P. Rama Rao

The density difference and generalized programs for two- and three-dimensional gravity modeling

Abstract Computer programs developed to model flat-topped bodies cannot accommodate models with a flat bottom, without a modification in the program. The concept of density difference is introduced and used to construct computer programs that can model gravity anomalies of any body—flat topped, flat bottomed, or undulating. The density difference is the density of the material lying below the interface minus the density of the material above. The suggested schemes determine the initial depths to the interface based on the Bouguer slab formula and improve them iteratively based on the differences between the observed and calculated anomalies. Two generalized computer programs in FORTRAN 77 for modeling two- and three-dimensional structures are presented.

Inversion of gravity anomalies of three-dimensional density interfaces

Abstract An inversion scheme to trace three-dimensional density interfaces from their gridded gravity anomalies is developed. The scheme requires the gravity anomaly values, profile and station spacings as the input, besides densities of the underlying and overlying materials and the mean or undisturbed depth to the interface. The scheme calculates the initial values of the depths to the interface below the internal grid points, and modifies them iteratively until a best fit is achieved between the observed and calculated anomaly values. The gravity anomalies of the density interface are generated by equating the material below the interface to a series of juxtaposing rectangular blocks, one centered below each of the internal gridded anomaly points. It is assumed that the interface flattens out to its undisturbed depth well ahead of the boundaries of the sampled area. The values of initial thicknesses of the prisms are calculated by equating the anomaly at a station to the algebraic sum of products of vertical gradients of gravity effects of the prisms and their thicknesses. The differences between the observed and calculated anomalies are then used to improve the thicknesses of the prisms by equating these differences to the algebraic sum of the products of the vertical gradients of gravity effects of prisms and increments to their thicknesses. The computer program, named GRAV3DIN coded in FORTRAN 77 and used to invert gravity anomalies of three-dimensional density interfaces, is presented.

A new method of interpreting self-potential anomalies of two-dimensional inclined sheets

A new method of interpreting self-potential anomalies of inclined sheet-like bodies of infinite strike length is presented in this study. In contrast to conventional schemes, the method does not explicitly make use of the magnitudes of the anomaly values during inversion. But, positions of a pair of points, at which the anomaly values differ from each other by a constant magnitude, are selected to construct some linear equations. The coefficients of these equations are functions of the model parameters, and hence the latter are solved from these coefficients. The method can be extended to gravity and magnetic anomalies of various models of simple geometry.

Automatic inversion of self-potential anomalies of sheet-like bodies

Abstract A computer program in FORTRAN 77 is presented to invert the self-potential anomalies of sheet-like bodies. The program needs as input only the self-potential anomalies and their distances measured from an arbitrary reference in the profile and does not require the initial estimates of the body parameters of the model as in classical inversion techniques. The program measures the maximum and minimum anomalies, scales a few characteristic distances, calculates the initial values of the body parameters and improves them iteratively until a best fit is obtained between the observed and calculated anomalies.

Inversion of gravity and magnetic anomalies of two-dimensional polygonal cross sections

Abstract Two computer programs GPOLYIN and TPOLYIN coded in FORTRAN 77 are presented to invert respectively gravity and magnetic anomalies of two-dimensional (2-D) bodies of polygonal cross section. The computer input consists of the observed anomalies, their distances relative to a convenient reference point and the density contrast or the dip and direction of magnetization, as well as the coordinates of the vertices of the initial model. The programs solve for increments to the initial values of the coordinates using Marquardts optimization technique. The partial derivatives are calculated by numerical differentiation. The program TPOLYIN is valid for any magnetization and for anomalies in any component.

Gravity anomalies of three-dimensional bodies with a variable density contrast

The exponential decrease in the density contrast of anomalous bodies is equated to a quadratic function. The three-dimensional body is divided into a series of parallel vertical polygonal sections of unit thickness each. The gravity effect of each side of the vertical polygonal cross-section with a quadratic density variation is derived in closed form. Gravity effects of a few equispaced and representative cross-sections are integrated to obtain the gravity anomaly of the entire body. When used in modelling gravity data, the method of considering vertical cross-sections has the unique advantage of easily allowing changes in theZ-coordinates of polygonal sections.

Magnetic Anomalies and Basement Structure Around Vizianagaram, Visakhapatnam and Srikakulam Districts of Andhra Pradesh, India

Abstract A systematic regional magnetic survey was carried out in the districts of Vizianagaram, Visakhapatnam and Srikakulam in Andhra pradesh, India comprising an area of 15, 000 sq. km of eastern migmatite zone of Eastern Ghat Mobile Belt. The magnetic anomalies are very noisy, varying between −1300 nT and +700 nT in amplitude and correlate very poorly with the surface geology. Upward continuation of these anomalies brought out distinct magnetic anomaly trends, running along NE-SW on the south and turning later to E-W on the north, consistent with the folding pattern of Eastern Ghats. Based on the termination of anomaly closures and displacement of anomaly trends, five faults, all striking approximately in the N-S direction, were inferred. From inversion of anomaly profiles, it is established that the anomalies are produced by structures in the magnetic basement composed of charnockites.