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

Gravimetry for monitoring water movements : the Classic Karst as a natural laboratory

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 \u160kocjan caves and then flows underground up to the Timavo Springs. The water path is very well known near the \u160kocjan 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

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

Tectonic and climate induced mass changes - competing signals in long term gravity signals

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

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

Orogenic mass changes detectable in satellite gravity missions

3Long term GNSS time series detect vertical crustal movement rates, which typically at orogens demonstrate uplift. The orogenic uplift can be ascribed to tectonic and post-glacial adjustments and crustal thickening. We investigate the sensitivity of satellite gravity change rate observations to detect the associated mass changes. Gravity change rate joint with uplift monitoring allows to distinguish the mechanism of uplift (Braitenberg and Shum, 2016). We use known vertical uplift rates over specific orogens to predict the gravity change for different geodynamic hypotheses of pure uplift and mantle inflow, or crustal thickening and isostatic Moho lowering. The sensitivity of gravity as a tool to distinguish the two mechanisms is investigated. The estimate of this tectonic signal is important, when the observed gravity change rates of GRACE and future missions are interpreted exclusively in terms of hydrologic changes tied to climatic variation. We find that in some areas, as the Tibetan plateau and the Himalayan- Alpine range, the tectonic signal is measurable by satellite gravity and contributes to a better understanding of the geodynamic processes leading to orogenesisEGU2017-16481openopenBraitenberg, C.; Pivetta, T.; Morsut, FBraitenberg, Carla; Pivetta, TOMMASO FERRUCCIO MARIA; Morsut, Federic

Geophysical Challenges for Future Satellite Gravity Missions: Assessing the Impact of MOCASS Mission

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

Geodetic monitoring in Nepal: preliminary results from Gorkha earthquake (25 April 2015)

The Himalaya arc is one of the most complex and tectonically active areas in the world, a very long (2500km) plate boundary capable of catastrophic earthquakes up to 8 Mw (Rajendran and Rajendran, 2011). Segments of the complex fault system, that accomodate the deformation between Asia and India, lie in correspondence of densely populated cities (i.e. 7.8 Mw on 25 April 2015). A good monitoring system, composed of seismographs and a geodetic network, is the indispensable scientific base to assess and mitigate the risk in this area and to get a better understanding of the dynamics of those geodynamic processes. In this contribution we present the preliminary data and analysis from two GNSS stations located in Nepal, one near to the Everest Pyramid (EvK2CNR), the other one near to the Nagarkot city. Both the antennas seem to have sensed and measured the deformation due to the last catastrophic quake occurred on 25 April 2015. The GNSS time series in the Nagarkot station showed an abrupt change in the displacement, that could be the effect of the near field deformation associated to the quake. A forward model approach, using the Okada model (1985), has been used to verify the compatibility of the observed field to the modeled deformation. The other station that is farther from the fault seems to have recorded a transient deformation. We further analyze the noise level of the station and possible atmospheric induced signals. Using the Okada model to simulate different displacement scenarios due to different earthquake parameters, we are able to assess the sensitivity of the network and efficiently program the installation of further stations

Seamount growth to be observed in future satellite gravity missions

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

Gravity as a tool to improve the hydrologic mass budget in karstic areas

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