The slope correction method of analysing magnetic data from a sloping survey plane

Abstract

When analysing magnetic data collected on a sloping survey plane, in areas with steep topography or in inclined boreholes, attention must be paid to the magnetic inclination and declination. An analysis of these quantities forms the basis of a new method of analysing magnetic data: the slope correction method. Two new quantities are used to describe the magnetic field. They are the apparent inclination and the apparent declination, both of which depend upon the magnetic field orientation, and the slope and the azimuth of the survey plane. They are readily computed for other orientations of the survey plane since they only involve the inclination and the declination of the induced and remanent magnetisations. A series of graphs showing how these elements vary with orientation of the survey plane, for the complete range of magnetic latitude, provide insight into the effect of the attitude of the survey plane on the measured field. The slope correction method has a wide range of applications which include analysis of magnetic survey data from boreholes and mine shafts, and ground and airborne surveys in rugged terrains. For a particular case of a ground magnetic survey over a steep mountain in Papua New Guinea, an application of the slope correction method revealed that the anomalous fields observed on two sides of the mountain could be produced by a single structure, and not two distinct structures as might be first thought from an inspection of the data. The method highlights a possible source of error in applying the reduction-to-the-pole filter to magnetic data from an undulating survey surface.