Gravity modeling of sedimentary basins by means of growing bodies and exponential density contrast-depth variations: A space domain approach (original) (raw)
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Gravity modeling of 21/2-D sedimentary basins — a case of variable density contrast
Computers & Geosciences, 2005
An algorithm and associated codes are developed to determine the depths to bottom of a 2 1/2-D sedimentary basin in which the density contrast varies parabolically with depth. This algorithm estimates initial depths of a sedimentary basin automatically and modifies thereafter appropriately within the permissible limits in an iterative approach as described in the text. Efficacy of the method as well as the code is illustrated by interpreting a gravity anomaly of a synthetic model. Further, the applicability of the method is exemplified with the derived density-depth model of the Godavari sub-basin, India to interpret the gravity anomalies due to the basin. Interpretations based on parabolic density profile are more consistent with existing geological information rather than with those obtained with constant density profile.
Computers & Geosciences, 2004
A method to model 3-D sedimentary basins with density contrast varying with depth is presented along with a code GRAV3DMOD. The measured gravity fields, reduced to a horizontal plane, are assumed to be available at grid nodes of a rectangular/square mesh. Juxtaposed 3-D rectangular/square blocks with their geometrical epicenters on top coincide with grid nodes of a mesh to approximate a sedimentary basin. The algorithm based on Newton's forward difference formula automatically calculates the initial depth estimates of a sedimentary basin assuming that 2-D infinite horizontal slabs among which, the density contrast varies with depth could generate the measured gravity fields. Forward modeling is realized through an available code GR3DPRM, which computes the theoretical gravity field of a 3-D block. The lower boundary of a sedimentary basin is formulated by estimating the depth values of the 3-D blocks within predetermined limits. The algorithm is efficient in the sense that it automatically generates the grid files of the interpreted results that can be viewed in the form of respective contour maps. Measured gravity fields pertaining to the Chintalpudi sub-basin, India and the Los Angeles basin, California, USA in which the density contrast varies with depth are interpreted to show the applicability of the method.
Pure and Applied Geophysics, 1995
The decrease in density contrast of sedimentary rocks with depth in many sedimentary basins can be approximated by a parabolic density function. Analytical gravity expression of an outcropping two-dimensional vertical step along which the density contrast decreases parabolically with depth is derived in the space domain. A modification ofBott's (1960) method of gravity interpretation is proposed by considering two outcropping vertical steps on either side of the first and last observation points in addition toN outcropping vertical prisms in order to interpret the gravity anomalies of nonoutcropping basins. The thicknesses of the two outcropping vertical steps are made equal to the thicknesses of the two outcropping vertical prisms placed below the first and last observation points. The initial depth estimates of the sedimentary basin are calculated by the infinite slab formula ofVisweswara Rao et al. (1993). The gravity effects of theN outcropping prisms and the two outcropping vertical steps are calculated at each anomaly point and the depth to the floor of the basin are adjusted based on the differences between the observed and calculated anomalies. A gravity anomaly profile of Los Angeles basin, California is interpreted.
Acta Geophysica, 2015
An automatic inversion using ridge regression algorithm is developed in the space domain to analyze the gravity anomalies of sedimentary basins, among which the density contrast decreases with depth following a prescribed exponential function. A stack of vertical prisms having equal widths, whose depths become the unknown parameters to be estimated, describes the geometry of a sedimentary basin above the basement complex. Because no closed form analytical equation can be derivable in the space domain using the exponential density-depth function, a combination of analytical and numerical approaches is used to realize forward gravity modeling. The depth estimates of sedimentbasement interface are initiated and subsequently improved iteratively by minimizing the objective function between the observed and modeled gravity anomalies within the specified convergence criteria. Two gravity anomaly profiles, one synthetic and a real, are interpreted using the proposed technique to demonstrat...
Near Surface Geophysics, 2009
An inversion using ridge regression to estimate simultaneously the parameters of pull-apart basins having finite strike length (2.5D) and regional gravity background from observed gravity anomalies is presented. A parabolic function is used to describe the density contrast variation with depth within the structure. The algorithm begins with initializing both the regional background and parameters of the basin and subsequently improves them iteratively until the modelled gravity anomalies mimic the observed ones. The applicability and efficacy of the inversion is demonstrated with a set of synthetic gravity anomalies 1) attributable entirely due to a theoretical model, 2) in the presence of pseudorandom noise and 3) in the presence of both pseudorandom noise and regional gravity background. It was found from the analysis of synthetic gravity anomalies that the modelled parameters of the structure closely mimic the true ones even when the gravity anomalies are corrupted with pseudorandom noise. In the presence of both random noise and regional background the estimated parameters deviate only modestly from the assumed ones. Furthermore, the applicability of the algorithm is exemplified with a derived density-depth model to analyse the Bouguer gravity anomalies observed over the Ranigunj basin, India. The estimated depth of the basin is consistent with the available borehole information. The interpretation of the basin supports the hypothesis that this basin might have been formed as a result of both E-W kinematics and orthogonal extension rather than simple local extensional tectonics.
INV2P5DSB—A code for gravity inversion of 2.5-D sedimentary basins using depth dependent density
Computers & Geosciences, 2007
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.
Revista Brasileira de Geofísica, 2005
In the present work, a computational methodology is implemented for the modeling of three-dimensional gravity sources inserted in a basement of variable density, such as granitic bodies and sedimentary basins. This procedure proceeds by separating the gravity data into two components: the component due to variations in density of underlying basement and the component caused by the basins themselves, which will be modeled. Another product of this inversion procedure is the gravitational attraction of the basement without the effects of the modeled source. It represents another important advantage of this method. Each gravity source can be isolated and modeled. Gravity synthetic data had been generated to simulate interfering sources and test the proposed methodology. The gravity effect of a sedimentary basin had been added the gravity anomalies of intrusive bodies, simulating interfering sources. The comparison between 3-D models obtained by techniques that assume the homogeneous basement and the proposed methodology demonstrates the efficiency in the isolation of the interfering sources and in the determination of its internal geometry. The application of this technique in the Guitiriz Granite (Spain) and the Rio do Peixe rift basin (Paraíba State) demonstrates an important resolution increment of the geophysical models in the regions where interfering sources are present. In the Guitiriz Granite case, its magmatic conduits are better defined without the gravity interference of others igneous bodies. In the Rio do Peixe basin, internal structures can be differentiated with more accuracy, free from gravity anomalies associated to the Paraná Granite, which outcrops in the northern portion of the basin.
Near Surface Geophysics, 2012
A method to compute gravity anomalies of multiple geological sources having variable but finite strike lengths with arbitrary density contrast variations is presented. An ensemble of variable but finite strike limited vertical prisms among which density contrasts vary randomly both with depth and lateral position describes the anomalous source(s). The proposed method has the advantage that it can be applied to compute the gravity anomalies of a variety of geologic sources at both local and crustal scale levels. The applicability of the method is demonstrated with synthetic and real field examples. In the synthetic example, the geology resembles that of a typical rifted sedimentary basin, where thick sectioned sediments with arbitrarily varying density contrasts are concealed under a thick pile of flood basalt. For the real field example, the method is applied to calculate the gravity anomalies of a crustal scale structure across the suture zone between the Superior and Churchill structural provinces in the Canadian Shield. Modelling of real field anomalies reveals that i) the Churchill block is much thicker and ii) the western boundary of the base of the fold belt within the Superior block is shallower than reported in previous studies.
IOP Conference Series: Earth and Environmental Science, 2019
East Java Basin is classified as a classical back-arc basin so that it is indicated that the basin has a complex structure and stratigraphy where there is an area called Rembang High. Rembang High is a gravity anomaly term that appears in Northern of East Java Basin area which has a higher value than other physiographic zone and is assumed to be associated with Rembang anticline. It is interesting to study the basement configuration and sedimentary sub-basin presumption. The gravity anomaly is supperposition of all anomaly sources in the subsurface, namely regional and residual anomaly. The separation of regional and residual anomaly is done by using upward continuation filter. Optimum continuity height can be obtained from the cross correlation of two continuity heights. The tesidual anomaly was obtained from the difference between Bouguer and regional anomaly. In addition to use the optimum continuity height, anomalous source depth estimation is obtained by using spectral analysis...