Romanian Reports in Physics, Vol. 57, No. 1, P. 151–163, 2005

PHYSICAL PROPERTIES OF THE QUATERNARY SEDIMENTARY ROCKS IN THE EASTERN 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 – Vãcãreºti (Fig. 1). Along the Dâmboviþa- 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 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. Yellow sandstone 21.8 5 1882 307 20.00 0.163 0.4863 11. Coarse yellow sand 24 2.2 1500 361 20.00 0.241 0.4693 12. Coarse yellow sand 28.5 4.5 1631 292 20.00 0.179 0.4834 13. Medium sand 34.2 5.7 1631 255 20.00 0.156 0.4875 14. Sandy blue shale 37.6 3.4 1631 449 20.50 0.275 0.4590 15. Sand with mica 39 1.4 1631 449 20.00 0.275 0.4590 16. Sandy blue shale 44.6 5.6 2588 449 21.00 0.173 0.4845 17. Yellow plastic shale 46.8 2.2 1455 449 20.50 0.309 0.4474 18. Sandy blue shale 48 1.2 1455 354 19.50 0.243 0.4685 19. Sand with mica 49 1 1455 354 20.00 0.243 0.4685 20. Blue shale 52.8 3.8 1455 354 20.50 0.243 0.4685 21. Silty sand with mica 54.8 2 1455 600 20.00 0.412 0.3976 22. Plastic blue shale 57.2 2.4 1455 600 20.00 0.412 0.3976 23. Yellow sandy shale 63.5 6.3 1904 215 20.00 0.113 0.4935 24. Silty sand with mica 64.6 1.1 1904 215 20.00 0.113 0.4935 25. Plastic blue shale 80 15.4 1904 302 21.00 0.159 0.4871 158 Andrei Bala et al. 8

In the borehole C2 the Vs/Vp ratio has reduced values of 0.155–0.230 (Fig. 4). The Poisson ratio has large values of 0.472–0.488 (Fig. 4). Below the hydrostatic level the Poisson ratio is gradually decreasing in a narrow band.

Fig. 4 – Vs/Vp and Poisson ratio for the borehole “Centura 2”.

4. SEISMIC PROFILING

A three component seismic profile of 600 m was recorded by “Prospecþiuni” SA, between the two boreholes, and having the following parameters [8]: 9 The Quaternary sedimentary rocks in the Bucharest area 159

– geophone spacing – 1.5 m; – shot spacing – 1.5 m; – number of channels – 100; – fold coverage – 50; – charge/shot – 12 g; – sampling rate – 0.5 msec; – recording time – 1 s.

Fig. 5 – Depth section on the seismic profile – P wave. Figures in the upper part are geophone numbers. 160 Andrei Bala et al. 10

– vertical and horizontal buried geophones; – recording station – I/O SYSTEM II. The time section for P wave was obtained first and then the time section for the conversion waves type P-SV were obtained by a computing procedure. The main computing sequence comprises the following processes: – static corrections; – velocity analysis; – dynamic corrections; – frequency filters; – F-K filters; – summation method. Due to the fact that the principal objective was shallow (0–100 m), high frequencies around 200 Hz were employed. Static and dynamic corrections as well as velocity analysis were applied with great care. Depth sections are presented in Fig. 5 (for the P wave) and Fig. 7a (for P-SV waves). Coherent reflected waves from all the interfaces which separate the layers with acoustic-impedance contrast (ρV) are present on the time section of the P waves. Layer thicknesses are between 1–10 m, which result in some interference of the seismic signal on the time section. Nevertheless the resolution can be considered as good. Reflected waves from the hydrostatic layer (h = 8 m) and from the depth of ~14, 24, 41, 48, 52 and 80–82 m are emphasized (Fig. 5 and Fig. 6). Even deeper reflected waves, from the layers with a weak slope from SW to NE are present: 125–127 m; 170–172 m; 230–233 m; 280–282 m; 300–305 m. Depth models resulted from the interpretation of the depth sections are presented in Fig. 6 (for P wave) and Fig. 7b (for P-SV waves). The signal/noise ratio and the resolution are better on the P-wave time section than on the S-wave time section for the depth range greater than 50 m. The P-wave and S-wave arrival times for the same interface vary by just a few ms due to the near horizontal layers (depth changes are not greater than 1–2 m for the same layer). The amplitude variations for the same reflected wave along the profile suggest that the acoustic impedance is variable along the same layer due to lithological changes. On the depth section of S waves reflections from the layers at the depth of 8, 14, 24, 28–30, 35 and 50 m are present (Figs. 7a and 7b). A better resolution is observed for the depth range shallower than 50 m (at 2, 8, 14, 24 and 28–30 m).

5. CONCLUSIONS

Some conclusions can be drawn on the 3 component seismic profile situated in the eastern part of Bucharest City, between the two measured boreholes: C1 and C2. 11 The Quaternary sedimentary rocks in the Bucharest area 161

Fig. 6 – Seismic model interpreted from P wave depth section on the profile. Values at the end of the model are interval seismic velocities for P waves obtained in the boreholes C1 and C2. Velocity values in the center are interpolated.

1. Between the two boreholes and along the seismic profile the sedimentary layers are almost horizontal (the depth differences are in the range of 1–2 m). 2. The upper sedimentary layers are interrupted by minor faults and fractures especially in the central part of the seismic profile, down to 50 m depth. 3. The amplitude changes of the same reflected wave along the profile prove that changes in lithology are present on most of the layers even in several tens of meters in length. 13 The Quaternary sedimentary rocks in the Bucharest area 163

4. The reduced values of the Vs/Vp ratio and the Poisson ratio values close to 0.5 prove that down to 70 m depth sedimentary layers must be considered as weak and very weak in the eastern part of Bucharest. 5. Accurate measurements in the boreholes of Vp, Vs and the density values of the sedimentary layers make possible further investigation of the frequency characteristics and transfer functions for the upper sedimentary layers in the eastern part of the Bucharest area.

Acknowledgements. This work is performed by the joint effort of research partners National Institute of Research and Development for Earth Physics and “Prospecþiuni” SA in the frame of the CERES Contract no. 34/11.11.2002, funded by Ministry of Education and Research. Special thanks are for “Metroul” SA, which granted the access for seismic measurements in the borehole “Centura 1”, and provided the density values of the sedimentary rocks. The authors wish to thank Marius Milea, Marketing Manager from “Prospecþiuni” SA, who constantly help the development of the project and co-sponsored the field works.

REFERENCES

1. G. Valsan, Câmpia Românã, BSRG, XXXVI, 1915. 2. P. Cotet, Unele date privind geomorfologia zonei oraºului Bucureºti, Probl. de geografie, VX, Bucureºti, 1963. 3. P. Cotet, 1976, Câmpia Românã, Ed. Ceres, Bucureºti, 1976. 4. V. Mutihac and L. Ionesi, Geologia României, Ed. Tehnicã, Bucureºti, 1974. 5. D. Paraschiv, The Moesian Platform and its Hydrocarbonous Deposits, Acad. Publ. House, Bucharest, 1979 (in Romanian). 6. E. Liteanu, Geology of the Bucharest city zone, Com. Geol. St. Tehn. Econ., seria E, 1, Bucharest (in Romanian), 1952. 7. N. Mandrescu, St. Cercet. Geol., Geogr., Geofiz., seria Geofizicã, 10, 1, 1972. 8. File reports no. II and III for CERES Contract no. 34/12.11.2002, “Models of the seismic velocity distributions in the sedimentary layers of the Moesian Platform, with details in the Bucharest City area”, director of the project dr. Andrei Bala.

Fig. 3 – Vs/Vp and Poisson ratio for the borehole “Centura 1”.

Table 2

Geologic and geophysical model of the borehole “Centura 2”; 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 2 2 300 150 19.50 0.500 0.3333 2. Sandy shale 8 6 600 230 19.50 0.383 0.4139 3. Sandy shale 14 6 1450 230 20.00 0.159 0.4871 4. Marl 24 10 2000 310 20.00 0.155 0.4877 5. Sandy shale 31 7 1650 330 20.00 0.200 0.4792 6. Marl 36 5 1850 400 21.00 0.216 0.4755 7. Sandy marl 41 5 1550 340 20.00 0.219 0.4747 8. Marl 50 9 1650 360 20.00 0.218 0.4750 9. Blue marl 60 10 2000 460 21.00 0.230 0.4721