Borehole measurements within highly magnetic bodies – corrections of measured magnetic fields and gradients

Abstract

This paper gives explicit expressions for the internal field and gradient components for a layered earth, a layer with gradational susceptibility, a dipping sheet, a sphere and a cylinder, that are exposed to an external field with a uniform gradient. However, the internal field and gradient components cannot be measured directly, as magnetic sensors must be placed within a cavity in the magnetic medium. This modifies the measured field and gradients. For low to moderate susceptibilities, the cavity effect can be calculated assuming that the magnetisation of the surrounding medium is essentially unperturbed by the presence of the cavity. This assumption is unacceptable when the surrounding medium has high susceptibility. I also give expressions that allow the true field and gradient components within a high susceptibility body to be calculated from measurements made in cylindrical cavities, such as boreholes, or in spherical or disc-like cavities.