The advantage of magnetic gradient tensor measurement in deep boreholes

Abstract

Deep borehole magnetic measurement provides additional dimensional information, compared with aeromagnetic survey and ground-based magnetic measurement. Borehole magnetic measurement mainly depends on determing the three components of the magnetic fields at present, which can be used to enhance oil recovery and find mineral. However, it is susceptible to the geomagnetic time variations, extremely sensitive to the azimuth error, and greatly affected by the magnetic shielding of steel casing pipes. The magnetic gradient tensor measurement can effectively suppress the effects of geomagnetic variations and is insensitive to the azimuth error. In this work, the magnetic shielding effect of a casing is simulated by COMSOL firstly, and the correctness of the simulations is verified by the experiments of placing the casing in the east-west and north-south directions respectively. Next, the magnetic field component B z and the magnetic gradient tensor component B zz on the casing axis are simulated by using a magnet as the anomalous source, and field experiments prove that the magnetic gradient tensor is relatively less affected by the magnetic shielding of the casing. Finally, through the simulations of ore bodies with different concentrations and grain sizes of magnetite, the advantage of magnetic gradient tensor measurement in deep boreholes is proved.