Creating Tourism Destinations of Underground Built Heritage—The Cases of Salt Mines in Poland, Portugal, and Romania

Salt mines, a significant category of local, regional, national, and/or European underground heritage, are becoming attractive tourism destinations. This paper examines three cases of salt mining in different European countries, namely Wieliczka in Poland, Campina de Cima in Portugal, and Turda in Romania. They are analyzed in the context of history, typical attributes of their attractiveness, and new uses after the salt extraction was or is going to be stopped, in order to detect their unique values as important assets for both Underground Built Heritage (UBH) and Salt Heritage Tourism (SHT). The results of their comparison show that despite a positive impact related to their protection as cultural and industrial heritage, there are also some negative aspects related to increasing costs of their maintenance and adaptation of salt mines to new functions and to meet the tourism needs. By putting in place measures to enhance the awareness of their values and for activating the local community, the three mines are showcases for the economic outputs for their sites and regions, as well as for increasing knowledge regarding UBH.

Man-made disaster on urban area: subsidence and underground salt dissolution in Maceio (Brazil) revealed by remote sensing and numerical modelling

<p>Land subsidence hazard affects many highly populated urban areas of the world as a consequence of natural and/or anthropogenic derived geomechanical rock alterations. Here we exploit the full archive of Synthetic Aperture Radar (SAR data) and present a 16-years history (2004-2020) of surface displacement affecting the federal capital of Maceió (Alagoas, Brazil), where sinkhole formation and fractures on infrastructures have been intensified since early 2018, forcing authorities to relocate the affected residence and pose the building under demolition. The geodetic result shows that precursory deformations were already visible in early 2000’s, reaching in November 2020 a maximum cumulative subsidence of approximately 2 m near the Mundaú lagoon coast. The maximum rate of subsidence is estimated at 27 cm/year. Numerical elastic source modelling proves that the subsidence is associated with localized, deep seated material removal at the location and depth where salt mining is performed. More sophisticated 2D distinct element method highlights the formation of cracks in sedimentary layers that eventually enables strong water percolation from rather superficial aquifers into the deeper underground, with potential increase of material dissolution and erosion. We discuss the accelerating subsidence rates, the influence of severe precipitation events to the aforementioned geological instability and the related dynamic evolution of the subsidence hazard by generating dynamic geohazard maps valuable for further infrastructure risk assessment.</p>

Monitoring surface deformation of deep salt mining in Vauvert (France), combining InSAR and leveling data for multi-source inversion

The salt mining industrial exploitation located in Vauvert (France) has been injecting water at high pressure into wells to dissolve salt layers at depth. The extracted brine has been used in the chemical industry for more than 30 years, inducing a subsidence of the surface. Yearly leveling surveys have monitored the deformation since 1996. This dataset is supplemented by synthetic aperture radar (SAR) images, and since 2015, global navigation satellite system (GNSS) data have also continuously measured the deformation. New wells are regularly drilled to carry on with the exploitation of the salt layer, maintaining the subsidence. We make use of this careful monitoring by inverting the geodetic data to constrain a model of deformation. As InSAR and leveling are characterized by different strengths (spatial and temporal coverage for InSAR, accuracy for leveling) and weaknesses (various biases for InSAR, notably atmospheric, very limited spatial and temporal coverage for leveling), we choose to combine SAR images with leveling data, to produce a 3-D velocity field of the deformation. To do so, we develop a two-step methodology which consists first of estimating the 3-D velocity from images in ascending and descending acquisition of Sentinel 1 between 2015 and 2017 and second of applying a weighted regression kriging to improve the vertical component of the velocity in the areas where leveling data are available. GNSS data are used to control the resulting velocity field. We design four analytical models of increasing complexity. We invert the combined geodetic dataset to estimate the parameters of each model. The optimal model is made of 21 planes of dislocation with fixed position and geometry. The results of the inversion highlight two behaviors of the salt layer: a major collapse of the salt layer beneath the extracting wells and a salt flow from the deepest and most external zones towards the center of the exploitation.

Shallow slip of blind fault associated with the 2019 Ms 6.0 Changning earthquake in fold-and-thrust belt in salt mines of Southeast Sichuan, China

SUMMARY An earthquake with a magnitude of Ms 6.0 and shallow focal depth of ∼4 km struck the Changning county, Sichuan province, China on 2019 June 17. The hypocentre is located in the fold-and-thrust belt with plentiful shale gas and salt mine resources. One hypothesis is that the shallow fault could be affected by the artificial pressure water injection including the disposal of wastewater, fracking shale gas extraction and salt mining in Changning area. In this study, SAR (Synthetic Aperture Radar) images, historical earthquakes, aftershocks and seismic reflection data were collected to jointly investigate the characteristics of the 2019 Changning earthquake. The source model inferred from the InSAR coseismic deformation observation reveals that the 2019 Changning earthquake is attributed to a blind fault dipping to southwest with dominant thrust and sinistral strike slip. Moreover, a small shallow fault developing within the Changning anticline was triggered by the main shock, which contributed to the surface displacements as observed in the north of the epicentre. The estimated maximum slip of 0.49 m is located at the depth of ∼1.9 km, ∼9 km northwest of the epicentre. The Coulomb failure stress change caused by the previous two large earthquakes, which occurred in the hydraulic fracturing area, suggesting that they have little effect on the initial rupture of the 2019 Changning earthquake. Despite this, they have a positive triggering effect on the fault rupture in the northwest of the seismogenic fault. In addition, the analysis on the relation between the positive Coulomb failure stress change and the aftershocks suggests that the aftershocks may have different motion patterns from the main shock. The analysis also shows the earthquakes occurrence in the seismogenic zone may be affected by the high pore pressure due to the long-term injection of salt mining for more than three decades.