Conference Agenda

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Session Overview
8.2-1 Gravity-based density models and their applications
Monday, 20/Sept/2021:
1:30pm - 3:00pm

Session Chair: Denis Anikiev, Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences
Session Chair: Hans-Jürgen Götze, CAU Kiel

Session Abstract

Gravity measurements contain important information about the subsurface at every spatial scale. Satellite missions provide highly accurate, global measurements, that allow building and testing 3D models of the Earth even for regions where other geophysical measurements are sparse. Depending on the envisaged model resolution and application, gravity data from regional or local surveys (e.g. ship and airborne) can be incorporated to provide higher resolution. However, solutions to the gravity inverse problem are non-unique and constraints by independent geological and geophysical observations need to be integrated to develop reliable subsurface density models. Such models, in turn, provide insights into subsurface variations in composition and thermo-mechanical state.In this session, we welcome contributions that integrate gravity and/or gradient data (at all scales) with other geophysical measurements and geological information to better understand the structure, properties and processes of the Earth’s subsurface. Hence, we would like to discuss diverse applications, from global scale, where mantle properties are assessed, down to the reservoir scale. We also welcome more technical contributions that address related topics like innovative joint inversion methods, model uncertainty estimation and determinations of parameter sensitivities, as well as temporal variations of the gravity field, and new interpretation software and techniques.

1:30pm - 2:00pm
Session Keynote

Solid Earth applications of global gravity data: from submarines to satellites

Bart Root

Delft University of Technology, Astrodynamics and Space Missions, Delft, the Netherlands

Prof. Vening Meinesz opened up the oceans for high precision gravimetric observations. Today, his submarine adventures are an inspiration to my science and education in gravimetric research. We wil follow him along his voyage aboard the K18, along which I will discuss several applications using global gravity field models. The theory of isostasy allows us the use the static gravity field to study GIA processes in Fennoscandia and North America. Also, observed crustal structure from active seismic experiments can be used to correct the gravity field and study the upper mantle. With a regional crustal model of the British Isles and surrounding oceans I was able to study the density variations in the lithospheric mantle underneath the crust. This study revealed a highly varying upper mantle density signature, but compared with seismic tomography large differences were seen. We show that this mismatch can be traced back to regularisation techniques used in seismology. This opened up the the study of mantle convection and its interaction with lithosphere. Seismic-derived mantle anomalies are still highly uncertain but might be improved with future gravity-rate datasets. Preliminary studies show potential in reducing the uncertainty in viscosity structure of the Earth. Finally, with the GOCE mission, a new boost has been given to the use of gravity gradients, I discuss an approach in inverting the full gravity gradient tensor estimating density structure of a subducting plate. By showing this variety of studies, I hope to inspire you to use satellite-derived global gravity fields.

2:00pm - 2:15pm

The compilation of the new Alpine gravity maps - from the work of the AlpArray Gravity Research Group

Pavol Zahorec1, Juraj Papco2, Roman Pasteka3, Miroslav Bielik3, Sylvain Bonvalot4, Carla Braitenberg5, Jörg Ebbing6, Gerald Gabriel7,8, Andrej Gosar9, Adam Grand3, Hans-Jürgen Götze6, György Hetényi10, Nils Holzrichter6, Edi Kissling11, Urs Marti12, Bruno Meurers13, Jan Mrlina14, Ema Nogová3, Alberto Pastorutti5, Corinne Salaun15, Matteo Scarponi10, Josef Sebera6, Lucia Seoane4, Peter Skiba7, Eszter Szűcs16, Matej Varga17

1Earth Science Institute, Slovak Academy of Sciences, Bratislava, Slovak Republic; 2Department of Theoretical Geodesy and Geoinformatics, Slovak University of Technology, Bratislava, Slovak Republic; 3Department of engin. geology, hydrogeology and applied geophysics, Comenius University, Bratislava, Slovak Republic; 4Bureau Gravimétrique International, Toulouse and GET, University of Toulouse, France; 5Department of Mathematics and Geosciences, University of Trieste, Italy; 6Institute of Geosciences, Christian-Albrechts-University Kiel, Germany; 7Leibniz Institute for Applied Geophysics, Hannover, Germany; 8Institute of Geology, Leibniz University Hannover, Germany; 9Slovenian Environmental Agency, Seismology and Geology Office, and University of Ljubljana, Slovenia; 10Institute of Earth Sciences, University of Lausanne, Switzerland; 11Department of Earth Sciences, Federal Institute of Technology (ETH), Zürich, Switzerland; 12Federal Office of Topography Swisstopo, Wabern, Switzerland; 13Department of Meteorology and Geophysics, University of Vienna, Austria; 14Institute of Geophysics, Czech Academy of Sciences, Prague, Czech Republic; 15Service Hydrographique et Océanographique de la Marine, Brest, France; 16Geodetic and Geophysical Institute, RCAES, Hungarian Academy of Science, Sopron, Hungary; 17Department of Civil, Environmental and Geomatic Engineering, Federal Institute of Technology (ETH), Zürich, Switzerland

The international networking initiative AlpArray Gravity Research Group (AAGRG) focused on the compiling homogeneous surface-based gravity datasets across the Alps and adjacent areas, on creating digital data sets for Bouguer and Free Air anomalies. In 2016/17 all ten countries around the Alps have agreed to contribute with point or gridded gravity data and data processing techniques to a recompilation of the Alpine gravity field in an area which is limited by 2° to 23° East and 41° to 51° North. For this recompilation, the group rely on existing national data.

The AAGRG decided to present the data set of the recalculated gravity fields on a 2km x 2km and 4km x 4km grid for the public. The densities used are 2670 kg/m3 for landmasses, 1030 kg/m3 for water masses above and -1640 kg/m3 below the ellipsoid. The correction radius was set to the Hayford zone O2 (167 km). The new Bouguer anomaly is compiled according to the most modern standards and reference frames (both location and gravity). Geophysical indirect effect and atmospheric corrections are also considered. In the Western Mediterranem (Ligurian Sea) completely reprocessed ship data of the Service Hydrographique et Océanographique de la Marine/Bureau Gravimétrique International were used. Marginal parts of the map were filled by GGM data.

Main aim of the work of the AAGRG is to release a gravity data base which can be used for high-resolution modelling, interdisciplinary studies from local to regional to continental scales, as well as for joint inversion with other datasets.

2:15pm - 2:30pm

New constraints on the Ivrea Geophysical Body at intra-crustal scales: a combination of gravimetry with passive seismology and rock’s physical properties

Matteo Scarponi, György Hetényi, IvreaArray Team

University of Lausanne, Switzerland

We present a high-resolution investigation of the Ivrea Geophysical Body (IGB) at intra-crustal scales in the Western Alps. The IGB is a sliver of Adriatic lower lithosphere, located at anomalously shallow depths, and presenting positive gravity and fast seismic anomalies. Despite comprehensive information from previous studies, structural questions persist on the IGB and on its structural relation with the Ivrea-Verbano zone (IVZ), which exposes lower-to-middle crustal composition outcrops at the surface. Therefore, we measured 207 new gravity data points, obtaining a coverage of ca. 1 point every 4-to-9 km2 across the IVZ, and we installed 10 broadband seismic stations (IvreaArray) along the linear West-East profile of Val Sesia, operated for 27 months. We compiled a surface rock-density map and used it to define the density-dependent terrain-corrected “Niggli” gravity anomaly to properly model the IGB density structure at depth. We modelled the IGB as a 3D, single density-contrast interface, obtaining 400 ± kg·m-3 as optimal density contrast and a 20-km wide protruding structure, as shallow as 1 ± 1 km below sea level. The seismic data was then used to constrain the IGB shape along the 2D Val Sesia cross-section by means of a joint inversion of seismic receiver functions and gravity anomaly data. This has confirmed the marked density contrast and shallow segments reaching 1 to 3 km depth below sea level, and provide agreement with the rock’s physical properties (ρ, vS) and the geological structures observed at the surface. These results are now published (doi:10.1093/gji/ggaa263 and doi:10.3389/feart.2021.671412).

2:30pm - 2:45pm

Residual gravity anomalies in the Western Mediterranean shed light on complex crust

Hans-Jürgen Götze, Philipp Tabelow

Christian-Albrechts-Universität zu Kiel, Germany

The data sets presented here are used for the preparations of a 3-dimensional modelling of the gravity field in the Western Mediterranean/Ligurian Sea. As part of the AlpArray initiative and the German priority program MB-4D, various compilations of the gravity field are available: Bouguer and Free Air Anomaly, as well as variously calculated residual fields that provide new insight into crustal and lithospheric structures. The anomalies have been processed according to modern standards. The recalculations were part of the research of the international AlpArray Gravity Group. In particular, the residual fields in the area of the Ligurian Sea show hitherto unknown small-scale anomalies after subtraction of long-wave components (satellite gravimetry). The short wavelengths in the gravity field of different magnitudes suggest strong structuring of the lithosphere e.g., offshore Marseille (with an anomaly of about 60 mGal and in the areas between the French-Italian mainland and Corsica-Sardinia (with up to 100 mGal). Furthermore, the new compilations suggest that the crustal underground of this area is not formed by a uniform basin, but by domains of rather different densities. The Italian coastal region between Genoa and Livorno is characterized by a belt of positive anomalies (up to 60 mGal). The subsurface beneath the two islands of Corsica and Sardinia is characterized by strong negative anomalies in the residual field, indicating density deficits beneath. The new findings are supported by applications of Euler deconvolution, gradient methods, directional filters, and curvature calculations considering also the evaluation of GOCE gradients.

2:45pm - 2:48pm

Lithospheric contact of the Western Carpathians with the Bohemian Massif in the light of seismic and new AlpArray gravity data

Dominika Godová1,2, Miroslav Bielik1,2, Pavla Hrubcová3, Roman Pašteka2, Pavol Zahorec1, Juraj Papčo4

1Earth Science Institute of the Slovak Academy of Sciences, Slovak Republic; 2Department of Engineering Geology, Hydrogeology and Applied Geophysics, Faculty of Natural Sciences, Comenius University in Bratislava, Slovak Republic; 3Department of Seismology, Institute of Geophysics of the Czech Academy of Sciences, Czech Republic; 4Department of Theoretical Geodesy, Faculty of Civil Engineering, Slovak University of Technology in Bratislava, Slovak Republic

The Bohemian Massif represents the largest exposure of rocks deformed during the Variscan orogeny. Western Carpathians form an arc-shaped mountain range related to the Alpine orogeny. In our study, the lithospheric structure of the key tectonic units in the area and their contact zone was analyzed by 2D gravity modelling along the NW-SE oriented CEL09 profile of the CELEBRATION 2000 seismic experiment. New gravity map compiled at the initiative of the AlpArray Gravity Research Group was used. This map is based on a uniform reprocessing of the national terrestrial gravimetric databases of ten countries of the wider Alpine region. The resultant 2D density model based on gravity data was constrainted by results of seismic reflection and refraction method. Applied densities were defined by transformation of the modelled P-wave velocities. A good correlation between the density and seismic models was shown. The resultant 2D density model consisting of five principal layers (sediments, upper crust, lower crust, lower lithosphere and asthenosphere) shows differences between the older, cooler and thicker Bohemian Massif (in average: ~32 km thick crust, and ~120 km thick lithosphere), and the younger, warmer and thinner Carpathian-Pannonian region (~28 km crust, ~95 km lithosphere). The detected contact is delimited by a change in the Moho and the LAB topography, and assumes an overthrusting of the Western Carpathians onto the Bohemian Massif by ~30 km resulting in a neo-transformation of the crust/mantle and related lithosphere after subduction.

2:48pm - 2:51pm

Pre-processing of gravity data for 3 D-modelling of the lithospheric underground in the Ligurian Sea

Philipp Tabelow, Hans-Jürgen Götze

Christian-Albrechts-Universität zu Kiel, Germany

The Ligurian Sea in the western Mediterranean Sea is a back arc basin created through the Apennines Calabrian subduction zone between 30 and 15 Ma ago. The inner geological structure of this basin is not well known. To improve the knowledge about the density distribution of the crust and lithosphere, we performed a pre-processing of gravity data prior to 3D-modelling. This work is related to research in the MB-4D priority and AlpArray project.

The satellite gravity gradients from GOCE were directly interpreted and used for filtering of different wavelengths to calculate residual fields, Bouguer and Free-Air anomalies as well as invariants and Euler-Deconvolutions. Furthermore, seismic profiles from several ship-borne surveys as well as OBS measurements of the AlpArray project (LOBSTER, GEOMAR, Kiel) and bathymetry data contributed additional information.

The processed data show an unknown anomaly offshore Marseille and the possibility of several underground structures with different densities. The basin itself is characterized by a mass surplus and positive anomalies with a maximum between Corsica and north-west Italia, while the anomalies underneath Corsica and Sardinia are neutral to negative.

The derived information will be used in the 3D-modelling software IGMAS+ to execute an inversion for the area and create a model of the mass distribution beneath the Ligurian Sea and its margins.

2:51pm - 2:54pm

Lithospheric-scale 3D model of Sicily domain based on gravity analysis

Giovanni Floridia1, Denis Anikiev2, Marco Viccaro1,3

1Università di Catania, Dipartimento di Scienze Biologiche Geologiche e Ambientali, Catania, Italy; 2GFZ German Research Centre for Geosciences, Potsdam, Germany; 3Istituto Nazionale di Geofisica e Vulcanologia – Sezione di Catania, Osservatorio Etneo, Catania, Italy

Sicily is a part of the central-Western Mediterranean area and represents a geotectonic boundary between the African and European plates. It is the result of a complex geological process based on a polyphasic evolution of a compressional step beginning with the Oligocene-Miocene clockwise rotation of Corsica-Sardinia simultaneously with the extensional processes of the Tyrrhenian basin. Consequently, the area is constrained by the continuing partial advance of the Sicilian-Maghrebian chain southwards and the Tyrrhenian extensional area towards the internal foreland areas (Hyblean domain). The study focuses on the creation of a 3D lithospheric-scale model of a 300 km x 400 km extended area in the central Mediterranean domain (Lat38°, Lat35°), which is consistent with the available geological and geophysical data, as well as with the observed gravity field. The reconstructed (simplified) geological setting consists of a lithospheric mantle, a crystalline basement (continental and oceanic crust), carbonates, the European margin and the Neogene-quaternary cover including volcanic bodies. The work aims to investigate the geometry of lithosphere integrating tomographic models in order to assess the major density contrasts and the lithospheric thermo-mechanical state. The regional 3D model provides also the boundary conditions for local thermal models to investigate afterwards.

2:54pm - 2:57pm

Gravity forward modelling and inversion based on the updated, enhanced gravity field solution in Antarctica

Theresa Schaller1, Mirko Scheinert1, Philipp Zingerle2, Roland Pail2, Martin Willberg2

1Geodetic Earth System Research, Technische Universität Dresden, Germany; 2Institute of Astronomical and Physical Geodesy, Technical University of Munich, Germany

Geoscientific studies in Antarctica are extremely challenging due to the remote location of the continent, its harsh environment and difficult logistics. Additionally, the continental crust is covered by an up to 5 km thick ice sheet, which makes surface based geoscientific studies extremely difficult. Gravity field measurements and gravity based subsurface models are therefore essential in studying the structure, properties and processes of the Antarctic subsurface.

In the last decades a large database of airborne, shipborne and ground based gravity data has been compiled. Recently, all existing and new gravity data were processed to infer an enhanced gravity field solution for Antarctica.

Subsequently, this new gravity field solution can be used for further geophysical studies. We use gravity disturbances to study subglacial topography, sediment thickness and Moho depths to improve respective existing models in Antarctica.

Studying these parameters on a continental scale, we apply 2D Parker-Oldenburg inversion in combination with results from other gravity based studies and further constraining data.

Additionally, we make use of the higher resolution of the new gravity grid (5 km) to study smaller regions in more detail, specifically the Weddell Sea area and Queen Mary Land. Here, we use gravity forward modelling constrained with ice penetrating radar and seismic data to infer geometric structure and densities of the subsurface.

In this contribution we present results of the Parker-Oldenburg Inversion and discuss the underlying parameters. Also, we show the resulting 3D forward models of the Weddell Sea area and Queen Mary Land.