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DOWNHOLE LITHOLOGICAL PROFILE OF RAMNICU VALCEA SEISMIC SITE BASED ON DRILL

Andrei P. Mihai, Relu-Dumitru Roban, Victorin Emilian Toader, Iosif Lıngvay, Ovidiu Ciogescu

Abstract

In absence of core samples and open-hole well-log data, the broken bits of rock removed from the borehole during the drilling process represent in many cases the only lithological data that is recovered from a well. The main objective of this study was to build up a lithological profile and to use the data in order to improve the knowledge about the local geology (Quaternary deposits). Lithological information at shallow depth is generally used to better understand the local site effects, known to play an important role in modifying the intensity of ground shaking. The Quaternary sediments of Ramnicu Valcea city were penetrated recently by a 12'' drill hole down to 40 m depth. During the drilling process, the drill cuttings were sampled, labeled, and packed up. Drill cuttings were caught as composite samples that reflect the various lithologies drilled over a 3 m interval. In the last 4 m, the sample rate increase to 1 sample per meter. To do this, the grain size analysis was performed in order to obtain the mass percentages for gravel, sand, mud for each sample. The wet sieve procedure was performed to obtain the textural classes, then the samples have been dried and weighed. The resulted lithology of the borehole profile is given as a mass percentage of different textural classes. The main propose of this study was to obtain lithological information of Ramnicu Valcea seismic site (RMGV) by analysing the drill cuttings. The grain size analysis was chosen method because it was the most reliable method to get lithological information from the soft quaternary sediments high mixed by the drilling process. The results show the mass concentration of gravel, sand, and mud of all sampled intervals from 0 to 40m depth. To see the predominant lithology of analysed samples, the results were plotted in a Folk triangular diagram. The result of the grain size analysis shows that at the upper top and at the bottom part of the profile predominates the gravel and that the middle part is mostly muddy. This study represents the first step in dynamic properties of quaternary sedimentary rocks of Ramnicu Valcea seismic site. The lithological information obtained in this study will be correlated with the future seismic measurements to define a better dynamic and elastic properties of the Quaternary sedimentary strata.

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DOI resolver: doi.org/10.5593/sgem2021/1.1/s01.004

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