Determination of Sedimentary Basin Basement Depth: A Space Domain Based Gravity Inversion using Exponential Density Function (original) (raw)
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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.
Journal of the Geological Society of India, 82, 561-569., 2013
An automatic modeling scheme is developed in the space domain to interpret the gravity anomalies of sedimentary basins, among which the density contrast decreases exponentially with depth. Forward modeling is realized in the space domain using a combination of both analytical and numerical approaches. A collage of vertical prisms having equal widths, whose depths are to be estimated, describes the geometry of a sedimentary basin. Initial depths of a sedimentary basin are predicted using the Bouguer slab formula and subsequently updated, iteratively, based on the differences between the observed and theoretical gravity anomalies until the modeled gravity anomalies mimic the observed ones. The validity and applicability of the method is demonstrated with a synthetic and two real field gravity anomalies, one each over the Chintalpudi sub-basin in India and the other over the San Jacinto graben, California. In case of synthetic example, the assumed structure resembles a typical intracratonic rift basin formed by normal block faulting and filled with thick section of sediments. The proposed modeling technique yielded information that is consistent with the assumed parameters in the case of synthetic structure and with the available/drilling depths in case of field examples.
Inversion of gravity‐field inclination to map the basement relief of sedimentary basins
GEOPHYSICS, 2004
This paper presents a method to map the basement relief of homogeneous sedimentary basins that does not require the knowledge of the basin density contrast. To reach this task, the proposed method relies on the invariance of the inclination of the anomalous gravity field with the density contrast caused by models constituted by two homogeneous media. This invariance occurs because the density contrast appears as a constant factor in both vertical and horizontal gravity components, therefore being canceled out when these components are divided during the evaluation of the field inclination. For such media, the field inclination is independent of the density contrast, thus allowing the source geometry reconstruction even when the density contrast is unknown. As the inclination is rarely measured, the gravity anomaly (i.e., the field vertical component) is initially used to compute the horizontal component of the gravity field by applying a suitable linear transform. The field inclination is estimated from both components and then used to invert the source geometry by fitting the inclination values under the geologic constraints attributed to the causative sources. In this process, the density contrast is not required nor introduced as an unknown parameter in the formulated inverse problem. Moreover, it can be estimated later by solving a new inverse problem where the source geometry determined from the inverted inclination is fixed and the constant density contrast is determined by fitting the gravity anomaly. This paper applies such ideas to map the basement relief of a sedimentary basin and to estimate its density contrast. The inversion is implemented by a random search procedure that excludes extreme models, and imposes constraints that the unknown interface is smooth everywhere and assumes known depth values at isolated points investigated by wells. The proposed technique is tested with synthetic noisy data from homogeneous and heterogeneous basin models and is applied to invert a gravity profile from the Recôncavo Basin, Brazil. The results from the real data application are compared with well data and previously published results.
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.
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.
2017
In this thesis, I applied Cauchy-type integral-based depth-to-basement estimation method to a variety of models to test the reliability of the method in different geological scenarios. I also inverted for three-dimensional (3D) subsurface anomalous density distribution with constrained model parameters in order to produce more compact inversion results. I demonstrated several singleblock and multiple-block synthetic model results produced by constrained 3D gravity inversion. I also display results for Cauchy-type integral-based 3D depthto-basement inversion of simple/complex basin models. The results from both methods are nicely consistent with true models at a very low misfit level and a fast convergence. A case study is presented at the end of our paper for both methods, and results for both methods are used to do interpretation jointly. In dedication to my mother for making me who I am, and my fiancé for her support throughout this thesis.
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.
Ridge-regression algorithm for gravity inversion of fault structures with variable density
Geophysics, 2004
We derive an analytical expression for gravity anomalies of an inclined fault with density contrast decreasing parabolically with depth. The effect of the regional background, particularly the interference from neighboring sources of a fault structure, is ascribed by a polynomial equation. We have developed an inversion technique employing the ridge-regression iterative algorithm to infer the shape parameters of the fault structure, in addition to the effect of regional background. We demonstrate the validity of the proposed technique by inverting a gravity anomaly of a theoretical model, both with and without adding a regional background. The technique is insensitive to the effect of regional background. Two density-depth models of the Godavari subbasin in India are used in our interpretation of the gravity anomalies of the Ahiri-Cherla master fault. The interpreted results of a parabolic density model are found to be more geologically reasonable in comparison with the constant density model. The variations of the misfit function of the theoretical and observed gravity anomalies, the damping factor, and the shape parameters of the fault against the iteration number indicate the reliability of the interpretation.
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.