Software Development of a Three-Dimensional Gravity Inversion and Application to Study of the Border Ranges Fault System, South-Central Alaska

The Border Ranges Fault (BRFS) system bounds the Cook Inlet and Susitna Basins, an important petroleum province within South-Central Alaska. Our research involves the testing of several plausible density models of structure along the Border Ranges Fault System by developing a novel, 3D inversion software package. The inversion utilizes gravity data constrained with geophysical, borehole, and surface geological information to produce a density solution. The novel inversion approach involves directly modeling known geology, initially free-air corrected data, and revising a priori uncertainties on the geologic model to allow comparisons to alternative interpretations. Previous modeling of the BRFS using geophysical data has been limited due to the complexity of local geology and structure, both of shallow crustal features and the deeper subduction zone. Since the inversion is based on a sequence of gridded surfaces, it is feasible to develop software to help build these gridded geologic models. While similar inversion software packages utilize rectangular, hexahedronal, and triangular prisms, ours is unique in the application of semi-infinite vertical line elements to deal with the complex geometry of surface and sub-surface structures. In addition, the algorithm uses an initial distance test from a gravity station to the area of study to eliminate insignificant computations. Interval computations, Kalman filtering, and covariance between forward modeling, process noise, and DEM uncertainties are considered in the model solution. Further considerations in the software development will include a user interface with grid visualization capability and the means to observe a block of earth structure from different perspectives.