Reduction to the pole and transformations of scattered magnetic data using Newtonian equivalent sources

FERNANDO GUSPI; IVAN NOVARA

GEOPHYSICS

SOC EXPLORATION GEOPHYSICISTS

Año: 2009 vol. 74 p. 67 - 73

0016-8033

We present an equivalent source method  for performing reduction to the pole and related transforms from magnetic data measured on unevenly spaced stations at different elevations. The equivalent source is composed of points located vertically beneath the measurement stations, and their magnetic properties are chosen in such a way that the reduced-to-the-pole magnetic field generated by them is represented by an inverse-distance Newtonian potential. This function, which attenuates least with distance, provides a better coverage for discrete data points. The magnetization intensity is determined iteratively, until the observed field is fitted within a certain tolerance related to the level of noise, and so advantages in computer time are gained over the resolution of large systems of equations. In the case of induced magnetization, the iteration converges well either for rather vertical or horizontal inclinations, and results are stable if noise is properly taken into account. However, for a range of intermediate inclinations around 35º, a factor tending to zero makes it necessary to perform the reduction through a two-stage procedure, using an auxiliary magnetization direction, without significantly affecting the speed and stability of the method. The performance of the procedure is illustrated in a synthetic example based on the field generated on randomly scattered stations by a random set of magnetic dipoles, contaminated with noise, which is reduced to the pole for three different magnetization directions. Results provide a good approximation to the theoretical reduced-to-the-pole field, either using a one or a two-stage reduction, showing minor noise artifacts when the direction is nearly horizontal. In a geophysical example with real data, reduction to the pole is used as a tool for correcting the estimated magnetization direction that originates an isolated anomaly over Sierra de San Luis, Argentina.