Gravity Variations and Ground Deformations Resulting from Core Dynamics

Fluid motion within the Earth’s liquid outer core leads to internal mass redistribution. This occurs through the advection of density anomalies within the volume of the liquid core and by deformation of the solid boundaries of the mantle and inner core which feature density contrasts. It also occurs through torques acting on the inner core reorienting its non-spherical shape. These in situ mass changes lead to global gravity variations, and global deformations (inducing additional gravity variations) occur in order to maintain the mechanical equilibrium of the whole Earth. Changes in Earth’s rotation vector (and thus of the global centrifugal potential) induced by core flows are an additional source of global deformations and associated gravity changes originating from core dynamics. Here, we review how each of these different core processes operates, how gravity changes and ground deformations from each could be reconstructed, as well as ways to estimate their amplitudes. Based on our current understanding of core dynamics, we show that, at spherical harmonic degree 2, core processes contribute to gravity variations and ground deformations that are approximately a factor 10 smaller than those observed and caused by dynamical processes within the fluid layers at the Earth’s surface. The larger the harmonic degree, the smaller is the contribution from the core. Extracting a signal of core origin requires the accurate removal of all contributions from surface processes, which remains a challenge. Article Highlights Dynamical processes in Earth's fluid core lead to global gravity variations and surface ground deformations We review how these processes operate, how signals of core origin can be reconstructed and estimate their amplitudes Core signals are a factor 10 smaller than the observed signals; extracting a signal of core origin remains a challenge

Temporal gravity variations in GOCE release 6 gravitational gradients

<p>As opposed to the level 1B release 5 GOCE gravitational gradient data, the newly reprocessed release 6 gradients provide reduced noise amplitudes in the low frequency-range, leading to reduced noise amplitudes of the derived gravity field models at large spatial scales, where temporal variations of the Earth’s gravity field have their highest amplitudes. This is the motivation to test the release 6 gradients for their ability to resolve temporal gravity variations.</p><p>For the gravity field processing, we apply a conventional spherical harmonics approach using the time-wise (TIM) processing method as well as a mass concentration (mascon) approach using point masses as base elements, which are grouped to land or ocean mascons by taking into account the coastlines.</p><p>By means of a closed-loop simulation study, we find that the colored instrument noise of the GOCE gravitational gradiometer introduces noise amplitudes into the derived gravity field models that lie above the amplitude of the gravity trend signal accumulated over 5 years. This indicates that detecting gravity variations taking place during the four-year GOCE data period from GOCE gradients only is challenging.</p><p>Using real GOCE data, we test bimonthly gradiometry-only gravity field models computed by both the spherical harmonic and the mascon approach for gravity signals that are resolved by GRACE data, being the temporal signals due to the ice mass trends in Greenland and Antarctica and the 2011 earthquake in Japan. Besides, corresponding GRACE/GOCE combination models are used to test whether the incorporation of GOCE data increases the resolution of temporal gravity signals.</p><p>We found that high-amplitude long-wavelength noise prevented the detection of temporal gravity variations among the bimonthly GOCE-only models. Using the SH approach, it was possible to detect the mean trend signal contained in the data by averaging multiple bimonthly models and considering their difference to a reference model. Using the mascon approach, trend signals contained in GOCE data could be recovered by including a GRACE model truncated to d/o 45 in a GRACE/GOCE combination model and thus let the GOCE data determine the short-scale signal structures instead of GRACE.</p><p>Finally, compared to the temporal gravity signal as resolved by GRACE data, no significant benefit of using or incorporating GOCE gravitational gradient data was found. The reason are the still rather high noise amplitudes in the derived models at large spatial scales, where the considered signal is strongest.</p><p>In order to detect temporal gravity variations in satellite gravitational gradiometry data, the measurement noise amplitudes in the low-frequency range would need to be reduced.</p>

Finding Centre of Gravity of Yoga Asana Postures to support Self-Assisted Yoga Practice

Self-assisted yoga practice should ensure safety. Being in a wrong posture while doing practice may cause problems like fall or fractures in the bones. Practitioners of activities like yoga, sports etc. should maintain correct postures all through their practice. Maintaining a correct posture is influenced by Centre of Gravity whose location is based on distributing the entire body weight evenly for stability. In this paper, we propose a method to find the Centre of Gravity of any asana sequence. The location of Centre of Gravity will be used by the assistive system to instruct the practitioner how to alter their postures to avoid fall and enable to take-up perfect practice. Motion capture systems will give the details in the form of Biovision hierarchy format, from which the coordinate values of human joints can be accessed. Based on the segmentation method, whole body Centre of Gravity is calculated. We have used Vrikshasana for our experiment and yoga posture data for ten participants are used to find the Centre of Gravity variations. Data for two trails were acquired. CM Variations in segments like thigh, angle are calculated. From the results the instructors may find whether ankle or thigh’s position should be changed to maintain correct posture.

Modelling the Gravitational Effects of Random Underground Density Variations

A mathematical model for small-scale spatial variations in gravity above the Earth’s surface is presented. Gravity variations are treated as a Gaussian random process arising from underground density variations which are assumed to be a Gaussian random process. Expressions for two-point spatial statistics are calculated for both the vertical component of gravity and the vertical gradient of the vertical component. Results are given for two models of density variations: a delta-correlated model and a fractal model. The effect of an outer scale in the fractal model is investigated. It is shown how the results can be used to numerically generate realisations of gravity variations with fractal properties. Such numerical modelling could be useful for investigating the feasibility of using gravity surveys to locate and characterise underground structures; this is explored through the simple example of a tunnel detection scenario.

ABOUT A POSSIBLE CONNECTION BETWEEN EARTHQUAKES AND LUNAR-SOLAR GRAVITY VARIATIONS

A possible correlation between the destructive earthquakes of magnitude M = 7 and above and luni-solar gravity variations between 1975 and 2015 has been analyzed. The lunar-solar variations are characterized by three extreme points: the maximum and minimum values of gravity, and the maximum rate of change of variations. At this time, there is an extreme impact of lunar-solar attraction on the earth’s crust and the Earth as a whole. Variations can be a source of irreversible deformation in the earth’s crust. If in this case, there is an additional external impact of space factors, the probability of an earthquake is increased. In a time, the earthquakes are grouped near extremes of lunar-solar variations: half of the events are associated with the maximum gradient of variations change, and the second half is equally confined to the maximum and minimum value of gravity variations. Lunar-solar variations of gravity in conjunction with other cosmic influences can cause earthquakes.