Fremgangsmåte og apparat for å bestemme et instruments orientering i forhold til jordens magnetfelt

The orientation of an instrument, e.g. a well logging instrument 10 in a well bore 2, is determined with respect to the earth's gravity and magnetic field by means of apparatus which includes a plurality of magnetometers 8,12 each including sensing coils. Each of the coils has a sensitive axis separated from the sensitive axis of the other coils by a known angle so that a direction of an apparent magnetic field can be determined. The apparent magnetic field includes both the earth's magnetic field and interference from magnetic anomalies. The apparatus also includes accelerometers 18 each having a sensitive axis separated from the sensitive axis of the other accelerometers by a known angle so that a direction of the earth's gravity may be determined. The apparatus includes means for calculating the magnitudes of the directional components of the apparent magnetic field caused by the magnetic anomalies from the measurements made by the sensing coils and the accelerometers, so that the directional components of the earth's magnetic field can be determined. In a preferred embodiment, the sensing coils of each magnetometer, and the accelerometers, are mutually orthogonal. One coil from each magnetometer is set parallel to the instrument axis 14 and in opposite polarity to the other axially parallel coil. The other coils therefore lie in a plane perpendicular to the axis, and are separated from each other by a known angle.