Note: Descriptions are shown in the official language in which they were submitted.
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8834
w ll~ FOR CO~TROLLING THE M~GNITUDE OF AMPLIFICATION
OF SIGN~IS ~ u~ BY A BOFE~IOLE TE~TL~WER
m e invention relates to borehole logging tools and particu
larly to a tool that is described as a borehole televiewer. A
borehole televiewer u~ a rotating acoustic tr~n~dll~Pr that
is peric~;r~lly pulsed to produce acoustic energy which tr~vels
outwardly and strikes ~he borehole wall and is rerlected back to
the tr~n~ r. The amplitude of the reflected signal is dis
played on a cathode ray tube whose face m~y be ph~LGyLd~iled. The
resulting photograph L~L~s~lLs a map of the borehole wall split
along the north direction and laid out flat. m e transducer is
pulsed at a relatively high rate, for ~x~mrle 1500 pulses per
second, while the tr~ns~l~r~r rotation period is 0.333 se~n~
The s~eep of the os~;lloscope is controlled by a north syn~
chronization signal ~hich is produced by a flux gate magneto-
me.ter which rotates in unison with the tr~n~ Pr.
While the above tools are useful in obtaining a display of
the borehole wall that indicates the loca~ions of fractures or
other features, it does have a serious limitation when used in a
non-ci~cular or eliptical borehole. m e same problem also arises
when the -tool moves off centre as a result of this travel through
the borehole. It ccm be shcwn that when the borehole televi~wer
is loccited off centre or in a non-circular hole~ the acoustic
energy will be scattered at all locati.ons except where the
acoustic energy strikes the borehole wall nor~al to a tangent a-t
this point. Thus, the reflected ~nergy received by the ~r~n~ r
will have four positions where it is at a m~;~ ~ and in the
r~m~;nln~ positions the reflected energy will be decreased. The
borehole tele~iewer as presently available has various amplifier
gain levels that are selected by a rotary .switch controlled frcm
the surface~ Since the amplitude is set from ~he surface, it
cannot be varied to provide the proper amplification as the
tr~n~tl~r rotates. Thus, the resulting display will have li~ht
and dark areas where details are lacking. It should be appreciated
that in normal operations a fracture or other ancmaly in the
borehole wall will not reflect energy and will appear as a dark
shadcw on the display.
The present invention solves the proble-ms of u~ ;nf3 a
borehole televiewer in a non-circular borehole by providing a
circuit for a~L~"~Lically controlling the magnitude of ampli-
fication of signals produced by the borehole televiewer, the
circuit ccmprising
amplifier means having an input and an output, the Lnput of
said am~lifier means being coupled to ~he acoustic transducer o
the televiewer,
gate ~eans having an input, an output and a gate, the input
of said gate means being coupled ~o the input of said ~m~lifiPr
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 ~mpl;~;Pr and to ~he gate of said gate means to
control said gate means in r~ n~e to the output signal of said
~mrl;f;~r. In a suitable ~mhc~;mPnt 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 autcmatic
gain contxol circult m the sense that the gain of the ~m~lif;pr
is changed but rather a voltage dividing circuit that shorts
part of the ;nc~m;ng signal to ground and thus decreases the
input signal to the ~m~l;fiPr. m is allows the ~rl;fier to
retain the gain setting selected from the surface while in-
creasing the signal level to provide details whe~ the reflected
signal is weak.
In an attractive ~mhc~im~nt of the invention the decay
constan-t for the circuit is less than the cycle time of the
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tr~n~ r~r so that the ~mrlif;~r will not saturate when the
reflected signal suddenly increases in amplit~de. Preferably,
the rise time of the circuit extends over several cycles so that
the ~mrl;f;~r Will not obscure fractures or other ~n~mAl;e~ in
the borehole wall that have a short circumferential extent along
the wall.
In a preferred embodiment of the i~vention 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.
m e invention will be more easily unders~ood from the
following description wh n 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 o-f the s:ignals produced by the
borehole televiewer.
Figure 3 is a graphic L~es~.Lation of the borehole
televiewer located in a non-circular borehole~
Figure 4 is a schematic drawing of the dU~ull~LiC gain
control circuit of the invention.
Referring now to Figure 1, there i5 shown the borehole
televiewer that includes an acoustic tr~n~ rPr 10 and a flux
gate magn~L~,~Ler ll which are mounted on a ccmmon shaft. m e
shaft is rotated by motor 12 about the axis 14 of the borehole
televiewer. Electronics for control of the p~ ;n~ of the
trAnq~l-rPr and the trAnYmi~ion of the reflected signals to the
surface i5 conta med in a section 16 of the tool while a
pressure equalization chamber 15 is mounted in the bottom
portion of the tool. The pressure ec~lalization ~hAmhPr serves to
equalize the pressure on the interior of the acoustic windcw 13
with the pressure of the fluid in the borehole. By e~~ ing
the pressure the required mP~h~niç~l ~LL~ly~h of the acoustic
window can be reduced and allow the use of materials which
provide a gcod acoustic match between the fluid filling the
borehole televiewer and the borehole fluid.
Referring to Figure 2, there is shcwn the waveforms of the
signals produced by the borehole televiewer. In particular, the
signal 20 represents the acoustic p~Lse produced by the trans-
ducer ~Thile the signal 21 represents the reflected signal
received by the tr~n~dll~Pr. As shownl a time interval of
approximately 100 microseconds elapses between the production of
the acoustic pulse and the receipt oi- the reflected signal. The
tlme lapse~ of course, ~PpPn~ upon the diameter of the borehole
but lO0 micro~ec~n~ is representati~Te. m e actual borehole
televiewer tool that is available commercially has a repetition
or cycle rate of the acoustic tr~n~ r that is approximdtel.y
1500 cycles per second while the rotational cycle of the trans-
ducer is 0.333 sec~n~.~. Thus, the borehole televiewer provides
approximately 500 acoustic pulses per rotation of the tr~n~ Pr
for sc~nnlng the borehole wall.
Referring to Figure 3, there is shcwn a schematic arrange-
ment of the borehole televiewer in a borehole wall that illus-
tra-tes the proble~ produced by a non-circular borehole or a
borehole televiewer positioned off centre. As shcwn, the trans-
ducer 10 is located at approxima~ely the y~ Lic ce~tre of the
non-circular borehole 31. When the acoustic pulse travels along
the path 32, it is dy~reciated that it strikes the borehole
normal to a tangent at the point at which it strikes and is
reflected back to the tr~n~ c~r 10. In contrast, when the
acoustic energy travels along the path 34, it strikes the
borehole wall at an angl.e to a tangent at the point at which it
strikes and instead of being re~lect~ back, the major portion
of the energy is scattered or reflect~d along the path 35. Frcm
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
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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 posi-tion 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 bac3s to the acoustic tr~ns~ Pr
due to a fracture or other anGmaly in the borehole wall. At
these positions, the gain of the amplifier must be decreased
very rapidly so tnat the ancmaly is not obscured in the final
display of borehole televiewer data.
Referring to Figure 4, there is shown a simple autcmatic
gain control circuit for controlling the transducer amplifier.
~ The input 41 of the circuit is coupled to the trAn~ Pr (not
shown). The circuit ~ es a field effect transistor 47 which
i5 operated as a voltage divider to shunt a portion of the input
signal of the amplifier 40 directly to ground. The gain control
circuit does not adjust the gain of an ~m~l;fi~r as is traditional
with duL~LLLcLLic gain control circuits but rather is a voltage
dividing circuit formed by the resistor 42~ and the resistance
of the field effect transistor 47. The resistance of the field
effect transistor 47 is ~Lo~v~Lional to the voltage applied to
the gate 46. The input 41 is both resistivel~ and capacitively
callpl~ to the Amplif;~r 40 through a circuit 42 ccmprising a
resist~nce 42A and a capacitance 42Bo The output 49 of the
~mrl;f;~r is coupled to an ;m~rl~nre 43 which is used to match
the output signal of the A~rl;fi~r to a logging cable 52. The
autcmatic gain control is provided by a fee~back lcop 44 which
signal is rectified by a diode 45. ~le rectified signal is
applied to the gate 46 o~ the field effect transistor 47 to
control the c~ ion thereof. By controlling -the conduction of
the field effect transistor 47 ~he input voltage ~L~Pli~ to the
amplifier 40 can ke controlled. The fee~hA~k si~nal charges a
resistance capacitance circuit comprising a resistance 50 and a
~2~3L7~3~
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capacitance 51, which circuit couples the f~h~k loop 44
resistively and capacitively to ground. The values of the
resistance 50 and the capacit~nce 51 are chosen to provide an
amplifier gain rise time of ~L~imately 20 m;ll;~r~n~ and a
decay time of 600 microseconds. Thus, the decay time is slightly
shorter than the time between the successive cycles of the
acoustic trAn~u~r lO. As can be appreciated if the output
vo1tage of the ~m~lif;~r 40 increases to a high value, the
capacitor 51 wculd be ;m~ tely charged and the voltaye Oll the
gate 46 will be an extre~ely high voltage thus re~ ;ng the
resistance of the field effect transistor 47 and the corre-
sponding signal voltage ~pllfA to the input of the amplifier
40. The charging rate of the capacitor 51 is controlled by the
~nt~rn~l resistance of the diode 45 which is relatlvely low and
thus the 600 microsecond decay tlme of the au~oma~ic gain
control circuit can be readily achieved. As the voltage on the
output of the amplifier 40 decreases the capacitor 51 will
~ h~rge to ground through the resistance 50 thus re~ ;ng the
voltage on the gate 46. This ~;~rh~rge rate is largely controlled
by the product of resistance 50 and capacitor 51 and is chosen
to be on the order of 20 m; 11;~econds. miS will increase the
resistance of the field effect transistor 47 and raise the
voltage le~el of the input to the a~plifier 40. When the voltage
on the capacitor is completely discharged the field effect
transistor 47 will act as a very hlgh resistance and apply the
m,~xlml~m voltage signal to the input of the ~mrl;fl~r 40.
From the above description, it is seen that the invention
provides a simple gain control for the tr~ns~ r ~rli~;~r
which has a slow rise time therefore increasing the output
voltage of the ~mrl;f;~r at a slcw rate but has a very fast
cutoff so that the amplifier will not: saturate. m e a~ ~Lic
gain cont~ol circuit will allow the borehole televiewer to
abtain an accurate representation of the wall of ~he borehole
and the presence of fractures an~ other ~nf~lie~ in the bore-
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hole wall will not be lost because of the saturation of theamplifier. Likewise, when the signal increases as the transducer
approaches a position w~ere 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 obta m ed. It is to be understood that
when the term "gain of the ~m~l; F;~r" is used, it is to describe
the voltage di~iding 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 ~ot a true autcmatic gain control circuit but
functions m the same manner as an autcmatic gain control
circuit.
