Inversão Gravimétrica Do Relevo Descontínuo De Uma Bacia Sedimentar

We present a new stable gravity inversion method applied to the mapping of an interface separating two homogeneous media. In contrast with previous similar methods, it does not impose an overall smoothness on the estimated interface to stabilize the solution. The density contrast between the media is assumed to be known. The interpretation model for the upper medium consists of rectangular juxtaposed prisms whose thicknesses represent the depths to the interface and are the parameters to be estimated. The true interface is assumed to be flat everywhere except at faults. To incorporate this attribute into the estimated relief, we developed an iterative process where three kinds of constraints are imposed on parameters: (a) proximity between values of adjacent parameters, (b) lower and upper bounds to parameters, and (c) proximity between the values of parameters and fixed numerical values. Starting with an initial solution which presents an overall smooth relief, the method enhances initially estimated geometric features of the interface; that is, flat areas will tend to become flatter and steep areas will tend to become steeper. This is accomplished by weighting the constraints, which requires proximity between adjacent parameters. The weights are initialized with values equal to unity and are automatically updated to enhance any discrepancy between adjacent depths that have been detected at the initial solution. Constraints (b) and (c) are used both to compensate for the decrease in solution stability due to the introduction of small weights, and to reinforce flatness at the basin bottom. Constraint (b) imposes that any depth be nonnegative and smaller than an a priori known maximum depth value whereas constraint (c) imposes that all depths be closest to a value greater than the maximum depth. The tradeoff between these conflicting constraints is attained with a final relief presenting flat bottom and steep borders. The method was tested with synthetic data produced by a simulated sedimentary cratonic extensional basin having a smooth floor, steep edges and known depth to the bottom. The results showed an improvement in the resolution of the relief, leading to a reliable mapping both of the sharp discontinuities at the borders and of the lateral extent of the base of the basin. The method was also applied to the Bouguer anomaly from the northern portion of Steptoe Valley, Nevada, delineating an isolated basin with a wider, flat base and more straight borders as compared with the estimate imposing overall smoothness on the relief.