Interpretation Of Gravity Anomalies Research Articles

A new technique based on the eigenvalue of the curvature tensor for enhancing gravity data

Enhancement of the boundaries of geologic structures is one of the main goals in interpretation of gravity anomalies. A range of different algorithms based on field derivatives has been introduced to solve this problem. Among these algorithms, the curvature gravity gradient tensor and its modifications are widely used in enhancing gravity data. These algorithms, however, have some main limitations such as the estimated edges are not compatible with the actual edges or bringing false information. In this study, I introduce a new technique based on the eigenvalue of the curvature tensor. The robustness of the presented technique is exemplified using model studies and a real-world dataset of the Vredefort dome (South Africa). The model studies show that the proposed filter can provide more accurate results and avoid artifacts in the output map. The real application result shows a good correlation between the edges highlighted by the presented algorithm and ring structures in the dome.

Gravity profiles interpretation applying a metaheuristic particle optimization algorithm of mineralized bodies resembled by finite elements

The interpretation of gravity anomalies is crucial for identifying subsurface mineralized targets and understanding the density variations between the targets and the surrounding structures. To confirm the presence of ore and mineral targets, simple geometric bodies are often used. One of the commonly used global metaheuristic algorithms for gravity data analysis is the particle optimization algorithm. In this study, we employed this method to determine the parameters of buried bodies that resemble finite vertical cylinders by inferring gravity anomalies profiles (amplitude coefficient, depth to top, depth to bottom, origin, and length of the target representing the difference between two depths). The algorithm utilizes particle movement to identify the best way to reach the global or optimum solution. The algorithm's performance was evaluated on synthetic-examples with and without noise (5 % and 10 % levels) and also verified on a real dataset for mineral exploration from Canada. The results showed that the algorithm's stability and accuracy were not affected by the presence of noise and multi-models. Moreover, the field case results were consistent with the existing geological information, borehole data, and previously published outcomes.

Crustal Composition and Moho Variations of the Central and Eastern United States: Improving Resolution and Geologic Interpretation of EarthScope USArray Seismic Images Using Gravity

AbstractEarthScope's USArray Transportable Array has shortcomings for the purpose of interpreting geologic features of wavelengths less than the Transportable Array station spacing, but these can be overcome by using higher spatial resolution gravity data. In this study, we exploit USArray receiver functions to reduce nonuniqueness in the interpretation of gravity anomalies. We model gravity anomalies from previously derived density variations of sedimentary basins, crustal Vp/Vs variation, Moho variation, and upper mantle density variation derived from body wave imaging informed by surface wave tomography to estimate Vp/Vs. Although average densities and density contrasts for these seismic variations can be derived, the gravity anomalies modeled from them do not explain the entire observed gravity anomaly field in the United States. We use the unmodeled gravity anomalies (residuals) to reconstruct local variations in densities of the crust associated with geologic sources. The approach uses velocity‐density relationships and differs from density computations that assume isostatic compensation. These intracrustal densities identify geologic sources not sampled by and, in some cases, aliased by the USArray station spacing. We show an example of this improvement in the vicinity of the Bloomfield Pluton, north of the bootheel of Missouri, in the central United States.

Assembly methods of solving inverse problems as an integral element of additive technologies for interpretations of gravity anomalies

In recent years, the theory of interpretation of gravitational anomalies has been replenished with fundamentally different from familiar, additive technologies for extracting information about the studied geo-density medium. The concept of «additivity» implies a summation in the results of the interpretation of information carried by each of the found admissible solutions to the inverse problem. At the same time, the results of the interpretation themselves are not expressed, as usual, in terms of one of these solutions. In the ore inverse problem of gravity exploration, an effective working tool for such technologies has become the assembly algorithms for constructing valid interpretation options. These algorithms have universal capabilities in the matter of accounting for a priori data and do not have high requirements for the formation of the initial approximation of the field source model, which is crucial from the point of view of their application in additive technologies. At the same time, all hitherto known modifications of the mounting method were designed for formulating inverse problems in which there are bodies with effective densities of the same sign. Due to the integration of the method with the procedure for separating gravitational fields, the problem of different sign densities was solved. The separation of the observed field into two components, due to the influence of sources with positive and negative effective density, is carried out by the approximation method. To approximate the discrete values of gravity, the sets of elementary sources under each point of observation are used. The masses of sources are determined by solving a system of linear algebraic equations. The sources are successively immersed at different depths corresponding to different variants of the selection of the interpreted field components. The article evaluates the current state and prospects for the further development of additive technologies for interpreting gravitational anomalies based on the methods that implement the concept of the mounting approach by V. N. Strakhov. Model of model examples are given illustrating the capabilities of the presented algorithms.

Interpretation of gravity anomalies over the eighty-five East Ridge in the Bay of Bengal

Several profiles of the negative gravity anomaly over the submerged 85o E ridge in the Bay of Bengal of the Indian Ocean have been interpreted in two dimension and results are combined together to give a detailed picture of its morphology and thickness of surrounding sediments. The observed negative gravity anomaly has been explained as the combined effect due to the positive mass anomaly caused by the replacement of sediments by high dense igneous rocks of the ridge, and the negative mass anomaly caused by replacing the high dense upper mantle material by the oceanic crust, which has been bended and sunk into the upper mantle due to the pressure exerted by the ridge. Downward migration of the oceanic crust has been also calculated assuming that the oceanic crust is behaving as a thin infinite elastic plate resting on inviscid fluid half space and is found to closely agree with the results of the gravity study. Both studies indicate that the thickness of the ridge vary from 11 km to 16 km while the oceanic crust has undergone a depression of 9 km to 14 km.

Generalized Solutions of the Inverse Problem and New Technologies for the Quantitative Interpretation of Gravity Anomalies

The approach, fundamentally different from the known ones, to estimating the spatial location of the domain filled with the disturbing masses based on the gravity field measurement data is suggested. The main idea of the approach is, using the set of the probable variants of the interpretation, to construct the distribution of a certain parameter associated with the estimate of probability of detecting the sources of the field in any point of the studied geological medium and then to apply this distribution to each domain eligible for being the true carrier of the anomalous masses. These constructions yield the generalized admissible solutions of the inverse problem with ranking the separate fragments of the model carrier in terms of the probability of detecting anomalous masses in them.

New possibilities of increasing the informativeness of quantitative interpretation of gravity anomalies

An effective method for structuring a spatial region studied with respect to the probability of filling its individual components with disturbing masses is proposed. The use of new mathematical forms for representing the results of interpretation of an anomalous gravity field that are fundamentally different from those known is a typical feature of the method.

Interpretation of gravity anomalies to delineate some structural features of Biga and Gelibolu peninsulas, and their surroundings (north-west Turkey)

On the basis of gravity data, derived anomaly traces were presented for the interpretation of some shallow structural features of Biga and Gelibolu peninsulas. Since building general understanding of subtle details about subsurface geology is of great importance considering that the study area is tectonically important, some advanced data processing techniques were implemented to gravity anomalies in a detailed manner. The procedures were performed using a MATLAB-based software package (Gravity and Magnetic Interpretation – GMINTERP). First, a finite element method was utilised to produce the residual data-set which is expected to reflect short wavelength anomalies arising from shallower geological structures, and thereafter some derivative-based algorithms were executed to analyse the residual data. The general anomaly patterns obtained from the applications clearly corresponded to the well-known surface geology map of the study area. Derivative-based anomaly maps put forward some findings about the existence of an old caldera structure in the western part of the Biga Peninsula. Additionally, abrupt lateral changes in anomaly amplitudes indicated the presence of some major structural discontinuities. Thus, findings yielded to make significant geological interpretations that might be important for further investigations. This study also showed that GMINTERP software package proved useful in assisting geological interpretation using geophysical potential field data-sets.

Density modeling of magmatic structures in the promising Kolpakova Nickeliferous district (Median massif, Kamchatka)

The deep structure is modeled for the crustal block in the Kolpakova River basin of the Kamchatka Peninsula. A technique for developing the 3D density model on the basis of the data derived from interpretation of gravity anomalies is described. The model is used for outlining the position and shape of the massive bodies in the upper crust, which are identified with intrusions of the Dukuk nickeliferous magmatic complex, with identification of areas promising with respect to metal deposits genetically associated with intrusive mafic and ultramafic magmatism. It is shown that the proposed technique represents an effective tool for the study of the inner structure in regions poorly studied by deep geophysical sounding methods or as yet completely lacking such data.

Решение геокартировочных и прогнозно-поисковых геологических задач методом гравиразведки с учетом сферичности Земли

Решение геокартировочных и прогнозно-поисковых геологических задач методом гравиразведки с учетом сферичности Земли

GUI based inversion code for automatic quantification of strike limited listric fault sources and regional gravity background from observed Bouguer gravity anomalies

A method coupled with a GUI based code in JAVA is developed in the space domain to simultaneously estimate the structures of strike limited listric fault sources and regional gravity background from a set of observed Bouguer gravity anomalies. The density contrast within the hanging wall of the structure is assumed to be varying continuously with depth based on a parabolic equation. The limiting surface of the fault plane is described with an exponential function. This method is automatic in the sense that it initializes both parameters of a strike limited listric fault source and regional gravity background from a set of observed Bouguer gravity anomalies and improves them iteratively until the modeled gravity anomalies fit the observed anomalies within the specified convergence criteria. The advantage of the code is that besides generating output in both ASCII and graphical forms it displays the animated versions of (i) the changes in model geometry, (ii) variation of each model parameter and misfit with iteration number, (iii) improvements in modeled gravity anomalies, and (iv) variation of density contrast with depth. The applicability of the code is demonstrated on both synthetic and real field gravity anomalies. In case of synthetic example pseudorandom noise is added to the residual gravity anomalies of the structure prior to inversion. The noisy anomalies are then inverted for the unknown parameters presuming (i) an ideal listric fault structure bounded by an exponential limiting surface with perfect flat top and bottom surfaces, (ii) non-ideal structure with uneven top and bottom surfaces with imperfect exponential limiting surface. Further, the robustness of the algorithm is exemplified by adding both regional gravity background and pseudorandom noise to the anomalies of the structure before inversion. In all cases, the interpreted parameters of the structure closely mimic the assumed parameters. The interpretation of gravity anomalies across the master fault of the Chintalpudi sub-basin in India has yielded information that is consistent with both DSS results and drilling information. The highlight of the code is that it can be used to interpret the gravity anomalies of listric fault sources even when the profile along which the interpretation is intended fails to bisect the strike length of the structure.

Interpretation of Gravity Anomalies by Multi-Scale Evaluation of Maxima of Gradients and 3D Modelling in Bipindi Region (South-West Cameroon)

The gravity maps display, in Bipindi zone, local oval culminations of low anomalies indicative of a presence of intrusive light body in a subsurface but the nature, the form and the position of this body are still unknown. The analyses of established gravimetric anomaly maps, the multi-scale evaluation of maxima of gradients and the quantitative interpretation of residual anomalies by 3D modelling permit characterizing the intrusive light body situated at Bipindi. The multi-scale evaluation of maxima of gradients shows that the modelling of the intrusive light body of Bipindi can be done without the problem of interference of anomalies from different sources. The 3D model of Bipindi zone shows two dissymmetrical blocks of the same type of rock with a density contrast of -0.095 g·cm-3 in comparison with the density of the surrounding metamorphic rocks. The two blocks are at a distance about 3 km from one to another. The upper surfaces of these blocks lie at a depth between 1 and 2 km. Their lower surfaces have two landings; one lies at a depth of about 8 km and another at a depth about 14 km. A consideration of the density of the modelled body, of the ranges of densities of specific rocks present in the general region indicates that the body may be composed of nepheline syenites. The intrusive body of Bipindi is situated in a senestral shear zone. The area situated between the two blocks of this intrusive body may be indicated for a detail study in the domain of mineral research.

Characterization of Density and Porosity of Rocks Samples from Ogun State of Nigeria

Knowledge of densities of rocks is essential in petrological and geological studies, interpretation of gravity anomalies and ground water exploration.Fifty samples were collected from Abeokuta, Sagamu, Odeda, Ewekoro, Ibese Yewa North Local Government, Ijebu East Local Government and Obafemi Owode in Ogun State and dry bulk density, saturated density, porosity and particle density were determined. Results showed that Ewekoro shale has the lowest mean density of 1.35g/cm3 while Ibese, Yewa North Local Government limestone has the highest mean density of 3.9g/cm3.Porosity ranges from 0.030 to 0.640 with the granite in Odeda local government having the highest porosity and the shale in Ewekoro local government having the lowest porosity.Mean porosity for all the rocks samples in the seven Local Government Areas was 0.34. Test of significance revealed that there is significant relationship in the values of density of rock samples within the state.

Prediction of gravity anomalies for geophysical exploration

Interpretation of gravity anomalies (determined on the earth’s surface) reveals information on mineral resources beneath the earth. The density of gravity stations (where gravity anomalies are determined) is critical to the successful interpretation of these anomalies. Where the density of the available gravity anomalies is not enough, for a particular purpose of geophysical exploration, more gravity stations can be established within the surveyed area and the gravity anomalies observed for these stations. In some cases, where observations of gravity anomalies are not possible due, probably, to inaccessibility of the newly chosen gravity stations, the required gravity anomalies for such stations can be estimated (predicted). Currently, classical least squares technique is used to accomplish such task. However, the technique does not produce optimum results because its formulation assumes that the observed gravity anomalies, used for the prediction, are error free, whereas, all observed quantities are affected by random errors. Therefore, in this study, an attempt is made to carry out prediction of gravity anomalies for geophysical exploration using least squares collocation technique. This is considered to be a better alternative because its formulation takes the presence of random errors of observations in the observed quantities into consideration and makes provision for filtering out these errors while predicting the signals of interest at the required number of stations.

Three-dimensional assembly technologies for the interpretation of gravimetric data

The assembly method of solving a three-dimensional ore-type inverse problem is considered. The method is based on the unity of a grid (finite-element) approximation of a dense medium and a special type of structuring the iteration process of selection which makes it possible to effectively observe all a priori restrictions on the model of field sources. The examples of interpretations of gravity anomalies are presented.

Three-dimensional deep density model of the Pautovaya Horst and its geological interpretation (Northeast Russia)

The quantitative interpretation of gravity anomalies reveals a concealed granitoid pluton within the core of the Pautovaya Horst (southeastern extremity of the Iniyali-Debin Synclinorium, northeast Russia). Using a new interpretative gravimetry method, a 3D density model is developed for the study area with its geological-petrological explanation. The model satisfies the necessary condition that the solution of inverse gravimetry problems provides the true solution.

Interpretation of Gravity Anomalies with the Normalized Full Gradient (NFG) Method and an Example

The Normalized Full Gradient (NFG) method which was put forward about 50 years ago has been used for downward continuation of gravity potential data, especially in the former Union of Soviet Socialist Republics. This method nullifies perturbations due to the passage of mass depth during downward continuation. The method depends on the downwards analytical continuation of normalized full gradient values of gravity data. Analytical continuation discriminates certain structural anomalies which cannot be distinguished in the observed gravity field. This method has been used in various petroleum and tectonic studies. The Trapeze method was used for the determination of Fourier coefficients during the application of this method. No other techniques for calculating these coefficients have been used. However, the Filon method was used for the determination of Fourier coefficients during the application of the NFG method in this work. This method, rather than the Trapeze method, should be preferred for indicating abnormal mass resources at the lower harmonics. In this study, the NFG method using the Filon method has been applied the first time to theoretical models of gravity profiles as example field at the Hasankale-Horasan petroleum exploration province where successful results were achieved. Hydrocarbon presence was shown on the NFG sections by the application of NFG downward continuation operations on theoretical models. Important signs of hydrocarbon structure on the NFG section for field and model data at low harmonics are obtained more effectively using this method.

INV2P5DSB—A code for gravity inversion of 2.5-D sedimentary basins using depth dependent density

A program, INV2P5DSB, has been developed to simultaneously estimate the depth of a 2.5-D sedimentary basin and regional background from observed gravity anomalies. The density contrast is assumed to be varying parabolically with depth above the basement interface. The main program is supported with two subroutines GRAV2P5D and SIMEQ and a function GPRM. The analysis of gravity anomalies due to a synthetic model of a 2.5-D sedimentary basin with and without regional background illustrates that the code is insensitive to regional background. Interpretation of gravity anomalies over the Gediz graben, Western Anatolia with parabolic density function yields geologically plausible model.

The roots of Mt. Vesuvius deduced from gravity anomalies

The structures of the Somma-Vesuvius volcanic complex are modelled on the basis of the interpretation of gravity anomalies obtained from data available in the literature and acquired along a new profile along the coastline from Naples to Castellammare di Stabia. In order to highlight the contribution of shallow crustal structures, the residual anomalies were considered. A marked gravity low was recognised in the eastern sector of Vesuvius. Furthermore data interpretation was carried out along two profiles centred on the low gravity region in question: a first profile crossing the Vesuvius crater in direction WNW-ESE, and a second one in NNE-SSW direction. The 2 ½ D model obtained reveals a crustal structure characterised by sediments of 2.3 Mg/m3 density, overlying bedrock with a density of 2.6 Mg/m3. Near the volcanic system the model becomes more complex due to the presence of light sediments with a density of 2.1 Mg/m3 overlying a body with a density of 2.4 Mg/m3 which extends into depth. The latter is thought to be closely related to the zone of magma ascent developed along the volcanic axis. Along the coast the volcanic component is reduced and the model shows that the layer with a density of 2.3 Mg/m3 ranges in thickness from 0 to about 3500 m. An additional body between 1500 and 3000 m with a density of 2.4 Mg/m3 was considered in order to account for the slight rise in the residual anomaly in the area in the vicinity of Mt. Vesuvius.

Structure of Réunion Island (Indian Ocean) inferred from the interpretation of gravity anomalies

Réunion is a volcanic edifice whose origin is related to a hot spot in the Indian Ocean. Only 3% of its volume is emergent. Many geological and geophysical studies were carried out on Réunion Island during the 1980's but few of them allow study of the internal structure of the edifice. Several gravity surveys have been carried out on the island since 1976 and we have compiled the available data set. The lack of data on the western side of the island led us to conduct a regional survey in 1993 to obtain a more homogeneous distribution of the stations. Computation of Bouguer anomalies for different correction densities accounts for the variable density of the rocks constituting the edifice and provides a distribution of gravity anomalies interpreted as dense bodies of intrusive rocks inside the edifice. Two very large intrusive complexes can be unambiguously recognised: one beneath Piton des Neiges and one beneath the Grand Brûlé area. Both have been penetrated by geothermal exploration drill holes and the first is also known from outcrop observations. 2.5D simple models were constructed to reveal the geometry and extent of the buried intrusives. They are deeply rooted, extending several kilometres below sea level, and extensive (20–25 km long and 10–13 km wide for the Piton des Neiges complex, 12–15 km long and some kilometres wide for the Grand Brûlé complex). The development of such complexes implies that the activity of the two volcanic centres was long lasting and remained stable while the volcanoes were growing. The Grand Brûlé complex has been interpreted as relics of an old volcano named Alizés Volcano. The interpretation of the gravity maps suggests the presence of a ridge of dense rocks to the North of the axis joining the centres of Piton des Neiges and Piton de la Fournaise volcanoes. By analogy with the other structures, 2.5D models show that this structure would culminate between 0 and 1 km below sea level and be 15 km wide. This complex induces a maximum anomaly in Takamaka Valley and we thus propose to name it Takamaka Volcano. No geological evidence of the nature of these dense rocks is available but the ridge coincides with structures revealed by magnetic and seismic data. Interpretation of the Bouguer anomaly maps suggests that the inner gravity structure of Piton de la Fournaise is not characterised by the presence of a voluminous dense body but probably by more restricted concentrations of dense rocks. Some structures can be recognised: along the present NE and SE rift zones and in the previous central part of Piton de la Fournaise to the West of the present summit. The recent eastward migration of the centre of activity of Piton de la Fournaise accounts for the lack of a large positive anomaly beneath the active craters.