DETERMINATION OF THE VERTICAL GRADIENT OF GRAVITY FROM GRAVITY ANOMALIES

In connection with the need to use the vertical gradient in the processing of the results of gravimetric measurements and their interpretation when solving problems of geology, geophysics, geodesy, geodynamics and navigation, in addition to the urgent problems of improvement, socalled indirect methods of measuring the vertical gradient, there is an equally urgent task of developing methods for determining the vertical gradient of gravity, using dependencies between different physical fields. The article presents the development and study of a method for determining the vertical gradient from gravity anomalies using the relationship between gravity anomalies and altitude based on field data obtained in the area of the Tashtagol field on Mount Boulanger in 2019 and 2020.

Refined prediction of vertical gradient of gravity at Etna volcano gravity network (Italy)

Predicted values of the vertical gradient of gravity (VGG) on benchmarks of Etna’s monitoring system, based on calculation of the topographic contribution to the theoretical free-air gradient, are compared with VGG values observed in situ. The verification campaign indicated that improvements are required when predicting the VGGs at such networks. Our work identified the following factors to be resolved: (a) accuracy of the benchmark position; (b) gravitational effect of buildings and roadside walls adjacent to benchmarks; (c) accuracy of the digital elevation model (DEM) in the proximity of benchmarks. Benchmark positions were refined using precise geodetic methods. The gravitational effects of the benchmark-adjacent walls and buildings were modeled and accounted for in the prediction. New high-resolution DEMs were produced in the innermost zone at some benchmarks based on drone-flown photogrammetry to improve the VGG prediction at those benchmarks. The three described refinements in the VGG prediction improved the match between predicted and in situ observed VGGs at the network considerably. The standard deviation of differences between the measured and predicted VGG values decreased from 36 to 13 μGal/m.

Prediction of vertical gradient of gravity and its significance for volcano monitoring – example from Teide volcano

We present a detailed calculation of the topographic contribution to the vertical gradient of gravity (VGG) based on high-resolution digital elevation model (DEM) and new developed software (Toposk) for the purpose of predicting the actual VGGs in the field. The calculations presented here were performed for the Central Volcanic Complex (CVC) of Tenerife. We aimed at identifying the most extreme VGGs within the CVC, as well as predicting the VGGs at benchmarks of the former microgravity/deformation network set up to monitor the 2004/5 unrest. We have carried out an observational campaign in June 2016 to verify the predicted VGG values, both the extreme ones and those at the benchmarks. The comparison between the predicted and the in-situ verified VGGs is presented here. We demonstrate the sensitivity of the VGG prediction to the choice of the topo-rock density, which is inherent to the volcanic areas with high variability of rock densities. We illustrate the significance of the use of actual VGG in volcano monitoring microgravimetric surveys on a couple of benchmarks of the CVC network.

INVESTIGATION INTO MEASUREMENTS USING GRAVIMETERS SCINTREX CG-5 / MATAVIMŲ SCINTREX CG-5 GRAVIMETRAIS ANALIZĖ / АНАЛИЗ ИЗМЕРЕНИЙ, ПРОВЕДЕННЫХ ГРАВИМЕТРАМИ SCINTREX CG-5

The article investigates four automatic gravimeters Scintrex CG-5. With reference to the measurements of the gravimetric calibration base set up in Lithuania, linear scale coefficients and the character of changes in the linear scale coefficients of gravimeters were estimated. The paper examines the vertical gradient of gravity acceleration in the eight-floor building, evaluates the range of changes into gravimeter drifts under laboratory conditions and describes daily gravimeter drift during gravimetric measurements. Santrauka Atlikti keturių Scintrex CG-5 gravimetrų tyrimai. Remiantis gravimetrinės bazės, įrengtos Lietuvos teritorijoje, matavimų duomenimis, nustatyta gravimetrų kalibravimo koeficientai ir jų kitimo pobūdis. Atlikti sunkio pagreičio vertikaliojo gradiento aštuonių aukštų pastate tyrimai. Laboratorinėmis sąlygomis nustatytas gravimetrų paros nulio slinkties kitimo diapazonas. Atlikti gravimetrų nulio slinkties matavimų metu tyrimai. Резюме Проведены исследования четырех гравиметров Scintrex CG-5. На основании измерений гравиметрической базы, находящейся на территории Литвы, получены коэффициенты калибровки гравиметров. Проведены исследования вертикального градиента ускорения силы тяжести в восьмиэтажном здании. Определен диапазон изменения лабораторного суточного смещения нуль-пункта гравиметров. Также оценено смещение нуль-пункта гравиметров во время измерений гравиметрической базы.

Ground-based gravimetry for measuring small spatial-scale mass changes on glaciers

Gravity change on a glacier surface is a composite of several effects (e.g. melting and accumulation of snow and ice, redistribution of mass with depth by refreezing of meltwater and height and thickness changes of the snow and ice layers). Models and equations necessary to estimate the measured gravity change due to different effects are presented, and the propagation of observational errors is evaluated. The paper presents experiences with ground-based gravity measurements carried out on Hardangerjøkulen, Norway, in spring and autumn 2007. It was found that the vertical gradient of gravity contributes most to the uncertainty in the determined mass change. With present instrumentation, gravity can be measured with the required accuracy to determine the mass loss to ∼10% of the loss determined by conventional mass-balance measurements. Improvements in field procedures to achieve the required accuracy for measuring the mass/density changes directly, combining gravity measurements and GNSS (Global Navigation Satellite Systems), are discussed.