## 3 Results  
The decompensative gravity anomalies have been employed to refine the initial model of the sedimentary cover. Initially, the whole anomaly has been attributed to the sedimentary cover. The gravity anomalies could be equally determined by changes of thickness and density of sediments. It is impossible to determine a priori of which factor dominates in each specific case. In general, we consider them as equally important and assume that half of the decompensative anomaly is caused by a change of the thickness and half--by density:  
\[(\mathrm{S}\ +\ \Delta\mathrm{S})\Delta\overline{\mathrm{g}}_{\mathrm{S}}=\ \Delta\mathrm{S}(\Delta\overline{\mathrm{g}}_{\mathrm{ini}}+\Delta\overline{ \mathrm{g}}_{\mathrm{S}}) \tag{6}\]  
where S is the initial thickness of sediments, \(\Delta\mathrm{S}\) is its change, \(\Delta\overline{\mathrm{g}}_{\mathrm{ini}}\) is the initial density of sediments (vertically averaged relative to the standard value 2.7 g/cm\({}^{3}\)) and \(\Delta\overline{\mathrm{g}}_{\mathrm{S}}\) is the density perturbation. Afterwards, several limitations were applied to the model to keep it realistic:  
1. It is supposed that the areas without sediments or with very low amount of them are determined reliably in the initial model. Therefore, we do not apply the correction to the areas where the thickness is less than 0.25 km. 2. It is assumed that sedimentary thickness should not exceed 25 km, the limit, which is determined based on existing seismic studies. 3. The maximal reduction of the sedimentary thickness is limited to 0.75 of the initial one. 4. The final density of sediments (averaged with depth) should be within the range 1.9-2.8 g/cm\({}^{3}\), which is consistent with experimental data, e.g., [54]. Therefore, the condition (6) is not always met and in some cases only part of the decompensative anomalies can be explained by modification of the initial model of sediments. In the last case, the rest is attributed to the top layer of the crystalline crust. The inversion for changes of the sedimentary thickness and density was performed in an iterative procedure. First, it is estimated for the 1D case as the simple Bouguer correction by fitting the equation:  
\[\Delta\mathrm{g}_{\mathrm{d}}\ =\ 2\pi\mathrm{G}\ [(\mathrm{S}\ +\ \Delta \mathrm{S})\Delta\overline{\mathrm{g}}_{\mathrm{S}}+\Delta\overline{ \mathrm{g}}_{\mathrm{ini}}\ \ \Delta\mathrm{S}\ +\ \Delta\overline{\mathrm{g}}_{\mathrm{uc}}\mathrm{h}_{\mathrm{uc}}] \tag{7}\]  
Figure 6: (**a**) Decompensative correction for the isostatic gravity anomalies; (**b**) decompensative gravity anomalies. where \(\text{h}_{\text{uc}}=15\) km is the top layer of the crystalline crust and \(\Delta\overline{\rho}_{\text{uc}}\) is the density change within this layer. The third term appears only when it is not possible to fit the decompensative anomaly solely by modification of the sedimentary layer parameters due to the above limitations. In the next step, the gravity effect of the modified mode was estimated by using a 3D modelling technique on the sphere [43]. The difference with the original decompensative anomalies was used then to estimate corrections for the initial model. The convergence is reached very fast (usually in the second step) because 3D effects are not significant for relatively small depths. The obtained corrections and final map of the sedimentary thickness are shown in Figure 7. The correction is quite significant in many areas and spans within the range \(-11{-}8.5\) km (Figure 7a). Consequently, the differences of the final map (Figure 7b) with the initial one (Figure 3a) are also significant. We will discuss them in the following section. In the same way, the corrections for the initial density of sediments were estimated (Figure 8a). It spans in the range from \(-0.6\) g/cm\({}^{3}\) to \(+0.6\) g/cm\({}^{3}\). The final map of the averaged density is demonstrated in Figure 8b. It should be noted that the average density of sediments does not directly relate to their gravity effect. The last one is also determined by thickness.