Laser-Scan and Gravity Joint Investigation for Subsurface Cavity Exploration — the Grotta Gigante Benchmark

GEOPHYSICS, VOL. 80, NO. 4 (JULY-AUGUST 2015); P. 1–12, 18 FIGS. 10.1190/GEO2014-0601.1 Laser-scan and gravity joint investigation for subsurface cavity exploration — The Grotta Gigante benchmark Tommaso Pivetta1 and Carla Braitenberg1 ABSTRACT obtained from laboratory measurements, was assigned to the prisms. We computed the gravity effect of the model in the We have studied a big karstic cave (Grotta Gigante) in same points at which the gravity field had measured. Excellent northern Italy using an innovative combination of laser-scan correlation was found for the cavity; some gravity anomalies and gravity data. We aimed to forward model the gravity were revealed in the surrounding area of the Grotta Gigante anomaly due to the cavity, verify its compatibility with the that could be effected by other underground karstic morphol- 1 Bouguer field, and identify the eventual presence of other ogies. We attempted to estimate the probable size and depth of sources of gravity anomalies. A sensitivity study was per- the causative bodies, compatible with the geologic environ- formed preliminarily to assess the minimum size of bodies that ment. This site testified to the goodness of gravity methods could be detected by the gravity surveys. The 3D density for the exploration of such structures, that is, particularly im- model of the Grotta Gigante was constructed using as a geo- portant for risk assessment in a karstic area. The cave itself, the metric constraint the laser-scan data set, which mapped the in- biggest tourist cave worldwide, represents an upper limit for ternal morphologies of the cave, and density measurements on expected gravity signals. The combination of exact knowledge collected rock samples. The laser point cloud was reduced in of the causative body and the related gravity anomalies com- data density, filtered from the outliers, and subdivided into two posed a unique data set (that we released to the public, as a surfaces representing the vault and the floor of the cave, to cor- benchmark), useful for testing inversion and forward model rectly define the prism model. Then, a mean density value, gravity algorithms. INTRODUCTION The genesis of this cave is related to the karstification process acting on the Aurisina limestones (Cucchi and Piano, 2013) during The Grotta Gigante, according to the 1995 Guinness World Re- the past 10 Ma and to different rock collapses that progressively cords, has been certified as the biggest tourist cave worldwide. It is enlarged the structure. In rainy (or warm) periods, the cave was al- located in the northeastern part of Italy, in the Classic Karst plateau most completely filled by clayey deposits, which currently cover the (Figure 1) near the city of Trieste, and it is principally composed of basement of the cave. a wide ellipsoidal hall of approximately 100 × 60 × 70 m, two main Since the eighteenth century, the Grotta Gigante cave has been galleries connecting it to the surface, and southward, a very deep intensively explored to get a better understanding of the complex karstic shaft (of approximately 180 m extension). For its drainage network and also to evaluate the structural risks connected isolated position and for its impressive dimensions, the cave is to the karst environment; several topographic surveys have been chosen by Marussi (1960) as the ideal site for the horizontal carried out over the past 70 years (Marussi, 1953; Busà, 1975), pendulum geodetic station. In addition to this, the Grotta Gigante and two long galleries (greater than 50 m) near the surface have holds a seismographic station (Costa et al., 2010) and other geo- been discovered and accessed. Some geophysical surveys were also physical instrumentations for environmental studies (Del Maschio attempted, but due to the rough topography and to logistic difficul- et al., 2011). ties inside the cave, only potential field methods (gravimetry) and Manuscript received by the Editor 19 December 2014. 1Università degli studi di Trieste, Dipartimento di Matematica e Geoscienze, Trieste, Italy. E-mail: [email protected]; [email protected]. © 2015 Society of Exploration Geophysicists. All rights reserved. 1 2 Pivetta and Braitenberg an innovative muon study (Caffau et al., 1997) could be exploited to orations on the laser-scan point cloud to obtain the 3D prisms rep- explore the underground structures. A negative gravity signal of ap- resentation and the density measurements are presented. A final proximately 1.5 × 10−5 m∕s2 amplitude spread over a region of comparison of the modeled and observed gravity fields and conclu- 300 × 200 m above the cave has been detected (Zanolla et al., sions constitute the last sections. 1996); however, only partial modeling studies on the signal have The combination of prisms and gravity observations is a unique been conducted in the past few years. data set representing a natural cave, which is a benchmark for test- In recent years, increased interest in using geophysical methods ing inverse and forward modeling algorithms. The cave is one of the for subsurface exploration and delineation of karstic structures is biggest caves worldwide, and therefore, it acts as an upper limit for testified by numerous publications (Kaufmann, 2014; Martinez- a gravity signal that can be expected for a natural cave. Moreno et al., 2014), and the Grotta Gigante is an ideal site for applying and testing such methodologies. METHODOLOGY AND DATA PRESENTATION Still, nowadays some uncertainties about the evolution in time of the cave and its complete extension remain. We currently do not Our study aims to quantify the gravity effect of the Grotta Gi- know how deep the sediment infilling is or if there are some other gante cave through a forward modeling approach and verify its karstic hypogeal morphologies linked to the cave. compatibility with Bouguer data. The gravity observation database In light of the geologic hazard evaluation connected to the karst consists of two surveys: the first one, realized in the 1970s by the environment and for a better characterization of the site, particularly Osservatorio Geofisico Sperimentale (OGS) (Zanolla et al., 1996), important also for the scientific studies conducted inside the cave, is composed of 200 microgravimetry measurements covering an we propose an investigation of the Grotta Gigante and the surround- area of 1000 × 1000 m around the cave. The data set at our disposal ing structures by a forward modeling approach of the gravity field. does not include the free air observations, only the Bouguer field is We use new laser-scan data acquired inside the cave and density reported; details on the acquisition and correction methodology measurements on rock samples to construct a 3D density model of could be found in Zanolla et al. (1996). the Grotta Gigante by means of prism representation (Nagy et al., The second survey, acquired in 2013 by the University of Trieste, 2000). Then, the modeled gravity field is computed and compared the Politecnico di Milano, and the OGS, gathers 75 new gravity with the recent superficial gravity observations carried out in the stations in a less widespread area with respect to the previous cam- surroundings of the cave. paign. These gravity measurements have not been linked to the geo- The workflow of the modeling study is as follows. We start pre- detic network; hence, we only deal with relative gravity values. senting the methodology used and the data available to construct This data set needs to be corrected for the topographic effect, so our physical model; then, a sensitivity analysis on synthetic exam- we calculate this correction using prism discretization of different ples of Karst morphologies, useful to assess the signal amplitude of digital elevation models available with decreasing resolution with various sources of gravity signals, is discussed. Subsequently, elab- distance. For the area immediately above the cave (450 × 450 m), we rely on an airborne laser- scanning (ALS) survey that presents a point cloud representation of the topography with more than 15 million points and a datum density of 15 points∕m2 (Paganini and Pavan, 2012). For our topographic correction, the data are re- gridded to a resolution of 1 × 1mby arithmetic averaging. The Regional Digital Elevation Model (http:// irdat.regione.fvg.it/CTRN/), with resolution of 2 10 m, is used for an external area of approximately 9km2 surrounding the ALS survey. The cave is only a few kilometers from the Slovenian border, which limits the use of the regional DTM. We, therefore, integrate the SRTM database (http:// dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/) for the reconstruction of onshore topography, whereas for the sea bathymetry, the EMODnet model (www.emodnet.eu/bathymetry) is used. Both of these models are resampled to a resolution of 200 m and are covering an area of approximately 1° around the Grotta Gigante. The topographic grids are converted into a Cartesian coordinate system (UTM33/WGS84), and the quotas are converted to ellipsoidal heights. The DEM quotas, provided in orthometric heights, Figure 1. A simplified geologic map of the Classic Karst. The main caves and springs are corrected for the geoidic undulation. The are reported. The red circle locates the Grotta Gigante cave (after Cucchi et al., 2001). gravity effects of all these topographies are Gravity and laser-scan cave detection 3 computed on each point of the gravity survey and then subtracted other karstic structures (Figures 3a and 4) are shown. The vertical from the free-air anomalies. The topographic effect in the area section of the model is shown in Figure 3b, the projection of the immediately above the Grotta Gigante ranges from 34.3 to cave to the surface is given in Figure 3c. Constraints on shape − 35.2 × 10 5 m∕s2. and density contrasts of the causative bodies are derived from liter- The Bouguer fields of these two gravity surveys show that ature data (http://www.catastogrotte.it/dettaglio_grotta.php?ID=2; the cave is associated with a negative gravity anomaly of Carulli and Onofri, 1969).

Load more