Physical Properties of the Quaternary Sedimentary Rocks in the Eastern Bucharest Area

Romanian Reports in Physics, Vol. 57, No. 1, P. 151–163, 2005 PHYSICAL PROPERTIES OF THE QUATERNARY SEDIMENTARY ROCKS IN THE EASTERN BUCHAREST AREA ANDREI BALA1, VICTOR RAILEANU1, NICOLAE MANDRESCU1, ION ZIHAN2, EUGEN DANANAU2 1 National Institute of Research and Development for Earth Physics, P.O. Box MG-2, Bucharest-Magurele, Romania, [email protected] 2 “Prospecþiuni” S.A., str. Caransebeº, nr. 1, Bucharest, Romania (Received October 4, 2004) Abstract. New seismic measurements are performed in the eastern Bucharest area with the purpose of defining better the physical properties of the shallow sedimentary rocks. A high resolution seismic profile, 600 m in length, was carried out in the area, as well as down-hole seismic measurements in two boreholes drilled at the ends of the profile. Computing and interpretation of the data lead to the conclusion that shallow sedimentary rocks can be considered weak and very weak in the area, down to 70 m depth. Seismic wave velocity values and bulk density values presented in the paper associated with local geology are useful primary data in the seismic microzonation of Bucharest City. This work was performed in the frame of the CERES Contract no. 34/11.11.2002, funded by the Ministry of Education and Research, Bucharest, Romania. Key words: seismic profile, down-hole seismic measurements, seismic microzonation. 1. GEOMORPHOLOGY OF THE BUCHAREST AREA Bucharest is situated in the central part of the Romanian Plain, a transitional area between the piedmont plains from the north and the Danube River to the south [1]. Some local plains can be distinguished in the city area: Bãneasa – Pantelimon, Giuleºti – Floreasca, Vergului and Cotroceni – Vãcãreºti (Fig. 1). Along the Dâmboviþa- Colentina interstream, the Giuleºti–Floreasca plain develops in the northwest, with heights between 80–95 m. The Vergului plain is located in the south-southeast; it has lower heights, between 70–80 m. Detailed geomorphologic studies made on this interstream underlined the existence of three steps, corresponding to three pseudo-terraces. The Vãcãreºti–Cotroceni plain, situated on the right bank of the Dâmboviþa River, in the south of the city has heights between 85–95 m in the Cotroceni area and 80–85 m in the Vãcãreºti area. The Dâmboviþa–Colentina interstream is situated 10–15 m lower than the Cotroceni–Vãcãreºti plain, because of the gliding of the Dâmboviþa River on its cone of dejection to its present flow [2], [3]. 3 The Quaternary sedimentary rocks in the Bucharest area 153 From the morphologic point of view, the Dâmboviþa River meadow is characterized by its flat bottom, by the irregular character of its banks which have a very small slope, and by very many windings of the river. The Colentina Valley, which is shorter and narrower, is very winding compared to the Dâmboviþa Valley. The Colentina Valley has a local alternant asymmetry throughout its waterway. The right bank of the Colentina Valley is higher and steeper, while the left bank is developed especially in the concavity of the river waterway. The Colentina River meadow is well defined, having generally high banks. 2. GEOLOGY OF THE BUCHAREST AREA Bucharest city is situated in the central part of the Moesian Platform, an important structural unit of the Romanian territory – which corresponds to the Romanian Plain from the morphological point of view. The Moesian Platform has a basement with 2 structural units: a lower one with chloritic and sericitic schists of Precambrian age and an upper one made up of old Paleozoic folded marine formations going back to the Middle Carboniferous age. The sedimentary cover of the Moesian Platform is relatively thick, exceeding 6,000 m. The Mesozoic cover, including also the Upper Permian, is almost continuous up to the Neozoic; however the Eocene, Oligocene and Lower Miocene deposits are lacking [4], [5]. The cohesionless Quaternary deposits are largely developed in the Bucharest area. The existing amount of geologic, hydrogeological and geotechnical data ([6], [7]) make it possible to know the lithological succession from the bottom upwards for the Lower and Upper Quaternary deposits: a) The Frãþeºti layers complex, common to all deep boreholes, consists of three layers of sand and gravel, named A, B and C separated by two intercalated layers of clay. The layers have a similar structure, with coarsely sands and gravel at the bottom and medium-fine sands transforming gradually into clays at the upper part. b) The Marl complex is represented by a succession of marl and clay, sometimes sandy marl with intercalation of fine sands. These deposits outcrop in the proximity of Uzunu locality on the Câlniºtea Valley. c) The Mostiºtea sands continuously cover the upper part of the marl complex. They are bank of brown-black sands, with rusted-colored intercalations. The thickness of the Mostiºtea sands is 10–15 m. A smaller thickness is present in the western part of Bucharest City (Cotroceni plain), in the Giuleºti zone and in the Bãneasa plain. d) The intermediate clay deposits are developed between the Mostiºtea sands and the Colentina sands and gravels. These clays deposits have variable thickness, between 5–20 m. In a few places, in the eastern part, the clays are completely 154 Andrei Bala et al. 4 laminated and this results in mixing the Mostiºtea sands with the Colentina sands and gravels. e) The Colentina sands and gravels show a transition from gravels, located at the bottom of this complex, to sands placed at the upper part. The entire deposit has a structure in lens, with increasing dimensions to the bottom layer. The thickness of the sands and gravels is 10–15 m in the Dâmboviþa–Colentina interstream, it is smallest in the Colentina–Vãcãreºti plain, (1–5 m on the Ghencea avenue), increases to 10 m eastward of the Antiaerianã Street and to 15 m in the Olteniþa Street area and eastward of the Mãgurele Street. f) The loess-like deposits have a lithology characterized by a large variety of granulation of the component elements, from clays and silt to fine sands and even coarse sands. These deposits have a variable stratification, a small percentage of fine particles, with coarse intercalations or even gravel elements and an uneven distribution of carbonates, which were deposited naturally in the initial stage. The geologic cross-sections from the eastern part of Bucharest City are presented in Fig. 2. 3. DOWN-HOLE SEISMIC MEASUREMENTS Down-hole seismic measurements were performed by “Prospecþiuni” SA in 2 boreholes which will be named “Centura 1” and “Centura 2” in the present paper (C1 and C2 in Fig. 1) [8]. “Centura 1” belongs to “Metroul” SA which gave access to the site, and “Centura 2” was drilled by “Prospectiuni” SA with the purpose of down-hole measurements at the NE end of the seismic profile (SP). The locations of the boreholes C1 and C2, as well as the position of the seismic profile (SP) are presented in Fig. 1, in the eastern site of Bucharest City. The boreholes were protected with plastic tubes and seismic measurements were performed down to 80 m and 62 m depth, respectively. The shot point was fixed at 3 m from the borehole. Wave generation was performed by hammer blows on a wooden block. A seismic station type OYO McSEIS 170 F.1122 with 24 recording channels was employed, with a sampling rate of 1 ms. The time length of each recording was 1 s. A three component sensor clamped on the borehole wall was used for recording, with a recording offset of 1 m. P wave onset was read on the vertical component and for the S wave the horizontal components of the sensor were used. Time and frequency criteria were used to correlate the S waves. Separate arrival times vs. depth graphs were constructed for P and S waves. In the t(h) graphs, below the hydrostatic level (~ 8 m), interval seismic velocities of the observed P waves are in the range 1455–2588 m/s in C1 and Vp = 1450–2000 m/s in C2. 156 Andrei Bala et al. 6 The interval seismic velocities for S waves are in the range 215–600 m/s in the borehole C1 and Vs = 230–460 m/s in the borehole C2. Detailed geologic data and geophysical measurements in the boreholes are presented in Table 1 for C1 and Table 2 for C2. In the borehole C1 the Vs/Vp ratio has small values of 0.113–0.412 below the hydrostatic level (~ 8 m – Fig. 3). The Poisson ratio has large values of 0.398–0.492 (Fig. 3). Both ratio values are good indicators that shallow sedimentary layers can be considered to be weak and very weak. Table 1 Geologic and geophysical model of the borehole “Centura 1”; γW is the bulk density Depth Thickness Vp Vs γ Poisson No. Layer W Vs/Vp [m] [m] [m/s] [m/s] kN/m3 ratio 1. soil 0.4 0.4 298 153 19.50 0.513 0.3210 2. Weathered shale 2 1.6 298 153 19.50 0.513 0.3210 3. Yellow shale 4 2 574 323 19.50 0.563 0.2683 4. Silty sand 4.5 0.5 574 323 20.00 0.563 0.2683 5. Sandy yellow shale 6.4 1.9 574 323 20.50 0.563 0.2683 6. Yellow sand with gravel 7.1 0.7 574 323 20.00 0.563 0.2683 7. Coarse sand 10 2.9 2344 340 20.00 0.145 0.4893 8. Plastic yellow shale 14 4 2344 340 20.50 0.145 0.4893 9. Plastic yellow shale 16.8 2.8 1882 307 20.50 0.163 0.4863 10.

Load more