In-situ magnetostratigraphy: interpretation of magnetic logging in sediments

The interest of magnetic logging in different geological settings: oceanic crust, sedimentary basins and deep continental crust is discussed. In all these cases, a good estimate of both the magnetic susceptibility and the remanence of the surrounding rocks could yield invaluable information about the geological formations penetrated in boreholes. A calculation of all the components of the magnetic field produced in a borehole by horizontal magnetized layers is presented. The variations of these components are calculated along the axis and off axis through simple geological formations and particularly through a zone of transition of polarity of the earth's magnetic field. Both susceptibility and remanence are involved in the interpretation of the field variations recorded when a magnetometer is passed through a borehole drilled in sediments; the separation of these variations is envisaged using magnetic field and susceptibility logs recorded simultaneously. We present an example of magnetic logging in sediments by the measurement of total field intensity. The results are compared with a model calculated from laboratory measurements of the natural remanent magnetization and magnetic susceptibility of cores.