Gravity for Detecting Caves: Airborne and Terrestrial Simulations Based on a Comprehensive Karstic Cave Benchmark
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Underground caves bear a natural hazard due to their possible evolution into a sink hole. Mapping of all existing caves could be useful for general civil usages as natural deposits or tourism and sports. Natural caves exist globally and are typical in karst areas. We investigate the resolution power of modern gravity campaigns to systematically detect all void caves of a minimum size in a given area. Both aerogravity and terrestrial acquisitions are considered. Positioning of the gravity station is fastest with GNSS methods the performance of which is investigated. The estimates are based on a benchmark cave of which the geometry is known precisely through a laser-scan survey. The cave is the Grotta Gigante cave in NE Italy in the classic karst. The gravity acquisition is discussed, where heights have been acquired with dual-frequency geodetic GNSS receivers and Total Station. Height acquisitions with non-geodetic low-cost receivers are shown to be useful, although the error on the gravity field is larger. The cave produces a signal of −1.5 × 10−5 m/s2, with a clear elliptic geometry. We analyze feasibility of airborne gravity acquisitions for the purpose of systematically mapping void caves. It is found that observations from fixed wing aircraft cannot resolve the caves, but observations from slower and low-flying helicopters or drones do. In order to detect the presence of caves the size of the benchmark cave, systematic terrestrial acquisitions require a density of three stations on square 500 by 500 m2 tiles. The question has a large impact on civil and environmental purposes, since it will allow planning of urban development at a safe distance from subsurface caves. The survey shows that a systematic coverage of the karst would have the benefit to recover the position of all of the greater existing void caves.
KeywordsTerrestrial gravity acquisition airborne gravity cave detection Grotta Gigante cave RTK GNSS positioning
We acknowledge the former director of the Grotta Gigante cave, Arch. A. Fabbricatore, for providing us the laserscan data and for the logistic support. The guides of the Grotta Gigante are gratefully thanked for their support in the density sampling campaign. The authors acknowledge Prof. Daisuke Yoshida of Osaka City University, and Dr. Eugenio Realini and Dr. Stefano Caldera of GReD for providing the u-blox data collected under the JSPS KAKENHI Grant Number 26730156. We thank Dr. S. Cirilli and Dr. F. Raicich for providing tide gauge data of Trieste harbor. We thank Thomas Jahr and an anonymous reviewer for the meticulous reviews.
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