Showing papers on "Bouguer anomaly published in 2017"

From Discrete Gravity Survey Data to a High-resolution Gravity Field Representation in the Nordic-Baltic Region

Abstract: The deduction of a regularly spaced gravity anomaly grid from scattered survey data is studied, addressing mainly two aspects: reduction of gravity to anomalies and subsequent interpolation by various methods. The problem is illustrated in a heterogeneous study area and contrasting test areas including mountains, low terrains, and a marine area. Provided with realistic error estimates, Least Squares Collocation interpolation of Residual Terrain Model anomalies yields the highest quality gravity grid. In most cases, the Bouguer reduction and other interpolation methods tested are equally viable. However, spline-based interpolation should be avoided in marine areas with trackwise survey data.

Numerical Calculation of Terrain Correction Within the Bouguer Anomaly Evaluation (Program Toposk)

Abstract: The new software solution Toposk was developed in order to recalculate the terrain corrections of the unified gravity database of the Slovak Republic. The program is designed primarily for the calculation of the gravitational effect of the topographic masses, and the terrain corrections are then derived from these effects. The program application is not restricted only to the territory of Slovakia. Several innovations were incorporated in the new software in comparison with previous programs used in Slovakia. The real position of the gravity meter sensor with regard to the surrounding terrain (above or below the surface) can be taken into account during the calculation. Program Toposk allows the use of interpolated heights for the calculation points in order to reduce the errors resulting from the elevation model inaccuracy. The choice of arbitrary subzones divisions as well as coordinate systems is allowed. By default, the program uses the following zone divisions: inner zone T1 (to a distance of 250 m from the calculation point), intermediate zone T2 (250–5240 m), and outer zones T31 (5240–28,800 m) and T32 (28,800–166,730 m). The computing algorithm was tested on several synthetic models, giving satisfactory results when compared with analytically evaluated values.

Interpretation of gravity data derived from the Earth Gravitational Model EGM2008 in the Center-North Cameroon: structural and mining implications

Abstract: This work presents an interpretation of gravity data derived from the Earth Gravitational Model EGM2008 in the Center north Cameroon and a correlation with previous tectonic geological and mining studies. EGM2008 Model provides high spatial resolution gravitational data integrating land, airborne, maritime gravity data, as well as satellite altimetry data. Significant similarities are underlined through Bouguer anomaly maps deduced from EGM2008 and land gravity data according to shape, orientation, and amplitude of anomalies. Furthermore, the statistical analysis of both datasets gives means of −38.25 and −38.21 mGal as well as standard deviations of 12.30 and 12.42 for EGM2008 and land data, respectively. Regional geological formations are largely interpreted and identified on EGM2008 Bouguer anomaly map. We applied regional/residual separation filtering, upward continuation and horizontal gradient methods to investigate the subsurface structure. Some known geological lineaments are highlighted such as Poli and Koum faults and Tchollire shear zone, while new ones are revealed. This work brings out main regional feature known as Poli-Ounianga-Kebir gravity heavy line trending SW-NE which contains uraniferous and auriferous deposits located along the same direction. This paper presents a coherent correlation between positive gravity anomalies, faults, and mineral indices in the Center-North Cameroon (Central Africa Fold belt), and thus brings new evidences of the efficiency of Earth Gravitational Model EGM2008 for structural investigation and mining exploration.

Receiver function and gravity constraints on crustal structure and vertical movements of the Upper Mississippi Embayment and Ozark Uplift

Abstract: The Upper Mississippi Embayment (UME), where the seismically-active New Madrid Seismic Zone resides, experienced two phases of subsidence commencing in the late Precambrian and Cretaceous, respectively To provide new constraints on models proposed for the mechanisms responsible for the subsidence, we computed and stacked P-to-S receiver functions recorded by 49 USArray and other seismic stations located in the UME and the adjacent Ozark Uplift and modeled Bouguer gravity anomaly data The inferred thickness, density, and Vp/Vs of the upper and lower crustal layers suggest that the UME is characterized by a mafic and high-density upper crustal layer of ∼30 km thickness, which is underlain by a higher-density lower crustal layer of up to ∼15 km Those measurements, in the background of previously published geological observations on the subsidence and uplift history of the UME, are in agreement with the model that the Cretaceous subsidence, which was suggested to be preceded by an approximately 2 km uplift, was the consequence of the passage of a previously proposed thermal plume The thermo-elastic effects of the plume would have induced wide-spread intrusion of mafic mantle material into the weak UME crust fractured by Precambrian rifting and increased its density, resulting in renewed subsidence after the thermal source was removed In contrast, the Ozark Uplift has crustal density, thickness, and Vp/Vs measurements that are comparable to those observed on cratonic areas, suggesting an overall normal crust without significant modification by the proposed plume, probably owing to the relatively strong and thick lithosphere

Gravity Data Contribution for Petroleum Exploration Domain: Mateur Case Study (Saliferous Province, Northern Tunisia)

Abstract: Triassic salt structures which are interesting for hydrocarbon exploration are widespread in northern Tunisia. A good knowledge of these structures’ evolution and geometry is required for prospection programs. In oil exploration, seismic method is the most adequate approach used to explore subsurface; the seismic quality deteriorates within salt provinces, so that gravity becomes more suitable. In this context, the present case study focuses on the interpretation of geological and computed gravity ground data to infer more accurate picture on the Triassic salt structures in Mateur region (northeastern Tunisia). First Vertical Derivative, Total Horizontal Derivative, and Total Horizontal Derivative of the tilt-angle maps were computed from the complete Bouguer anomaly to decipher fault patterns by enhancing shallow structures and approximating edges of source bodies. Euler Deconvolution and spectrum analysis are also elaborated to estimate the burial depths of delineated structures. Transformed maps highlight, nearby the Lansarine–Baouala Triassic outcrops, the presence of circular gravity lineaments around positive anomalies that reflect the shape of concentric faults edging Triassic subsalt features. Causative source depths of these anomalies exceed 1500 m, thus indicating deep-rooted salt structures. Our results corroborate that fact that the Lansarine–Baouala Triassic outcrops form a huge diapir surrounded by large but buried subsalt dome features. The obtained results help refining the previous structural map that could be of significant use for petroleum exploration in the Mateur area.

High resolution Slovak Bouguer gravity anomaly map and its enhanced derivative transformations: new possibilities for interpretation of anomalous gravity fields

Abstract: The paper deals with the revision and enrichment of the present gravimetric database of the Slovak Republic. The output of this process is a new version of the complete Bouguer anomaly (CBA) field on our territory. Thanks to the taking into account of more accurate terrain corrections, this field has significantly higher quality and higher resolution capabilities. The excellent features of this map will allow us to re-evaluate and improve the qualitative interpretation of the gravity field when researching the structural and tectonic geology of the Western Carpathian lithosphere. In the contribution we also analyse the field of the new CBA based on the properties of various transformed fields – in particular the horizontal gradient, which by its local maximums defines important density boundaries in the lateral direction. All original and new transformed maps make a significant contribution to improving the geological interpretation of the CBA field. Except for the horizontal gradient field, we are also interested in a new special transformation of TDXAS, which excellently separates various detected anomalies of gravity field and improves their lateral delimitation.

National Gravimetric Database of the Slovak Republic

Abstract: Compilation of the Slovak gravimetric database with the actual amount of about 320,000 observation points is presented. Gravity data were collected during more than 50 years, which yields a very heterogeneous dataset, with large variations in the station coverage and processing methods. The regional gravimetric database (more than 212,000 points) was resumed in 2001. The compilation discussed herein (with more than 107,000 detailed gravity measurements) was made during 2011–14. Quality-control process and complete recalculation of the Bouguer anomalies is presented. Primary focus of this project was on a proper recalculation of the terrain corrections. New detected linear features in the Bouguer anomaly map were verified by the field measurements. A new software solution for reconstruction of the gravity acceleration values from the Bouguer anomaly map was developed for geodetic applications.

Continuity of subsurface fault structure revealed by gravity anomaly: the eastern boundary fault zone of the Niigata plain, central Japan

Abstract: We have investigated gravity anomalies around the Niigata plain, which is a sedimentary basin in central Japan bounded by mountains, to examine the continuity of subsurface fault structures of a large fault zone—the eastern boundary fault zone of the Niigata plain (EBFZNP). The features of the Bouguer anomaly and its first horizontal and vertical derivatives clearly illustrate the EBFZNP. The steep first horizontal derivative and the zero isoline of the vertical derivative are clearly recognized along the entire EBFZNP over an area that shows no surface topographic features of an active fault. Two-dimensional density structure analyses also confirm a relationship between the two first derivatives and the subsurface fault structure. Therefore, we conclude that the length of the EBFZNP as an active fault extends to ~56 km, which is longer than previously estimated. This length leads to an estimation of a moment magnitude of 7.4 of an expected earthquake from the EBFZNP.

Gravity Anomalies and Isostasy Deduced From New Dense Gravimetry Around the Tsangpo Gorge, Tibet

Abstract: We built the first dense gravity network including 107 stations around the Tsangpo Gorge, Tibet, one of the hardest places in the world to reach, and conducted a gravity and hybrid GPS observation campaign in 2016. We computed the Bouguer gravity anomalies (BGAs) and free-air gravity anomalies (FGAs), and increased the resolution of the FGAs by merging the in situ data with EIGEN-6C4 gravity model data. The BGAs around the Tsangpo Gorge are in general negative and gradually decrease from south (-360 mGal) to north (-480 mGal). They indicate a uniformly-dipping Moho around the Tsangpo Gorge that sinks from south to north at an angle of 12°. We introduced a method to compute the vertical tectonic stress of the lithosphere, a quantitative expression of isostasy, using BGA and terrain data and applied it to the area around the Tsangpo Gorge. We found that the lithosphere of the upstream of the Tsangpo Gorge is roughly in an isostatic state, but the lithosphere of the downstream exhibits vertical tectonic stress of ~50 MPa, which indicates the loss of a large amount of surface material. This result doesn’t support the deduction of the valley bottom before uplift of the Tsangpo Gorge by Wang et al. (2014).

Gravity Survey at the Ceboruco Volcano Area (Nayarit, Mexico): a 3-D Model of the Subsurface Structure

Abstract: Ceboruco volcano (−104°30′, 21°7′, 2150 m asl) is located in the western portion of the trans-Mexican volcanic belt and NW extreme of the Tepic-Zacoalco rift zone, a structure composed of a series of NNW-trending en echelon fault-bounded basins constituting the NE boundary between the north-American plate and the Jalisco block (JB). Ceboruco experimented a Plinian eruption about 1000 years ago and several more of different styles afterward; the last one in 1870 CE. This volcano poses a significant risk because of the relatively large population in its surroundings. Ceboruco has been studied by mostly from the point of view of petrology, geochemistry, and physical volcanology; however, no geophysical studies about its internal structure have been published. In this paper, we present the results of a gravimetric survey carried out in its surroundings and a model of the internal structure obtained from inversion of the data. The Ceboruco area is characterized by a negative Bouguer anomaly spanning the volcanic structure. The probable causative body modeled with the data of the survey is located about 1 km below mean sea level and has a volume of 163 km3. We propose that this body is the magma chamber from where the products of its eruptions in the last 1000 years ensued.

Investigating Moho depth, Curie Point, and heat flow variations of the Yozgat Batholith and its surrounding area, north central Anatolia, Turkey, using gravity and magnetic anomalies

Abstract: This paper proposes an interpretation of the Bouguer gravity and magnetic anomaly map of the Yozgat Batholith and its surrounding area (north central Anatolia) for determination of crustal and geothermal structures. In the study area, the Moho and Conrad depths based on a relationship between Bouguer gravity anomaly and seismically determined crustal thickness were estimated as 34-39 km and 19.5-21.5 km, respectively. The crustal thickness of the study area increases from west to east. The estimated Curie point depths in the study area by means of spectral analysis of the magnetic data vary between 10.4 and 19.5 km. The shallow depths indicate that the magnetization is restricted to the upper crust in the study area. In addition, the heat flow values calculated for a thermal conductivity of 2.7 W m-1 K-1 range from about 80 to 150 mWm-2. The shallow velocity structures in central Anatolia are consistent with the estimated high heat flow values in the upper crust and associated with comparatively high negative Bouguer gravity anomalies.

The Study of Geological Structures in Suli and Tulehu Geothermal Regions (Ambon, Indonesia) Based on Gravity Gradient Tensor Data Simulation and Analytic Signal

Abstract: In early 2017, the geothermal system in the Suli and Tulehu areas of Ambon (Indonesia) was investigated using a gravity gradient tensor and analytic signal. The gravity gradient tensor and analytic signal were obtained through forward modeling based on a rectangular prism. It was applied to complete Bouguer anomaly data over the study area by using Fast Fourier Transform (FFT). The analysis was conducted to enhance the geological structure like faults as a pathway of geothermal fluid circulation that is not visible on the surface because it is covered by sediment. The complete Bouguer anomaly ranges of 93 mGal up to 105 mGal decrease from the southwest in Suli to the northeast in Tulehu. A high gravity anomaly indicates a strong magmatic intrusion below the Suli region. The gravity anomalies decrease occurs in the Eriwakang mountain and most of Tulehu, and it is associated with a coral limestone. The lower gravity anomalies are located in the north to the northeast part of Tulehu are associated with alluvium. The residual anomaly shows that the drill well TLU-01 and geothermal manifestations along with the Banda, and Banda-Hatuasa faults are associated with lowest gravity anomaly (negative zone). The gravity gradient tensor simulation and an analytic signal of Suli and Tulehu give more detailed information about the geological features. The gzz component allows accurate description of the shape structures, especially the Banda fault associated with a zero value. This result will be useful as a geophysical constraint to subsurface modeling according to gravity gradient inversion over the area.

Importance of the Decompensative Correction of the Gravity Field for Study of the Upper Crust: Application to the Arabian Plate and Surroundings

Abstract: The isostatic correction represents one of the most useful “geological” reduction methods of the gravity field. With this correction it is possible to remove a significant part of the effect of deep density heterogeneity, which dominates in the Bouguer gravity anomalies. However, even this reduction does not show the full gravity effect of unknown anomalies in the upper crust since their impact is substantially reduced by the isostatic compensation. We analyze a so-called decompensative correction of the isostatic anomalies, which provides a possibility to separate these effects. It was demonstrated that this correction is very significant at the mid-range wavelengths and may exceed 100 m/s2 (mGal), therefore ignoring this effect would lead to wrong conclusions about the upper crust structure. At the same time, the decompensative correction is very sensitive to the compensation depth and effective elastic thickness of the lithosphere. Therefore, these parameters should be properly determined based on other studies. Based on this technique, we estimate the decompensative correction for the Arabian plate and surrounding regions. The amplitude of the decompensative anomalies reaches ±250 m/s2 10−5 (mGal), evidencing for both, large density anomalies of the upper crust (including sediments) and strong isostatic disturbances of the lithosphere. These results improve the knowledge about the crustal structure in the Middle East.

Structural Architecture of the Hydrothermal System from Geophysical Data in Hammam Bouhadjar Area (Northwest of Algeria)

Abstract: We determine the structural architecture of the hydrothermal system of Hammam Bouhadjar area (Northwest of Algeria) by the use of geophysical data. New gravity and electrical surveys covered an area of about 48 km2 in 2009. There were 350 gravity measurements made with a sampling of 500 m and 45 electrical soundings (Schlumberger type, AB = 1000 m). The Bouguer anomaly map shows a regression of gravity field towards the NW and SE. All of the observed anomalies are elongated in NE–SW direction. The results obtained from different processing methods (gradients, upward continuation, Euler deconvolution, wavelet transform and modelling) of gravity data were used to generate structural map of the studied area. The vertical and horizontal variations of resistivity confirm the presence of superficial and deeper faults system. Following the geophysical (gravity and electrical) analysis and modelling, we propose a model to explain the origin of the Hammam Bouhadjar thermal waters. We suggest that the hot spring water comes from an aquifer located in sandstones lenses in the Senono-Oligocene Tellian unit. Following the gravity modelling the aquifer is identified at about 800 m, the same depth where the geothermal gradient is insufficient to heat the water. In these circumstances, the aquifer is probably heated by volcanic processes connected with a hot compartment by faults and contacts affecting structures identified in depth. The presence of a conductor along of the horseshoe area suggests that the water percolates into this area and then is drained by the different accidents to invade the whole area.

Gravity anomaly grids for the New Zealand region

Abstract: We present 1 arc-minute Bouguer, Faye, free air and topography corrected gravity anomaly grids for the New Zealand region, 25°S to 60°S and 160°E to 170°W. The grids were compiled from existing terrestrial, marine and satellite altimetry-derived gravity data enhanced with new airborne gravimetry data that were acquired for improvement of the New Zealand vertical datum. The airborne data seamlessly cover onshore and offshore areas over New Zealand’s North, South and Stewart islands with a uniform flight line spacing of 10 km. All data were corrected for the gravitational effect of the Geodetic Reference System 1980 (GRS80) reference ellipsoid and tied to the International Gravity Standardization Net 1971 (I.G.S.N.71) gravity datum. The gravity anomaly data from all sources were combined using the method of least squares collocation with a three dimensional logarithmic covariance function. Terrain corrections for gravity anomaly grids were calculated using an 8 m digital elevation model for topograp...