Interpretation Of Gravity Data Research Articles

Joint interpretation of gravity and magnetic data in the Kolárovo anomaly region by separation of sources and the inversion method of local corrections

Abstract We present a new interpretation of the Kolárovo gravity and magnetic anomalies in the Danube Basin based on an inversion methodology that comprises the following numerical procedures: removal of regional trend, depth-wise separation of signal of sources, approximation of multiple sources by 3D line segments, non-linear inversion based on local corrections resulting in found sources specified as 3D star-convex homogenous bodies and/or 3D contrasting structural contact surfaces. This inversion methodology produces several admissible solutions from the viewpoint of potential field data. These solutions are then studied in terms of their feasibility taking into consideration all available tectono-geological information. By this inversion methodology we interpret here the Kolárovo gravity and magnetic anomalies jointly. Our inversion generates several admissible solutions in terms of the shape, size and location of a basic intrusion into the upper crust, or the shape and depth of the upper/lower crust interface, or an intrusion into the crystalline crust above a rise of the mafic lower crust. Our intrusive bodies lie at depths between 5 and 12 km. Our lower crust elevation rises to 12 km with and 8 km without the accompanying intrusion into the upper crust, respectively. Our solutions are in reasonable agreement with various previous interpretations of the Kolárovo anomaly, but yield a better and more realistic geometrical resolution for the source bodies. These admissible solutions are next discussed in the context of geological and tectonic considerations, mainly in relation to the fault systems.

Локальное прогнозирование рудоносных интрузивных образований на основе современных технологий интерпретации геопотенциальных полей

The article shows the effectiveness of the modern techniques of gravity and magnetic data interpretation in geological difficult terrain conditions of the northwest of the Siberian platform. It describes the results of local assessment of the deep-laying intrusive bodies bearing a copper-nickel mineralization obtained with the use of probabilistic and statistical methods, and an interpretative tomography transformation.

Characteristics of the crystalline basement beneath the Ordos Basin: Constraint from aeromagnetic data

Aeromagnetic anomaly zonation of the Ordos Basin and adjacent areas was obtained by processing high-precision and large-scale aeromagnetic anomalies with an approach of reduction to the pole upward continuation. Comparative study on aeromagnetic and seismic tomography suggests that aeromagnetic anomalies in this area are influenced by both the magnetic property of the rock and the burial depth of the Precambrian crystalline basement. Basement depth might be the fundamental control factor for aeromagnetic anomalies because the positive and negative anomalies on the reduction to the pole-upward-continuation anomaly maps roughly coincide with the uplifts and depressions of the crystalline basement in the basin. The results, together with the latest understanding of basement faults, SHRIMP U-Pb zircon dating of metamorphic rock and granite, drilling data, detrital zircon ages, and gravity data interpretation, suggest that the Ordos block is not an entirety of Archean.

Hidden equivalence and efficiency of gravity data interpretation

Analytical properties of the solutions of the inverse problem of gravimetry are studied in the context of the approximative approach and method of linear integral representations. A new effect of the inheritance of specific analytical properties in the solutions, which is named the effect of hidden equivalence, is revealed and investigated. This effect significantly influences the interpretation informativity in the complex models of a medium, and it should be taken into account in the geological interpretation of gravity data. Hidden equivalence is studied for both linear and nonlinear inverse problems. As an example of a nonlinear problem, the inverse problem of structural gravimetry is analyzed. The correlation between the boundary equations and boundary values of the harmonic functions is demonstrated. Methods are suggested to allow for the effects that occur by expanding the approximative approach for complex conditions (the models of the media with spatially distributed parameters) by the dynamical and criterion principles.

A Gravity Survey and Data Interpretation of Tawke Structure (Iraqi Kurdistan Region)

In this study 780 gravity measurements were conducted in the Tawke area along the paved and accessible unpaved roads. The obtained gravity data was subjected to the necessary corrections and analyses qualitatively and quantitatively. For that purpose the Bouguer anomaly map was transformed analytically to regional and residual fields. Moreover two profiles trending NS were limited to a 2-D gravity models. The main structural features of the studied area as are recognized utilizing two main lines of evidence, namely, geological map and geophysical interpretations have one distinctive trend; it is the E-W Taurus trend. The Bouguer map of the Tawke area shows gravity high in the central part. It trends E-W. It is indicative of the main anticline. Two gravity lows indicating the two synclines bounding the structure from north and south are clearly shown. Six faults are inferred from gravity data. When inferred faults and gravity lows and highs are plotted on the geological map they show good agreement. A considerable density contrast of 0.36 gm/cm3 was observed between the Jeribe, Ana, Pila Spi, and Avana formation group against Lower Bakhtiari, Upper Fars and Lower Fars formation group and used in the modeling of two profiles. One main positive and two negative features characterize these profiles. The positive anomaly represents Tawke Anticline, the south negative anomaly represents Khabore syncline and the other Zakho syncline. Geological model for profiles show that the Tawke Anticline is an asymmetrical Anticline producing a structural trap by tilting of the Miocene formations comprising sealing rocks (Fatha Formation) and reservoir rock represented by the Jeribe carbonates. Two small faults on the northern limb of Tawke Anticline have indications on the surface as seepages of oil.

GRAVITY EVIDENCE ON THE THUAN HAI-MINH HAI FAULT AND FAULTS SYSTEM IN SOUTH AND SOUTH -CENTRAL VIETNAM CONTINENTAL SHELF

The South and South-central Vietnam continental shelf is a region with the diversely complicated conditions of geology. In spite of over the past many years of investigation, the fault systems in many places have remained poorly understood. In this paper, the author has studied and applied some techniques for interpretation of the newest gravity data and others, which are frequency filtering, gravity horizontal gradient and maximum horizontal gradient, in order to determine plainly the main faults systems in the South and South-central Vietnam continental shelf and also clear up whether or not as well as the location of the fault named ‘Thuan Hai - Minh Hai'.

GRAVITY EVIDENCE ON THE THUAN HAI-MINH HAI FAULT AND FAULTS SYSTEM IN SOUTH AND SOUTH -CENTRAL VIETNAM CONTINENTAL SHELF

The South and South-central Vietnam continental shelf is a region with the diversely complicated conditions of geology. In spite of over the past many years of investigation, the fault systems in many places have remained poorly understood. In this paper, the author has studied and applied some techniques for interpretation of the newest gravity data and others, which are frequency filtering, gravity horizontal gradient and maximum horizontal gradient, in order to determine plainly the main faults systems in the South and South-central Vietnam continental shelf and also clear up whether or not as well as the location of the fault named ‘Thuan Hai - Minh Hai'.

Use of gravimetry in detailed geophysical prospection of potential geothermal energy exploitation sites: Case studies in the Czech Republic

Methodological aspects of gravity data processing and interpretation for assessing the possibility of exploiting geothermal energy for electricity production are presented. Gravimetry is one of the most economical and most effective geophysical methods for exploration of deep geologic structures. Unfortunately the gravity inversion ambiguity problem introduces the necessity of constraining gravity models by other independent data. The procedures interpreting 2D gravity models constrained by the results of an integration of other suitable geophysical methods are described in three case studies in the Czech Republic. Gravity, reflection and refraction seismics, magnetometry, resistivity, and vertical electric sounding data were collected along profiles at each locality. Two-dimensional gravity models based on complete Bouguer anomalies were constructed taking into account the results of processing and interpretation of other geophysical data. Relevant factors influencing the potential for geothermal energy exploitation were evaluated from the interpreted geophysical models. Promising zones suitable for future exploration were established at each locality. The most important structures for locating geothermal production and injection drills are deep tectonic fault zones which can freely support fluid flow. The presented examples show that the gravity anomalies caused by fracture zones can be significant enough to unambiguously interpret and evaluate their importance in terms of hydrological permeability. In addition, the presented case histories demonstrate that two-dimensional geophysical surveys can effectively be used to reduce exploration costs in geothermal projects.

Application of the continuous wavelet transform of gravity and magnetic data to estimate sub‐basalt sediment thickness

ABSTRACTThe Continuous Wavelet Transform was recently proposed for the interpretation of gravity and magnetic potential data. We utilize the Continuous Wavelet Transform of gravity and magnetic data to address one of the most common issues in exploration geophysics: mapping of sub‐basaltic sedimentary strata. We observe that the magnetic response of the basaltic layer is dominant in a three‐layer case of a basalt‐sediment‐basement, whereas the gravity signal is dominated by the base of the sediment. Thus the Continuous Wavelet Transform of the magnetic data is related to the thickness of the basalt and the Continuous Wavelet Transform of the gravity data is related mostly to the bottom of the sediment. These observations are demonstrated with a synthetic model and a few field examples. Derived depths using Continuous Wavelet Transform are in good agreement with known vertical cross‐sections. Therefore, Continuous Wavelet Transform analysis of both gravity and magnetic data offers a possibility for primary information of sub‐basaltic sediment thickness, which can provide a basis for further detailed modelling.

Cenozoic basement structure of the South China Sea and adjacent areas by modeling and interpreting gravity data

The South China Sea and adjacent areas is a large region with diversely complicated geological conditions. In spite of investigations carried out over the past many years, the marine geological structure in many places has remained poorly understood because of the deficient data, a thick seawater layer as well as of the sensitive areas among the countries in the region.

Interpretation of gravity and magnetic data and development of two-dimensional cross-sectional models for the Border Ranges fault system, south-central Alaska

Extensive Quaternary glacial cover and a lack of dense geophysical data within the Cook Inlet basin (CIB) of south-central Alaska make locating and determining the geometry of the Border Ranges fault system (BRFS), a major feature of the Alaska-Aleutian forearc region, difficult. We use recently collected gravity data, available aeromagnetic data, and other geophysical information as constraints to develop plausible two-dimensional cross-section models that better image the BRFS and related geologic structures of the CIB. Our integrated models show a thick sequence of late Mesozoic sedimentary rocks and the Peninsular terrane basement (6–20 km depth) overlying a serpentinized body at a depth of 16–34 km. The late Mesozoic rocks and serpentinite are interpreted as possible sources of the south Alaska magnetic high over the CIB. The eastern boundaries of the CIB are characterized by gravity and magnetic highs of the emplaced Border Range ultramafic and mafic assemblages (BRUMA). Formation of the BRUMA may be related to the serpentinized rocks that composed a Jurassic oceanic arc. Our models constrain the BRFS as a structural boundary between the overthrusted BRUMA and the Chugach terrane to the east. The BRFS dips 50°–70° toward the west-northwest and extends to at least 15 km. The BRFS may penetrate steeply or shallowly to a form a décollement at greater depths. A model that includes underplated sediments at the base of the accretionary complex (12–40 km) is consistent with the observed gravity low over the Chugach Mountains (Chugach terrane). The underplating may be associated with the subducting and shortening of the Yakutat microplate in south-central Alaska.

Imaging of subsurface lineaments in the southwestern part of the Thrace Basin from gravity data

Linear anomalies, as an indicator of the structural features of some geological bodies, are very important for the interpretation of gravity and magnetic data. In this study, an image processing technique known as the Hough transform (HT) algorithm is described for determining invisible boundaries and extensions in gravity anomaly maps. The Hough function implements the Hough transform used to extract straight lines or circles within two-dimensional potential field images. It is defined as image and Hough space. In the Hough domain, this function transforms each nonzero point in the parameter domain to a sinusoid. In the image space, each point in the Hough space is transformed to a straight line or circle. Lineaments are depicted from these straight lines which are transformed in the image domain. An application of the Hough transform to the Bouguer anomaly map of the southwestern part of the Thrace Basin, NW Turkey, shows the effectiveness of the proposed approach. Based on geological data and gravity data, the structural features in the southwestern part of the Thrace Basin are investigated by applying the proposed approach and the Blakely and Simpson method. Lineaments identified by these approaches are generally in good accordance with previously-mapped surface faults.

A priori information and admissible complexity of the model of anomalous objects in the solution of inverse problems of gravity

We study the problem of selecting an admissible complexity of the model of gravity sources, which largely governs the quality of the inverse problem solution and has been poorly explored to date. The reasons why the interpretation theory of anomalous gravity has not yet suggested clear guidelines on this subject are analyzed. Among these reasons is the inadequacy of the prerequisites that are used in the theory of inverse problems to the real conditions of a geophysical experiment. The standard scheme of constructing the algorithms for quantitative interpretation of gravity data is expanded by the missing elements that are necessary for the solution of the problem. The most important role in the improved scheme is played by the algorithmic approaches that allow the interpreter to assess the size of the set of admissible solutions of the inverse problem, the corrections for the inadequacy of the used model of gravity sources to the actual physical distribution of the anomalous masses, and the new forms of representation of the results, which provide monotonic dependence of the solution’s quality on the amount of the a priori information.

Interpretation of gravity data to delineate structural features connected to low-temperature geothermal resources at Northeastern Portugal

A great number of low-temperature geothermal fields occur in Northern-Portugal related to fractured rocks. The most important superficial manifestations of these hydrothermal systems appear in pull-apart tectonic basins and are strongly conditioned by the orientation of the main fault systems in the region. This work presents the interpretation of gravity gradient maps and 3D inversion model produced from a regional gravity survey. The horizontal gradients reveal a complex fault system. The obtained 3D model of density contrast puts into evidence the main fault zone in the region and the depth distribution of the granitic bodies. Their relationship with the hydrothermal systems supports the conceptual models elaborated from hydrochemical and isotopic water analyses. This work emphasizes the importance of the role of the gravity method and analysis to better understand the connection between hydrothermal systems and the fractured rock pattern and surrounding geology. ► Analysis of a gravity survey over an area with important hydrothermal systems. ► Directional derivatives lead to detection of faults related to hydrothermal system. ► The 3D model gives an image of the evolution in depth of the density contrast. ► Results support the conceptual models from hydrochemical and isotopic analyses. ► This work stresses the role of the gravity method to study hydrothermal systems

A least-squares window curves method to interpret gravity data due to dipping faults

We have developed a least-squares method to determine simultaneously the depth and the dip angle of a buried fault from first moving average residual gravity anomalies using filters of successive window lengths. The method involves using a relationship between the depth and the dip angle of the source and a combination of windowed observations. The relationship represents a family of curves (window curves). For a fixed window length, the depth is determined for each dip angle value by solving one nonlinear equation of the form f(z) = 0 using the least-squares method. The computed depths are plotted against the dip angle values representing a continuous curve. The solution for the depth and the dip angle of the buried fault is read at the common intersection of the window curves. The method involves using a dipping faulted thin slab model convolved with the same moving average filter as applied to the observed data. As a result, this method can be applied to residuals as well as to measured gravity data. The method is applied to theoretical data with and without random errors. The validity of the method is tested on gravity data from Egypt. In all cases examined, the model parameters obtained are in good agreement with the actual ones and with those given in the published literature.

V. N. Strakhov's mounting approach to solving inverse problems of gravity prospecting: the current state and real possibilities

Strakhov's mounting algorithms for solving nonlinear inverse gravity problems are considered from the point of view of the concept of problem-oriented methods of conditional minimization, which make up the core of any technology for quantitative interpretation of gravity data. The comparative analysis of editing algorithms has revealed their main advantage, which lies in the ability to bypass the richly acute angles in the theory and practice of interpretation of potential fields, have become a stumbling block for most known approaches to solving inverse problems, which rely on classical methods to solve the conditions of no-extremal problems . At the same time, the question of the actual resolution of the editing method remains unresolved, the answer to which can only be given by a large-scale computational experiment. In this article, this gap is partially filled.

Lithospheric structure in Central Europe: Integrated geophysical modelling

We present four new 2D lithosphere-scale transects crossing central Europe from the West European Platform in the North to the Aegean Sea in the South and from the Adriatic Sea in the West to the East European Platform in the East. Modelling is based on the joint interpretation of gravity, geoid, topography and surface heat flow data with temperature-dependent density. Wherever possible, crustal structure is constrained by seismic data. The thickness of the lithosphere decreases from the older and colder platforms to the younger and hotter Pannonian Basin with a maximum thickness under the Eastern and Southern Carpathians. Similar to earlier studies, the thickness of the Carpathian arc lithosphere varies between 150km in the North (the Western Carpathians) and about 300km in the Vrancea zone (the Eastern and Southern Carpathian junction). In the Platform areas it is between 120 and 150km and in the Pannonian Basin it is about 70km. The lithosphere thickens strongly underneath the Transylvanian Basin reaching locally values of nearly 200km which is thicker than in earlier studies. New results are given for the Southern Carpathians and the Moesian Platform. The models show that the Moesian Platform is overthrust from the North by the Southern Carpathians and from the South by the Balkanides and characterised by bending of this platform. In all transects, the thickest crust is found underneath the Carpathian Mountains or, as in the case of the Vrancea area, under their immediate foreland. The thickest crust outside the orogens is modelled for the Moesian Platform with Moho depths of up to 45km. The thinnest crust is located under the Pannonian Basin with about 26–27km.

Interpretation of gravity data by the continuous wavelet transform: The case of the Chad lineament (North-Central Africa)

Abstract A slightly bended gravity high along the Chad lineament in Central North Africa is analyzed and interpreted by the continuous wavelet transform (CWT) method. We use scale normalization on the continuous wavelet transform, allowing analysis of the gravity field in order to determine the sources at different depths. By focusing on homogenous standard sources, such as sphere or cube, horizontal cylinder or prism, sheet and infinite step, we derive the relationships between the source depth and pseudo-wavenumber. Then the source depth can be recovered from tracing the maximal values of the modulus of the complex wavelet coefficients in the CWT-based scalograms that are function of the pseudo-wavenumber. The studied area includes a central gravity high up to 75 km wide, and a secondary high that occurs at the southern part of the anomaly. The interpretation of the depth slices and vertical sections of the modulus maxima of the complex wavelet coefficients allows recognition of a relatively dense terrane located at middle crustal levels (10–25 km depth). A reasonable geological model derived from the 2.5D gravity forward modelling indicates the presence of high density bodies, probably linked to a buried suture, which were thrusted up into the mid-crust during the Neo-Proterozoic terrane collisions between the Saharan metacraton and the Arabian-Nubian shield. We conclude that the Chad line delineates a first order geological boundary, missing on the geologic maps.

Modelagem gravimétrica no Lineamento Guapiara, sul do Estado de São Paulo

O Lineamento Guapiara é uma importante feição geológica no Estado de São Paulo, Brasil, associada ao vulcanismo mesozoico na Bacia do Paraná. O presente trabalho teve por finalidade a interpretação e modelagem de dados gravimétricos coletados entre as cidades de Capão Bonito e Apiaí (SP), em escala de semi-detalhe, para reconhecimento da geometria da compartimentação crustal ao longo desse lineamento. Os mapas da anomalia Bouguer na área revelam uma notável anomalia gravimétrica positiva de direção NW-SE, que coincide com a orientação da maior concentração de diques associados ao Lineamento Guapiara. O modelo crustal elaborado a partir das anomalias caracteriza um significativo afinamento da crosta na área e corrobora trabalhos anteriores. Esse afinamento seria decorrência da subida do manto, reflexo da atividade epirogenética gerada pela propagação dos esforços tectônicos divergentes que ocorreram na margem da Placa Sul-Americana durante o Cretáceo.

Interpretation of aeromagnetic and gravity data from western New South Wales – identifying basin and basement geology

SUMMARY High resolution aeromagnetic data and regional gravity data have been interpreted by the Geological Survey of New South Wales for the western half of the state. The region lies between the Curnamona Province and Koonenberry Belt in the far west, and the Lachlan Orogen in the central west, and it extends from the Queensland border to the Victorian border. Overlapping sedimentary basins – the Darling Basin, Eromanga Basin, and Murray Basin – cover virtually all the basement rocks throughout the region. The primary objective is to identify lithological and structural variations within the basement layer, using them as the basis for creating an interpretive geophysical–geological map of the basement surface. Samples and information from drill holes (including water bores) provided limited geological control on the basement layer. Major results are improved classification of the numerous interpreted granite intrusions and recognition of major structures. Regions have been identified where basement occurs at depths viable for mineral exploration. The detailed study has also defined subtle features in the aeromagnetic data that relate to basin structure and stratigraphy. These sequences include the Cobar Supergroup and the Darling Basin, and provide information which could be applicable to petroleum exploration, and groundwater and geosequestration investigations.