Gravity Anomaly Maps Research Articles

THE USE OF THE LOGARITHMIC IMAGE PROCESSING APPLIED TO ANALYSING BOUGUER GRAVITY ANOMALY MAP (TANGIER-TETUAN’S AREA -MOROCCO)

Image processing is a powerful tool for the enhancement of edges in images used in the interpretation of geophysical potential field data. Arial and terrestrial gravimetric surveys were carried out in the region of Tangier-Tetuan. From the observed and measured data of gravity Bouguer gravity anomalies map was prepared. This paper reports the results and interpretations of the transformed maps of Bouguer gravity anomaly of the Tangier-Tetuan area using the logarithmic image processing. Filtering analysis based on classical image process was applied. Operator image process like the logarithmic operator and the associated gamma correction tool are used. This paper also present the results obtained from this image processing analysis of the enhancement edges of the Bouguer gravity anomaly map of the Tangier-Tetuan zone.

Preparation of the Free‐Air Gravity Anomaly Map in the Land and Seas of China and Adjacent Areas Using Multi‐Source Gravity Data and Interpretation of the Gravity Field

AbstractMulti‐source gravity data including measurements on land, shipboard surveys, satellite altimetry, Bouguer gravity anomalies and EGM2008 global gravity model data were collected and analyzed for compiling the free‐air gravity anomaly map on a scale of 1:5000000 in the land and seas of China and adjacent areas. The data merging and integration method was established based on analysis of the accuracy and resolution of different sources of gravity data. Evenly distributed land gravity data was gridded in flat land areas. A synthetic method using Bouguer gravity and high precision topographic data to calculate the free‐air gravity data was proposed to fill the land areas where the gravity measurement is hardly or impossible to implement due to complex terrain. Data comparison between the synthetic free‐air gravity data, measured data and EGM2008 data proves that using the synthetic data is a better choice for filling the areas lacking real data. This method was also applied to the continental and island areas outside mainland China where there are no real data collected. The STD analysis method was applied to the northern part of the South China Sea to compare the accuracy of three independent gravity data, the shipboard gravity data and two kinds of satellite altimetry derived gravity data, the SS series from Scripps, US and DNSC08GRA from DTU, Denmark. SSV18.1 and DNSC08GRA data, which show very good consistence in terms of accuracy and resolution. The standard deviation of the latest two satellite gravity data in the SCS is less than 2 mGal, which is a major improvement compared with the previous SS16.1. With the rectification of shipboard gravity data, the wide coverage of satellite gravity data could be used for small‐scale free‐air gravity mapping. In the coastal zone, the EGM2008 global potential model was used as a reference field providing a seamless gravity transition from land to ocean. Wavelet transform and multiscale analyses were applied to decompose the free‐air gravity field in China and its adjacent areas to help gravity field interpretation. Eight anomaly areas were demarcated by three horizontal and four vertical first‐order gravity gradient belts. Anomaly subareas and the second‐order gravity gradient belts were delineated in the gravity field to provide evidence for establishing the tectonic framework in the block tectonic scheme.

CNNEDGEPOT: CNN based edge detection of 2D near surface potential field data

All anomalies are important in the interpretation of gravity and magnetic data because they indicate some important structural features. One of the advantages of using gravity or magnetic data for searching contacts is to be detected buried structures whose signs could not be seen on the surface. In this paper, a general view of the cellular neural network (CNN) method with a large scale nonlinear circuit is presented focusing on its image processing applications. The proposed CNN model is used consecutively in order to extract body and body edges. The algorithm is a stochastic image processing method based on close neighborhood relationship of the cells and optimization of A, B and I matrices entitled as cloning template operators. Setting up a CNN (continues time cellular neural network (CTCNN) or discrete time cellular neural network (DTCNN)) for a particular task needs a proper selection of cloning templates which determine the dynamics of the method. The proposed algorithm is used for image enhancement and edge detection.The proposed method is applied on synthetic and field data generated for edge detection of near-surface geological bodies that mask each other in various depths and dimensions. The program named as CNNEDGEPOT is a set of functions written in MATLAB software. The GUI helps the user to easily change all the required CNN model parameters. A visual evaluation of the outputs due to DTCNN and CTCNN are carried out and the results are compared with each other. These examples demonstrate that in detecting the geological features the CNN model can be used for visual interpretation of near surface gravity or magnetic anomaly maps.

Extraction of lineaments from gravity anomaly maps using the gradient calculation: Application to Central Anatolia

Linear anomalies are important in the interpretation of gravity and magnetic data because they indicate some important structural features. In this study, a method is proposed to extract lineaments which may be difficult to recognize. It is based on the convolution between the templates obtained from the second horizontal derivative of the theoretical anomaly due to a truncated horizontal plate and the gravity anomaly. Local maxima of the gradient magnitude determine lineaments in a gravity anomaly map. The method is used in the edge detection of geological structures closer to the surface, which are masked by other structures with various depths and dimensions. A source code in Matlab format for the proposed algorithm is implemented. Firstly, it was tested on synthetic examples and satisfactory results were obtained. Secondly, it was tested on gravity-anomaly maps of the Tuzgolu (Salt Lake) basin in Central Anatolia, Turkey. Subsequently, the proposed algorithm and normalized standard deviation (NSTD), another edge-detection filter, were evaluated together to obtain the horizontal location of potential-field source boundaries. A visual evaluation of the outputs was carried out and the results compared with each other. These examples demonstrate that the proposed method can be used in the visual interpretation of gravity-anomaly maps in order to detect geological features such as major and minor faults.

Detailed gravity survey to help seismic microzonation: Mapping the thickness of unconsolidated deposits in Ottawa, Canada

Abstract In this study, measurements of gravity were made to map and model the thickness of Quaternary deposits (sand and clay) overlying Ordovician limestones in a suburb of Ottawa (Orleans, Ontario). Because ground motion amplification is partly related to the thickness of unconsolidated deposits, this work helps refine the assessment of the earthquake damage potential of the area. It also helps the mapping of clay basins, which can locally exceed 100 m in thickness, where ground motion amplification can occur. Previous work, including well log data and seismic methods, have yielded a wealth of information on near-surface geology in Orleans, thereby providing the necessary constraints to test the applicability of gravity modeling in other locations where other methods cannot always be used. Some 104 gravity stations were occupied in an 8 × 12 km test area in the Orleans. Stations were accurately located with differential GPS that provided centimetric accuracy in elevation. Densities of the unconsolidated Quaternary deposits (Champlain Sea clay) determined on core samples and densities determined on limestone samples from outcrops were used to constrain models of the clay layer overlying the higher density bedrock formations (limestone). The gravity anomaly map delineates areas where clay basins attain > 100 m depth. Assuming a realistic density for the Champlain Sea clays (1.9–2.1 g/cm3), the thickness over the higher density bedrock formations (Ordovician carbonate rocks) was modeled and compared with well logs and two seismic reflection profiles. The models match quite well with the information determined from well logs and seismic methods. It was found that gravity and the thickness of unconsolidated deposits are correlated but the uncertainties in both data sets preclude the definition of a direct correlation between the two. We propose that gravity measurements at a local scale be used as an inexpensive means of mapping the thickness of unconsolidated deposits in low-density urban areas. To obtain meaningful results, three conditions must exist. Firstly, elevations of gravity stations must be measured accurately using differential GPS; secondly, that the regional gravity field must be well defined, and thirdly, that the local geology be simple enough to be realistically represented with a two-layer model.

Chapter 3 Circum-Arctic mapping project: new magnetic and gravity anomaly maps of the Arctic

Abstract New Circum-Arctic maps of magnetic and gravity anomalies have been produced by merging regional gridded data. Satellite magnetic and gravity data were used for quality control of the long wavelengths of the new compilations. The new Circum-Arctic digital compilations of magnetic, gravity and some of their derivatives have been analyzed together with other freely available regional and global data and models in order to provide a consistent view of the tectonically complex Arctic basins and surrounding continents. Sharp, linear contrasts between deeply buried basement blocks with different magnetic properties and densities that can be identified on these maps can be used, together with other geological and geophysical information, to refine the tectonic boundaries of the Arctic domain.

Structural and tectonic interpretation of geophysical data along the Eastern Continental Margin of India with special reference to the deep water petroliferous basins

The study area encompasses the Eastern Continental Margin of India (ECMI) and the adjoining deep water areas of Bay of Bengal. The region has evolved through multiple phases of tectonic activity and fed by abundant supply of sediments brought by prominent river systems of the Indian shield. Detailed analysis of total field magnetic and satellite-derived gravity data along with multi channel seismic reflection sections is carried out to decipher major tectonic features, basement structure, and the results have been interpreted in terms of basin configuration and play types for different deep water basins along the ECMI. Interpretation of various image enhanced gravity and magnetic anomaly maps suggest that in general, the ENE–WSW trending faults dominate the structural configuration at the margin. These maps also exhibit a clear density transition from the region of attenuated continental crust/proto oceanic crust to oceanic crust based on which the Continent Ocean Boundary (COB) has been demarcated along the margin. Basement depths estimated from magnetic data indicate that the values range from 1 to 12 km below sea level and deepen towards the Bengal Fan in the north and reveal horst–graben features related to rifting. A comparison of basement depths derived from seismic data indicates that in general, the basement trends and depths are comparable in Cauvery and Krishna–Godavari basins, whereas, in the Mahanadi basin, basement structure over the 85°E ridge is clearly revealed in seismic data. Further, eight multichannel seismic sections across different basins of the margin presented here reveal fault pattern, rift geometries and depositional trends related to canyon fills and channel–levee systems and provide a basic framework for future petroleum in this under explored frontier.

Tectonic modeling of Konya-Beysehir Region (Turkey) using cellular neural networks

In this paper, to separate regional-residual anomaly maps and to detect borders of buried geological bodies, we applied the Cellular Neural Network (CNN) approach to gravity and magnetic anomaly maps. CNN is a stochastic image processing technique, based optimization of templates, which imply relationships of neighborhood pixels in 2-Dimensional (2D) potential anomalies. Here, CNN performance in geophysics, tested by various synthetic examples and the results are compared to classical methods such as boundary analysis and second vertical derivatives. After we obtained satisfactory results in synthetic models, we applied CNN to Bouguer anomaly map of Konya-Beysehir Region, which has complex tectonic structure with various fault combinations. We evaluated CNN outputs and 2D/3D models, which are constructed using forward and inversion methods. Then we presented a new tectonic structure of Konya-Beysehir Region. We have denoted (F1, F2, , F7) and (Konya1, Konya2) faults according to our evaluations of CNN outputs. Thus, we have concluded that CNN is a compromising stochastic image processing technique in geophysics.

Inferring radial models of mantle viscosity from gravity (GRACE) data and an evolutionary algorithm

Convective flow in the mantle can be thought of (and modeled) as exclusively driven by density heterogeneities in the mantle itself, and the resulting lateral variations in the Earth’s gravity field. With this assumption, and a model of mantle rheology, a theoretical relationship can be found between 3D mantle structure and flow-related quantities that can be measured on the Earth’s surface, like free-air gravity anomalies. This relationship can be used to set up an inverse problem, with 1D mantle viscosity as a solution. In the assumption that seismic velocity anomalies be of purely thermal origin, and related to density anomalies by a simple scaling factor, we invert the large-scale length component of the above-mentioned measurements jointly with seismic observations (waveforms and/or travel times) to derive an accurate 5-layer spherically symmetric model of upper- and lower-mantle viscosity. We attempt to account for non-uniqueness in the inverse problem by exploring the solution space, formed of all possible radial profiles of Earth viscosity, by means of a non-deterministic global optimization method: the evolutionary algorithm (EA). For each sampled point of the solution space, a forward calculation is conducted to determine a map of gravity anomalies, whose similarity to GRACE (gravity recovery and climate experiment) is then measured; the procedure is iterated to convergence, according to EA criteria. The robustness of the inversion is tested by means of synthetic tests, indicating that our gravity data set is able to constrain less than 6 radial layers, each with uniform viscosity. Independently of the tomographic model or the scaling factor adopted to convert seismic velocity into density structure, the EA optimization method finds viscosity profiles characterized by low-viscosity in a depth range corresponding to the transition zone, and relatively uniform elsewhere.

Gravity Anomaly Map showing Intersections of Fracture Zones with Ocean Ridges in the Central Part of the Pacific Ocean(Photo)

Gravity Anomaly Map showing Intersections of Fracture Zones with Ocean Ridges in the Central Part of the Pacific Ocean(Photo)

Modelling Potential Field Sources in the Gelibolu Peninsula (Western Turkey) Using a Markov Random Field Approach

In this study, a Markov Random Field (MRF) approach is used to locate source boundary positions which are difficult to identify from Bouguer gravity and magnetic maps. As a generalized form of Markov Chains, the MRF approach is an unsupervised statistical model based algorithm and is applied to the analysis of images, particularly in the detection of visual patterns or textures. Here, we present a dynamic programming based on the MRF approach for boundary detection of noisy and super-positioned potential anomalies, which are produced by various geological structures. In the MRF method, gravity and magnetic maps are considered as two-dimensional (2-D) images with a matrix composed of N1 × N2 pixels. Each pixel value of the matrix is optimized in real time with no a priori processing by using two parameter sets; average steering vector (θ) and quantization level (M). They carry information about the correlation of neighboring pixels and the locality of their connections. We have chosen MRF as a processing approach for geophysical data since it is an unsupervised, efficient model for image enhancement, border detection and separation of 2-D potential anomalies. The main benefit of MRF is that an average steering vector and a quantization level are enough in evaluation of the potential anomaly maps. We have compared the MRF method to noise implemented synthetic potential field anomalies. After satisfactory results were found, the method has been applied to gravity and magnetic anomaly maps of Gelibolu Peninsula in Western Turkey. Here, we have observed Anafartalar thrust fault and another parallel fault northwest of Anafartalar thrust fault. We have modeled a geological structure including a lateral fault, which results in a higher susceptibility and anomaly amplitude increment. We have shown that the MRF method is effective to detect the broad-scale geological structures in the Gelibolu Peninsula, and thus to delineate the complex tectonic structure of Gelibolu Peninsula.

Neotectonics of the Somogy hills (Part II): Evidence from seismic sections

The Somogy hills are located in the Pannonian Basin, south of Lake Balaton, Hungary, above several important tectonic zones. Analysis of industrial seismic lines shows that the pre-Late Miocene substratum is deformed by several thrust faults and a transpressive flower structure. Basement is composed of slices of various Palaeo-Mesozoic rocks, overlain by sometimes preserved Paleogene, thick Early Miocene deposits. Middle Miocene, partly overlying a post-thrusting unconformity, partly affected by the thrusts, is also present. Late Miocene thick basin-fill forms onlapping strata above a gentle paleo-topography, and it is also folded into broad anticlines and synclines. These folds are thought to be born of blind fault reactivation of older thrusts. Topography follows the reactivated fold pattern, especially in the central-western part of the study area. The map pattern of basement structures shows an eastern area, where NE–SW striking thrusts, folds and steep normal faults dominate, and a western one, where E–W striking thrusts and folds dominate. Folds in Late Neogene are also parallel to these directions. A NE–SW striking linear normal fault and associated N–S faults cut the highest reflectors. The NE–SW fault is probably a left-lateral master fault acting during–after Late Miocene. Gravity anomaly and Pleistocene surface uplift maps show a very good correlation to the mapped structures. All these observations suggest that the main Early Miocene shortening was renewed during the Middle and Late Miocene, and may still persist. Two types of deformational pattern may explain the structural and topographic features. A NW–SE shortening creates right-lateral slip along E–W faults, and overthrusts on NE–SW striking ones. Another, NNE–SSW shortening creates thrusting and uplift along E–W striking faults and transtensive left-lateral slip along NE–SW striking ones. Traces of both deformation patterns can be found in Quaternary exposures and they seem to be consistent with the present day stress orientations of the Pannonian Basin, too. The alternation of stress fields and multiple reactivation of the older fault sets is thought to be caused by the northwards translation and counter-clockwise rotation of Adria and the continental extrusion generated by this convergence.

A Gravity Anomaly Map of the Irish Western Seaboard

This paper describes the results of two marine gravity surveys undertaken by the Dublin Institute for Advanced Studies and the Institut ffir Geophysik, University of Hamburg. The surveys cover a large part of the continental shelf to the west of Ireland and east of the Rockall and Porcupine basins between latitudes 51.2oN and 55.2°N. The two data sets have been combined with the land data of the Dublin Institute for Advanced Studies to produce a unified gravity anomaly map of the west of Ireland and its continental shelf. The offshore extension of various onshore anomalies, notably those due to Caledonian granites and Carboniferous basins, are delineated by the new map.

A Gravity Anomaly Map of the Irish Western Seaboard

A Gravity Anomaly Map of the Irish Western Seaboard

A Gravity Anomaly Map of the Irish Western Seaboard

This paper describes the results of two marine gravity surveys undertaken by the Dublin Institute for Advanced Studies and the Institut ffir Geophysik, University of Hamburg. The surveys cover a large part of the continental shelf to the west of Ireland and east of the Rockall and Porcupine basins between latitudes 51.2°N and 55.2°N. The two data sets have been combined with the land data of the Dublin Institute for Advanced Studies to produce a unified gravity anomaly map of the west of Ireland and its continental shelf. The offshore extension of various onshore anomalies, notably those due to Caledonian granites and Carboniferous basins, are delineated by the new map.

Regional gravity analysis of the crustal structure of Tunisia

Gravity data were integrated with seismic refraction/reflection data, well data and geological investigations to determine a general crustal structure of Tunisia. The gravity data analysis included the construction of a complete Bouguer gravity anomaly map, residual gravity anomaly maps, horizontal gravity gradient maps and a 2.5-D gravity model. Residual gravity anomaly maps illustrate crustal anomalies associated with various structural domains within Tunisia including the Sahel Block, Saharian Flexure, Erg Oriental Basin, Algerian Anticlinorium, Gafsa Trough, Tunisian Trough, Kasserine Platform and the Tell Mountains. Gravity anomalies associated with these features are interpreted to be caused either by thickening or thinning of Palæozoic and younger sediments or by crustal thinning. Analysis of the residual gravity anomaly and horizontal gravity gradient maps also determined a number of anomalies that may be associated with previously unknown structures. A north-south trending gravity model in general indicated similar subsurface bodies as a coincident seismic model. However, thinner Mesozoic sediments within the Tunisian Trough, thinner Palæozoic sediments in the Gafsa Trough, and a greater offset on the Saharian Flexure were required by the gravity data. Additionally, basement uplifts under the Kasserine Platform and Gafsa Trough, not imaged by seismic data, were required by the gravity data. The gravity model revealed two previously unknown basins north and south of the Algerian Anticlinorium (5 km), while the Erg Oriental Basin is composed of at least two sub-basins, each with a depth of 5 km.

Gross differences between two isostatic gravity anomaly maps of China

Comparison of the 1° × 1° Isostatic Gravity Anomaly Map of China in the Lithosphere Dynamics Atlas of China, International Lithosphere Committee Publication No. 125 with the Isostatic Gravity Anomaly Map of China in the Atlas of Geophysics of China, International Lithosphere Committee Publication No. 201, shows that there are quite substantial differences between the two maps.

Qualitative interpretation of Bouguer anomaly in the southern part of the Korean peninsula

Regional gravity anomaly maps of Korea have been used for geologic information on the nature of crust and isostasy. We made an attempt to interpret the Bouguer map based on the nature, quality, and characteristics of anomalies and integrate the data with the geologic and tectonic features. Seismicity is also considered for the correlation with the Bouguer map. It reveals that the gravity anomaly map helps delineate major tectonic boundaries. It is observed that the gravity high zones are associated with earthquake activity and lineaments in their proximity.

New gravity and magnetic anomaly maps of Turkey

The General Directorate of the Mining Research and Exploration Aeromagnetic data were collected along flight lines spaced at Company of Turkey (MTA) recently completed gravity and 1–5 km intervals at an elevation of 600 m above ground level, aeromagnetic surveying over the whole of Turkey. The gridded which was controlled by radar altimetry. The data were provided data (see below) have been released to the University of by MTA in the form of a 10 km grid. We subsequently removed Ankara, but the original data remain confidential and the the International Geomagnetic Reference Field (IGRF) for the property of MTA. We present the information in the form of 1982.5 epoch from the aeromagnetic anomalies using geocontour maps which reveal the potential field anomalies of the magnetic field coefficients contained in a computer program entire country. developed by the International Union of Geodesy and Geophysics There was an active geomagnetic observatory in Istanbul Association of Geomagnetism and Aeronomy (Baldwin & Langel from the late 1880s; geophysical investigations in the form of 1993). Digital topographic height data, supplied by MTA in local surveys for prospecting have been undertaken in Turkey the form of a 10 km grid, were used for the IGRF computation. since the 1940s, e.g. Yungul (1956). The earliest large-scale The resulting aeromagnetic anomalies were contoured at geophysical investigations of Turkey were the aeromagnetic 200 nT intervals (Fig. 2). A version of this map is available at surveys for metalliferous minerals described by Hutchison et al. the URL http://www.mta.gov.tr/tymdm/harita/manyetik.jpg. (1962). Ozelci (1973) investigated gravity anomalies in the Although entitled ‘Total Magnetic Intensity’, this appears to eastern Mediterranean Sea and Turkey. Small-scale gravity be a magnetic anomaly map, but the nature of the regional and magnetic anomaly maps based on limited data sets have field removed from the observations is not stated. been presented by Akdogan (1995) and Aydin & Karat (1995). The gravity anomaly map of Turkey (Fig. 1) in general MTA has made 62 000 gravity observations in Turkey at shows broadly E–W-trending contours. Most of the land area station intervals of 2–5 km. The locations and elevations of is represented by negative anomalies, reflecting the isostatically the gravity stations were obtained by the Geodesy Department thickened continental crust which increases in thickness of MTA from 1 : 25 000 scale topographic maps. Gravity values towards the east. There are strong gradients along the northern were tied to MTA and General Command of Mapping and southern coastlines, with gravity anomalies increasing base stations related to the Potsdam 981260.00 mGal over the quasi-oceanic crust of the Black and Aegean seas. (1 Gal=1 cm s−2) absolute gravity value accepted by the The western Aegean coastline is characterized by a less steep, International Union of Geodesy and Geophysics in 1971. coast-parallel gradient, indicating a more gently sloping Moho. The gravity values are thus related to the International E–W-trending anomalies in the SW of Turkey (28.4°E, 37.3°N) Gravity Standardization Network (Erden 1979). The following correlate with graben in the West Anatolian Extensional corrections were applied by the MTA to the observed gravity Province (Ates et al. 1996). Surprisingly, the graben of the Sea values: (1) latitude corrections using the Gravity Formula of Marmara region in the NW of the country does not appear of 1967; (2) Bouguer corrections assuming a density of to be reflected in the gravity anomalies. To the east of the 2.40 Mg m−3; (3) topographic corrections completed to a West Anatolian Extensional Province an extensive residual distance of 167 km assuming a terrain density of 2.40 Mg m−3; gravity anomaly closure centred on (32.0°E, 38.5°N) correlates (4) free-air and tidal corrections. It is estimated that the error with the largely sedimentary units of the western Anatolides. in the gravity anomalies is of the order of 5 per cent. The data The Kirsehir Block in the centre of Turkey (33.9°E, 39.0°N) were provided by MTA in the form of a 10 km grid, the appears to correlate approximately with a zone of positive original data remaining confidential. The Bouguer gravity residual anomalies, reflecting its cratonic character. A complex anomalies supplied were contoured at 10 mGal intervals (Fig. 1). suite of positive and subsidiary negative residual anomalies A version of this map, which incorrectly uses a Bouguer in NE Turkey correlates with the largely Tertiary volcanic correction density of 2.67 Mg m−3 in conjunction with a rocks of the East Anatolian Contractional Province and NE terrain correction density of 2.40 Mg m−3, is available at the Anatolian Block, reflecting the high density of these rocks and URL http://www.mta.gov.tr/tymdm/harita/bouger2.gif. indicating that they have significant vertical extent. The aeromagnetic anomaly map (Fig. 2) shows that the country is divided into a southern region of subdued magnetic * Corresponding author.

New Gravity and Magnetic Anomaly Maps in the Taiwan-Luzon Region and Their Preliminary Interpretation

New Gravity and Magnetic Anomaly Maps in the Taiwan-Luzon Region and Their Preliminary Interpretation