Frontiers of Earth Science最新文献

After the construction of cascade reservoirs in the upper reaches of the Three Gorges Reservoir (TGR), the sediment load outflow of the upper Yangtze River Basin (YRB) has been significantly altered, decreasing from 491.8 Mt/yr (1956–2002) to 36.1 Mt/yr (2003–2017) at Yichang station. This has widely affected river hydrology, suspended sediment grain size distribution, and channel morphology. This study analyzed hydrological variations in water discharge and sediment load of the upper YRB over the past 62 years (1956–2017) by employing a double mass curve. The variations in the source areas of sediment yielding for the upper YRB were quantified, and field measurement data of the cross-channel profile were collected to investigate the sedimentation process in the TGR from 2003 to 2017. More than 90% of the sediment load reduction in the upper YRB may be explained by human activities. The Jinshajiang River was no longer the largest sediment source area for the Zhutuo station (accounting for 5.23%) in the 2013–2017 time span, and the sediment rating rates for the inflow and outflow of the TGR shifted to negatively correlated. A longitudinal fining trend was revealed in the suspended sediment size. Still, the mean median grain size of suspended sediment in the TGR had an increasing trend in the 2013–2017 period. This result may be closely related to sediment regulation in reservoirs and incoming sediment load reduction. Sedimentation in the TGR decreased sharply from 299.8 Mt/yr in 2003–2012 to 47.2 Mt/yr in 2013–2017, but the sedimentation rate of the TGR remained at > 80% annually. Moreover, some cross sections in the fluctuating backwater zone experienced scouring.

This study aims to propose an empirical prediction model of hydraulic aperture of 2D rough fractures through numerical simulations by considering the influences of fracture length, average mechanical aperture, minimum mechanical aperture, joint roughness coefficient (JRC) and hydraulic gradient. We generate 600 numerical models using successive random additions (SRA) algorithm and for each model, seven hydraulic gradients spanning from 2.5 × 10⁻⁷ to 1 are considered to fully cover both linear and nonlinear flow regimes. As a result, a total of 4200 fluid flow cases are simulated, which can provide sufficient data for the prediction of hydraulic aperture. The results show that as the ratio of average mechanical aperture to fracture length increases from 0.01 to 0.2, the hydraulic aperture increases following logarithm functions. As the hydraulic gradient increases from 2.5 × 10⁻⁷ to 1, the hydraulic aperture decreases following logarithm functions. When a relatively low hydraulic gradient (i.e., 5 × 10⁻⁷) is applied between the inlet and the outlet boundaries, the streamlines are of parallel distribution within the fractures. However, when a relatively large hydraulic gradient (i.e., 0.5) is applied between the inlet and the outlet boundaries, the streamlines are disturbed and a number of eddies are formed. The hydraulic aperture predicted using the proposed empirical functions agree well with the calculated results and is more reliable than those available in the preceding literature. In practice, the hydraulic aperture can be calculated as a first-order estimation using the proposed prediction model when the associated parameters are given.

A new digital elevation model (DEM) upscaling method based on high accuracy surface modeling (HASM) is proposed by combining the elevation information of DEM and the valley lines extracted from DEM with different flow accumulation thresholds. The proposed method has several advantages over traditional DEM upscaling methods. First, the HASM ensures the smoothness of the upscaled DEM. Secondly, several DEMs with different topographic details can be obtained using the same DEM grid size by incorporating the valley lines with different flow accumulation thresholds. The Jiuyuangou watershed in China’s Loess Plateau was used as a case study. A DEM with a grid size of 5 m obtained from the local surveying and mapping department was used to verify the proposed DEM upscaling method. We established the surface complexity index to describe the complexity of the topographic surface and quantified the differences in the topographic features obtained from different upscaling results. The results show that topography becomes more generalized as grid size and flow accumulation threshold increase. At a large DEM grid size, an increase in the flow accumulation threshold increases the difference in elevation values in different grids, increasing the surface complexity index. This study provides a new DEM upscaling method suitable for quantifying topography.

The identification of superimposed gas-bearing systems in coal measures is the basis for expediting the optimization of coal measure gas co-production. Through the analysis of drill cores and log data of Upper Carboniferous Benxi Formation to the member 8 of Middle Permian Lower Shihezi Formation in Daning-Jixian block, eastern margin of Ordos Basin, four distinct superimposed coal measure gas-bearing systems were identified, and their formation mechanism was discussed from the sequence stratigraphic perspective. Type I system mainly contains multiple coal seams, shales and sandstone layers. Type II system is dominated by multiple coal seams and shales. Type III is characterized by multiple sandstone layers, and type IV system is dominated by limestones and mudstones. In general, the gas-bearing systems deposited in barrier-lagoon are type II, those deposited in carbonate tidal flats are type IV, and those deposited in the delta front are types I and III. The marine mudstone, acting as a key layer near the maximum flooding surface, exhibits very low permeability, which is the main factor contributing to the formation of superimposed gas-bearing systems. The sedimentary environment plays a significant role in controlling the distribution of gas-bearing systems. Notably, the vertical gas-bearing systems in the south-western region, where delta front and lagoon facies overlap, are more complex than those in the north-eastern delta front facies.

The Mesozoic fan deltas in the north-west margin of the Junggar Basin, as important petroleum reservoirs, exhibited complex facies change and internal structures with strong heterogeneity which were controlled by the transformation of slope-patterns, bringing great challenges to the study of sedimentary characteristics. The Upper Karamay Formation at north-west margin of the Junggar Basin was the objective in this paper which attempts to clarify the mechanism of sedimentary response and sand-body distribution of fan delta systems under the control of slope-pattern change. Based on a data set of cores, well logs and seismic, two types of slope-pattern were identified in the study area, which include steep-to-gentle in the south and gentle-to-steep in the north. The control of difference slope-patterns on the sand-body distribution was clarified based on the analysis of the sedimentary dynamics, facies characteristics, and depositional evolution of the fan deltas. The study shows that the transport mechanism of sediments on the steep-slope was dominated by debris flows, developing coarse-grained, thick-bedded lobes with poor structural maturity of clasts. On the gentle-slope, the deposition was dominated by hyperconcentrated-traction currents, forming relatively fine-grained, thin-bedded lobes with increased sandy matrix. The sand-bodies show frequent bar-channel transformation and channel down-cutting under the steep slope setting, which exhibit migration of isolated river channels on the gentle slopes. Under the steep-to-gentle pattern, the coase-grained sediments were mainly accumulated at slope toe, generally developed equiaxial lobes. However, the coarse-grained clasts were preserved both at proximal and distal lobes on the gentle-to-steep slopes, showing obvious lateral extension of the fan delta. The slope patterns controlled sedimentary respond rates of the fan deltas during lake level change. By comparing the modern cases of fan systems worldwide, the control of slope patterns on deposition of coarse-grained fans was clarified, providing insight into hydrocarbon exploration on basin margins.

Accurate approaches for estimating flow resistance in large alluvial rivers are fundamental for simulating discharge, sediment transport, and flood routing. However, methods for estimating riverbed resistance and additional resistance in the channel-bar landscapes remain poorly investigated. In this study, we used in situ river bathymetry, sediment, and hydraulic data from the Shashi Reach in the Yangtze River to develop a semi-empirical approach for calculating flow resistance. Our method quantitatively separates flow resistance into riverbed resistance and additional resistance and shows high accuracy in terms of deviation ratio (∼20%), root-mean-square error (∼0.008), and geometric standard deviation (∼3). Additional resistance plays a dominant role under low-flow conditions but a secondary role under high flows, primarily due to the reduction in momentum exchange in channel-bar regions as discharge increases. Riverbed resistance first decreases and then increases, which might be attributed to bedform changes in the lower and transitional flow regimes as flow velocity increases. Overall, our findings further the understanding of dynamic changes in flow resistance in the channel-bar landscapes of large river systems and have important implications for riverine ecology and flood management.

The uniaxial compressive strength (UCS) of rocks is a critical index for evaluating the mechanical properties and construction of an engineering rock mass classification system. The most commonly used method for determining the UCS in laboratory settings is expensive and time-consuming. For this reason, UCS can be estimated using an indirect determination method based on several simple laboratory tests, including point-load strength, rock density, longitudinal wave velocity, Brazilian tensile strength, Schmidt hardness, and shore hardness. In this study, six data sets of indices for different rock types were utilized to predict the UCS using three nonlinear combination models, namely back propagation (BP), particle swarm optimization (PSO), and least squares support vector machine (LSSVM). Moreover, the best prediction model was examined and selected based on four performance prediction indices. The results reveal that the PSO–LSSVM model was more successful than the other two models due to its higher performance capacity. The ratios of the predicted UCS to the measured UCS for the six data sets were 0.954, 0.982, 0.9911, 0.9956, 0.9995, and 0.993, respectively. The results were more reasonable when the predicted ratio was close to a value of approximately 1.

The accurate determination of geological age is a key to understanding the history and process of paleolake evolution and oil and gas exploration in continental lake basin. However, improving the accuracy of geological age has always been a difficult scientific problem. A 609-m-thick, continuous lacustrine mudstone and sandstone succession in Chezhen Sag (eastern China) provides an ideal middle Eocene sedimentary record for establishing a high-resolution stratigraphic chronology framework. Based on spectrum analysis and sliding window spectrum analysis of the natural gamma (GR) logging data of well Che 271 (C271) in Chezhen Sag, the periods of 405 kyr and 40.1 kyr were filtered by a Gaussian bandpass filter, and a “flngting” astrochronological time scale (ATS) was established. The total number of 405 kyr eccentricity cycles were 13.6 and 40.1 kyr obliquity cycles were 138 which recorded from the upper member 4 (Es4U) to the member 3 (Es3) of the Eocene Shahejie Formation, and the depositional duration was 5.53 Myr. Correlation Coefficient (COCO) analysis and evolutionary Correlation Coefficient (eCoCo) analysis found that the optimal sedimentary rate of different strata. Sedimentary noise simulation revealed the history of paleolake water changes in the Middle Eocene in the Chezhen Sag, according to which four sequences are divided. The study shows that the lake level change of Chezhen Sag in the middle Eocene shows prominent 1.2 Myr cycles and an antiphase well-coupled relationship with obliquity modulation. Finally, we propose a model to explain the relationship between the orbital cycle and lake level change in the continental lake basin. When the obliquity of the earth increases, the middle and high latitudes of the earth will be closer to the sun, the direct sunlight will be higher, and the meridional sunshine will increase, thus accelerating the evaporation process of lake basin water. When the seasonal changes are obvious (maximum period of 1.2 Myr ultra-long obliquity), this effect is more significant. The relative lake level change based on the restoration of high-precision ATS has significant scientific and economic value for understanding the vertical evolution of continental stratigraphic sequences and the formation and distribution of oil and gas resources.

Organic matter is the basis for oil and gas generation, and the depositional environment controls its enrichment. The first member of the Qingshankou Formation (K₂qn¹) in Songliao Basin has a thick organic-rich shale and so is an important target section for shale oil exploration and development. In the Gulong Sag, shale samples from this unit were collected over the full length of the section. The characterization of the environments of deposition (EOD) of K₂qn¹ was improved by utilizing lithological characteristics, thin section observations, elemental compositions, and organic carbon concentrations. Combined with the normalization coefficients proposed in this paper, an organic matter correlation model was established to elucidate the factors that influence organic matter enrichment. From the bottom to the top of K₂qn¹, the lake depth gradually becomes shallower, the primary productivity first decreases and then increases, the reducing conditions become stronger and then weaker, the water salinity gradually decreases, the climate first becomes semi-humid and then warm and humid, and the input of terrigenous debris first decreases and then increases. A major marine transgression at the base of the K₂qn¹’s brought in nutrients to increase primary productivity, and the density-stratified reducing environment preserved and enriched organic matter. High primary productivity occurred during the middle of the deposition of the K₂qn¹, while terrigenous input is low. Organic matter is preserved in reduced deep lake environments, resulting in organic matter-rich black shale. The lake became shallower, and the salinity decreased in the upper part of K₂qn¹. Benthic organisms rapidly multiplied, consuming large amounts of oxygen and destroying the previously depositional environment, resulting in a reducing environment disturbed by benthic organisms with poor preservation conditions and the lowest organic matter content.

The growth and breakup processes of raindrops within a cloud influence the rain intensity and the sizes of raindrops on the surface. The Doppler velocity spectrum acquired by a vertically pointing radar (VPR) contains information on atmospheric turbulence and the size classification of falling hydrometeors. In this study, the four types of Convective Cells (CC) during precipitation events with more than 700 mm of precipitation in southern China are described. The characteristics of four types of CCs correspond to the isolated convection, the early stage, the mature stage, and the decline stage of organizational convection, in that order. Microphysical analysis using retrieval of vertical air motion (Vair) and raindrop evolution in clouds from Doppler velocity spectra collected by C-band VPR revealed the growth and breakup of falling raindrops with dynamic impact. Larger raindrops appear in the early stages and are accompanied by ice particles, which are impacted by the falling path’s downdraft. Raindrop aggregation, which is primarily related to the alternation of updraft and downdraft, accounts for the mature stage’s high efficiency of surface rainfall. The CCs in the decline stage originate from the shallow uplift in the weak and broad downdraft under conditions of enough water vapor. The updraft dominates the stage of isolated convection. Observations of convective cells could be more accurately represented in model evaluations.