Based on the results of the area-based gravity survey of an average scale, performed for the first time on the territory of the Nalaihin coaliferous depression (Mongolia), a map of gravity anomalies in the Bouguer reduction was obtained. Nalayhinskaya basin of the Mesozoic age is located within the Hentyi folded area. It is located 35 km southeast of the city of Ulanbaatar and is controlled by a system of deep faults. The basin has a sublatitudinal orientation and is filled with Lower Cretaceous sedimentary deposits. The medium-Paleozoic sedimentary rocks that enclosed it are collected in the steep isoclinal folds of the northeasterly strike. The expediency of carrying out gravimetric operations is due to the fact that in the depressions seismic activity is directly related to the depth of the crystalline basement. The survey was performed using gravimeters scintrex autograv CG-5 according to the standard procedure. The heights of the observation points were determined by the Trimble 5700 GPS devices. The rms accuracy of calculating the gravity anomalies and the density of the observational network correspond to the technical requirements for gravimetric surveys of a scale of 1:100 000. The correction for the relief was introduced within a radius of 200 km. A quantitative interpretation of the gravitational anomalies was carried out to estimate the thickness of the sediments. The interpretation was carried out by the method of selection of three-dimensional models by inversion of residual gravitational anomalies of the Bouguer. The method of least squares is used. This is a method of approximating geological bodies with three-dimensional prisms with vertical sections that are polygon-shaped and are limited in extent. As a result of the interpretation it was established that the foundation of the depression has a block structure and is divided by faults of the north-east and north-west directions. The maximum thickness of sedimentary deposits in the basin is 800 m. Data on the thickness of sediments and the morphology of the basement can be used in engineering seismological studies to clarify the seismic hazard of certain sections of the Ulanbaatar region. In addition, the data obtained can be useful for clarifying the prospects for the operation of the Nalaiha coal deposit.

Turutanov Evgeny Khrisanfovich, Doctor of Sciences (Geology and Mineralogy), Chief Researcher, Head Laboratory Complex Geophysics, Institute of the Earth's Crust SB RAS, 128, Lermontov st., Irkutsk, 664033, Russian Federation, tel.: (3952) 42-87-92, e-mail: tur@crust.irk.ru 

Buyantogtokh Bazarin, Postgraduate, Institute of the Earth's Crust SB RAS, 128, Lermontov st., Irkutsk, 664033, Russian Federation, Researcher Center for Astronomy and Geophysics of MAS, PO-788, Ulaanbaatar-210613, Mongolia, tel .: 976-11-458024, e-mail: bazaryn_b@yahoo.com 

Tengis Badma, Postgraduate, Institute of the Earth's Crust SB RAS, 128, Lermontov st., Irkutsk, 664033, Russian Federation, Researcher Center for Astronomy and Geophysics of MAS, PO-788, Ulaanbaatar-210613, Mongolia, tel .: 976-11-458024

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