Gravimetric Interpretation Research Articles

Gravity forward modelling software with user‐friendly interface

AbstractThe gravimetric forward method is crucial in geophysical applications for a gravimetric interpretation of the Earth's inner structure. In this study, we present the gravimetric forward modelling open‐source software that incorporates a graphical user interface. This software allows data preparation, manipulation and result interpretation both spatially and spectrally. For spatial domain modelling, it uses prism and tesseroid elements, whereas in the spectral domain, it extends Parker's formulas within specified boundaries. The software's utility is demonstrated through synthetic models and real‐world applications, including calculating corrections for topography, sediments and consolidated crust using ETOPO1 and CRUST1.0 models. Performance comparisons show that Parker's method delivers computation speed superior to that of the prism, tesseroid and Terrain gravity forward (TGF) software, with variances ranging within ±12 mGal for and ±0.3 E for across different geological scenarios.

Investigation of the Superficial Structures of the Foumban Area (West Cameroon) Based on Spectral Analysis and 3D Modelling

The main objective of this study is to characterise the superficial structures of Foumban area. To achieve this, a sample of data from the gravity map of Cameroon is used to build a new Bouguer map to the studied area. The finite element method is used to separate the Bouguer anomalies into regional and residual anomalies. Given that the signatures of gravity anomalies related to surface structures are perceptible on the residual anomaly map, a qualitative analysis of this map helped to identify three anomaly domains in the north-western, central and north-eastern parts of the studied area. These three anomalies peak at 19 mGal, -24 mGal and 27 mGal, respectively. Spectral analysis is used to obtain the appropriate energy spectrum for different profiles that were selected, to estimate the average depth of the intrusive bodies. Thus, thanks to 3D modelling of the anomalous structures of the crust, a gravimetric interpretation is carried out. This interpretation leads correlatively to the geology of the region to conjecture the presence of sediments (clays, conglomerates, sandstones, sands) in the center of the study area; metamorphic rocks (gneisses, micaschists) and magmatic rocks (granites) responsible for positive anomalies in the rest of the area

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.

Structuration profonde dans la région de Bir M’Chergua (Nord-Est de la Tunisie). Implications hydrogéologiques

[fr] La région de Bir M’Chergua sise à cinquante kilomètres de la capitale Tunis constitue un pôle économique important en Tunisie. Cependant la pénurie en eau résultante de la succession de nombreuses périodes de sécheresse a généré une notable dégradation de diverses activités industrielles, touristiques et agricoles dans cette région. L’exploitation des eaux souterraines peut pallier à cette situation mais elle nécessite une meilleure caractérisation des aquifères. Dans ce contexte, est réalisée la présente étude qui vise à apporter des précisions au schéma hydrogéologique de la région de Bir M’Chergua en déterminant la structuration profonde à partir de l’interprétation des données gravimétriques. Des cartes d’anomalie résiduelle, de la dérivée verticale et du gradient horizontal total sont calculées à partir de l’anomalie de Bouguer pour identifier et délimiter les sources perturbatrices. La technique de déconvolution d’Euler est aussi appliquée pour estimer la profondeur de ces sources. La carte structurale issue de l’interprétation gravimétrique permet de mettre en évidence des structures enfouies et des discontinuités de différentes directions et profondeurs qui peuvent influencer énormément la géométrie et le fonctionnement du système aquifère de Bir M’Chergua. En effet, la mise en évidence des accidents tectoniques profonds de plus de 1500 m aux alentours du Dôme du Jebel Oust permet d’expliquer l’origine et la forte salinité des eaux thermales dans ce secteur; les accidents ont favorisé une pénétration profonde des eaux de recharge ainsi que leur contact avec les terrains triasiques.

Study of salt structures from gravity and seismic data in Santos Basin, Brazil

Seismic is one of the main methods used for the identification of structures and stratigraphic studies in sedimentary basins. In the Santos Basin, numerous 2D and 3D seismic surveys are being conducted in order to get a better ima-ge of the geological section to depths beyond the base of salt layer. Crustal modelling studies that make joint interpretation of seismic and gravity data are found in the literature, however there are few studies that relate gravity anomalies directly to salt structures. This work aims to associate gravity anomalies with salt structures from seismic and gravimetric interpretation. For studies aimed to model the crustal structure from gravity field data, the knowledge of two major discontinuities is required, the basement and the Moho. Such interfaces are often not easily seen by seismic and so, during this study, they were found by different methods involving analysis of gravity anomalies. The other interfaces involving density contrasts were analyzed based on seismic interpretation. The results showed that the obtained seismic geological interpretations may provide additional information when compared to gravity anomaly data. In all the modelled profiles, some geological information of the Santos Basin that are not visible in the seismic, could be interpreted according to the geological model and the adjustment of gravity anomaly curves. As a final conclusion of this work, it is suggested that the combined analysis of the two cited geophysical methods, can provide important information about the crustal structure and to assist in modelling the salt layer.

Study of salt structures from gravity and seismic data in Santos Basin, Brazil

Seismic is one of the main methods used for the identification of structures and stratigraphic studies in sedimentary basins. In the Santos Basin, numerous 2D and 3D seismic surveys are being conducted in order to get a better ima-ge of the geological section to depths beyond the base of salt layer. Crustal modelling studies that make joint interpretation of seismic and gravity data are found in the literature, however there are few studies that relate gravity anomalies directly to salt structures. This work aims to associate gravity anomalies with salt structures from seismic and gravimetric interpretation. For studies aimed to model the crustal structure from gravity field data, the knowledge of two major discontinuities is required, the basement and the Moho. Such interfaces are often not easily seen by seismic and so, during this study, they were found by different methods involving analysis of gravity anomalies. The other interfaces involving density contrasts were analyzed based on seismic interpretation. The results showed that the obtained seismic geological interpretations may provide additional information when compared to gravity anomaly data. In all the modelled profiles, some geological information of the Santos Basin that are not visible in the seismic, could be interpreted according to the geological model and the adjustment of gravity anomaly curves. As a final conclusion of this work, it is suggested that the combined analysis of the two cited geophysical methods, can provide important information about the crustal structure and to assist in modelling the salt layer.

On the impact of topography and building mask on time varying gravity due to local hydrology

We use 3 yr of surface absolute gravity measurements at three sites on the Larzac plateau (France) to quantify the changes induced by topography and the building on gravity time-series, with respect to an idealized infinite slab approximation. Indeed, local topography and buildings housing ground-based gravity measurement have an effect on the distribution of water storage changes, therefore affecting the associated gravity signal. We first calculate the effects of surrounding topography and building dimensions on the gravity attraction for a uniform layer of water. We show that a gravimetric interpretation of water storage change using an infinite slab, the so-called Bouguer approximation, is generally not suitable. We propose to split the time varying gravity signal in two parts (1) a surface component including topographic and building effects (2) a deep component associated to underground water transfer. A reservoir modelling scheme is herein presented to remove the local site effects and to invert for the effective hydrological properties of the unsaturated zone. We show that effective time constants associated to water transfer vary greatly from site to site. We propose that our modelling scheme can be used to correct for the local site effects on gravity at any site presenting a departure from a flat topography. Depending on sites, the corrected signal can exceed measured values by 5-15 μGal, corresponding to 120-380 mm of water using the Bouguer slab formula. Our approach only requires the knowledge of daily precipitation corrected for evapotranspiration. Therefore, it can be a useful tool to correct any kind of gravimetric time-series data.

Spatial and Spectral Analysis of Refined Gravity Data for Modelling the Crust–Mantle Interface and Mantle-Lithosphere Structure

We analyse spatial and spectral characteristics of various refined gravity data used for modelling and gravimetric interpretation of the crust–mantle interface and the mantle-lithosphere structure. Depending on the purpose of the study, refined gravity data have either a strong or weak correlation with the Moho depths (Moho geometry). The compilation of the refined gravity data is purely based on available information on the crustal density structure obtained from seismic surveys without adopting any isostatic hypothesis. We demonstrate that the crust-stripped relative-to-mantle gravity data have a weak correlation with the CRUST2.0 Moho depths of about 0.02. Since gravitational signals due to the crustal density structure and the Moho geometry are subtracted from gravity field, these refined gravity data comprise mainly the information on the mantle lithosphere and sub-lithospheric mantle. On the other hand, the consolidated crust-stripped gravity data, obtained from the gravity field after applying the crust density contrast stripping corrections, comprise mainly the gravitational signal of the Moho geometry, although they also contain the gravitational signal due to anomalous mass density structures within the mantle. In the absence of global models of the mantle structure, the best possible option of computing refined gravity data, suitable for the recovery/refinement of the Moho interface, is to subtract the complete crust-corrected gravity data from the consolidated crust-stripped gravity data. These refined gravity data, that is, the homogenous crust gravity data, have a strong absolute correlation of about 0.99 with the CRUST2.0 Moho depths due to removing a gravitational signal of inhomogeneous density structures within the crust and mantle. Results of the spectral signal decomposition and the subsequent correlation analysis reveal that the correlation of the homogenous crust gravity data with the Moho depths is larger than 0.9 over the investigated harmonic spectrum up to harmonic degree 90. The crust-stripped relative-to-mantle gravity data correlate substantially with the Moho depths above harmonic degree 50 where the correlation exceeds 0.5.

Gravitational attraction and potential of spherical shell with radially dependent density

Solutions to the direct problem in gravimetric interpretation are well-known for wide class of source bodies with constant density contrast. On the other hand, sources with non-uniform density can lead to relatively complicated formalisms. This is probably why analytical solutions for this type of sources are rather rare although utilization of these bodies can sometimes be very effective in gravity modeling. I demonstrate an analytical solution to that problem for a spherical shell with radial polynomial density distribution, and illustrate this result when applied to a special case of 5th degree polynomial. As a practical example, attraction of the normal atmosphere is calculated.

Viscoelastic characterization of benzo-crown ether functionalized electroactive films

Electrochemical, acoustic wave and imaging (AFM) methods were used to characterize electroactive poly[Ni(3-Mesalophen-b15-c5)] films. The distinct physical principles of these techniques generate different responses to film characteristics, such that their simplistic correlation in the context of a single uniform film model does not adequately describe the data. Film behaviour can only be rationalised using a two layer model comprising a dense (poorly solvated) inner layer and an outer layer comprising islands of diffuse (highly solvated) polymer. The two layers can be observed directly by AFM imaging. The balance of their contributions shifts with polymer coverage: the inner layer dominates thin films and the outer layer dominates thick films. The inner layer is acoustically more rigid and, even if ion transport is slow, is sufficiently thin as to pose no major barrier to electrochemical communication with the underlying electrode. For very thin films, gravimetric interpretation of the acoustic wave response correlates simply with coulometric data; thicker films require viscoelastic phenomena to be considered. The low polymer segment density of the outer layer facilitates ion transport but results in weak acoustic coupling, such that the acoustic and coulometric assays differ. By combining the AFM, acoustic wave and electrochemical responses one can establish a quantitative picture of component layer populations and characteristics.

Temporal resolution of ion and solvent transfers at nickel hydroxide films exposed to LiOH

A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor mobile species transfers on short time scales following the application of a potential step to potentiostatically deposited α-nickel hydroxide films exposed to aqueous LiOH solution. Upon film oxidation, hydroxide ions enter the film and protons are deintercalated. However, the protons are not detected directly since, upon ejection from the film, they immediately combine with hydroxide ions from the solution to yield water. The relatively rigid film lattice (which allows gravimetric interpretation of the EQCM response) imposes volume constraints. As ions are driven into/out of the film in order to satisfy electroneutrality, this volume constraint causes solvent to be driven out of/into the film by internal pressure. Consequently, solvent transfer is forced into the same time scale as ion transfer and is no longer simply diffusionally controlled. Combining the EQCM/PBD-derived fluxes of individual mobile species with their molar volumes, one can estimate the implied changes in film volume driven by redox switching. These volume changes are relatively small and correspond to a monotonic expansion on film oxidation and a transient expansion then eventual contraction upon film re-reduction. These observations are in the opposite sense to those predicted on the basis of unit cell volumes for crystalline α-Ni(OH) 2 and γ-NiOOH, suggesting that the material studied is poorly represented by these idealised structures.

Geophysical interpretation of a high‐resolution seismic refraction profile in the Northern Apennines

ABSTRACTA combined seismic and gravimetric interpretation in the Northern Apennines area (Italy) is presented. To the knowledge of the authors, this is one of the few attempts to apply tomographic methodology to a seismic refraction profile. This procedure, together with the classical interpretation for defining lower reflectors, led to the formulation of quite an accurate model of the upper crust. A gravity analysis was performed concurrently taking into account the seismic results at different depths which correspond to different frequency domains in the gravity signal. While the medium‐ and high‐frequency patterns have been solved by trial‐and‐error, the regional trend has been modelled applying the collocation procedure to the gravity data.

The Nordkalottproject: A combined petrophysical, magnetic and gravimetric interpretation profile in southeastern Norrbotten, Sweden

The Nordkalottproject: A combined petrophysical, magnetic and gravimetric interpretation profile in southeastern Norrbotten, Sweden

On the use of the gauss seidel method on linear parameter interpretation in magnetometry and gravimetry

Approximate magnetization and density distributions are found by dividing the ground into bodies with homogeneous properties. The resulting linear equation system for the material constants of the bodies are efficiently solved by the iterative Gauss—Seidel method, which has theoretically simple convergence criteria and small requirements on computer capacity. An efficient use of the method requires, that: 1. (1) The number of measurements equals the number of bodies. 2. (2) The bodies are placed so that the maximum influence of the body appears at corresponding field point. 3. (3) Bodies of similar size and with “narrow” anomalies should be preferred. 4. (4) The iteration converges from arbitrary starting values, if the horizontal distance between bodies of similar form exceeds a minimum value. This minimum value is approximately 1.5 times the depth for infinitely deep magnetic plates. The value depends on depth extent and it increases strongly when direction of magnetization differs more from the direction of the plate. Minimum distances between thin plates or cylinders are smaller than for thick plates, minimum distances for corresponding bodies in gravimetric interpretation are more than twice the minimum distances of magnetic bodies. 5. (5) Using starting values clearly less than the correct values gives convergent solutions for horizontal body distances a half to one quarter of the theoretical minimum values.

A CONTRIBUTION TO THE QUANTITATIVE ERROR ANALYSIS IN MAGNETIC AND GRAVIMETRIC INTERPRETATION *

AbstractThe paper discusses the smallest obtainable parameter errors (variances) in the interpretation with the least‐squares method. Useful approximations of the sum of squares contained in the minimum error expressions are obtained using results of numerical integration. The approximations lead to especially simple results for long interpretation profiles, when the parameter errors are proportional to the square root of the point separation.Formulae are developed and examples shown for minimum error calculation in gravimetric interpretation with the cylindrical model and in magnetic interpretation with the two‐dimensional plate model. Smallest errors are obtained when the interpretation profile is chosen around the anomaly maximum except for dip and depth extent interpretation of magnetic plates.