Patent GB-1306781-A (GB)
Title: Method and apparatus for borehole directional logging
IPC: E21B47/022
Filed: 1971-03-08
Abstract: 1306781 Electro physical measurements TEXACO DEVELOPMENT CORP 8 March 1971 6262/71 Headings G1F G1N [Also in Division G4] The position of a borehole at any given depth is determined from electrical signals representative of the inclination and azimuth angles at the given depth. In a first arrangement, Fig. 3 the logging apparatus includes a magnetic shielding case 13 having an inclination coil 10 coupled with a constant speed motor 11 rotatably mounted therein. Helmholtz coil 40, Fig. 5 (not shown) is gimbally mounted to the upper case 13 so as to subject the coil 10 to a vertical field so that if the instrument case is tilted through an angle due to the deviation of the borehole an A.C. is induced in the inclination coil which is proportional to sin # 1  where # 1  is the angle of inclination of the borehole. Attached to the lower end of shaft 19 by a flexible shaft 21 is a compass coil 20, Fig. 4 (not shown) which rotates about a vertical axis at the same rotational rate as the inclination coil 10 and which is mounted in non-magnetic lower housing 15. Thus an A.C. signal is induced in the compass coil 10 as the latter cuts the earth's magnetic field as it rotates about its vertical axis. It is shown that the measured phase angle α between the A.C. induced in the compass coil 20 and the A.C. induced in the inclination coil 10 is the azimuth angle # 1  of the borehole directly. The compass coil 20 may be mounted on a rigid shaft so that its axis of rotation is the same as coil 10 in which case the phase angle α is not uniquely a function of the angle # 1  because the coil is subject to both vertical and horizontal components of the earth's field and correction of angle α is needed to give uniquely # 1  In an alternative arrangement Fig. 7 a gyrocompass 70 is incorporated to provide a horizontal magnetic field of predetermined azimuth direction intersecting the compass coil 20a in place of the earth's magnetic field. The horizontal magnetic field is derived from permanent ring magnet 82 having a pair of diametrically opposed pole pieces 83,84 and which has complete pivotal freedom and is maintained horizontal by plumb bob 85. The gyro compass is aligned with a reference direction before the instrument is lowered into the borehole. As in the Fig. 3 embodiment the A.C, induced in the inclination coil 10, not shown in Fig. 7, as it rotates about the axis of the borehole is indicative of the inclination angle # 1 . The azimuth angle # 1  is obtained by measuring the phase angle angle α between the voltages induced in the inclination coil 10 and compass coil 20a and solving the equation sin # 1 = sin α (1+cos<SP>2</SP> α tan<SP>2</SP># 1 )<SP>-¢</SP> Fig. 11 (not shown) In a modification of the arrangement of Fig. 7, Fig. 9 (not shown) the horizontal magnetic field of predetermined azimuth direction is provided by mounting the compass coil 10a in a pivotally free gimbal ring having a pair of diametrically opposed bar magnets mounted thereon. Interaction of the earth's magnetic field with the bar magnets causes the arrangement to act as a compass to maintain the gimbal ring in a constant horizontal azimuth direction. In order to determine the position of the borehole at any depth an integration or summing up of the effect of all the instantaneous values of inclination and of azimuth angle over the entire borehole path from the earth's surface to the depth L of the borehole is made. Signals representative of the voltages induced in the inclination coil and compass coil are transmitted by the logging cable to apparatus, Fig. 1 (not shown) where they are processed to provide azimuth # 1  and inclination angle # 1  signals which are recorded at incremental lengths # L of the cable together with the total length L of the cable at the respective points. Analogue, see Division G4 Fig. 2 (not shown) digital computations are made to also derive signals representing y and # by trial and error using successively determined values of y and # where γ is the distance to any point P along the borehole axis in a cylindrical co-ordinate system measured from the vertical Z axis of a Cartesian co-ordinate system with its origin at the beginning of the borehole and # is the angular displacement of point P with reference to the x-axis of the co-ordinate system.