Patent GB-1217544-A (GB)
Title: Acoustic borehole logging
Inventor: ZEMANEK JOSEPH
IPC: E21B47/08
Filed: 1968-01-31
Abstract: 1,217,544. Acoustic borehole logging; gating; switching; sawtooth generators; cathode-ray tube Circuits. MOBIL OIL CORP. 31 Jan., 1968, No. 5018/68. Headings H3T, H4D and H4T. [Also in Division G1] In a system for scanning the inner surface of a borehole, acoustic energy propagated towards said surface in a rotatable beam is reflected to a receiver whose output is displayed in sequences, each corresponding to a 360 degrees rotation, an electrical signal representative of the beam position in each rotation is applied to the display device, and the display sequences are spaced from each other in dependence on the position of the receiver along the borehole. In the Fig. 2 embodiment for determining the diameter of a borehole, the borehole wall is scanned by rotating directional acoustic transmitter-receiver (Tx-Rx) assembly 11, 12 while logging tool 13 is moved through the borehole. Echo signals arising from transmitted acoustic pulses are fed uphole as bright-up signals for the beam of a cathode-ray tube (CRT) 26. The wiper of downhole potentiometer 15 is rotated with Tx-Rx assembly 11, 12 and provides a sawtooth-waveform rotational position signal which is applied to an uphole self-balancing servo system which drives potentiometer 24 to give outputs at 22, 23 proportional to the sine and cosine of the rotation angle. Sawtooth waveforms derived from these outputs deflect the CRT beam and give a recurrent radial timebase sweep which is triggered by each acoustic transmission and rotates in synchronism with the assembly 11, 12, thus providing a P.P.I. display of the borehole cross-section. By delaying the start of the radial sweep (and hence reducing the sweep length) in dependence on the depth of the tool 13 in the borehole a pseudo three-dimensional display of the borehole crosssection (Fig. 6, not shown) may be obtained, in which cracks (e.g. 119) extending down the borehole can be detected. The display may be photographed. Modifications of the downhole potentiometer 15 and uphole servo-system, using continuous-wound potentiometers with one or two wipers to eliminate " dead spots "  n the resistive tracks, are described (Figs. 4 nd 5, not shown). Instead of the P.P.I. display linear raster-type display (Fig. 17, not shown) may be used: the sawtooth output from potentiometer 15 provides horizontal beam deflection, and vertical deflection (or movement of a film used to photograph the display) is related to the movement of tool 13 through the borehole. Each line of the resulting display has a length corresponding to one revolution of Tx-Rx assembly 11, 12 and the display enables the dip of an interface traversed by the borehole to be determined. To provide borehole diameter information on this display a low-amplitude sawtooth voltage modulates the vertical deflection so that diameter variations appear (Fig. 9f, not shown) as deviations of the display lines from the straight. Scanning of the borehole wall may be effected by sequentially energizing several transmitting transducers spaced around the circumference of tool 13, instead of by rotating transmitter 11 mechanically. Indication of the geographical orientation of the Tx-Rx assembly 11, 12 is provided by a downhole magnetic or gyro compass, a flux gate magnetometer, or a Hall effect device, Fig. 13, which rotates with assembly 11, 12 and comprises Hall plate 200 and magnetic flux concentrators 200a, 200b. A 10 kc. input signal from source 202 produces an amplitude modulated 10kc output signal, of amplitude dependent on the orientation of plate 200 with respect to the earth's magnetic field. This output signal is synchronously detected by phase-detector 204 in the downhole equipment of Fig. 18 and a positive pulse corresponding to the envelope voltage crossing the zero axis in a positive direction (e.g. corresponding to the Tx-Rx assembly facing north) is produced by amplitude discriminator 210, monostable circuit 211 and pulse generator 502 for transmission uphole. Also transmitted uphole are negative sync. pulses (from generators 11a and 507) corresponding to each acoustic transmission, and the envelope of the receiver signals (derived by demodulator network 511, 512, 513). In the uphole equipment, Fig. 19, the positive orientation pulses, negative sync. pulses and demodulated receiver signals are separated and are applied to CRT 26 for respectively initiating the horizontal deflection, conditioning the CRT for intensity-modulation by the receiver signals (thereby preventing bright-up of the display by spurious signals), and modulating the display intensity. Vertical deflection is related to movement of tool 13 through the borehole, giving a linear raster-type display. Circuitry may be provided for intensifying or blanking the CRT beam at vertical intervals corresponding to 10 ft. depth intervals in the borehole. A downhole switch or the waveform from potentiometer 15 (Fig. 2) may be arranged to initiate the horizontal deflection in the event of failure of the Hall effect device. The receiver signals may be subjected to automatic gain control or to amplitude range compression prior to display. Sawtooth waveforms for the Fig. 2 P.P.I. display are produced by the Fig. 3 circuit. Transistor 55 in sampling amplifier 25 is normally conducting, but is made non-conducting by application to its base of a sync. pulse (from monostable delay circuit 33 and buffer amplifier 36), whereupon capacitor 37 charges approximately linearly through resistor 56 towards a voltage E v  proportional to the sine of the rotation angle of Tx-Rx assembly 11, 12 (Fig. 2). The capacitor voltage is applied via buffer stage 39 to the vertical deflection circuit of the CRT. The horizontal deflection waveform is similarly derived from a capacitor 38 which charges towards a voltage EH proportional to the cosine of the rotation angle. Amplitude discriminator 210 in the downhole equipment of Fig. 18 comprises, Fig. 14, a tunnel diode 400 provided with an adjustable D.C. bias voltage from network 403, 404 and gives an output pulse from amplifier 407 when the input, fed over emitter follower 401, passes through zero in a positive direction. A gating circuit, Fig. 15, for blanking the CRT display for the duration of a 50 Ás pulse comprises stages 409, 410 supplied with the receiver signal over resistor 408. The 50 Ás pulse makes transistor 410 conducting, thereby preventing the receiver signal from passing via stages 409 and 411 to the CRT. At the same time, the 50 Ás pulse makes transistor 412 non-conducting so that the 50 Ás pulse passes via stages 413 and 411 to the CRT for beam blanking.