Borehole magnetometry; detection of a steel liner from a nearby observation borehole

Abstract

Steel casings and liners in boreholes produce magnetic fields which can be observed in adjacent boreholes and on the earth's surface. In this application, we attempted to measure the perturbation to the magnetic declination caused by a steel liner at depth. At one site the presence of the liner was clearly revealed by the horizontal and vertical magnetic fields in boreholes located 20 m and 25 m from the hole containing the liner. However, reproducibility of the declination among subsequent runs was never better than ±2 degrees, and was as much as ±5 degrees. Despite the poor repeatability, constraints can be placed upon the declination by using a monopole model of the liner which duplicates the character of the magnetic field components.