CONTINUOUS AQUATIC SOUNDINGs OF THE LAKE TERE-KHOL’ WATER AREA IN THE REPUBLIC OF TUVA (RUSSIA)

1. CONTINUOUS AQUATIC SOUNDINGs OF THE LAKE TERE-KHOL’ WATER AREA IN THE REPUBLIC OF TUVA (RUSSIA) Arzhantseva I.A., Andreev M.A., Modin I.N. Moscow State University Geophysical deptartment

2. During the summer seasons 2007-2008 in the Republic of Tuva a complex geophysical survey was performed • The main purpose of the research wasto investigate an ancient fortress on the island on lake Tere-Khol’ for future restoration • The fortress Por-Bajin (“Clay House”, Uighur culture) is more than an archaeological site with monumental architectural remains: it is situated in a unique environmental zone which is also deserving studies

3. Geographical position of lake Tere-Khol’ Moscow Abakan Kyzyl Tere-Khol M o n g o l i a

4. Lake Tere-Khol QuickBird Image August 7, 2007

5. The Por-Bajyn island under excavation

6. 0,55 m/year Destruction of the island beach 1970 2007 120 лет 0,3 m/year 140 лет 130 лет 0,7 m/year

7. Although known perhaps as early as the beginning of the eighteenth century, the site was not properly explored until the middle of the twentieth century when the island was recorded as measuring six hectares; by 2006, the island had been reduced to some four hectares in area. • This gradual destruction of the island prompted the large-scale archaeological investigation of Por-Bajin from May 2007 through September 2008, with researchers the Russian Academy of Sciences, from Moscow State University and the State Museum of Oriental Art, Moscow.

8. Bathymetric map of the lake Around island

9. The east part of Tere - Khol lake

10. Continuos Aquatic Soundings • CAS method is based on multiple vertical electric soundings (VES) method, but data acquisition is done via a special marine streamer and multichannel receiver • The method is intended to produce Electrical Resistance Tomography (ERT) imaging on shallow water areas • With modern receivers and transmitters CAS allows data acquisition with a very high rate which guarantees high quality results

11. IMVP-8 Streamer: This inversed asymmetric Schlumberger array consists of 11 electrodes: 2 current (in the centre) and 8 pairs of potential for which “N” electrode is common Spacing1 OM1=2.5 m Spacing8 OM8=38 m AB=1.5 m Work frequency - 4.8 Hz Current in AB =300 mA Boat: Low-frequency transmitter “Astra-100” max. current – 1A, Multichannel receiver IMVP-8, GPS receiver for further positioning of achieved results

12. The main feature of the array is mutual substitution of current and potential electrodes. Together with use of a multichannel receiver, it provides an enormous advantage: in a short time period equal to several periods of generating a signal in the current line, we can get a DC resistivity electrical sounding (ES)curve. • During the acquisition, all the signals are continuously recorded on the computer's hard disk. This makes it possible to do all the subsequent accurate data processing in the laboratory.

13. DATA PROCESSING X(t), Y(t) DU(t) OM1 {rk}n(t) {rk}n(x,y) OM7

14. The high observation density of CAS allows application of two-dimensional inversion mechanism for the interpretation. • On lake Tere-Khol’ the maximum distance between the furthest potential electrode and the center of current electrodes of 38 m allowed to research depth of 20 m • A 35 km survey was carried out covering the central and southern areas of the lake. Here the observation network was uniformly distributed to research the whole water area of the lake.

15. The observation network map Total length of profile-35 km Point of observation

16. The further interpretation was carried out with respect to the division into 7 electrophysicallithotypes: • Lake water : 45-50 W.m • Sandy loams : 45-50 W.m • Clay bonded sand : 50-80 W.m • Medium-clayed sands : 80-100 W.m • Sorted low-clayed sands : 100-150 W.m • Sorted sands : 150-200 W.m • Rough sands : >200 W.m

17. 2D inversion results along profile 4 Line CD. Sounding step 1 m Field pseudo-section ra Calculated pseudo – sectionra Geoelectrical section r(x,z) Geological-geophysical section

18. The acquired results show that at the time of the fortress construction, the open water area of the lake covered only the western part of today's extent of Lake Tere-Khol, and the fortress itself stood at the tip of a large peninsula that included the present-day adjoining island and a group of islands about 2 km to the northeast of Por-Bajin island. The southern shoreline of the lake was highly indented and appeared to have been a complex array of straits, islands and bays.

19. That time there was a water area in the northeastern part of the lake (eastern conduction zone) that probably was joined to the south-western water area by a narrow strait. We estimate the width of the strait as 200 m at its eastern part and 500 m at its western part (to the north of the adjoining island).

20. 2D inversion results

21. Paleo Reconstraction

22. Conclusions • Continuous electrical soundings made a paleogeographical reconstruction possible. It showed that the time Por-Bajin was built, open water existed only in the western part of the present-day extent of Lake Tere-Khol', to the west of the island of Por-Bajin. • The fortress was therefore built on the tip of a large peninsula. At that time, there was also a lake area in the northeast part of Tere-Khol', and this was linked to the southwestern water area by a narrow strait.

23. Conclusions • A linear object was found between the fortress and adjoining island which was probably an isthmus linking the islands.

24. Thank you for your attantion