GB936461A - Apparatus for depth measurement in cased boreholes - Google Patents

Abstract

936,461. Locating magnetic bodies. KOOLAJIPARI TROSZT. Oct. 1, 1959, No. 33359/59. Class 37. Apparatus for depth measurement in cased boreholes in which casing collars link successive longitudinal sections of the casing tube, comprises a sonde having a first pair of differentially connected coils 2, 2<SP>1</SP>, Fig. 1, around permanent magnets 4, 4<SP>1</SP> mounted perpendicular to the axis of the sonde and spaced by a distance equal to the collar casing, for measuring the casing reluctance whilst the sonde is being moved in the borehole and thus developing a pulse as the coil assembly passes each collar, and a second pair of coil 5, 5<SP>1</SP>, Fig. 2, which develop a signal proportional to casing reluctance whilst the sonde is stationary in response to an alternating current applied to a pair of coils 4, 4<SP>1</SP>. Single cables with earth return are used for supplying the coil systems and for returning the vindicating signals to apparatus on the surface. In a circuit arrangement, Fig. 4 (not shown), for the A.C. arrangement, D.C. is chopped by a pulsator for feeding to the inducing coils, and commonly driven pulsating contacts rectify the current in the measuring circuit, a variable resistor being provided in the feeding circuit for current adjustment, and a smoothing circuit being inserted between the rectifying contacts and the recorder in the measuring circuit. Three wire arrangements for depth measurement and operation of a casing perforator including either a remotely controlled switch or a blocking condenser are described with reference to Figs. 5 and 6 (not shown). The arrangements are described in combination with a radio active welllogging sonde. The depth measurement coils serving also as the radiation pick-up coils.