Institute of Geology, Karelian Research Center, Russian Academy of Sciences, Petrozavodsk, Russia:

P. A. Ryazantsev, Senior Researcher, Candidate of Geologo-Mineralogical Sciences, chthonian@yandex.ru
R. V. Sadovnichii, Researcher, Candidate of Geologo-Mineralogical Sciences

The article is focused on application of geophysical methods to studying and valuating deposits of high-carbon (schungite) rocks. The promising technique for the analysis of spatial heterogeneities in tock mass is seismic tomography—the well-reputed method in geological engineering survey. The background for site survey of schungite deposits is the velocity contrast of elastic waves between schungite and enclosing rocks. Moreover, antecedent researchers found the change in the density of schungite depending on carbon content. The selected object for the study is Maksovo deposit situated in the center of the Republic of Karelia. It is a stratified dome in cross-section, with the maximum thickness in the middle. The basic rocks are predominantly high-carbon schungite varieties. Subsurface geology is complicated by outcrops of a few dolerite sills in the center and in the west of the deposit. Aiming to study the northern area of the deposit, 6 lines of seismic tomography are laid. In each line, the sources and receipts were arranged every 5 m. In each line, a smooth velocity model of P-waves (V_p), reflective of spatial heterogeneity of rocks, is obtained. The models distinctly map a sill of dolerite with V_p = 36–8.1 km/s and show velocity variability in schungite rocks with V_p ranging from 0.8 to 3.0 km/s. For the interpretation purposes, the velocity maps are built for the surface (level +40 m) and depth (level +30 m). These maps are compared with carbon weight content and drilling data. It is finally found that seismic tomography sufficiently accurately images geological variety of the deposit. Cross-plotting aimed to correlate P-wave velocities and carbon content exhibits such relation. As a results of the accomplished work, efficiency of the seismic tomography is actually tested in solving problems of geological mapping, as well as the possibility of determining types of schungite rocks based on carbon content using the velocity models is analyzed.
The study was carried out in the framework of R&D project No. 213 of the Institute of Geology, Karelian Research Center, Russian Academy of Sciences.

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