Plate reconstruction studies show that the Neotethys Ocean was closing due to the convergence of Africa and Eurasia toward the end of the Cretaceous. The period around 75 Ma reflects the onset of continental collision between the two plates as convergence continued to be taken up mostly by subduction of the Neotethys slab beneath Eurasia. The Owen transform plate boundary in the northeast accommodated the fast northward motion of the Indian plate relative to the African plate. The rest of the plate was surrounded by mid‐ocean ridges. Africa was experiencing continent‐wide rifting related to northeast‐southwest extension. We aim to quantify the forces and paleostresses that may have driven this continental extension. We use the latest plate kinematic reconstructions in a grid search to estimate horizontal gravitational stresses (HGSs), plate boundary forces, and the plate's interaction with the asthenosphere. The contribution of dynamic topography to HGSs is based on recent mantle convection studies. We model intraplate stresses and compare them with the strain observations. The fit to observations favors models where dynamic topography amplitudes are smaller than 300 m. The results also indicate that the net pull transmitted from slab to the surface African plate was low. To put this into context, we notice that available tectonic reconstructions show fragmented subduction zones and various colliding micro‐continents along the northern margin of the African plate around this time. We therefore interpret a low net pull as resulting from either a small average slab length or from the micro‐continents' resistance to subduction.

<p>This work is part of the "Subsidence" DeepNL project which aims to identify subsurface drivers of subsidence above the Groningen (the Netherlands) gas field and to forecast future subsidence. The hydrocarbon extraction in Groningen induces a pressure reduction in the gas reservoir which triggers compaction and land subsidence. This deep-subsurface process is modeled by a disc-shaped reservoir model, which is a superposition of individual nuclei of strain based on the Geertsma's approach. We estimate the surface deformation and the strength of the disc strain using a particle method. We apply the method to one single nucleus of strain at 3 km depth and extend to a disc-shape geometry. Synthetic experiments with a single nucleus of strain and with discs of varying sizes, 2.2 km to 13.3 km diameter, at 3 km depth are performed to assess the performance of the method for an increasing degree of complexity. Sequential Importance Resampling prevents the sample degeneracy when the number of nuclei increases. Adding a jitter noise in the resampling step avoids an impoverishment of the ensemble values. The results indicate that the method estimates the surface deformation and the strength for a large number of sources and for a relatively small effective ensemble size. In further investigations, localization can provide an additional means to deal with increasing dimensions and a relatively small ensemble size.</p>

<p>Hydrocarbon production may cause subsidence as a result of the pressure reduction in the gas-producing layer and reservoir compaction. To analyze the process of subsidence and estimate reservoir parameters, we use a particle method to assimilate Interferometric synthetic-aperture radar (InSAR) observations of surface deformation with a conceptual model of reservoir. As example, we use an analytical model of the Groningen gas reservoir based on a geometry representing the compartmentalized structure of the subsurface at the reservoir depth.</p><p>The efficacy of the particle method becomes less when the degree of freedom is large compared to the ensemble size. This degree of freedom, in turn, varies because of spatial correlation in the observed field. The resolution of the InSAR data and the number of observations affect the performance of the particle method.</p><p>In this study, we quantify the information in a Sentinel-1 SAR dataset using the concept of Shannon entropy from information theory. We investigate how to best capture the level of detail in model resolved by the InSAR data while maximizing their information content for a data assimilation use. We show that incorrect representation of the existing correlations leads to weight collapse when the number of observation increases, unless the ensemble size growths. However, simulations of mutual information show that we could optimize data reduction by choosing an adequate mesh given the spatial correlation in the observed subsidence. Our approach provides a means to achieve a better information use from available InSAR data reducing weight collapse without additional computational cost.</p>

<p>Since the start of production in 1968 in the Groningen gas field (Netherlands) considerable land subsidence (>30 cm) has occurred above the field. The PS-InSAR technique provides surface deformation data with a high spatial and temporal resolution. However, using this data to obtain a detailed solution for the reservoir compaction distribution is not trivial because of: (1) uncertainties in the data (e.g., atmospheric noise), (2) soil deformation (e.g., clay shrinkage and peat oxidation), and (3) non-uniqueness of different reservoir compaction distributions leading to identical surface signals. This study is part of the DeepNL/Subsidence project, where we aim to identify the drivers of subsidence in the Groningen field by assimilating geodetic time series in geomechanical models of the subsurface, to disentangle the signals from deep (reservoir and overburden) and shallow (soils) drivers. In this study we perform a sensitivity analysis to investigate the level of complexity that can be resolved for the reservoir geometry, for the reservoir compaction, and for the mechanical properties of the reservoir and overburden.</p><p>We employ a semi-analytical mechanical model for the Groningen subsurface using the PSGRN/PSCMP code by Wang et al. (2006). The model geometry is based on the geological model constructed by the producer (NAM, 2020). We simplify the geology by using lateral uniform material properties and we construct multiple versions of the mechanical model with different levels of complexity (of reservoir geometry, reservoir compaction and mechanical properties). The synthetic surface deformation results produced by the models are used in an inversion to investigate whether the applied complexities can be recovered from inverting the surface signal. Information theory analysis is used to determine how much of the originally applied information is recovered by the inversion and to define a threshold complexity. Model versions that are more complex than the threshold complexity lead to non-uniqueness inhibiting a robust solution. We finally analyse the spatial characteristics of the surface deformation results, to determine whether the InSAR data uncertainty warrants an additional lowering of the complexity threshold.</p><p> </p><p>NAM (2020). Petrel geological model of the Groningen gas field, the Netherlands. Open access through EPOS-NL. Yoda data publication platform Utrecht University.</p><p>Wang, R., Lorenzo Martín, F., Roth, F. (2006): PSGRN/PSCMP - a new code for calculating co- and post-seismic deformation, geoid and gravity changes based on the viscoelastic-gravitational dislocation theory. Computers and Geosciences, 32, 4, 527-541. https://doi.org/10.1016/j.cageo.2005.08.006</p>

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