## 2 Geoenvironmental Setting
Identification of the effects of the phenomena and processes ongoing in the salt rock mass of the Inowroclaw dome is a complex problem. These actions caused deterioration of geomechanical properties of rock mass and have presented a threat to objects on the surface. As such, this issue requires comprehensive research using various research methods. Such comprehensive research methods have been successively used over the years in some of the localities in the area of Inowroclaw. During the mining era, when intensive exploitation of the salt deposit was being performed, geophysical measurement works were carried out for the purpose of obtaining a reliable geological recognition of the salt structure, while geodetic works were carried out to assess the impact of mining exploitation. After the end of the mining operations, research works were carried out to assess the post-mining effects and also the effects of the natural processes (subrosion, uplift movements of the salt dome). Hence, in the years 2002-2005 and in 2018, surveying/geodetic (leveling, GPS/GNSS, tachimetric) and geophysical measurements (gravimetric, seismic, GPR) were performed [20; 21; 22; 23; 24; 25; 26]. The most valuable results for the city area were obtained thanks to the leveling and gravimetric measurements; however, among the last two methods, only gravimetric measurements were taken in the study area [23]. In the years 1968-1969 and 1980, gravimetric measurements were carried out in the area of the Inowroclaw salt dome [20]. In total, nearly 2000 points were measured along 74 profile lines (Figure 3). The initial aim of these measurements was to determine the boundaries of the salt structure on the basis of a gravimetric anomaly induced by a smaller unit mass of the salt rocks forming the dome than forming the surrounding rocks [20]. In the next stage, the basic task of the gravimetric work was to determine changes in the physical state of the rock mass [16]. These gravimetric measurements were carried out using SCINTREX 202 and SCINTREX 375 gravimeters. The established network consisted of nine local base points. The average error of a single gravity measurement amounted to \(\pm\)0.023 mGal, and it was found that the mean square error of determining the Bouger anomaly was \(\pm\)0.037 mGal [20]. Figure 3: Distribution of gravity anomalies according to field measurements in 1968–1969 (cyan line) and in 1980 (blue line). The anomaly values are presented in milligals. The distributions based on the data obtained from [20]. 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—bandoned mine shafts; 5,6—isolines of gravity anomaly; 7—points of gravimetric measurements. The results of these works were reported in a technical elaboration [20] and they enabled the assessment maps of Bouger anomalies and maps of differential anomalies for the time interval of 1969-1980 (Figure 3). Figure 3 also presents the measurement points, along with distributions of the Bouger anomalies. The presented distribution of gravity anomaly isolines clearly corresponds to the geometry of the salt dome boundary in the horizontal projection (Figure 2). Small distances between isolines outline the fact that the exterior walls of the salt dome are steeply sloping, and the higher concentration of the isolines at the east side refers to the greater incline of the eastern wall of the dome. A difference in the distributions for the years 1968-1969 and 1980 was noticeable. The main reason for the changes is the gravitational effect induced by the underground workings of the \"Solno\" mine [20,27] and the subrosion and other hydrogeological reasons which changed a mass distribution in the geological substrate.