EGM2008 Gravity Data Research Articles

Structural and tectonic interpretation of EGM2008 gravity data around the Laccadive ridge in the Western Indian Ocean: an implication to continental crust

The present study attempts to structural and tectonic interpretation around the Laccadive ridge in the Western Indian Ocean using EGM2008 Bouguer gravity data. The different derivatives of EGM2008 Bouguer gravity data indicate that the tectonic boundaries are predominantly confined in the N-S and NNW-SSE directions followed by E-W and NE-SW trends. The Euler deconvolution result suggests that the deformities in the region are of both crustal- and lithospheric-scales. The gravity highs/lows in the study area are analyzed based on sediment/crustal thickness and Moho depth estimated using radially averaged power spectrum technique. The study highlights the Moho undulation followed by crustal thickening/thinning underneath the Laccadive ridge and its surroundings for their anomalous gravity signatures. The affinity of the structural pattern observed in the Laccadive ridge region with continent trends, moderate crustal thickness and shallower Moho observed below the Laccadive ridge axis, suggest thinned rifted continental crust with shallow basement.

Using EGM2008 Gravity Data for Mapping and Detecting Structural Subsurface Anomalies in the Mayo-Kani Division (Cameroon)

To resolve the problem of gravity mapping in the Mayo-Kani Division, we extracted a sample of data from the high-resolution Bouguer anomaly database EGM2008. The objective of this process was to map and detect subsurface anomalies in the region. Regional-residual separation was applied to the data using a third-order polynomial fit. This separation enables to obtain regional and residual maps. Other filtering actions, such as horizontal and vertical derivatives, have been applied to the data to enhance sources of anomalies in the study area. The residual map of the study area was superimposed on the geological map to delineate and interpret the correlation with the shallow geological structures. The main sources of residual gravity anomalies have been recognized. The interpretation of the derived maps revealed the N-S, E-W, NE-SW and NW-SE structural patterns. These trends have been associated with the major structural directions observed in the Far North Cameroon region. The Euler Deconvolution allowed the detection of sources of anomalies and to determine their depths. The fracture map obtained from this method has improved knowledge on the search for buried ores and the geological structures associated with oil and gas deposits. Overall, the generated gravity maps provided a better understanding of the Mayo-Kani geological setting.

Isopycnicity of cratonic mantle restricted to kimberlite provinces

The isopycnicity hypothesis states that the lithospheric mantle of ancient platforms has a unique composition such that high density due to low lithosphere temperature is nearly compensated by low-density composition of old cratonic mantle. This hypothesis is supported by petrological studies of mantle xenoliths hosted in kimberlite magmas. However, the representativeness of the kimberlite sampling may be questioned, given that any type of magmatism is atypical for stable regions. We use EGM2008 gravity data to examine the density structure of the Siberian lithospheric mantle, which we compare with independent constraints based on free-board analysis. We find that in the Siberian craton, geochemically studied kimberlite-hosted xenoliths sample exclusively those parts of the mantle where the isopycnic condition is satisfied, while the pristine lithospheric mantle, which has not been affected by magmatism, has a significantly lower density than required by isopycnicity. This discovery allows us to conclude that our knowledge on the composition of cratonic mantle is incomplete and that it is biased by kimberlite sampling which provides a deceptive basis for the isopycnicity hypothesis.

Lineament Mapping over Sir Creek Offshore and its Surroundings using High Resolution EGM2008 Gravity Data: An Integrated Derivative Approach

ABSTRACT The present study deals with the delineation of lineaments over Sir Creek offshore and its surroundings from EGM2008 gravity data using various derivative techniques owing to their cost-effectiveness in prospective hydrocarbon exploration. Initially, 2-D and 3-D synthetic models have been generated with vertical prismatic objects at different depths. The effectiveness of total horizontal derivative (THD) technique has been established by comparing with E-W and N-S Horizontal derivatives and First Vertical derivative techniques. The residuals of Bouguer gravity data have been estimated with different cut-off wavelengths. Further, the residual anomaly map has been enhanced by the derivative techniques for the delineation of the structural features. Possible depths of the delineated lineaments have been estimated using Euler deconvolution of the Bouguer gravity data, which indicates maximum clustering over the delineated lineaments. It is observed that most of the lineaments are in the depth range of 1.0 km to 5.5km, which correlate well with the previous seismic studies. The present study reveals that the major lineament trends in the N-S, E-W and NNW-SSE directions followed by NE-SW, NW-SE and ENE-WSW directions. These major lineament trends are due to the tectonic activities occurred during Precambrian and Cretaceous period. Different small circular features, rectangular features and shorter wavelength features have also been identified, which could be the key parameter for mapping potential location for hydrocarbon exploration.

Geometrical Characterisation of the Mamfe Basin, Cameroon, from the Earth, Gravitational Model (EGM 2008)

Gravimetric studies using the ETOPO1-corrected high resolution satellite-based EGM2008 gravity data was used to define the surface extent, depth to basement and shape of the Mamfe basin. The Bouguer anomaly map was produced in Surfer 11.0. The Fast Fourier Transformed data was analyzed by spectral analysis to remove the effect of the regional bodies in the study area. The residual anomaly map obtained was compared with the known geology of the study area, and this showed that the gravity highs correspond to the metamorphic and igneous rocks while the gravity lows match with Cretaceous sediments. Three profiles were drawn on the residual anomaly map along which 2D models of the Mamfe basin were drawn. The modeling was completed in Grav2dc v2.06 software which uses the Talwini’s algorithm and the resulting models gave the depth to basement and the shape of the basement along the profiles. After processing and interpretation, it was deduced that the Mamfe basin has an average length and width of 77.6 km and 29.2 km respectively, an average depth to basement of 5 km and an overall U-shape basement. These dimensions (especially the depth) theoretically create the depth and temperature conditions for petroleum generation.

Edge enhancement of potential field data using spectral moments

Edge enhancement in potential-field data helps geologic interpretation, where the lineaments on the potential-field frequently indicate subsurface faults, contacts, and other tectonic features. Therefore, a variety of edge-enhancement methods have been proposed for locating edges, most of which are based on the horizontal or vertical derivatives of the field. However, these methods have several limitations, including thick detected boundaries, blurred response to low-amplitude anomalies, and sensitivity to noise. We have developed the spectral-moment method for detecting edges in potential-field anomalies based on the second spectral moment and its statistically invariable quantities. We evaluated the spectral-moment method using synthetic gravity data, EGM-2008 gravity data, and the total magnetic field reduced to the pole. Compared with other edge-enhancing filters, such as the total horizontal derivative (TDX), profile curvature, curvature of the total horizontal gradient amplitude, enhancement of the TDX using the tilt angle, theta map, and normalized standard deviation, this spectral-moment method was more effective in balancing the edges of different-amplitude anomalies, and the detected lineaments were sharper and more continuous. In addition, the method was also less sensitive to noise than were the other filters. Compared with geologic maps, the edges extracted by the spectral-moment method from gravity and the magnetic data corresponded well with the geologic structures.

Spectral harmonic analysis and synthesis of Earth’s crust gravity field

We developed and applied a novel numerical scheme for a gravimetric forward modelling of the Earth’s crustal density structures based entirely on methods for a spherical analysis and synthesis of the gravitational field. This numerical scheme utilises expressions for the gravitational potentials and their radial derivatives generated by the homogeneous or laterally varying mass density layers with a variable height/depth and thickness given in terms of spherical harmonics. We used these expressions to compute globally the complete crust-corrected Earth’s gravity field and its contribution generated by the Earth’s crust. The gravimetric forward modelling of large known mass density structures within the Earth’s crust is realised by using global models of the Earth’s gravity field (EGM2008), topography/bathymetry (DTM2006.0), continental ice-thickness (ICE-5G), and crustal density structures (CRUST2.0). The crust-corrected gravity field is obtained after modelling and subtracting the gravitational contribution of the Earth’s crust from the EGM2008 gravity data. These refined gravity data mainly comprise information on the Moho interface and mantle lithosphere. Numerical results also reveal that the gravitational contribution of the Earth’s crust varies globally from 1,843 to 12,010 mGal. This gravitational signal is strongly correlated with the crustal thickness with its maxima in mountainous regions (Himalayas, Tibetan Plateau and Andes) with the presence of large isostatic compensation. The corresponding minima over the open oceans are due to the thin and heavier oceanic crust.

A three‐dimensional Moho depth model for the Tien Shan from EGM2008 gravity data

The Tien Shan in Central Asia is the largest intracontinental mountain range in the world, but it is 1500 km away from the collision zone between the Indian and Eurasian plates. This region has been and still is the focus of numerous geoscientific studies, mainly because of its evolutionary history and its unique position in the Eurasian lithosphere plate. So far, mainly seismological data have been used to explore the origin of and ongoing seismic activity in this region, but only one study has investigated terrestrial gravity data. In this study, a new gravity data set, EGM2008, is used to determine the crust‐mantle boundary (Mohorovičić discontinuity, Moho) of the Tien Shan using inversion of gravity data. In addition, an isostatic Moho is calculated from topographic data, which by comparison to the results of the gravity inversion illuminates the effects of isostatic compensation. The results of the gravity inversion generally agree with results of previous seismic studies and indicate that the Tien Shan has a mountain root with a thickness of about 75 km. Furthermore, the Moho is shallow under the basins, e.g., in the Tarim and Ili basins. The comparison with the isostatic Moho indicates an over‐compensation of the orogen and an under‐compensation of the basins. The over‐compensation results from the former subduction of the Tarim Basin terrane in the south. The under‐compensation of the Tarim Basin is generated by support of the terrane between the Tien Shan in the north and the Pamir mountains, Tibet and Himalayas in the south.

Global Crust-Mantle Density Contrast Estimated from EGM2008, DTM2008, CRUST2.0, and ICE-5G

We compute globally the consolidated crust-stripped gravity disturbances/anomalies. These refined gravity field quantities are obtained from the EGM2008 gravity data after applying the topographic and crust density contrasts stripping corrections computed using the global topography/bathymetry model DTM2006.0, the global continental ice-thickness data ICE-5G, and the global crustal model CRUST2.0. All crust components density contrasts are defined relative to the reference crustal density of 2,670 kg/m3. We demonstrate that the consolidated crust-stripped gravity data have the strongest correlation with the crustal thickness. Therefore, they are the most suitable gravity data type for the recovery of the Moho density interface by means of the gravimetric modelling or inversion. The consolidated crust-stripped gravity data and the CRUST2.0 crust-thickness data are used to estimate the global average value of the crust-mantle density contrast. This is done by minimising the correlation between these refined gravity and crust-thickness data by adding the crust-mantle density contrast to the original reference crustal density of 2,670 kg/m3. The estimated values of 485 kg/m3 (for the refined gravity disturbances) and 481 kg/m3 (for the refined gravity anomalies) very closely agree with the value of the crust-mantle density contrast of 480 kg/m3, which is adopted in the definition of the Preliminary Reference Earth Model (PREM). This agreement is more likely due to the fact that our results of the gravimetric forward modelling are significantly constrained by the CRUST2.0 model density structure and crust-thickness data derived purely based on methods of seismic refraction.