## 2 Geoenvironmental Setting
That is to say that the residual anomalies, which are the effect of disturbances in the near-surface part of the rock mass, were estimated. For this purpose, the Griffin method, with a small radius for collecting data (50 m), was used, which made it possible to subtract the gravitational effect from the salt dome and extract the image of the residual anomalies (Figure 4). The distribution of the residual gravity anomalies varies in the area from \(-\)0.30 mGal to +0.3 mGal [23]. But the greater part of the microgravimetric network is characterized by residual anomalies of low values. Particular attention is drawn to the clearly marked anomaly in the area of the shallow underground workings which are a remnant of the historical mining operations which were carried out continuously at the border of the rock salt and the cap rock up until its catastrophic collapse in 1907. But subsequent borehole exploitation was also conducted in this area (Figure 4). The high negative value of the residual anomaly with an amplitude of above 0.2 mGal indicates a significant missing mass that is usually associated with the presence of sub-surface cavity. This may indicate a still-incomplete filling of old excavations or a presence of a new activity of subrosion [23]. However, the occurrence of anomalous zones associated with old sinkholes was not observed, which indicates that they were effectively filled with saline and that karst  
Figure 4: Distribution of residual gravity microanomalies (in milligals) in the area under study (the distribution after [23], modified). The legend presents the signatures used on the map and their numerical designations; their meaning is as follows: 1—boundary of the area under study; 2—sinkholes existed before 1861 according to [18]; 3—sinkholes occurred after 1861 according to [18]; 4—boundary of the salt dome at the depth of 470 m; 5—historic, currently non-existent railway lines; 6—abandoned mine shafts; 7—boreholes. phenomena had not developed within them. Nevertheless, as was mentioned, the outline of the anomaly confirmed the reliability of the information about the existence of flooded underground workings from the beginning of the twentieth century that which are outlined on mining maps from that period of time (Figure 1). While anomalous zones in the area under study were identified thanks to the use of gravimetric measurements, in the absence of geodetic measurements, it was impossible to characterize the displacements of the ground surface there due to the absence of geodetic measurements. In another part of the town, however, a very good understanding of such characteristics was obtained. Shortly after the closing down of the \"Solno\" mine and after the liquidation of the Inowroclaw mining plant (a term defined by the country's mining laws and dealing with concessions and granted rights for mining operations) the monitoring of ground surface deformation was halted. In the years 2002-2005 and in 2018, the authors, within a research study were carrying out measurements of the network of benchmarks located in Inowroclaw using the precise leveling method. This network of the benchmarks was made using reference points located outside the former mining area of the \"Solno\" mine [21; 22; 26]. Areas of the vertical displacements of negative values were identified and despite the fact that they can be still observed, it was determined that they are vanishing. Recently the dominant trend has been noted of a decrease in ground subsidence and a clear increase in uplifts (in terms of their value and range of occurrence). Uplifts occur in the central and north-eastern parts of the dome [26]. The maximum annual changes for uplifts (positive values of vertical displacements) and subsidence (negative values of vertical displacements) in recent years are about 2 mm/year. It should be noted here that these values refer to changes recorded for benchmarks that are mounted on walls of large-volume buildings and for which the characteristics of height change are different than for those mounted in the ground.