Residual Gravity Research Articles

Origin of coseismic anelastic deformation during the 2016 Mw 6.4 Meinong Earthquake, Taiwan

An unexpectedly large uplift of 91 mm was detected by geodetic observations during the rupture of 2016 Mw 6.4 Meinong earthquake, Taiwan, which has been attributed as either a triggered anelastic and hydrologic related deformation from a proposed duplex/mud diapir or a triggered aseismic slip on a proposed backthrust. Here, using both high-rate GNSS and free-field strong motion data, we first estimated the coseismic source model. The locations of high PGV were inferred to explain the unexpected distribution of damaged buildings approximately 25–30 km west of the epicenter. Then, the aseismic surface displacements during the earthquake was differentiated using the coseismic source model. The aseismic extension of 3.8 μstrain is inferred at the unexpectedly large uplift region. Combining with local geology, residual gravity anomaly, seismic tomography, interseismic leveling vertical velocities, coseismic leveling uplifts and proposed Coulomb stress changes, the accelerated mud diapirism during the earthquake triggered by slip on the deep seismogenic fault was identified as the cause of unexpectedly large coseismic uplift.

Gravity Prediction Method and the Application of Full-Space Tunnels in the Deep Extension of High-Density Vertical Orebodies

A full-space tunnel gravity prediction method for evaluating the deep extension of high-density vertical orebodies has been developed based on the significant density differences between surrounding rocks and their characteristics as large-scale orebodies. This method uses the residual gravity anomaly parameters in the tunnel to determine whether the orebody is upright based on the symmetry of the anomaly. Moreover, it identifies the deep extensional characteristics of high-density upright orebodies based on the positive and negative anomalies. This method has characteristics such as strong anti-interference ability, slight influence from the terrain, and high detection accuracy. It has produced promising outcomes in the detection of deep, concealed orebodies in the Huangshaping skarn-type tin–tungsten polymetallic deposit located in southern Hunan. This method not only resolves the problem of rapid identification of the deep extension of single-hole high-density orebodies and provides reference basis for deep exploration but also proposes a new method for mineral resource evaluation.

Gravity study of the Western Bahira Basin and the Gantour Phosphatic Plateau, central Morocco: Interpretation and hydrogeological implications

The Western Bahira Basin (WBB), located in Central Morocco, is known for its large reserves of phosphate, which are currently being extracted through open pit mines within the Gantour Phosphatic Plateau (GPP). This mining activity, as well as the increasing agricultural development that this region has been experiencing during the last few decades, has subjected the groundwater in this semi-arid region to intense exploitation through pumping. Consequently, thorough knowledge of the underground water resources is essential for sustained activity.This study uses gravity data to investigate the deep geologic structure of the western Bahira and Gantour plateau area. Its main purpose is to improve the knowledge of this structure that influences the groundwater dynamics.The methodology involves, in addition to the qualitative and quantitative interpretation of the gravity data, the use of various filtering and analysis techniques to highlight the major geological structures. In fact, a qualitative analysis of the residual gravity map shows that the observed anomalies can be explained in terms of the bedrock topography of the study area. Positive anomalies spatially correlate with the outcrops of this bedrock in the Hercynian massifs of Jebilet and Rehamna, as well as with the Douar Rhirat structural high. The negative anomalies, on the other hand, tend to reflect increased thicknesses of sedimentary sequences, such as in the southern part of the WBB. Moreover, the Brikyine granite in the north of the study area is noticeably associated with the highest amplitude negative anomaly.Analysis of the residual gravity data using the total horizontal gradient, upward continuation, and Euler deconvolution techniques provides a quantitative assessment of the geologic structure. These methods help delineate various structures oriented in two main directions: N45E and N105E. Among these structures, several major faults have been identified. They are rooted at a maximum depth in excess of 2800 m. These faults are responsible for the general structure of the study area and the morphology of the basement.Furthermore, the 2D forward modeling, performed across the main negative gravity anomalies of the WBB, helps characterize the Paleozoic bedrock topography and provides more constraints on its variation. The resultant gravity models clearly show an asymmetric structure for the western Bahira trough. This depression is filled by sedimentary formations whose thickness at the depot-center can locally reach 1200 m. Additional analysis of piezometric data shows that the groundwater flows are controlled by the gravity structures highlighted in this study.

Crustal configuration of the Naama and El Bayadh region of northwest Algeria: Inferences from gravity and magnetic analysis

Northwestern Algeria is located where the Saharan Atlas borders the Saharan Platform along the South Atlas Front. The origin of the South Atlas Front is controversial, thus a detailed gravity and magnetic analysis constrained by seismic reflection profiles and well data were conducted to determine the structural configuration of this region. A residual gravity anomaly map created using upward continuation and a reduced to the pole magnetic anomaly map indicated a series of northwest-trending maxima anomalies parallel to the Atlassic orogeny folds and faults and east-trending maxima within the Benoud foreland basin. These maxima mostly coincide with Paleozoic basement uplifts based on seismic reflection profiles. Depth analyses based on upward continuation, and two-dimensional forward modeling of the gravity and magnetic data indicated that the source of the maxima are mainly 5 km in depth with the magnetic sources being approximately 0.5 km deeper than the gravity sources. The gravity and magnetic models indicate that the source bodies are steep-sided and coincide with interpreted faults from the seismic reflection profiles. The maxima anomalies are mainly caused by basement uplifts instead of variations in density and/or magnetic susceptibility in the Paleozoic or older basement lithologies. The South Atlas Front and the Saharan Atlas on the central and eastern portions of Algeria is governed by a thin-skinned tectonics style involving the Mesozoic-Cenozoic sedimentary cover. While the western part was probably controlled by a mixed thin- and thick-skinned tectonic style with the reactivation of deeply rooted Triassic and Jurassic faults during Tertiary compressive events in the interpreted Paleozoic basement.Our gravity and magnetic models illustrate a crustal architecture model of the Paleozoic basement which is consistent with a north-dipping basement normal fault between less deformed Saharan Platform than the more deformed Atlassic domain. Such a crustal model aids in determining the nature of the Atlassic orogeny as geological and geophysical studies have been determining the variations of structural styles along its entire length.

Support vector machine-based integration of AVIRIS NG hyperspectral and ground geophysical data for identifying potential zones for chromite exploration – A study in Tamil Nadu, India

In the present study, we have integrated the spectral anomaly map derived from airborne visible-infrared imaging spectrometer-next generation (AVIRIS-NG) data and residual ground geophysical anomaly maps (gravity and magnetic) for identifying the mafic cumulates and chromitites in the Sittampundi Layered-Complex (SLC), Tamil Nadu, India. A spectral map of chromite bearing mafic cumulates was prepared by implementing the matched filtering (MF) algorithm on the selected spectral bands of AVIRIS-NG data using the mixed image spectra of mafic cumulate and chromitite as an end member. A low pass filter was applied in the spectral map to extract the clusters of coherently distributed spectrally anomalous pixels representing the surface exposure of mafic cumulates. Concurrently, the ground gravity and magnetic data were processed for deriving the residual gravity and magnetic anomaly maps. Subsequently, an integration of spectral map, residual gravity, and magnetic anomaly maps was attempted using the support vector machine (SVM) algorithm to identify the potential sites of chromitite occurrence. Results were verified using the geological map and the field data on surface exposures of mafic cumulates. This research demonstrates that integration of remote sensing data and geophysical anomaly maps has immense potential to detect the surface distribution of chromite bearing mafic cumulates in the layered complex and the same methods can be replicated for carrying out mineral prospecting in the other parts of the world.

Interpretation of gravity\u2013magnetic anomalies to delineate subsurface configuration beneath east geothermal province along the Mahanadi rift basin: a case study of non-volcanic hot springs

Gravity and magnetic studies have been carried out over a non-volcanic hot spring zone consisting of Atri and Tarabalo hot springs along the intracontinental Mahanadi rift basin to delineate the subsurface structures and to understand their effect on the geothermal activities over a stable continental region. Calculated gravity and magnetic anomaly maps unveil the presence of hot springs along Mahanadi fault. The four-layer subsurface configuration as observed using radially averaged power spectrum analysis and 3D Euler solutions of both gravity and magnetic data indicates occurrence of multi-phases sedimentation and tectonic events. 2D forward, 2D inverse, and 3D inverse residual gravity models have delineated high-density igneous intrusive bodies surrounded by comparatively less dense Khondalites and Charnockites rich altered zones. The sharp high to low density transition zones are identified as the regional Mahanadi fault. The India–Antarctica rifting, existence of two hot springs along the Mahanadi fault of the rifted basin, and similarity in water chemistry strongly indicates an interconnection between these two hot springs. Igneous intrusions and radiogenic element-rich metamorphosed shallow formations combinedly acting as the heat source. Deciphered altered zone, deeper intrusion and deeply connected regional fault along the hot springs confirmed that this regional fault is providing the major pathway for water circulation through radioactive element-rich altered zones while the local and shallow fractures connecting the Mahanadi fault feed the hot springs of the study area.

The integrated history of repeated caldera formation and infill at the Okataina Volcanic Centre: Insights from 3D gravity and magnetic models

Multistage collapse caldera create complex geological structures that are often buried by kilometers of pyroclastic infill and late stage lava domes, making study of their origins difficult from outcrop alone. Here we present new gravity and aeromagnetic data compilations derived from terrestrial, lake and airborne surveys to investigate the buried internal structure of the Okataina Volcanic Centre (OVC) and interpret its stages of development. Magnetic highs (1300 nT) are caused by a combination of thick lava flows and domes that infill the collapse structures, and a deeper feeder structure/dyke complex that extends below the basement to at least 6 km depth. Magnetic lows caused by hydrothermal alteration are associated with topographically low areas near the topographic and structural collapse margins suggesting fluid circulation within the caldera exploits both deep and shallow structures. Several large rhyolite lava flows contain low magnetisation rims reflecting the glassy outer carapace and crystalline core.The gravity data show a − 62 mGal residual gravity low associated with the OVC, with the steepest gradients occurring inside the topographic margins. The gradient of the gravity low is stepped towards its lowest point near the outlet of Lake Tarawera. Each step is interpreted as relating to the buried structural collapse margins of the Utu, Matahina, Rotoiti and possibly Kawerau, catastrophic caldera forming eruptions, creating an overlapping and nested caldera structure, however there is no evidence for an OVC encircling ring structure. Buried caldera margins associated with oldest Utu eruption may play a role in the location of the youngest Tarawera eruptions. We propose smaller amplitude gravity lows that extend outside the topographic margins of the caldera are related to lateral magma migration to the south-west, exploiting regional tectonic stress regimes, towards eruption vents within the caldera. 3D gravity inversion, including models numerically constrained by a 3D magnetotelluric model, suggest a caldera depth of 5000 ± 500 m that accumulated over multiple collapse episodes along with rifting induced subsidence in the past 550 ka.Our multiphysics approach to investigating caldera structure has revealed buried structural margins and their relationships to eruptive vents and hydrothermal features at the OVC. Detailed geophysical models of caldera structure are rare globally, and our findings are consistent with analogue and numerical models of caldera formation that accompanies the most violent volcanic eruptions on Earth.

Delineation of structural and tectonic features in the Mahanadi basin, eastern India: New insights from remote sensing and land gravity data

• An integrated analysis (ALOS PALSAR , SRTM-DEM, and Ground Gravity data) is performed to delineate the structural and tectonic framework at the Mahanadi graben , India. • The 2-D gravity model incorporates the rifting model of the basin formation along the preexisting basement shear zones. • The structural trends with pronounced geomorphic expressions represent the reactivated structures formed under the evolving stress regimes of the Mahanadi graben. During Gondwana dispersal, the Mahanadi basin experienced rifting and reactivation of preexisting structures, causing terrain deformation with faulting. The landform around Proterozoic Sukinda Thrust in the Mahanadi basin provides an opportunity to map the geomorphic expressions and correlate them with the tectonics of the graben using remote sensing images, digital elevation model, and gravity data. The subsurface extensions of the identified structures using SAR data and relief image analysis are modeled with the Bouguer gravity anomaly. The mapped tectonic features correlate well with the gravity gradient trends in residual gravity anomalies. The E-W and NW-SE trending residual gravity lineaments match well with the mapped faults and suggest their extension within the shallow crust. The two-dimensional (2-D) gravity model realistically constrains the subsurface geology, and by incorporating other published data, we suggest that the Gondwana rifting had reactivated the preexisting shear zones in the cratonic basement down to a depth of ∼ 5 km and subsequently deposited sediments into the fault-bound shallow basin of the Mahanadi graben.

Structure of the upper crust at the axis segmentation stage of rift evolution as revealed by gravity data: Case study of the Gedemsa magmatic segment, Main Ethiopian Rift

The upper crustal structure of the individual tectono-magmatic segments within the Main Ethiopian Rift is poorly understood. The Gedemsa tectono-magmatic segment (GTMS) is considered to be within an intermediate stage of rift development with magma-assisted rifting contributing to overall extension. The interpretation of gravity data from Global Gravity Model plus2013 has provided an insight on the nature and structure of the shallow upper crust beneath the GTMS within the Central Main Ethiopian Rift. A detailed investigation of the structure of the shallow upper crust based on 2D gravity models, 3D geologic interpretation maps, and residual and upward continued gravity anomaly maps indicate the occurrence of dense, mafic intrusions at depths between 10-20 km and 5–10 km beneath the Tullu Moye-Gedemsa and Boku volcanic centers, respectively. The gravity data indicate at deeper depths, the GTMS (25 km wide and 60 km long) is elongated parallel to the Main Ethiopian Rift axis. Our 2D gravity modeling indicates that the Quaternary faults within the Wonji Fault Belt are closely associated with the dense magmatic intrusions as the faults aid in transporting melt from the deeper crustal melt areas to the shallow magma chambers.

Crustal thickness and structural pattern evaluation of Sinai Peninsula using three-dimensional density modeling with aeromagnetic and earthquake data

The Sinai Peninsula is a subplate located between the African and Arabian plates. It is an important subplate in the world; however, this region is poorly understood because of the shortage of geophysical data. Thus, this study focuses on built a three-dimensional (3D) high-resolution forward model of the crustal thickness with a new tectonic model and structural evaluation of the Sinai Peninsula to understand this area tectonically and determine its lithospheric and crustal thicknesses. Qualitative and quantitative interpretations of Bouguer gravity, reduced-to-the-pole (RTP) aeromagnetic, and earthquake data are explained to achieve the goals of this study. Two-dimensional (2D) interactive sequential modeling of gravity data is performed along with some well-selected profiles with existing crustal layer depth points from previous works in the studied region. Seventeen 2D models are constructed and used to determine the basement, Conrad (lower crust boundary), and Moho (upper mantle boundary) depths and to build a 3D model. Accuracy of the 3D built model is evaluated by extracting observed, calculated, and residual gravity anomaly maps. The results show that the basement rocks appear on the surface in the South of Sinai and deepen (down to 4.5 km) toward the North. The Conrad surface varies approximately from 16 km (in the northeast and southwest of Sinai) to 22.8 km (in the southeast). The Moho surface depth (i.e., crustal thickness) varies approximately from 28.8 to 34.2 km, and it deepens toward the northeastern and southwestern parts of Sinai. A tentative basement structure map is constructed from the horizontal gradient (H-gradient) filter and the 3D Euler deconvolution of the Bouguer gravity and RTP aeromagnetic data. Correlation between the geological background of the study area and the obtained results suggests that the Sinai Peninsula is tectonically active, mainly in its southern portion due to the tectonic movements along the Gulf of Suez and the Gulf of Aqaba. The crustal thickness increases southward and thins northward.

A Thermo‐Compositional Model of the African Cratonic Lithosphere

AbstractRecently, the continually increasing availability of seismic data has allowed high‐resolution imaging of lithospheric structure beneath the African cratons. In this study, S‐wave seismic tomography is combined with high resolution satellite gravity data in an integrated approach to investigate the structure of the cratonic lithosphere of Africa. A new model for the Moho depth and data on the crustal density structure is employed along with global dynamic models to calculate residual topography and mantle gravity residuals. Corrections for thermal effects of an initially juvenile mantle are estimated based on S‐wave tomography and mineral physics. Joint inversion of the residuals yields necessary compositional adjustments that allow to recalculate the thermal effects. After several iterations, we obtain a consistent model of upper mantle temperature, thermal and compositional density variations, and Mg# as a measure of depletion, as well as an improved crustal density model. Our results show that thick and cold depleted lithosphere underlies West African, northern to central eastern Congo, and Zimbabwe Cratons. However, for most of these regions, the areal extent of their depleted lithosphere differs from the respective exposed Archean shields. Meanwhile, the lithosphere of Uganda, Tanzania, most of eastern and southern Congo, and the Kaapvaal Craton is thinner, warmer, and shows little or no depletion. Furthermore, the results allow to infer that the lithosphere of the exposed Archean shields of Congo and West African cratons was depleted before the single blocks were merged into their respective cratons.

Effects of Microorganisms on Drop Formation in Microgravity During a Parabolic Flight with Residual Gravity and Jitter

Effects of Microorganisms on Drop Formation in Microgravity During a Parabolic Flight with Residual Gravity and Jitter

Optimised gravity anomaly fields from along-track multi-mission satellite altimeter over Malaysian seas

Marine gravity anomalies are crucial parameters and elements for determining coastal and ocean geoid, tectonics and crustal structures, as well as offshore studies. This study aims to derive and develop a marine gravity anomaly model over Malaysian seas from multi-mission altimetry data. Universiti Teknologi Malaysia 2020 Mean Sea Surface Model is computed based on along-track data from nine satellite missions, incorporating TOPEX, Jason-1, Jason-2, ERS-2, Geosat Follow on (GFO), Envisat-1, CryoSat-2, SARAL/AltiKa, and Sentinel-3A. The data exploited are from 1993 to 2019 (27 years). Residual gravity anomaly is computed using Gravity Software, and two-dimensional planar Fast Fourier Transformation method is applied. The evaluation, selection, blunder detection, combination, and re-gridding of the altimetry-derived gravity anomalies and Global Geopotential Model data are demonstrated. Cross-validation procedure is employed for data cleaning and quality control using the Kriging interpolation method. Then, cross-validation procedure is applied to the tapering window width 200, which adopting the GECO model denotes the optimum gravity anomaly with root mean square errors in the range of ± 4.2472 mGal to ± 6.0202 mGal. The findings suggest that the estimated marine gravity anomaly is acceptable to be implemented in the marine geoid determination and bathymetry estimation over Malaysian seas. In addition, the results of this study are valuable for geodetic and geophysical applications in marine areas.

Development of tailored gravity model based on global gravitational and topographic models and terrestrial gravity data for geophysical exploration of southern benue trough in southeast Nigeria

A major challenge in geosciences is the acquisition of high-resolution geophysical data that could help to better understand the Earth's interior and processes. In this study, we develop a Tailored Gravity Model (TGM) to fit local terrestrial gravity data collected throughout the Southern Benue Trough (situated in southeast Nigeria) by using Global Gravitational Models (GGMs), terrestrial gravity data, and Digital Elevation Models (DEMs). Specifically, we develop the TGM by computing gravity values from the XGM2019e_2159 global gravitational model on the topographic surface generated by using the Multi-Error-Removed Improved-Terrain (MERIT) DEM topographic heights, and algebraically adding the residual gravity effects obtained from the SRTM2gravity global model of spherical gravimetric terrain correction, while removing a detected systematic bias. An independent statistical evaluation of the terrestrial and tailored gravity data shows that most of the differences between these two datasets are within ±10 mGal, while the RMS of differences improved from 16.7 to 6.4 mGal and a systematic bias was removed. We used the tailored gravity data to compile the Bouguer gravity map. The gravity map produced from a high-resolution absolute gravity data reveals the spatial gravity pattern that is likely associated with aquifers and magmatic structures within the Southern Benue Trough. In agreement with published studies, the Bouguer gravity map demonstrates the folding, faulting, and upwelling or uplifting of the crust. Our gravimetric interpretation suggests that the elongated gravity high along the flexurally inverted and circular Abakaliki Anticlinorium in the Bouguer gravity map could indicate a possible existence of a reservoir of mineral ore bodies or mineral-bearing rocks.

Insights into Moho depth beneath the northwestern Andean region from gravity data inversion

SUMMARYThe complex Moho topography beneath the northwestern Andes is the result of multiple geodynamic processes during the Cenozoic. To contribute to our understanding of the Moho depth distribution beneath this region, we inverted gravity data from two widely used satellite-derived data sets (EGM2008 and EIGEN-6C4) and one regional airborne Bouguer gravity anomaly map (ANH2010). Their inversion allowed choosing the ANH2010, based on lower residual gravity and a higher agreement with seismic estimations, as the most suitable data set to gain insights into the Moho depth beneath the northwestern Andes and its relationship with previously identified tectonic features. The inverted Moho argues for a 40–50 km depth beneath the Central and Eastern cordilleras, reaching depths beyond 50 km below the Eastern Cordillera, and shallower depths between 30 and 40 km mainly along the foreland region to the east, the Western Cordillera and the coastal plains. Three main thickened crust features of regional extent were identified: (1) a deep Moho expression with a crustal thickness greater than 40 km in the northwesternmost foreland region, which we consider a direct consequence of the adjacent thickened Eastern Cordillera involving the fold and thrust deformation migration from the range towards the foreland, and the flexural deformation proposed for the eastern foothills; (2) a regional deep Moho expression (50–60 km) along the axis of the Eastern Cordillera, related to its shortening history including multiple phases of Cenozoic thick-skinned deformation and magmatic underplating; and (3) a Moho deeper than 60 km in a southern latitude (1°S–1°N) beneath the modern magmatic arc, whose interpretation is more complex, likely a combined result of mafic addition to the base of the crust, foundering tectonics, and lateral displacement of the lower crust prompted by the subducting Carnegie ridge.

Regional gravity field distribution over cratonic domains of the Indian shield: Implications for lithospheric evolution and destruction

• Detailed analysis of regional gravity field over Indian cratons. • Extremely low Bouguer gravity of −120 mGal associated with Dharwar Craton. • Gravity field is largely positive over the north Indian cratonic segments. • Cratons with positive gravity have thinner lithosphere and higher heat flow. The nature of crust and lithospheric mantle evolution of the Archean shields and their subsequent destruction through intraplate tectonic processes are important in understanding continental dynamics and resources. The Indian shield, which has remained dynamic throughout the past for more than three billion years of geological history, is unique in terms of its lithospheric architecture compared to other Archean terranes on the globe. It consists of five major cratons, the Dharwar, Singhbhum, Bundelkhand, Bastar and Aravalli, which are separated by active rift valleys, mega lineaments and sutures zones. Here, we present a detailed analysis of gravity field over these areas, and their possible relationship with lithospheric thickness variations. Our results show a large variation in residual gravity field among the different cratons. A highly negative anomaly of around -70 mGal occurs over the western part of the Dharwar Craton, beneath which the lithosphere is about 160–200 km thick. In comparison, the anomalies are conspicuously positive, reaching around (i) +10 mGal over Central Dharwar Craton, Eastern Ghats Belt and the eastern part of Singhbhum Craton, (ii) +35 mGal over the Nellore Schist and Aravalli-Delhi Fold Belts, and (iii) +60 to +70 mGal over eastern part of SGT. In these regions, the lithosphere is markedly thinner between 70 and 135 km. Such areas have been associated with episodic magmatism, moderately high heat flow and elevated Moho temperatures, indicating a large-scale upwarping of isotherms leading to lithospheric destruction to the tune of 100–150 km, caused by mechanical, thermal and chemical erosion. Our study confirms that many of the Archean cratons on the globe, like the Indian shield, have undergone substantial erosion of their roots through subsequent geotectonic processes.

GTOOLS, an open-source MATLAB program for processing high precision, relative gravity data for time-lapse gravity monitoring

gTOOLS is an open-source software for the processing of relative gravity data. gTOOLS is available in MATLAB and as a compiled executable to be run under the free MATLAB Runtime Compiler. The software has been designed for time-lapse (temporal) gravity monitoring. Although programmed to read the Scintrex CG-5 and CG-6 gravimeters output data files, it can be easily modified to read data files from other gravimeters. The software binds together single-task processing modules within a very simple user interface that is based on one text file. Gravity processing involves three modules: (a) gravimeter calibration; (b) automatic processing of gravity data to find adjusted gravity differences; and (c) post processing of results. Each module is optional and runs independently from the others. Data processing includes (a) averaging out the measurements noise, and correction for solid Earth tides, and ocean loading, and residual instrumental drift, and (b) calculate the residual instrumental drift and gravity differences between the base station and monitoring sites, and their uncertainties, by a weighted least square analysis of the gravity data. The software allows the automatic processing of a gravity campaign spanning multiple days in a single run. The software is tested on gravity data from 2015 eruption at Cotopaxi volcano, Ecuador.

Topography and Gravity Reveal Denser Cratonic Lithospheric Mantle Than Previously Thought

AbstractThe density structure of the cratonic lithospheric mantle (CLM) remains debated. We suggest that one important reason for which many geodynamic studies favor neutrally buoyant CLM is that they adopted separate reference frames when estimating the isostatic effects of continental and oceanic lithosphere, while instead a globally consistent one should be used. This reflects a misconception that continental crust perfectly balances the surrounding oceanic lithosphere. Using a unified global reference frame with recent constraints on crustal properties and residual topography, we show that assuming neutrally buoyant CLM leads to prominent negative residual topography (∼−1.3 km) and positive residual gravity (∼354 mGal) within cratons relative to oceans, neither of which can be explained by the effects of the convecting mantle. This requires the CLM, especially that with thick keels, to be less compositionally buoyant and denser than previously thought, a conclusion supporting recent observations on CLM deformation.

Spatial distribution characteristics and mechanism of nonhydrological time-variable gravity in mainland China

The purpose of this study is to explore nonhydrological mass transfer in China continent. For this purpose, gravity recovery and climate experiment (GRACE) data were obtained to study the spatial distribution of time variant gravity signals in China continent. Then, from auxiliary hydrological data processed according to the current hydrological model, a new more comprehensive hydrological model of China continent was constructed. Finally, the time variant signals of this new hydrological model were removed from the time variant gravity field computed from GRACE data, thus obtaining a description of the nonhydrological mass transfer of China continent. The physical sources and mechanisms of the resulting mass transfer are then discussed. The improved, more realistic, hydrological model used here was created by selecting the hydrological components with the best correlations in existing hydrological models, by use of correlation calculation, analysis, and comparison. This improved model includes water in soils and deeper strata, in the vegetation canopy, in lakes, snow, and glaciers, and in other water components (mainly reservoir storage, swamps, and rivers). The spatial distribution of the transfer signals due to nonhydrological mass in China continent was obtained by subtracting the combined hydrological model from the GRACE time-variable gravity field. The results show that the nonhydrological signals in China continent collected in GRACE data were mainly positive signals, and were distributed in the Bohai Rim and the northern and eastern parts of the Tibetan Plateau. The above nonhydrological mass transfer signals have been studied further and are discussed. The results show that the nonhydrological mass migration signals in the Bohai Rim region originate primarily from sea level change and marine sediment accumulation. The mass accumulation from Indian plate collision in the Tibetan Plateau appears to be the main reason for the increase in the residual gravity field in that region.

The Use of Edge Enhancement Methods and Euler Deconvolution to Estimate an Ore Deposit Depth from Gravity Data

The Hercynian massif of the central Jebilet (Morocco) is characterized by the outcrop of many gossans with great economic importance. This work focuses on interpreting gravity data of Benslimane gossan, located about thirty kilometres to the North-West of Marrakech. The residual gravity map of the study area highlights several anomalies which coincide with the mining and geologi-cal contexts. Applying edge detection methods, for example, tilt angle derivative (TDR), the total horizontal derivative of the tilt angle derivative (HDR_TDR) and the 3D Euler deconvolution, allowed us to estimate the depth of the Benslimane deposit. As a result, the average depth of the ore deposit was estimated to exceed 200 m. The results are promising, and the processing methods must be applied to the other gossan in the Jebilet massif for further exploration studies.