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In this contribution, activities of the AlpArray Gravity Research Group (AAGRG) are introduced. Since 2018, this group in the frame of the AlpArray project (http://www.alparray.ethz.ch/en/home/) prepares gravity and other data sets to support multidisciplinary goals of the project. It is focused on the mantle, plate and surface processes in the Alps-Apennines-Carpathians-Dinarides orogenic system. In 2018, the AAGRG set up its own methodology guidelines and assembled available land-gravity data and digital elevation model (DEM) data from all the participating countries. Our presentation targets especially three goals: 1) to introduce AAGRG activities as an integral part of the AlpArray project for studying Alpine orogeny – a goal that by definition requires international cooperation, 2) to review the input data and the works accomplished so far, and, 3) to discuss the steps to be taken to produce detailed gravity maps of the region – the first Alps-dedicated gravity field data set. We plan to prepare gravity grids in a homogeneous processing approach of either 2x2 km or 4x4 km resolution, depending on the coverage and data quality. The final data sets will be made public in late 2019. A special emphasis is put on the calculation of the Bouguer anomaly using ellipsoidal rather tha normal heights. For calculating topographic effects the preference is given to local DEMs, where available, as they often provide higher quality and spatial resolutions. The public gravity data sets are evaluated with the high-resolution geopotential models like EIGEN-6C4 or EGM2008 – a useful means for identifying biases in the data coming from various countries and campaigns.

A pan-Alpine gravity data map, homogeneous regarding input data sets, applied methods and corrections as well ascommon reference frames, is not available yet. Therefore, all countries around the Alps have agreed to contributewith gravity data and/or gravity data processing techniques to a recompilation of the alpine gravity field in theframe of the AlpArray Gravity Research Group (AAGRG). Also to collaborate with other groups in AlpArray.In the last year group members met two times to set up guidelines for joint processing and homogenizationof existing gravity data sets. Following the results and appointments of the last technical AAGRG meeting inBratislava October 2018, the members of the 9 participating countries decided to present a first data set of theBouguer gravity field in September 2019 on a 2km x 2km or 4km x 4km grid for the public. Other compilationswill follow: maps of Free Air Gravity, regional field and the corresponding residual gravity field. These new datasets will be available to all AlpArray partners for interdisciplinary work and modelling.This new Bouguer anomaly will be station completed with at least 1 point/4 km2 resolution and compiledaccording to the most modern criteria. In general, even though global models like SRTM1 perform well, thepreference was given to local DEMs - if available. Two software packages for topographic corrections on thebase of ellipsoidal heights are available: “TOPOSK”, developed by Slovak colleagues, and the “adaptive masscorrection” from the Kiel group. The results of both methods are being tested and compared. The geophysicalindirect effect and its determination are also taken into account. Special emphasis is put on the lakes in the studyarea. They have a considerable effect on the gravity of stations that lie at their edges (for example, the partiallyvery deep reservoirs in the Alps). In the Ligurian and the Adriatic seas ship data of the Bureau GravimétriqueIinternational will be used. Although not unproblematic, these data got the preference over satellite data.In the AAGRG, long discussions were centered on the calculation of long-distance effects of topogra-phy/bathymetry and its compensating masses (root). The first compilation of the AA Bouguer anomaly mapwill, however, be prepared WITHOUT taking long-distance effects in account. The group agreed to extendthe correction radius to the Hayford zone O2(167 km). All topographic corrections will be calculated as masscorrections between the physical surface and the ellipsoidal reference. The group further discussed the necessityof a 3D interpolation of the gravity grid values. Although a 3D interpolation would be required from themethodological-theoretical point of view (the problem is the non-identical gravity and height grids), it was decidedto perform a 2D interpolation in the first compilation - the method of Kriging shall be applied. The poster will visualize the mentioned correction effects and illuminate the necessity of a complete revision of the existing land based Alpine gravity data.

AlpArray Gravity Research Group (AAGRG) compiles a homogeneous gravity data set across the Alpine area, creates related gravity products and uses them for studies from small to regional to continental scales, as well as for joint inversion with other datasets.

Für die öffentliche Kommunikation geophysikalischer Arbeiten spielt eine anschauliche Präsentation der Sachverhalte eine entscheidende Rolle. Seit in den 1980er Jahren erste Ansätze für Rapid Prototyping vorgestellt wurden, hat sich die Technologie von 3D Druckern stetig weiterentwickelt und wird inzwischen regelmäßig in verschiedenen wissenschaftlichen Disziplinen von den Ingenieurswissenschaften bis hin zu Medizin und Chemie eingesetzt, um Vorserien-Prototypen, Bau- und Ersatzteile sowie Anschauungsmodelle günstig und schnell produzieren zu können. In den Geowissenschaften und speziell in der Geophysik wurden diese Verfahren bisher sehr selten, zur Anfertigung von Ersatz- und Zusatzteilen für verschiedene Messinstrumente, verwendet. Im Rahmen des DGMK-Projektes TiPot3D wurde in der Arbeitsgruppe Geophysik und Geoinformation der Christian-Albrechts-Universität zu Kiel ein 3D Drucker Ultimaker² angeschafft und steht dort seit 2013 für spezielle geophysikalische Anwendungen bei der Interpretation und für die Visualisierung von 3D Modellen der Erdkruste zur Verfügung. Für die Präsentation von Daten nach einem 3D Druck ist kein zusätzliches Equipment notwendig. Diese neue Visualisierungsmöglichkeit bietet sich somit vor allem dann als Kommunikationsmedium an, wenn die Präsentation z.B. mittels 3D Computergrafik nicht angemessen ist oder technisch nicht in Frage kommt. Um Daten und Modelle mit Hilfe des 3D Drucks zu repräsentieren, müssen die Eingabedaten als digitale triangulierte Geometriemodelle vorliegen oder aber in diese überführt werden. Es wird anhand mehrerer unterschiedlicher Beispiele (z.B. 3D Untergrundmodelle aus Seismik und Gravimetrie, sowie Erdbebendeformationskarten) gezeigt, in welcher Weise geophysikalische Daten und Ergebnisse unterschiedlicher Komplexität für den 3D Druck aufbereitet und repräsentiert werden. Generell lassen sich die meisten geophysikalischen Datensätze mittels 3D Druck repräsentieren. Der zu betreibende Aufwand hängt dabei sowohl von der Komplexität der Eingabedaten, als auch vom kommunikativen Zweck und der beabsichtigten Größe des zu druckenden Datensatzes ab. Die gezeigten Ergebnisse haben sich vor allem für fachfremdes Publikum als ein eindrucksvolles Präsentationsmittel erwiesen.

The new techniques are user-friendly because they are highly interactive, ideally real-time and topology conserving and can be used for both flat and spherical models in 3D. These are important requirements for joint inversion for gravity and magnetic modelling of fields and their derivatives, constrained by seismic and structural input from independent data sources. A borehole tool for magnetic and gravity modelling will also be introduced. We are already close to satisfying the demand of treating several geophysical methods in a single model for subsurface evaluation purposes and aim now for fulfilling most of the constraints: consistency of model results and measurements and geological plausibility as well. For 3D gravmag modelling, polyhedrons built by triangles are used. All elements of the gravity and magnetic tensors can be included. In the modelling interface, after geometry changes the effect on the model is quickly updated because only the changed triangles have to be recalculated. Because of the triangular model structure, our approach can handle complex structures very well and it is flexible (e.g. overhangs of salt domes or plumes). For regional models, the use of spherical geometries and calculations is necessary and available. 3D visualization is performed with a 3D-printer (Ultimaker 2) and gives new insights into even rather complicated Earth subsurface structures. Inversion can either be run over the whole model, but typically it is used in smaller parts of the model, helping to solve local problems and/or proving/disproving local hypotheses. The basic principles behind this interactive approach are high performance optimized algorithms (CMA-ES: Covariance-matrix-adoption-evolution-strategy). The efficiency of the algorithm is rather high in terms of stable convergence due to topological model validity. Potential field modelling is always influenced by edge effects. To avoid this, a simple but very robust method has been developed: Derive a density/susceptibility-depth function by taking the mean value of the borders of depth slices through the model. The focus of the presentation is set on two practical study examples: one from the international KTB – Project, Germany´s deep continental borehole, as well as a very complex salt structure in the Northwest German Basin.

Any integrative interpretation by the aid of independent geophysical disciplines benefits from modern computational techniques. Today 3D seismic surveying and imaging, gradients of potential field (mostly aero- and/or satellite observations) are combined with on-/off-shore electro-magnetic surveys. In this paper we aim to describe new algorithms and software tools for 3D potential field and gradients of remanent and induced magnetization of complicated underground structures, the local stress field, and the distribution of stress beneath salt domes on base of constrained 3D models. E.g. the presence of salt domes causes a significant perturbation of in situ stress, which may cause serious implications e.g. for the stability of boreholes drilled in the vicinity of salt structures. Geological structures will be approximated by extremely flexible homogeneous polyhedrons (with respect to the domain’s density and/or susceptibility) and their fields and gradients are calculated by the transformation of volume integrals into a sum of line integrals. Alternatively, the approximating polyhedrons consist of variable density distributions which are recalculated from studies of seismic velocities. Magnetic modeling is possible by either applying Poisson’s Theorem or a modification of the derived formulas for gravity modeling. The inversion of potential fields is performed by the CMA-ES, the “covariance-matrix-adoption Evolution Strategy”. A new robust method was introduced for the reduction of edge effects of potential field data. Although the main structure of the new software already existed for 3D gravity modeling, it had be extended by a tool for magnetic modeling, a borehole gravity/susceptibility modeling tool and an immersive visualization of even complicated geological underground structures by 3D printing. Examples from the Gifhorn Trough and KTB are presented here to illustrate the user friendly newly developed software tools.

The new techniques used here are user-friendly because they are highly interactive, ideally real-time and topology conserving and can be used for both flat and spherical models in 3D. These are important requirements for joint inversion not only for gravity and magnetic modelling of fields and derivatives, constrained by seismic and structural input from independent data sources, but also essential toward a true integration of Full Waveform Inversion. A borehole tool for magnetic and gravity modelling will also be introduced. We are close to the demand of treating several geophysical methods in a single model for the subsurface and aim for fulfilling most of the constraints: measurements and geological plausibility. For 3D modelling, polyhedrons built by triangles are used. All elements of the gravity and magnetic tensors can be included. In the modelling interface, after geometry changes the effect of the model can quickly be updated because only the changed triangles have to be recalculated. Because of the triangular model structure our approach can handle complex structures very well (e.g. overhangs of salt domes). For regional models the use of spherical geometries and calculations can be necessary and is provided. 3D visualization is performed with a 3D-printer (Ultimaker 2) and provides new insights in even rather complicate Earth subsurface structures. Inversion can either be run over the whole model, but typically it is used in smaller parts of the model, helping to solve local problems and/or proving/disproving local hypotheses. Instead of optimizing the position of model vertices, interactive inversion uses a different parameterization of the model. The inner points of a lattice are used to define a distortion of space. The user can monitor model updates live on screen and stop the process at any time. The base principles behind this interactive approach are highly performance optimized algorithms (CMA-ES: Covariance-matrix-adoption-evolution-strategy). The efficiency of the algorithm is very good in terms of stable convergence due to topological model validity. Potential field modelling is always influenced by the area outside the core area of investigation, causing edge effects. To avoid these effects a simple but very robust method has been developed: Derive a density/susceptibility-depth function by taking the mean value of the borders of depth slices through the model. The focus of the poster presentation is set on practical examples from the international KTB – Project, Germany´s deep continental borehole.

In the frame of a PHD-work in the subproject F1/F4 of the Collaborative Research Center "Deformation processes in the Andes" (SFB 267) the analytical solution for estimation of the flexural rigidity (equivalent the elastic thickness) of the lithosphere based on given topography and moho undulations e.g. taken from a 3D density model or computed by an inversion from gravity data, was developed. The flexural rigidity is the parameter that governs the flexural response of the lithosphere in the meaning of a theoretical thin plate flexure model. This proposed analytical solution is an alternative methode to the widely used calculation of admittance of topography and gravity and the coherence methode, with some good advantages. Instabilities of the numerical admittance evaluation in wavenumber domain with low spectral energy of topography are overcome. The analysis can be made over an area which is much smaller than 350 km comparable to the coherence methode. The proposed methode allows a higher space resolution of elastic thickness than any other spectral methode. Instead of calculating the admittance function using spectral analysis, a set of point-load response functions is used in order to retrieve the optimal flexure parameter. Essential for the modeling is furthermore the isostatic flexure model, which is evaluated in terms of a convolutional approach (Braitenberg), thus overcoming the problems connected with a spectral analysis. This convolution methode was successfully applied in South China Sea and the Eastern Alps. Now the analytical solution is included within this convolution approach and was for the validation compared with the FFT-solution after Vening-Meinesz. This new methode has been applied in the SFB 267 working area of the Central Andes (15-$33\deg$S) and Southern Andes (36-$42\deg$S). Based on this Database the gravity field over the Andes has been used to construct 3D density models (IGMAS, Schmidt and Götze) from which the crustal thickness variations are obtained. Constraining data of this density models come from results of other projects of the SFB 267 e.g. the crust-mantle interface depth values recovered from seismic studies, reciever function study and the shipborne bathymetry measurements. The computation of spatial variations of the flexural rigidity leads us to understand the differences between the northern and the southern part of the Andes and to distinguish e.g. some tectonical provinces. The analytical computed flexure model and the gravity model of moho depth variations agree very well over all the areas.

The gravity field of the southern Central Andes and their eastern foreland between 20° and 30° S has been investigated with regard to the isostatic state, the crustal density structure of the orogeny and the rigidity of the lithosphere. The gravity database and constraining information for a 3D density model are derived from geophysical field data acquired in the Central Andes over many years. The data cover both the area of the northern steep subduction zone and the flat-slab area in the south. All gravity data are tied to the IGSN71 gravity datum and are terrain corrected. Topographic corrections and analysis of Andean topography is based on the 1 km × 1 km mean elevation data grid of the USGS (GTOPO30) and our own data. The gravity effect of the downgoing Nazca Plate was removed from both Bouguer and isostatic residual anomalies (Airy and Vening-Meinesz type) and then the anomalies were correlated with mean topographic heights to identify areas of disturbed isostatic equilibrium. Most of the morphological Andean units are close to isostatic equilibrium and, in particular, we find Airy-type equilibrium in the Main Cordillera. Departures from isostatic equilibirum are found in the Bolivian Altiplano/Argentina Puna, the SE foreland, in the Coastal Cordillera and along a large NW- to SE-striking zone crossing the volcanic arc. This structure can be linked to high density crustal remnants of Mesozoic rifting. Novel methods for 2D modelling of lithospheric rigidity which can account for surface and subsurface loads have been applied to calculate flexural rigidity. Rather low values (about 10 21 to 10 23 Nm, corresponding to 10–15 km effective elastic thickness) were obtained in the area of active Andean volcanoes. These low values reflect the elevated temperatures and crustal melting associated with the arc. Rigidities of 10 23 to 5×10 23 Nm (effective elastic thickness 35–45 km, and in some cases more than 50 km) are found for the backarc region. The Andean forearc region is characterized by a relatively high flexural rigidity of about 10 24 Nm, which is consistent with the existence of a cold and rigid lithospheric domain.