Gravity inversion of a discontinuous relief stabilized by weighted smoothness constraints on depth

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 in which three kinds of constraints are imposed on parameters: (1) proximity between values of adjacent parameters, (2) lower and upper bounds to parameters, and (3) 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 updated automatically to enhance any discrepancy between adjacent depths that have been detected at the initial solution. Constraints 2 and 3 are used both to compensate for the decrease in solution stability caused by the introduction of small weights and to reinforce flatness at the basin bottom. Constraint 2 imposes that any depth be nonnegative and smaller than an a priori known maximum depth value, whereas constraint 3 imposes that all depths be closest to a value greater than the maximum depth. The trade-off between these conflicting constraints is attained with a final relief presenting flat bottom and steep borders. The method was tested with a synthetic gravity anomaly produced by a simulated sedimentary cratonic extensional basin whose basement consists of steep edges and a hat 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. Additionally, the method produced excellent estimates for the average dip angles of the basin edges (presumably controlled by normal faults), indicating, in this way, its potential in interpreting data produced by this kind of basin. The method was applied to the Bouguer anomaly from the northern portion of Steptoe Valley, Nevada, delineating an isolated basin with a wider, flat base and relatively straight borders as compared with the estimate imposing overall smoothness on the relief.