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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to the measurement of the internal dimensions
of an elongated borehole, such as within an oil well.
Apparatus are known for measuring the inner diameters of cylindrical tubes
or of wells bored in the ground. For example, inside calipers or sondes
capable of being moved in these tubes or boreholes can be equipped with
fingers adapted to spread away from the body of the caliper or sonde until
they come into contact with the walls of the tubes or boreholes, the
measurement of this outward spread of the fingers thus furnishes
information relative to the sought diameter.
Such apparatus are also used in uncased boreholes to check the condition of
their walls and notably to detect the presence of caving irregularities
which can affect the logging measurements to be made within the boreholes.
The condition of the internal wall of a tube, such as a steel casing
supporting the walls of a well, or a production tubing designed to bring
to the surface the fluids produced by a given zone of the earth formations
traversed by the well can also be checked. These casing and tubing undergo
numerous abrasion and corrosion phenomena with time. The monitoring of
their internal dimensions makes it possible to check their degree of wear.
Mechanical caliper apparatus are however relatively complex and difficult
to design, especially when one wishes to obtain a large number of
measurements in holes of small diameter. Mechanical calipers also have the
drawback of probably scratching the wall of the steel tubes into which
they are introduced.
Acoustic-type caliper devices are known in which a transducer mounted on
the sonde transmits pulses in the direction of the borehole wall. These
pulses are reflected by this wall with the resulting echo being detected
either by the transducer producing the pulses or by another transducer
specialized in the reception of these signals. The time elapsing between
the transmission of each pulse and the detection of the corresponding echo
provides a measurement of the distance of the transducer from the borehole
wall. By repeating similar measurements around the longitudinal axis of
the sonde, for example by having a transmitting-receiving transducer
rotate around this axis, it is possible to obtain plotting of the form of
the hole, the accuracy of which is dependent on the number of measurements
made during any given rotation cycle. By sequentially moving the sonde
longitudinally after a rotation cycle, it is possible to obtain an image
of the form of the hole over any chosen depth interval. Such apparatus
however require the use of a drive device for rotating a relatively
complex transducer. They must also operate within severe environments such
as those encountered in oil well boreholes in which extremely high
temperature and pressure conditions often prevail and in which any
contacting media can be very abrasive.
Devices have been proposed for measuring one transverse dimension of a
borehole wall in a well by means of a transducer mounted in a stationary
manner on the sonde. Acoustic transducers designed for this type of
application are relatively voluminous. They generally include a
piezoelectric disc, one face of which is designed to transmit and receive
acoustic signals, the other faces being lined with an absorbant material
in order to attenuate the effect of the echoes reflected by the borehole
wall in directions other than that of the transmitting face. Owing to
their dimensions, it is difficult to consider the use of a large number of
such transducers on the same tool, for exploring the dimensions of the
borehole in several directions around the axis of the well. In addition,
because of their volume, these transducers cannot be used in sondes
intended to go through production tubing which is typically of a smaller
diameter.
SUMMARY OF THE PRESENT INVENTION
Considering these difficulties and deficiencies, the object of the present
invention is to provide an acoustic method and apparatus for measuring the
transverse dimensions of a hole, notably in a well, capable of being
implemented with small-diameter sondes that are capable easily of being
introduced into production tubing that are within the oil wells and which
lend themselves to performing a large number of measurements.
A method according to the invention is characterized in that, by means of a
sonde lowered into a well, simultaneous acoustic energy pulses are
transmitted in two directions from the same transducer and the echoes
retransmitted to the transducer by the borehole wall in response to the
pulses transmitted in these two directions are picked up successively in
order to obtain a measurement of the respective distances. According to
one embodiment, these two directions are opposite and aligned along a line
substantially diametrical in relation to the axis of the well. The sum of
the distance measurements obtained along these two directions therefore
provides a measurement of a hole diameter. It is then possible to
determine the diameter variations within the hole as the tool is moved
longitudinally within the well. Preferably several transducers which are
oriented in different directions around the longitudinal axis of the well
are used to perform such measurements.
An apparatus for measuring the transverse dimensions of a borehole wall in
a well according to the invention comprises a sonde capable of being moved
longitudinally in the well and designed to cooperate with means for
positioning this sonde in relation to the borehole wall, having at least
one acoustic transducer capable of transmitting acoustic signals toward
the borehole wall and picking up the signals reflected by this wall. The
transducer is capable of transmitting acoustic signals in two aligned and
opposite transverse directions, and of picking up the echo signals
respectively reflected by the borehole wall along these two directions.
The transducer is mounted on the sonde by positioning means such that the
first echoes reflected by the borehole wall in response to the signals
transmitted simultaneously in these two directions are received by the
transducer to different times. According to a preferred embodiment, means
are also provided for analyzing the amplitude of the echoes received from
the borehole wall so as to provide an indication of the condition of the
surface of the wall.
The electro-acoustic transducer comprises a piezoelectric material which
vibrates mechanically when it is subjected to an oscillation of
appropriate electric voltage. To obtain transducers which are sufficiently
directive, both as regards transmission and reception, use is generally
made of coatings in absorbant material which leave only one face of the
piezoelectric material exposed for the transmission in the two directions
of acoustic waves between the transducer and the surrounding medium. But,
in general, the attenuation of the acoustic waves transmitted and received
by the transducer on its face opposite its active face is particularly
difficult to achieve and calls for the application on this face of
relatively thick linings of absorbant material which make the transducer
voluminous. According to one feature of the invention, no attempt is made
to eliminate the effects of the acoustic stimulation of this opposite face
and, in order to separate the echoes received by two opposite face of the
transducer in response to the same excitation, provision is made so that
it operates in a position which is dissymmetric in relation to the walls
of the hole with which is cooperates. It is thus possible to obtain a
transducer of small overall dimensions adaptable on sondes of small
diameter. Moreover, when conditions allow, advantage is taken to the
presence of two echoes in response to each transducer firing pulse to
carry out two measurements. Thus, the apparatus according to the invention
can not only be made more compact, but moreover makes it possible to
achieve a high information density. Furthermore, the echoes received by
the transducer are transformed into signals which are not greatly affected
by the internal noise of the transducer, unlike what takes place with
monodirectional transducers in which an attempt is made to attenuate, in a
manner which is inevitably incomplete, the signals emitted and received by
one of its faces.
According to a preferred embodiment, the transducer comprises a block of
piezoelectric material having active faces on opposing sides of the block,
which faces are substantially parallel to the axis of the sonde. Each face
is advantageously covered with a quarter-wave plate at the excitation
frequency of the transducer in order to limit the number of oscillations
transmitted in response to each excitation of said piezoelectric block.
According to a preferred embodiment the sonde comprises a plurality of such
bidirectional transducers whose orientations are distributed around the
sonde as to allow the measurement of multiple radially transverse
dimensions. These transducers can be superposed on each other along the
sonde. The sonde can moreover be equipped with a reference transducer,
identical in its composition to the measurement transducers, but which is
mounted so as to transmit acoustic pulses in two opposite directions
toward two respective reflectors placed at different distances from this
transducer. The space between the transducer and each of the reflectors is
in contact with the medium surrounding the sonde. Thus, the measurement of
the time intervals between the transmission of a pulse by this reference
transducer and the reception of the respective echoes provides a precise
measurement of the propagation velocity of the acoustic waves in the
medium in which the sonde is immersed. It can also provide a measurement
of the attenuation constant of the acoustic waves propagating in this
medium.
In the case of small-diameter tubes such as production tubing, in which it
is possible to detect only the first echo received by the transducer and
thus obtain a radius measurement, it is advantageous to use an odd number
of transducers distributed regularly over the periphery of the sonde.
The following explanations and description are of an illustrative nature
and are given with reference to the appended drawings in which:
FIG. 1 represents a sonde according to the invention in operation in an oil
well;
FIG. 2 is an elevation view of the tool of FIG. 1;
FIG. 3 is a sectional view of the tool along the line III--III of FIG. 2;
FIG. 4 is a sectional view along the line IV--IV of FIG. 2;
FIGS. 5a and 5b represent diagrams of the signals used in the operation of
the invention;
FIG. 6 is a signal diagram illustrating a detail of the explanations
provided;
FIG. 7 is a functional diagram of the measurement circuits associated with
the sonde;
FIG. 8 is a diagram illustrating the analysis of the amplitude of the echo
signals received;
FIG. 9 is a more detailed circuit diagram of certain parts of the diagram
of FIG. 7;
FIG. 10 is a sectional view of a second embodiment of the tool taken
perpendicular to the axis of the sound;
FIG. 11 is a partial sectional view of the second embodiment taken along
XI--XI of FIG. 10; and
FIG. 12 is a schematic representation of the arrangement of the transducer
assemblies within the second embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, a sonde 10 is lowered into a well 11 whose walls are lined with
a steel casing 12 over its entire depth. This casing 12 is connected to
the formation defining the well by an impervious cement filling 14.
At its upper part, the well 11 is equipped with a well head 15 having
shutoff valves (not shown) connected to a production tubing 16 having a
diameter substantially smaller than that of the well 11. The tubing 16
goes down into the well to a pre-determined depth. The annular interval
between the lower end 20 of the tubing 16 and the casing 12 is closed off
by a plug or packer 18. With this construction it is possible to convey to
the surface the fluids produced by the oil-bearing formations traversed by
the well below the packer 18.
As illustrated, the sonde 10 is suspended in the well by a cable 22 which
passes through the tubing and the well head 15. This cable is run back to
the surface, passed around a pulley 25, connected to a winch 26 of a
control unit 28 which is used for controlling the measurements operation.
The cable 22 is used for both mechanical suspension of the sonde 10 and
for the electrical transmission of signals between the sonde 10 and the
control unit 28.
In one embodiment, the outer diameter of the sonde 10 is about 43 mm, a
value which allows its passage through tubing of small inner diameter.
This sonde 10 is equipped, in the vicinity of its upper and lower ends,
with centralizing devices 30 and 32 that make is possible to keep the
longitudinal axis of the sonde 10 substantially in coincidence with the
longitudinal axis of the cased well 11 during the longitudinal movements
of the sonde 10 within the well. The centralizers 30 and 32 are equipped
with arms having rollers 35 distally mounted thereon which rollers are
made of a rubbery material that can minimize the scratching of the casing
12 or the tubing 16 when the sonde 10 is moved vertically in the well 11.
These arms are loaded by springs (not shown) which tend to spread them
away from the sonde and to apply the rollers 35 against he walls of the
well 11. The stiffness of these springs and the number of arms are
determined in order to maintain any offcentering of the arms in relation
to the centerline of the well within specified tolerance limits, for
example when the well is inclined with respect to the vertical. The number
of arms usually varies from four to six depending on specific utilization
conditions.
As shown in FIG. 2, the sonde 10 further comprises, separately or in
combination with other logging devices in the well, an acoustic caliper
module 40 which includes nine electro-acoustic transducers 42.sub.1 to
42.sub.9 superposed in a configuration which will be explained in greater
detailed below.
Connected to the caliper module 40 is a reference module 44 designed to
carry out measurements of the speed of the acoustic waves in the well 11.
The sonde 10 terminates at its upper end with a signal-preprocessing
electronic cartridge 46 that is fixed directly to a head 48 for connecting
the sonde 10 to the cable 16.
FIG. 3 represents a cross-section at the level of the transducer 42.sub.1
of the sonde 10. For simplicity the sonde 10 is assumed to be centered in
relation to the internal wall 50 of the casing 12. The sonde body 52
includes a through passage 54.sub.1 which is rectangular in longitudinal
section, i.e. higher than it is wide. The dimensions of the passage 54,
are about 25 millimeters in height (see dimension h in FIG. 2) and 15
millimeters in width (dimension l in FIG. 3). The passage 54.sub.1
terminates in two diametrically opposite openings 55.sub.1 and 56.sub.1 in
the outer wall of the tool body 52.
Within the passage 54.sub.1 is mounted the piezoelectric transducer
42.sub.1 which includes essentially a rectangular ceramic block 59 (FIG.
3) with a height of h and a width of l. The block 59 is lined on each of
its faces 60 and 61 with a metallic coating. The coatings are connected to
conductors (not shown) for the piezoelectric excitation of this block by
the application of voltage pulses. The metallized faces 60 and 61 are
lined respectively with suitable coatings 62 and 63 whose thickness is
determined so as to correspond to one-fourth of the propagation wavelength
in this material of the acoustic signals produced by the piezoelectric
ceramic block 59 when it is excited for example by a 500-kHz electric
voltage signal. The coatings 62 and 63 play the role of quarter-wave
plates having the effect of producing a relatively sharp cutoff of each
burst of acoustic oscillations transmitted from the faces 60 and 61 of the
block 59 under the action of a brief block excitation signal at the
frequency indicated for sending a corresponding energy pulse into the
medium surrounding the sonde body 52 and in which are immersed the outer
faces of the transducer 42.sub.1. Coatings 62 and 63 can be formulated
with various materials such as a high performance thermo plastic (e.g.
poly ether ether ketone) or an epoxy resin.
The plane of the block 59 is parallel to the longitudinal axis 65 of the
sonde body 52 and perpendicular to the direction of the passage 54.sub.1.
The transducer 42.sub.1, whose total thickness is about 6 millimeters, is
mounted in a position which is offcentered in relation to the axis 65, by
distance e which is about 12.5 millimeters.
The transducers 42.sub.1 is symmetric from the geometrical and electrical
standpoints. The value of the offcentering e is determined such that the
echoes received by the transducer in response to a pulse transmited in the
two opposite directions in which the transducer is facing are received at
moments of time sufficiently far appart so as to be detectable with
accuracy by the transducer and the processing electronics to which it is
connected.
The transducers 42.sub.1 to 42.sub.9 (FIG. 2) are mounted eccentrically in
nine passages 54.sub.1 to 54.sub.9 which are all shaped identically but
the orientations of which are offset angularly around the longitudinal
axis of the sonde by 40 degrees in relation to each other. Each of these
transducers makes it possible, by detecting the different return times of
the two echoes observed in response to each pulse transmitted, to obtain
measurements of the transverse dimensions of the well on nine diameters
spaced angularly by 40 degrees. As represented in FIG. 2, the openings
55.sub.1, 55.sub.2, and 55.sub.3 corresponding to the passages 54.sub.1 to
54.sub.9 are about 50 millimeters from each other in the axial direction,
the total height of the acoustic caliper module 40 thus being about 45
centimeters.
The reference module 44 includes an axial slot 70 provided in the body 53
of the module (FIG. 4) and defined by respective longitudinal plane walls
71 and 72 on each side of the sonde axis. A symmetric transducer 75
identical in its make-up to that of each of the transducers 42.sub.1-9 is
mounted between the walls 71 and 72 so that its active faces are
perpendicular to the axis of the sonde and located at unequal distances,
respectively X.sub.2 and X.sub.1, from the longitudinal end walls 76 and
78 of the slot 70. The measurement of this time interval makes it possible
to precisely determine the velocity of the acoustic waves in the fluid
filling the well and in which the transducer 75 and the reflectors 76 and
78 in the longitudinal slot 70 are immersed. The attenuation constant of
this fluid can also be determined. In this example, the distances X.sub.1
and X.sub.2 are chosen equal to 45 mm and 75 mm, respectively
corresponding to round-trip transit times of 60 .mu.s and 100 .mu.s
respectively in a fluid such as water. These values will constitute the
typical values for the other transducers 42.sub.1-9.
The total height of the reference module 44 is about 15 centimeters. The
total length of the assembly made up of the modules 40 and 44 is thus
about 60 centimeters. The resulting tool therefore not only has small
transverse dimensions but also a relatively reduced length.
In the module bodies 52 and 53 the passages 54 and the longitudinal slot 70
are traversed by channels in which are placed the conductors connecting
the transducers 42.sub.1 to 42.sub.9 and 75 to the electronic cartridge
46.
The general operation is the following: The transducers 42.sub.1 to
42.sub.9 are supplied successively by excitation pulses. Clock pulses
transmitted at a frequency of 10 MHz are counted from the instant T.sub.0
of the excitation pulse of each transducer 42 (FIGS. 5a and 5b). Each
transducer 42 transmits two symmetric pulses which propagate in opposite
directions toward the walls of the casing 50. After reflection, the
resulting echo signals are detected by the transducer at respective times
T.sub.1 and T.sub.2 which correspond to a time interval .DELTA.T
approximately equal to four times the time taken by the acoustic waves
transmitted to cover the distance e equal to the offcentering of the
transducers. FIG. 5a shows the first echo 81 coming from the nearest well
wall portion 50 and received by the transducer as of the time T.sub.0.
FIG. 5b shows the first echo 82 coming from the farthest portion of this
wall. Each of the detected signals 81, 82 is made up of a succession of
very rapid oscillations of short but not negligible duration and whose
amplitude, after having undergone a sudden increase, decreases owing to
the effect of the respective quarter-wave plate.
The detection of the time T.sub.1 and T.sub.2 is carried out as illustrated
in FIG. 6 by detecting the first crossing of a threshold, symbolized by
the broken line 83, by each oscillating signal 81 and 82 coming from the
transducer (point 84 of the waveform of this signal) as received by the
electronics 46 and by noting the number of pulses counted upon the
following zero-crossing of the waveform as illustrated by point 85 of FIG.
6. In addition, the amplitude of the echoes is measured to obtain an
indication of the surface condition of the wall reflecting the signal.
This indication is furnished by the level of gain necessary for
maintaining the signal 81 or 82 after amplification within a given
amplitude range. In particular, the echo received from the farthest wall
of the well is generally amplified to a greater extent since the
corresponding acoustic signal has travelled a longer distance and has
undergone greater attenuation.
These functions are implemented by a electronic circuit 6 (FIG. 7) in which
the conductors 100.sub.1 to 100.sub.10 coming from the nine transducers
42.sub.1 and 42.sub.9 and from the transducer 75 are each connected, on
the one hand, to the output of a corresponding transmitter 102.sub.1 to
102.sub.10 and, on the other hand, to a corresponding input 104.sup.1 to
104.sub.10 of a multiplexer 105.
The transmitters 102.sub.1 to 102.sub.10 are controlled by the
corresponding outputs 106.sub.1 to 106.sub.10 of a demultiplexer 107 wose
control input 108 is capable of receiving pulses from the firing control
output 109 of a control logic 110. The demultiplexer 107 is adapted to
carry out the distribution of the firing pulse to the transmitters
102.sub.1 to 102.sub.10 according to the information transmitted to its
address input 112 by an addressing bus 114 connected to an addressing port
115 of the control logic 110. The port 115 is also connected to an
addressing input 116 of the multiplexer 105 so as to control the
transmission, on the output 118 of this multiplexer, of the signals
present on whichever of its inputs 104.sub.1 to 104.sub.10 is designated
by the addressing signal. The control logic 110 is a sequencer wired to
cyclically address the transducers (schematically designed TR.sub.1 to
TR.sub.10 in FIG. 7) by means of the demultiplexer 107 and the multiplexer
105.
After each transducer firing pulse, the multiplexer 105 and the
demultiplexer 107 are positioned by an identical address signal
respectively on the input 104.sub.i and the output 106.sub.i corresponding
to the same transducer 42.sub.i. The firing pulse is an enabling logic
signal with a duration of about 1 microsecond that is transmitted every
millisecond. This pulse is transmitted by the demultiplexer 107 to excite
one of the transmitters 102.sub.i. Each transmitter includes a pulsing
circuit operating at a frequency of 500 kHz so as to deliver, in response
to the control pulse, a bipolar electric voltage pulse with a duration of
2 microseconds and of about 400 volts peak-to-peak. This voltage excites
the corresponding piezoelectric transducer TR.sub.i which then transmits
an acoustic energy pulse into the medium surround the sonde 10. The output
109 of the control logic 110 is also connected to an inhibition input 119
of the multiplexer 105 through a timing circuit 120. Thus, no signal
appears on the output 118 of this multiplexer throughout the duration of
the firing pulse and during an additional period lasting a few
microseconds following the first period, in order to prevent the reception
of noises related to transmission. As of the end of this inhibition
period, the output 118 of the multiplexer 105 listens for signals
transmitted by the transducer TR.sub.i on the corresponding input
104.sub.i. The output 118 of the multiplexer 105 is connected to the input
of a variable-gain amplifier 130 which includes three series-connected
amplification stages whose gain is adjustable by discrete values. The
first stage 132 includes, for example, two gain values, 0 and 30 dB,
respectively. The second stage 134 is adjustable with five gain values, 0,
6, 12, 18 and 24 dB, respectively. The gain of the third stage 136 is
adjustable in steps of 1.5 dB between 0 and 4.5 dB. Thus, the gain of the
amplifier 130 is adjustable in steps of 1.5 dB over a gain range extending
from 0 to 58.5 dB. These gain values are controlled by the outputs of a
decoder 133 which decodes a digital signal set on a multi-bit input 138 of
the amplifier 130, through a connection 135 coming from a gain control
circuit 140 whose function is explained below.
The output 139 of the amplifier 130 is connected to the input of a
threshold and zero-crossing detector 142 which includes two comparators
triggered successively by the crossing of the threshold 83 and zero of
FIG. 6. The signal appearing on the outut 144 of the detector at the zero
crossing point 85 of the output signal of the amplifier 130 is processed
by a detection, counting and transmission circuit 150 which will be
described below and which receives the pulses from a clock 152 at 10 MHz
which is also connected to a timing input 151 of the control logic 110.
The output 139 of the amplifier 130 is also connected to the input of an
amplitude change detection circuit 160 capable of delivering a signal on
an input 162 of the circuit 140 when the amplitude of the received echo
signal has exceeded a level L2 (FIG. 8), and a signal on an input 164 of
the gain control circuit 140 when the received signal 155 has not exceeded
a lower level L1 (FIG. 8), the ratio between the two levels L2 and L1
being 1.5 dB.
The circuit 140 includes a register (not shown) connected to a four-bit
input 165 of the circuit 140 which in turn is connected to a gain bus 167.
The latter makes it possible to transfer into this register, from a memory
during the preceding operating cycle of the transducer TR.sub.i in a
position whose address corresponds to this transducer. Upon the firing of
the transducer TR.sub.i, the control logic triggers, by applying the
output signal 109 on an input 271 of the circuit 140, the transfer of a
gain value G.sub.1 stored in memory for this transducer into the gain
register of the circuit 140. The multibit output of the register sets the
gain of the amplifier 130 through the connection 135. With the end of an
echo signal detected at the output of the latter, as will be explained
below, the value contained in the register is incremented in response to a
signal on the input 164 or decremented in response to a signal on the
input 162 to modify the gain of the amplifier 130 in the corresponding
direction, for example by means of an adder, or by loading the value of
the gain into an up-down counter whose up-down counting inputs are placed
under the control of the inputs 162 and 164. The new numerical value of
the gain G.sub.1 stored in memory 168 by the bus 167 is transmitted by the
latter to the input 269 of the detection, counting and transmission
circuit 150. Thus, the gain of the amplifier 130 is set for each fired
transducer according to the value it had during the preceding firing of
this transducer.
The same operation for setting a gain stored in memory is undertaken at the
end of the first echo detected by the transducer TR.sub.i awaiting the
next echo. Another gain value G.sub.2 determined after the reception of
this echo is stored in memory 168 in a second position assigned to the
transducer TR.sub.i.
A more detailed description will now be given of the detection, counting
and transmission circuit 150 during the firing of a transducer TR.sub.i by
the control circuit 110 (See FIG. 9 in which the multiplexing and
demultiplexing circuits 105 and 107 respectively have been omitted). At
the time T.sub.0 corresponding to the firing of the transducer TR.sub.i, a
counter 170 receiving pulses from the clock 152 on a count input 171 is
triggered by a signal coming from the control logic 110 on its input 172.
When a first echo 81 (FIG. 5a) is received by the transducer TR.sub.i and
transmitted by the amplifier 130, the threshold the zero-crossing
detection circuit 142 produces, on its output 144, a signal which causes
the changeover of a D-type flip-flop 175, to the input 176 of which it is
connected, so that the output Q 177 of this flip-flop changes over from 0
to 1 and applies, through an inverter 300, a blocking signal on an input
179 of an AND gate 180 whose other input 181 receives directly the pulse
from the output 144. This produces, just before the blocking of the AND
gate 180, a brief signal at the output of the latter which triggers the
instantaneous reading of the contents of the counter 170 in a buffer
register 182 by a multibit link 183, enabling the read input 184 of this
register. At the same time, the brief signal coming from the AND gate 180
is applied to the reset input 185 of the counter 170 which, still supplied
by the pulses from the clock 152, begins counting again from zero.
The echo 81, amplified at the output of the amplifier 130 with the gain
G.sub.1 previously stored in memory 168 for the first echo received by the
transducer TR.sub.i during its preceding firing, as explained earlier, is
analyzed by the amplitude change detector 160. This device comprises a
first threshold detector or level comparator L1, 191, which triggers a
flip-flop 192 if the threshold L1 is crossed and a second comparator 193
which triggers a flip-flop 194 when the threshold L2 is crossed. The
inverting output of the flip-flop 192 is connected to the input 164 and
the direct output of the flip-flop 191 is connected to the input 162 of
the gain control circuit 140. After a time T.sub.1 +p (FIGS. 5a and 5b),
corresponding to a predetermined count level of the counter 170 after its
resetting by the AND gate 180, the gain control circuit 140 increments or
decrements the gain, or leaves it unchanged, depending on the signals
present on its inputs 162 and 164 under the control of a signal coming
from a decoding output 302 of the counter and applied, through an OR gate
304 and a synchronization circuit 300, to an input 195 of the gain control
circuit 140. The time p is chosen so that this gain value modification
takes place as soon as the maximum deviations of the first echo are
passed. The numerical value of the gain G.sub.1 thus obtained is placed in
a memory position 168 corresponding to the first pulse detected by the
transducer TR.sub.i. The gain value G.sub.1 is also transmitted into a
position of the buffer register 182 along with the count value
corresponding to the time T.sub.1 by the gain value bus 167 connected to
an input 269 of this register. As soon as the digital word corresponding
to the time T.sub.1 and to the gain G.sub.1 is transferred to the buffer
memory 182, it is loaded into a first in-first out register 220 through a
multibit link 222. This register is connected to the telemetering circuit
230 for the word-by-word transmission of the measurements carried out
following the firing of each transducer TR.sub.1 and TR.sub.10. The
telemetering circuit 230 transmits these measurements along the cable to
the surface.
The memory 168 has two storage positions for each transducer TR.sub.1, one
for storing the gain value G.sub.1 corresponding to the first echo
received following a firing and the other for the value G.sub.2 of the
next echo. In addition to the four-bit addressing of its input 169, this
memory includes an input 198 controlled in response to the outut Q 177 of
the flip-flop 175 through the synchronization circuit 305. The signal
present on this input indicates, depending on whether its logic level is 1
or 0, if the first pulse has been received or not. Consequently, the input
198 plays the role of a fifth addressing bit for the positions of the
memory 168 designated for each transducer by the address bus 169. This
additional bit designates the positions corresponding to the gains G.sub.1
and G.sub.2 according to the detection of the first pulse by the flip-flop
175.
The synchronization circuit 300 triggers the readjustment operation for the
gain G.sub.1 by sending a signal to the input 195 of the gain control
circuit 140 as soon as the output 302 of the counter 170 has indicated the
end of a period p after the first resetting of the counter and then
controls the transfer of the new value G.sub.1 to the corresponding memory
position for the first pulse received by the transducer TR.sub.i.
At the end of a short time interval following this storage in memory, the
circuit 300 applies the logic level 1 to the input 198 of the memory in
response to the output signal Q 177 present on its input 305. It then
brings about the extraction of the value G.sub.2 of the gain previously
stored in memory for the second echo 82 which is transferred by the bus
167 to the register of the circuit 140. Thus, after a short time interval
following T.sub.1 K+p, the gain of the amplifier 130 is set at a suita | | |
