Fast linearized forecasts for subsurface flow data assimilation with ensemble Kalman filter

Ensemble methods present a practical framework for parameter estimation, performance prediction, and uncertainty quantification in subsurface flow and transport modeling. In particular, the ensemble Kalman filter (EnKF) has received significant attention for its promising performance in calibrating heterogeneous subsurface flow models. Since an ensemble of model realizations is used to compute the statistical moments needed to perform the EnKF updates, large ensemble sizes are needed to provide accurate updates and uncertainty assessment. However, for realistic problems that involve large-scale models with computationally demanding flow simulation runs, the EnKF implementation is limited to small-sized ensembles. As a result, spurious numerical correlations can develop and lead to inaccurate EnKF updates, which tend to underestimate or even eliminate the ensemble spread. Ad hoc practical remedies, such as localization, local analysis, and covariance inflation schemes, have been developed and applied to reduce the effect of sampling errors due to small ensemble sizes. In this paper, a fast linear approximate forecast method is proposed as an alternative approach to enable the use of large ensemble sizes in operational settings to obtain more improved sample statistics and EnKF updates. The proposed method first clusters a large number of initial geologic model realizations into a small number of groups. A representative member from each group is used to run a full forward flow simulation. The flow predictions for the remaining realizations in each group are approximated by a linearization around the full simulation results of the representative model (centroid) of the respective cluster. The linearization can be performed using either adjoint-based or ensemble-based gradients. Results from several numerical experiments with two-phase and three-phase flow systems in this paper suggest that the proposed method can be applied to improve the EnKF performance in large-scale problems where the number of full simulation is constrained.     

Assimilation of time-averaged observations in a quasi-geostrophic atmospheric jet model

The problem of reconstructing past climates from a sparse network of noisy time-averaged observations is considered with a novel ensemble Kalman filter approach. Results for a sparse network of 100 idealized observations for a quasi-geostrophic model of a jet interacting with a mountain reveal that, for a wide range of observation averaging times, analysis errors are reduced by about 50% relative to the control case without assimilation. Results are robust to changes to observational error, the number of observations, and an imperfect model. Specifically, analysis errors are reduced relative to the control case for observations having errors up to three times the climatological variance for a fixed 100-station network, and for networks consisting of ten or more stations when observational errors are fixed at one-third the climatological variance. In the limit of small numbers of observations, station location becomes critically important, motivating an optimally determined network. A network of fifteen optimally determined observations reduces analysis errors by 30% relative to the control, as compared to 50% for a randomly chosen network of 100 observations.     

Permittivity in the microwave region of some natural minerals

The permittivity measured at a frequency of 10 GHz normalized to constant density is the same within the experimental error for talc, baryte and dolomite. In the case of clay and kaolin, it is about 10% higher. This is attributed to the high content of Al₂O₃ (30–40%) in them, which interpretation is proved by experiment.     

Catchment scale hydrology of an irrigated cropping system under soil conservation practices

Soil erosion by water is a pressing environmental problem caused and suffered by agriculture in Mediterranean environments. Soil conservation practices can contribute to alleviating this problem. The aim of this study is to gain more profound knowledge of the effects of conservation practices on soil losses by linking crop management and soil status to runoff and sediment losses measured at the outlet of a catchment during seven years. The catchment has 27.42 ha and is located in a commercial farm in southern Spain, where a package of soil conservation practices is an essential component of the farming system. The catchment is devoted to irrigated annual crops with maize–cotton–wheat as the primary rotation. Mean annual rainfall‐induced runoff coefficient was 0.14 and mean annual soil loss was 2.4 Mg ha^?1 y^?1. Irrigation contributed to 40% of the crop water supply, but the amount of runoff and sediment yield that it generated was negligible. A Principal Components Analysis showed that total soil loss is determined by the magnitude of the event (rainfall and runoff depths, duration) and by factors related to the aggressiveness of the events (rainfall intensity and preceding soil moisture). A third component showed the importance of crop coverage to reduce sediment losses. Cover crops grown during autumn and early winter and crop residues protecting the soil surface enhanced soil conservation notably. The role of irrigation to facilitate growing cover crops in Mediterranean environments is discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

Neogene sediment deformations and tectonic features of northeastern Tunisia: evidence for paleoseismicity

The Neogene stratigraphic series is characterized by predominant clayey facies alternated by other sand layers. The outcrop and subsurface studies show varied and complex styles of deformations and lead to relate the structures to paleoseismic events. The seismicity of eastern onshore and offshore Tunisian margin follows the master fault corridors oriented globally N–S, E–W, and NW–SE that correspond to the bordering faults of grabens and syncline corridors and associated faulted drag fold structures oriented NE–SW. Epicenters of magnitudes between 3 and 5 are located along these border fault corridors. The Neogene strata record brittle structures, including numerous and deep faults and fractures with straight and high-angle dipping planes. The structuring of NE–SW en echelon folds and synclines inside and outside NW–SE and E–W right lateral and N–S and NE–SW left lateral tectonic corridors indicates the strike-slip type of bordering faults and their seismogenic nature. Wrench fault movements that induce mud and salt diapirs, mud volcanoes, and intrusive ascensions are related to seismic shocks. Seismic waves caused by activity along one, or most likely, several tectonic structures would have propagated throughout the Quaternary cover producing seismites. The similarity of deposits, structuring, and seismites between the Tunis-Bizerte to the North and Hammamet-Mahdia to the South accredits the hypothesis that the seismic episodes might have affected sedimentation patterns along the Sahalian large geographic area. The paleoseismic events in northeastern Tunisia might be related to tectonic fault reactivations through time. This hypothesis is consistent with the geomorphologic context of the study area, characterized by several morphostructural lineaments with strong control on the sediment distribution, as well as uplifted and subsiding terrains. The estimated magnitude of the seismic events and the great regional tectonically affected areas demonstrate that the northeastern Tunisia experienced stress through the last geological episodes of its evolution. This Neogene kinematic reconstruction highlights the neotectonic system inducing the actual seismicity on this margin. Therefore, there is a straight relationship between deepseated faults and seismicity.     

Correction to: Imagery of the metamorphic bedrock roof of the Sahel active fault in the Sablettes (Algiers) reclaimed area by ambient vibration HVSR

The original version of this paper was published with incomplete affiliation. Given in this article are the complete affiliations.     

Diagenetic evolution of the volcaniclastic deposits: an example from Neoproterozoic Dokhan Volcanics in Wadi Queih basin,central Eastern Desert,Egypt

Wadi Queih basin hosts a ～2,500-m thick Neoproterozoic volcanoclastic successions that unconformably lie over the oldest Precambrian basement. These successions were deposited in alluvial fan, fluviatile, lacustrine, and aeolian depositional environments. Diagenetic minerals from these volcaniclastic successions were studied by X-ray diffractometry, scanning electron microscopy, and analytical electron microscopy. The diagenetic processes recognized include mechanical compaction, cementation, and dissolution. Based on the framework grain–cement relationships, precipitation of the early calcite cement was either accompanied or followed by the development of part of the pore-lining and pore-filling clay cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cement and feldspar grains. In addition to calcite, several different clay minerals including kaolinite, illite, and chlorite with minor smectite occur as pore-filling and pore-lining cements. Chlorite coating grains helps to retain primary porosity by retarding the envelopment of quartz overgrowths. Clay minerals and their diagenetic assemblages has been distinguished between primary volcaniclastics directly produced by pyroclastic eruptions and epiclastic volcaniclastics derived from erosion of the pre-existing volcanic rocks. Phyllosilicates of the epiclastic rocks display wider compositional variations owing to wide variations in the mineralogical and chemical compositions of the parent material. Most of the phyllosilicates (kaolinite, illite, chlorite, mica, and smectite) are inherited minerals derived from the erosion of the volcanic basement complex, which had undergone hydrothermal alteration. Smectites of the epiclastic rocks are beidellite–montmorillonite derived from the altered volcanic materials of the sedimentary environment. Conversely, phyllosilicate minerals of the pyroclastic rocks are dominated by kaolinite, illite, and mica, which were formed by pedogenetic processes through the hydrothermal influence.     

Contribution of multivariate statistical techniques in the hydrochemical evaluation of groundwater from the Ouargla phreatic aquifer in Algeria

A hydrochemical evaluation of the hydrogeological surface aquifer of Ouargla was conducted using 17 samples of water. The analysis of the samples focused on the determination of calcium, magnesium, sodium, potassium, chloride, sulfate, bicarbonate, and nitrate, while electrical conductivity, temperature, and pH were measured on the ground. The obtained data are processed by multivariate techniques with a varimax rotation approach after standardization. The chemical data follow four factorial axes that provide a cumulative total variance explained by 67 %. Considering the EC as an additional variable, the matrix components after varimax rotation have identified a first axis related to NaCl, a second axis associated with CaSO₄, a third axis of HCO₃ and a fourth axis of NO₃. These variables control a significant part of the chemistry of the groundwater in the region of Ouargla.     

Origin and evolution of post-collisional volcanism: an example from Neoproterozoic Dokhan volcanics at Gabal Nugara area, Northeastern Desert, Egypt

Nugara volcanics are one of the northernmost outcrops of the Arabian?CNubian Shield. Two distinct volcanic successions are found in the Nugara basin: (1) old volcanic sequence composed of voluminous medium- to high-K calc-alkaline lavas and minor alkali basalt and (2) young volcanic sequence composed of subordinate tholeiitic mafic lavas. Their eruptions were punctuated by occasional volcaniclastic deposits that generated fall, flow, or reworked suites compositionally identical to the lava flows. These volcanics are a part of a post-subduction and extensional-related magmatic event in Northeastern Desert of Egypt. The volcanic rocks of the Nugara basin are characterized by strong enrichment in LILE relative to HESF, high LILE/HFSE ratios, and depletions of Nb on MORB-normalized multi-element diagrams. The geochemical features of the volcanic rocks suggest that they experienced fractional crystallization, along with mixing processes. Crustal contributions to the magma sources may also have occurred during magmatic evolution. These processes have resulted in scattered major and trace element variations with respect to increasing silica contents. The model proposed for their origin involves contrasting ascent paths and differentiation histories through crustal columns with different thermal and density gradients. The geochemical features of the most mafic samples suggest that the volcanic rocks in the region were derived from a mainly lithospheric mantle source that had been heterogeneously metasomatized by previous subduction events during convergence between the East and West Gondwanaland. The volcanic activity in the region is best explained by the delamination of lithospheric mantle slices that were heterogeneously enriched by previous subduction-related processes.