Borehole magnetics: Navigation, vector components, and magnetostratigraphy

Abstract

Navigation surveys of boreholes record three pieces of information: direction, dip, and depth. In magnetic compass devices generally the magnetic sensor is used to define the orientation of the borehole relative to local magnetic North. No consideration is given to the possibility of local deviations of the magnetic field vector. The dip of the borehole is derived from either a tiltmeter, or bubble level. Depth is actually measured as distance along the hole. Using a magnetic navigation probe that consists of a three-axis fluxgate magnetometer and orthogonal tiltmeters it is possible to separate rotations of the magnetic declination from changes in the orientation of the borehole. Also since the full magnetic vector is measured it is possible to use this information to locate the source of magnetic bodies since they give rise to diagnostic magnetic vector fields. As variations in the orientation and magnitude of the magnetic vector also reflect the changes in susceptibility and remanence orientation with lithology it is also possible to use the same information to obtain a map of subsurface lithological variations as defined by total magnetic field variations. Examples are provided from a survey of three boreholes through the Onaping-Onwatin contact of the Sudbury Structure.