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Subduction of the Neotethys Ocean resulted in collisions of continental plates, with formation of Cenozoic orogens. The different shapes and structures of various segments of these young orogens, such the Zagros Collisional Zone, suppose a complex interplay of shallow and deep structures, producing different degrees and styles of deformation. As part of the PRIN 2017 project, we analyze several types of recently acquired data (e.g., seismic tomography models of the crust and upper mantle, Moho depth, seismicity distribution, and surface topography). We find that the NW and central Zagros is characterized by a zone of thickened crust of variable width and overlain by topography that exhibits large height variations over small distances, in the central part of the collisional zone. These variations are accompanied by sharp lateral changes in the number of seismic events and velocities/temperatures at depths of ~100 km. We attribute these observations to relamination processes (i.e., the detachment of Arabian crust from the subducting lithospheric mantle and its underthrusting beneath the crust of the overriding plate), which are controlled by the variable geometry and stiffness of the overriding and subducting plates. This hypothesis is tested by performing a series of numerical experiments, using the numerical code I2VIS [1], that simulates relamination processes, occurring during continental collision. The consistency of the results is also verified through forward models of the static gravity field of the modelled structures, which are compared with the present-day observed gravity.References[1] Gerya, T.V., 2019. Cambridge University Press. ISBN 978-0-521-88754-0., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The statistical evaluation of the correlation between gravity and topography bears information on the isostatic state and the density inhomogeneities of the lithosphere. In appropriate spectral bands, the relation between gravity and topography is linear, and the coefficients depend on the underlying crust and subcrustal densities. We first explore the mutual linear gravity-topography spectral-dependent relations with synthetic lithosphere models, considering different geodynamic situations involving lateral crustal density variation, superficial density changes in topography, and different types of isostatically compensated models. We demonstrate the sensitivity of the regression parameters to the subsurface structure. A second important application is the residualization of the gravity field, useful in areas in which other geophysical constraints are scarse. On other planets topography and gravity field are globally available, and the regression analysis can be used for subsurface exploration and structural characterisation. We demonstrate the usefulness on the African and South American continent, and across the Atlantic ocean, using the gravity data of GOCE. One focus are the Greenstone belts, which are geologically interesting structures to which often mineralizations are associated.

The Classical Karst is a limestone plateau stretching between Italy and Slovenia over an area of about 600km2.The aquifer of Classical Karst contains a complex network of conduits, shafts and large voids that are fed by theautogenic recharge and allogenic input of the Reka River. The Reka River sinks underground in the Škocjan Cavesand continues its underground flow for almost 40km until it reaches the Adriatic Sea at the Timavo Springs. Theriver shows high discharge variations; with minimal discharge below 0.3 m3/s and maximal discharge reachingover 350 m3/s. As the conduit system cannot efficiently drain large discharge, huge water masses are stored inthe epiphreatic voids of the aquifer during flood events. Škocjan Caves present such storage, where a vast amountof water is temporary stored during intense rain. Further evidences of the impressive water movements in thissystem could be found in the geodetic time-series recorded by the Grotta Gigante horizontal pendulums, whichshow deformation transients during Reka flood events. Gravimetry could be a useful tool to obtain local mass balances of such complex system, contributing together withthe other classical hydrologic prospections in depicting the water dynamics in this karstic environment. In additionto this gravimetry represents the ideal completion to the already set up geodetic instrumentation in the ClassicalKarst region. In the last year, we installed two continuous recording gravity stations, near the Škocjan caves and inside the GrottaGigante cave. The Škocjan caves serve as a test site because the cave geometry and the hydraulic system here arewell known. The Grotta Gigante site offers a quiet place and long term geodetic time-series but on the other hand,the hydrodynamics here are less clear. In this contribution, we present the simulations in support to the placement of the instruments and the first analysisconducted on the observed gravity time-series. For the Škocjan caves our simulations estimate that this mass accu-mulation could generate gravity signals up to 30 microGal for extreme events with peak discharge over 250m3/slasting for 1-1.5 days, accumulating over 35 106m3of water. The recorded data in Škocjan supports our simula-tions: a prominent peak up to 5 microGal of amplitude was recorded during a 14 106m3flood event on October2018. We believe that the Classical Karst represents an interesting study case for both the geodetic and hydrologic com-munities; the Škocjan cave offers a natural laboratory to optimally assess the contribution of gravimetry as a toolfor monitoring underground fluid mass movements.

Growing seamounts bear a hazard to navigation, especially if their summit reaches shallow depths and they reach the ocean surface. A seamount that expands up to the surface and creates an island, is detectable by remote sensing images, but not if the island retracts below the surface. Real time gravity observations detect the mass change independently of the optical detection, the limiting factor being only the noise level of the data acquisition in relation to the signal generated by the mass change. Starting from realistic size-frequency distributions of seamounts, we estimate the expected signals of seamount growth. We develop a method to compare the signal to the spectral noise characteristics of a GRACE-type mission, expandable to a possible mission with improved noise curve. We evaluate the expected gravity changes of seamounts and find that a noise curve of GRACE improved by a factor 10 would be sufficient to detect a realistic sea mount growth with a latency of 1 year. The detection threshold though has a tradeoff with the time resolution, since resolution improves for increased time periods over which the satellite observation can be averaged.

Several mountain ranges as Alps, Himalaya and Tibet are presently subject to uplift, as documented by GNSSvertical movement rates. Uplift occurs in response to climatic mass loss (deglaciation or hydrologic mass loss)or due to the dynamic forces (crustal compression or mantle inflow below uplifting crust). The uplift generates amass change, which produces a time variation of the gravity field. The deglaciation and changes in the subsurfacehydrologic budget, also generate a mass change, which sums to the tectonic change. Satellite remote sensingis useful in determining the shrinking outlines of glaciers, using both multispectral imaging as well as Radarobservations, thus allowing to determine the surface geometry change. The essential value for climate change andestimate of the hydrologic budget is though the total volume budget estimate, which requires also the thicknessvariation. Remote sensing catches the surface height changes, but these must be corrected for the crustal uplift. Thegeodetic measurements of the crustal dynamics of the Alpine and Himalayan mountain ranges in terms of heightand gravity changes, are therefore in close relation to the estimate of the climatic changes inducing glacier andhydrologic budget changes. We estimate the hydrologic and glacier signal for the Alps and Himalaya-Tibet, usingresults from remote sensing and subsurface hydrologic observations, where available (for the methodologicalrationale see Chen et al. 2018). We estimate the contribution of the dynamic uplift by direct observations ofGNSS. We find that the hydrologic and glacier gravity signal calculated at satellite heights of GRACE andGOCE are superposed to the tectonic signal, and discuss to which amount the signals can be resolved by gravitymeasurements. We compare the predicted signals with the satellite observations of GRACE and GOCE, findingthat the tectonic uplift signal is small relative to the expected glacier/hydrologic signals, but that it cannot beneglected. We define the requirements to future gravity satellites in order to make a significante contribution to thedetection of hydro-glacial mass changes and the separation of the tectonic signal.Reference:Chen W., Braitenberg, C., Serpelloni, E. (2018) Interference of tectonic signals in subsurface hydrologic monitor-ing through gravity and GPS due to mountain building, Global and Planetary Change, Volume 167, August 2018,Pages 148-159.

The observation of crustal deformation is a means to calculate the strain rates and the stress loading at faults. The strain rate is expected to vary in time during the earthquake cycle, but also due to hydrologic masses and fluxes. Hydrologic mass is an elastic loading of the crust, with a consequent lowering and return to the starting position. The opposite effect occurs in places in which the subsurface waters are constrained to flow in channels with consequent buildup of pressure of the water, which determines a surface uplift and deformation. This latter effect is present in karst areas, and in particular in the classical karst shared between Italy and Slovenia, where crustal deformation is measured with tiltmeters in caves and GPS at the surface.

The Himalaya and Alps mountain ranges, and Tibetan plateau are presently subject to uplift, as documented by GNSS vertical movement rates. Uplift occurs in response to climatic mass loss (deglaciation or hydrologic mass loss) or due to the dynamic forces (crustal compression or mantle inflow below uplifting crust). The uplift generates a mass change, which produces a time variation of the gravity field. The deglaciation and changes in the subsurface hydrologic budget, also generate a mass change, which sums to the tectonic change. The shrinking outlines of glaciers are revealed by remote sensing, but the total volume budget estimate requires also the thickness variation. Remote sensing catches the surface height changes, but these must be corrected for the crustal uplift. The geodetic measurements of the crustal dynamics of the Alpine range in terms of height and gravity changes, is therefore in close relation to the estimate of the climatic changes inducing glacier and hydrologic budget changes. We show that for the Alps and Himalayas the hydrologic gravity signal is superposed to the tectonic signal, and discuss to which amount the signal can be resolved by gravity measurements (Braitenberg and Shum, 2015; Chen et al., 2018). We consider the satellite observations of GRACE and GOCE and propose what kind of error curve would be necessary for possible future gravity satellites to significantly increase the resolution.

MOCASS (Mass Observation with Cold Atom Sensors in Space) is an on-going study project funded by the Italian Space Agency in the framework of preparatory activities for future missions and payloads of Earth Observation. The object of the proposal is an innovative satellite gravity mission based on advanced cold atom interferometry (CAI) accelerometers, with the aim of modelling the static and time-variable gravity field of the Earth with high accuracy and resolution and of monitoring mass variations that occur on and below the Earth surface. The basic idea is a GOCE mission follow-on, launching a unique spacecraft with an on-board instrument capable of measuring some functionals of the Earth gravitational potential. The improvement with respect to the GOCE mission concept can only be achieved by going beyond the technology of electrostatic gradiometers, taking advan- tage of a new generation of sensors, such as cold atom interferometers. In the framework of the MOCASS study, the instrument characteristics are defined in terms of long-term stability, accuracy, and spectral responses. Then simulations on gravity field recovery based on the space-wise approach already used for the GOCE data processing are implemented. Finally an analysis on the geophysical signals that can be detected given the simulated mission performance are performed, with particular attention to hydrologic and tectonic modelling of changing masses. First simulations have already been performed by considering the GOCE orbit parameters but assuming that a CAI gradiometer is on board the spacecraft. This allows direct comparisons between GOCE and MOCASS per- formances. Instrument error spectra have been defined depending on the orbit and CAI configurations, all of them characterized by a flat error spectrum in the low frequencies, differently from the one of the GOCE electrostatic accelerometers. Given the error spectrum and the interferometer integration spectral response, simulated obser- vations have been produced and processed by the space-wise approach, which basically consists in a sequential application of a Wiener filter, a local collocation gridding and a spherical harmonic analysis. From sample statis- tics, the accuracy of the recoverable gravity field model can then be evaluated and compared with the expected gravity signal from selected geophysical phenomena, e.g. orogen geodynamics and glacier melting. Although the study is at this time not complete, these preliminary investigations show promising results.

The Alpine and neighboring Dinarides and Appennine orogens are subject to uplift, as documented by the GNSS vertical movement rates. The uplift is caused by crustal compression and thickening and/or mantle inflow below an uplifting crust. The two end-members can be distinguished by the amount of gravity change rate, the thickening having a smaller gravity signal than the crustal uplift, because the mass surplus at the surface is compensated at the level of lower crust. We calculate the expected rates using the observed GNSS determined uplift rates as geometrical surface constraints, and defining the two end-member geodynamic models. We analyze the gravity change rates of GOCE and GRACE and compare them with the expected rates. Hydrology must be considered in this context, as it contributes to a competing gravity change rate signal that is superposed to the tectonic signal. We find that the predicted signal is measurable, and that lateral changes in the sign of the observed gravity rates over the Alpine range have no counterpart in the ongoing uplift. We analyze possible candidates for this latter signal.

The Karst environment is characterized by a peculiar water system circulation, governed by a network of conduits in which the water flows. The name Karst is derived from the Classic Karst region which is located across Italy, Slovenia and Croatia borders. This area gave name to the phenomenon because it was one of the first worldwide to be studied and it is still object of many researches and hosts an important monitoring network. In this area the water is supplied mainly by infiltration during the autumn-spring rainfall events but also from the Reka river that sinks in the Škocjan caves and then flows underground up to the Timavo Springs. The water path is very well known near the Škocjan cave where the water inflow from the Reka river and the rain fall are continuously monitored and also the karst conduits have been mapped directly by speleology inspection. Such data are indispensable in order to construct and constrain 2D hydraulic models that explain very well the water dynamics in the area. However in Skocjan the water circulation is superficial while in other parts of the Karst the water flows deeper underground: in the Grotta Gigante, a natural cave, the water flow is located over 200m below the surface. Its movement could be hardly monitored by direct observation and also modelling is limited due to the lack of a 3D model of the aquifer. Indirect geophysical methods, in particular gravimetry, could be exploited in order to obtain some constraints for the underground conduits and cavities and also to gain information about the water mass movements through time. In this contribution we present some preliminary synthetic models for assessing the gravity signals expected for the underground cavities typical for the karstic area. In addition we evaluate the time gravity field change during strong rainfall events where the water is expected to fill the conduits and cavities. In future we will take advantage of these models to place a continuous gravity meter that cuold be useful to constrain the water fluxes in area where a direct observation of the water is difficult.

The satellite GOCE has produced an extraordinary global gravity field with a spatial resolution of 80 km at a precision of 1-2 mGal. When considering geologic structures, the wavelengths of interest for exploration purposes are smaller. We show that the GOCE data produce a new quality assessment tool for fields of higher resolution, which all necessarily rely on terrestrial data. The space-borne control of terrestrial data is necessary in order to obtain 100% reliability of existing data or to assess the quality of newly acquired data. We propose a scheme for controlling and testing the quality of higher resolution data, for example airborne campaigns or the EGM08 global gravity field. EGM08 has the higher resolution of 10 km, but with varying quality, depending on the terrestrial data availability. We show how the quality assessment can be made using the GOCE data, giving confidence in the successive modelling results. Specifically we consider the African continent. Africa is an example where terrestrial data area scarce and where the reliability of EGM08 is very variable, jeopardizing the usefulness of the field if the quality assessment with the GOCE da is not fulfilled.

The GRACE/GRACE-FO satellites have observed large scale mass changes, contributing to the mass budget calculation of the hydro-and cryosphere. The scale of the observable mass changes must be in the order of 300 km or bigger to be resolved. Smaller scale glaciers and hydrologic basins significantly contribute to the closure of the water mass balance, but are not detected with the present spatial resolution of the satellite. The challenge of future satellite gravity missions is to fill this gap, providing higher temporal and spatial resolution. We assess the impact of a geodetic satellite mission carrying on board a cold atom interferometric gradiometer (MOCASS: Mass Observation with Cold Atom Sensors in Space) on the resolution of simulated geophysical phenomena, considering mass changes in the hydrosphere and cryosphere. Moreover, we consider mass redistributions due to seamounts and tectonic movements, belonging to the solid earth processes. The MOCASS type satellite is able to recover 50% smaller deglaciation rates over a mountain range as the High Mountains of Asia compared to GRACE, and to detect the mass of 60% of the cumulative number of glaciers, an improvement respect to GRACE which detects less than 20% in the same area. For seamounts a significantly smaller mass eruption could be detected with respect to GRACE, reaching a level of mass detection of a submarine basalt eruption of 1.6 109 m3. This mass corresponds to the eruption of Mount Saint Helens. The simulations demonstrate that a MOCASS type mission would significantly improve the resolution of mass changes respect to existing geodetic satellite missions.

Both GRACE and GOCE have proven to be very successful missions, providing a wealth of data which are exploited for geophysical studies such as climate changes, hydrology, sea level changes, solid Earth phenomena, with benefits for society and the whole world population. It is indispensable to continue monitoring gravity and its changes from space, so much so that a GRACE follow-on mission has been launched in 2018. In this paper, a new satellite mission concept named MOCASS is presented, which can be considered as a GOCE follow-on, based on an innovative gradiometer exploiting ultra-cold atom technology and aimed at monitoring Earth mass distribution and its variations in time. The technical aspects regarding the payload will be described, illustrating the measurement principle and the technological characteristics of a cold atom interferometer that can measure gravity gradients. The results of numerical simulations will be presented for a one-arm and a two-arm gradiometer and for different orbit configurations, showing that an improvement with respect to GOCE could be obtained in the estimate of the static gravity field over all the harmonic spectrum (with an expected error of the order of 1 mGal at degree 300 for a 5-year mission) and that estimates are promising also for the time-variable gravity field (although GRACE is still performing better at very low degrees). Finally, the progress achievable by exploiting MOCASS observations for the detection and monitoring of geophysical phenomena will be discussed: the results of simulations of key geophysical themes (such as mass changes due to hydrology, glaciers and tectonic effects) with expected gravity change-rates, time constants and corresponding wavelengths, show that an improvement is attainable and that signals invisible to past satellites could be detected by exploiting the cold atom technology.

Monitoring the water movements in karstic areas is a fundamental but challenging task due to the complexity of the drainage system and the difficulty in deploying a network of observations. Gravimetry offers a valid complement to classical hydrologic measurements in order to characterize such systems in which the recharge process causes temporarily accumulation of large water volumes in the voids of the epi-phreatic system. We show an innovative integration of gravimetric and hydrologic observations that constrains a hydrodynamic model of the Škocjan cave system (Slovenia). We demonstrate how the inclusion of gravity observations improves water mass budget estimates for the Škocjan area based on hydrological observations only. Finally, the detectability of water storage variations in other karstic contexts is discussed with respect to the noise performances of spring and super-conducting gravimeters.

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.

Water supply from karst sources is a worldwide natural resource and the exploitation is tied to the knowledge of the positions of the hydrologic channels. We show that surface deformation induced by flood events in karst conduits is observable, and consists in uplift and outward movement from the hydraulic channel. Precipitation events produce the natural occurrence of subsurface hydraulic overpressure up to 1 MPa. Numerical modeling shows that the stresses are so strong to uplift and dislocate the surface by several mm and induce tilts in the order of microradians. The naturally induced deformation is compatible with a transient internal pressure loading of a channel. The results can be used to find new channels with dense GNSS networks. Sea water incursion and channels accessed for tourism could be monitored. Seismicity has been shown to have a seasonal variation in some areas, which could be explained by the subsurface stresses induced by the natural subsurface overpressure. The pressure induced deformation is expected to be observed in all karstic systems worldwide.

Depicting the water dynamics in a Karst environment frequently represents a challenge due to the more complex drainage system compared to porous media and to logistic difficulties in deploying an efficient monitoring network. The observations of key physical parameters such as discharge are usually carried out sparsely and only if a direct access to the drainage channels is possible. For this reason, indirect geophysical methods, such as gravimetry, could be a valid complement to hydrological prospections to study the hydrodynamics of such systems. A typical karst environment is the “Carso/Kars” region, which is a 600km2 limestone aquifer, shared between Italy and Slovenia. The aquifer is constituted by an extended network of caves, shafts, and conduits fed by autogenic water and the allogenic contribution of the Reka river. The Reka river sinks in the Škocjan caves and continues its underground flow for over 30 km, finally outflowing in the Adriatic sea at the Timavo Springs. The river shows high discharge variability between dry and wet periods (0.3- 350 m3/s) and as the karst conduit system cannot efficiently drain large discharge, huge water masses are temporarily stored in the epiphreatic voids during flood events. In July 2018 we installed a gravimeter near the Škocjan caves with the aim to monitor continuously the water variations. In this contribution, we present the gravity data and the processing in order to remove tidal and other non-hydrologic components and, thus, to decipher the gravity variations linked to the Karst water circulation. Some challenges in the processing arise since the study area is close to the Adriatic sea, where marine tidal and non-tidal contributions are unneglectable. Our gravity measurements revealed many effects that are linked to the local hydrology: one important event flooded the caves in February 2019 causing a gravity change of 40 microGal. The event is compatible with the gravity estimate derived from a hydraulic model of the cave which reported an accumulation of about 10 106 m3 of water in the whole cave system in 1.5 days. The case discussed in this contribution represents a first promising application of gravimetry for tracking the water paths in the Classical Karst, easily applicable to other sectors of the Karst for which only little information about the hydrological system exist.