Geophysical Phenomena Research Articles

Micro Cantilever-Based Optical Gravimeter

Geophysical phenomena like volcanic eruptions, tectonic plate movements, and earthquakes directly influence the gravitational pull in the region of occurrence. Measurement of anomalies in the gravitational pull has been a useful technique in analyzing such activities quantitatively. Gravitational variations also carry indications of the presence of the earth’s minerals and other such useful resources. In this paper, a novel on-chip micro-cantilever-based optical gravimeter has been proposed for the precise measurement of gravitational changes. Simulations comprising both three-dimensional full-vector finite element and finite difference time domain methods have been carried out for analyses, demonstrating an extensive reading range of 6000 mGal with a resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.17~\mu $ </tex-math></inline-formula> Gal and sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1~\mu $ </tex-math></inline-formula> Gal/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sqrt {Hz}$ </tex-math></inline-formula> at 1 Hz.

Velocity anisotropy and trend in Niger Delta, Nigeria

In geophysical data interpretation, matching the vertical velocity direction from seismic data with borehole-derived velocities is a challenging task because seismic-derived velocities are faster than borehole recorded velocities. This geophysical phenomenon is caused by velocity anisotropy. In this study, we used an empirical approach to estimate the degree of velocity anisotropy in the study area. The results showed that the delta anisotropy in sandstone beds varies from − 2.5% to 7.2% while most of them concentrate between 3.2% and 6.1%. The epsilon ranges between -6.4% and 9.3% while many of them concentrate between 3.2% and 7.2%. The gamma varies from − 6.3% to 7.3% while most of them concentrate between 1.2% and 5%. At shale beds, delta anisotropy varies from − 11.2% to 11.1% but most of them concentrate between 4.3% and 10.5%. The epsilon varies from − 7.2% to 14.5% while most of them concentrate between 4.5% and 10.5%. The gamma varies from 6.4% to 8.2% while majority of them concentrate between 2% and 5.3%. The results indicate that the study area is weakly to moderately anisotropic with shale beds having higher anisotropy values than sandstone beds. This probably results from preferential alignment of clay mineral orientations which also affect in situ velocity propagation. Three distinct velocity gradients (low, moderate and very high) were identified in the study area. These velocities vary erratically but showed northeast–southwest increase in velocities. Thus, the need to derive correction factors for individual wells for improved exploration success.

NOANET: A Continuously Operating GNSS Network for Solid-Earth Sciences in Greece

Abstract The NOANET network is a continuously operating high-rate Global Navigation Satellite System (GNSS) network in Greece whose primary role is to enhance and support geophysical research employing GNSS data. This network is operated by the Institute of Geodynamics of the National Observatory of Athens and currently (September 2020) consists of 26 stations, most of which are located close to major seismogenic structures of Greece to optimally measure tectonic and seismically induced motions. All NOANET receivers are configured to record and collect data with a sampling rate of 1 Hz, although some of them also collect data every 5 Hz on their ring buffer. The network is committed to free and open data sharing within the scientific community, and the collected data are made available via the NOANET data repository and distribution point for all interested parties with no limitations. Integrity, validity, and quality checks of the acquired data are performed using a variety of software tools along with in-house developed programs to supervise the network performance and detect ill-formed data and/or awkward station behavior. In addition, the conventional low-rate GNSS observation data of all NOANET stations are routinely processed on a daily basis to supervise their performance through their position time series. Since the beginning of its establishment, the NOANET network has recorded a variety of deformation signals, and a large number of published papers have used GNSS data from stations that are part of NOANET to constrain, model, and interpret the nature of the associated geophysical phenomena.

Steady-state similarity velocity profiles for dense granular flow down inclined chutes

In this paper we exploit the fact that dense granular flow down an inclined chute exhibits a simple scaling property. This implies that the fundamental governing partial differential equation admits similarity profiles which are applicable to any general friction law $$\mu (I)$$ of the material irrespective of the particular functional form under consideration, where $$\mu (I)$$ is a function of the dimensionless parameter, called the inertial number I. We adopt a widely accepted constitutive model describing the material response that draws its original inspiration from the realms of classical visco-plasticity. The governing steady-state similarity solutions are however applicable for any friction law describing dense granular media. These steady-state solutions embody the key aspects of dense granular flow down an inclined chute that find wide usage in various industrial applications and will further aid in modelling natural geophysical phenomenon. We provide steady-state analytical solutions for some special cases and we emphasize the importance of similarity solutions as a benchmark standard for various numerical studies concerning the complex material behaviour of dense granular flows.

A Significant Change in Ocean Bottom Pressure Off Eastern Taiwan, Southwestern Ryukyu Subduction Zone

Three real-time permanent observatories (EOS02–EOS04) connected by a submarine cable were constructed and deployed to monitor seismicity, tsunamis, and other geophysical phenomena off eastern Taiwan, southwestern Ryukyu subduction zone. In this study, we process the pressure data (January 26, 2017, to December 31, 2019) recorded by three ocean bottom pressure gauges. After removing of tidal and short-period oceanographic effects and the linear trend caused by long-term sensor drift, overall, the variation patterns of the ocean bottom pressure (OBP) records at stations EOS02 and EOS03 are similar, while that at station EOS04 (located on a slope) exhibits different characteristics. The amplitude fluctuations of the OBP records are very large, even reaching approximately 3 × 102 mbar at station EOS02. In addition, we observe a significant permanent change in the OBP data at station EOS04 where the pressure variation reaches approximately 4.2 × 102 mbar (approximately equals a 4.2 m water column variation). Considering the seismicity and background tectonic characteristics of station EOS04, we speculate that this permanent pressure change may have been caused by submarine landslides and/or slumps (triggered by nearby earthquakes).

Regional characteristics of the lithospheric structure in the southeast of North China revealed by explosion seismology

Based on the blasting seismic detection data obtained in the southeast of North China in recent years, this paper comprehensively analyzes and studies the crust-mantle lithospheric structure and seismological characteristics of different tectonic regions, such as offshore basins, west Shandong uplift, Tanlu fault zone and Jiangsu-Shandong orogenic belt. The low-velocity Pg waves in Dongying depression and Northern Jiangsu basin reveal the unstable basement structure with extremely thick sediments. The travel time of Pg wave is characterized by relatively low propagation velocity and small crustal thickness of offshore continental margin; the first break time and high apparent velocity of Pg wave in west Shandong uplift indicate that the sedimentary basement is relatively thin. The Pm wave shows the characteristic of dominant wave in the first-order velocity discontinuity of the crust-mantle interface, which reflects the high crustal velocity and stable structure in west Shandong uplift. The Pm and Pl wave are obviously complicated, which can reflect the crust-mantle lithospheric structure of the transitional zone between Tanlu fault zone and Jiangsu-Shandong orogenic belt. The small time difference between Pn and PL waves can be regarded as the highly destructive seismological manifestation of Tanlu fault zone on the crust-lithosphere scale. Based on many geophysical phenomena such as electrical structure, density structure and terrestrial heat flow, it is believed that the lithospheric destruction degree of Tanlu fault zone and Jiangsu-Shandong orogenic belt was high during the destruction of the North China Craton.

Comparison of Digital Image Correlation Methods and the Impact of Noise in Geoscience Applications

Digital image correlation (DIC) is a commonly-adopted technique in geoscience and natural hazard studies to measure the surface deformation of various geophysical phenomena. In the last decades, several different correlation functions have been developed. Additionally, some authors have proposed applying DIC to other image representations, such as image gradients or orientation. Many works have shown the reliability of specific methods, but they have been rarely compared. In particular, a formal analysis of the impact of different sources of noise is missing. Using synthetic images, we analysed 15 different combinations of correlation functions and image representations and we investigated their performances with respect to the presence of 13 noise sources. Besides, we evaluated the influence of the size of the correlation template. We conducted the analysis also on terrestrial photographs of the Planpincieux Glacier (Italy) and Sentinel 2B images of the Bodélé Depression (Chad). We observed that frequency-based methods are in general less robust against noise, in particular against blurring and speckling, and they tend to underestimate the displacement value. Zero-mean normalised cross-correlation applied to image intensity showed high-quality results. However, it suffers variations of the shadow pattern. Finally, we developed an original similarity function (DOT) that proved to be quite resistant to every noise source.

South Atlantic Anomaly Areal Extent as a Possible Indicator of Geomagnetic Jerks in the Satellite Era

Geomagnetic jerks are sudden changes in the geomagnetic field secular variation related to changes in outer core flow patterns. Finding geophysical phenomena related to geomagnetic jerks provides a vital contribution to better understand the geomagnetic field behavior. Here, we link the geomagnetic jerks occurrence with one of the most relevant features of the geomagnetic field nowadays, the South Atlantic Anomaly (SAA), which is due to the presence of reversed flux patches (RFPs) at the Core-Mantle Boundary (CMB). Our results show that minima of acceleration of the areal extent of SAA calculated using the CHAOS-7 model (CHAOS-7.2 release) coincide with the occurrence of geomagnetic jerks for the last 2 decades. In addition, a new pulse in the secular acceleration of the radial component of the geomagnetic field has been observed at the CMB, with a maximum in 2016.2 and a minimum in 2017.5. This fact, along with the minimum observed in 2017.8 in the acceleration of the areal extent of SAA, could point to a new geomagnetic jerk. We have also analyzed the acceleration of the areal extent of South American and African RFPs at the CMB related to the presence of the SAA at surface and have registered minima in the same periods when they are observed in the SAA at surface. This reinforces the link found and would indicate that physical processes that produce the RFPs, and in turn the SAA evolution, contribute to the core dynamics at the origin of jerks.

G-Tric: generating three-way synthetic datasets with triclustering solutions

BackgroundThree-way data started to gain popularity due to their increasing capacity to describe inherently multivariate and temporal events, such as biological responses, social interactions along time, urban dynamics, or complex geophysical phenomena. Triclustering, subspace clustering of three-way data, enables the discovery of patterns corresponding to data subspaces (triclusters) with values correlated across the three dimensions (observations times features times contexts). With increasing number of algorithms being proposed, effectively comparing them with state-of-the-art algorithms is paramount. These comparisons are usually performed using real data, without a known ground-truth, thus limiting the assessments. In this context, we propose a synthetic data generator, G-Tric, allowing the creation of synthetic datasets with configurable properties and the possibility to plant triclusters. The generator is prepared to create datasets resembling real 3-way data from biomedical and social data domains, with the additional advantage of further providing the ground truth (triclustering solution) as output.ResultsG-Tric can replicate real-world datasets and create new ones that match researchers needs across several properties, including data type (numeric or symbolic), dimensions, and background distribution. Users can tune the patterns and structure that characterize the planted triclusters (subspaces) and how they interact (overlapping). Data quality can also be controlled, by defining the amount of missing, noise or errors. Furthermore, a benchmark of datasets resembling real data is made available, together with the corresponding triclustering solutions (planted triclusters) and generating parameters.ConclusionsTriclustering evaluation using G-Tric provides the possibility to combine both intrinsic and extrinsic metrics to compare solutions that produce more reliable analyses. A set of predefined datasets, mimicking widely used three-way data and exploring crucial properties was generated and made available, highlighting G-Tric’s potential to advance triclustering state-of-the-art by easing the process of evaluating the quality of new triclustering approaches.

Subcretionary tectonics: Linking variability in the expression of subduction along the Cascadia forearc

A number of seismic and other geophysical phenomena exhibit significant lateral heterogeneity along the strike of the Cascadia forearc. Both the overriding and downgoing plate have been invoked to play the dominant role in controlling along-strike correlations between seismogenic behavior, potential field measurements, morphological/tectonic characteristics, and seismic structure; however, significant feedbacks likely exist between the two. In this study, we apply a 3D velocity correction to receiver function data and interpret the resulting discontinuity model alongside a recently published shear-wave velocity model to understand the possible causative relationships between correlative along-strike variations. Our discontinuity model indicates that the forearc crust thickens as it approaches the mantle wedge corner before progressively thinning toward the magmatic arc, likely due to the basal accretion of material from the downgoing plate to the overriding plate. In the northern and southern portions of the forearc, this “subcreted” material is characterized by thick (∼10 km) anomalously low shear-wave velocity zones. The thickness, high internal reflectivity, and low Bouguer gravity signatures associated with the low velocity zones likely indicate that this subcreted material is composed of dominantly (meta)sedimentary material that has been emplaced through successive subcretion events over geologic timescales. Furthermore, the anomalously low velocities and spatial correlation with high non-volcanic tremor (NVT) density and short slow slip recurrence intervals indicate that these regions are fluid-rich. While first-order variations in the fluids that control NVT and slow slip likely result from differences in the permeability of the downgoing slab as inferred from its stress state and the distribution of intraslab seismicity, these subcreted packages likely represent thick, vertically-impermeable regions in the lower crust that further accentuate this correlation. Variability in the amount of subcretion explains patterns of exhumation and uplift along the Cascadia margin and the resulting forearc topography over geologic timescales, and is likely controlled by some combination plate interface geometry/rheology and overriding plate architecture.

Accuracy Analysis of Point Velocities Determined by Different Software Packages and GNSS Measurement Processing Methods

The paper analyzes the geodynamic network of the City of Zagreb for periodic campaigns carried out from 2006 to 2009 which were processed by different software packages. The first computations and processing results were obtained by using the scientific software Gamit/Globk and indicate the ongoing tectonic activity of the area. In this paper, all calculations were performed by using the scientific software Bernese. Processing strategies and applied error models in the Gamit and Bernese solution are analyzed. The results of the previous analyses show the need to perform GNSS measurements at intervals of up to one year, which is necessary for understanding the mechanism of the structural frame of the wider Zagreb area. The research and analysis performed in this paper indicate certain uncertainties in determining the velocities from periodic one-year GNSS measurements. When periodic GNSS observations are analyzed at time intervals shorter than 2.5 years, annual signals can cause significant errors in determining point velocities. The accuracy of determining velocities between annual time intervals depends on a number of factors: noise level in GNSS measurements, measurement sessions quantity and applied computation strategies. Previously, the time series analysis of observations was a key procedure in the context of geodynamic and geokinematic research and the FODITS algorithm was used for the analysis. A noise analysis on the daily time series of coordinates was performed for the purpose of understanding all influences on geodynamic points. Moreover, a correlation between the time series of observations was determined in order to estimate the final velocity uncertainty error. The purpose of this paper is to demonstrate the applicability of the methods and procedures used to determine the coordinates and velocities of points that can be reliably used for geodynamic and geokinematic analyses and consequently, timely responses to various geophysical phenomena due to earthquakes or other natural phenomena.

Diplomats in Science Diplomacy: Promoting Scientific and Technological Collaboration in International Relations**

Diplomats in Science Diplomacy: Promoting Scientific and Technological Collaboration in International Relations**

Microscopic Picture of Erosion and Sedimentation Processes in Dense Granular Flows.

Gravity-driven flows of granular matter are involved in a wide variety of situations, ranging from industrial processes to geophysical phenomena, such as avalanches or landslides. These flows are characterized by the coexistence of solid and fluid phases, whose stability is directly related to the erosion and sedimentation occurring at the solid-fluid interface. To describe these mechanisms, we build a microscopic model involving friction, geometry, and a nonlocal cooperativity emerging from the propagation of collisions. This new picture enables us to obtain a detailed description of the exchanges between the fluid and solid phases. The model predicts a phase diagram including the limits of erosion and sedimentation, in quantitative agreement with experiments and discrete-element-method simulations.

Comparison of Terrestrial Water Storage Changes Derived from GRACE/GRACE-FO and Swarm: A Case Study in the Amazon River Basin

The mass changes in the Earth’s surface internally derived from the Gravity Recovery and Climate Experiment (GRACE) and the GRACE Follow-On (GRACE-FO) missions have played an important role in the research of various geophysical phenomena. However, the one-year data gap between these two missions has broken the continuity of this geophysical research. In order to assess the feasibility of using the Swarm time-variable gravity field (TVGF) to bridge the data gap, we compared Swarm with the GRACE/GRACE-FO models in terms of model accuracy, observation noise and inverted terrestrial water storage change (TWSC). The results of the comparison showed that the difference between the degree-error root mean square (RMS) of the two models is small, within degree 10. The correlation between the spherical harmonic coefficients of the two models is also relatively high, below degree 17. The observation noise values of GRACE/GRACE-FO are smaller than those of Swarm. Therefore, the latter model requires a larger filter radius to lower these noise levels. According to the correlation coefficients and the time series map of TWSC in the Amazon River basin, the results of GRACE and Swarm are similar. In addition, the TWSC signals were further analyzed. The long-term trend changes of TWSC derived from GRACE/GRACE-FO and the International Combination Service for Time-variable Gravity Fields (COST-G)-Swarm over the period from December 2013 to May 2020 were −0.72 and −1.05 cm/year, respectively. The annual amplitudes of two models are 15.65 and 15.39 cm, respectively. The corresponding annual phases are −1.36 and −1.33 rad, respectively. Our results verified that the Swarm TVGF has the potential to extract TWSC signals in the Amazon River basin and can serve as a complement to GRACE/GRACE-FO data for detecting TWSC in local areas.

InSAR Greece with Parallelized Persistent Scatterer Interferometry: A National Ground Motion Service for Big Copernicus Sentinel-1 Data

Advances in synthetic aperture radar (SAR) interferometry have enabled the seamless monitoring of the Earth’s crust deformation. The dense archive of the Sentinel-1 Copernicus mission provides unprecedented spatial and temporal coverage; however, time-series analysis of such big data volumes requires high computational efficiency. We present a parallelized-PSI (P-PSI), a novel, parallelized, and end-to-end processing chain for the fully automated assessment of line-of-sight ground velocities through persistent scatterer interferometry (PSI), tailored to scale to the vast multitemporal archive of Sentinel-1 data. P-PSI is designed to transparently access different and complementary Sentinel-1 repositories, and download the appropriate datasets for PSI. To make it efficient for large-scale applications, we re-engineered and parallelized interferogram creation and multitemporal interferometric processing, and introduced distributed implementations to best use computing cores and provide resourceful storage management. We propose a new algorithm to further enhance the processing efficiency, which establishes a non-uniform patch grid considering land use, based on the expected number of persistent scatterers. P-PSI achieves an overall speed-up by a factor of five for a full Sentinel-1 frame for processing in a 20-core server. The processing chain is tested on a large-scale project to calculate and monitor deformation patterns over the entire extent of the Greek territory—our own Interferometric SAR (InSAR) Greece project. Time-series InSAR analysis was performed on volumes of about 12 TB input data corresponding to more than 760 Single Look Complex Sentinel-1A and B images mostly covering mainland Greece in the period of 2015–2019. InSAR Greece provides detailed ground motion information on more than 12 million distinct locations, providing completely new insights into the impact of geophysical and anthropogenic activities at this geographic scale. This new information is critical to enhancing our understanding of the underlying mechanisms, providing valuable input into risk assessment models. We showcase this through the identification of various characteristic geohazard locations in Greece and discuss their criticality. The selected geohazard locations, among a thousand, cover a wide range of catastrophic events including landslides, land subsidence, and structural failures of various scales, ranging from a few hundredths of square meters up to the basin scale. The study enriches the large catalog of geophysical related phenomena maintained by the GeObservatory portal of the Center of Earth Observation Research and Satellite Remote Sensing BEYOND of the National Observatory of Athens for the opening of new knowledge to the wider scientific community.

The future of endangered crayfish in light of protected areas and habitat fragmentation

The long-term survival of a species requires, among other things, gene flow between populations. Approaches for the evaluation of fragmentation in the frame of freshwater habitats consider only a small amount of the information that combined demography and geography are currently able to provide. This study addresses two species of Austropotamobius crayfish in the light of population genetics, spatial ecology and protected areas of the Carpathians. Advancing the classical approaches, we defined ecological distances upon the rasterised river network as a surrogate of habitat resistance to migration, quantifying the deviations from the species´ suitability range for a set of relevant geospatial variables in each cell of the network. Molecular analyses revealed the populations of the two Austropotamobius crayfish species are clearly distinct, lacking hybridisation. Comparing pairs of populations, we found, in some cases, a strong disagreement regarding genetic and ecological distances, potentially due to human-mediated translocations or the geophysical phenomena of regressive erosion, which may have led to unexpected colonisation routes. Protected areas were found to offer appropriate local habitat conditions but failed to ensure connectivity. The methodology applied in this study allowed us to quantify the contribution of each geospatial (environmental) variable to the overall effect of fragmentation, and we found that water quality was the most important variable. A multilevel approach proved to reveal a better understanding of drivers behind the distribution patterns, which can lead to more adequate conservation measures.

Solute dispersion in bifurcating networks

Abstract

Aeolian Creep Transport: Theory and Experiment

AbstractAeolian sand transport drives geophysical phenomena, such as bedform evolution and desertification. Creep plays a crucial, yet poorly understood, role in this process. We present a model for aeolian creep, making quantitative predictions for creep fluxes, which we verify experimentally. We discover that the creep transport rate scales like the Shields number to the power 5/2, clearly different from the laws known for saltation. We derive this 5/2 power scaling law from our theory and confirm it with meticulous wind tunnel experiments. We calculate the creep flux and layer thickness in steady state exactly and for the first time study the relaxation of the flux toward saturation, obtaining an analytic expression for the relaxation time.

Illumination of Damage in Intact Rocks by Ultrasonic Transmission‐Reflection and Digital Image Correlation

AbstractLaboratory‐scale experiments on intact rocks are critical to the development of physics‐based fundamental understanding of various geophysical phenomena. In this work, the capability of ultrasonic wave transmission (T‐mode) and reflection (R‐mode) to monitor damage progression in uniaxially loaded prismatic intact rock specimens has been analyzed, as it is imperative to study the observations and document the capabilities of these techniques in a controlled environment. This study is novel in the sense that the R‐mode linear ultrasonic testing (LUT) has been rarely employed in studying intact rock damage processes in a laboratory setting, with most of the studies utilizing a direct transmission (T‐mode) approach or focusing on macroscopically fractured material. The two‐dimensional digital image correlation (2‐D DIC) full‐field strain measurement approach was also used in‐sync with the LUT monitoring to explicitly correlate the stress‐induced damage in the specimens with the changes observed in the ultrasonic (T‐mode and R‐mode) signals. The results show that both the T‐mode and R‐mode LUT approaches are sensitive to detect the evolution of tensile and shear damage in the specimens, with the R‐mode ultrasonic signals showing higher degree of sensitivity to the damage in the rocks, immediately following the initiation of damage in the rock volume.

Improving AI System Awareness of Geoscience Knowledge: Symbiotic Integration of Physical Approaches and Deep Learning

AbstractModeling dynamic geophysical phenomena is at the core of Earth and environmental studies. The geoscientific community relying mainly on physical representations may want to consider much deeper adoption of artificial intelligence (AI) instruments in the context of AI's global success and emergence of big Earth data. A new perspective of using hybrid physics‐AI approaches is a grand vision, but actualizing such approaches remains an open question in geoscience. This study develops a general approach to improving AI geoscientific awareness, wherein physical approaches such as temporal dynamic geoscientific models are included as special recurrent neural layers in a deep learning architecture. The illustrative case of runoff modeling across the conterminous United States demonstrates that the physics‐aware DL model has enhanced prediction accuracy, robust transferability, and good intelligence for inferring unobserved processes. This study represents a firm step toward realizing the vision of tackling Earth system challenges by physics‐AI integration.