Mapa rozkładu gęstości skał występujących nad poziomem morza w Polsce

Czesław Królikowski, Jerzy Twarogowski


Przedstawiono wyniki pomiarów gęstości skał występujących nad poziomem morza. Mapa w oryginalnej skali 1:500 000 została opracowana na podstawie wyników oznaczeń w 1370 punktach na całym obszarze Polski. Stanowi to podstawę kompleksowych interpretacji geofizycznych metodą strippingu i modelowania grawimetrycznego.


The maps of the gravity anomalies using Bouguer reduction are the starting gravimetric material for all the qualitative and quantitative analyses. To achieve the reduction, the knowledge of the density of the rock masses lying above sea level is necessary.

The first trial of the serious synthesis of the data about the rock density above sea level was done in the year 1965. The laboratory measurements of the samples of rocks were done from the cores of about 90 boreholes drilled in the period 1954-1961. After elaborating of the dependence of the effective porosity on the density of the dry sample, the data from the fifties were corrected and the map of the density of rocks above sea level was put together in the scale 1:1 000 000. The map showed the existence of great regional differences of the density between 2.1-2.80 g/cm3. The weighted average for Poland was 2.25 g/cm3.

In the end of the seventies, the work upon preparing the gravimetric bank of data, and evaluating of the uniform gravimetric map of Poland in the scale 1:50 000 and 1:200 000 has begun. To calculate the Bouguer anomalies it was accepted for the area of Polish Lowland the density 2.25 g/cm3,and for Southern Poland the density 2.60 g/cm3.

The present work is the synthesis of data achieved in the years 1962-1988 using the uniform method of measurements of the density. To put together the maps, the densities achieved in the laboratory measurements, executed mainly by the Enterprise for Geophysical Investigations, were used. The samples of rocks were achieved from the boreholes and from the exposures. For the scientific aims, the data achieved using the gamma-gamma radiometric method near the surface were used .

The laboratory density measurements were made for the insufficient amount of boreholes. In the greater amount of boreholes, the density values were estimated, using the knowladge of the lithological profile and using the density characteristics for separate rocks from the defined region. After the determination of the bulk density of Cainozoic rocks lying above sea level, the state of full saturation of pore space and argillaceous rock contraction during drying were taken into account. In the case of the pre-Cainozoic rocks these elements were not taken into consideration.

The results were represented on the map in scale 1:500 000. The isolines corresponding to the increase of 0.05 g/cm3, and in the part of Sudetes and Fore-Sudetic Block of 0.10 g/cm3 were drawn. This map contains the data from the measurements of density in 413 boreholes, the estimation of density in 957 boreholes and the measurements of density in the samples from about 1000 exposures. Omitting the exposures, we obtain the data frequency of 1 point to about 230 km2, that is in the accepted scale, 1 point to 9 cm2.

The image of the distribution of the density is connected with the geological structure of the area. The density of rocks lying above sea level depends mainly on their age, lithology, water content and so on. The density of rocks in Polish Lowland is contained in the intervall 2.00-2.30 g/cm3, and in Lower Silesia - above 2.50 g/cm3, in Holy Cross Mountains - above 2.40 g/cm3, in Carpathian Mountains - in the interval 2.60-2.70 g/cm3, and in the transiton regions such as Silesian-Cracovian Monocline and Upper Silesian Trough - it achieves 2.60-2.70 g/cm3.

The average densities for the separate sheets of the map 1:500 000 amount to: for NW -2.11 g/cm3, for NE - 2.16 g/cm3, for SW - 2.20 g/cm3, and for SE - 2.30 g/cm3, what shows the strict dependance on the type of rocks, laying above sea level.

The average density for the whole Poland equals 2.22 g/cm3.

While drawing the isolines the rule of linear interpolation, and in the mountainous regions the geological structure, were beeing taken into account.

In summary, it can be stated that the great differentiation of the density of rocks lying above sea level shows the necessity of using the variable density in the Bouguer reduction. The constant density may be used for maps in greater scales and it is proposed to use for the lowland area the density 2.15 g/cm3, on the Silesian-Cracovian Monocline - 2.30-2.50 g/cm3, and in the Sudetes, part of the Fore-Sudetic Block, in the Carpathian Mountains and Holy Cross Mountains - about 2.60 g/cm3.

Contemporaneously, the observation of the amount of points of measurements shows the insufficient degree of the survey, and allows to state, that farther investigation of the density of rocks is necessary.

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