Abstract: A sensor for detecting a magnetically permeable mass by the disturbance of the earth's magnetic field in the vicinity of the sensor comprises a ferromagnetic strip with a conductive winding wrapped thereabout, a means for magnetically biasing the strip such that regardless of orientation the earth's magnetic field and the strip placing the ferromagnetic strip in a position on its BH curve where the slope is substantially linear, a means for generating an analog signal indicative of the inductance of the winding as the earth's magnetic field is disturbed in the vicinity of the ferromagnetic strip causing a change in the inductance of the winding, a means for digitizing the analog signal at spaced time intervals to produce a series of digitized values, and a digital computer means for processing the digitized signal values to characterize the presence and/or motion of the permeable mass.