculated using the formula:
where V is the speed of propagation of radio waves in the located medium;
∆ t- delay time.
Technical means of subsurface sounding
The customs authorities of the Russian Federation are supplied with a small-sized radio-technical device for subsurface sounding (RPPS) "Zond".
According to this order ZOND - a small-sized radio-technical device for subsurface sounding is an operational and technical means of customs control and is designed to detect contraband investments (metal and non-metal objects) in loose, bulk and homogeneous cargoes placed in the bodies of vehicles, railway cars (platforms) and holds ships 8 .
Such cargoes can be sand, crushed stone, ore, grain, vegetable and mineral raw materials, timber, etc., including in packaged form and in containers (bags, bales, boxes, etc.).
Cargoes consisting of metal products, scrap metal, as well as cargoes packed in metal (metallized) containers are not subject to probing. The composition of the device can be described by the following components:
1. Antenna receiving-transmitting device (UART) of the I range.
2. Antenna receiving and transmitting device of the II range.
3. Device for processing, control and indication (summer / winter version).
4. Folding rod - handle.
5. Rechargeable battery (capacity 1.2 Ah and 3.5 Ah).
6. ZIP-0 kit (individual).
7. Operational documentation.
9. Packaging 9 .
The main technical characteristics are:
1. Depth of sounding (detection), m not less than 3
2. Accuracy of determining the depth of the detected object, m not worse than 0.1
3. Resolution when detecting metal objects, m not worse than 0.1
4. The speed of movement of the UART over the surface of the probed load, m/s, not more than 1
5. Operating frequency, MHz 700.0 and 1200.0
b. Capacity of the internal storage device, MB not less than 3.5
7. Radiation power of the transmitting device, W
I range not less than 150
II range not less than 8
8. The duration of the probing pulse, not 1.5
9. Repetition frequency of probing pulses, kHz 25 - 50
10. Sensitivity of the receiving device, μV not worse than 300
11. Time of continuous operation (with two independent power sources), hour. at least 8
12. Operating temperature range, deg. C from -20 to +45
13. Weight of RPPZ (summer / winter version), kg 3.5 / 4.5
14. Service life, years 5 10
5. Introscopic technique: - - inspection x-ray equipment - inspection and screening complexes
Resolution of the radar- the ability to separately observe and measure the coordinates and movement parameters of closely spaced targets.
Let's allocate in the field of view four adjacent elementary volumes 1, 2, 3, 4 (Fig. 2.1, b), each of which contains one point target. Elements 1 and 2 have the same angular coordinates, but differ in range by , elements 1, 3 differ only in azimuth, and 1,4 only in elevation, and all targets are observed separately. We will reduce each of the quantities until separate observation of targets becomes impossible. Then volume 1 will be permitted volume and its elements, the resolution of the station is estimated in terms of range, azimuth and elevation.
Range resolution is estimated by the minimum distance between two targets located in the same direction, at which these targets are observed separately.
Resolution in azimuth is estimated by the minimum difference in azimuths of two targets with the same range and elevation angle, at which these targets are still observed separately.
Elevation Resolution is estimated by the minimum difference in the elevation angles of two targets with the same range and azimuth, at which these targets are still observed separately.
Speed resolution is estimated by the minimum difference in the radial velocities of two targets with the same coordinates at which these targets are still observed separately. In principle, it is enough to resolve targets in one coordinate or in speed.
Thus, the resolution of the radar is a tactical characteristic that determines the ability of the radar to separate closely spaced objects and their elements during their sounding.
1.3. Measurement accuracy of coordinates and parameters of objects
Measurement errors are divided into gross, systematic and random.
Gross mistakes- the result of a gross miscalculation of the operator or a consequence of a malfunction in the equipment. Such errors are much larger than other errors in a given series of measurements, and therefore they are easily detected.
Systematic errors are caused by long-term factors, such as signal delay for radar purposes or imperfection of the measurement method. Such errors are compensated to some extent by the calibration of the equipment.
Random bugs depend on random circumstances that are not amenable to preliminary accounting.
For example, 100 target range measurements were made using some hypothetical radio range finder. At the same time, random errors were distributed into groups according to Table. 2.1. Errors m were observed in 4 experiments, which corresponds to the frequency of errors, errors were observed in 6 experiments, i.e. their frequency, etc.
Table 2.1 - Distribution of random measurement errors by intervals
|
Error groups, m | ||||||||
|
Average value of error interval, m | ||||||||
|
Number of measurements | ||||||||
|
Error rate |
Mean Random Errors(dash - averaging sign) is equal to the sum of the products of errors, more precisely, the average values of the intervals, by their frequency:
The result obtained is inaccurate, since it differs from mathematical expectation. This is the name of the average error, which should be expected from the results of multiple measurements and which is calculated by dividing the error range X over infinitely small intervals. Only then will a strict regularity of random errors be revealed and their frequency will be called the probability of errors.
The data in the table are illustrated by rectangles with a base equal to the error interval and an area equal to the corresponding error rate (Fig. 2.2). When u, the areas of the rectangles express the probabilities of occurrence of a random error in a given infinitesimal interval of errors, and the vertices of the rectangles fall on a curve called error distribution curve. The ordinates of this curve (since the height of the rectangle is equal to the error rate divided by) are the derivative of the error probability with respect to its current value X.
The resulting derivative can be called probability density random errors
(2.1)
Rice. 2.2. Histogram and Error Distribution Curve
The area under the distribution curve, expressing the total probability of errors, equal to one. This amount is distributed continuously over all random error values. Consequently, the probability of occurrence of this error is infinitely small, but the probability density, as the limit of the ratio of two infinitesimal quantities, is a finite value. That is why the probability density is plotted along the y-axis of the distribution curve, and not the error probability itself.
Random measurement errors, like many other random phenomena in radar, arise under the influence of many independent and insignificant factors, the probability of occurrence of each of which is subject to normal distribution law:
(2.2)
where is the probability density of the appearance of a random variable X; - average value (mathematical expectation) of the value X; - variance of a random variable X; σ - standard deviation equal to the square root of the dispersion; e is the base of the natural logarithm.
If the measurements are made under constant conditions, then each value of the random error X appears with equal probability both with the sign + and with the sign -. Therefore, the distribution curve (density of probability distributions) is symmetrical about the y-axis and the average value, i.e. the distribution center of random errors is zero. Then
(2.3)
From formulas (2.2), (2.3) it follows that the quality of measurements is higher, the smaller the dispersion (scattering) of errors, since then the probability of large random errors decreases. In addition, the variance of the sum of random variables is equal to the sum of the variances of the terms, which is true not only for the normal distribution law. For example, let, under unchanged conditions, P independent measurements of any parameter. Then the random errors of each measurement have the same variance, which we denote as , and the variance of the sum of errors in P times more. The rms value of this sum. But the result P measurements is defined as the arithmetic mean of the results of individual measurements, so the standard error
i.e. with an increase in the number P experiments (measurements) of a certain value, made under constant conditions, the root-mean-square error decreases by a factor, since the scattering of random errors affects less and less.
Thus, radar measurement errors are divided into coarse, systematic, and random. The most difficult to compensate for random errors. To do this, you need to increase the number of measurements.
The invention relates to the field of radar sounding using single ultra-wideband (UWB) pulse signals and can be used when sounding several closely spaced objects, such as asphalt pavement layers. The method consists in emitting an N-leaf probing radio pulse, continuously receiving the reflected signal, integrating it N-1 times in the selected time window, detecting and evaluating signals from the objects of study. Achievable technical result of the invention is to increase the accuracy of the resolution of UWB sounding. 6 ill.
The invention relates to the field of radar sounding using ultra-wideband (UWB) pulse signals of duration T and can be used when probing several objects, the distance between which L is comparable to cT, where c is the speed of light in the medium, i.e. in conditions when the signals reflected from several objects of study are superimposed on each other. Such a problem arises, for example, when probing subsurface layers of soil, in particular, multi-layer asphalt pavement.
It is known, page 24, that any signal S(t) that can be emitted by an antenna must satisfy the condition:
Including a single multi-leaf probing UWB radar signal.
In UWB radar sounding of several nearby objects of study, the problem of resolving signals received from one and the other objects arises. This problem is exacerbated by the presence of interference, the imperfection of the transceiver equipment and many other factors.
The traditional way of preliminary processing of the radar signal reflected from the object of study is its detection - the extraction of a low-frequency function - the amplitude (complex) envelope of the radio pulse. When working with UWB signals, the amplitude envelope of a UWB signal, obtained using the Hilbert transform, does not always correctly reflect the features of its shape p.17. In this case, the potentially high resolution of UWB signals is not realized.
Known Patent RU 2141674 - a method of ultra-wideband radar sounding, which consists in emitting a pulse with one antenna, receiving this pulse with another remote antenna, the received pulse is delayed, re-radiated and received by an antenna located in the place of primary radiation. This method allows you to spread in time the signals received from the antenna and from the structural elements surrounding it. With this method, the problem of resolution is solved by the time diversity of the reflected signals.
The disadvantage of this method is the limited scope associated with the fact that the possibility of artificial separation in time of the reflected signals from several objects of study rarely occurs.
Closest to the claimed method is, which consists in the fact that they emit an N-leaf, probing radio pulse, continuously receive the reflected signal in the selected time window, detect and evaluate signals from the objects of study. To solve the problem of permission, determine:
The signal of direct transmission from the emitting to the receiving antenna (when probing open space), which is subtracted from the received signal during subsequent soundings of the medium;
Total reflection signal when probing a metal sheet, which is used to calibrate subsequent probings.
The forward signal is subtracted from the signal received from the research objects. Then, in turn, the closest response is detected, and taking into account the attenuation of the known total reflection signal, it is subtracted from the received signal. Thus, it is theoretically possible to resolve the received signals.
The disadvantage of this method is the low accuracy. First, the signal passing through the medium changes the frequency spectrum, and consequently, not only the amplitude, but also its shape. As a result, it turns out to be incompetent to use the total reflection signal as a calibration signal. Secondly, the recursive nature of processing, in which each new object is detected based on the results of the detection of the previous one, leads to the accumulation of errors.
The problem solved by this invention is to increase the resolution of UWB sounding reflected from nearby objects, and consequently, to obtain more and better quality information from radar sounding.
To solve the problem in a method for increasing the resolution of ultra-wideband radar sounding, which consists in emitting an N-leaf probing radio pulse, continuously receiving the reflected signal in the selected time window, detecting and evaluating signals from the objects of study, integrating the reflected signal in the selected time window N -1 times, and use the integration results to detect and evaluate signals from the objects of study.
The essential difference between the proposed method and the prototype is that when sounding with an N-leaf radio pulse, the reflected signal is integrated in the selected time window N-1 times.
The prototype uses the operation of subtracting known responses from the received signal.
The use of N-1 multiple integration - a linear method for converting received signals, allows you to convert their multi-lobe temporal structure into a single-lobe. Figure 1 shows that the three-lobe radio pulse after a single probing becomes two-lobe, and after the second integration - single-lobe. If such a pulse could be emitted by an antenna, then the problem of resolving nearby objects would be greatly simplified. Integrating the received signal for a linear system is equivalent to integrating the input signal. Thus, integrating the output signal greatly simplifies the resolution of nearby objects.
The inventive method is illustrated by the following graphics.
Fig.1 - the results of the successive integration of the three lobe signal.
Fig.2 - partial signals reflected from three objects.
Fig.3 - the total signal reflected from the three objects.
Fig.4 - the result of a single integration of the reflected signal.
Fig.5 - the result of two-fold integration of the reflected signal.
Consider the possibility of implementing the proposed method.
When radar sounding can be used single radio pulses with a small number of time lobes N=2-5, such as three-lobe pulse S(t), shown in Fig.1. Such signals have a UWB spectrum. Their processing is possible in the frequency or time domain. In both cases, it is necessary to detect the signals reflected from the objects of study, to evaluate their amplitude, polarity, temporal position, and other parameters. Such soundings are used, for example, in the study of pavement layers. In this case, the objects of study are the boundaries of the coating layers, which reflect the probing signal and have different permittivities ε. Depending on the ratio of dielectric permittivities ε of the media, the reflected signals can have different polarities.
If the objects of study (layers of the road surface) are located close to each other, then the reflected signals are superimposed on each other. Figure 2 shows partial signals S 3i (t), (i=1, 2, 3) reflected from three different layers. Each of them has its own amplitude and shape. Signal S 32 (t) has reversed polarity. The total reflected signal S 3 (t)=S 31 (t)+S 32 (t)+S 33 (t), figure 3, of little use for analysis. To solve the resolution problem, it is possible to reduce the duration of the probing signal S(t), but this will lead to an unjustified increase in the development cost or to technical unfeasibility.
Single integration of the signal reflected from objects
Figure 4 does not solve the resolution problem, but reintegration
Fig.5, allows you to fairly accurately assess how the temporal position, polarity and amplitude of the reflected signals. Said score can be obtained visually or using a computer.
Note that using the proposed linear transformation, the restoration of the ratio of the amplitudes of partial signals and the distance between them is possible even in the case when the signals are delayed relative to each other for a time less than the duration of the period of the central harmonic of the signal spectrum, i.e. in the conditions of realizing the potential range resolution .
Thus, the proposed method allows for UWB radar sounding to detect objects of study, approaching the potential resolution.
Consider the possibility of practical implementation of the proposed method. Figure 6 shows a diagram of a device that implements the proposed method, where:
1. UWB signal generator.
2. Transmitting antenna.
3. Receiving antenna.
4. Investigated multilayer medium.
5. Stroboscopic receiver.
6. Controlled delay line.
7. Analog-to-digital converter (ADC).
8. Computer.
The signal from the computer 8 starts the UWB signal generator 1, which is emitted by the antenna 2. The UWB signal reflected from the studied multilayer medium 4 enters the antenna 3. The delay line 6 controlled by the computer 8 triggers the stroboscopic receiver 5, which selects one instantaneous amplitude of the reflected signal. Analog-to-digital converter 7 converts this value into a code that is readable by computer 8. The start frequency of generator 1 can be tens of kilohertz, which does not require high speed ADC 7. The delay value 6 sets the reception window and the position of the reference point in it. By repeatedly repeating the measurements, one can average the values of this reflected signal count, and by changing the delay value, one can obtain the entire realization of the reflected signal in the selected time window with an accuracy up to the scale-time transformation. Thus, as a result of multiple probing, the memory of the computer 8 stores the instantaneous amplitudes of the reflected signal in the reception window. The integration of the obtained digital readings is carried out by successive summation of the readings, and multiple - by the successive application of this procedure. In Fig.1-5, the abscissa shows the numbers of samples of the UWB signal. The obtained integration results can be processed visually by the operator, or by known processing methods in the computer 8.
Thus, the proposed method is technically feasible and allows you to increase the resolution of ultra-wideband radar sounding.
List of used literature
1. Astanin L.Yu., Kostylev A.A. Fundamentals of ultrawideband radar measurements. - M.: Radio and communication, 1989. - 192 p.: ill.
2. Patent RU 2141674.
3. Patent FR 2626666.
4. Theoretical Foundations of Radar / Ed. V.E. Dulevich. - M.: Sov. radio, 1978. - 608 p.
Claim
A method for increasing the resolution of radar ultra-wideband sensing, which consists in emitting an N-lobe probing radio pulse, where N=2, 3, 4, 5..., continuously receiving reflected signals in a selected time window, detecting signals from objects of study, measuring and evaluate the parameters of the signals reflected from the objects of study, characterized in that the probing of the object of study with an N-leaf radio pulse is carried out repeatedly, when receiving the reflected signals with a controlled delay value, a reception window is set with the ability to receive the entire implementation of the reflected signal in the selected time window and the position of the reference point in it, integrate the received samples of the reflected signal in the selected time window of reception N-1 times, converting the N-lobe temporal structure of the signal into a single-lobe, providing resolution of closely spaced objects of study, use the results of integration to detect objects of interest ion, measurement and evaluation of the parameters of signals from the objects of study.
5.1 Methods of field work.
Field work 1980-86 on radar research of peat and sapropel deposits were carried out with the participation of the author and according to the method developed by him /23/. The experience of radar profile surveys developed at RKIIGA and sounding used at AARI was taken as a basis.
The method of radar research provides for the integration of two independent methods of subsurface radar, namely: the method of radar sounding and the method of radar profiling. The difference between these methods is that profiling is carried out along the survey route with a constant distance between the receiving and transmitting antennas. Radar sounding provides for a gradual increase in the spacing between the antennas in opposite directions from the point of study. Probing, in essence, is an electrical prospecting analogue of the SEM.
Radar profiling is the main method of exploration for peat deposits. In practice, two modifications of the method are used: continuous and discrete radar profiling. Continuous profiling is carried out by moving the antenna systems and the device along the survey route evenly, without stopping. For transportation, a GAZ-71 caterpillar conveyor or a Buran-type snowmobile is usually used. On small, up to 500 meters, areas of the field, it is possible to conduct continuous profiling using a portable modification of the S-023 georadar. Discrete profiling is used only when it is impossible to use vehicles, it consists in measuring the times of arrival of reflected signals and recording them at certain points of the research route. In this case, the profiling step is usually 10-25 meters.
The sequence of work in the exploration of a peat deposit using radar equipment is as follows.
At the starting point of research, it is necessary to set the depth to the main reflecting boundary, which is the bottom of the deposit, as accurately as possible, identify the reflected signal visible on the locator screen with it, and calibrate the device. To do this, it is necessary to carry out mechanical sounding of the deposit from sampling the underlying soil, if the mineral bottom of the peat deposit is composed of dense sandy or sandy loam deposits, and there is no smooth transition from peat to the underlying soil, then the result of mechanical sounding should be taken as the depth of the reflected boundary. If sapropel is present in the bottom layer of the deposit, or there is a smooth transition from peat to the underlying soil, then radar sounding should be carried out at the starting point of the study.
Radar sounding is performed as follows: the receiving and transmitting antennas are installed on the surface of the deposit a meter apart. After recording the received signals on magnetic tape, the antennas are simultaneously spaced in the opposite direction from the center, at a distance comparable to the depth of the deposit bottom. At the start and end points, the time of arrival of the reflected signals will be measured.
The propagation velocity of electromagnetic waves (V) and the depth (h) of the reflecting boundary is calculated by the formulas:
| V = [(x 2 2 -x 1 2)/(t 2 2 -t 1 2)] 1/2 |
| H \u003d 1/2 (t 1 2 (x 2 2 -x 1 2) / (t 2 2 -t 1 2) - x 1 2 ) 1/2 \u003d 1/2 (t 2 2 (x 2 2 -x 1 2)/(t 2 2 -t 1 2) - x 2 2 ) 1/2 |
where:
x 1 and x 2 - the distance between the antennas,
t 1 and t 2 - time of arrival of the reflected waves.
When determining t 1.2, it may be difficult to identify the signal reflected from the mineral bottom. For preliminary identification of reflections, one should use the data of manual drilling and calculate the time of arrival of the reflected wave using the formula
Or use the conversion tables t (ns) to h (m).
After determining the distance to the reflecting boundary and the speed of wave propagation in the deposit, it is necessary to calibrate the device in terms of depth and start working on the method of radar profiling of the peat deposit. During the movement of vehicles with a georadar installed on it, a continuous recording of the received signals is carried out. Places of intersection of the route with characteristic forms of relief, as well as the planned binding, are fixed on a magnetic tape by briefly switching on calibration marks. Each inclusion of labels is recorded in the observation log.
Calculation of the average depth of a peat deposit and raw material reserves from radar data requires control over changes in the propagation velocity of electromagnetic waves in different parts of the deposit. To do this, it is necessary to create a reference network of direct measurements of the depth of the reservoir or radar soundings.
It is most convenient to combine the points of the reference network with sampling points. In addition, when creating a core network, it is necessary to be guided by the following provisions:
- there should be at least two reference points per typical peat deposit.
- Along the profile or highway, reference points should be located at least 1000 m apart,
- Reference points should be located in places with different depths of the deposit,
- With a strong variability in the depth of the deposit, the points of the reference network should be located in places with a horizontal position of the bottom of the deposit.
The movement of the all-terrain vehicle along the peat deposit is carried out along the routes planned in accordance with the instructions for the exploration of peat deposits.
The methodology for exploration of sapropel deposits is carried out in winter, similar to the methodology described above. During the summer field period, radar sounding is replaced by pulsed electromagnetic logging.
The technique of field studies of peat and sapropel can be used practically unchanged for subsurface radar of other geological media. But before starting field work, it is necessary to evaluate the capabilities of the available equipment /3/. This can be done based on the calculations given in Section 3. Nomograms have been developed by the author specifically to assess the limits of radar capabilities for geological research. Figure 5.1 shows a nomogram that allows, based on the duration of the probing pulse and the speed of propagation of electromagnetic waves, to determine the value of the "dead zone" of the locator (H).
The length of the probing pulse in nanoseconds along the vertical is plotted on the lower scale of the nomogram - the length of the "dead zone" in meters. The nomogram is built on a logarithmic scale for various velocities from 3.35 to 30 cm/ns and covers all cases of velocity changes that can be encountered in the study of geological media. According to the main nomogram (solid lines), Nm can be determined for the location of the receiving and transmitting antennas at one point. When using antenna diversity, the size of the "dead zone" increases, because. the difference between the arrival of the direct and reflected waves decreases. To introduce corrections for antenna spacing, an additional family of dashed lines is constructed. The corrected values for the depth of the "dead zone" are shown on the upper scale.
The nomogram should be used as follows. Let the duration of the emitted pulse be 60 ns, the studies are carried out in a medium with V=3.35 cm/ns (rel=80) and antenna diversity is used for 2 m antennas. On the OX axis, we find the point 60 ns and restore the perpendicular to the intersection with the straight line with V=3.35 cm/ns. The ordinate of this point is the value H=1m. The corrected value of Nm is found by the abscissa of the intersection Nm=1m. And a dotted line with an index of 2mU on the upper scale of the nomogram. In the considered sign, the final value of the depth of the "dead zone" is 1.8 m.
The nomogram is universal and can be used to convert time sections into depth ones. For this, instead of imp. Take the delay time tref. of the reflected signal relative to the moment of emission. The range resolution is defined similarly.
The actual values of the dead zone and range resolution and may be lower, because radar profiling is more informative than single observations of the wave pattern.
The maximum range of subsurface radar can be estimated from the nomogram in Fig. 5.2. To do this, it is necessary to know the value of the specific attenuation of radio waves in the medium under study (dB/m). If there are no measurements on samples, the value of specific attenuation should be estimated from the literature or use the graphs for converting ohm to dB / m (Fig. 5.3, 5.4).
The nomogram in Fig. 52 takes into account the influence of only attenuation and geometric losses on the range of location and is calculated for various values of the locator potential in decibels.
As an example of the use of a nomogram, we will give an estimate of the maximum depth of radar sounding of a peat deposit. Average values of radar sounding of a peat deposit. The average values of the electrical resistivity of high-moor peat fluctuate around 100 Ohm, which corresponds to a specific electrical attenuation of 4 dB/m. The potential values of the radars developed so far are 80-100 dB. But the nomograms give a depth of 5.5 to 15 m, which is in good agreement with the data of field measurements. For lowland peat with attenuation greater than 5 dB/m, the maximum sounding depth is less than 5 m. type, carried out according to the existing methodology, are possible only in rare cases.
Such a limitation on the depth of the method negatively affects its practical usefulness, since low-lying peat deposits and sapropel deposits. Having similar electrical properties, as follows from the consideration of the nomogram, perhaps in two ways, namely? By increasing the potential of the georadar and reducing the frequency of probing pulses.
An increase in the locator potential is possible primarily through the use of more powerful generators of nanosecond pulses. But this is due to an increase in the dimensions of the generator and the power it consumes. Although a 200 kW generator weighs about 1.5 kg and consumes 30 W, this is not suitable for wearable GPR modifications. for equipment options carried on the conveyor, weight and dimensions do not matter, which makes it possible to use powerful generators. The currently developed pulse generators can develop power measured in gigawatts and increase the locator potential by more than 60 dB, i.e. the problems of increasing the depth of research can be solved in this way.
Another, more preferable, method is to reduce the average frequency of the probing pulse from 80 MHz to 10-20 MHz. But this causes an increase in the "dead zone! And a decrease in the range resolution, which becomes equal to 1.5-1 m. Technically, it is easier to reduce the frequency of the probing pulse. As follows from Section 4, it is enough to increase the effective length of the antenna for this. Although the lengthening of the antennas somewhat tighten the requirements to the generator and will require special signal processing, this method is more convenient.
The given nomograms make it possible to quickly determine the possibilities of using the available radar equipment for solving specific geological problems. And in the absence of a positive decision, determine ways to modernize the equipment or evaluate the possibilities of using other GPRs.
