## 2 Materials and Methods
### Method  
Sediments are usually less dense than surrounding crystalline rocks, therefore, gravity data were often used for studying of sedimentary basins. Commonly, the isostatic gravity anomalies are used for these purposes. In these anomalies, the effect of deep density heterogeneity (e.g., of the Moho variations and density variations in the subcrustal layers), which dominate in the Bouguer gravity anomalies, is largely reduced, e.g., [32; 33]. It was shown that the isostatic model, which is used for this correction, should include two basic parameters, namely the average depth to the compensating density anomalies, successfully approximated by the depth to the Moho, and flexural rigidity of the lithosphere. The last parameter controls transition between the local and regional isostatic compensation, it is directly related to the effective elastic thickness (EET) of the lithosphere [34]. Both parameters can be determined a-priory by independent methods. Therefore, the isostatic anomalies of the gravity field were widely used for studying of sedimentary basins, e.g., [35; 36; 37]. Unfortunately, the gravity effect of density variations within sedimentary basins might be substantially reduced (up to about 1 order of magnitude for wide basins), since these density anomalies are also isostatically compensated by deep masses and by elastic deformation of the lithosphere [38]. Long ago, it has been suggested to refine the full effect the upper crust and, in particularly, sediments by calculating a decompensative correction [38; 39]. The efficiency of this correction has been demonstrated in many previous studies [38; 40; 41; 42]. These authors employed essentially local isostatic compensation models (chiefly Airy). It has been demonstrated that ignoring flexural rigidity of the lithosphere may principally bias the results. Later on, [PERSON] et al. [13] and [PERSON] and [PERSON] [14] have improved this method to account for the EET of the lithosphere. It is also used in the present study. Therefore, the method applied in this paper consists of two principal steps. In the first step, we determine the isostatic correction and calculate the isostatic gravity anomalies. In the next step, the decompensative correction and decompensative gravity anomalies are calculated. These anomalies are used to correct an initial density model of the sedimentary cover. The isostatic correction in the spectral domain can be calculated according to [43] as:  
\[\Delta\text{gic}\left(k_{x},k_{y}\right)=G_{\text{is}}\left(k_{x},k_{y} \right)\cdot\text{t}_{\text{adj}}\left(k_{x},k_{y}\right)=-2\;\pi\text{G} \rho\;\text{C}\cdot\text{exp}\left(-\text{k}\cdot\text{M}\right)\dots\;\text {t}_{\text{adj}}\left(k_{x},k_{y}\right), \tag{1}\]  
where M is the depth to the Moho, \(k=\sqrt{k_{x}^{2}+k_{y}^{2}}\) is the wavenumber, \(k_{x}=2\pi\text{/}\lambda_{x}\) and \(k_{y}=2\pi\text{/}\lambda_{y}\), G is the gravitational constant, \(\text{t}_{\text{adj}}\) is the adjusted topography, which is introduced in the sea area to equalize the bathymetry (\(\text{t}_{\text{b}}\)) and topography variations for the constant density of the topography \(\rho\). In the present study, the adjusted topography is also corrected for the initial model of the sedimentary cover:  
\[t_{\rm adj}=t_{\rm b}-\frac{\rho_{\rm w}}{\rho}t_{\rm b}-\frac{\rho\ -\rho_{\rm s}}{\rho}t_{\rm s}, \tag{2}\]  
where \(\rho_{\rm w}=1.03\ \rm g/cm^{3}\) is the water density, \(\rho_{\rm s}\) and \(t_{\rm s}\) are the vertically averaged density and thickness of sediments according to the initial model.