Magnetic data are used on a routine basis to help geological mapping. Although geologists often use them in a qualitative fashion to map lithological units, they can be used to determine the depth and dip of these units. Magnetic interpretation techniques can be grouped in three major categories. Firstly, magnetic data can be interpreted by interactively fitting 2D models to profile data. Secondly, inversion techniques can be used where one attempts to fit, in a least squares sense, the calculated response of a model to the measured data. Finally, automated techniques, such as Euler deconvolution (Reid, 1990) and the analytic signal (Roest et al., 1992), allow quick estimation of source location and depth.