Incorporating geologic structure into the inversion of magnetic data

Abstract

Magnetic field inversions are non-unique but a realistic goal is to find a causative earth structure that is compatible with the geophysical data, the petrophysical constraints, and with geology. Invariably the inversion results are improved as the number and diversity of constraints are increased. In this paper we concentrate upon the inclusion of geologic structural information. Geologic structural modelling programs can import faults, boundaries, and strike and dips of geologic units and interpolate this sparse information in space. When provided with a 3D voxel mesh, they can compute a strike, dip, and plunge for each cell. Following previous work, structural geologic information is incorporated into the inversion as a weak constraint by encapsulating it into the model objective function. The model objective function is formed such that each prism has its own set of rotated vectors to enforce smoothness along the direction of the geology. User-controlled parameters specify the degree of smoothness throughout the 3D volume and thus allow additional geologic insight to be directly incorporated. In addition to structural geology, the inversion algorithm utilizes reference models and bound constraints that help us realize our goal of incorporating all available information. The efficacy of the inversion is demonstrated through a synthetic and a field example.