Gravimetrics
Abnormal gravitation field. Abnormal density
 Ukraine |
 Abnormal gravitational field of the territory of Ukraine |
 Central part of Ukraine on the Ukrainian Shield |
Abnormal gravitational field
Abnormal gravitational field. Abnormal Earth crust and upper mantle thickness.
Earth gravitational field or field of gravity force – is a force field caused by Earth gravity and centrifugal force as a result of the Earth axial rotation. It is conventionally divided into abnormal and basic-normal parts, calculated according to the distribution formulae of acceleration of normal gravity. Abnormal part of field is considerably smaller in magnitude, has complicated structure and reflects Earth form characteristics and its subsoil inhomogeneity. On a map abnormal gravitational field structure is shown by the similar size lines of its values, which are called isoanomalous lines.
Abnormal gravitational field changes in wide ranges in connection with great diversity of Earth crust and upper mantle material thickness distribution along the lateral when natural gradient stratified increasing at the same time with depth; in some regions thickness inhomogenuities take place in the upper mantle also. Gravitational field morphology and intensity visually in the first place reflect thickness distribution characteristics in the subsurface Earth crust part.
In the Western Ukraine (Precarpathian bending), where sedimentary strata is thick and bounder crust-mantle (or the Mokhorovichich discontinuity) is immersed greatly, minimal magnitude of the field in Bouguer reduction (up to -100mGal). To the West from Carpathians Moho discontinuity rises sharply, and field becomes positive (up to +30 mGal). In the north-western part of the territory, where the sediment thickness reaches 1-3 km, the field is weakly negative (about -10-20mGal).
Central part of Ukraine is situated on the Ukrainian Shield. Field level there is weakly positive. And granitic massifs are characterized by the subzero field (except anomaly in -30mGal over the massif in the region of Novoukrainka-Kirovohrad), and basic materials and granulites are characterized by positive field magnitude, the most intensive of which is (higher than +60 mGal) are observed in the region of Golovanivsk. On the south East of the Shield great positive anomalies are made by the ferruginous structures in the regions of Kryvyi Rih, Dnipropetrovs’k, Nikopol’, Zaporizhzhia, Melitopol’, Mariupol’.
In the Eastern part of Ukraine the Dniprovs’ko-Donets’kyi avlakogene (DDA) is situated. The field there depends on correlation of sedimentary strata effects and basic Devonian material implanted along the axis of the avlakogene. In general, between the depressions due to the sedimentary strata thickness decreasing lifting of the Moho discontinuity is observed. High field is observed in Donbass (higher than +40mGal), where thick sedimentary strata materials are very metamorphized and have high thickness. From the North DDA is characterized by the minimal field, that is caused by light sedimentary strata, and the local intensive minimums in the regions of Sieverodonets’k (-40 mGal), Svatove, Shevchenkove, Okhtyrka are caused by the sole material low thickness.
On the South, in Crimea, very high field is observed, which maximum (over than +150mGal) is situated in the Crimean Mountains. Earth crust there usually has the basic structure and there might be ultrabasic materials.
In the tectonically active structures (Carpathian, Donbass, Crimea) in gravitational field effects of mantle material thickness decrease, caused by high temperature influence.
Gravity isoanomalies (mGal)
The connection between structure characteristics and Earth crust substantial structure (upper mantle in some regions) and gravitational field is in most cases inverse. That is why features of abnormal Earth crust and upper mantle thickness distribution were discovered only with the help of methodology and technology of gradient stratified three-dimensional deep structure gravitational modeling worked out in Ukraine, which is based on rapid openings, correlation dependences r=f (Vp) for crystalline materials, which are based on experiments under PT conditions of deep structures subject to material substantial structure specific amendments, studying of theoretical models of different types of blocks, modern software.
Due to that, Earth crust and upper mantle abnormal thickness areas are singled out and regularities in its distribution were established. It was found out that due to isostatic compensation tendency of the separate blocks and its groups heavier blocks has immersing of the Earth crust bottom, and the lighter ones – lifting of the Moho surface. That is, on the Ukrainian Shield the Golovanivs’k structure, separate blocks on Volhynia-Podolya and in other regions have Earth crust raised thickness when bending, and granitoid blocks (Kirovohrad, Korosten’ and others) are characterized by the material lightened thickness and lifting of the Moho bounder.
By results of gravitational modeling areas of upper mantle materials with raised thickness under the DDA, Crimea, Precarpathian, and in Transcarpathian region- area of upper mantle material with lightened thickness are predicted.
Authors: V. I. Starostenko, S.S Krasovsky
Catalogue of gravity pole measurements. Publications.
Earth gravity and anomalys maps. Publications.
Gravity measurings of Antarctic
During the season works of the 9th (2004) and 10th (2005) Ukrainian Antarctic expeditions workers of the Institute of geophysics NASU on the islands of Argentine archipelago measuring of the gravitational field at 17 points were conducted. Scheme of the observation points placing is given in fig. 1. British Antarctic Service has already performed works on the creation of bearing gravimetric points’ network, which is situated on the area with coordinates between southern latitudes of 51 and 71 degrees (Renner, 1982). In the area, which borders station “Academic Vernadsky” there are 16 bearing gravimetric points, two of which are situated on Galindez Island. All measuring were performed with one of them (VS01) taking into account, where gravitation force angle is 982339,6 mGal (Renner, 1982). Results of the field observations are given in table 1. Gravitation force abnormalities are obtained through the use of International formula of the normal Earth field 1930 with Potsdam system’s amendment. Map of abnormalities in the free air (Δgс.в.) of the Argentine islands region is given in fig.1. Value of Δgс.в., which were obtained as a result of field works, absolutely fall in with regional field of the research region, which confirms the high quality of these works.
|
№
|
Point’s name |
Geographical coordinates
|
Absolute value, mGal
|
Height (m)
|
Abnormality in the free air |
Bouguer abnormality |
|
|
Latitude
|
Longitude
|
||||||
|
1
|
GS-30
|
-64,07877417
|
-65,15790834
|
982360,86
|
18,226
|
66,68
|
64,65
|
|
2
|
GS-40
|
-64,07475466
|
-65,20767627
|
982340,275
|
18,173
|
46,36
|
44,34
|
|
3
|
GS-50
|
-64,13603279
|
-65,1749509
|
982349,8
|
17,519
|
51,32
|
49,37
|
|
4
|
GS-60
|
-64,0920
|
-65,1409
|
982339,27
|
25,9396
|
46,5
|
43,6
|
|
5
|
GS-70
|
-64,08327785
|
-65,24750238
|
982334,31
|
17,036
|
39,44
|
37,54
|
|
6
|
GS-80
|
-64,07031
|
-65,16022
|
982332,305
|
20,906
|
39,55
|
37,23
|
|
7
|
GS-90-а
|
-64,1846
|
-65,3370
|
982330,925
|
22,121
|
30,40
|
27,94
|
|
8
|
GS-110
|
-64,1118669153
|
-65,1259208263
|
982354,615
|
17,11
|
56,73
|
55,82
|
|
9
|
GS-120
|
-64,21801164
|
-65,23468524
|
982340,694
|
18,934
|
36,81
|
34,70
|
|
10
|
GS-130
|
-64,22176364
|
-65,23976194
|
982341,914
|
23,078
|
39,05
|
36,47
|
|
11
|
GS-200
|
-64,27407679
|
-65,23688375
|
982339,875
|
18,595
|
31,91
|
29,84
|
|
12
|
GS-230
|
-64,27902489
|
-65,24379049
|
982339,32
|
17,532
|
30,67
|
28,72
|
|
13
|
GS-240
|
-64,3174766
|
-65,23923264
|
982339,41
|
18,503
|
28,33
|
26,27
|
|
14
|
GS-250
|
-64,30787211
|
-65,239728
|
982370,755
|
16,818
|
29,84
|
27,97
|
|
15
|
GS-260
|
-64,31717175
|
-65,2357799
|
982339,885
|
17,819
|
28,62
|
26,64
|
|
16
|
GS-310
|
-64,3976
|
-65,5112
|
982340,035
|
17,305
|
51,12
|
49,19
|
|
17
|
VS01
|
-64,153
|
-65,147
|
982339,2
|
9,076
|
36,90
|
35,9
|
Solving gravimetrics and magnetometrics intepretation task.
Approximational approach has been used in practice of interpretational works for a long time already. Let us pay attention to some sides of this important and many-sided question.
The first one. It is a question of information basis replacement - about transition from fields' elements maps to analytical approximation of these elements. Initial gravitational or the magnetic field is replaced with a field of some auxiliary model. The interpreter receives the whole set of geophysical fields. It essentially raises the level of qualitative interpretation.
In the discussed works it is possible to create an auxiliary model in such a way, that along with the analytical description of the initial field, the interpreter receives geometrical characteristics of abnormal weights distribution (fig. 1). Otherwise the problem becomes more complicated. It becomes nonlinear. The steady mathematical apparatus of such problems decision was developed.
The second one. Problems of geological interpretation are often solved with the help of trial-and-error method. At the first stage the interpreter creates the hypothetical geological model, whose weights could create the needed abnormal field. Such geological model should be parametrized. It means that some sequence of parameters with numerical values, which define geometrical features of the model, density or magnetic characteristics of weights, is established. There is a possibility to receive a theoretical field which is conditioned by weights of geological model. The theoretical field is compared with the initial. Now the problem consists in finding new numerical values of model parameters, which minimise the divergence of compared fields.
Thus, interpretational problem is always solved in model class, fixed beforehand. It is possible to allocate works, which are devoted to the general questions of the theory and practice of gravimetric and magnitometric fields’ interpretation.
In some works cycle problems of analytical model of initial abnormal field creation are considered (fig. 2).
Results of the inverse problems solution in various model classes are considered. These are inverse problems for contact surfaces. Here it is necessary to divide both two-dimensional problems, and problems in which by the abnormal field the relief of interface of geological formations division is restored.
Solution series of return problems is given, if the abnormal field is caused by star objects or bodies of L.N.Sretenskiy class. In this class the average horizontal plane is allocated and the configuration of body cover and its sole is defined. The solution of problems from one class allows to define change of density or magnetisation of rocks which are placed in the fixed horizontal layer.
Main results are published in periodical scientific journals. Mainly, they are NASU Reports; Earth physics, Moscow: Ed. RAS; Geophysical magazine. Kiev: Ed. NASU; Geoinformatics. Kiev: Ed. NASU.
Fig. 1. Inverse gravimetric problem solution for two contact surfaces by the variation field of gravitation force.
Bulah E.G.