Thickness Of Sediment Layer Research Articles

The geology and geophysics of Lake Tarawera, New Zealand: Implications for sublacustrine geothermal activity

The Okataina Volcanic Centre is a large caldera system in the Taupo Volcanic Zone, New Zealand, which includes the Haroharo caldera, where the geothermally active Lake Tarawera is located at its southwestern boundary. Here, we show the results of high-resolution bathymetric mapping of the lake floor combined with gravity, magnetic and heat-flow surveys carried out over the lake, constrained by geochemical sampling. Our combined data elucidate the detailed geometry of the SW structural margin of the Haroharo caldera bisecting Lake Tarawera, and the relationship with the regional faults related with the Taupo Volcanic Zone rift.In this context, the spatial distribution of heat-flow in Lake Tarawera appears controlled by fluid-focusing permeable zones associated with the structural lineaments and by peripheral, gravity-driven circulation of meteoric water associated with cooling of highly permeable rhyolite domes around the lakeshore. The structural boundary of the Haroharo caldera provides first-order control of heat-flow in the lake, with WNW striking normal faults to a lesser degree. An estimated total conductive output of ∼5.3 MW escaping through the floor of Lake Tarawera is refracted by the thick layer of sediments, providing an impermeable layer to the convection of fluids, which in turn diminishes heat output compared to other lakes in this region.Our results contribute to modelling of hydrothermal circulation at the Okataina Volcanic Centre, and when integrated with data from the other lakes and the terrestrial geothermal output, it will contribute to determine the global geothermal output at this caldera system. The relationships to eruptive history and structural setting are the key to modelling similar geothermal system of other silicic calderas worldwide.

Crustal structure beneath the central and western North China from receiver function analysis

The North China Craton (NCC) is one of the oldest cratons on earth. Several important tectonic transformations of Mesozoic-Cenozoic tectonic regime led to the destruction of the North China craton. The knowledge of crustal structure can provide important constraints for the formation and evolution of cratons. New maps of sediment thickness, crustal thickness (H) and vP/vS (κ) in the central and western NCC were obtained using sequential H-κ stacking. P-wave receiver functions are calculated using teleseismic waveform data recorded by 405 stations from ChinArray project. Benefiting from the densely distribution of temporary seismic stations, our results reveal details of the crustal structure in the study area. The thickness of sedimentary layer in North China ranges from 0–6.4 km, and the thickest sedimentary layer is in Ordos block and its surroundings (about 2.8–6 km); The thickness of sedimentary layer in the Mongolia fold belt and Yinshan orogenic belt is relatively thin (less than 1 km). The crustal thickness of the study area varies between 27–48 km, of which the crust of the North China Plain is about 30–33 km, the central NCC is about 33–40 km, and the Ordos block is 40–48 km thick. The average vP/vS ratios in the study area is mostly between 1.66 and 1.90, and that in the Yanshan-Taihang mountain fold belt is between 1.70 and 1.85, and that in the Ordos block is between 1.65 and 1.90, with an average value of 1.77, indicating the absence of a thick basaltic lower crust. The obvious negative correlation between crustal thickness and average vP/vS ratio within Ordos and Central Asia orogenic belt may be related to magmatic underplating during the crustal formation. There is no significant correlation between the crustal thickness and the vP/vS ratio in the Lüliang-Taihang mountain fold belt, which may be related to the multiple geological processes such as underplating and crustal extension and thinning in this area. The lack of correlation between crust thickness and topography in the central orogenic belt and the North China Basin indicates the topography of these areas are controlled not only by crustal isostatic adjustment but also by the lithospheric mantle processes.

Differential tectonic evolution and formation mechanism of three sub-sags in Wenchang Sag of Pearl River Mouth Basin, South China Sea

Notable differences in the structural characteristics and evolution of three adjacent sub-sags, i.e., the Wenchang sub-sags A, B, and C, on the downthrown side of the Zhu III South Fault in the Wenchang Sag, are significant as they affect the formation and distribution of the oil and gas in these three sub-sags. However, the differences in their tectonic evolutions and formation mechanisms have not yet been adequately explained. In this paper, stress analysis, equilibrium profiles, and paleogeomorphic restoration, are used to investigate the dynamic settings, formation mechanisms, and influencing factors of the structural deformation related to the formation of the Wenchang Sag based on interpretation of seismic data. The results of the stress analysis suggest clockwise deflection of the regional tensile stress direction from a WNW-ESE trend during the Early Paleocene to NW-SE and NNW-SSE trends during the Eocene, to a nearly N–S trend during the Oligocene, and finally to a NNE-SSW trend during the Miocene. This clockwise rotation of the regional tensile stress direction led to the formation of a dextral strike-slip stress component parallel to the NE-trending Zhu III South Fault. This strike-slip stress component formed a releasing bend in sub-sag A, and may be associated with the continuous subsidence of a thick sedimentary layer in sub-sag A. It also created a restraining bend in sub-sag B, which underwent multiple structural inversions during its extension and subsidence and has a relatively small sedimentary thickness. The double restraining bend in sub-sag C is considered to have been strongly uplifted and eroded in response to this strike-slip stress component. Four obvious structural inversions in sub-sag B are identified in this paper. These structural inversions correspond to the last four regional tectonic movements. This interpretation suggests that the formation of the structural inversions was likely related to the strong tensile stress and the small intersection angle between the direction of the regional tensile stress and the pre-existing boundary fault. The rotation of the tensile stress direction was responsible for the strike-slip movement on the pre-existing boundary fault and the formation of the releasing bend and restraining bend, which controlled the structural evolutions of the sub-sags. This reasonably explains the differential tectonic evolution of these three sub-sags in the Wenchang Sag, and provides a crucial idea for structural analysis of similar basins.

Marine Geophysical Data and Its Application to Assessment of Crustal Structure along the Northern Egyptian Passive Continental Margin

The Egyptian passive continental margin is considered a remarkable geologic setting, in addition to being an occupation and manufacturing locality in north Egypt. This work used accessible potential field data, such as marine gravity data, to provide a wider vision of the potential field of the area under investigation. The results of the two-dimensional (2D) gravity modelling revealed a good agreement between the modelled gravity and observed gravity fields, including known regional structures found in the investigated area. The findings revealed that crustal modelling was affected by the tectonic structure and the huge thickness of sedimentary layers, which act as barriers to the crystal crust. The results revealed that the crustal thickness and density are spread among the deposited layer and the inferior mantle in the Moho range. Furthermore, it was found that the basement extent lies nearly 6–9 km lower in the northern Egyptian coastline to approximately 13 km under the Herodotus abyssal plain. Moreover, it was shown that the thickness of the sedimentary layers deposit increases near the East Mediterranean Ridge.

Heat flow data and thermal structure in northeastern Japan

New heat flow data corrected for climate change over Northeastern Japan were obtained using the temperature profile of the borehole of the High Sensitivity Seismograph Network (Hi-net). The obtained spatial distribution of heat flow shows low heat flow on the forearc side, high heat flow along the Ou Backbone Range, and low heat flow in the plains on the back-arc side. However, the distribution is not clearly divided into high and low heat flow along the VF front; for example, the low heat flow extends from near the northern Kitakami Mountains on the forearc side to the Ou Backbone Range crossing the VF, while the high heat flow extends to the central Kitakami Mountains and Sendai plain on the forearc side. In addition, a crustal temperature structure model was developed that considers into account the presence of sedimentary layers, the temperature dependence of thermal conductivity, and differences in heat generation due to lithology. There is a good correlation between this temperature structure and the lower limit of the seismogenic layer, which is between 400 and 450 °C. Compared to previous studies, the crustal thermal structure calculation method assumed is a model whose estimated temperature distribution is sensitive to structural differences; however, a more accurate estimation of the temperature structure is possible if detailed structural information is available. On the other hand, it seems necessary to treat fluid behavior in more detail in areas of high heat flow. However, the estimation of crustal temperature structure, especially in regions with thick sedimentary layers, is considered an improvement over the previous study.Graphical

Gravity and magnetic anomalies of earthquake-prone areas in the southwestern Ulleung basin margin, East Sea (Sea of Japan)

Submarine earthquakes have increased in the southwestern Ulleung Basin adjacent to the Korean Peninsula. This study analyzed the gravitational and magnetic properties of the three earthquake-prone areas (Hupo Bank and offshore regions near Pohang and Ulsan) in the basin. The basin was affected by tensile and compressive stresses during the formation of the East Sea. The southern Hupo Bank and the Pohang offshore exhibited high gravity anomalies and strong magnetic anomalies. Hupo Bank was separated from the peninsula and earthquakes in this region have been influenced by crustal fractures that facilitated igneous activities during the formation of the basin. Dense volcanic rocks and seaward dipping reflectors along the Pohang coast and continental slope suggest magmatic activities during the formation of the East Sea. Comparatively, the Ulsan offshore, with a thick sedimentary layer, exhibited a slightly higher gravity anomaly than the surrounding area, but no significant differences in the magnetic anomaly. Sequential tensile and compressive stresses related to the creation of the basin produced complex tectonic structures in this region. The magnetic tilt derivative results suggest that earthquakes were located near magnetic source boundaries. The results show that it is important to monitor earthquake-prone areas with gravity and magnetic anomalies.

Assessment of Stormwater Infiltration Basins Models Developed in Gaza Strip

Stormwater remains the sole source of aquifer recharge in the Gaza strip, which should be utilized properly through artificial infiltration. The study objective is to investigate and analyze the infiltration efficiency of three large exiting infiltration basins in the Gaza strip (Alamal, Asadaqa, and Waqf) using different infiltration techniques. The technique applied in Ala-mal basin is the natural surface spreading of stormwater while Asadaqa basin used the surface spreading combined with graveled boreholes. Waqf basin used non-graveled boreholes (empty shafts cased with UPVC pipes). The infiltration rate and efficiency were recorded and estimated for each basin during the 2021-2022 wet season and compared to a past 2017-2018 wet season at a water depth of 1.70 m. The study revealed that, the actual infiltration capacity of Waqf basin was estimated as 2,000 m3/day in the 2021-2022 wet season, twice that in the 2017-2018 wet season, with an infiltration efficiency of 57.47 %, that was attributed to the 18 drilled non-graveled boreholes, which enhanced the seepage of stormwater into the underlying soil. Asadaqa basin has the lowest infiltration efficiency of 3.90 % due to the continuous accumulation of thick and dense sediment layer on the basin floor, with nonchanged actual infiltration capacity (around 2,800 m3/day) between the two studied wet seasons. On the oth-er hand, Alamal basin infiltration efficiency was only 4.60 %, with actual infiltration capacity of 629 and 105.4 m3/day dur-ing the two wet seasons, respectively where some repair and upgrade works were performed at Alamal basin which enhanced the actual infiltration capacity but still far from the design infiltration capacity. For future studies, Waqf basin technique should be thoroughly studied and investigated as a novel artificial infiltration method, with deep study on the factors af-fecting the infiltration process.

A Temporary Broadband Seismic Array in the Largest Desert of China: TASTE

Abstract In early Permian, a mantle plume heated up the Tarim block and formed the Tarim large igneous province. It is an interesting phenomenon to explore the interaction between mantle plume and a thick continent lithosphere, because mantle plume has been proposed to be an important route of material and energy transportation in the earth. Currently, the Tarim block is surrounded by Tibetan plateau to its south edge and Tian Shan orogeny to its north edge. The combined effects of both mantle plume activity and plate tectonics should have significantly changed the structure and composition of the Tarim block because of its amalgamations in Neoproterozoic. Seismic imaging plays a key role on revealing the deep structure of the Earth, which could help unravel the questions mentioned before. However, the harsh natural environment in the central part of the Tarim basin, the Taklimakan desert—the largest desert in China—has seriously hampered the broadband seismological observation. From July 2017 to November 2019, the Institute of Geology and Geophysics, Chinese Academy of Sciences deployed a 2D broadband seismic array named the Tarim seismic Array for lithoSpheric signaTure of mantle plumE (TASTE) over the inner part of the Tarim basin with an averaged spacing of 60–70 km. The primary target of this project is to obtain a detailed lithospheric structure to resolve the possible lithospheric signatures of Permian mantle plume activity. Here we introduce the basic information of the TASTE network and the performance of these instruments by analyzing the background noise level. Preliminary results of receiver function and ambient noise analyses are also shown, which may suggest a thick sedimentary layer, as well as complicated crust and lithospheric feature.

Characterising sediment thickness beneath a Greenlandic outlet glacier using distributed acoustic sensing: preliminary observations and progress towards an efficient machine learning approach

AbstractDistributed Acoustic Sensing (DAS) is increasingly recognised as a valuable tool for glaciological seismic applications, although analysing the large data volumes generated in acquisitions poses computational challenges. We show the potential of active-source DAS to image and characterise subglacial sediment beneath a fast-flowing Greenlandic outlet glacier, estimating the thickness of sediment layers to be 20–30 m. However, the lack of subglacial velocity constraint limits the accuracy of this estimate. Constraint could be provided by analysing cryoseismic events in a counterpart 3-day record of passive seismicity through, for example, seismic tomography, but locating them within the 9 TB data volume is computationally inefficient. We describe experiments with data compression using the frequency-wavenumber (f-k) transform ahead of training a convolutional neural network, that provides a ~300-fold improvement in efficiency. In combining active and passive-source and our machine learning framework, the potential of large DAS datasets could be unlocked for a range of future applications.

Flow analysis on carbonaceous deposition of heavy oil droplets and catalyst particles for coking formation process

The flow state has an important influence on the carbonaceous deposition for coking formation process. To study the flow of heavy oil droplets and catalyst particles in FCC process, we used the scanning-electron microscope (SEM) to analyze the microstructure of coke and simulated the flow by computational fluid dynamics (CFD) method. A new iso-vortex surface method according to swirling strength was used to analyze the fluid motion. The results showed that the dissolution of powdered coke sample in ethanol was more uniform than that in water. Swirling strength on the outer wall of vortex finder was large, and there was a depression in the flow layer at about 270°. Under the swirling strength of 654.59 s−1, the flow layer in the outer wall of vortex finder was thick in the range of 0–180°. A stable stagnant layer in the barotropic gradient boundary layer was easier to form a deposition layer of fine particles or droplets. When the fluid velocity outside the boundary layer increased, the shear force increased. The thickness of sedimentary layer and the size of sedimentary particles in the boundary layer tended to decrease. In addition, the backflow also affected the deposition of particles and droplets.

Surface wave imaging using deep reflection seismic data: a study on the Cuonadong dome

As interference waves in deep reflection data processing, surface waves are often suppressed as noise, but surface waves carry considerable underground media information, including structural information and the physical properties of rocks. Reasonable extraction and use of surface wave signals are of great significance when studying shallow characteristics. Deep reflection data are collected using large offsets, trail spacing, and explosive sources. The surface wave energy tends to be stronger, and the high-frequency surface wave signal is abundant. After extraction and inversion, the shallow shear wave velocity structure can be obtained. Near the Cuonadong dome in the southern Tibetan detachment system (STDS), a large number of leucogranites are developed in the core, containing important rare metal minerals and high metallogenic potential. However, studies regarding the shallow structure in this region are rare. In this paper, we use deep reflection data from a profile through the Cuonadong dome to obtain the S-wave velocity structure of the study area by extracting the surface wave fundamental-mode dispersion curve and inversion. Combined with regional geological and magnetotelluric data, we supposed that the thickness of the Cuonadong dome sediment layer (< 1.4 km/s) varies greatly from east to west, the thickness of the sediment layer is the deepest near the Cuona fault and Jisong fault (more than 1 km), and the core of the dome is the thinnest. Under the Cuonadong dome, there are obvious high-velocity anomalies (> 2.2 km/s), and the horizontal S-wave velocity changes greatly, which is mainly related to the destruction of magmatic activity since the Miocene. These understandings of the structure and velocity field of the Cuonadong dome can provide a powerful geophysical basis for establishing the dome structure model and searching for hidden ore bodies.Graphical abstract

Seismic wave propagation simulations in Indo-Gangetic basin using spectral element method

SUMMARY This study focuses on developing a 3-D computational model of the Indo-Gangetic basin (IG basin) using the spectral element method. The region includes the subcontinent's most densely populated areas. The basin is unique as it consists of geologically younger sedimentary layers along with several ridges and depressions in its domain. However, the proximity of great Himalayan earthquakes and the presence of thick sedimentary layers of the basin results in higher seismic hazards. The limited instrumentation of the domain poses challenges in understanding the response of the basin due to a seismic event. This motivated us to develop a computational model of the IG basin by incorporating the best-known geometry, material properties and fine resolution topography. In the lateral direction, the modelled part of IG basin spans over ∼6° × 4° (between longitude 80.5°–86.5°E and latitude 25°–29°N). The validation of the developed basin model is performed by simulating the ground motions for the 2015 Mw 7.9 Nepal main shock and five of its aftershocks. Both qualitative and quantitative comparison of the simulated time histories suggests that the developed model could accurately simulate ground motions over a frequency range of 0.02–0.5 Hz. The developed basin model is then used to understand the seismic wavefield characteristics during the 2015 Mw 7.9 Nepal main shock. The spatial variation of peak ground velocity (PGV), as well as amplification, are investigated at a 0.2° × 0.2° grid and selected cities in the basin. The contours of PGV amplification indicate a higher value of ∼8–10 in the horizontal direction and ∼2.5–3.5 in the vertical direction for sediment depth &amp;gt;4 km. A comprehensive comparison of the simulated PGVs and the ground motion prediction equations shows that, while the simulations agree with the prediction, they also show heterogeneity of ground-motion distribution that cannot be fully described by empirical prediction relations. Hence the results from this study are more reliable and find applications in seismic hazard assessment of the cities in the basin. Besides, the results can be used to guide the installation of future seismic stations in the region.

Thermal Lithospheric Thickness of the Sichuan Basin and its Geological Implications

AbstractThermal lithospheric thickness is an important parameter in studying the tectonic‐thermal evolution of basins and plate dynamics. Based on the measured geothermal data and thermophysical properties of the rocks, the thermal lithospheric thickness of the Sichuan Basin was calculated according to the principles of heat conduction in the crust and lithospheric mantle. The calculation results revealed that the thickness of the thermal lithosphere in the Sichuan Basin is 140–190 km and is unevenly distributed. The thickness of the thermal lithosphere in central Sichuan and southwestern Sichuan is less than 160 km, while that in the western Sichuan depression and eastern Sichuan is larger (∼180 km). The distribution of the thermal lithospheric thickness in the basin has a good correlation with the geological units and the thickness of the sedimentary layers. The thickness of the thermal lithosphere in the depression area, which has thick sedimentary layers and the fault‐fold zone with shallow crustal deformation and thickening, are larger than that in the basement uplifted area, which has thin sedimentary layers. The calculated thermal lithospheric thickness is in good agreement with the geophysical data and reflects the stable conduction temperature field in the Sichuan Basin. The present thermal regime and thermal lithospheric thickness of the Sichuan Basin indicate that flexural thickening of the lithosphere occurred in the eastern Sichuan fault‐fold belt and the Longmen Mountain–Western Sichuan depression foreland basin system, while asthenospheric uplift occurred in the central Sichuan region, which were the result of the expansion of the Xuefeng orogeny from the east and the compression of the Tibetan Plateau from the west.

Crustal structure and subsidence mechanisms of the Williston Basin: New constraints from receiver function imaging

Mechanisms responsible for the long-term subsidence of intracontinental basins such as the Williston Basin in North America remain enigmatic, partly due to the thick sedimentary layer commonly found in the basins that prevents reliably imaging the deep crustal and upper mantle structures using some of the most-commonly employed seismic methods such as receiver function analysis. In this study, we used receiver functions recorded by 274 USArray and other stations in the Williston Basin and adjacent areas to investigate the layered structure of the crust in the hydrocarbon-rich intracontinental basin. After the removal of strong reverberations on the receiver functions associated with a low-velocity sedimentary layer using a recently developed time-domain deconvolution approach, two positive arrivals representing downward increases of seismic velocities are imaged beneath the basin and the area to the west. The top interface has a depth of about 40 km at the depocenter of the basin, and gradually shallows eastward to about 30 km beneath the Superior Craton, and the deeper interface has a mean depth of about 65 km beneath the Williston Basin. The layer between the two interfaces may represent an eclogitized or garnet-rich lower crustal layer. The results are consistent with the hypothesis that continuous retrograde metamorphic reactions in the previously-thickened lower crust during the Paleoproterozoic Trans-Hudson Orogeny resulted in the subsidence of the intracontinental Williston Basin.

Regional-scale investigation of pile bearing capacity for Canadian permafrost regions in a warmer climate

Climate change is being experienced particularly intensely in the Arctic and therefore adaptation of engineering systems for this region cannot be further delayed. However, one of the major barriers to studies focused on adapting northern engineering systems is the lack of information at the spatial and temporal scales required for engineering applications. This study investigates pile bearing capacity for selected pile configurations for the Canadian permafrost regions (Nunavut and Northwest Territories), for current and future climates, using the very first ultra-high resolution (4 km) climate change simulation developed for the region using the Global Environmental Multiscale (GEM) model, for a high emission scenario.Comparison of the ultra-high-resolution GEM simulation, driven by reanalysis, with available observations confirms the model's ability in representing near-surface permafrost and related climate variables. The estimated adfreeze contribution to the total bearing capacity, for current climate, informed by the reanalysis-driven GEM simulation, for a 5-m cement pile, is found to be of the order of 15% for regions with shallow bedrock and 80% for regions with deeper bedrock. Application of the GEM climate change simulation outputs, for RCP8.5 scenario, suggest decreases to adfreeze contribution in the 5–30% range by 2040, with the largest differences noted for regions with deeper bedrock. For steel piles of same configuration, although the adfreeze contributions are only about 70% of that for cement piles, the projected relative changes are of similar magnitude.Further downscaling to 250 m resolution using the land model of GEM for the Slave Geological-Grays Bay corridor, where future developments are planned, including an all-season road, enables better estimation of bearing capacity for realistic pile scenarios such as those for bridges (in thick layer of sediments) used for river crossings. Due to the wide variation of pile materials, lengths and installation methods, site specific information can be developed from the framework developed in this study. The results of this study, including the ultra-high resolution climate change information, will thus form the basis for additional detailed investigations on climate-infrastructure interactions and climate resiliency studies.

Acoustic resonances within the surficial layer of a muddy seabed.

This is an investigation of sound propagation over a muddy seabed at low grazing angles. Data were collected during the 2017 Seabed and Bottom Characterization Experiment, conducted on the New England Mud Patch, a 500 km2 area of the U.S. Eastern Continental Shelf characterized by a thick layer of muddy sediments. Sound Underwater Signals (SUS), model Mk64, were deployed at ranges of 1-15 km from a hydrophone positioned 1 m above the seafloor. SUS at the closest ranges provide measurements of the bottom reflection at low grazing angles (<3 deg). Broadband analysis from 10 Hz to 10 kHz reveals resonances in the bottom reflected signals. Comparison of the measurements to simulated signals suggest a surficial layer of mud with a sound speed lower than the underlying mud and overlying water. The low sound speed property at the water-mud interface, which persists for less than 1 m, establishes a sound duct that impacts mid-frequency sound propagation at low grazing angles. The presence of a low-speed surficial layer of mud could be universal to muddy seabeds and, hence, has strong implications for mid-frequency sound propagation wherever mud is present.

Biogeochemistry of three different shallow gas systems in continental shelf sediments of the South-Eastern Baltic Sea (Gulf of Gdańsk): Carbon cycling, origin of methane and microbial community composition

The objective of the study was to describe the biogeochemistry of three different shallow gas systems, with particular emphasis on the content of carbon forms, origin of gas and microbial community composition of sediments. Surface sediments (1–1.2 m) in the areas above geophysically different shallow gas structures located in south-eastern Baltic Sea (Gulf of Gdańsk) were investigated: MET1-MP – an active deepwater pockmark with gas seepage and upward infiltration of freshwater, MET2 – shallow gas regular accumulation with lower availability of sulphates and high input of terrestrial organic matter, and MET3 – shallow gas elongated sub-bottom furrow with sulphate presence in the whole sediment profile. For comparison purposes, a reference site (ZGG) without any gas structures in sediments was included in the study. At each station, the following parameters were examined: CH4, CORG and CINORG in sediments; DIC, DOC, H2S, SO42−, NH4+, Cl− in pore and near-bottom waters; stable isotopes in DIC (δ13C-DIC), in gases (δ13C-CH4, δ13C-CO2, δ2H-CH4), and microbial communities in sediments (16S rRNA). In addition, general physico-chemical conditions in bottom waters (dO2, T, S, pH and Eh) as well as molecular composition of gas emitted from MET1-MP pockmark were determined. In order to be able to better compare the geological background of these shallow gas systems, the sediments were investigated by non-invasive acoustic methods: multi-beam echosounder and sub-bottom profiler. Moreover, relative AOM rates were estimated. The first system (MET1-MP) was characterised by a dramatic decrease in the availability of sulphate down the sediment, acetoclastic methanogenesis taking place in surface sediments, the AOM limited to a very thin topmost sediment layer, average stable isotope values of δ13C-CH4 = −58.9‰, δ2H-CH4 = −310‰, δ13C-CO2 = −6.9‰, carbonate precipitation and both methanogenic as well as methanotrophic Archaea present in the whole sediment profile without clear zonation. In the shallow gas regular accumulation, in a close proximity to the coast (MET2), methane was virtually trapped in subsurface sediment layers, even despite not-high sulphate concentrations resulting from lower salinity (~7.5 PSU). In that area, the bacterial community was, unlike typical marine sediments, dominated by Firmicutes, and average stable isotope values were: δ13C-CH4 = −67.0‰, δ2H-CH4 = −243‰, δ13C-CO2 = −7.9‰. The third shallow gas system (MET3) was characterised by most efficient AOM, mostly due to sulphate presence in a relatively thick sediment layer, even several dozen cm below the SMT. The average stable isotope ratios for gases were as follows: δ13C-CH4 = −75.8‰, δ2H-CH4 = −241‰ and δ13C-CO2 = −11.9‰. Despite high concentrations of methane, the methanogenic Archaea were virtually absent in the sediment profile, while the methanotrophic ones showed clear zonation, with ANME-2a-2b present in shallower sediments and ANME-1 in deeper layers. From the environmental point of view, active pockmark shallow-gas systems can be considered as the least efficient in preventing methane from migration to surface sediment layers and the water column, especially when freshwater infiltration occurs, which additionally dilutes the pool of sulphates available for AOM. In our study, the MET1-MP active pockmark was the most “leaky” in terms of methane release to the water column. In contrast, the shallow gas systems being under influence of additional sources of oxidants (in our case – MET2; influence of riverine discharge introducing Fe(III), together with high concentrations of oxygen in bottom waters) or having sulphate present in a relatively thick sediment layer (e.g. due to sulphide reoxidation below the SMT; in our case – the MET3 system) can be considered efficient in stopping methane within sediments, by its almost complete oxidation before reaching the water column.

Crustal structure of the Volgo–Uralian subcraton revealed by inverse and forward gravity modelling

Abstract. Volgo–Uralia is a Neoarchaean easternmost part of the East European craton. Recent seismic studies of the Volgo–Uralian region provided new insights into the crustal structure of this area. In this study, we combine satellite gravity and seismic data in a common workflow to perform a complex study of Volgo–Uralian crustal structure, which is useful for further basin analysis of the area. In this light, a new crustal model of the Volgo–Uralian subcraton is presented from a step-wise approach: (1) inverse gravity modelling followed by (2) 3D forward gravity modelling. First, inversion of the satellite gravity gradient data was applied to determine the Moho depth for the area. Density contrasts between crust and mantle were varied laterally according to the tectonic units present in the region, and the model is constrained by the available active seismic data. The Moho discontinuity obtained from the gravity inversion was consequently modified and complemented in order to define a complete 3D crustal model by adding information on the sedimentary cover, upper crust, lower crust, and lithospheric mantle layers in the process of forward gravity modelling, where both seismic and gravity constraints were respected. The obtained model shows crustal thickness variations from 32 to more than 55 km in certain areas. The thinnest crust with a thickness below 40 km is found beneath the Precaspian basin, which is covered by a thick sedimentary layer. The thickest crust is located underneath the Ural Mountains as well as in the centre of the Volgo–Uralian subcraton. In both areas the crustal thickness exceeds 50 km. At the same time, initial forward gravity modelling has shown a gravity misfit of ca. 95 mGal between the measured Bouguer gravity anomaly and the forward calculated gravity field in the central area of the Volgo–Uralian subcraton. This misfit was interpreted and modelled as a high-density lower crust, which possibly represents underplated material. Our preferred crustal model of the Volgo–Uralian subcraton respects the gravity and seismic constraints and reflects the main geological features of the region with Moho thickening in the cratons and under the Ural Mountains and thinning along the Palaeoproterozoic rifts, Precaspian sedimentary basin, and Pre-Urals foredeep.

Derivation of a near-surface damping model for the Groningen gas field

SUMMARY Seismic damping of near-surface deposits is an important input to site-response analysis for seismic hazard assessment. In Groningen, the Netherlands, gas production from a reservoir at 3 km depth causes seismicity. Above the gas field, an 800 m thick layer of unconsolidated sediments exist, which consists of a mixture of sand, gravel, clay and peat strata. Shear waves induced at 3 km depth experience most of their anelastic attenuation in these loose sediments. A good estimate of damping is therefore crucial for modelling realistic ground-motion levels. In Groningen, we take advantage of a large network of 200 m deep vertical arrays to estimate damping from recordings of the induced events. As a first step, we apply seismic interferometry by deconvolution to estimate local transfer functions over these vertical arrays. Subsequently, two different methods are employed. The first is the ‘upgoing’ method, where the amplitude decay of the retrieved upgoing wave is used. The second is the ‘up-down’ method, where the amplitude difference between retrieved up- and downgoing waves is utilized. For the upgoing method, the amplitude of the upgoing direct wave is affected by both elastic and anelastic effects. In order to estimate the anelastic attenuation, it is necessary to remove the elastic amplification first. Despite the fact that elastic compensation could be determined quite accurately, non-physical damping values were estimated for a number of boreholes. Likely, the underlying cause was small differences in effective response functions of geophones at different depths. It was found that the up-down method is more robust. With this method, elastic propagation corrections are not needed. In addition, small differences in in situ geophone response are irrelevant because the up- and downgoing waves retrieved at the same geophone are used. For the 1-D case, we showed that for estimating the local transfer function, the complex reverberations need to be included in the interferometric process. Only when this is done, the transfer function does not contain elastic transmission loss and Q estimation can be made without knowing the soil profile in detail. Uncertainty in the estimated damping was found from the signal-to-noise ratio of the estimated transfer function. The Q profiles estimated with the up-down method were used to derive a damping model for the top 200 m of the entire Groningen field. A scaling relation was derived by comparing estimated Q profiles with low-strain damping profiles that were constructed using published models for low-strain damping linked to soil properties. This scaling relation, together with the soil-property-based damping model, allowed up-scaling of the model to each grid-cell in the Groningen field. For depths below 200 m, damping was derived from the attenuation of the microseism over Groningen. The mean damping model, over a frequency band between 2 and 20 Hz, was estimated to be 2.0 per cent (0–50 m depth), 1.3 per cent (50–100 m), 0.66 per cent (100–150 m), 0.57 per cent (150–200 m) and 0.5 per cent (200–580 m).

Contribution of the Airborne Magnetic Field to the Structural Study of the Variscan Granitoid of Rehamna, Morocco

At the Moroccan Western Meseta, the study of airborne magnetic data from the Rehamna massif, using the potential aeromagnetic methods, has allowed us to improve and describe the geometry of the geological structures of the Rehamna variscan granitoids and their subsurface components. These methods have proven to be an adequate geophysical technique to study the deep and subsurface structures of this area covered by a thick Mesozoic-Cenozoic sedimentary cover. The calculation of the reduced map to pole and upward continuation of the reduced residual magnetic field to the pole at 2 km to 30 km shows the close relationship between the positive anomalies and the deep magma bodies that outcrop locally in the tectonic contacts; thus, a strong correlation between the aeromagnetic lineaments and the regional crustal-scale faults-oriented N-S to NE-SW. The calculation of the Euler deconvolution of these lineaments, using the different structural indices, shows a depth of aeromagnetic lineaments about ten kilometers due to the presence of a very thick sedimentary layer. The final structural map reveals four groups of faults oriented, respectively NNW-SSE, N-S, NNE-SSW, NE-SW, and E-W.