Lateral Density Variations Research Articles

Geophysical investigation of the ground fissures and ground subsidence near Karla lake (eastern Thessaly basin, Greece)

Ground fissures have occurred in the last four decades across the eastern Thessaly basin (Greece) resulting in damage to the villages of the area. Several studies for the area refer to the over-pumping of ground water as the main reason for their occurrence, causing sediment compaction due to the reduction of the aquifer level. In this paper, we depict the results of a joint geophysical survey trying to determine the subsurface regime of the basin and the exact reasons contributing to the existence of the ground fissures. The focal part of the survey includes gravity measurements for deeper investigation, combined with existing and already presented geoelectrical and electromagnetic data. Several borehole data have also been used for the calibration of the geophysical interpretation. The differential GNSS data of the gravity campaign revealed the ground subsidence of the area, reaching up to 9.16 m. The alpine basement of the area is comprised mainly of metamorphic rocks, such as marbles, mica schists and gneiss-schists, covered by thick fluviοterrestrial and alluvial deposits. Several structural maps were generated in order to delineate the lateral density variations that could be related to fault zones along with the interpretive sections for geological modelling. The alpine bedrock was adumbrated in relatively great depths, with a large anticline of NW-SE direction, rising and separating the basin in two parts. In the east part, the fluvioterrestrial deposits, which are expected to play an important role in the compaction due to their water aquifer, are located only west of this anticline. At the east part, where the old lake Karla was hosted, the alluvial deposits lay directly on the alpine basement in smaller depths. This complicated regime is responsible for differential sediment compaction and the ground subsidence of the surface.

On the impact of true polar wander on heat flux patterns at the core–mantle boundary

Abstract. The heat flux across the core–mantle boundary (CMB) is a fundamental variable for Earth evolution and internal dynamics. Seismic tomography provides access to seismic heterogeneities in the lower mantle, which can be related to present-day thermal heterogeneities. Alternatively, mantle convection models can be used to either infer past CMB heat flux or to produce statistically realistic CMB heat flux patterns in self-consistent models. Mantle dynamics modifies the inertia tensor of the Earth, which implies a rotation of the Earth with respect to its spin axis, a phenomenon called true polar wander (TPW). This rotation must be taken into account to link the dynamics of the mantle to the dynamics of the core. In this study, we explore the impact of TPW on the CMB heat flux over long timescales (∼1 Gyr) using two recently published mantle convection models: one model driven by a plate reconstruction and a second that self-consistently produces a plate-like behaviour. We compute the geoid in both models to correct for TPW. In the plate-driven model, we compute a total geoid and a geoid in which lateral variations of viscosity and density are suppressed above 350 km depth. An alternative to TPW correction is used for the plate-driven model by simply repositioning the model in the original paleomagnetic reference frame of the plate reconstruction. The average TPW rates range between 0.4 and 1.8° Myr−1, but peaks up to 10° Myr−1 are observed. We find that in the plate-driven mantle convection model used in this study, the maximum inertia axis produced by the model does not show a long-term consistency with the position of the magnetic dipole inferred from paleomagnetism. TPW plays an important role in redistributing the CMB heat flux, notably at short timescales (≤10 Myr). Those rapid variations modify the latitudinal distribution of the CMB heat flux, which is known to affect the stability of the magnetic dipole in geodynamo simulations. A principal component analysis (PCA) is computed to obtain the dominant CMB heat flux pattern in the different cases. These heat flux patterns are representative of the mantle convection cases studied here and can be used as boundary conditions for geodynamo models.

Reconstruction of the Subsurface of Al-Hassa Oasis Using Gravity Geophysical Data

Al-Hassa city, located in Eastern Saudi Arabia, boasts the world’s largest oasis and the most expansive naturally irrigated lands. Historically, a total of 280 natural springs facilitated significant groundwater discharge and irrigation of agricultural land. Furthermore, the water in certain springs formerly had a high temperature. The spatial variability of the water quality was evident. At the same time, Al-Hassa Oasis is situated on the northeastern side of the Ghawar field, which is the largest conventional onshore oil field in the world in terms of both reserves and daily output (approximately 3.8 mmb/d). The aforementioned traits suggest an intricate subsurface that has not yet been publicly and thoroughly characterized. Due to the presence of significant cultural noise caused by agricultural and nearby industrial activities, a robust, easy-to-use, and accurate geophysical method (gravity) was used to cover an area of 350 km2, producing the 3D subsurface model of the study area. A total of 571 gravity stations were collected, covering the whole Al-Hassa Oasis and parts of the nearby semi-urban areas. The gravity data were corrected and processed, and a 3D inversion was applied. The resulting 3D geophysical subsurface modeling unveiled an intricate subterranean configuration and revealed lateral variations in density, indicating the presence of a potential salt dome structure, as well as fracture zones that serve as conduits or obstacles to the flow of the subsurface fluids. This comprehensive modeling approach offers valuable insights into the subsurface dynamics of the broader study area, enhancing our understanding of its qualitative tectonic and hydraulic features and their impacts on the area’s natural resources, such as groundwater and hydrocarbons.

Plume‐Modified Lithosphere Mantle Controlled the Cenozoic Sediment Thickness in the Tarim Basin

AbstractThe Cenozoic sediments are very thick in the southwest Tarim Basin and very thin in the northwest, but what controls these variations is unclear. Here, we use two‐dimensional thermo‐mechanical models to investigate how the lateral variations in rheological strength and depletion density of cratonic lithosphere mantle affect the cratonic basin deformation. Model results show that the basin basement uplift occurs above either the region with crustal thickening or high depletion in the mantle. A model with a stronger and density‐depleted northern half of cratonic lithosphere mantle in the context of compression matches the differential Cenozoic subsidence and deformation observed in the Tarim Basin well. We propose that a Permian plume led to the lateral heterogeneity of the lithosphere mantle under the Tarim craton, and the modified lithosphere mantle characteristics caused the differential Cenozoic sediment thickness in the Tarim Basin.

ПОБУДОВА ТА ПЕРШІ РЕЗУЛЬТАТИ ІНТЕРПРЕТАЦІЇ ТРИВИМІРНОЇ МОДЕЛІ ЩІЛЬНОСТІ МАНТІЇ ПІД УКРАЇНСЬКИМ ЩИТОМ

Background. Mantle density models are key tools for understanding fundamental geological and physical processes occurring within the Earth. Many parameters used in mantle density models remain poorly understood and undefined. Among others, these include data on the composition and rheology of the mantle, which can vary significantly. Methods. The method of creating density models (density) significantly influences the final result. Modeling with one-dimensional models simplifies the calculation process but generalizes the distribution of mantle density, assuming it is homogeneous in the horizontal direction. This limitation does not allow for the consideration of lateral variations in mantle density, which can be important at the regional level. In this study, we present a quasi-three-dimensional model of mantle density beneath the Ukrainian Shield, obtained on the basis of a set of one-dimensional density curves. Polynomial corrections for heterogeneity were applied during the calculations, compensating for the shortcomings of one-dimensional models. This three-dimensional model was derived by recalculating one-dimensional velocity curves obtained by seismic tomography for 21 mantle domains in the depth range of 50 to 2600 km. The process of transforming P-wave velocity curves into a density model includes the following steps: determining seismic boundaries in the mantle as points of inflection of the first derivative of P-wave velocity curves for each mantle domain; creating a synthetic S-wave mantle model beneath the Ukrainian Shield by recalculating P-wave velocity curves; solving the Adams-Williamson equation using seismic velocities (P, S) for each domain with subsequent polynomial correction to account for heterogeneity; selecting a reference mantle model that would serve as the basis for converting velocity curves into density through the comparison of gravitational potential on the Earth's surface and calculated values from existing reference mantle models (PREM, PREMA, PREMC, IASP91 AK135). The AK135 model was chosen as the reference model based on the comparison of calculated and observed gravitational potential at the central point of the Ukrainian Shield. This study focuses on the final stages of constructing the mantle density model, taking into account mass balancing of the upper and lower mantle for each domain when determining density using the Adams-Williamson equation and introducing polynomial corrections relative to the AK135 reference model; calculating densities for each of the 21 mantle domains and their three-dimensional integration. Results. In this study, we present a quasi-three-dimensional model of mantle density beneath the Ukrainian Shield, obtained on the basis of a set of one-dimensional density curves, with polynomial corrections for inhomogeneity incorporated into the calculations, compensating for the shortcomings of one-dimensional model calculations. This three-dimensional model was obtained by recalculating one-dimensional velocity curves obtained by the seismic tomography method for P-waves, calculated for 21 mantle domains in the depth range from 50 to 2600 km. Conclusions. This study focuses on the final stages of constructing the mantle density model, considering balancing the mass of the upper and lower mantle for each domain in determining density using the Adams-Williamson equation and introducing polynomial corrections relative to the AK135 reference model; calculating densities for each of the 21 mantle domains and their three-dimensional integration. The obtained mantle density model of the Ukrainian Shield is well aligned with the division of the mantle into three main layers: lithosphere, upper mantle, and lower mantle. Each of the structural layers has its own visual pattern of density heterogeneity.

Inverting Gravity Data to Density and Velocity Models for Selected Area in Southwestern Iraq

The gravity method is a measurement of relatively noticeable variations in the Earth’s gravitational field caused by lateral variations in rock's density. In the current research, a new technique is applied on the previous Bouguer map of gravity surveys (conducted from 1940–1950) of the last century, by selecting certain areas in the South-Western desert of Iraqi-territory within the provinces' administrative boundary of Najaf and Anbar. Depending on the theory of gravity inversion where gravity values could be reflected to density-contrast variations with the depths; so, gravity data inversion can be utilized to calculate the models of density and velocity from four selected depth-slices 9.63 Km, 1.1 Km, 0.682 Km and 0.407 Km. The depths were selected using the power spectrum analysis technique of gravity data. Gravity data are inverted based on gravitational anomalies for each depth slice or level and the extracted equivalent depth data from available wells using a connection curve between densities and velocities, which were mostly compatible with Nafe and Drake's standard curve. The inverted gravity data images highlight the behavior of anomalies/structures in the model and domain of density/velocity, which can be utilized in the processing of the recorded seismic data and time to depth conversion, in parallel with available well's data information within the intended study area of South-Western Iraq.

Impact of Stochastic Ocean Density Corrections on Air‐Sea Flux Variability

AbstractAir‐sea flux variability has contributions from both ocean and atmosphere at different spatio‐temporal scales. Atmospheric synoptic scales and the air‐sea turbulent heat flux that they drive are well represented in climate models, but ocean mesoscales and their associated variability are often not well resolved due to non‐eddy‐resolving spatial resolutions of current climate models. We deploy a physics‐based stochastic subgrid‐scale parameterization for ocean density, that reinforces the lateral density variations due to oceanic eddies, and examine its effect on air‐sea heat flux variability in a comprehensive coupled climate model. The stochastic parameterization substantially modifies sea surface temperature (SST) and latent heat flux (LHF) variability and their co‐variability, primarily at scales near the resolution of the ocean model grid. Enhancement in the SST‐LHF anomaly covariance, and correlations, indicate that the ocean‐intrinsic component of the air‐sea heat flux variability is more consistent with high‐resolution satellite observations, especially in Gulf Stream region.

Global Variability of Density Contrast Across the 660‐km Discontinuity

AbstractThis study investigates lateral variations in density contrast across the 660‐km discontinuity, which is critical for understanding mantle convection and composition. We produce a global map of density jump, with values ranging from 3.8% to 9.3%, at the mineralogical phase boundary by applying inversions of amplitude variations with offset to underside S reflections. Our observations reveal a global average velocity jump of 4.1% and a density jump of 5.3%, favoring a pyrolitic bulk composition. Near major subduction zones, most notably the western Pacific and South America, we identify reduced density jumps in regions of depressed 660‐km discontinuity, consistent with basalt fractions greater than 30% in a mechanically mixed mantle. The causal association between density jump and compositional changes is further supported by a moderate correlation between lateral variations in density jump and those of water content anomalies.

Surface Convergence Zones due to Lagrangian Residual Flow in Tidally Driven Estuaries

Abstract Buoyant material, such as floating debris, marine organisms, and spilled oil, is aggregated and trapped within estuaries. Traditionally, the aggregation of buoyant material is assumed to be a consequence of converging Eulerian surface currents, often associated with lateral (cross-estuary) density gradients that drive baroclinic lateral circulations. This study explores an alternative aggregation mechanism due to tidally driven Lagrangian residual circulations without Eulerian convergence zones and without lateral density variation. In a tidally driven estuary, the depth-dependent tidal phase of the lateral velocity varies across the estuary. This study demonstrates that the lateral movement of surface trapped material follows the tidal phase, resulting in a lateral Lagrangian residual circulation known as Stokes drift for small-amplitude motions. For steeper bathymetry, the lateral change in tidal phase is greater and the corresponding lateral Lagrangian residual flow faster. At local depth extrema, e.g., in the thalweg, depth does not vary laterally, so that the associated tidal phase is laterally constant. Therefore, the Stokes drift is weak near depth extrema resulting in Lagrangian convergence zones where buoyant material concentrates. These ideas are evaluated employing an idealized analytic model in which the along-estuary tidal flow is driven by an imposed barotropic pressure gradient, whereas cross-estuary flow is induced by the Coriolis force. Model results highlight that convergence zones due to Lagrangian residual velocities are efficient in forming persistent aggregation regions of buoyant material along the estuary. Significance Statement Our study focuses on the aggregation of buoyant material (e.g., debris, oil, organisms) in estuaries. Traditionally, the aggregation of buoyant material is assumed to be a consequence of converging Eulerian surface currents, often associated with lateral (cross-estuary) density gradients that drive baroclinic lateral circulations. Our study explores an alternative aggregation mechanism due to tidally driven Lagrangian residual circulations without Eulerian convergence zones and without lateral density variation. Our results highlight that convergence zones due to Lagrangian residual velocities are efficient in forming persistent aggregation regions of buoyant material along the estuary.

Global Moho Gravity Inversion from GOCE Data: Updates and Convergence Assessment of the GEMMA Model Algorithm

Since its discovery in 1909, the Moho was routinely studied by seismological methods. However, from the 1950s, a possible alternative was introduced by gravimetric inversion. Thanks to satellite gravity missions launched from the beginning of the 21st century, a global inversion became feasible, e.g., leading to the computation of the GEMMA model in 2012. This model was computed inverting the GOCE second radial derivatives of the anomalous potential by a Wiener filter, which was applied in the spherical harmonic domain, considering a two-layer model with lateral and vertical density variations. Moreover, seismic information was introduced in the inversion to deal with the joint estimation/correction of both density and geometry of the crustal model. This study aims at revising the GEMMA algorithm from the theoretical point of view, introducing a cleaner formalization and studying the used approximations more thoroughly. The updates are on: (1) the management of the approximations due to the forward operator linearization required for the inversion; (2) the regularization of spherical harmonic coefficients in the inversion by proper modelling the Moho signal and the gravity error covariances; (3) the inclusion of additional parameters and their regularization in the Least Squares adjustment to correct the density model by exploiting seismic information. Thanks to these updates, a significant improvement from the computational point of view is achieved too, thus the convergence of the iterative solution and the differences with respect to the previous algorithm can be assessed by closed-loop tests, showing the algorithm performance in retrieving the simulated “true” Moho.

Regional variations of Mercury's crustal density and porosity from MESSENGER gravity data

A new solution of Mercury's gravity field to degree and order 160, named HgM009, is retrieved through a reprocessing of MESSENGER radio science measurements. By combining our latest gravity field with topography data, localized spectral admittance analyses are carried out to investigate Mercury's crustal and lithospheric properties across the northern hemisphere. The measured spectra are compared with admittances predicted by lithospheric flexure models. The localized gravity/topography admittance analyses yield key information on the lateral variations of the bulk density of the upper crust. Elastic and crustal thicknesses are also adjusted in our study, but the local admittance spectra allow us to constrain these parameters only over a few regions. The average bulk density across the observed areas in the northern hemisphere is 2540 ±60 kg m−3. The crustal porosity is then constrained by using an estimate of the pore-free grain density of surface materials with our measured bulk density. Our estimate of the mean porosity is 14.7 ±1.6 %, which is comparable to, but slightly higher than, the average value measured on the Moon. Larger crustal porosities are observed over heavily cratered regions, suggesting that impact bombardment is the main cause of the crustal porosity.

Capacitive probing of electronic phase separation in an oxide two-dimensional electron system

Electronic phase separation strongly affects the rich phenomenology of two-dimensional electron systems in complex oxides, but its origins and magnitude are not fully understood. Here, the authors use capacitance measurements to quantify the lateral variation of the carrier density associated with EPS at the LaAlO${}_{3}$/SrTiO${}_{3}$ interface. The very high value of several 10${}^{13}$ cm${}^{\ensuremath{-}2}$ and the pattern of capacitance variation imply that charged defects are the main factor causing phase separation. Measurements of the full superconducting dome show that phase separation suppresses global ${T}_{c}$, especially at low nominal carrier density.

Correction to: Impacts of Using the Rigorous Topographic Gravity Modeling Method and Lateral Density Variation Model on Topographic Reductions and Geoid Modeling: A Case Study in Colorado, USA

Correction to: Impacts of Using the Rigorous Topographic Gravity Modeling Method and Lateral Density Variation Model on Topographic Reductions and Geoid Modeling: A Case Study in Colorado, USA

Seismological observation of Earth’s oscillating inner core

We investigate the differential rotation of Earth’s inner core relative to the mantle using pairs of precisely located nuclear explosions. We find that the inner core subrotated at least 0.1° from 1969 to 1971, in contrast to superrotation of ~0.29° from 1971 to 1974. These observations contradict models of steady inner core rotation and models that posit much faster rotation rates. The reversal of polarity, timing, and rotation rates is consistent with a model of oscillations about an equilibrium with gravitational locking of the mantle and inner core due to lateral density variations. The model, which has a 6-year period, can explain the variation in the length of day, which has oscillated fairly steadily for the past decades. Inner core oscillation would also allow interpretations of causal connections between inner core and mantle lateral variations, which are problematic if the inner core consistently superrotates.

Impacts of Using the Rigorous Topographic Gravity Modeling Method and Lateral Density Variation Model on Topographic Reductions and Geoid Modeling: A Case Study in Colorado, USA

Impacts of Using the Rigorous Topographic Gravity Modeling Method and Lateral Density Variation Model on Topographic Reductions and Geoid Modeling: A Case Study in Colorado, USA

Characterising the contemporary stress orientations near an active continental rifting zone: A case study from the Moatize Basin, central Mozambique

AbstractThis paper presents the first comprehensive analysis of the present‐day stress from boreholes near an active continental rifting zone in the Moatize Basin, Mozambique. The state of present‐day stress in this area that is located ca. 100 km away from the Eastern African Rift System (EARS) is poorly understood, and most of our knowledge is from earthquake focal mechanisms that provide stress information from the deeper part of the lithosphere, and to a lesser extent from surface geological features in the Malawi region. Considering the limited reliability of earthquake‐derived stress orientations near plate boundaries, poor coverage of low to moderate magnitude earthquakes in eastern Africa, and ambiguity about the latest activity of geological structure; other well‐established methods are required to shed light on the active tectonics of EARS. In this study, we analyse stress orientation using log data from 95 vertical boreholes in a mine site to investigate the neotectonic stress pattern of the region. Analysis of 17.9 km of televiewer logs resulted in interpretation of 1188 stress‐related borehole failures. The results indicate a mean regional trend of 045° ± 31° for the maximum horizontal stress (SHmax). Our investigation reveals that the regional state of stress in the study area is controlled by superposition of stress sources that act at very different spatial scales. The consistency between our results and predictions by plate‐scale geodynamic models of stress orientations confirm that the regional pattern of stress in this area is mainly controlled by first (>500 km) and second (distances between 500 and 100 km) order stress sources (i.e., large tectonic forces and lateral density variations). However, high‐resolution data used in this study reveal that third (between 100 and 1 km) and fourth (<1 km) order stress sources from stiffness contrasts, rock fabric and geological structures have a great impact on the stress perturbations at smaller and local scales.

Lithospheric-mantle modification beneath the thick sedimentary fan of Bay of Bengal: Inference from the 3D gravity model

A lithospheric density structure of the Bay of Bengal (BoB), which includes the thickest sedimentary layers deposited after continental rifting, is determined from constrained 3D/2D gravity modeling, to understand intricate tectonics processes of the region. The results suggest lateral density variation in the sub-crustal lithospheric mantle and high-density upper mantle is found beneath the thick sedimentary region of BoB. The present density model also predicts the thick lithosphere beneath the huge pile of sediments. The dense and thick lithospheric mantle results are supported by global tomographic and other geophysical results. It is also noticed that the dense mantle and thick sedimentary region have high elastic strength. We propose that the density and strength of the lithospheric mantle in the northern BoB are increased with age and sedimentation since rifting. The cause of density changes in the mantle is indistinct and could occur due to numerous reasons such as the transformation of mantle material catalyzed by a change in pressure and temperature conditions, or mantle flow.

The use of «native» wavelet transform for determining lateral density variation of the Volgo-Uralian subcraton

In the last two decades in conjunction with the development of satellite gravimetry, the techniques of regional-scale inverse and forward gravity modeling started to be more actively incorporated in the construction of crustal and lithospheric scale models. Such regional models are usually built as a set of layers and bodies with constant densities. This approach often leads to a certain difference between the initially used measured gravity field and a gravity field that is produced by the model. One of the examples of this kind of models is a recent lithospheric model of the Volgo-Uralian subcraton. In the current study, we are applying the method of «native» wavelet transform to the residual gravity anomaly for defining the possible lateral density variations within the lithospheric layers of Volgo-Uralia. Keywords: wavelet transform; gravity field inversion; forward gravity modeling; Volgo-Uralian subcraton; satellite gravimetry.

Integration of gravity, magnetic, and seismic data for subsalt modeling in the Northern Red Sea

Rifts and rifted passive margins are often associated with thick evaporite layers, which challenge seismic reflection imaging in the subsalt domain. This makes understanding the basin evolution and crustal architecture difficult. An integrative, multidisciplinary workflow has been developed using the exploration well, gravity and magnetics data, together with seismic reflection and refraction data sets to build a comprehensive 3D subsurface model of the Egyptian Red Sea. Using a 2D iterative workflow first, we have constructed cross sections using the available well penetrations and seismic refraction data as preliminary constraints. The 2D forward model uses regional gravity and magnetic data to investigate the regional crustal structure. The final models are refined using enhanced gravity and magnetic data and geologic interpretations. This process reduces uncertainties in basement interpretation and magmatic body identification. Euler depth estimates are used to point out the edges of high-susceptibility bodies. We achieved further refinement by initiating a 3D gravity inversion. The resultant 3D gravity model increases precision in crustal geometries and lateral density variations within the crust and the presalt sediments. Along the Egyptian margin, where data inputs are more robust, basement lows are observed and interpreted as basins. Basement lows correspond with thin crust ([Formula: see text]), indicating that the evolution of these basins is closely related to the thinning or necking process. In fact, the Egyptian Northern Red Sea is typified by dramatic crustal thinning or necking that is occurring over very short distances of approximately 30 km, very proximal to the present-day coastline. The integrated 2D and 3D modeling reveals the presence of high-density magnetic bodies that are located along the margin. The location of the present-day Zabargad transform fault zone is very well delineated in the computed crustal thickness maps, suggesting that it is associated with thin crust and shallow mantle.

Predicting Global Marine Sediment Density Using the Random Forest Regressor Machine Learning Algorithm

AbstractMarine sediment density is of vital importance due to its influence on many geoacoustic and nongeoacoustic parameters, including but not limited to: acoustic impedance, sediment grain size, sediment attenuation, gravity, and isostatic and dynamic responses of oceanic crust. Additionally, it plays a fundamental role on geomechanical behavior which is important for pore pressure generation, seafloor slope stability, and seafloor infrastructure. Subsurface drilling from the Deep Sea Drilling Project, Ocean Drilling Program, International Ocean Discovery Program, a myriad of scientific research cruises, and a number of petroleum companies have amassed large quantities of invaluable seafloor geologic parameters. These point sampling location data yield accurate vertical constraints at a given location, but extrapolating that information away from the location is very difficult. To address this, we have taken a machine learning approach (the random forest regressor [RFR] in this instance) to predict global seafloor density and its associated uncertainty, both at a 5 × 5‐arc minute resolution. The RFR algorithm accepts a sparsely sampled observational data set with densely gridded relatable predictors and predicts statistically optimal estimates where no physical measurements have been made. The final prediction has median, mean, and root‐mean‐square errors of 0.058, 0.076, and 0.1009 g/cm3, respectively. Results show density predictions that coincide with expected lateral density variation (albeit at a very coarse spatial scale) with respect to ocean depth and uncertainties that are low for most of the seafloor. Finally, a parametric isolation indicates locations where additional samples are needed to improve the overall prediction.