Residual Gravity Research Articles

Implementasi Metode Gravitasi Untuk Pemodelan Konseptual Sistem Panas Bumi Daerah Karaha-Talaga Bodas, Provinsi Jawa Barat

The high usage of non-renewable energy and the uncertainty of heat sources in the Karaha- Talaga Bodas area are the background of this research. The high usage of non-renewable energy over-increased necessity can cause the reserve become depleted. Therefore, a different perspective is needed by switching the use of non-renewable to renewable energy, which is geothermal energy. To optimize the potential of geothermal, simillar research about geothermal’s system component is needed, which is heat source. This piece of information regarding the probability of the heat source being foreseeable from the surface, but the detail is still less-effective since most of the possible heat sources do not correspond to those indicated from the surface information, thereby, the gravity method can validating the probability of the heat source. The research purposely indicates the heat source and foresees the conceptual geothermal system in Karaha-Talaga Bodas area using the gravity method. The surface’s study results shows a probability of heat source as the eruption centers in the Karaha-Talaga Bodas area. The gravity’s study results shows a probability of heat source by the Bouguer anomaly, regional gravity anomaly, residual gravity anomaly, and high-density in the Talaga Bodas subsurface area. Meanwhile, the gravity’s study results shows a probability of heat source in Karaha subsurface by the residual gravity anomaly and high density contrast. Based on the conceptual model of Karaha- Talaga Bodas geothermal system, it can be interpreted that the geothermal system in Karaha and Talaga Bodas has different heat source.

Gravity estimation of groundwater mass balance of sandy aquifers in the land subsidence-hit region of Yunlin County, Taiwan

Land subsidence will decrease the safety factor of bridges and structures. Many highways and railways constructed decades ago are experiencing damage due to continual land subsidence in several alluvial fans of Taiwan. Groundwater over-pumping for industrial and agricultural uses leads to severe (>3 cm/year) land subsidence in the middle to distal Choushui River Alluvial Fan (CRAF), the largest alluvial fan in Taiwan. The Taiwan High Speed Rail passes through CRAF and land subsidence is now a major concern. Replenishing groundwater with artificial recharge lakes is a potential solution to mitigate land subsidence impacts. Using gravimetry, we examined two undetermined regional unconfined aquifers (RUAs) in the land subsidence-hit region that could host potential artificial recharge lakes to replenish groundwater. We established seven absolute gravity sites and measured time-lapsed gravity values in Yunlin in southern CRAF in 2021, including five sites in the subsidence-hit region and over two unconfined aquifers in the proximal fan. A consistent pattern of residual gravity changes associated with water storage changes at all the gravity sites confirms the recharge potential of the two RUAs in the land subsidence-hit region. Here we estimated groundwater storage changes by residual gravity changes around gravity sites without using prior hydrology information such as groundwater levels and storage coefficients. Of all the gravity sites in the land subsidence-hit region, the most significant March-to-September residual gravity change (26.6 μgal) and vertical displacement (−4.2 cm) were observed at Siutan elementary school (STES). The estimated groundwater storage change around STES is significantly large to increase the water balance in Yunlin, despite the site's severe land subsidence in 2021. We used electrical resistivity imaging (ERI) to aid the identification of the RUA near STES and discussed a potential joint gravimetry-ERI study of the RUAs for subsidence-mitigation engineering works such as constructions of recharge lakes.

Bouguer gravity anomalies and the three-dimensional density structure of a thick mudstone area: A case study of southwestern Taiwan

Gravity surveying is one of the best methods to deduce the subsurface structures in areas covered by thick mudstone in southwestern Taiwan where seismic data analysis is limited. We obtained regional and residual gravity anomalies from the Bouguer anomaly map using the wavelength filtering method to image the gravitational effects of subsurface sources at different depths. The distribution of the residual gravity anomalies corresponds well with the locations of faults and anticlines. Through the change of residual gravity anomalies due to sources at different depth, we estimated the inclination and extension of the anomalous bodies at different depths. Then, the three-dimensional density structure was inferred from inversion of the gravity data to reveal the distribution of mud diapirs and changes of depth to the basement. The positive residual gravity anomalies of approximately 1–2 mGal indicate that the material density of the diapirs is 0.1–0.2 g/cm3 higher than the surrounding rock. The origin of positive residual gravity anomalies can be roughly divided into three types corresponding to the possible subsurface structures. The depth of the positive gravity anomaly below the major anticlines is approximately 4–5 km, corresponding to the bottom of the mudstone formation, from which the mud diapirs may originate. The residual gravity anomaly pattern also indicates a connection of the Chegualin and Youchang faults and the nearly vertical Zuochen fault above 10 km depth. Finally, the low gravity in the Pingtung Plain results from the high-density contrasts on both sides of the two major faults and the deepening of the basement in the plain. The high-angle Chauchou fault, one of two major faults, was also identified at shallow depths based on analysis of the gravity data.

UTILIZATION OF SATELLITE GRAVITY ANOMALY DATA FOR TWO-DIMENSIONAL MODELING OF SUBSURFACE STRUCTURE OF SLAMET VOLCANO, CENTRAL JAVA, INDONESIA

Study using the satellite gravity method has been carried out in the Slamet Volcano area, Central Java, Indonesia. The gravimetric satellite has produced gravity anomalies data which have been corrected up to free air correction. This study purposes to determine the subsurface structure of Slamet Volcano by modeling residual gravity anomalies data. Data processing which has been conducted includes bouguer correction, terrain correction, data reduction to a horizontal surface, separation of regional and residual anomalies data, modeling, and interpretation. The residual gravity anomalies data obtained from the processing range from -40.62 – 66.22 mGal. The anomalies data are modeled in two dimensions (2D) to obtain the subsurface structure model of Slamet Volcano. The 2D-subsurface models on the AB and CD trajectories show the presence of magma chamber and several lava rocks which form the body of the volcano. The rocks are interpreted as andesitic lava with a density value of 2.72 g/cm3, andesitic-basaltic lava with a density value of 2.89 g/cm3, basaltic lava with a density value of 2.97 g/cm3, and magma chamber with a density value of 1.32 g/cm3. Such low value of magma chamber density relative to the surrounding rocks, indicates that Slamet Volcano is still active.

Microgravity Monitoring in Fractured-Vuggy Carbonate Reservoirs

With the development of Tahe Oilfield entering the high water cut stage, gas channeling occurs in fractured-vuggy system during nitrogen injection, resulting in some inefficient wells. To improve the development effect of gas flooding, how to define the distribution of fracture, vuggy, and remaining oil has become one of the urgent problems to be solved at present. Microgravity monitoring technology uses high-quality data, the residual gravity anomaly of the target layer is obtained by depth recursion processing, the density distribution of the target layer is obtained by layer density inversion, and the fractured-vuggy distribution is depicted by edge detection. The results show that the lower part of the fractured-vuggy system in the north is connected to the middle, while the fractured-vuggy system in the south is directly connected to the middle, which leads to different effects of injected nitrogen. The early injected nitrogen in the SX area is mainly distributed in the north. Nitrogen injection in the north needs to reach a certain amount of gas before the middle can be effective. Nitrogen is injected in the south, and the central part is effective quickly. The research results provide a basis for adjusting injection-production scheme and improving reservoir development effect. Compared with the production performance and seismic interpretation results, it verifies the accuracy of ultradeep microgravity monitoring in depicting the development of fractured-vuggy system, which provides a new technology and idea for characterizing fractured-vuggy carbonate reservoirs.

Gravity and magneto-telluric study of Manuguru geothermal field, Telangana state, India

Manuguru geothermal field is located on the southeastern margin of the Godavari River valley. A sequence of rocks of the Gondwana group unconformably lies over a quartzite-phyllite basement of the Pakhal Supergroup in the present study area. A geophysical investigation deploying gravity and magneto-telluric (MT) survey was carried out covering an area of 200 km2 to characterize the Manuguru geothermal system and locate the probable reservoir source. Bouguer gravity anomaly shows a noticeable variation of 25 mGal ranging from -72 mGal to -47 mGal. Residual gravity and derivative analysis exhibit that there are several basement faults controlling the deep subsurface hot water flow in the area. In the MT study, a total of 27 MT soundings data were acquired at varied frequency ranges (20 kHz to DC). The MT data were analyzed for 1D as well as 2D modeling using the inversion technique to configure the subsurface resistivity distribution along three selected profiles (P1 to P3). Two exploratory drillings conducted based on our studies validate the inferred low resistivity anomalies. To configure basement and fault systems, a 2D forward gravity inversion modeling was performed along a selected profile which showed that the depth of the Archaean basement underlain by the Gondwana and Pakhal sediments ranges from 1.2–2.2 km. The results inferred a 2D resistivity structure with varying resistivity from <2 Ohm-m to >4000 Ohm-m. The study identified a distinguished low resistivity zone (<4 Ohm-m) at a depth ≥2.4 km indicating deep hot water geothermal storage in the area.

Fast imaging for the 3D density structures by machine learning approach

Residual Bouguer gravity anomaly inversion can be used to imaging for local density structures or to interpret near-surface anomalous mass distribution. The reasonable prior information is the crucial recipe for obtaining a realistic geological inversion result, especially for the ill-posed geophysical inversion problem. The conventional strategies introduce the prior constraints or joint multidisciplinary information in object function as regularization, and then use some optimization algorithm to minimize the object function. This process is called model-driven approach and is usually time-consuming. In recent years, the rapid development of machine learning technology has provided new solutions for solving geophysical inversion problems. Machine learning methods can reduce the dependence on prior information in the inversion process through setting special training datasets, and the time consumption of an inversion process executed by the trained model can be shortened by several orders of magnitude, which is conducive to fast inversion for the same type of application scenarios. In this study, we were inspired by the U-net model and develops the GV-Net (Gravity voxels inversion network) model using the convolutional neural network for the inversion of residual gravity anomalies. We first discussed the effects of different loss functions on the convergence speed of model training and prediction accuracy. Then, we analyzed the robustness of our model by changing noise levels of the datasets. At last, we employed this model in a real scenario. The results have demonstrated that the GV-Net model has the ability to deal with specific inverse problems by predefined training datasets.

Implications on Gravity Anomaly Measurements Associated with Different Lithologies in Turkana South Subcounty

The use of gravity data has demonstrated capability for monitoring lithological changes on a large scale as a consequence of differentiating basement and sedimentary of buried valleys. Gravity anomalies are associated with lateral contrasts in density and therefore deformation by faulting or folding will be manifested if accompanied by lateral density changes, otherwise, the vice versa is true. The study’s objective is to evaluate the effectiveness of gravity method in establishing different lithologies in an area. The study has revealed that regional anomaly gravity map presents high anomalies in the Northern region in the NW-SE trend and low anomalies in the southern trend in NW-SE, while the residual anomaly gravity map shows different trends for the low and high gravity anomalies. The gravity anomalies are well interpreted in line with the lithologies of the study area rather than the deformation of the same lithologies. There are observed high values of gravity anomaly values (ranging from -880.2 to -501.2 g.u.) where there are eolian unconsolidated rocks overlying the basement compared to low gravity anomaly values (ranging from -1338.9 to -1088.7 g.u.) where the andesites, trachytes and phonolites overly the basement. The different regional gravity anomalies relate well with different rock densities in the study area along the line profile for radially averaged power spectrum. The gravity highs are noted in the eastern point and are associated with andesites, trachytes, basalts and igneous rocks, while the gravity lows are associated with sandstone, greywacke, arkose, and eolian unconsolidated rock. The utilization of the information from the Power spectrum analysis demonstrates that the depth to the deepest basement rock is 12.8 km which is in the eastern flank, while the shallowest to the basement of 1.1 km to the western flank.

Determination of a new gravimetric geoid modelling for Sudan using the least-squares collocation technique

The main purpose of this study is to compute a new gravimetric geoid model for Sudan by using the least-square collocation technique (LSC method) and applying the remove-compute-restore (RCR) technique. The computation of the model contains different datasets which are the gravity contribution of the model GO_CONS_GCF_2_TIM_R6e degree/order 300, BGI free-air gravity dataset in Sudan, GPS/levelling data, and high-resolution topographic information from ASTER digital elevation model. The “residual gravity anomalies” were run through the GEOCOL program using the GRAVSOFT software package, and the effects were restored to calculate the quasi-geoid surface (height anomalies). The gravimetric geoid was computed by adding the (N − \U0001d701 h ) separation term to the quasi-geoid and was fitted to the GPS and levelling data provided by Sudan. The accuracy of our gravimetric geoid model SDN-LSC-G22 of the area of Sudan and some areas of bordering countries has been investigated by using geoid undulations computed from GPS and levelling data and by investigating the differences between the geoids of the GGM models which are EGM2008 and SGG-UGM-2. Our gravimetric geoid model (SDN-LSC-G22) has indicated an accuracy of 17.4 cm, in terms of a standard deviation compared with 66 GPS and Leveling data distributed in the area of Khartoum (most of these points are control points and benchmarks). Also, we evaluated our gravimetric geoid model by using 19 points distributed in the area of Sudan, and they indicated a standard deviation of 51.3 cm. The overall accuracy of SDN-LSC-G22 compared with the geoid undulation of all GPS and levelling has indicated an STD of 34.1 cm. The model SDN-LSC-G22 has shown better accuracy and significant differences compared with the GGM models EGM2008 and SGG-UGM-2 in terms of the differences with the available GPS and levelling data which have shown ~17 cm differences using (Abdalla, 2009) GPS and levelling data. It has demonstrated STD of ~17 cm differences by using 66 GPS and levelling data. Therefore, the model SDN-LSC-G22 provided better improvements and reliable geoid heights over Sudan compared to EGM2008 and SGG-UGM-2 gravity field models.

Utilization of Gravimetric Satellite Data for Delineating of Subsurface Model of The Purwokerto-Purbalingga Groundwater Basin

The utilization of gravimetric satellite data has been carried out to delineate the subsurface model of the Purwokerto-Purbalingga Groundwater Basin. Access and processing of satellite gravity anomalies data were carried out to obtain the residual gravity anomalies data. Modeling of the residual gravity anomalies data was conducted along the AA′, BB′, and CC′ trajectories. The modeling results show a basin model filled by alluvial deposits (1.75 g/cm3 and 2.28 g/cm3) with a maximum depth of about 402 m for the AA′ trajectory, 543 m for the BB′ trajectory, and 463 m for the CC′ trajectory. The modeling results show that this alluvial basin is delimited by impermeable and semi-impermeable layers, which include laharic deposits of Slamet Volcano (2.61 g/cm3), andesite lava deposits (2.90 g/cm3), Tapak formation rocks (2.50 g/cm3), breccia rocks of Tapak formation (2.70 g/cm3), and breccia rocks of Halang formation (2.80 g/cm3). The fairly large thickness of alluvial deposits supported by dug-well water tables data and resistivity data indicates that the potential of groundwater in the Purwokerto-Purbalingga Groundwater Basin area is very large. The results of the study are expected to be a solution to overcome droughts that often occur in the Banyumas and Purbalingga regencies, as well as for the development of groundwater-based irrigation.

Gravity yearly transient change around the epicenter of 2022 MS 6.9 Menyuan earthquake, China and seismotectonic implications

The MS 6.9 Menyuan earthquake occurred in the northeastern Tibetan Plateau on 8 January 2022. High-precision absolute gravity observations provide a unique perspective on seismotectonic feature of strong continental earthquakes. An obvious and coincident gravity increase about 5 years before the Menyuan earthquake with the rate up to 3.94 μGal/yr is observed at four absolute gravity stations within 350 km away from the epicenter. These four stations have been carried out repeat gravity observations by the Crustal Movement Observation Network of China (CMONOC) for more than a decade. The observed gravity changes could not be reasonably explained by the land water storage change nor crustal vertical deformation and thus might be related to the processes in the crust. A disc-shaped equivalent source region with a radius of 175 km is then modeled, based on the residual gravity rates. We find that the seismicity increases gradually in the modeled source region from about 2017 to 2021, similar to the pattern of absolute gravity change. Furthermore, the spatiotemporal distribution of epicenters is consistent with the fluids diffusion model, indicating that this could be related to the redistribution of deep fluid mass. With consideration of the near-surface processes, regional crustal structure, and seismicity, multiple lines of evidence consistently refer to the presence of deep fluids migration before the Menyuan earthquake. Our results suggest that high-precision absolute gravimetry can be used to monitor crustal fluid environment and determine mass changes in the source regions of potential large earthquakes.

Geophysical assessment for deep structuring mapping and water resources perspectivity in semi-arid land region (northeastern Tunisia, Mediterranean Basin)

In Mediterranean semi-arid regions, shallow aquifers are generally over-exploited. However, several deeper groundwater boreholes exhibit serious problems of low hydraulic yield and dry wells. The existence of reliable information about the regional geological structure can enhance the success of deep well programmes. Geophysics has proved very useful as a fast and efficient tool to understand the structuring of deep hydrogeology aquifers. Indeed, the identification and characterization of the deep structures of the Bir Drassen–Sidi Toumi region (Cap-Bon, northeastern Tunisia) contribute to a better quantification of the groundwater potential, allowing more sustainable management. This province can be considered as the SW extension of the NE–SW Jebel Abderrahman Atlasic anticline, one of the most prominent geological structures in northeastern Tunisia. To identify the Miocene and Oligocene reservoirs, a geophysical study was carried out using advanced gravity analysis, supported by boreholes, and seismic and geoelectrical calibration. Different gravity filters were applied to reveal the sub-basin architecture and faulting network in the region. By calculating the maxima of the total horizontal gravity derivative and the distribution solutions for Euler's equations, a set of deep faults of different directions was divulged. Time-domain electromagnetic mapping illustrated the influence of these fault zones on the regional arrangement of sub-basin aquifers and their recharge processes. Finally, a 3D residual gravity inversion was carried out to better image the tectonically controlled depocentres associated with prolific thick fluvial–deltaic reservoirs. These ‘deep aquifer valley’ can be suggested as strategic targets for deep hydrogeological exploration planning.

Mapping the structural configuration of the western Gulf of Guinea using advanced gravity interpretation methods

The western Gulf of Guinea has important implications for seismic-hazard estimates of the adjoining sub-Sahara West Africa region because of the presence of four active transform faults, namely the St. Paul, Romanche, Chain and Charcot fault zones. A satellite gravity data analysis was performed over the western Gulf of Guinea to delineate the structural configuration of the area. Initially, the energy spectrum analysis was used to determine regional and residual gravity effects. Further, the advanced techniques such as the tilt angle of the gradient amplitude (TAHG), the improved horizontal tilt angle (ITDX), the enhanced gradient amplitude (EHGA), and the fast sigmoid-based edge detection filter (FSED) were applied to the residual gravimetric anomaly to extract lineaments of the western Gulf of Guinea. The major trend of the detected lineaments has an ENE-WSW direction and depths of the gravity sources range from 3.4 to 13 km corresponding to widespread altered basalt, breccia, serpentinized peridotite and minor gabbro. Our findings closely agree with known information but are also able to confirm the presence of various other structures. Our results thus provide a structural map that helps us to have a better understanding of the tectonic and structural framework of the western Gulf of Guinea.

Multiphysics Modeling of Volcanic Unrest at Mt. Ruapehu (New Zealand)

AbstractPre‐eruptive signals at the crater lake‐bearing Mt. Ruapehu (New Zealand) are either absent or hard to identify. Here, we report on geophysical anomalies arising from hydrothermal unrest (HTU) and magmatic unrest (MU) using multiphysics numerical modeling. Distinct spatio‐temporal anomalies are revealed when jointly solving for ground displacements and changes in gravitational and electrical potential fields for a set of subsurface disturbances including magma recharge and anomalous hydrothermal flow. Protracted hydrothermal injections induce measurable surface displacements (&gt;0.5 cm) at Ruapehu's summit plateau, while magmatic pressurization (5–20 MPa) results in ground displacements below detection limits. Source density changes of 10 kg/m3 (MU simulations) and CO2 fluxes between 2,150 and 3,600 t/d (HTU simulations) induce resolvable residual gravity changes between +8 and −8 μGal at the plateau. Absolute self‐potential (SP) anomalies are predicted to vary between 0.3 and 2.5 mV for all unrest simulations and exceed the detection limit of conventional electric surveying. Parameter space exploration indicates that variations of up to 400% in the Biot‐Willis coefficient produce negligible differences in surface displacement in MU simulations, but strongly impact surface displacement in HTU simulations. Our interpretation of the findings is that monitoring of changes in SP and gravity should permit insights into MU at Ruapehu, while HTU is best characterized using ground displacements, residual gravity changes and SP anomalies. Our findings are useful to inform multiparameter monitoring strategies at Ruapehu and other volcanoes hosting crater lakes.

A long-term charge/discharge cycle at Mt. Etna volcano revealed through absolute gravity and GPS measurements

We present results of repeated absolute gravity and GPS measurements, carried out at Mt. Etna volcano between 2009 and 2018. Absolute gravity measurements are rarely performed along arrays of stations on active volcanoes and, through our unprecedented dataset, we highlight the possibilities of this method to track underground mass changes over long time-scales. Analysis of the residual absolute gravity data and ground deformation reveals a cycle of gravity increase and uplift during 2009 to 2011, followed by gravity decrease and subsidence during 2011 to 2014. Data inversion points to a common mass and pressure source, lying beneath the summit area of the volcano, at depth of ~ 5 km b.s.l. The bulk volume change inferred by the inversion of the deformation data can account for only a small portion of the mass change needed to explain the correspondent gravity variations. We propose that the observed relationship between gravity and vertical deformation was mostly due to the compressibility of the magma in the inferred reservoir, which, in turn, was enhanced by the presence of exsolved gas. Overall, the gravity and deformation data we present reveal a cycle of magma recharge (2009–2011) and discharge (2011–2014) to/from the inferred storage zone. During the recharge phase only degassing occurred from the summit craters of Mt. Etna. During the following phase of discharge, the magma lost from the reservoir at ~ 5 km b.s.l. fed the exceptional phase of volcanic activity during 2011–2014, when tens of lava fountaining episodes took place.

Depicting the Underground River Systems in Karst Mountains of Buayan and Ayah Subdistricts Using GGMPlus Data and Springs Distribution

The karst mountain area in Buayan and Ayah subdistricts is a mountainous area that stretches from North to South and ends in the South coast region. In this area, many springs are found which are a source of clean water for residents. The existence of springs in the area is an indicator of the existence of underground rivers flowing under the mountainous area, this is also reinforced by the presence of interconnected caves and the existence of river flows in these caves. Analysis the presence of underground river flows in the Karst mountain area in the study area can be identified by looking at the type of rock formations that dominate the area. The density contrast shown through the residual gravity data also shows changes in rock formations related to the presence of water sources in the study area. GGMPlus data was obtained from forward modeling in order to get a residual map. This study shows that rock formations in the Karst area in the study area conform to the topographical shape.

An improved altimeter-derived gravity anomaly from shipborne gravity based on the mean sea surface height constraint factors method

Coastal marine gravity modeling faces challenges due to the degradation of the quality and poor coverage of altimeter data in coastal regions. The effective fusion of shipborne gravity data and altimeter-derived marine gravity data can make shipborne gravity data more useful for the accurate estimation of altimeter-derived coastal marine gravity. A mean sea surface height constraint factor (MSSHCF) method based on the ordinary kriging method and the remove-restore technique is proposed to fuse altimeter-derived gravity model with shipborne gravity data. In this method, all data are standardized during the interpolation process to reduce the error and mean sea surface as a vertical variable is added to the semi-variance function in ordinary kriging to obtain the residual shipborne gravity as corrected data source. The coastal marine gravity models V2.1 and V3.1 which fused altimeter-derived gravity data with shipborne gravity data and V1.1 without shipborne gravity data at a spatial resolution of 1′×1′ can be obtained. Validation experiments show that the accuracy of the gravity model V3.1 obtained by the MSSHCF method more closely agrees with the validated gravity model DTU17 and SS V31 than the model V2.1 obtained by the ordinary kriging interpolation method and the V1.1 model. Our results were validated against shipborne gravity data; the accuracy of model V3.1 was 4.95 % higher than the model V1.1 in South China Sea area A and 2.48 % higher in South China Sea area B. Meanwhile, the accuracy of model V3.1 was 2.07 % higher than model V2.1 in South China Sea area A and 2.42 % higher in South China Sea area B. The effects of distance from the coast and sea depth on the marine gravity model were also evaluated. The results show that the gravity model V3.1 has higher accuracy with the change in ocean distance and depth than the V2.1 and V1.1 gravity models. Thus, our study shows that the MSSHCF method effectively refines coastal altimeter-derived gravity using shipborne gravity data.

Delineation of Aquifer Boundary by Two Vertical Superconducting Gravimeters in a Karst Hydrosystem, France

Mass distribution on Earth is continuously changing due to various physical processes beneath the Earth's surface or on the surface. Some of the primary sources for these mass displacements are tidal forces, atmospheric and oceanic loading, and seasonal changes in continental water distribution. The development of relative cryogenic gravimeters, the Superconducting Gravimeters (SGs), has made it possible to characterize and monitor such mass variations at orders of magnitudes as small as a few nm/s2 (1 nm/s2–10–10 g where g is the mean gravity at the Earth’s surface). Our study focuses on the hydrodynamics of the 900 m thick unsaturated zone of the low-noise underground research laboratory (Laboratoire Souterrain à Bas Bruit, LSBB) located in Rustrel (France) using a unique configuration of two SGs vertically arranged 520 m depth apart. The installation of an SG (iGrav31) at the site surface several years after installing the first (iOSG24) inside a tunnel has provided several new insights into the understanding of the hydrological processes occurring in the LSBB. By comparing differential and residual gravity time-series together with global hydrological loading models, we find that most water-storage changes occur in the unsaturated zone between both SGs. The misfit between the observed gravity time-series and the gravity effect corresponding to local hydrological contribution calculated from global hydrological models can be explained by large lateral fluxes and rapid runoff occurring in the LSBB site. Finally, we implement a rectangular prism method to compute forward gravity responses to water storage changes for a homogeneous water-layer following the site topography using a 5-m digital elevation model. In particular, we analyse the sensitivity of the differential record from both SGs to the extent and depth of the water storage changes by computing the corresponding 2D admittances. This gravity difference is sensitive to an extension up to about 2500 m laterally before tending towards an asymptotic value corresponding to the Bouguer plate approximation. We show that the zone of water-storage changes that best fits observed differential gravity signal is located at depths larger than 500 m (below iOSG24). This fitting is improving when the integration radius increases with depth. This is the first time that hydrological processes are investigated when the baseline configuration of two SGs is vertical.

Mesozoic magmatic arc in East Asian continental margin triggered by Paleo-Pacific plate subduction: Constraints from gravity and magnetic anomalies

The Mesozoic East Asian Continental Margin (EACM) is generally considered as a continental magmatic arc triggered by Paleo-Pacific Plate subduction. However, the magmatic arc was not evident for a long time. This paper tries to use the gravity and magnetic responses along the magmatic arc to deeply explore its spatio-temporal distribution. Based on the gravity and magnetic anomalies of the Mesozoic magmatic arc on the EACM and combined with the integration of previous geological and geochronological studies, it is found that the tectonic features of the magmatic arc are more obvious in the fourth-order approximation/detail gravity anomaly fields by using wavelet multi-scale analysis. Furthermore, the residual crustal gravity anomalies containing the medium-wavelength components of the complete Bouguer gravity anomalies are analyzed for the identification of the Jurassic magmatic arcs. Finally, line-drawing analysis is performed on the fourth-order detail gravity anomaly field to explore the response of the Mesozoic magmatic arcs to Paleo-Pacific subduction on the EACM. Our results support that the magmatic arcs on the EACM mainly developed in the Late Triassic-Early Cretaceous Andean Active Continental Margin and in the late Early Cretaceous-Eocene transtensional continental margin. Our study contributes to the formation and refinement of the Paleo-Pacific Plate subduction mechanism and the Mesozoic tectonic evolutionary model of the EACM, and also provides geophysical evidence for the reconstruction of dynamic processes of the EACM.

Improving the Force Display of Haptic Device Based on Gravity Compensation for Surgical Robotics

Haptic devices are applied as masters to provide force displays for telemedicinal robots. Gravity compensation has been proven to be crucial for the accuracy and capability of force displays, which are critical for haptic devices to assist operators. Therefore, the existing method suffers from an unsatisfactory effect, a complex implementation, and low efficiency. In this paper, an approach combining active and passive gravity compensation is proposed to improve the performance of a force display. The passive compensation is conducted by counterweights fixed with the moving platform and pantographs to offset most of the gravity and reduce the loads of the motors, while the peak capability of the force display is enhanced. The required weight is optimized by a multi-objective genetic algorithm in terms of the maximum torque of the motors in the global workspace. As a supplement, the residual gravity is eliminated by active compensation to extend the accuracy of the force display. The balancing forces in the discretized workspace are entirely calibrated, and the required force for the arbitrary configuration is calculated by interpolations. The decisions regarding the algorithm parameters are also discussed to achieve a compromise between the effect and elapsed time. Finally, the prototype with a compensation mechanism is implemented and experiments are carried out to verify the performance of the proposed method. The results show that the peak capability of the force display is enhanced by 45.43% and the maximum deviation is lowered to 0.6 N.