Results in Geophysical Sciences最新文献

This study focusses on the sedimentological and aeromagnetic characteristics within the Agbaja Ironstone Formation to establish the sedimentary depositional history and map oolitic ironstone zones in the Lokoja district of the Nupe Basin. Forty sandstone samples were collected for granulometric, sedimentary facies and petrographic analyses to reconstruct depositional processes of the Agbaja Ironstone Formation. Aeromagnetic data analysis was engaged to map the oolitic ironstone rich zones. The iron-rich sandstones in the Agbaja Ironstone Formation occur as oolitic ironstone, ironstone concretion and laterites. The bivariate grain size plots and multivariate discriminant analyses suggest that 92% of the sandstone in the formation were deposited in beach and coastal dunes under fluvial-river actions, while 8% are of the marine environment deposited under wave actions. The quartz-feldspar lithic (QFL) ternary diagram and palaeocurrent analyses of the samples revealed that the matured, sub-arkosic sandstones originated from the cratonic interior (Abakaliki uplifts). The bioturbation and reactivation by Ophiomorpha burrows and herringbone on the fine-medium grained sandstones in the formation contributed to the observed high magnetic intensity values. High magnetic signatures (−94.784 to 1.191 nT), analytic signal peaks and depth levels from the 2-D source parameter images are the basis for the produced oolitic ironstones prospectivity map in this study.

Despite numerous subsurface studies of mass-transport deposits (MTDs) using seismic data, internal characters of MTDs at the seismic scale are still not well understood, largely because of the limitation of seismic resolution. This study investigates Miocene-Pliocene MTDs in an understudied, hydrocarbon-rich region of the northern Gulf of Mexico. With the help of quantitative seismic geomorphology techniques, we utilized a high-quality 3D seismic dataset. We document sinuous channel and lobe features hidden within individual MTDs. This provides evidence for considering a seismically-defined MTD unit as amalgamated deposits of multiple events with different flow types (e.g., turbidity currents and cohesive flows), rather than an en masse deposit of a singular event. Additionally, we document an unshielded erosional remnant, which is generated by the bifurcation of a megascour marking the base of an MTD unit. Remnant strata are interpreted as sandy sediment waves. Channel, lobe, and erosional remnant features examined in this study demonstrate the presence of reservoir-prone facies encased within MTD units, forming stratigraphic traps. This research enhances our understanding of the intermingling nature of MTDs and other typical deep-water deposits, and the reservoir potential of MTDs and associated strata.

In this study, eighteen (18) vertical electrical sounding (VES) data points were located in the Federal Capital Territory (FCT), Abuja, Nigeria with ABEM Terrameter using Schlumberger array to delineate potential groundwater development areas. Groundwater flow direction map was produced using the elevation of the water level from sea level with GIS Arc/Info Point coverage. The investigated areas have four geoelectric sections, categorized as topsoil, clay, fractured/weathered basement, and fresh basement. The groundwater is mainly confined in the fractured/weathered basement. It generally flows from NW to SE, but this flow is not steady. It has been obstructed and reoriented towards the south, whereas some portion flows towards the northern parts of the aquifer system peradventure due to the variation in aquifer depth from place to place. Out of the 18 VES points, 9 points located around the NE and SE peripherals of the study area have groundwater development potentials at depths ranging from 40 - 80 m and aquifer thickness of 25 – 65 m, characterized by resistivity values that varied from 80 – 680 Ωm, whilst the other nine in the SW and central parts do not show promising groundwater development potentials because they lack fractured/weathered zones. The northern and SE zones, which comprise areas such as Bwari, Kuje, Idu CITEC, Sherete, and GTSSC are the preferred targets for water well drilling than the southern zone. However, other communities including Gwagwalada and Gugugu in the southern zone also offer promising groundwater potentials.

Bath’s empirical law is derived from the magnitude-difference statistical distribution of earthquake pairs. The pair (two-event, bivariate) distribution related to earthquake correlations is presented. The single-event distribution of dynamically-correlated earthquakes is derived, by means of the geometric-growth model of energy accumulation in the focal region. It is shown that the dynamical correlations may account, at least partially, for the roll-off effect in the Gutenberg-Richter distributions. The extension of the magnitude difference to negative values (by observing the order of the members of the pair) leads to a vanishing mean value of the magnitude difference and to the standard deviation as a measure of its variations. It is suggested that the standard deviation of the magnitude difference is the average difference in magnitude between the main shock and its largest aftershock (foreshock), thus providing an insight into the nature and the origin of Bath’s law. Earthquake purely statistical correlations and deterministic time-magnitude correlations of the accompanying seismic activity are also presented.

The region ∼40 km north-west of Johannesburg, South Africa, known locally as the Cradle of Humankind, is of global significance as the caves preserve Plio-Pleistocene faunal and early hominin fossils. Despite a long history of research, there is still a need to contextualise and date the remarkable collection of fossils. An important but understudied palaeontological site, Bolt's Farm, may provide a key to addressing this as it preserves a series of >20 separate eroded palaeocave remnants occurring across a 1 km length of hillside. This is in contrast to highly concentrated deposits representing a single site, as is the case at the majority of the sites in the region. Historically, a lithostratigraphic approach to South African palaeocaves made reconstruction and comparison within, and between, deposits difficult or impossible. Here, we present a sequence stratigraphic approach and simple facies model for three palaeocave remnants at Bolt's Farm collectively termed the Aves Cave Complex (ACC), and a chronology based on combined uranium lead (U-Pb) dating, of basal and capping flowstones, and palaeomagnetic analysis. Results indicate that these currently discrete localities, formed together from a single entry dating to the end of the Gauss Normal Polarity Chron between 3.03 and 2.61 Ma, making ACC one of the oldest directly dated fossil deposits in the Cradle. The ACC contains the earliest occurrence of a key biochronological species, Metridiochoerus andrewsi, in the region. This work reinforces the model that clastic sedimentation and flowstone precipitation do not occur concurrently in Cradle caves; rather their mutually exclusive formation is driven by allocyclic changes in hydroclimate. This research contributes to understanding how Bolt's Farm developed the unprecedented high density of palaeokarst observed today, by offering the first evidence that currently discrete localities were once connected as a single cave system.

Fracture arrest in layered rock sequences is important in many geodynamic processes, such as dyke-fed volcanic eruptions, earthquake ruptures, landslides, and the evolution of plate boundaries. Yet it remains poorly understood. For example, we do not fully understand the conditions for dyke arrest (preventing potential eruptions) or hydraulic-fracture arrest in gas shales (preventing potential aquifer pollution). Here we present new numerical results on the conditions for arrest of fluid-driven (mode-I) vertical fractures in layered rock sequences when the tips of the fractures approach the interface between two layers of contrasting mechanical properties. In particular, we explore the stress-field effects of variations in layer stiffness, proximity of fracture tip to layer interface, and layer thickness. When the layer hosting the fracture tip is stiffer, fracture arrest normally occurs at the interface with the more compliant layer. By contrast, when the layer above the interface is stiffer, fracture arrest may occur within the host layer well below the interface. These conclusions are supported by field observations of arrested fluid-driven joints and dykes and, therefore, provide a better understanding of the mechanical conditions for dyke-fed eruptions.

Salt tectonics has important implications for hydrocarbon exploration in salt-bearing basins since salt deformation can directly or indirectly form hydrocarbon traps, influence hydrocarbon migration, and can control deepwater depositional systems. In various basins around the globe, extensive research has been conducted on initiation of salt mobilization, subsequent deformation, and eventual cessation, typically from subsurface two-dimensional (2D) sections. However, 3D seismic data has dominated the petroleum industry for the last 30 years. Despite the plethora of 3D seismic data acquired in salt-bearing basins, there has been hardly any published work displaying the 3D geometries of complex salt bodies. 3D salt mapping in the subsurface can reveal true distribution of salt bodies and their detailed intricacies of geometrical variations, aiding in the overall salt system interpretation. Using a large 3D seismic survey (3,350 km²), this study presents the first 3D salt mapping in the Gulf of Mexico, demonstrating how 3D visualization of the entire Louann Salt system within the Middle Jurassic to present-day stratigraphy can improve interpretation of salt feeder geometries, allochthonous salt canopies, initial salt distribution, and salt weld locations in the study area.

The tectonic evolution of the European Eastern Alps within the Alpine orogeny is still under debate. Open questions include: the link between surface, crustal and mantle structures; the nature of the Moho gap between the two plates; the relationship between the Alps, the adjacent foreland basin and the Bohemian Massif lithospheric blocks. We collected one year of continuous data recorded by ~250 broadband seismic stations –55 of which installed within the EASI AlpArray complementary experiment– in the Eastern Alpine region. Exploiting surface wave group velocity from seismic ambient noise, we obtained an high-resolution 3D S-wave crustal model of the area.

The Rayleigh-wave group-velocity from 3 s to 35 s are inverted to obtain 2-D group velocity maps with a resolution of ~15 km. From these maps, we determine a set of 1D velocity models via a Neighborhood Algorithm, resulting in a new 3D model of S-wave velocity with associated uncertainties. The vertical parameterization is a 3-layer crust with the velocity properties in each layer described by a gradient. Our final model finds high correlation with specific geological features in the Eastern Alps up to 20 km depth, the deep structure of the Molasse basin and important variations of crustal thickness and velocities as a result of the Alpine orogeny post-collisional evolution. The strength of our new information relies on the absolute S-wave crustal velocity and the velocity gradient unambiguously sampled along the Moho, only limited by the amount and quality distribution of the data available.

We investigated Coulomb stress transfer (CST) between the two normal, parallel nearest-neighbour seismogenic Monte Vettore and Norcia Faults (central Italian Apennines). Soon after the Mount Vettore Fault System earthquake of 30 October 2016 (Mw 6.6), we surveyed surface faulting evidence along both this and the Norcia Fault System, wondering on the mechanism that could have triggered the discontinuos surface ruptures formed along the latter. Really, seismological data show that the Norcia Fault did not release earthquakes during the 2016–2017 central Italy seismic, i.e., did not slip at depth. First, we mapped and defined the coseismic slip vector of the surface coseismic ruptures. Geological-structural data and geological cross-sections have been integrated to identify kinematic constraints and the relationships between the two faults systems. Then, we calculated the Coulomb stress generated by the Monte Vettore Fault System on 30 October. Results show that a positive lobe formed in the uppermost volume of the hanging-wall of the Norcia Fault System. Then, we simulated the Coulomb Stress Transfer in the case of the activation of the Norcia Fault System. From our analyses, it is possible to hypothesize that during a high magnitude seismic event (Mw 6.6–6.9) these two parallel fault systems can interact by transferring Coulomb stress reciprocally, although at different crustal level, and with different implications.