We have studied a big karstic cave (Grotta Gigante) in
northern Italy using an innovative combination of laser-scan
and gravity data. We aimed to forward model the gravity
anomaly due to the cavity, verify its compatibility with the
Bouguer field, and identify the eventual presence of other
sources of gravity anomalies. A sensitivity study was performed
preliminarily to assess the minimum size of bodies that
could be detected by the gravity surveys. The 3D density
model of the Grotta Gigante was constructed using as a geometric
constraint the laser-scan data set, which mapped the internal
morphologies of the cave, and density measurements on
collected rock samples. The laser point cloud was reduced in
data density, filtered from the outliers, and subdivided into two
surfaces representing the vault and the floor of the cave, to correctly
define the prism model. Then, a mean density value,
obtained from laboratory measurements, was assigned to the
prisms. We computed the gravity effect of the model in the
same points at which the gravity field had been measured. Excellent
correlation was found for the cavity; some gravity
anomalies were revealed in the surrounding area of the Grotta
Gigante that could be effected by other underground karstic
morphologies. We attempted to estimate the probable size
and depth of the causative bodies, compatible with the geologic
environment. This site testified to the goodness of gravity
methods for the exploration of such structures, that is, particularly
important for risk assessment in a karstic area. The cave
itself, the biggest tourist cave worldwide, represents an upper
limit for expected gravity signals. The combination of exact
knowledge of the causative body and the related gravity
anomalies composed a unique data set (that we released to
the public, as a benchmark), useful for testing inversion and
forward model gravity algorithms.
