DELINEATION OF LINEAMENTS IN THE REGION OF OUADDAI (CHAD) USING GRAVITY DATA

The gravity map of Ouaddai in the eastern region of Chad exhibits important anomalies that are often delimitated by the high gravity gradients, resulting from the density contrasts between various anomaly sources. This presumes an important tectonic activity in zone. Because of its arid and desert nature, the region is water-poor or even lacking water resources. The main source of water in the region is made of very deep aquifers. The gravity analysis from this study helps to better understand the network of faults in the area. Many complementary approaches of the gravity data processing have been applied, namely the horizontal gradient coupled to the upward and downward continuation and the Euler method. Results from the data filtring have allowed highlighting faults network in various directions (SSW-NNE S-N E-W, NE-SW) and the main direction. In total, 37 major faults were detected with various lengths including 11 (F6, F9, F10, F12, F17, F29, F31, F34, F35, F37) for a cumulative total length of 353 km oriented towards Bongor basin and 15 others (F1, F4, F7, F8, F11, F13, F18, F20, F23, F25, F26, F27, F28, F30, F33), for a cumulative total length of 621 km, oriented towards Doba basin. These faults are of paramount importance with great potential impacts on the regions hydrocarbon reservoirs. Thise results in one hand confirmed some known faults from the previous investigations. In the other hand, the study helps to identify other new and unknown tectonic signatures.

Effect of gravity curvature on large-scale atomic gravimeters

<p>In terrestrial geodesy, absolute gravimetry is a tool to observe geophysical processes over extended timescales. This requires measurement devices of high sensitivity and stability. Atom interferometers connect the free fall motion of atomic ensembles to absolute frequency measurements and thus feature very high long-term stability. By extending their vertical baseline to several meters, we introduce Very Long Baseline Interferometry (VLBAI) as a gravity reference of higher-order accuracy.</p><p>By using state-of-the-art vibration isolation, sensor fusion and well controlled atomic sources and environments on a 10 m baseline, we aim for an intrinsic sensitivity σ<sub>g</sub> ≤ 5 nm/s² in a first scenario for our Hannover VLBAI facility. At this level, the effects of gravity gradients and curvature along the free fall region need to be taken into account. We present gravity measurements along the baseline, in agreement with simulations using an advanced model of the building and surroundings [1]. Using this knowledge, we perform a perturbation theory approach to calculate the resulting contribution to the atomic gravimeter uncertainty, as well as the effective instrumental height of the device depending on the interferometry scheme [2]. Based on these results, we will be able to compare gravity values with nearby absolute gravimeters and as a first step verify the performance of the VLBAI gravimeter at a level comparable to classical devices.</p><p>The Hannover VLBAI facility is a major research equipment funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation). This work was supported by the DFG Collaborative Research Center 1464 “TerraQ” (Project A02) and is supported by the CRC 1227 “DQ-mat” (Project B07), Germany’s Excellence Strategy EXC-2123 “QuantumFrontiers”, and the computing cluster of the Leibniz University Hannover under patronage of the Lower Saxony Ministry of Science and Culture (MWK) and the DFG. We acknowledge support from “Niedersächsisches Vorab” through the “Quantum- and Nano-Metrology (QUANOMET)” initiative (Project QT3), and for initial funding of research in the DLR-SI institute, as well as funding from the German Federal Ministry of Education and Research (BMBF) through the funding program Photonics Research Germany.</p><p>[1] Schilling et al. “Gravity field modelling for the Hannover 10 m atom interferometer”.  Journal of Geodesy 94, 122 (2020)</p><p>[2] Ufrecht, Giese,  “Perturbative operator approach to high-precision light-pulse atom interferometry”. Physical Review A 101, 053615 (2020).</p>

Possibilities of transdimensional inversion for estimating deep Earth velocity and mantle structure

<p>Commonly, the physical properties of the Earth (e.g., velocity, density) are parameterized as continuous fields. The most popular representation are grids and basis functions like spherical harmonics or splines. In an inversion context it is quite common that not all the parameters are fully constrained by the available inputdata. This relates to the common issues of insufficient resolution, incomplete coverage, and trade-offs due tonon-uniqueness. By applying some form of regularization to the inverse problem, a well-behaved and unique solution can be obtained, but this solution depends on the details of the chosen regularization.</p><p>Transdimensional approaches address the regularization problem by using a model representation with a variable number of parameters. The number of parameters is adjusted according to the requirements of the input data using the reversible jump Monte Carlo Markov Chain (rj-MCMC) algorithm. The output is an ensemble of variable resolution models that provides insight into the required model complexity and trade-offbetween parameters.</p><p>Here, I present synthetic tests from a joint inversion of satellite gravity gradients and normal modes for the Earth's velocity and density structure. The mantle's seismic velocity and density inside a 2-D spherical annulus are described by a variable number of discrete anomalous volumes, each with a variable size, shape, location and strength of velocity and density anomaly. The discrete anomalies are adjusted using the transdimensional approach in order to fit the gravity and normal mode data. This synthetic example shows promising results, because the synthetic model can recovered reasonably well.</p>

Quantum sensors for space-borne earth observation

<p>Atom interferometry enables quantum sensors for absolute measurements of gravity (1) and gravity gradients (2). The combination with classical sensors can be exploited to suppress vibration noise in the interferometer, extend the dynamic range, or to remove the drift from the classical device (3). These features motivate novel sensor and mission concepts for space-borne earth observation e.g. with quantum gradiometers (4) or hybridised atom interferometers (5). We will discuss developments of atom optics and atom interferometry in microgravity in the context of future quantum sensors (6) and outline the perspectives for applications in space (4,5).</p><p>The presented work is supported by by the CRC 1227 DQmat within the projects B07 and B09, the CRC 1464 TerraQ within the projects A01, A02 and A03, by "Niedersächsisches Vorab" through "Förderung von Wissenschaft und Technik in Forschung und Lehre" for the initial funding of research in the new DLR-SI Institute, and through the "Quantum and Nano- Metrology (QUANOMET)" initiative within the project QT3.</p><p>(1) V. Ménoret et al., Scientific Reports 8, 12300, 2018; A. Trimeche et al., Phys. Rev. Appl. 7, 034016, 2017; C. Freier et al., J. of Phys.: Conf. Series 723, 012050, 2016; A. Louchet-Chauvet et al., New J. Phys. 13, 065026, 2011; A. Peters et al., Nature 400, 849, 1999.</p><p>(2) P. Asenbaum et al., Phys. Rev. Lett. 118, 183602, 2017; M. J. Snadden et al., Phys. Rev. Lett. 81, 971, 1998.</p><p>(3) L. Richardson et al., Comm. Phys. 3, 208, 2020; P. Cheiney et al., Phys. Rev. Applied 10, 034030, 2018; J. Lautier et al., Appl. Phys. Lett. 105, 144102, 2014.</p><p>(4) A. Trimeche et al., Class. Quantum Grav. 36, 215004, 2019; K. Douch et al., Adv. Space. Res. 61, 1301, 2018.</p><p>(5) T. Lévèque et al., arXiv:2011.03382; S. Chiow et al., Phys. Rev. A 92, 063613, 2015.</p><p>(6) M. Lachmann et al., arXiv:2101.00972; K. Frye et al., EPJ Quant. Technol. 8, 1, 2021; D. Becker et al., Nature 562, 391, 2018; J. Rudolph et al., New J. Phys. 17, 065001, 2015; H. Müntinga et al., Phys. Rev. Lett. 110, 093602 , 2013.</p>

The thermochemical structure of West Antarctica from multi-observable probabilistic inversion

<p>The interaction between ice sheets and mantle dynamics is crucial to understanding the present-day topography in many regions (Antarctica, Patagonia, North America, Scandinavia) and recent ice mass losses on a large scale. A better knowledge of mantle rheology and the physical properties beneath these regions will improve our understanding of this interaction. To better characterize these processes, we investigate the present-day thermochemical structure (temperature and major-element composition) of the lithospheric and sub-lithospheric mantle. The thermal structure provides indirect information on variations in mantle viscosity, key parameter in glacial isostatic adjustment models (GIA). Recent geophysical studies in Antarctica show a relationship between mantle viscosity inferred from GIA and seismic velocity anomalies. Here we use a 3-D multi-observable probabilistic inversion method to retrieve estimates of the thermal and lithological structures (velocities and densities) beneath West Antarctica at a resolution of 1°x1°. The method is based on a probabilistic (Bayesian) formalism and jointly inverts Rayleigh wave dispersion data, bouguer gravity anomalies, satellite‐derived gravity gradients, geoid height, absolute elevation and surface heat flow. With the Markov chain Monte Carlo procedures applied here, we use highly optimized forward problem solvers to sample the parameter space and determine geological structure and feature with full characterization of their uncertainties. In this presentation, we will discuss the main results, interpretation in terms of mantle rheology, and its implication for GIA model in this region.</p>

Validation of calibrated GOCE gravity gradients GRD_SPW_2 by least-squares spectral weighting

<p>The Gravity field and steady-state Ocean Circulation Explorer (GOCE) was the first mission which carried a novel instrument, gradiometer, which allowed to measure the second-order directional derivatives of the gravitational potential or gravitational gradients with uniform quality and a near-global coverage. More than three years of the outstanding measurements resulted in two levels of data products (Level 1b and Level 2), six releases of global gravitational models (GGMs), and several grids of gravitational gradients (see, e.g., ESA-funded GOCE+ GeoExplore project or Space-wise GOCE products). The grids of gravitational gradients represent a step between gravitational gradients measured directly along the GOCE orbit and data directly from GGMs. One could use grids of gravitational gradients for geodetic as well as for geophysical applications. In this contribution, we are going to validate the official Level 2 product GRD_SPW_2 by terrestrial gravity disturbances and GNSS/levelling over two test areas located in Europe, namely in Norway and former Czechoslovakia (now Czechia and Slovakia). GRD_SPW_2 product contains all six gravity gradients at satellite altitude from the space-wise approach computed only from GOCE data for the available time span (r-2, r-4, and r-5) and provided on a 0.2 degree grid. A mathematical model based on a least-squares spectral weighting will be developed and the corresponding spectral weights will be presented for the validation of gravitational gradients grids. This model allows us to continue downward gravitational gradients grids to an irregular topographic surface (not to a mean sphere) and transform them into gravity disturbances and/or geoidal heights in one step. Before we compared results obtained by spectral downward continuation, we had to remove the high-frequency part of the gravitational signal from terrestrial data because in gravitational gradients measured at GOCE satellite altitude is attenuated. To do so we employ EGM2008 up to d/o 2160 and the residual terrain model correction (RTC) has been a) interpolated from ERTM2160 gravity model, b) synthesised from dV_ELL_Earth2014_5480_plusGRS80, c) calculated from a residual topographic model by forward modelling in the space domain.  </p>

GeoGravGOCE: A GOCE SGG processing software for datum transformations and filtering

<p>Whilst GOCE SGG data have been widely processed and used in geodetic research, one of the key points of their use is to have a one-stop software for their pre-processing and basic manipulations in terms of frame transformations and filtering operations. Within the GeoGravGOCE project, funded by the Hellenic Foundation for Research Innovation, the main goal is the optimal combination of GOCE Satellite Gravity Gradiometry (SGG) data with in-situ observations for geoid determination. During the project development, it became apparent that GOCE SGG data after using the GOCEPARSER, had to be pre- and post-processed via several own-developed routines in order to perform data quality checks, data consistency tests, reference frame transformations, data reductions and filtering. With that in mind, a standalone open-source software has been developed in MATLAB consisting of a Graphical User Interface (GUI) to perform the aforementioned operation. The software is divided in four tabs and is designed to process the original GOCE gravity gradients, which are the second-order derivatives of the gravitational potential. The first tab of the software is designed to allow the pre-processing of the Level 2 Electrostatic Gravity Gradiometer nominal gravity gradients (EGG_NOM) and Satellite to Satellite Tracking Precise Science Orbits (SST_PSO) products. The second tab enables the transformation of gravity gradients from a Global Geopotential Model (GGM) from the Local North Oriented Frame (LNOF) to the Gradiometer Reference Frame (GRF). The third tab provides filtering options for the reduced SGG observations and encompasses three different methods: Finite Impulse Response (FIR), Infinite Impulse Response (IIR), and Wavelet Multi-Resolution Analysis (WL-MRA). Finally, the fourth tab allows the transformation of SGG data from the GRF to the LNOF and vice versa. In this work, we present the basic software development procedure and outline its basic functionality and results.</p>

Continental Hotspots Tracks from Analysis of GOCE Gravity Gradients Data

<p>Hotspots are thermal instabilities that originate in the mantle and manifest themselves on the surface by volcanism, continental breaks or "traces" observed in the oceans. Theirs effects under the continents are still debated: in addition to a phase of activity associated with surface volcanism, a residual thermal anomaly could persist durably under the lithosphere along the trajectory of the hotspot.<br>For a simple model of thermal anomaly (parallelogram aligned in a fixed direction), we compute the perturbations of the geoid, of the gravity vector and of the associated gravity gradients. We show that in a coordinate system aligned with the parallelogram, gravity gradients have a characteristic signal with an order of magnitude of a few hundred mEotvos, well above the current data detection level. Thus for four real cases: in North Africa (with the Hoggar, Tibesti, Darfur and Cameroon hotspots), in Greenland (Iceland and Jan Mayen), in Australia (Cosgrove) and in Europe (Eifel), we calculate the paleo-positions of the hotspots during the last 100 Ma in a reference frame linked to the lithospheric plates, and we build maps of gravity gradients at different altitudes filtered at the spatial scale of a few hundred kilometers (scale of the hotspot) and oriented along the direction of the trajectory.<br>We clearly find signals aligned in the direction of the movement of the plates on spatial scales of a few hundred kilometers.<br>This signal is sometimes correlated with the topography and it is difficult to separate the sources resulting from volcanic edifices and their associated isostatic crustal roots from that induced by residual thermal anomaly. These results show that gradiometric data are able to detect and follow the tracks of hotspots in the continental lithosphere, during at least a few tens of millions of years, providing new clues to constrain their trajectory and improve reference frame tied to the mantle.</p>