GB2133882A

GB2133882A - Circuit for controlling the magnitude of amplification of signals produced by a borehole televiewer

Info

Publication number: GB2133882A
Application number: GB08401298A
Authority: GB
Inventors: Frederick Henry Kreisle Rambow
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1983-01-20
Filing date: 1984-01-18
Publication date: 1984-08-01
Also published as: GB2133882B; FR2539880B1; CA1201797A; JPH0432961B2; FR2539880A1; GB8401298D0; JPS59137808A

Abstract

An electronic circuit is provided for improving the response of a borehole televiewer (10,11) in an out-of-round or off-centre borehole. The circuit comprises an automatic gain control circuit (40,45,47,50,51) having time constants that allow detection of borehole anomalies (50,51) while having a fast enough rise time (45,51) to adjust for the effects of out-of- roundness or the televiewer being off- centre in the borehole. The borehole televiewer comprises a rotating acoustic transducer (10) which is periodically energized to produce acoustic energy that is reflected from the borehole wall back to the transducer. The amplitude of the reflected energy, indicative of fractures, etc., is displayed on a CRT and may be photographed. The CRT sweep is synchronised with a north synchronisation signal produced by a flux gate magnetometer (11) rotating with the acoustic transducer (10). The transducer may be pulsed at a rate of 1500 pps with a rotation period of 0.333 seconds. <IMAGE>

Description

SPECIFICATION Circuit for controlling the magnitude of amplification of signals produced by a borehole televiewer The invention relates to borehole logging tools and particularly to a tool that is described as a borehole televiewer. A borehole televiewer utilizes a rotating acoustic transducer that is periodically pulsed to produce acoustic energy which travels outwardly and strikes the borehole wall and is reflected back to the transducer. The amplitude of the reflected signal is displayed on a cathode ray tube whose face may be photographed. The resulting photograph represents a map of the borehole wall split along the north direction and laid out flat. The transducer is pulsed at a relatively high rate, for example 1 500 pulses per second, while the transducer rotation period is 0.333 seconds.The sweep of the oscilloscope is controlled by a north synchronization signal which is produced by a flux gate magnetometer which rotates in unison with the transducer.

While the above tools are useful in obtaining a display of the borehole wall that indicates the locations of fractures or other features, it does have a serious iimitation when used in a noncircular or elliptical borehole. The same problem also arises when the tool moves off centre as a result of this travel through the borehole. It can be shown that when the borehole televiewer is located off centre or in a non-circular hole, the acoustic energy will be scattered at all locations except where the acoustic energy strikes the borehole wall normal to a tangent at this point.

Thus, the reflected energy received by the transducer will have four positions where it is at a maximum and in the remaining positions the reflected energy will be decreased. The borehole televiewer as presently available has various amplifier gain levels that are selected by a rotary switch controlled from the surface. Since the amplitude is set from the surface, it cannot be varied to provide the proper amplification as the transducer rotates. Thus, the resulting display will have light and dark areas where details are lacking. It should be appreciated that in normal operations a fracture or other anomaly in the borehole wall will not reflect energy and will appear as a dark shadow on the display.

The present invention solves the problems of utilizing a borehole televiewer in a non-circular borehole by providing a circuit for automatically controlling the magnitude of amplification of signals produced by the borehole televiewer, the circuit comprising: amplifier means having an input and an output, the input of said amplifier means being coupled to the acoustic transducer of the televiewer, gate means having an input, an output and a gate, the input of said gate means being coupled to the input of said amplifier and the output of the gate means being coupled to ground, and a feed back loop, said feed back loop being coupled to the output of said amplifier and to the gate of said gate means to control said gate means in response to the output signal of said amplifier.In a suitable embodiment of the invention the circuit has a relatively slow rise time and a very fast decay or fall time. It is observed that the circuit is not a true automatic gain control circuit in the sense that the gain of the amplifier is changed but rather a voltage dividing circuit that shorts part of the incoming signal to ground and thus decreases the input signal to the amplifier.

This allows the amplifier to retain the gain setting selected from the surface while increasing the signal level to provide details when the reflected signal is weak.

In an attractive embodiment of the invention the decay constant for the circuit is less than the cycle time of the transducer so that the amplifier will not saturate when the reflected signal suddenly increases in amplitude. Preferably, the rise time of the circuit extends over several cycles so that the amplifier will not obscure fractures or other anomalies in the borehole wall that have a short circumferential extent along the wall.

In a preferred embodiment of the invention the gate means of the circuit is a field effect transistor whose gate voltage is controlled to control its conduction to ground. Thus, the field effect transistor acts as a voltage divider circuit in the input circuit of the amplifier.

The invention will be more easily understood from the following description when taken in conjunction with the attached drawings in which: Figure 1 is a schematic elevation view of a borehole televiewer.

Figure 2 is a waveform of the signals produced by the borehole televiewer.

Figure 3 is a graphic representation of the borehole televiewer located in a non-circular borehole.

Figure 4 is a schematic drawing of the automatic gain control circuit of the invention.

Referring now to Figure 1, there is shown the borehole televiewer that includes an acoustic transducer 10 and a flux gate magnetometer 11 which are mounted on a common shaft. The shaft is rotated by motor 12 about the axis 1 4 of the borehole televiewer. Electronics for control of the pulsing of the transducer and the transmission of the reflected signals to the surface is contained in a section 1 6 of the tool while a pressure equalization chamber 1 5 is mounted in the bottom portion of the tool. The pressure equalization chamber serves to equalize the pressure on the interior of the acoustic window 1 3 with the pressure of the fluid in the borehole.By equalizing the pressure the required mechanical strength of the acoustic window can be reduced and allow the use of materials which provide a good acoustic match between the fluid filling the borehole televiewer and the borehole fluid.

Referring to Figure 2, there is shown the waveforms of the signals produced by the borehole televiewer. In particular, the signal 20 represents the acoustic pulse produced by the transducer while the signal 21 represents the reflected signal received by the transducer. As shown, a time interval of approximately 100 microseconds elapses between the production of the acoustic pulse and the receipt of the reflected signal. The time lapse, of course, depends upon the diameter of the borehole but 100 microseconds is representative. The actual borehole televiewer tool that is available commercially has a repetition or cycle rate of the acoustic transducer that is approximately 1 500 cycles per second while the rotational cycle of the transducer is 0.333 seconds.Thus, the borehole televiewer provides approximately 500 acoustic pulses per rotation of the transducer for scanning the borehole wall.

Referring to Figure 3, there is shown a schematic arrangement of the borehole televiewer in a borehole wall that illustrates the problem produced by a non-circular borehole or a borehole televiewer positioned off centre. As shown, the transducer 10 is located at approximately the geometric centre of the non-circular borehole 31.

When the acoustic pulse travels along the path 32, it is appreciated that it strikes the borehole normal to a tangent at the point at which it strikes and is reflected back to the transducer 10. In contrast, when the acoustic energy travels along the path 34, it strikes the borehole wall at an angle to a tangent at the point at which it strikes and instead of being reflected back, the major portion of the energy is scattered or reflected along the path 35. From the above description, it can be readily appreciated that the energy received by the transducer along the path 32 will greatly exceed the acoustic energy received along the path 34. Thus if details of the borehole are to be displayed some means must be taken for increasing the gain of the transducer amplifier as it rotates from the position 32 to the position 34.

While the gain must be adjusted, it must be adjusted slowly since the change in the path of acoustic energy occurs at a slow rate. While the gain must be adjusted slowly there are positions where the energy may be reflected directly back to the acoustic transducer due to a fracture or other anomaly in the borehole wall. At these positions, the gain of the amplifier must be decreased very rapidly so that the anomaly is not obscured in the final display of borehole televiewer data.

Referring to Figure 4, there is shown a simple automatic gain control circuit for controlling the transducer amplifier. The input 41 of the circuit is coupled to the transducer (not shown). The circuit utilizes a field effect transistor 47 which is operated as a voltage divider to shunt a portion of the input signal of the amplifier 40 directiy to ground. The gain control circuit does not adjust the gain of an amplifier as is traditional with automatic gain control circuits but rather is a voltage dividing circuit formed by the resistor 42A and the resistance of the field effect transistor 47.

The resistance of the field effect transistor 47 is proportional to the voltage applied to the gate 46.

The input 41 is both resistively and capacitively coupled to the amplifier 40 through a circuit 42 comprising a resistance 42A and a capacitance 42B. The output 49 of the amplifier is coupled to an impedance 43 which is used to match the output signal of the amplifier to a logging cable 52. The automatic gain control is provided by a feedback loop 44 which signal is rectified by a diode 45. The rectified signal is applied to the gate 46 of the field effect transistor 47 to control the conduction thereof. By controlling the conduction of the field effect transistor 47 the input voltage applied to the amplifier 40 can be controlled. The feedback signal charges a resistance capacitance circuit comprising a resistance 50 and a capacitance 51 , which circuit couples the feedback loop 44 resistively and capacitively to ground.The values of the resistance 50 and the capacitance 51 are chosen to provide an amplifier gain rise time of approximately 20 milliseconds and a decay time of 600 microseconds. Thus, the decay time is slightly shorter than the time between the successive cycles of the acoustic transducer 10. As can be appreciated if the output voltage of the amplifier 40 increases to a high value, the capacitor 51 would be immediately charged and the voltage on the gate 46 will be an extremely high voltage thus reducing the resistance of the field effect transistor 47 and the corresponding signal voltage applied to the input of the amplifier 40. The charging rate of the capacitor 51 is controlled by the internal resistance of the diode 45 which is relatively low and thus the 600 microsecond decay time of the automatic gain control circuit can be readily achieved.As the voltage on the output of the amplifier 40 decreases the capacitor 51 will discharge to ground through the resistance 50 thus reducing the voltage on the gate 46. This discharge rate is largely controlled by the product of resistance 50 and capacitor 51 and is chosen to be on the order of 20 milliseconds. This will increase the resistance of the field effect transistor 47 and raise the voltage level of the input to the amplifier 40. When the voltage on the capacitor is completely discharged the field effect transistor 47 will act as a very high resistance and apply the maximum voltage signal to the input of the amplifier 40.

From the above description, it is seen that the invention provides a simpie gain control for the transducer amplifier which has a slow rise time therefore increasing the output voltage of the amplifier at a slow rate but has a very fast cutoff so that the amplifier will not saturate. The automatic gain control circuit will allow the borehole televiewer to obtain an accurate representation of the wall of the borehole and the presence of fractures and other anomalies in the borehole wall will not be lost because of the saturation of the amplifier. Likewise, when the signal increases as the transducer approaches a position where the path of acoustic energy will be normal to the borehole wall the gain of the amplifier will be reduced. Thus, signals providing an accurate representation of the borehole wall will be obtained. It is to be understood that when the term "gain of the amplifier" is used, it is to describe the voltage dividing circuit formed by the resistance 42 and the field effect transistor 47 that effectively control the voltage level of the input signal supplied to the amplifier 40. As explained, this is not a true automatic gain control circuit but functions in the same manner as an automatic gain control circuit.

Claims

1. A circuit for automatically controlling the magnitude of amplification of signals produced by a borehole televiewer in response to acoustic waves reflected by a borehole wall, said waves being received by and emanated from a rotating acoustic transducer that is periodically pulsed to produce acoustic energy, the circuit comprising: amplifier means having an input and an output, the input of said amplifier means being coupled to the acoustic transducer; gate means having an input, an output and a gate, the input of said gate means being coupled to the input of said amplifier and the output of the gate means being coupled to ground; and a feedback loop, said feedback loop being coupled to the output of said amplifier and to the gate of said gate means to control said gate means in response to the output signal of said amplifier.

2. The circuit of claim 1, wherein said gate means provides a variable resistance to ground, the magnitude of said variable resistance being proportional to the feedback signal being transferred to the gate of the gate means via the feedback loop.

3. The circuit of claim 2, wherein said gate means is a field effect transistor.

4. The circuit of claim 3, wherein the feedback loop includes control means for providing a feedback signal with a variable voltage for controlling the gate of the field effect transistor.

5. The circuit of claim 4, wherein the control means provides a feedback signal with a slow rise time and a rapid decrease.

6. The circuit of claim 5, wherein the rise time does not exceed one-quarter of the time required for the borehole televiewer to make one complete scan of the borehole.

7. The circuit of claim 6, wherein the rapid decrease does not exceed the time elapsing between successive pulses of the borehole televiewer.

8. The circuit of claim 4, wherein the control means couples the feedback loop resistively and capacitively to ground and comprises a resistance capacitance circuit whose time constant provides a slow rise time and a fast decay time for the feedback signal.

9. A method for improving the response of a borehole televiewer in an out-of-round borehole, the method comprising: amplifying the signal of the borehole televiewer downhole; and controlling the magnitude of said amplification in response to the amplified signal.

10. The method of claim 9, wherein said controlling increases the amplification slowly in response to a decrease in the amplified signal and decreases the amplification rapidly in response to an increase in the amplification above a desired level.

11. A circuit for automatically controlling the magnitude of amplification of signals produced by a borehole televiewer substantially as described hereinbefore with reference to the accompanying drawings.

12. A method for improving the response of a borehole televiewer in an out-of-round borehole, substantially as described hereinbefore with reference to the accompanying drawings.