PURPOSE: To improve magnetic induction and controllability by providing a single or a plurality of polar pieces having a shape for controlling generated magnetic field, in cooperation with a magnetism source.

CONSTITUTION: This magnetic structure 3 has two polar pieces 5 and 6 secured to a magnetism source 4 and one of the magnetic poles thereof. The other pole of the magnetism source 4 is shaped to fill the internal space of a well 1 as much as possible. Also, the magnetic structure 3 is applied to the measurements of the relaxation time of protons of water or hydrocarbon for determining the porosity and magnetic permeability of the atmosphere. In addition, an antenna 8 having the capability of transmitting a high frequency electromagnetic excitation signal to the atomic nucleus of a geological layer zone 7 is installed between the two polar pieces 5 and 6, and at a position faced to the zone 7. In this case, the antenna 8 detects signals from the nucleus in response to the excitation signals. Furthermore, an antenna 37 having a phase orthogonal with the antenna 8 may be used. In this case, the antenna 37 is coupled to useful signals, similar to the case of the antenna 8, but not coupled electromagnetically.