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The coseismic surface variations and crustal deformation in the Central Precordillera region of San Juan (Argentina) as a consequence of the Mw 6.5 earthquake of January 19th, 2021 (2:46 UTC) were characterized by a multidisciplinary team, integrating information from Gravity, Seismology, GNSS and DInSAR (Synthetic Aperture Radar Differential Interferometry) techniques. Prior to the occurrence of the event, several periodic measures were taken at fixed reference points throughout San Juan province. The results of the data analysis show significant changes in height corresponding to the uplifting of a few centimeters of the hanging wall block in accordance with the estimated deformation obtained by the focal mechanism. In addition, the transcurrent component is evident in the GNSS vectors calculated after the earthquake. These results are consistent with the Gravity variations measured at the control points. This indicates the uplifting of the basement of Central Precordillera while the elastic Andean compressional deformation migrates N-E related to the Tulum lineament. The joint analysis of 4D gravity with GNSS heights shows an accumulation of deformation involving the lower crust prior to the earthquake, and the release of stresses with permanent deformation, which partially complies with the laws of isostasy in the Precordillera, after the earthquake. • Gravity Velocity Changes as Earthquakes Precursors. • GNSS Analysis to determinate the change on the velocity and positions GNSS stations, before and after seismic event. • DInSAR Analysis to calculate the main surface deformation due to 2021 Tectonic Earthquake. • Analysis the deformation pre and post seismic, through geodetic method and gravity. [ABSTRACT FROM AUTHOR]

We present the annotation to a genetic algorithm (GA) method for an inverse synthetic subsurface density model using surface and borehole time-lapse gravity data. The objective of the inversion is to find the boundaries of the object area and background, where one bit of the chromosome represents the densities. The model that was used in this paper was a simple homogeneous body anomaly and a simplified real water mass injection model in order to argue that the code is suitable for field modeling. We show the influences of the existence of borehole gravity data and location towards the inversion, where the result indicates that an additional good borehole location could increase the success rate up to 13.33% compared to without gravity borehole data for the simple model and up to 4.39% for the field model. The inversion produced the best results when the borehole positions were placed in a state of symmetry towards the body object's mass. [ABSTRACT FROM AUTHOR]

We investigate the feasibility of the hybrid gravity technique applied to two geothermal reservoirs in northern Alsace, France, namely the Soultz-sous-Forêts and Rittershoffen sites. Soultz-sous-Forêts site is the first enhanced geothermal system demonstration site producing electricity in France. Here a geothermal fluid at 165 °C allows to produce around 1.5 MW thanks to one injection well and one production well 5 km deep. Rittershoffen site is dedicated to an industrial use and it is designed to produce 24 MWth heat power with 2 wells around 2.5 km deep. The most recent production episodes of Rittershoffen and Soultz-sous-Forêts geothermal plants have started respectively on May and June 2016. To study underground mass redistribution, time-lapse relative microgravity measurements have been done since 2014 on a network designed ad hoc for Soultz-sous-Forêts site and since 2015 for Rittershoffen site. After tide and drift correction, double differences are calculated to retrieve the gravity variation at each measuring point compared to a reference time and station. Absolute gravity measurements have been also collected at one of the reference stations. Before the beginning of the production in 2016, the stability of the Soultz-sous-Forêts reference station was monitored through the repetition of absolute measurements and continuous gravity records. In 2016, regular ties between the reference stations and the Strasbourg gravity observatory STJ9 have been done. Several superconducting gravimeters operate continuously in STJ9. Thus, we approach the concept of hybrid gravity. Vertical deformation is also controlled thanks to six continuous GPS measurements: the height changes are less than 1 cm. So we consider that our gravity variations are only due to subsurface mass changes. For the Soultz-sous-Forêts network, we notice significant changes in agreement with the position of the injection and the production wells. The maximum gravity change is 31 ± 8 μGal. On the contrary, we do not detect any similar signal for the Rittershoffen network. A simplistic model using two spherical sources located at 5 and 2 km for Soultz-sous-Forêts and Rittershoffen sites respectively shows negligeable surface gravity changes when taking into account the known injection and production flow rates. [ABSTRACT FROM AUTHOR]

Hydraulic conductivity [ABSTRACT FROM AUTHOR]

Groundwater-level measurements in monitoring wells or piezometers are the most common, and often the only, hydrologic measurements made at artificial recharge facilities. Measurements of gravity change over time provide an additional source of information about changes in groundwater storage, infiltration, and for model calibration. We demonstrate that for an artificial recharge facility with a deep groundwater table, gravity data are more sensitive to movement of water through the unsaturated zone than are groundwater levels. Groundwater levels have a delayed response to infiltration, change in a similar manner at many potential monitoring locations, and are heavily influenced by high-frequency noise induced by pumping; in contrast, gravity changes start immediately at the onset of infiltration and are sensitive to water in the unsaturated zone. Continuous gravity data can determine infiltration rate, and the estimate is only minimally affected by uncertainty in water-content change. Gravity data are also useful for constraining parameters in a coupled groundwater-unsaturated zone model (Modflow-NWT model with the Unsaturated Zone Flow (UZF) package). [ABSTRACT FROM AUTHOR]

As an advanced application of soft computation in the oil and gas industry, genetic algorithms (GA) can contribute to geophysical inversion problems in order to achieve better results and efficiency in the computational process. Time-lapse gravity responses to pore-fluid density changes can be modeled to provide the density distribution in the subsurface. This paper discusses the progress of work in inverse modeling of time-lapse gravity data using value encoding with alphabet formulation. The alphabet formulation was designed to provide the solution for positive and negative density change with respect to a reference value (0 gr/cc). The inversion was computed using a genetic algorithm as the optimization method. Working with genetic algorithms, time-intensive computational processes are a challenge, so the algorithm was designed in steps through the evaluation of a GA operator performance test. The performances of several combinations of GA operators (selection, crossover, mutation, and replacement) were tested with a synthetic model of a single-layer reservoir. Sharp boundaries of density changes in the reservoir layer were derived from interpretation of the averaged calculation of several model samples. Analysis showed that the combination of stochastic universal sample-multipoint crossover-quenched simulated annealing per generation-no duplicity achieved the most promising results. [ABSTRACT FROM AUTHOR]

Unconsolidated sediments in alpine watersheds can store glacier melt and snowmelt as groundwater, which helps sustain flow in mountain rivers during dry periods. However, the amount and distribution of groundwater storage in rugged alpine terrain is not well understood, hindering our ability to predict the rate and timing of groundwater discharge into alpine streams. We show how non-invasive time-lapse microgravity surveys can be used to gauge the spatial distribution of groundwater storage changes within a large ( ca 1500 × 1000 m) moraine-talus field of the Lake O'Hara alpine watershed of the Canadian Rockies. Additional ground-penetrating radar (GPR) and seismic refraction surveys provide complementary information on subsurface bedrock topography and reveal the location of a major northwest-southeast trending depression that likely controls groundwater flow to an alpine lake contiguous with the moraine-talus field. Repeat relative gravity measurements made on a network of 80 gravity stations over and around the moraine-talus field during the summers of 2009 and 2010 reveal gravity changes of up to 25 µgal. Although the small gravity changes associated with groundwater flowing out of storage areas are noisy, significant changes are evident on the eastern side of the moraine-talus field. © Her Majesty the Queen in Right of Canada 2012. [ABSTRACT FROM AUTHOR]

Summary: Total water storage change in the subsurface is a key component of the global, regional and local water balances. It is partly responsible for temporal variations of the earth’s gravity field in the micro-Gal (1μGal=10−8 ms−2) range. Measurements of temporal gravity variations can thus be used to determine the water storage change in the hydrological system. A numerical method for the calculation of temporal gravity changes from the output of hydrological models is developed. Gravity changes due to incremental prismatic mass storage in the hydrological model cells are determined to give an accurate 3D gravity effect. The method is implemented in MATLAB and can be used jointly with any hydrological simulation tool. The method is composed of three components: the prism formula, the MacMillan formula and the point-mass approximation. With increasing normalized distance between the storage prism and the measurement location the algorithm switches first from the prism equation to the MacMillan formula and finally to the simple point-mass approximation. The method was used to calculate the gravity signal produced by an aquifer pump test. Results are in excellent agreement with the direct numerical integration of the Theis well solution and the semi-analytical results presented in [Damiata, B.N., and Lee, T.-C., 2006. Simulated gravitational response to hydraulic testing of unconfined aquifers. Journal of Hydrology 318, 348–359]. However, the presented method can be used to forward calculate hydrology-induced temporal variations in gravity from any hydrological model, provided earth curvature effects can be neglected. The method allows for the routine assimilation of ground-based gravity data into hydrological models. [Copyright &y& Elsevier]

On August 14, 2015 Cotopaxi Volcano (Ecuador) erupted with several phreatomagmatic explosions after nearly 135 years of quiescence. Unrest began in April 2015 with an increase in the number of daily seismic events and inflation of the flanks of the volcano. Time-lapse gravity measurements started at Cotopaxi volcano in June 2015. Although minor gravity changes were detected prior to eruptive activity, the largest gravity variations at Cotopaxi were measured between October 2015 and March 2016, when other geophysical parameters had reached background levels. Inverse modelling of GPS data suggests a deep intrusion prior to the eruptive activity, while inverse modelling of post-eruptive gravity changes suggests variations in the volcano hydrothermal system. Deformation, seismicity, and gravity changes are consistent with the intrusion of a deep magmatic source between April and August 2015. Part of the magma rose from depth and interacted with the hydrothermal system, causing the phreatomagmatic activity and pushing hydrothermal fluids from a deep aquifer into a shallow perched aquifer. • Gravity changes were detected at Cotopaxi before and after eruptive activity. • Pre-eruptive geophysical changes are consistent with a deep magmatic intrusion. • Post-eruptive gravity changes suggest changes in the volcano's hydrothermal system. • An interaction with a shallow magmatic source likely caused upward fluid migration. [ABSTRACT FROM AUTHOR]