Abstract: A method and device for detecting inversions of the earth's magnetic field by taking measurements in a drill shaft. The device includes a first probe used to measure the magnetic induction B.sub.1 during its movement in the shaft, a sensor to measure magnetic induction B.sub.2 on the surface of the ground at a stationary point at the same time as B.sub.1 is measured, a second probe to measure magnetic susceptibility X of the rocks as it moves through the shaft, and a data-recording and data-processing unit. The data-recording and data-processing unit further includes a device for calculating a value of B.sub.1 -B.sub.2, filtering the calculated value to isolate the induction fraction B.sub.L, which is only composed of a component B.sub.I resulting from an induced magnetization, and the component B.sub.R resulting from the residual magnetization. The induction B.sub.I based on the induction B.sub.1 in the shaft and on the magnetic susceptibility X can be calculated.