Glikman A.G. - "About seismics".

Material source: http://www.newgeophys.spb.ru/en/article/report/index.shtml.

Перевод: Терещенко А.С.

 

About seismics

            Anyone investigation method, including, and the geophysical method can be effective only in the event that it is based on quite concrete physical effect. And on the contrary, if the basis of a research method is not observed experimentally this method it will be inevitable to develop in a deadlock direction.

            In other words, potential ability of a geophysical method is in rigid dependence on as far as real properties of a used physical field are taken into account.

            But it was so, that the most important in geophysics the method - seismic prospecting - was created without taking into account real properties of a field of elastic fluctuations in solid materials, and in a basis of this method there was just such, nonexistent effect. Poisson has offered idea echolocation in massif as model of seismic prospecting. He thought, shock generates an elastic (sound) pulse which should be spread in all directions in massif and also be reflected from borders taking place in terrestrial thickness according to laws of geometrical optics.

            In the beginning of XX century, approximately in 70 years after Poisson formalized his idea, experiments with realization of seismic works was begun. However from this time, and down to our days nobody saw echo-signal as well as primary outgoing sound pulse.

            Instead of an expected echo-signal which as presumptive, should have the same kind and the same spectrum, as well as a primary pulse, seismic signal from the very beginning was long, slowly fading oscillatory process of unfairly big amplitude. Long oscillatory process is observed in immediate proximity from a point of shock influence, not allowing to see a primary probing impulse. All this did not correspond to the mathematical description of all processes which, on presentation of scientists, should occur at seismic works.

            In a usual situation when results of experiment do not correspond to an initial hypothesis, it is necessary to find the reasones of this discrepancy. In this case it has not taken place. Furthermore, at the beginning of XX century it was declared, acoustics of solid materials and, a part of it, seismic prospecting as the science, has finished the development as any situation in acoustics can be described by mathematically.

            It was very strange declaration. To declare end of cognition of any area is a nonsense as the knowledge has no the end. And as to acoustics of solid materials here the similar application and in general should be perceived with humour as a minimum. The matter is that in this area of knowledge till now there is no fundamental position which could be proved experimentally.

            The constancy of speed of movement of elastic waves in homogeneous materials, and even presence of those or other types of elastic fluctuations can not be proved. In acoustics of solid materials any sensor of basic parameters of a field of elastic fluctuations is not created till now. And it means, that any of the arguments that are included in the equations, describing a field of elastic fluctuations, cannot be determined experimentally. But the mathematical equation only in that case concernes to physics if its arguments can be determined in experiment. Hence, from the point of view of methodology, using of the mathematical equation for the description of a field of elastic fluctuations, incorrectly. The consequence of these moments is the neither acoustics of solid materials as a whole, nor seismic prospecting - are not in the competence of metrological services.

            During 1909-1910 years, at the same time, when it was declared end of development of acoustics of solid materials,, has almost simultaneously taken place epoch-making events in destiny of seismic prospecting. Despite absence an echo - signals, and also that the amplifiers did not exist yet, was made a number of the opening which have put seismic prospecting at once on height, inaccessible for any other geophysical methods. By means of seismic prospecting Mohorovichich, Konrad, Golitsin, Gutenberg surfaces and a number others which are taking place on depths from 10 up to 800 (!) km were open. On the basis of these discoveries the really global conclusions were made: about a liquid condition of a nucleus of the Earth, that the earth's crust under continents is thicker, than under the seas … Similar statements are very convenient: they can not be checked up.

            The origin of long oscillatory process which is observed at seismic prospecting, was perceived as result of an interference between set of reflections from set of borders deposited in terrestrial thickness. Geophysics began to count this oscillatory process a noise which prevents detection of an echo - signal. And the main tendency of development of seismic prospecting became struggle against this noise. Method of struggle - perfection of the equipment, and then, programs of processing.

            To have money to perfection of the equipment, it was necessary to present seismic prospecting as an effective geophysical method. And consequently the second tendency is a creation of system of the falsifications directed on presenting seismic prospecting as an effective geophysical method. For this purpose seismic prospecting apply only then when already there is a geological information. It can be received with the help of prospecting drilling or (and) with the help of various geophysical methods, and in the report disappears, that this information was already known at realization of seismic prospecting.

            Thus, the opening of many deposits which were open with the help of other means, is attributed to seismic prospecting.

            So, all forces were thrown in these two directions, and directly the physics of formation and spreading of elastic fluctuations are away from interest of seismic prospecting. And therefore, probably, nobody paid attention to a spectrum of this long oscillatory process, that is, directly, seismic signal.

            When in 1977 we have carried out research of spectrum of a seismic signal it was found out, that it have image as a damped harmonic motion, and is more often, several damped harmonic motions. It very much an important point because any interference can not result in occurrence of such signal.

            Physicists, as against mathematicians, are not free in their own actions. If the mathematician, describing mental model, can accept any hypotheses concerning this model, physicist is obliged to move forward in dependance of results of already made experiments. Therefore the researcher who is finding out, that reaction to impulse looks like damped harmonic motion, can not conduct the further work differently any more as on the following, quite certain logic.

            Only the oscillatory system can transform an impulse to a fading harmonic process. Hence, the first, that it was necessary to do after detection of a similar sort seismic signal - is to search, what physical objects act as oscillatory systems. This problem(task) was solved rather quickly, in 1977 when it was found out, that parallel-sided objects almost from all solid materials, including geological structures have this properties. Besides it was found out, that a fading harmonic process is spread along this parallel-sided structure-resonator but not a initial pulse.
If presence of oscillatory system is revealed, the following step, that it was necessary to make was to find conformity between characteristics of geological structures and their properties as oscillatory systems. This problem also was solved in the same 1977 when empirical dependence between thickness h parallel-sides geological structure and own frequency f0 this structure as oscillatory system was found out:

h=k/f0 (1)

            Disclosure of this dependence started the spectral seismic prospecting, that is a method, is allowed to define thickness of geological layers that are in terrestrial thickness, on the basis of a spectrum of seismic signal.

            Further, it was necessary to find out, first, what is factor k, has dimension of speed and why this factor is marvellously constant for all types of rocks the value equal 2500m/s with a deviation from this value, not exceeding 10%. It was very much an important point because all questions connected to kinematic characteristics of a field of elastic fluctuations, are key. According to directories, there are no such speeds of spreading of elastic fluctuations which would have so identical value in various rocks. On the contrary, as it is specified in these directories, even in the same stratum the speed of a sound can change in some times.

            Besides it was very important to understand, due to that there is a transformation of short impulse impact to a harmonic signal. Development of the theory of spectral seismic prospecting was impossible without understanding of physics of this phenomenon. It was possible to answer all these questions approximately in 1982 when already there was an application of the first generation of the equipment of spectral seismic prospecting. It was the coal mine equipment for forecasting stability of rock of a roof "Resonance". Essential role that we could answer these questions, has played found out then effect of acoustic resonant absorption (ARA).

            In physics it is not much effects of resonant absorption - ferromagnetic, paramagnetic, electronic, gamma-resonance … Now to these already known fundamental effects it is possible to add and acoustic resonant absorption effect.

            I would like to remind, that the physics is first of all, is sum of physical effects and the phenomena, and each of them is equally important for development of physics. Each of them has value for process of knowledge irrespective of as far as it is appeared understandable at its detection. As known, many of known physical effects for today are perceived on only phenomenological level, however it does not prevent to use them. Resonant absorptions take a special place among physical effects because by their help there is a understanding of other effects and the phenomena. Because effect ARA was found out, it was possible to find out sense of factor k in the formula (1) which is a speed of shear waves Vsh. And thus, has arisen at last a metrologically correct way of definition of speed Vsh. And the formula (1) has the next kind now:

h=Vsh/f0 (1’)

            Thus the physical sense of the shear waves consists in the following.

            The shear process is an imaginary part of a field of elastic fluctuations. And at result there were understandable some effects are observed at seismic prospecting. In particular, extremely low attenuation of elastic waves at their spreading along geological structures. Here the remarkable analogy to an electromagnetic field turns out.

            The electromagnetic field is characterized by real and imaginary components. The real part is responsible for active losses - heating, mechanical work. The imaginary part is responsible for spreading of a field. Attenuation of this (imaginary) part of a field is very unsignificant.
It is known, for example, that wireless enthusiasts with the help the one-watt transmitter communicate on as much as possible distances, till to opposite points of the Earth.

            Precisely as occurs and at spreading of elastic waves. Seismic signal (it always has image of fading harmonious process) is formed by imaginary part of field (by shear waves), and attenuation of this waves is rather unsignificant. That is all seismic signals, are accepted various seismographs at seismic prospecting, are generated by imaginary, shear waves. But if the elastic wave arising as a result of shock influence is spread not in all directions, but only along rocky layers the echo-signal comes not from depth reflections of an elastic wave but from border of a rocky layer.

            Once again we shall note, that in result of shock influence (at seismic prospecting) the initial pulse is not spread in all directions. The own fluctuations which have arisen in each, separate, in elementary case, horizontal geological structure are spread under laws of geometrical optics, but only within the limits of these geological structures-resonators, in a horizontal direction. And when at realization seismic works they speak that the echo-signal is received from any depth, not so. The received signal is really an echo-signal, but from vertical border of that parallel-sided geological structure in which the given, concrete oscillatory process has arisen.

            It can be checked up very simply. If displacement of seismic cable to any side lead to temporal changes of the moment of arrival of an echo-signal for a while, proportional to this displacement, that, obviously, it is necessary to come to a conclusion that the echo-signal comes not from depth, but from some side. But if the echo-signal comes not from below, but from sides it becomes clear why seismic cut never corresponds to a real geological cut.

            As a result of representation of a field of elastic fluctuations as sum of real and imaginary components and the revealings of that fact, that in terrestrial thickness are spread the own elastic processes generated by an imaginary component of this field, it is possible to see very deep analogy between acoustics and electrodynamics. When on replacement the electrical of a direct current electrodynamics came and the electrical of a direct current has come as its rather insignificant part of electrodynamics. Also and generally accepted acoustics with its only mechanism - an interference - is included as a particular and little part into complex acoustics what is spectral acoustics (naturally, and spectral seismic prospecting).

 

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© ДонНТУ, Терещенко Алексей Сергеевич, 2009