A window constrained nonlinear inversion method for interpretation of aeromagnetic data

Abstract

We introduce a nonlinear constrained inversion technique for 2D interpretation of aeromagnetic data along flight lines using a simple dike model. We first estimate the strike direction of a quasi 2D structure based on the eigenvector corresponding to the minimum eigenvalue of the pseudogravity gradient tensor (PGGT) derived from gridded magnetic field anomalies, assuming that the magnetization direction is known. Then the measured magnetic field can be transformed into the strike coordinate system and all magnetic dike parameters horizontal position, depth to the top, dip angle, width and susceptibility contrast can be estimated by nonlinear least squares inversion of the magnetic field data along the flight lines. We use the Levenberg-Marquardt algorithm together with the trust-region-reflective method which enables users to define inequality constraints on model parameters such that the estimated parameters always lie in a trust region. Assuming that the maximum of the calculated gzz (vertical gradient of the pseudogravity field) is approximately located above the causative body, data points enclosed by a window, along the profile, centered at the maximum of gzz are used in the inversion scheme for estimating the dike parameters. The size of the window is increased until it exceeds a predefined limit. Then the solution corresponding to the minimum data fit error is chosen as the most reliable one. Application of our method is demonstrated on a new aeromagnetic data set from the Särna area, West Central Sweden. Constraints from laboratory measurements on rock samples from the area are used in the inversion scheme.