remote sensing Article Non-Invasive Methodological Approach to Detect and Characterize High-Risk Sinkholes in Urban Cover Evaporite Karst: Integrated Reflection Seismics, PS-InSAR, Leveling, 3D-GPR and Ancillary Data. A NE Italian Case Study Alice Busetti 1, Chiara Calligaris 1,* , Emanuele Forte 1 , Giulia Areggi 1 , Arianna Mocnik 2 and Luca Zini 1 1 Mathematical and Geosciences Department, University of Trieste, Via Weiss 2, 34128 Trieste, Italy; [email protected] (A.B.); [email protected] (E.F.); [email protected] (G.A.); [email protected] (L.Z.) 2 Esplora Srl, Spin-Off University of Trieste, Via Weiss 1, 34128 Trieste, Italy; [email protected] * Correspondence: [email protected] Received: 22 October 2020; Accepted: 18 November 2020; Published: 20 November 2020 Abstract: Sinkholes linked to cover evaporite karst in urban environments still represent a challenge in terms of their clear identification and mapping considering the rehash and man-made structures. In the present research, we have proposed and tested a methodology to identify the subsiding features through an integrated and non-invasive multi-scale approach combining seismic reflection, PS-InSAR (PSI), leveling and full 3D Ground Penetrating Radar (GPR), and thus overpassing the limits of each method. The analysis was conducted in a small village in the Alta Val Tagliamento Valley (Friuli Venezia Giulia region, NE Italy). Here, sinkholes have been reported for a long time as well as the hazards linked to their presence. Within past years, several houses have been demolished and at present many of them are damaged. The PSI investigation allowed the identification of an area with higher vertical velocities; seismic reflection imagined the covered karst bedrock, identifying three depocenters; leveling data presented a downward displacement comparable with PSI results; 3D GPR, applied here for the first time in the study and characterization of sinkholes, defined shallow sinking features. Combining all the obtained results with accurate field observations, we identified and mapped the highest vulnerable zone. Keywords: sinkhole; PSI; 3D-GPR; reflection seismics; leveling; evaporites; geo-hazard 1. Introduction Several European regions are affected by ground subsidence phenomena due to the presence of highly soluble evaporite rocks. The dissolution of soluble rocks and deposits at the surface, or in the subsurface, combined with internal erosion and deformational processes can produce depressions named sinkholes or dolines that represent a severe geo-hazard, as has occurred, among others, in France [1], Germany [2], Lithuania [3], Russia [4], Spain [5,6], United Kingdom [7,8], Albania [9], USA [10], South Africa [11] and Italy [12–15]. Sinkholes mainly occur when the bedrock is a soluble rock, mainly carbonates and/or evaporites and in terms of geohazard, there are noteworthy differences between lithologies [16,17]. In fact, gypsum and halite have much lower mechanical strength and more ductile rheology than most carbonate rocks, and their solubility is definitively higher, so in evaporitic terrains, the process is typically much faster and sinkhole frequency is commonly higher ([18] and the references within). These features Remote Sens. 2020, 12, 3814; doi:10.3390/rs12223814 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 3814 2 of 28 Remote Sens. 2020, 12, x FOR PEER REVIEW 2 of 30 can reach depths of tens of meters and diameters of more than hundreds of meters. The largest These features can reach depths of tens of meters and diameters of more than hundreds of meters. known collapse sinkhole is Xiaozhai Tiankeng in China, 662 m deep and reaching 119.3 million m3 The largest known collapse sinkhole is Xiaozhai Tiankeng in China, 662 m deep and reaching 119.3 in volume [19]. Crveno Jezero in Croatia has a vertical extension of 528 m, including the 280 m deep million m3 in volume [19]. Crveno Jezero in Croatia has a vertical extension of 528 m, including the lake at its bottom [20]. The sinkhole discovered in Malta is a circular to elliptical collapse structure up 280 m deep lake at its bottom [20]. The sinkhole discovered in Malta is a circular to elliptical collapse to 600 m in diameter and with an average area of 56,000 m2 [21]. Italy is also affected by this type of structure up to 600 m in diameter and with an average area of 56,000 m2 [21]. Italy is also affected by geohazardthis type of [22 geohazard–27], and the[22–27], results and of the recent results investigations of recent investigations have shown thathave Friuli shown Venezia that Friuli Giulia Venezia region (hereGiulia after region noted (here as FVG)after noted is one as of FVG) the most is one aff ofected the areasmost affected in northern areas Italy in northern [18] with Italy 1199 [18] sinkhole with phenomena1199 sinkhole inventoried phenomena just inventoried considering just the consider evaporateing bedrocks the evaporate [15]. Here, bedrocks sinkholes [15]. Here, are well-known sinkholes naturalare well-known phenomena natural and have phenomena been recognized and have since been the recognized end of the since 1800s the [18 end,28 ,of29 the], as 1800s they represent[18,28,29], a seriousas they threat represent to man-made a serious threat structures, to man-made such as buildings structures, and such roads as buildings [14]. and roads [14]. GypsumGypsum crops crops out out inin almostalmost allall ItalianItalian regions, over about 1% 1% of of the the total total national national territory territory [13]. [13]. SinceSince most most of of these these evaporite evaporite outcropsoutcrops areare veryvery small,small, more or less detailed studies studies on on gypsum gypsum karst karst havehave been been carriedcarried outout inin onlyonly aa fewfew regionsregions asas Piedmont,Piedmont, Friuli Venezia Giulia, Emilia-Romagna, Emilia-Romagna, Calabria,Calabria, andand Sicily.Sicily. InIn detail,detail, karstkarst areas in FVG are very common [14,30–35], [14,30–35], but but only only 1% 1% of of the the karstifiablekarstifiable lithologies lithologies areare representedrepresented byby evaporites.evaporites. These crop crop out out along along the the Alta Alta Val Val Tagliamento Tagliamento valleyvalley and and along along the the northern northern alignment, alignment, encompassing encompassing Pesarina, Pesarina, Pontaiba Pontaiba and and Lumiei Lumiei valleys. valleys. The The Alta ValAlta Tagliamento Val Tagliamento valley valley (Figure (Figure1) is the 1) mostis the amostffected affected with with hundreds hundreds of inventoried of inventoried sinkholes sinkholes [14]. Here,[14]. theHere, combination the combination of an intensivelyof an intensively karstified karsti evaporitefied evaporite bedrock, bedrock, the presence the presence of regional of regional thrust, thethrust, high the amount high amount of annual of annual rainfall rainfall (1600–2000 (1600–2 mm000/y) mm/y) and the and large the fluctuationslarge fluctuations of the of water the water table (oftentable greater(often greater than 10 than m) 10 seem m) seem to be to responsible be responsible for the for above-mentioned the above-mentioned phenomena. phenomena. One One of theof mostthe most notable notable sinkhole sinkhole areas inareas the FVGin the is locatedFVG is in located Quinis, in a hamletQuinis, of a thehamlet Enemonzo of the municipality Enemonzo (Figuremunicipality1). (Figure 1). FigureFigure 1. 1.(a ()a Study) Study area area location. location. In red,In red, the Friulithe Friuli Venezia Venezia Giulia Giulia region region (FVG); (FVG); (b) focus (b) on focus the Carnianon the evaporiteCarnian outcropsevaporite (in outcrops green) at (in the green) NW side at ofthe the NW FVG, side mainly of the within FVG, the mainly Alta Valwithin Tagliamento the Alta valley. Val InTagliamento blue, the Tagliamento valley. In blue, River. the The Tagliamento red star is River. placed The on Quinisred star hamlet. is placed on Quinis hamlet. EvenEven in in a a small small placeplace suchsuch asas Quinis,Quinis, thethe identificationidentification and precise mapping of of sinkholes sinkholes are are didifficultfficult tasks. tasks. TheThe detectiondetection andand adequateadequate characterizationcharacterization of sinkholes and and potentially potentially unstable unstable groundground commonly commonly require require thethe applicationapplication ofof multiple surface and and subsurface subsurface investigation investigation methods. methods. GutiGutiérrezérrez et et al. al. [ 36[36]] have have describeddescribed somesome ofof them:them: aerial and satellite images, images, topographic topographic maps, maps, field field surveys,surveys, paleokarst paleokarst analysis, analysis, subsidence subsidence damage damage maps, maps, light detectinglight detecting and ranging and ranging (LIDAR), (LIDAR), synthetic aperturesynthetic radar aperture interferometry radar interf (InSAR),erometry microseismicity, (InSAR), microseismic ground-basedity, monitoring,ground-based hydrochemical monitoring, modeling,hydrochemical speleological modeling, explorations, speleological geophysical explorations surveys,, geophysical probing surveys, and drilling probing and and trenching. drilling and trenching.The most innovative methodological advances developed in recent years are related to the acquisitionThe most of interferometric innovative methodological SAR (InSAR) dataadvances that measure developed the displacementin recent years of theare groundrelated surfaceto the acquisition of interferometric SAR (InSAR) data that measure