Laser-scan and gravity joint investigation for subsurface cavity exploration \u2013 The Grotta Gigante benchmark

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

Parameter sensitivity in satellite - gravity - constrained geothermal modelling

2The use of satellite gravity data in thermal structure estimates require identifying the factors that affect the gravity field and are related to the thermal characteristics of the lithosphere. We propose a set of forward-modelled synthetics, investigating the model response in terms of heat flow, temperature, and gravity effect at satellite altitude. The sensitivity analysis concerns the parameters involved, as heat production, thermal conductivity, density and their temperature dependence. We discuss the effect of the horizontal smoothing due to heat conduction, the superposition of the bulk thermal effect of near-surface processes (e.g. advection in ground-water and permeable faults, paleoclimatic effects, blanketing by sediments), and the out-of equilibrium conditions due to tectonic transients. All of them have the potential to distort the gravity-derived estimates.We find that the temperature-conductivity relationship has a small effect with respect to other parameter uncertainties on the modelled temperature depth variation, surface heat flow, thermal lithosphere thickness. We conclude that the global gravity is useful for geothermal studies.EGU2017-16678openopenPastorutti, A.; Braitenberg, CPastorutti, Alberto; Braitenberg, Carl

Paran\ue1-Etendeka lithosphere modeling according to GOCE observations and geophysical constraints: improvement of PERLA project

One of the challenges of the European Space Agency (ESA) is to improve knowledge of physical properties and geodynamic processes of the lithosphere and the Earth\u2019s deep interior, and their relationship to the Earth-surface changes. PERLA project is a part of the challenge of ESA\u2019s Living Planet program to investigate the Solid Earth, and in particular the lithosphere of the Paran\ue1-Etendeka Large Igneous Province (LIP). At the present stage the study is focusing on the upper mantle, the source of the magma. The aim is to motivate the asymmetry of the shallow volcanic effusion of the Early Cretaceous tholeiitic magmatism, that in Paran\ue1 is wide, thick and represented by the basaltic layer of Serra Geral Formation, while in Etendeka it is rare and spanned. Viceversa the alkaline magmatism shows similar effusions along the region with dyke swarms and associated alkaline and alkaline-carbonatite complexes from Early Creataceous to Paleogene age. ESA\u2019s Living Planet program offers a suite of scientific satellites, the Earth Explorers, and in this context PERLA adopts the newest GOCE satellite mission products. The Marussi tensor field and especially its vertical component show a positive anomaly along the coastline sector of both the western and eastern Atlantic Ocean. Positive anomalies are also related to the deeper Moho under the northern part of Paran\ue1 basin, in South America (SAM) and the Etendeka continental part. Here we aim to define the detail of masses between crust and upper mantle by modeling the Marussi Tensor components and the invariants. The invariants are easier to understand because they are independent of the reference system. The forward model uses Tesseroids. The density model is compared with recent seismologic models, and is performed according to the results provided by the physical laws governing rock densities and seismic velocity of lithosphere in function of temperature and pressure combined with laboratory measurements of a great number of mineral samples. Also the age of the mantle is included, according to the standard petrological classification of mantle with the percentages of four lead minerals: Olivine, Orthopyroxene, Clinopyroxene and Garnet. Studying the GOCE gravimetric data with the integration of geophysical and also petrological constraints is useful to investigate the lithosphere and to improve the geodynamics of complex geologic areas like LIPs

La gravimetria da satellite come vincolo nelle stime di flusso di calore : primi risultati

Un confronto tra modelli globali di gravit\ue0 (quali quelli ottenuti dai dati del satellite GOCE) e mappe di flusso di calore in superficie -due osservabili geofisiche non legate da semplici leggi- suggerisce un legame tra anomalia di Bouguer e diversi regimi di trasporto del calore. In un quadro finalizzato a valutare quanto sia possibile quantificare in maniera rigorosa tale relazione, abbiamo verificato come un semplice modello in cui valga una relazione diretta spessore crostale - produzione radiogenica di calore in crosta continentale possa essere utilizzato per stimare la componente di flusso sub-continentale. A causa dei vincoli logistici ed economici associati alle misure dirette del flusso di calore, la distribuzione di queste non \ue8 omogenea: in particolare \ue8 presente un bias verso i flussi elevati, associato all'interesse per lo sfruttamento della risorsa geotermica ad alta entalpia. Persistono aree prive di misure anche in zone non remote dell'Europa centro-occidentale. Compensare questi vuoti d'informazione tramite interpolazione pu\uf2 comportare la sovrastima dell'estensione delle zone ad alto flusso. Una possibile strategia per ovviare a ci\uf2 \ue8 la separazione tra componenti di flusso a diverse profondit\ue0, con l'obiettivo di isolare le componenti pi\uf9 profonde (rappresentate dal flusso attraverso la base della crosta), alla quale sono associate lunghezze caratteristiche delle anomalie termiche misurate in superficie maggiori rispetto a quelle dovute a strutture pi\uf9 localizzate. Otteniamo questo tramite una strategia di backstripping, stimando la componente crostale del flusso con la profondit\ue0 di due interfacce crostali, usate come fattore di scala, ottenute tramite inversione del dato di gravit\ue0. Il risultato \ue8 una mappa di flusso a scala regionale (risoluzione di circa 100 km), che presentiamo in un area studio (includente Alpi e bacini adiacenti, massiccio renano, Graben del Reno), confrontandola col risultato di un'interpolazione non vincolata. Questo prodotto, meno suscettibile all'influenza di fenomeni locali, ha permesso di isolare i fattori e le criticit\ue0 su cui andr\ue0 indirizzata una pi\uf9 sofisticata modellazione

Geothermal estimates from GOCE data alone: assessment of feasibility and first results

The characteristics of the available global gravity models derived from satellite gravity suggest that they could be applied in modelling the downward continuation of the temperature field at a continental scale. To obtain this, we quantified how and to which extent the mass distribution that we can obtain from inverse modelling of gravity can be linked to the factors affecting the temperature field, such as the radiogenic heat production and the thermal conductivity of rocks. Since there is no direct physical law linking the two fields, we resort to a reference lithosphere, built up on a set of lithological parameters \u2013including their associated uncertainties. A central and most critical assumption is that the crustal heat production can be tied to crustal thickness, a relationship which strength shows extreme variability in different geodynamic domains. We take this into account, including it as a parameter uncertainty and propagating it to the results. Pursuing the search for a reliable method to isolate the component of the heat flow due to the crustal heat production from the available measurements, we test this framework on the go_cons_gcf_2_tim_r5 release of the GOCE-derived field. We so obtain a satisfactory distinction between different heat transport domains (dominated by heat production, conduction from the mantle, or shallow plays), which proved helpful in interpolating regional heat flow maps at the resolution of the gravity data (about 140 km)

Sensitivity of gravity and topography regressions to earth and planetary structures

The availability of global gravity fields and topography through calculation services like the International Centre for Global Earth Models, allows easy access to gravity data, greatly enlarging the spectrum of users. The applications extend much farther than the classic modeling through the gravity-specialist. We investigate the sensitivity of the joint analysis of topography and gravity data based on linear regression analysis and clustering of the response to particular characteristics of the lithosphere structure. The parameters of the regression analysis are predicted to have characteristic values, which allow to distinguish continental crust from oceanic crust, and signalize the presence of crustal inhomogeneity. Predictions are made through theoretical considerations and on synthetic models. We use the South Atlantic Ocean and the confining South American and African continents for illustration, where the regression parameters distinguish oceanic crust from the ridge up to the bathymetric inflection point, from the transitional crust and the continental crust, allowing to map these units. The general properties of the parameters are statistically relevant, since the errors on the parameters are less than 10% the amplitude of the parameters. We compare the regression parameters with those produced by a global crustal model (CRUST1.0), and find good correspondence between the observed and predicted fields. The analysis can be applied with machine learning algorithms, without the need of specific forward or inverse gravity modeling skills. It is therefore particularly useful in view of the enhanced access to the data through the calculation service, and could be implanted as an add-on tool, since it allows to efficiently distinguish isostatic contribution to the gravity field from crustal sources. Given the experience on the gravity field of the Earth, the analysis can be analogously extended to other planets. For illustration, we show that for Mars a coherent class of Martian crust can be identified

Geodynamic implications of temporal gravity changes over Tibetan Plateau

The Tibetan Plateau is one of the most geologically dynamic systems and the highest plateau in the world with ongoing three-dimensional crustal deformation. The Plateau is uplifting and deforming horizontally as observed by present-day global navigation satellite system (GNSS) and repeated leveling measurements. Crustal mass is conservative and less dense than the mantle, thus the horizontal shortening must be accompanied by crustal thickening and horizontal extrusion. According to the level of isostatic compensation, the thickening is partitioned into topographic uplift and Moho deepening. Here, we investigate the mass change induced gravity signal observed at or near the crust, and discuss whether this signal could be detected using terrestrial or satellite gravity observations. We set up a model for the Tibetan Plateau crustal thickening and calculate the expected gravity signal. The predictions are then compared with the present-day gravity changes observed by GRACE and with published in situ absolute gravity rates. We conclude that the crustal thickening signal cannot be neglected and that it contributes significantly to the observed signal. Those studies with focus on the mountain glacier and hydrologic mass fluxes should be aware that, if neglected, the crustal signal could introduce a significant bias. The observations give a positive gravity rate over central Tibetan Plateau, unexplained by the hydrologic or cryospheric signals, and a negative rate over the Himalayas and at its foothill, which is attributable to terrestrial hydrologic signals including human depletion of groundwater. Our model shows that the positive gravity rate could be explained by elevation uplift, and a stable or upwelling Moho. The negative gravity change signal is due primarily to the strong elevation-gradient at the foothill of the Himalayas, and to an uplift accompanied by crustal thickening and Moho lowering. The estimated gravity rates can be used when defining the requirements on future gravity missions, as the tectonic signal should be resolved in order to improve its separation from hydrologic and/or cryospheric processes generating a gravity change

Monitoraggio delle acque di fondo del Bus de la Genziana (Pian Cansiglio, Nord-Est Italia)

In this paper we present the results of groundwater monitoring of the Bus de la Genziana (Cansiglio, North-Eastern Italy). It is a cave 587 m deep and about 7 km long with a siphon hanging in the bottom. For the fi rst time a diver instrument was installed here from March 2013 to December 2014 to discover the underground hydrodynamics. The level has been raised with a pattern of impulsive average of 4-5 meters with a few millimeters of rain: it has reached 27 meters in one event and twice it has exceeded 50 meters with heavy rainfall lasting for several days. A direct relationship is between tiltmeters recording (installed here at 25 m deep) and water level in the siphon. Observing the conductivity and the temperature, before the events of fl ood, the conductivity values (EC, at 25\ub0C) were an average of 215 and 260 \u3bcS/cm, while the parameter fell between 150 \u3bcS/cm and 190 \u3bcS/cm when the karstic system fi lls. On the contrary the temperature changes a little bit: the variations are sometimes near the error measurement of the instrument with 0.04 to 0.3\ub0 C, minimum values 7.48\ub0 C and maximum value 8.34\ub0 C. These parameters show that the place can be affected by mixing of new infi ltration water, which is however very fast as a trigger, as less outfl ow. This type of dynamic helps us to understand that the underground water circuits are highly conductive, as confi rmed by tiltmeters signal, and enrich the knowledge of the Cansiglio complex hydrogeology. Prospects for the future are to continue monitoring by installing a diver instrument also in the Abisso del Col della Rizza, a cave near Genziana, 800 m deep, in order to understand better the underground hydrodynamics, which are much more complex than they look

Sediment basin modeling through GOCE gradients controlled by thermo-isostatic constraints

Exploration of geodynamic and tectonic structures through gravity methods has experienced an increased interest in the recent years thank\u2019s to the possibilities offered by satellite gravimetry (e.g. GOCE). The main problem with potential field methods is the non-uniqueness of the underground density distributions that satisfy the observed gravity field. In terrestrial areas with scarce geological and geophysical information, valid constraints to the density model could be obtained from the application of geodynamic models. In this contribution we present the study of the gravity signals associated to the thermo-isostatic McKenzie-model (McKenzie, 1978) that predicts the development of sedimentary basins from the stretching of lithosphere. This model seems to be particularly intriguing for gravity studies as we could obtain estimates of densities and thicknesses of crust and mantle before and after a rifting event and gain important information about the time evolution of the sedimentary basin. The McKenzie-model distinguishes the rifting process into two distinct phases: a syn-rift phase that occurs instantly and is responsible of the basin formation, the thinning of lithosphere and the upwelling of hot asthenosphere. Then a second phase (post-rift), that is time dependent, and predicts further subsidence caused by the cooling of mantle and asthenosphere and subsequently increase in rock density. From the application of the McKenzie-model we have derived density underground distributions for two scenarios: the first scenario involves the lithosphere density distribution immediately after the stretching event; the second refers to the density model when thermal equilibrium between stretched and unstretched lithospheres is achieved. Calculations of gravity anomalies and gravity gradient anomalies are performed at 5km height and at the GOCE mean orbit quota (250km). We have found different gravity signals for syn-rift (gravimetric maximum) and post-rift (gravimetric minimum) scenarios and that satellite measurements are sufficiently precise to discriminate between them. The McKenzie-model is then applied to a real basin in Africa, the Benue Trough, which is an aborted rift that seems to be particularly adapt to be studied with satellite gravity techniques. McKenzie D., 1978, Some remarks on the development of sedimentary basins, Earth and Planetary Science Letters, 40, 25-3

Control of deep lithospheric roots on crustal scale GOCE gravity and gradient fields evident in Gondwana reconstructions

The GOCE gravity field is globally homogeneous at the resolution of about 80km or better allowing for the first time to analyze tectonic structures at continental scale. Geologic correlation studies based on age determination and mineral composition of rock samples propose to continue the tectonic lineaments across continents to the pre-breakup position. Tectonic events which induce density changes, as metamorphic events and magmatic events, should then show up in the gravity field. Therefore gravity can be used as a globally available supportive tool for interpolation of isolated samples. Applying geodynamic plate reconstructions to the GOCE gravity field places today\u2019s observed field at the pre-breakup position. In order to test the possible deep control of the crustal features, the same reconstruction is applied to the seismic velocity models, and a joint gravity-velocity analysis is performed. The geophysical fields allow to control the likeliness of the hypothesized continuation of lineations based on sparse surface outcrops. Total absence of a signal, makes the cross-continental continuation of the lineament improbable, as continental-wide lineaments are controlled by rheologic and compositional differences of lithospheric mantle. It is found that the deep lithospheric roots as those found below cratons control the position of the positive gravity values. The explanation is that the deep lithospheric roots focus asthenospheric upwelling outboard of the root protecting the overlying craton from magmatic intrusions. The study is carried out over the African and South American continents. The background for the study can be found in the following publications where the techniques which have been used are described: Braitenberg, C., Mariani, P. and De Min, A. (2013). The European Alps and nearby orogenic belts sensed by GOCE, Boll. Bollettino di Geofisica Teorica ed Applicata, 54(4), 321-334. doi:10.4430/bgta0105 Braitenberg, C. and Mariani, P. (2015). Geological implications from complete Gondwana GOCE-products reconstructions and link to lithospheric roots. Proceedings of 5th International GOCE User Workshop, 25 - 28 November 2014. Braitenberg, C. (2015). Exploration of tectonic structures with GOCE in Africa and across-continents. Int. J.Appl. Earth Observ. Geoinf. 35, 88-95. http://dx.doi.org/10.1016/j.jag.2014.01.013 Braitenberg, C. (2015). A grip on geological units with GOCE, IAG Symp. 141, in press