In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.



Volume 51 No. 1 March 2002

Special issue on geophysics



Preface; 3–4

Alla Shogenova

Thermal conductivity of Cambrian siliciclastic rocks from the Baltic basin; 5–15

Argo Jõeleht, Kalle Kirsimäe, Alla Shogenova, Saulius Šliaupa,

Ilmo T. Kukkonen, Vita Rasteniene, and Angelina Zabele

Abstract. The thermal conductivity of Cambrian siliciclastic rocks, mainly sandstones and silt­stones, was studied together with their chemical composition and other petrophysical properties in Estonia, Latvia, and Lithuania. The thermal conductivity is mainly controlled by porosity and rock composition. It was found that the Wiener and layered mixing and geometric mean models approximated best the thermal conductivity vs. porosity relationship of quartz-rich rocks within the measured porosity range (0–30%). The matrix thermal conductivity is positively correlated only with the silica content which reflects the content of quartz, whereas other rock-forming minerals decrease the matrix thermal conductivity. The thermal conductivity of siliciclastic rocks correlates well with porosity-dependent parameters (wet density, apparent resistivity, elastic properties), but has no relation to magnetic properties.

Key words: thermal conductivity, petrophysical properties, siliciclastic rocks, Cambrian, Baltic basin.

Revision of aeromagnetic data: ground magnetic investigations of altered sedimentary rocks (Hatrurim basin, Israel); 16–32

Boris Khesin and Sophia Itkis

Abstract. Aeromagnetic survey revealed two local anomalies with a magnitude of up to 20 nT within the Hatrurim basin, where altered sedimentary (mainly carbonaceous) rocks of the Hatrurim Formation subjected to combustion metamorphism (CM) are widespread. Aeromagnetic data analysis and ground magnetic prospecting show that the CM model cannot relate all the magnetic characteristics. On one hand, high-amplitude (up to 4000 nT) high-wave number ground anomalies could be a by-product of the combustion (surface) metamorphism. On the other hand, poor correlation between magnetic susceptibility  values (reached up to 4000 ´ 10–5 un. SI) and metamorphic grade of Hatrurim Formation rocks, between magnetic anomalies and/or metamorphic grade/thickness of the rocks may suggest involvement of an additional source or presence of high remanent magnetization.

Key words: magnetic prospecting, altered sedimentary rocks, combustion metamorphism, Hatrurim basin, Israel.

Porosity structure of Icelandic basalt; 33–46

Omar Sigurdsson and Valgardur Stefansson

Abstract. Systematic sampling of igneous rocks from extinct geothermal systems in the Icelandic crust has resulted in a database covering most types of geothermal reservoir rocks presently under exploitation in Iceland. At present, the database includes more than 500 samples spanning the basaltic to rhyolitic composition and hydrothermal alteration from literally fresh rocks to greenschist facies rocks. The database is used to study the relationship between different reservoir parameters, such as thermal conductivity and porosity, alteration, and permeability. The present paper describes the relationship between porosity and permeability and the information that can be derived from this data set. It is found that the average difference between effective and total porosities is about 2% and that this difference is independent of porosity over the range 0–50%. This demonstrates that the whole pore volume in these igneous rocks is effective for fluid flow through them. The permeability measured with liquid brine is systematically lower than that obtained by measurements using air or gas. Brine permeability is on average three times lower than gas permeability measured on the same sample. We explain this discrepancy as a result of the formation of a connate liquid film that adheres to the rock in combination with capillary forces that reduce and block portions of the flow paths for the liquid flow. Reasonable conversion of gas permeability to brine permeability can be obtained by taking into account the effect of immobilization of part of the brine. This supports the view that a film of connate brine causes the difference in the observed permeability values. The bundle capillary tube model seems to be the best model describing the porosity structure of the Icelandic rocks, whereas the Kozeny–Carman model shows very weak correlation to the data. The calculated tube diameters of the bundle capillary tube model are in the range 0.03–80 mm, while laboratory measurements on only a few samples gave values in the range 0.05–100 mm. The calculated diameters of the majority of the samples are less than 0.7 mm, or smaller than the wavelength of visible light.

Key words: igneous rock, porosity structure, permeability, basalt, capillary tube model, rock database, geothermal systems.

Gravity and magnetic modelling of anomalous sources of complex configuration; 47–59

Mykola Yakymchuk, Ignat Korchagin, Tatjana Mikheeva, Marina Orlova, and Yuriy Yakymchuk

Abstract. The automated fitting method was applied to the modelling of gravity and magnetic anomalies. This technique allows us to use for interpretation the observed components of gravity and magnetic fields and their linear transformations, the geoid anomalies, and the course gradient data of magnetic fields. The software is based on the optimization of the sum of square differences between measured and modelled field components. The universal approximation procedure for parametrization of anomalous sources allows us to describe effectively different elements of the modelled space. The optimization procedure for the minimization of objective functions includes gradient-based algorithms and singular value decomposition technique. The technology is tested on practical geological problems.

Key words: gravity, magnetic, modelling, optimization, automated fitting.


Instructions to authors; 60–62

Copyright Transfer Agreement; 63