by Yuri TSVETKOV, Dr. Sc. (Phys. & Math.), department head, RAS Institute of Terrestrial Magnetism, Ionosphere and Radio Waves Propagation, and Nina ROTANOVA, laboratory head of the same Institute
With all of the indisputable and truly impressive achievements in the studies of plutonic geodynamics, many questions connected with the inner structure of our planet still remain unanswered. And one can hardly expect easy and quick solutions to these problems because penetrating directly into the bowels of our planet is something well-nigh impossible. Therefore, of great importance today are what experts call geophysical studies, including seismological, magnetotelluric and other similar studies based on the interpretation of geophysical fields. But, strange as this may seem, our notions of the rock composition and processes occurring in the deeper layers of the earth crust remain largely of hypothetical nature. One of the most "productive" geophysical approaches within this general context-and its potential is far from being exhausted-are studies of the magnetic fields of the earth.
ANOMALOUS GEOMAGNETIC FIELD
There are 3 kinds of geomagnetic fields generated by different sources. The first, and the main, or normal one (from 20 to 60 x 10 3 nTl) is generated by currents in the liquid part of the core of the earth. The patterns of its lines of force is traced by our compasses. The second-variable, alternating one - is produced by currents in the ionosphere and magnetosphere. Its typical manifestation are magnetic storms. For the temperate latitudes its diurnal solar variations are around 30 nTl, and of its disturbances (magnetic storms ) 100- 1,000 nTl. And, finally, there is the third anomalous field which owes its existence to the magnetism of the earth crust all through its thickness (~ 40 km) with the deeper rocks being nonmagnetic. That is why this anomalous magnetic field is the most instructive among the physical fields of the continents and oceans for our studies of the earth crust. Its mean value is about 200 nTl, exceeding 1,000 nTl in major magnetic anomalies (such as the Kursk magnetic anomaly of 10 3 -10 4 nTI).
This seemingly fundamental problem has, however, some very tangible applied aspects to it. The plutonic structure has an important part to play in the distribution of minerals in the surface layer of the earth crust; what is more, in a not too distant future mankind will have to delve deeper and deeper into the earth crust in search of mineral raws.
But how should one investigate that anomalous field? For its studies we need rich experimental material obtained in the course of ground, aeromagnetic and satellite studies. In aeromagnetic surveys, due to low flight altitudes (0.5-3 km), surface sources of magnetic fields suppress signals emanating from the deeper core strata. Besides, such surveys are usually conducted on limited areas, at different levels and at different time. Thus it is difficult, if possible at all, to intertie all of these data and obtain a regional map reflecting the fields of plutonic sources. The thing is that anomalous magnetic fields are not stationary and have what experts call age, or secular track, variation.
A notable contribution to the studies of the anomalous magnetic field has been provided by satellite surveys.
Starting from the 1960s, the data obtained by the Soviet COSMOS-49 and -321 satellites and the American POGO-2, -4, -6 and MAGSAT (1980) has appreciably altered our notions of the spatial structure of anomalous fields.
In studies of the structure of the lower part of the core by magnetic methods it is important to have data obtained from probes operating at altitudes of 20-40 km and equal to the vertical thickness of the earth crust. In this case signals from depth are not suppressed by surface sources' emissions and the anomalous magnetic field undergoes the normal process of filtration and formation of its structure. And here Mother Nature herself has intervened on behalf of the scientists. The point is that at altitudes above 20 km there exist in the earth atmosphere stable air currents along the parallels. And an ideal flying machine fitted for flights in such conditions is a free floating balloon.
French researchers, for example, prepared and carried out an almost round-the- world flight of a balloon equipped with a magnetometer from South Africa to Australia (across South America). And the Japanese have been conducting such flights around Antarctica. And even though these missions by French and Japanese scientists were specially designed for studies of anomalous magnetic fields, their practical value, to our mind, has been minimal. Using but a single magnetometer it is practically impossible to solve the problem of separation of magnetic fields and single out an anomalous field reliably.
So, how can one obtain the picture of an anomalous magnetic field in its "pure form"? And how can one isolate from it the variable and the main, normal fields? We, on our part, suggested using two sensors placed on board one and the same balloon, but operating at different altitudes, fathoming both the field itself and its gradients. Since the sources of a variable magnetic field are located a great distance away from the measurement level, it is having the same effect on both these sensors. And since the sources in the earth crust are brought closer, we get a difference of magnetic fields, the gradient. The main magnetic field, however, can be singled out by mathematic modelling. If the main field differs from the model one by about 20 nTl, then its spatial gradients practically fit the model ones. This makes it possible to identify the anomalous magnetic gradients from the fields determined in the stratosphere, which helps their reliable geological and geophysical interpretation.
MEASURING MAGNETIC GRADIENTS FROM BALLOONS
We at our Institute have accumulated a wealth of experience on gradient magnetic surveys from balloons and this experience is unrivalled anywhere in the world. We passed the stages of measuring vertical magnetic gradients by two magnetometers - first on the base of one and later, of two kilometers. We have developed for the first time in the world, and cleared for operation on board big stratospheric drifting balloons, a system of three magnetometers spaced four and later six kilometers apart. And we used a reliable method of automatic deployment of the system during balloon ascent.
In the operating condition the airborne magnetic gradientometer is a system of three autonomous proton (nuclear-precession) magnetometers (based on hydrogen nuclei precession in the earth's magnetic field), evenly spaced in the vertical direction and tugged at altitudes of 26, 28 and 30 km.
The main problems of using such systems are associated with their descent and especially with lifting the intermediate container-sensor package. In this experiment it is not necessary to place instrument containers into the balloon basket. This makes it possible to find an original solution for the system of automatic deployment of the gradientometer. And during the landing, separate parachutes can be used for the main pack and its retractable part. The coordinates of the balloon in flight are measured by a device consisting of a receiver and a storing device, or accumulator, of navigational data.
The anomalous magnetic field abounds in puzzles. In recent time experts in various countries have been focusing on long-wave (from 500 to 3,000 km) magnetic anomalies. So far we know but little about their sources, although the existence of such anomalies, according to balloon and satellite data, is out of the question. A number of balloon flights have been conducted across Russia
with one of the first ones carried out back in 1975 near the Kursk Magnetic Anomaly. On that occasion just one proton magnetometer was used. In combination with satellite and ground measurements, the night demonstrated how the anomalous field weakens depending on the altitude of measurements. At altitudes of about 700 km it amounts to 3 nTl only even over the Kursk Anomaly.
The 1980s saw a significant breakthrough in the conduct of experiments of this kind. Balloons, equipped with two and then even three proton magnetometers, were launched from Kamchatka in the western direction along the 56 parallel. These nights made it possible to measure not just the field itself and its vertical gradient, but variations of the latter along the vertical. The rate of dwindling of the anomalous field was calculated in the upward direction from the source.
Now, a question arises about the numerical values of the anomalous field, its vertical gradient and the errors involved. We demonstrated that the root-mean- square value of the vertical gradient of the anomalous field all along the route from the Kamchatka to the Urals amounts to 2.5+/-0.3 nTl/km. And the anomalous field itself is estimated there at about 50 nTl with an error of 20 nTl. The latter is determined by an error of the main field model. Errors introduced by the alternating magnetic field are small thanks to the correction in which we used data of magnetic observatories located along the
flight path of the balloons. Thus the anomalous field itself and its vertical gradient are significant values which are appreciably greater than their measurement errors, and the vertical gradient is assessed more precisely than the field itself.
And now we have approached the most difficult problem in studies of anomalous magnetic fields - the problem of the nature of magnetic anomalies. Practically all methods used for the identification of the sources of magnetic anomalies are based on the results of their spatial spectral analysis. It has been conducted using anomalous magnetic field profiles obtained at near-ground, stratospheric and orbital altitudes. The dynamic characteristics of the spectra were determined by the method of narrow-band filtration based on the identification of harmonic components with the help of adaptive filters. The methods of spectral analysis, based on adaptive filtration, make it possible to obtain not only unidimensional, but also two-dimensional and even three- dimensional pictures of a series under examination in the spectral area. One can see not only characteristic periods in the spectra, but also their variations along the whole length of the profile. Long-period changes L = 500-3,000 km are conspicuous at all altitudes. What is more, magnetic anomalies with such periods have maximum intensity not all along the profile. These three areas are identified: 60-70 0 , 120-140 0 , 150-160 0 E, which relate to the regions of the Urals, the Aldan shield and the Sea of Okhotsk plate.
INTERPRETATION OF MAGNETIC ANOMALIES
A new and unexpected solution was found for the presentation of the anomalous field in the frequency area with the help of wavelet-analysis. Today this "mathematical microscope" can give us not only the general structure of a signal under investigation, but makes it possible to study its local features. Within the structure of the coefficients, different inhomogeneities are apparent: small-scale (4-5 0 ); longitudinal (10-12 0 ) and, finally large-scale (20- 30 0 ).
The energy density distribution plot shows seven large-scale details which characterize anomalous magnetic fields. Their comparison with the tectonic map of the region indicates that they reflect actual tectonic structures.
One of the major problems involves determining the depths of the upper and bottom edges of the magnetoactive layer. We shall cite the spectral analysis data on the borders of this layer from what we call the balloon profile of the magnetic field as measured over the territory of Eastern Siberia. These generalized data represent a statistical assessment of the depths of anomaly sources.
If one measures a magnetic field at different altitudes, as in the case of balloon gradient surveys, the magnetic anomalies obtained will also contain information about the depth of their source. Thus for the two magnetic anomalies of the Vitim plateau, while calculating the depths of their sources, we used the measured values of the field and its vertical gradient in which the attenuation factor remained unchanged within the range of altitudes under consideration. That for both anomalies this condition is found to be observed, with the depth of the bottom edge of the magnetic field being about 32 km. This is in good agreement with the results of spectral measurements. The new approach made it possible to plot a depths profile for the sources of individual magnetic anomalies. The degree of its correlation with the profiles of different geophysical fields can provide answers to many outstanding questions, such as the role of the structural and thermal peculiarities of the earth crust in the formation of the lower border of the magnetoactive layer.
Wavelet-transformation of anomalous magnetic field profile:
top-wavelet coefficients distribution;
on the vertical-scale factor;
According to satellite data the entire dynamics of the anomalous field is focused in the bottom section of the diagram;
in the upper section-two large-scale details;
below-energy density distribution;
seven large-scale inhomogeneity characterizing anomalous magnetic fields which reflect real tectonic structures.
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