GB774098A - Acoustic well logging - Google Patents

Abstract

774,098. Geophysical prospecting. SOCONY MOBIL OIL CO., Inc. Aug. 16, 1954, No. 23783/54. Class 118 (2). A system for measuring the interval between the generation of an elastic impulse at a transmitter positioned in a well bore and its reception at a receiver supported in a predetermined spaced relation from said transmitter comprises means for generating a voltage that varies monotonically from a predetermined initial value following the generation of said elastic impulse, a capacitive element, a switching circuit including normally non-conductive electron discharge means interconnecting said capacitive element and said sources of monotonically varying voltage, means effective in response to the arrival of said elastic impulse at said receiver for momentarily closing said switching circuit for flow of current therethrough to charge said capacitive element to a voltage equal to the magnitude of said monotonically varying voltage at the instant of arrival of said elastic impulse at said receiver, and means for measuring the voltage across the capacitive element. A cable 413, which may have a tension member in addition to transmitter and receiver conductors 414 and 415, carries a transmitter 410, comprising a relaxation oscillator pulsing a piezo-electric crystal 428, and a receiver 411. Transmitter 410 and receiver 411 are maintained at a predetermined spacing and may be moved up and down within the bore-hole 412. Transmitter 410 produces a pulse as indicated at 440 (Fig. 2) which may repeat, for example, 100 times per second. A gating unit 433 produces a negative voltage 441, in response to triggering by the transmitter 410, and controls a voltage generator 434 which provides a saw-tooth voltage as at 442. The length of the gate 441 is made greater than the time of travel of an elastic pulse in the lowest velocity formations to be investigated, but shorter than the period of the pulse repetition rate. The gain of amplifier 430 is also controlled by gating unit 433 to vary in the manner indicated at 443. A signal from the receiver 411 (as at 444) is thus amplified and fed to blocking oscillator 431 which provides a pulse 445 to operate an electronic switch 432. This momentarily connects a condenser to the voltage generator 434 so that it takes up the instantaneous voltage as indicated at 446 in Fig. 2. This occurs at the pulse repetition frequency and the voltage on the condenser will vary in accordance with the time interval #t (Fig. 2) dependent upon the velocity of the elastic impulses in the part of the well bore wherein the transmitter and receiver are situated. The voltage on the condenser is recorded on a recorder 435 driven by a pulley 437 whereby the movement of the recording chart is made proportional to the movement of the cable 413. Conventional circuitry for units 430 to 434 inclusive is given in Fig. 3 (not shown). A modification (Fig. 4, not shown) utilizes two receivers at a predetermined spacing and the signal from the receiver nearer to the transmitter triggers the gating unit 433. An auxiliary gating unit is provided to prevent triggering of unit 433 by the transmitter pulse due to cross-feed in the cable 413. Alternatively the battery 421 may be located down-hole in the housing of the transmitter, and thereby eliminate one conductor in the cable 413 and cross-feed therefrom. Instead of a saw-tooth voltage from generator 434 a voltage decreasing according to a rectangular hyperbole which is the reciprocal of a sawtooth may be provided (Fig. 6, not shown). Thus, the voltage is representative of velocity instead of time and the recorder 435 plots the velocities in the various formations directly. Conventional circuitry for generation and utilization of such a hyperbolic waveform is disclosed in Fig. 5 (not shown).