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Claims  |
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We claim:
1. Apparatus for measuring the transverse dimensions of a borehole,
comprising:
a sonde adapted for longitudinal displacement through said borehole, said
sonde including transducer means for transmitting and detecting
diametrically oppositely directed acoustic pulses radially of the sonde,
the pulses being directed towards and reflected from opposed wall sections
of the borehole, said transducer means including at least one transducer
having diametrically opposing active faces substantially parallel to and
eccentrically disposed with respect to a longitudinal axis of said sonde.
2. The apparatus of claim 1, wherein said sonde further comprises reference
module means for measuring and determining a velocity of the acoustic
pulses during transmission radially of the sonde within said borehole.
3. The apparatus of claim 2 wherein sid reference module means comprises a
reference transducer means for transmitting a first reference acoustic
pulse from one side of said reference transducer means and simultaneously
transmitting a second reference acoustic pulse from the other side of said
reference transducer means, a first reference echo and a second reference
echo being successively received by said reference transducer means in
response to the simultaneous transmission of the first and second
reference acoustic pulses, a first time interval between transmission of
the first reference acoustic pulse and reception of the first reference
echo being determined, a second time interval between transmission of said
second reference acoustic pulse and reception of the second reference echo
being determined, the first and second time intervals being combined to
determine the velocity of the acoustic pulses during transmission radially
of the sonde within said borehole.
4. The apparatus of claim 1, wherein said sonde further comprises
positioning means for maintaining the longitudinal axis of the sonde
substantially parallel to or coincident with a longitudinal axis of said
borehole.
5. The apparatus of claim 1 wherein said at least one transducer comprises
a piezoelectric block.
6. The apparatus of claim 5 wherein each of said faces of said
piezoelectric block is covered by a coating transparent to said acoustic
signals, said coating forming a quarter-wave plate.
7. The apparatus of claim 1, wherein said at least one transducer comprises
a plurality of the transducers distributed about the longitudinal axis of
the sonde.
8. The apparatus of claim 7 wherein the number of such plurality of
transducers is odd.
9. The apparatus of claim 7, wherein each of said plurality of transducers
is mounted within a through passage within said sonde, each said passage
being disposed completely through said sonde along a diameter thereof and
having an orientation affording substantially unobstructed transmission of
an acoustic signal from each active face of the transducer toward a wall
of the borehole, the passages being distributed longitudinally along the
axis of the sonde.
10. The apparatus of claim 9 wherein at least one of the passages have an
elongated rectangular cross section.
11. The apparatus of claim 9 wherein at least one of the passages have
walls which angle outwardly away from the transducer mounted therein.
12. The apparatus of claim 7 wherein said sonde further comprises:
electronic cartridge means connected to said transducer means for
energizing said transducer means thereby transmitting said acoustic pulses
toward said opposed wall sections of the borehole and for detecting the
acoustic echo pulses received by said transducer means which are reflected
from said opposed wall sections of the borehole.
13. The apparatus of claim 7 wherein said electronic cartridge means
comprises: control means connected to each of said plurality of
transducers for generating a control pulse which energizes a selected
transducer, said selected transducer transmitting an acoustic pulse toward
said opposed wall section of the borehole in response thereto; detection
means connected to each of said plurality of transducers for detecting the
generation of said control pulse and the reception of said acoustic echo
pulse received by said selected transducer; and count and transmission
detector means connected to said detection means for receiving said
control pulse and said acoustic echo pulse from said detection means, and
generating a count representative of a time interval between reception of
said control pulse and reception of said acoustic echo pulse, said time
interval being used to determine the transverse dimensions of the
borehole.
14. The apparatus of claim 13 wherein said count and transmission detector
means comprises: counter means responsive to said control pulse for
commencing said count and responsive to said acoustic echo pulse for
terminating said count, the value of said count upon termination of said
count being said time interval; and buffer means connected to an output of
said counter means for storing said value of said count, stored in said
counter means, therein, the value of said count being stored in said
buffer means prior to a reset of said counter means and a restart of said
count in response to another control pulse.
15. The apparatus of claim 13 wherein said electronic cartridge means
further comprises: amplifier means interconnected between said detection
means and said count and transmission detector means for amplifying said
acoustic echo pulse received by said selected transducer; and gain control
means connected to said amplifier means for varying a gain of said
amplifier means in accordance with an amplitude of said acoustic echo
pulse received from said selected transducer, whereby said gain is
maintained at a proper level thereby allowing said count and transmission
detector means to detect the existence of said acoustic echo pulse and to
continue the generation of said count in response thereto.
16. The apparatus of claim 1 wherein said sonde further comprises means for
measuring a time interval between the transmission of a first pulse by
said transducer and the reception of a first echo by said transducer, and
a time interval between the simultaneous transmission of a second pulse by
said transducer and the reception of a successively received second echo
by said transducer.
17. A method for measuring the transverse dimensions of a borehole
comprising the steps of:
(a) transmitting from transducer oppositely directed acoustic pulses
oriented perpendicularly to the longitudinal axis of the borehole toward
diametrically opposed sectors of the borehole wall, said transducer being
eccentered from said longitudinal axis of the borehole;
(b) detecting at said transducer first and second successively received
acoustic echo pulses after reflection of the transmitted acoustic pulses
from said opposed sectors of the borehole wall;
(c) determining an elapsed time for each of said first and second echo
pulses to return to said transducer after transmission of said acoustic
pulses;
(d) transmitting a reference pulse from a reference transducer and
receiving a reference echo pulse in response thereto;
(e) determining an elapsed time for said reference echo pulse to return to
said reference transducer; and
(f) combining the elapsed times of said first echo pulse, said second echo
pulse, and said reference echo pulse to determine the transverse
dimensions of the borehole.
18. The method of claim 17, wherein the echoes reflected from both sectors
of the wall are detected, and the time elapses between the transmission
and the detections of the respective echoes are summed.
19. The method of claim 17 wherein a plurality of the transducers are
distributed diametrically around a longitudinal axis of the borehole, the
method for measuring the transverse dimensions of a borehole comprising
the further steps of: simultaneously performing the transmitting step (a),
the detecting step (b), the determining step (c) and the combining step
(f) for each of said plurality of transducers. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to the measuement 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 notable 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 casings and tubing
undrego 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
applications 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
borehols 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 of 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 of 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 faces 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 it cooperates. It is thus possible to obtain a
transducer of small overall dimensions and which is adaptable to sondes of
small diameter. Moreover, when conditions allow, advantage is taken of 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 so 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 propagating velocity of the acoustic waves in the
medium in which the sonde is immersed. It can also provide a measurement
of the attentuation constant of the acoustic waves propagation 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 sonde 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 it 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 sprad them away
from the sonde and to apply the rollers 35 against the 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 terminatse 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.sub.1, are about 25 millimeters in height (see dimension h in FIG. 2)
and 15 millimeters in width (dimension 1 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 1. 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 quarterwave 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
a distance e which is about 12.5 millimeters.
The transducer 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 transmitted in
the two opposite directions in which the transducer is facing are received
at moments of time sufficiently far apart 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 determined 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 passage 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 with 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 simultaneouus 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 a function 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 signals 81 and 82 at times 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 oscilating 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 an electronic circuit 46 (FIG. 7) in
which the conductors 100.sub.1 to 100.sub.10 coming from the nine
transducers 42.sub.1 to 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.sub.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 whose
control input 108 is capable of receiving pulses from the firing control
output 109 of a control logic circuit 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 intput 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 designated
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.1 and the output 106.sub.1 corresponding
to the same transducer 42.sub.1. 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 tramsmitters 102.sub.1 to 102.sub.10. 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.1 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 firs 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.1 on the corresponding input
104.sub.1. 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 multibit 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 output 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 | | |
