GB908467A - Improvements relating to borehole survey apparatus - Google Patents

Abstract

908,467. Electric indicating systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. Dec. 18, 1958 [Sept. 24, 1957], No. 29897/57. Class 40 (1). [Also in Groups XXXVI and XL (c)] In a bore-hole surveying apparatus (see Group XXXVI), the attitude of the testing sonde is determined by representing the components of the bearing angle from signals derived from an inductor compass mounted on a gimbal platform and the components of tilt derived from means responsive to the setting platform with respect to one gimbal member and to the setting of the gimbal member itself with respect to the sonde. As shown in Fig. 2, two inductor units 20, 21 are mounted with their longitudinal axes at right angles to each other, thus forming an inductor compass. The units 20, 21 are mounted on a platform 22 pivoted about one transverse axis of one gimbal ring 23, which is itself pivoted about another transverse axis of another gimbal ring 24, the two gimbal axes being mutually perpendicular. A rotary potentiometer 25 has its shaft rotated in dependence upon the movement of ring 23 with respect to platform 22 so that the output of potentiometer 25 (X tilt potentiometer) is a measure of one component of tilt of the sonde with respect to the vertical. Similarly, the output from potentiometer 29 (the Y tilt potentiometer) will give a second component of the tilt of the sonde. The ring 24 is fixed to the sonde casing and if the sonde should twist about its longitudinal axis in the bore hole, inductor unit 20, 21 will rotate in a horizontal plane with respect to magnetic north and signals will be derived from them representing two components of the bearing angle of a fixed reference in the sonde with respect to north. As shown in Figs. 3 and 4, each inductor unit 20, 21 consists of two thin wires 40, 41 each overwound with the primary exciting winding 42, 43, energized with alternating current of, for example, 1 kc/s. frequency. The primary windings are enclosed in a common secondary winding 44 from which an output is derived. When the inductor unit is exposed to the earth's magnetic field, wires 40, 41 are magnetized in proportion to the component of the field along their direction, the amplitude of the sinusoidal output varies in dependence upon the angle between the longitudinal axis of the unit and magnetic north, the phase of the output indicating the sign of the angle, each unit of the inductor being followed by a phasesensitive rectifier to resolve the ambiguity of the sign. Signals indicative of the bearing angle are transmitted from the sonde to a receiving equipment situated on the ground, Fig. 10 (not shown). see (Group XXXVI), via a time multiplexed pulse position modulation system. At the receiver the demodulated signals are fed to an electro-mechanical resolver which has two stators, the magnetic axes being mutually perpendicular and having a D.C. energized rotor winding. The resolver also includes a servomotor which is responsive to a signal derived from the rotor to rotate it to a position where the output voltage is approximately zero, the angular displacements of the rotor being then representative of the bearing angle. The rotor shaft is calibrated to give an indication of the sonde orientation, also a potentiometer collector shaft produces a voltage analogous to the bearing angle. This voltage triggers a scanning waveform generator of a cathode-ray tube on which the indication obtained from the sonde is displayed so that the scan is always synchronized with the signals from the probe electrode of the sonde which is orientated in the direction of magnetic north.