NMR sensing apparatus, which may be used in well logging, includes a magnet arrangement (10) for generating a static magnetic field in a toroidal region (9) containing materials to be analyzed. The magnet arrangement (10) includes at least four magnets arranged along a longitudinal axis (11) which may coincide with the axis of, for example, a bore hole (7) being investigated. A coil arrangement (16) is used both for generating an r.f. magnetic field in the region (9) so as to excite nuclei of the materials to be analyzed, and for receiving NMR signals from the excited nuclei. An indication of the properties of the materials being analyzed is then obtained from the received NMR signals by means of a computer (32).
