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9НАУКИ О ЗЕМЛЕ
THE PREDICTION MAP OF GEOELECTRIC SECTIONS OF
AUSTRALIA
Bashkuev Yu.B.
Institute of Physical Materials Science of the Siberian Branch of the Russian Academy of Sciences, Doctor
of Technical Sciences, Professor, Head of Laboratory Ulan-Ude, Russia Angarkhaeva L.Kh.
Institute of Physical Materials Science of the Siberian Branch of the Russian Academy of Sciences, Candidate of Physical and Mathematical Sciences, Assistant Professor, Leading Researcher Ulan-Ude, Russia Buyanova D. G.
Institute of Physical Materials Science of the Siberian Branch of the Russian Academy of Sciences, Candidate of Physical and Mathematical Sciences, Assistant Professor, Leading Researcher Ulan-Ude, Russia Advokatov V.R.
Institute of Physical Materials Science of the Siberian Branch of the Russian Academy of Sciences, Candidate of Physical and Mathematical Sciences, Senior Researcher Ulan-Ude, Russia
ABSTRACT
Predictive map of geoelectric sections of the Australia, necessary for calculation of propagation of VLF-MF radiowaves, is constructed. Taking into account the layered structure of the underlying medium, this map is capable of increasing the accuracy of electromagnetic field calculations by 1,5-3 times as compared to the Morgan-Maxwell map and ITU-R Recommendation P.832-2. The methodology of the geoelectric mapping is described. The studies of electrical properties of layered media by combined radio and geophysical methods in a variety of natural and geological conditions, and the proposed method of geoelectric mapping have resulted in the construction of a new generation of maps showing the electrical properties of the underlying medium that account for the layered structure of the crust and have no analogues in the world.
Keywords: geoelectric section, rocks, electrical properties, predictive map, surface impedance.
Introduction
The prediction of characteristics of excitation and propagation of radiowaves in low frequency radio range area is realized with the regard for electric properties of underlying medium, as a rule. Operational effectiveness of different telecommunication systems depends considerably on good knowledge of electric characteristics of underlying medium and it's time and space variations. Considering it, the background electric characteristics of large areas are of the great interest for practice because they let the researchers make the exact calculations of electromagnetic fields [1-4]. Electric properties of stratified underlying medium in low frequency area of radio range are regulary changed in space and depend on a type of geo-electric structure (depression, massif, fault zone) and belonging to this or that complex of crystal and sedimentary rocks. The method of prediction of stratified medium electric properties of uninvestigated territories is based on the classification of geo-electric structures and on quantitative results of diagnostics of territories-analogues (key-areas). It permits to make an goal-directed quest for areas with predetermined electric characteristics.
The R. Morgan-E. Maxwell's conductivity map of the globe [1] doesn't often satisfy the needs of practice, because it is composed on homogeneous underlying medium model. There are considerable differences with measured results in some regions, here at electric boundaries of the map don't coincide with real electric and geological boundaries. Therefore a creation of new prediction map of geoelectric sections of world's continents has been necessary.
Geoelectric mapping method
In 1971-2015 our laboratory worked out the basis of radiophysical diagnostics of electric state of the stratified underlying medium in the low frequency area of radio range. With the help of ground and remote (from plane's board) methods of radio-impedance sounding and profiling the general and regional regularities of space-time and frequency variation of Euroasia stratified underlying medium geoelectric characteristics have been established [3-5]. Electric characteristics of investigated regions are typical for continental crust. Surface impedance belongs to inductive area and it varies within the wide limits from minimal for salines to maximal for granitoids and glacial shields. On the basis of objective classification of geoelectric structures of
the Earth's crust and the interpretation algorithms of the radioimpedance sounding data the new effective method of physical-statistics prediction of the geoelec-tric sections of stratified natural media basic types are developed.
The methodology of geoelectric prediction is based on the following propositions:
1. There exists a regular connection between resistivity (conductivity) and lithoiogical, hydrogeo-logical, frozen ground parameters of mountain rocks, its composition and the peculiarities of the spatial var-yability of geoelectric section (GES).
2. The resistivity of the certain type of mountain rock is a lognormal value.
3. The contours of the areas with similar electric properties coincide with the geological boundaries, the thickness of mountain rocks is sustained.
4. Prediction is carried cut on the geological maps of basic mountain rocks and quaternary deposits with consideration for available resistivity sounding and boring data.
5. GES within the limits of skin-layer for VLF range (f = 10 kHz) in the general case is presented as 2- or 4-layered.
6. In the areas with relatively simple geological structure the prediction of GES with the accuracy, sufficient for practice, is necessary to carry out basing on the key-areas study (territories-analogues) with the following prognostic spreading of the characteristics on allied in geological respect territories.
By composing the prediction map of GES parameters we mean the determination of the area distribution of different types of GES, the evaluation of resistivity Pj and thickness hj of separate certain layers of the section on the whole scale of the map basing on limited quantity of the initial information. The problem is to determine the GES type (for instance px > p2 or px < p2) of the homogeneous area and its boundaries and statistic evaluation of p7 and hj parameters.
The information on GES maps is described as codes, determining resistivity Pj and thickness hj of the layers on the logarithmically-equal scale. The logarithm of the discretization step of pj and hj is equal to 0,333. The median values of Pj, hj are calculated according to step number N by the following formula:
Pj = ohm • m, N = 0 + 15;
hj = w0333(N-35) m, N = 1 + 15.
The dielectric permeability £j of layers according to the experimental data is equal to £j = 10.
Main results and discussion
The Australia GES mapping has been carried out on a little scale because of the experimental data limits. Under creating the map published works' materials on electric and radioimpedance sounding of Australia and New Zealand territories were used [6-9]. As the geological and topographical basis we used "The geological map of the Continents", scale 1:5000000.
Explanatory note closed to the prediction map of GES of Australia on 7,6 million square kilometers area contains:
- tables of the 19 founded geoelectric structures and frequency dependences of surface impedance in the 10-1000 kHz range;
- qualitative estimation of truth of the composed map;
- review of utilized materials about electric properties of upper part of Earth's crust of the investigated territories.
On the Fig. l the fragment of the prediction map of GES of Australia is presented. On this fragment the area distribution of geoelectric sections is reflected.
According to Morgan-Maxwell's map [1] Australia has 3 gradations of conductivity. The general background of Western and Northern Australia is 10-3 S/m, Southern and Eastern Australia - 10-2 S/m.
Fig. 1. The fragment of the prediction map of GES of Australia.
The normalized surface impedance of the «-layered medium is presented as S(n) = • Q(n), which is suitable to be computer calculated [3]. Here S± is homogeneous medium's surface impedance with the first
layer parameters; Q(n> = F(f,pj, £j, hj) is a correcting
Analysis of the modulus |5| and phase <ps o of impedance on GES map shows the considerable limits of its variations. So, at / = 10 kHz the values of |5| are changed from 0,0026 up 0,027, cps o - from -34° up to -51°; and at f = 1000 kHz the values of |5| vary from 0,019 up 0,18, cps o - from -26° up to -50°.
The maps of GES help us to determine an radio field attenuation function W in wide radio wave range with regard for relief and forest. The values of W for model of impedance multi-sectional radio path are calculated on the basis of the numerical solution of Hufford's integral equation [3]. The prediction mistake of field attenuation is ±(15^30)%.Prediction maps of GES are composed for a dry season. The conditions in upper layer of GES are changed in a year cycle, also there are the change of temperature distribution character and rock's structure, humidity, saline composition and phase state of water in rock. The mapping territories belong to the equatorial and the sub-tropic areas basically, for which an un-regular distribution of precipitations in a year is typical. The variation of electric condition of GES for these areas is conditioned mainly by mass-exchange (moisture-ion-transfer). The stratified medium model with properties and structure varying according to climatic conditions [3] allows to take into account season changes of impedance of boundary surface and near-earthly radio field.
Conclusion
The method of physical-statistics prediction of GES of stratified natural medium basic types has been developed. It takes into account physical processes in medium of different space-time scale. The methodology of small and large-scale geo-electric mapping has been grounded and elaborated in details. The prediction maps of GES of Australia have been compiled. In order to precise maps it is necessary to test the prediction by direct measurements of surface impedance.
Acknowledgment
factor taking into account lower earth's layers. On the Fig. 2 the frequency dependences of the impedance modulus |5| and phase (ps o are given for more spreading types of GES of Australia.
and (figo for typical GES of Australia.
This study was financially supported by the state budget project "Radio wave propagation in inhomoge-neous impedance channels".
References
1. Morgan R., and Maxwell E. Omega Navigation System Conductivity Map. Washington: Office of Naval Research, 1965.
2. ITU-R Recommendation P.832-2: World atlas of ground conductivities, 1999.
3. Bashkuev Yu.B. Electrical properties of natural layered media. Novosibirsk: SB RAS publishing house, 1996.
4. Bashkuev Yu.B., Advokatov V.R., Angarkhaeva L.Kh., Dorzhiev V.S., and Hayakawa M. Maps of geoelectric sections of Turkey, Iran, Afghanistan, Pakistan, Korea, and Japan // Natural Hazards and Earth System Sciences, 8, pp. 861-868, 2008.
5. Bashkuev Yu.B., Advokatov V.R., and Angarkhaeva L.Kh. Predictive Map of Geoelectric Sections of North and South America // Universal Journal of Geoscience, 1(2), pp. 84-89, 2013.
6. Thiel D.V. Surface-Impedance Changes in the Vicinity of an Abrupt Lateral Boundary at the Earth's Surface // IEEE Trans. Geosci. Remote Sensing, vol. 28, no. 4, pp. 500-502, July 1990.
7. Thiel D.V. A Preliminary Assessment of Glacial Ice Profiling Using VLF Surface-Impedance Measurements // Journal of Glaciology, vol. 32, no. 112, pp. 376-382, 1986.
8. Bibby H.M. Electrical Resistivity Mapping in the Central Volcanic Region of New Zealand // New Zealand Journal of Geology and Geophysics, vol. 31, pp. 259-274,1988.
9. Constabl S.C. Resistivity Studies over the Flinders Conductivity Anomaly, South Australia // Geophys. J. Roy. Astron. Soc., 83, no. 3, pp. 75-86, 1985.
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