## 2 Geoenvironmental Setting
Despite the considerable depth at which the \"Solno\" mine was operating, the observationable changes in the gravity field were to be expected, taking into account the amount of the rock salt output which was removed from the interior of the rock mass. It should be noted here that rock mass under mining generates a gravitational effect. In the case of rock salt mass (unlike, for example, coal rock mass) this effect mainly consists of the gravitational impact of existing or closed excavations (while coal rock mass is also affected by caving zones and by fracture zones [28, 29, 30, 31, 32]. Mining activity causes the formation of voids and, as a result, a loss of mass in the rock mass, which results in the final gravitational effect being recorded as a negative gravity anomaly. Additional factors associated with the additional gravitational effect in the conditions of mining exploitation include rock mass deformations in the area of tectonic dislocations, and also hydrogeological changes in the rock mass [30]. The huge mass of rock salt extracted by the Solno mine induced a significant gravitational effect whose maximal value amounted to 0.3 mGal [27]. The liquidation of the Solno salt mine and the flooding of the workings with fully saturated brine reduced that effect by about 40% [27]. Due to the distances between the measurement points in surveys carried out in 1968-1969 and 1980, it was impossible to identify smaller anomalous structures (caverns, old sinkholes, etc.). The microgravimetric measurements carried out in 2002-2005 in various areas of the town allowed for the determination of anomalous zones, which are associated with many phenomena including fault zones, old sinkholes, and underground suffusion [23]. These various phenomena have been the subject of many papers [27, 28, 29, 30, 31, 32]. Gravimetric measurements which were obtained in the years 2002-2005 were carried out using the Scintrex CG-3 Autograv gravimeter. It is characterized by its ability to provide highly accurate measurements, i.e., approximately \(\pm\)0.010 mGal, with repeatability of results to within \(\pm\)0.005 mGal. Calculation of the adjusted values was carried out with an accuracy of \(\pm\)0.001 mGal [23]. The accuracy of the gravimetric measurements carried out in the period was much higher than those carried out previously, and the range of error of determining the relative values of gravity did not exceed \(\pm\)0.015 mGal [23], which was a value that equaled 1/20 of the expected maximal microanomalies caused by mining. Most of the measurements were obtained using the microgravimetry technique due to the fact that the basic research objective was the detection of weakness zones in the upper part of the rock mass, these weakness zones having been caused by mining/postmining and natural processes such as the creation or presence of caves, voids, or discontinuities. It should be noted here that microgravimetry is a powerful means of approach for the detection of many small geological and anthropogenic bodies [28]. It is popular and is widely used, especially to map shallow subsurface karstic features. Studies on the principles of the microgravimetric technique have been published by many authors [28, 29, 30, 31, 32]. The majority of the town area was covered by points of local microgravimetric networks, and the results of the measurements have already been discussed in other papers [23,27]. One of the networks was established in the area of study in 2003. On the basis of the field measurements, the distribution of the Bouguer anomaly--a gravity anomaly which was corrected for the height at which it was measured and the gravitational attraction of the terrain--was determined. This distribution was approximated by a fourth-degree polynomial, obtaining the values of the regional field, which were assumed as a trend. Next, the determined trend was used as the basis for the analytical determination of the residual gravity field.