Patent GB-911412-A (GB)
Title: Depth measuring system
IPC: E21B47/04
Filed: 1958-12-11
Abstract: 911,412. Electrical measuring systems. SOCONY MOBIL OIL CO. Ltd. Dec. 11, 1958 [Feb. 15, 1958], No. 40047/58. Class 40(1). [Also in Groups XIX and XXXVI] Apparatus for determining the true length l 0  of an extensible cable 12 (Fig. 1) supporting a geophysical exploration tool 10 inside a bore hole 11 is shown to be given by the equation where l m  = length as measured by sheave 18 at the surface, K = coefficient of increase in length of the cable, T m = total tension in cable as measured at surface by strain gauge system 16, x m  = instantaneous value of l m , t m  = instantaneous value of T m , and l<SP>1</SP> 0  = length of cable with a predetermined value of tension. The system of Fig. 1 develops each of the terms of this equation in the form of angular rotations of shafts of the appropriate amplitude and sign using disc and wheel integrators 39, and algebraically adds them by differential gears 37, 41 or selsyn devices to provide a record on a chart 21 of the true depth of the tool 10. While the chart 21 is driven by a signal representing the tool depth, the recording pen 23 is simultaneously given a lateral displacement in accordance with a potential difference representative of a selected characteristic of the strata at the level of the tool 10. The system automatically takes into account the magnitude and direction of the frictional forces on the cable, and may be modified to allow for (a) The apparent decrease in weight when the bore hole is full or partly full with liquid (Fig. 2, not shown). (b) An approximation which considers the integral term of the equation as effectively equal to #<SP>lm</SP> 0 Kt m  dx m , and the last term as K/2T m l m  (Fig. 5, not shown). By making other practical approximations the true length l 0  of the cable is shown to be given by the equation the system of Figs. 6 and 7 (not shown) developing the terms of this equation in the form of proportional voltages, and algebraically adding them, using devices similar to those shown in Fig. 1. Fig. 9 shows a system for solving the latter equation using pulse techniques, the length l m  being measured by a pulse generator 300, which produces a pulse for each increment of length of cable as measured at the surface (dx m ), which is positive or negative depending on whether the cable is being wound up or unwound, the length of the pulse being determined by the tension t m  as measured by the strain gauge system 16. These pulses gate a train of pulses from a further generator 313, which are algebraically added in a counter 314 to provide the second term of the equation. Similarly, the tension t m  produces control pulses at a proportional rate from generator 300<SP>1</SP>, whose durations are determined by the length x m , to produce corresponding trains of pulses from generator 320, which are algebraically counted at 324 to provide the third term of the equation. The sum of the two integrals is converted to an angular rotation by a reversible stepping relay 343, and combined with the first term of the equation in a differential selsyn 345, feeding the recorder drive. In a further modification, the sum of the first two terms of the equations may be checked by adding pulses obtained from a series of magnetic marks placed on the cable at predetermined positions. The tension measuring system 16 comprises four strain gauge resistors 250-253 (Fig. 8) mounted on a ring-shaped element 15 (Fig. 1) so that tension in the cable produces tension in resistors 250, 251 and compression in resistors 252, 253. The resistors are connected in an automatically balanced bridge circuit 16a, whose zero output conditions are determined by potentiometer 257 across the supply 254, and the bridge unbalance current is calibrated by a rheostat 273, and produces an equivalent output signal from a separate, isolated bridge 280 for use in the computing systems. Specification 774,098 is referred to.