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by Natalya KAPUSTYAN, Cand.Sc.(Phys.&Math.), Otto Schmidt Joint Institute of Physics of the Earth, RAS
It was back in 1944 when the great Russian scientist Academician Vladimir Vernadsky wrote on a philosophical note: "Noosphere is a novel geological phenomenon on our planet. Within it man becomes for the first time a major geological force... The face of the Earth is changing and its virgin nature is fading away."
"BIOGRAPHY" OF TECHNOGENIC EROSION
The role of erosion, including technogenic one. in the formation of what we call the face of the Earth has long been a commonly accepted fact. "The birth and death" of geological rocks and minerals and their transformations have preoccupied some of the greatest minds of all times, demanding an explanation. But the rather primitive experimental base available in the past, on the one hand, and the vision of the world as the result of the Act of Creation as well as the anthropocentric nature of the Renaissance science, on the other, precluded any reasoning on a geological time scale, with Nature being expected to produce quick and "forceful" actions. This approach was backed by seemingly more than sufficient evidence, such as earthquakes, hurricanes, volcanic eruptions, and so on. At the same time the need to have building materials, coal, ores and precious stones urged specialists to look for some order and regularities within our geological environment. At that time, however, people could hardly identify, and try to understand, any slow-going processes and generally tended to ignore any "weak" impacts upon geological formations (atmospheric effects and changes of temperature). Theories of evolution of the lithosphere were largely of speculative nature or else rested on the results of some chemical tests which bred all sorts of fantasies, such as the formation of sands, for example, being explained
as the result of salt precipitation from sea water, the ores being viewed as some "living entities" which "sprouted up" through rock and precious stones being of "male" and "female" nature, and capable of "breeding" progeny when in contact with water.
A meaningful insight into the scope of geological processes and their time scale called for a scientific mind of proper caliber, so to speak, and the first such person who noted the role of erosion in studies of river valleys, was Leonardo da Vinci (1452-1519). In the middle of the 16th century, on the strength of some natural studies, similar ideas were voiced by the German mineralogist Georgius Agricola (Bauer) and the French scientist Bernard Palissy In his dialogues - "Theory"and"Practice" - Palissy pointed out that rocks are being gradually eroded by water, wind and man.
That was the crossing line from which human activities came to be regarded also as a force with a geological impact. Later on, thanks to the steadily growing requirements and the advent of new technological developments, this anthropogenic impact continued to grow both in strength and scope. During the outgoing century, apart from such traditional objects as mines and quarries, cities and roads, dams and man-made lakes, large areas of land have been occupied by all sorts of industrial sites with powerful electric machines and equipment which radiate into the ground all kinds of mechanical vibrations. And there have been some really striking changes in the density and workloads of our motor- and railways and in the range and intensity of uses of our urban areas.
Today we are all aware of the erosive impacts upon our environment of all sorts of mechanical vibrations (such as those produced by industrial units and factories). Most of these are what we call "strong" oscillations which have to be taken into account by architects and industrial and building designers. And there are also some weaker mechanical vibrations which are inseparable from the "technosphere" of this century and which seem to be having no "damaging impact" on the environment such as the lithosphere to the depths of some tens of kilometers from the surface. On the other hand, however, this mechanism has been in action for decades on end - for no less than half a century
One of the "measures" of the scale of this impact is the size of the territory "sacrificed" to technology. The total length of railways in the world has now reached 1.5 min km, and the volume of rock used for the building of embankments which are directly exposed to vibrations is comparable to the present-day volume of river deposits.
We all regard our cities as the focal points of growth of our civilization. And this growth has been truly impressive, as demonstrated, for example, by the statistics quoted by Academician Viktor Osipov The first city whose population topped the one million mark at the start
of the 19th century was Shanghai, and by the end of the 20th century Mexico City boasted 22 min residents already In Russia, the rate of growth of the number of cities and towns runs ahead of the growth of their population. Today from 70 to 80 percent of our population are city dwellers. The area occupied by our capital Moscow is around 1,000 km2, but the impact of the megalopolis is felt on a much broader surrounding area.
Thus, a considerable area of dry land today is occupied by linear (roads and railways) and spatial (urban) handiwork of human activity all of which can be aptly described as giant vibrators which are constantly pumping mechanical energy into the earth crust. The ratio between this "noospheric" impact and the natural processes depends on the properties and activities of the lithos-phere which range from vigorous induced seismic effects to slow degradation of the environment-its gradual erosion.
ATTENTION: WEAK SIGNALS
Both natural and anthropogenic erosions of the upper lithosphere have been traditionally investigated by what we call the engineering-geological disciplines. The processes occurring within its interior are examined by mining experts: in rock excavation, gas and water filtration and mechanical impacts (shocks and vibrations). In the construction of hydrotechnical projects building experts take into account the ratios of statistic and dynamic stresses. Thanks to that a vast store of both theoretical and practical knowledge has been accumulated on the results of the human impact on the lithosphere: the intensity and nature of the processes involved, and the depth of their occurrence.
But there still remains a whole range of impacts whose role in causing some irreversible changes in deep geological structures still remains obscure. This includes what we call weak and protracted mechanical vibrations of the medium- frequency band (1-20 Hz). And while high-frequency oscillations are largely absorbed in the upper strata of the earth crust, seismic waves of greater length (such as in the above frequency range) penetrate much deeper, which is one of the reasons why they are used for soundings of the crust and the upper mantle.
Close attention to such weaker signals (comparable to, or much weaker than the "background" of the planet itself) as a likely instrument of seismic studies was given for the first time by the authors of the project Vibration Scanning of the Earth launched in the 1970s. The project headed by the Institute of Physics of the Earth (USSR Academy of Sciences) involved scientific and technical cooperation and included a chain of experiments some of which are still in progress under the direction of the Corresponding Member of the Russian Academy of Sciences Alexei Nikolayev One of the main ideas was that the traditional explosions used in such studies can be
replaced with non-explosive low-power sources which cause no damage to the environment and generate easily controlled signals. Using these sources, scientists "sent out" into the environment either a prolonged vibrosignal, or a chain of emissions. Over large distances the incoming response was "filtered out" from among the seismic background noise.
Now, what is the ultimate distance of registration, that is the possibility of signal transmission and reception with an amplitude less than the background noise of the planet itself? In search of an answer in 1983 a team of scientists staged an experiment in which they registered at a distance of 600 km from a vibrator monochromatic signals at frequencies of 2 and 4 Hz (amplitudes of soil displacement at observation points were 2-10^-10 and 5-10^-11 respectively). And now scientists of one of this country's biggest centers of vibroseis-mic research in Novosibirsk are picking up vibrosignals at a distance of 1,500 km. As an expert in this field Academician Anatoly Alexeyev points out, high- precision data can be registered at vibroprofiles of 200 to 300 km, including some deep-lying boundaries in the crust.
The problem of remote registration of weak vibrosignals is by no means accidental. The point is that what we call technogenic sources of this kind are similar to vibrators in the amplitudes and frequencies of their emissions. By analogy technogenic signals should be present not just in the uppermost layers of the lithosphere but penetrate throughout its depth.
GEOPHYSICAL MEDIUM: NEW MODEL
One of the main results of the experiments with weak seismic signals has been the development of a new method of studies of this planet. But the most important achievement is in the radical change of our notions on the properties of geological formations. Weak sounding signals of a certain shape have enabled scientists to visualize the "subtleties" of the geological medium: its non- linear properties and effects of the interaction of the interior fields with seismic emissions. The possibility of controlling the duration of a signal and its rhythm of emission has made it possible to observe the processes of establishment of a seismic field in the lithosphere and its "chaotization" with the changing natural stresses (geodynamic, tidal). Geological rocks in the upper part of the crust have long been regarded by engineering geology as a multi-component system consisting of a solid, liquid and gaseous phases and microorganisms. Today these notions can be extended to more deeper-lying "floors" of the hard shell of the Earth.
A generalization of the results of structural studies of the lithosphere in the 20th century and of the seismic regime data has made it possible to work out a new model of the geophysical medium. Studies conducted by Academician Mikhail Sadovsky and his colleagues have proved that the solid matter of the Earth consists of a hierarchy of components of different size with every level possessing different types of contact between its constituents. In this general picture the field of internal stresses is of complicated and mosaic nature, changing with changes in the system, with
nonhomogeneities acting as "concentrators" of stresses (such, for example, as borders of mineral grains, cracks and pores). Thus at the ends ofmicrocracks several millimeters in size the amplitude of a seismic signal can be boosted by tens and, given a certain mutual arrangement of cracks, by even hundreds of times. Even with minor variations of the fields of stress the medium can behave in different ways: either "dumping off' energy with some minor changes in the inner structure hierarchy (in the form of specific weak emission - seismic emission), or there occurs a growth of cracks within it which is accompanied by a marked energy release. What is more, the solid, liquid and gaseous components of the medium show a different "response" to dynamic impacts (including vibration - in the velocities of the response and in its directions. There can occur a transfer of matter, dissolution of salts or sedimentation, and so forth.
Thus the contemporary model of the lithosphere postulates a set of possible mechanisms whereby the seismic energy "pumped in" from technogenic sources is "processed" by it. As a result, the complex hierarchical system irreversibly alters its properties: there can be some "healing" of the defects of the medium-and its erosion as well.
TECHNOGENIC SIGNALS: DETAILS OF A PICTURE
Mechanical devices of various kinds generate three main types of oscillations: pulsed, random and those close to monochromatic (what are called "fine lines" or "peaks" in the spectra of microseisms). The latter are mostly generated by powerful electric machinery. The basic peak frequency (also registered are the harmonicas of the doubled, triple, etc. frequency) can be calculated by dividing the frequency of an electrical network (50 Hz) by a number equal to the number of the pairs of poles of an electric machine. Since the first parameter "is floating" in time, the seismic peaks "keep track" of these variations.
The first observations of the "fine lines" and the subsequent "boom" of the 1960- 70s when one searched high and low for them all over the globe helped detect a set of respective signals.
As it turned out, peaks of this or that frequency are present not only practically everywhere on dry land, but also on the ocean floor, on the surface and in the interior regions of the lithosphere (at a depth of 3 km). This factor demonstrates the "long range" of their sources.
Technogenic noises are part and parcel of the microseismic field of the planet and predominate in the high (over 20 Hz) and medium frequency (1-20 Hz) bands. Another and natural component of these same noises is produced by the surface - exogenic (wind, surf, cyclones) and internal - endogenic (seismic and acoustic emissions) sources.
In the 1950s James Brune and Jack Oliver of the Lament Observatory (USA) generalized data on the strength of seismic noises on dry land; that helped identify the "quiet", "medium" and "noisy" places. Our own observations of recent years conducted in appreciably different geological, geophysical and technogenic situations indicate that the power of microseisms in "noisy" places remains at the former level, and in places where there are no obvious sources of interference it has increased and is approaching the maximum level of the 1950s. Thus over the half of a century the number of "quiet" places on this planet has dwindled considerably This could be due to the "spread" of the technosphere over the surface of this planet and to the "long-range" of the sources which inject energy processed by the medium.
And now back to the technogenic peaks. Changes in the amplitude of seismic signals in the vicinity of industrial units such as a high-capacity pump of the Leningradskaya Atomic Power Station or turbogenerators of the Nurek Hydro Power Station - produced some minor shifts of soil of the order of 0.5-10-6 m; these are weak signals similar to the monochromatic emissions of seismic vibrators. Changes of their amplitude in time during the normal operation of the above units are less than 10 percent, which is slightly worse than with the special seismic vibrators, but much better than with all the other non-explosive sources.
Thus technogenic peaks can be used for the same tasks as vibrators in the monochromatic regime, i.e. for structural studies and for geological environment monitoring. Monochrome is especially convenient for the latter objective since during the signal reception there takes place what we call a vec-toral summing up of all the waves, and data on temporal changes in the environment are accumulated.
Generally speaking, the sources of technogenic peaks are the prototypes of seismic vibrators (even in terms of their design). And although the latter are a specialized tool of research (they are "tuned up" and repositioned), some of the problems concerned can be tackled with the help of sources of technogenic peaks. Their advantages are in continuous operation and in the possibility of being installed on sites with complex communications and certain specific restrictions (say, relative to nuclear power stations). It is also important that one does not have to install a source of emissions since the technogenic seismic signal is provided "free of charge".
And, what may the most interesting side of the matter, is that the technogenic source "irradiates" the same parts of the environment which are exposed to its effect.
TECHNOGENIC VIBRATION-ECHO IN LITHOSPHERE
Speaking about technogenic peaks, let us now take a closer look at out experience with sites on the grounds of the Leningradskaya Nuclear Power Station. Here "samples" of microseisms were obtained at various points, peaks identified and soil displacement amplitudes measured. Thereupon isolines of equal peak amplitudes were plotted on charts. Using these charts, researchers identified in the left wing of the station a source emitting simultaneously seismic signals at frequencies of 8.3, 12.5 and 16.6 Hz. When the amplitude charts were compared with soil charts the researchers saw that a "tongue" of partial flooding of the site made a nearly full circle around the source of vibration (with a radius of about 300 m).
The following explanation may be suggested. As we know, in the process of their operation vibrators first compact the soil and then produce a zone of
tensile stresses underneath. The presence of this zone (as proved, for example, by studies in coal mines) causes a migration of fluids with dust-like particles in a fissured-porous medium. Some of the particles are sedimented in the process, sealing up the transport channels so that the rate of filtration is decreased. In this way a technogenic source of vibration located on certain soils dries up the ground beneath.
Similar phenomena are observed in case of some random seismic signals, such as those "emitted" by automobile roads, for example. On loose ground, as proved by observations in Arkhangelsk, the ground water table drops steadily near such roads.
The above examples dealt with the effect of vibration on a limited area and only upon the uppermost layer of the lithosphere. And there are also other examples one of which demonstrates changes of the geodynamic regime within an area of about 5,000 km2 and at depths down to 30 km. This is true of the Nurek Hydro Station in Tajikistan which has a dam some 300 meters high and an artificial lake of 10.5 km3 of water, and all that located in a highly seismic-prone area. The operating regime of the station includes a seasonal water discharge (with the level dropping down by 70 m) for field irrigation. The construction of the station and the filling of the man-made lake induced seismicity in this region so that the number of tremors increased with a slight drop in their power and changes in the foci positions.
Academician Mikhail Sadovsky and his Tajik colleagues have established that a torrent of water flowing over the dam generates some mechanical "shake-ups" which take off some of the stresses building up in the resilient-elastic material (salt deposits to a depth of 5 km). Our own investigations have considerably expanded the picture of the processes involved. For the bottom layer of seismicity (from 5 to 30 km) we have been able to design a geodynamic model of the environment by means of studies of the seismic regime of the crust of the region and its monitoring with the help of remote expositions. According to this model the crust in this region is subdivided into several structures which differ by the type of the processes which occur in them and by the energy release-the magnitude of earth tremors and their sequence.
The most active structure is a narrow vertical zone located directly under the dam of the station - a kind of a "nail" or peg driven into the crust with its "cap" consisting of a fan-like pattern of fine ruptures. Along the stem of the nail seismicity moves up and down in step with the rising or dropping level of water. This is where most of the "impact energy" of the lake is "processed". At a high water table or its rapid alteration the vertical structure is unable to cope with the impact, and then the rupture-induced disturbances in the "cap" are activated
to generate a series of tremors in structures under the bed of the lake.
In this way the "nail" structure under the dam "triggers" seismicity and passes it on to the inlet of the lake! This effect also covers a powerful tension rupture which "sidetracks" seismicity to a distance of 50 to 70 km from the station where the tensions are discharged for good.
The above seismogenic structures came into being after the construction of the Nurek Hydro Station (late 1970s) and took body and shape in about 3 years. After that the region shifted to the aforesaid pattern which was basically different from the pattern ofgeodynam-ic processes before the start of the project. In this way anthropogenic interference-the building of a lake and mechanical vibration linked with the dam - have altered the natural pattern of events. In the decade that followed it was the vibration which "set the trend" for the local geodynamic scenario. Of course the main cause of these geodynamic changes are fluctuations in the water level, but mechanical vibrations provided a "contribution" of their own which determined the spatial pattern of the ongoing processes.
And here is one more example to demonstrate the effect of weak mechanical oscillations on the "life" of geological faults, but this time not in a seismic-prone, but in a quiet region. On the border between the Moscow and Kaluga regions, where there is a developed network of automobile roads, we found a convenient "fork" or crossing from which five highways branch out with traffic of different intensity, but with pavements of a similar type. Researchers engaged in the study measured the levels of microseisms, or ground unrest, along these roads in the absence of traffic, and in quiet and windless weather. Well, the roads happened to "remember" the conditions they had been exposed to - the levels of noises on them were quite different.
And it is also interesting to note that roads with heavy traffic are not the "noisiest" ones in seismic terms. In this respect everything depends on the structure of the upper layers of the crust. It is known that, in the absence of external
factors, one can "hear" the noise of the environment through microseisms, or ground noise, because of the changing stresses at formation fracturings in the crust. The pattern of the latter is reflected by a grid of lineaments (elements of relief visible on photos taken from outer space and linked with rupture processes). By superimposing upon this grid the map of automobile roads we were able to clarify the results of our measurements:
the density and direction of lineaments under the roads differed appreciably A comparison of similar situations indicated that the levels of microseisms increased inversely to traffic intensity
In the above case, like in the area of the Nurek Hydro Power Station, mechanical vibrations (in this particular instance from road traffic) relieve tensions accumulating in geological faults, and the level of seismic noises is thus reduced. A similar situation was observed in the Pribaikalye Region (near Lake Baikal) by our colleagues from the Institute of the Earth Crust of the Siberian Branch of the Russian Academy of Sciences: in the conditions of artificial mechanical "shake-ups" (by minor explosions, vibro and techno-genic signals) they measured tensions along the Angara fault of the crust upto the limit of detectable shifts*.
The above examples are but the first results of our investigations of relevant phenomena. Nevertheless technogenic vibration erosion of the lithosphere can be regarded as a new planetary phenomenon. The following lines of research in this field arc now in progress. Novosibirsk scientists have developed a method of assessment of the conditions of buildings and major structures (including the Sayano-Shushenskaya Station) exposed to microseisms, or ground unrest.
* See: S. Pshirkov, "R&D in Eastern Siberia", Science in Russia, No. 6, 1998,- frf.
Structures of this kind "transform" signals coming from the lithosphere and amplify them with their own oscillations so that the "weak spots" of the structure under investigation become detectable (the impact of microseisms upon buildings and structures can be compared to processes in geological rocks).
Similar studies are also under way in the Moscow area. Researchers of the Institute of Geoecology of the Russian Academy of Sciences have done a great amount of work tracing interconnections of the geological environment with the complex technogenic system of the capital. They drew up a large-scale map of what we call structural-geomorphologi-cal zoning of the city area and are conducting detailed studies of the hydrogeo-logical situation, including the geody-namics of subsurface river valleys. The GEON Center of the Ministry of Natural Resources of the Russian Federation is analyzing the seismological situation in Moscow, particularly, the levels of microseisms in various districts. All this provides a geologo-geophysical basis without which it is impossible to identify some fine phenomena, such as the impact of technogenic vibration erosion.
Another area of research covers studies ofendogenic seismic noises. Scientists of the Institute of Geospheric Dynamics (Russian Academy of Sciences) arc mapping out areas of geological environment activation by means of seismic noises and microtremors. Experts of the Physics Department of Moscow State University have been studying temporal variations ofmicroseism parameters. It was established that what they call "microscale seismology" reflects the geodynamic regime of the lithosphere.
Over the past 25 years scientists of the Joint Institute of Physics of the Earth (RAS) named after Otto Schmidt have been focusing their attention on the impact of weak vibrosignals upon geological formations. During this time they have amassed a wealth of data and experience in theoretical, modeling and on- the-spot studies (the above examples represent some of these results).
Today experts are sufficiently clear on methods of investigating the impact of technogenic signals on the lithosphere. These are known as "on-schedule soundings" (observations at fixed time intervals) using either "our own" technogenic sources (such as powerful electrical machines on site) or external sources - turbines of remote hydro-electric stations or some special geophysical equipment (vibrators, pneumatic machines). Researchers have already decided on registration instruments, data processing and interpretation and have built a flexible system of observations at various structures and in different geodynamic situations. In other words, the experimental set-up is ready
GEOECOLOGY OF THE 21ST CENTURY
Summing it all up, what are the most challenging and interesting aspects of the problem under review? First, we have to find out what elements of the lithosphere are most susceptible to weak impacts and what the rate of the induced changes is. And there are several possible scenarios in prospect. Here's one: among the multifarious defects of the environment irradiated by a vibrosignal, only the "resonance" elements can be picked up. The strongest impact falls upon the fissures, especially their ends, and the appearance of additional tensions thereupon can precipitate further growth of these formations until they exceed the "resonance" dimensions. Then the balance in the hierarchy of defects in the environment is upset; for its preservation either some of the fractures have to be "healed up", or the disintegration will continue up until the growth of the biggest ruptures therein. The place occupied by the growing fractures in the hierarchy will be filled either by smaller or new ones. To have a full understanding of the mechanism of these two opposite processes the emphasis in the investigations should be placed on experiments with rock samples and on-the-spot observations of ruptures or fracturing.
The next important question concerns the nature of "effective range" of a source of vibration. Corresponding Member of the Russian Academy, Sergei Krylov, put forward an interesting hypothesis which has since been developed by his followers. A layer of decreased velocity of resilient waves in
the crust (waveguide) is regarded as a kind of "trap" which accumulates seismic energy that escapes through rock fracturing or ruptures. This concept can also be applied to technogenic signals. It appears that the internal channels within the crust and the surface "waveguide" have been "mastered" by them: the energy accumulated there is passed on to the weakened zone. One of the facts in support of this mechanism is a sharp amplification of the underground sound with depth.
It is also important to elucidate the following practical question: how technogenic signals interact with elements of the relief or with surface geological formations (such as landslides). Over the past few decades man-made relief on this planet has grown considerably (including high-rise buildings, big residential areas and industrial sites). Since all of such structures possess resonance frequencies of their own, including those in the seismic band, the added impact of microseisms (including tech-nogenic ones) can cause an amplification of oscillations of certain frequencies by buildings. Good adhesion of a building foundation with the soil turns the building into a kind of vibrator which acts upon the subsoil and becomes a relay, or retransmitter, of seismic energy
All of the above examples show the importance of studies of weak technogenic emissions. Until now they have not been taken into account in the development of urban or industrial projects and units. At times the consequences thereof have been really deplorable. To give just one example - the installation of a traffic light at one of the exits from a road bridge across the Inya River in Novosibirsk (which was the cause of increased soil vibration produced by cars slowing down to a halt) practically ruined the whole bridge within 35 years since it was put into operation.
Lithospheric erosion caused by technogenic vibrations constitute a new ecological problem which is on our agenda in the new millennium.
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