U.S. patent number 3,789,355 [Application Number 05/213,061] was granted by the patent office on 1974-01-29 for method of and apparatus for logging while drilling.
This patent grant is currently assigned to Mobil Oil Corporation. Invention is credited to Bobbie J. Patton.
United States Patent |
3,789,355 |
Patton |
January 29, 1974 |
METHOD OF AND APPARATUS FOR LOGGING WHILE DRILLING
Abstract
A method and apparatus is described for monitoring at a remote
location downhole conditions encountered while drilling a well. A
sensed downhole condition is represented by a binary coded acoustic
signal which is transmitted by way of a liquid path, provided by
drilling liquid, to the surface of the earth. The acoustic signal,
whose phase state represents bit values, is detected at the surface
and decoded by way of a coherent system. Coherency is provided by
deriving from the received signal a reference signal which is
compared with the received signal to produce an output
representative of the sensed condition.
Inventors: |
Patton; Bobbie J. (Dallas,
TX) |
Assignee: |
Mobil Oil Corporation (New
York, NY)
|
Family
ID: |
22793597 |
Appl.
No.: |
05/213,061 |
Filed: |
December 28, 1971 |
Current U.S.
Class: |
367/137;
367/83 |
Current CPC
Class: |
E21B
47/18 (20130101); E21B 47/20 (20200501) |
Current International
Class: |
E21B
47/18 (20060101); E21B 47/12 (20060101); G01v
001/40 () |
Field of
Search: |
;340/18P,18LD |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Benjamin A.
Assistant Examiner: Moskowitz; N.
Attorney, Agent or Firm: Andrew L. Gaboriault et al.
Claims
I claim:
1. In a logging-while-drilling system for transmitting downhole
measured conditions to the surface of the earth during the drilling
of a well utilizing a flowing drilling liquid including an acoustic
generator having a movable member which when driven at a constant
speed produces in the liquid a continuous acoustic wave signal
having a first phase state and having a frequency proportional to
the speed of movement of said movable member, driving means for
said member, and a downhole transducer, the improvement
comprising:
means responsive to the output of said transducer for momentarily
changing the speed of said member and then re-establishing said
constant speed to change the phase of said acoustic signal from the
first phase state to a second phase state.
2. The system of claim 1 wherein said acoustic signal is coded in
m-ary coded bit lengths, where m is any integer greater than 1,
means for initiating said bits at a selected rate, and means for
varying the bit initiation rate to vary the time duration over
which said acoustic signal representing each bit is generated.
3. The system of claim 2 where m = 2, 4 or 8.
4. The system of claim 1 wherein the movable member for generating
the acoustic signal comprises a rotating valve for interrupting
liquid flow, means for generating pulses at a predetermined
repetition rate to control the rotation of said valve, means
responsive to the rotation of said valve for producing a signal
representative of the rate of rotation of said valve, means for
comparing said pulses and said signal to produce an error signal
when said valve is operating at a phase other than a preselected
phase, and means responsive to said error signal to adjust the rate
of rotation of said valve to regain said preselected phase and
return said error signal to zero.
5. The system of claim 1 including a plurality of means for sensing
downhole conditions and for generating analog signals
representative of said conditions, cycling means for multiplexing
said signals, an analog-to-digital converter for converting said
analog signals into digital words represented by two states "0" and
"1," an encoder responsive to said digital words to produce an
output signal only upon the occurrence of one of said states, means
for introducing to said system a sync signal at the beginning of
each cycle of said means for multiplexing, and wherein said means
for momentarily changing the speed of said member responds to each
output signal from said encoder for reversing the phase of said
acoustic signal.
6. The system of claim 5 in which said means for introducing said
sync signal introduces alternately two sync signals, one the
complement of the other.
7. The system of claim 5 including means for generating said states
at a selected rate, and means for varying the state generation rate
to vary the time duration over which said acoustic signal
representing each state is generated.
8. In the drilling of a well employing a drilling liquid and a
drill string within the well, the method of logging said well
comprising driving a movable member of an acoustic generator at a
constant speed to produce in the liquid a continuous acoustic wave
signal having a first phase state and having a frequency
proportional to the speed of movement of said movable member,
sensing at least one downhole condition and in response to said
sensed condition changing the speed of said member and then
re-establishing said constant speed to change the phase of said
acoustic signal from the first phase state to a second phase
state.
9. The method of claim 8 wherein said acoustic signal is modulated
in a nonreturn to zero mode in which phase conditions are different
phase states of said acoustic signal with reference to the phase
state of said signal during a preceding time interval.
10. In a well logging tool, the combination comprising:
an elongated housing adapted to be inserted into a well and into
contact with liquid within said well, said housing having a liquid
flow passage therein,
an acoustic generator supported by said housing and including a
movable member which when driven at a constant speed produces in
the liquid a continuous acoustic wave signal having a frequency
proportional to the speed of movement of said movable member,
means for regulating the operation of said generator at a selected
frequency,
a plurality of transducers supported by said housing for producing
analog signals representative of sensed downhole conditions,
means for converting said analog signals to serial digital
signals,
multiplexer means for serially applying the signals from said
transducers to said converting means, and
means for momentarily changing the speed of said member and then
re-establishing said constant speed to change the phase of said
acoustic signal in response to said digital signals.
11. The combination of claim 10 wherein said means for regulating
the operation of said generator comprises:
clock means for generating a primary timing function representative
of said selected frequency,
pulse generating means associated with said generator for
generating a secondary timing function representative of the actual
operating frequency of said generator,
means for comparing said primary and secondary timing functions,
and
means for adjusting the operating frequency of said generator in
response to the deviation of said secondary timing function from
said primary timing function.
12. In a well logging system having a logging tool movable
lengthwise through a well filled with liquid and including an
acoustic generator for imparting to the well liquid an acoustic
signal whose frequency is proportional to the speed of a motor
driving the acoustic generator, and having one or more transducers
for producing output signals representative of sensed downhole
conditions, the improvement comprising:
a signal source of constant frequency,
signal means driven by said motor for generating within said well
liquid an acoustic output signal having a frequency proportional to
the speed of said motor,
means for producing a signal proportional to the speed of the
motor,
means responsive to said constant frequency and to said signal
representative of motor speed for producing a resultant output
signal varying with phase differences between them,
means responsive to said last-named output signal for maintaining
said motor at a constant speed with a resultant fixed-phase
relationship between said constant frequency and said acoustic
output signal,
an uphole transducer acoustically coupled to said well liquid for
generating an output signal having the same frequency as that of
said acoustic output signal,
an oscillator for producing a reference frequency,
means including a phase detector for adjusting said oscillator for
maintaining a fixed-phase relationship between that of said
reference frequency and said acoustic output signal,
means responsive to each transducer carried by said logging tool
for applying to said driving motor a series of control pulses of
magnitude and duration for producing transient changes in the speed
of said motor to change by predetermined amounts the phase of said
acoustic output signal relative to that of said reference
frequency, and
means including a phase detector responsive to said changes in
phase due to said transient speed changes for producing output
pulses corresponding with said control pulses.
13. In a logging-while-drilling system for transmitting data via a
flowing drilling liquid between spaced points during the drilling
of a well utilizing an acoustic generator for producing a
continuous wave having a movable member which when driven at a
constant speed produces in the liquid a continuous wave signal
having a first phase state and having a frequency proportional to
the speed of movement of said movable member, driving means for
said member, and a signal source, the improvement comprising:
means responsive to the output of said signal source for
momentarily changing the speed of said driving means and then
re-establishing said constant speed to change the phase of said
acoustic signal from the first phase state to a second phase state.
Description
BACKGROUND OF THE INVENTION
This invention relates to the logging of wells during drilling and
more particularly to the telemetry of data relating to downhole
conditions by means of an acoustic signal transmitted through
drilling liquid within a well while concomitantly drilling the
well.
It has long been the practice to log wells, that is, to sense
various downhole conditions within a well, and concomitantly
therewith transmit the acquired data to the surface. Well logging
operations performed by service companies today utilize wireline or
cable-type logging procedures. In order to conduct the operations,
drilling is stopped and the drill string removed from the well. It
is costly to stop drilling operations in order to log. The
advantages of being capable of logging while drilling are obvious.
However, the lack of an acceptable telemetering system has been a
major obstacle to a successful logging-while-drilling
operation.
Various telemetering methods have been suggested for use in
logging-while-drilling procedures. For example, it has been
proposed to transmit the acquired data to the surface electrically.
Such methods have in the past proven impractical because of the
need to provide the drill pipe with a special insulated conductor
and means to form appropriate connections for the conductor at the
drill pipe joints. Other techniques proposed for use in
logging-while-drilling operations involve the transmission of
acoustic signals through the drill pipe. Exemplary of such
telemetering systems are those disclosed in U.S. Pat. Nos.
3,015,801 and 3,205,477 to Kalbfell. In the Kalbfell systems, an
acoustic energy signal is imparted to the drill pipe and the signal
is frequency modulated in accordance with a sensed downhole
condition. Frequency shift keying is employed to transmit the
acquired data in a digital mode. Yet other telemetering procedures
proposed for use in logging-while-drilling systems employ the
drilling liquid within the well as the transmission medium. Of
these perhaps the most promising is the technique described in U.S.
Pat. No. 3,309,656 to Godbey. In the Godbey procedure, an acoustic
wave signal is generated in the drilling liquid as it is circulated
through the well. This signal is modulated in order to transmit the
desired information to the surface of the well. At the surface the
acoustic wave signal is detected and demodulated in order to
provide the desired readout information.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a new
and improved logging-while-drilling process wherein telemetry of
information to the surface of the well is accomplished by phase
modulation of an acoustic signal. In carrying out the invention, an
acoustic signal is generated within the drilling liquid within the
well and transmitted upwardly through the drilling liquid to a
remote uphole station. One or more downhole conditions are sensed
and the acoustic signal is modulated in response to such sensed
conditions by varying the phase of the signal between a plurality
of phase conditions to produce an m-ary encoded signal. The encoded
signal is received at the uphole station and then demodulated or
decoded to produce readout functions corresponding respectively
with the phase conditions and thus the sensed conditions.
In a further aspect of the invention there is provided a new and
improved downhole logging tool for use in the telemetry of
information by phase shift keying of an acoustic signal in a liquid
medium within a well. The logging tool comprises an elongated
housing adapted for insertion into a well. The housing has a
passage therein through which liquid within the well may flow. An
acoustic generator supported by the housing functions to
periodically obstruct the passage in order to impart an acoustic
energy signal to the adjacent fluid. A plurality of logging
transducers supported by the housing produce analog signals
representative of sensed downhole conditions. The transducer
signals are sequentially applied by means of a multiplexer to an
analog-to-digital converter where the analog signals are converted
to serial digital signals. A control unit operates the generator at
a selected carrier frequency and responds to the digital signals to
change the phase of the acoustic signal. In one embodiment this is
accomplished by momentarily changing the frequency of the generator
in order to shift the phase of the acoustic signal between at least
two phase conditions.
In yet another aspect of the present invention there is provided a
receiving system for recovering the acoustic signal from the
drilling liquid and demodulating the signal to obtain the
information carried thereby. The receiving system is coherent--that
is, it derives the demodulating signal from the received signal.
The receiving system comprises an acoustic transducer which is
responsive to the acoustic signal within the drilling liquid to
produce an output signal representative of the phase and frequency
of the signal. The system further comprises a generator which
produces a control signal whose phase is controlled so to be
substantially constant and to match one phase state of the output
signal from the transducer. The transducer output signal and the
control signal are compared in a phase detector which produces a
plurality of readout functions corresponding respectively with a
plurality of phase relationships between the control signal and the
transducer output signal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a well drilling system equipped simultaneously
to drill and to log;
FIG. 2 is a cross section of the logging sonde;
FIG. 3 is a block schematic of the downhole encoding and
transmitting equipment;
FIGS. 4 and 5 are waveforms associated with the operation of the
equipment shown in FIG. 3;
FIG. 6 is a block schematic of the uphole receiving system; and
FIGS. 7 and 8 are waveforms associated with the operation of the
system of FIG. 6.
DESCRIPTION OF SPECIFIC EMBODIMENTS
In accordance with the preferred embodiments of the invention as
described in more detail hereinafter, an acoustic signal is
transmitted through the drilling liquid employed in normal drilling
operations. As the well is drilled, at least one downhole condition
within the well is sensed and a signal, in most cases analog, is
generated which is representative of the sensed condition. An
analog signal is converted to a serial digital signal. The acoustic
signal generated within the drilling liquid is modulated in
response to the digital signal by correlating the phase of the
acoustic signal during sequential time periods corresponding to the
digit intervals of the digital signal with a plurality of phase
conditions representative of the respective digit values of the
digital signal. The acoustic signal is received at an uphole
station and demodulated in order to produce appropriate readout
functions corresponding with the respective phase conditions. These
readout functions then may be applied to appropriate utilization
means such as recording and/or data processing systems, such as
computers, from which desired information may be derived.
Turning now to FIG. 1, there is illustrated a well 10 which is
being extended through the earth's crust by means of a rotary
drilling technique. Drilling operations are carried out utilizing a
drill bit 12 attached to the lower end of a drill string 14. The
drill string terminates at its upper end in a kelly 16 which is
polygonal in cross section and extends through a rotary table 18 on
the floor of the drilling rig. The kelly is received in the rotary
table in a slidable torque-applying relationship by means of a
kelly bushing (not shown). The rotary table is powered by a prime
mover 20 through a suitable drive mechanism such as a chain drive
21 as will be readily understood by those skilled in the art. The
drill string is suspended from the derrick structure of the
drilling rig by means of a hook 23 which is attached to a traveling
block 24. The traveling block in turn is suspended by a suitable
cable arrangement 26 from the crown block (not shown) which is
located in the upper portion of the derrick structure. The kelly is
connected to the hook through a rotary swivel 28 which permits
rotation of the drill string relative to the hook and traveling
block. Drilling liquid from a container 30 (commonly called a "mud
pit") is circulated by a pump 31 through a conduit 32 into the
swivel 28 and thence downwardly through the interior passage of the
drill string to the bit 12. The drilling liquid then passes
outwardly into the wellbore through appropriate ports in the drill
bit and is circulated to the surface of the well through the
annulus between the drill string and the wall of the well. At the
surface the mud is withdrawn from the annulus through a conduit 33
and recirculated to the mud pit 30. The conduit 32 is equipped with
a desurger system 34 which functions to suppress undesirable
"noise" in the drilling liquid introduced by pulsations produced by
the pump 31.
Typically, the drilling liquid takes the form of a "mud," that is,
a liquid having solids suspended therein. These solids function to
impart desired rheological properties to the drilling liquid and
also to increase the density thereof to a value adequate to provide
sufficient hydrostatic pressure at the bottom of the well. In some
instances, a relatively clear liquid may be employed which contains
little or no suspended particulate materials. The drilling liquid
may be either an aqueous-base liquid or an oil-base liquid.
Located within the drill string 14 near the drill bit is a downhole
logging tool 36 which comprises one or more logging transducers for
sensing downhole conditions and an acoustic generator for imparting
an acoustic signal to the drilling liquid. Typically, the logging
tool will be provided with transducers to measure a number of
downhole conditions such as weight on bit, torque at the bit,
pressure drop across the bit, pressure in the annulus, formation
pressure, vibration, drilling mud temperature, and natural gamma
ray count. The acoustic generator employed may be of any suitable
type which will impart a pressure wave signal to the drilling
liquid which is of sufficient amplitude for transmission to the
desired uphole location. A particularly appropriate generator for
use in carrying out the present invention is a rotary valve
transmitter of the type disclosed in the aforementioned patent to
Godbey.
In carrying out the present invention, an acoustic signal is
imparted to the drilling liquid by means of the acoustic generator
within tool 36 and thence transmitted upwardly through the drilling
liquid. This signal is modulated in response to a downhole
condition sensed by a logging transducer by varying the phase of
the acoustic signal. At the surface of the acoustic signal is
recovered from the drilling liquid by means of one or more
receiving transducers and converted to an electrical output signal.
For example, a transducer 38 may be mounted on the upper section of
swivel 28 as illustrated in FIG. 1. The signal from transducer 38
is applied to an uphole receiving system 40 where it is demodulated
to produce the readout functions representative of the measured
downhole conditions.
In accordance with the present invention it is preferred to employ
an m-ary pulse code format, where m equals 2, 4, 8, in which the
acoustic signal generated downhole is phase shift keyed to transmit
the desired information in a serial digital format. For example, th
phase conditions employed to impart intelligence to the wave may be
the phase state of the signal, i.e., the phase angle of the signal
relative to a reference wave. Thus, for a binary code (m=2) an
inphase state (phase angle of zero) may be employed for one bit
value and a phase reversal state (phase angle of 180 degrees) may
be employed for the other bit value. For a base four code (m=4) the
phase conditions corresponding to the respective digit values may
be phase states of 0, 90, 180, and 270 degrees. Alternatively, a
nonreturn to zero scheme may be employed in which the phase
conditions corresponding to the several digit values are several
phase states of the signal relative to the phase state of the
signal during a previous digit interval. For example, for a binary
coded system, a phase continuity from one digit interval to the
next may be indicative of a zero bit value whereas a reversal of
phase from one digit interval to the next may be indicative of a
bit value of one. For a ternary coded system the digit values may
be indicated by a constant phase relationship from one digit
interval to the next for one digit value, at 120-degree phase shift
for another digit value, and a 240-degree phase shift for the third
digit value. The nonreturn to zero coding technique is preferred
for reliability in decoding inasmuch as bit value is indicated by
phase change. The invention will be described in detail with
reference to a binary nonreturn to zero code.
The logging tool 36 is shown in more detail in FIG. 2. Tool 36
comprises elongated inner and outer housings 42 and 44 which define
an annulus 45 through which the drilling liquid passes. The upper
and lower ends of the outer housing are threaded for connection in
the drill string. As illustrated, the logging tool is composed of
three basic sections 46, 48, and 50. Associated with the upper
section 46 is an acoustic generator or transmitter 54 comprising a
slotted stator 52 and a rotor 54a with complementing slots which is
driven relative to the stator. The rotor is shown in the open
position in which slot 54b in the rotor and slot 52a in the stator,
respectively, are in communication. A prime mover 56 applies power
for rotation of the rotor 54a through a gear reduction unit 57. As
is evident from an examination of FIG. 2, drilling liquid will pass
through the slots in the rotor and stator. As the rotor is driven
by the prime mover, the liquid stream is repeatedly interrupted to
impart the desired acoustic wave signal to the drilling liquid. The
diameter of rotor 54a is slightly less than the inner diameter of
housing 44. Thus, some drilling liquid will pass around the rotor
and through the slots in the stator when the rotor is in a closed
position.
A tachometer 58 is mounted on the lower end of the motor shaft in
order to provide a signal which is representative of the speed of
the motor, and thus the speed at which valve 54 is driven. As
described hereinafter, the tachometer signal is employed in a
feedback loop to control the frequency and phase of the acoustic
signal. Tachometer 58 comprises a coil 59 and and rotor 60 which is
lugged to the motor shaft. Rotation of element 60 relative to coil
59 induces an alternating voltage signal in the coil of a frequency
proportional to the operating speed of motor 56. The signal from
coil 59 is then applied to conventional pulse shaping and frequency
dividing circuits (not shown) to provide a pulse signal utilized to
control the speed of the motor.
The intermediate section 48 of the tool is sealed off from the
upper and lower sections by means of bulkheads 62 and 63 through
which electrical leads pass. This section contains the electronic
components associated with the logging tool, including those for
receiving the output signals from the several logging transducers
utilized to sense downhole conditions. A collar 64 surrounds
housing 44 to provide an outer compartment 65 within which logging
transducers may be located.
The lower section 50 of the logging tool houses a generator 68
which provides electrical power for the motor 56. Mounted within
this section is an electrical generator 68 which is driven by a
turbine 70. The turbine 70 is rotated by the drilling liquid which
flows downwardly through the tool through the annulus 45 between
the inner and outer housings 42 and 44. For a more detailed
description of the mechanical characteristics of the logging tool
shown in FIG. 2, reference is made to the aforementioned patent to
Godbey.
Turning now to FIG. 3, the major components of logging tool 36 are
illustrated in block schematic form. This system includes a
sync-word generator 74 and a plurality of logging transducers
D.sub.1, D.sub.2 . . . D.sub.N for sensing downhole conditions such
as those described above and producing outputs representative of
such conditions. The outputs (typically analog voltage signals)
from units D.sub.1, D.sub.2 . . . D.sub.N are applied through a
multiplexer 80 to a coder 82. The multiplexer functions to apply
the analog signal to coder 82 in any suitable sequence pattern. The
reference character SW identifies a channel of the multiplexer 80
associated with a sync word introduced in another part of the
system from generator 74. If desired, the multiplexer may be
equipped with sufficient channels so as to provide for more
frequent sampling of one or more signals from units D.sub.1,
D.sub.2 . . . D.sub.N. For example, the signal from transducer or
unit D.sub.1 may be applied to two input channels in the
multiplexer so that this parameter is transmitted twice for each
multiplexer cycle.
The coder 82 is an analog-to-digital converter which produces a
digital word in response to each analog signal from transducers or
detectors D.sub.1, D.sub.2 . . . D.sub.N. The output from coder 82
is applied to an encoder 84 by way of an 10-element gate 75 and the
output from sync-word generator 74 is applied to encoder 84 by way
of 10-element gate 76. Encoder 84 is a parallel-in series-out shift
register which functions to convert the parallel signal from coder
82 and from sync-word generator 74 to a serial digital signal which
is then applied sequentially to a motor control unit 85. The motor
control unit, multiplexer, coder, and encoder are controlled for
synchronous operation by a master clock 86.
Sync-word generator 74 is a hardware unit that outputs on command
from master clock 86 and counter 87 a predetermined word or words
utilized to identify the beginning of a frame of data words.
Preferably, the system is provided also with a parity generator 88
which functions to add a parity bit to the word output of encoder
84, thus providing a parity check for each word transmitted. In
operation, the parity generator produces a parity bit of one state
in response to an odd number of bits in a word of a given state and
a parity bit of another state in response to an even number of bits
in the word of the given state. For example, if an odd parity check
is employed for bit values of one, the parity generator 88 will
count the number of "1's" in the word and make the eleventh bit a
"1" if the number of "1's" in the first ten bits is even and a "0"
if the number of "1's" in the first ten bits is odd. Thus, each
word frame transmitted will contain an odd number of "1's," thus
providing a check for transmission or reception errors.
The motor control unit 85 functions to maintain a phase lock
transmission mode by comparing a feedback signal from the
tachometer with the primary timing signal from the clock 86. In
addition, motor control unit 85 functions to execute a phase
reversal of the acoustic signal in response to each signal "1"
command from the encoder 84. In both instances, control voltages
are applied from unit 85 to voltage-controlled unit 90 which
supplies power to a two-phase induction motor 56. The motor 56 is,
as described in FIG. 2, mechanically connected to drive the
transmitter 54.
The operation of the motor control unit 85, and associated
circuits, in carrying out the aforementioned functions will be
described with reference to the waveforms shown in FIGS. 4 and 5.
The several waveforms illustrated and the respective points at
which they appear in the circuitry of FIG. 3 are designated by
reference characters in FIGS. 4 and 5. For the purpose of this
description, it is assumed that the transmitter 54 is a 10-slot
rotary valve of the type described previously which is operated at
an acoustic carrier frequency of 25 hertz, equivalent to a
transmitter speed of 150 rpm. In one embodiment where the gear
reduction unit 57 had a ratio of 25:1, the motor 56 was operated at
a speed of 3750 rpm to produce from transmitter 54 an output at 25
hertz. In addition, the encoder output was framed to produce 11-bit
(10 information bits plus 1 parity bit) binary words.
In operation, the master clock 86 and counter 87 provide timing
signals a, b, c, d, and g (FIG. 5) to the multiplexer, coder,
encoder, motor control unit, and sync-word generator, respectively.
In addition, the pulse generator 58a produces a train of signal
pulses e which is applied to the motor control unit. The repetition
rate of pulses e is a measure of the speed of motor 56. The
generator 58a includes the tachometer 58 (FIG. 2) and related pulse
shaping circuits. The frequency (pulse repetition rate) of the
primary timing signal d is 50 pulses per second. Signal e likewise
will have a repetition rate of 50 pulses per second when the
transmitter 54 is operating at 25 hertz.
The various signals or pulses a, b, c, d, e, and g occur at
repetition rates related in the following manner:
a = b = (f.sub.s /BP)
c = (f.sub.s /P)
d = e = 2f.sub.s
g = (f.sub.s /BPW)
where, in the specific embodiment being described,
f.sub.s = sonic frequency = 25 hertz
B = number of bits per word = 11
P = periods or cycles per bit = 25
W = number of words per frame = 16
The speed of motor 56 is determined by the amplitude of signal
output from unit 85. The output in turn is a function of the
summation of four (4) voltages; V.sub.1, the value of which is
determined by the phase relationship between pulses d and e;
V.sub.2, a constant amplitude d-c voltage set at a value determined
to produce a 25 hertz acoustic signal; V.sub.3, a constant
amplitude d-c voltage (a negative bias whose value is equal to
positive V.sub.1 when pulses d and e are 180 degrees out of phase);
and V.sub.4, a phase reversal voltage, generated to change the
speed of motor 56 in order to reverse the phase of signal generated
by transmitter 54.
The voltages V.sub.2 and V.sub.3 are shown provided, respectively,
from batteries 91 and 93. The voltage V.sub.1 is derived in the
following manner. Signal d from the clock is applied to a bistable
multivibrator 92 and to a linear integrator 94. The signal e is
also applied to multivibrator 92 and to hold circuit 96. The clock
signal d operates to turn the multivibrator 92 to the ON state
where it applies a constant amplitude d-c voltage V.sub.1 to
integrator 94. The signal e from the pulse generator 58a operates
to return the multivibrator to the OFF state. Thus, the output from
multivibrator 92 is a pulse train of constant amplitude, variable
duration pulses in which the pulse duration is proportional to the
interval between the clock pulse d and pulse e. This pulse train is
applied to the integrator 94 which produces a voltage signal whose
amplitude is proportional to the time duration of the respective
pulses applied to it. The integrator holds the integrated value
until it is reset to zero by the next-occurring clock pulse d,
whereupon it is conditioned to respond to the succeeding pulse from
multivibrator 92. Each final, integrated value at the integrator is
sampled and held by sample and hold circuit 96 which functions in
response to pulse e. Accordingly, a voltage representing the phase
difference between pulses d and e is held from one pulse e to the
next. This voltage, the voltage V.sub.1, and voltages V.sub.2 and
V.sub.3 are applied to a summing amplifier 98.
The output from amplifier 98 is applied to motor power supply 90 to
change the frequency of its output and thus control the speed of
induction motor 56. More particularly, the power supply 90
comprises an inverter 102, a voltage-controlled oscillator 104, and
a source of d-c illustrated as battery 103. The inverter 102
translates the d-c to a-c at a frequency of signal from oscillator
104, which frequency is determined by the output from amplifier
98.
As thus far described, the motor control circuitry drives the motor
56 and therefore the acoustic transmitter 54 in a phase locked mode
in which the clock pulses d are 180 degrees out of phase with
respect to pulses e from the pulse generator 58a. In this mode the
positive voltage V.sub.1 from sample and hold circuit 96 is equal
in amplitude to the negative voltage V.sub.3 and the motor speed is
determined by voltage V.sub.2. A change in motor speed will change
the phase relationship between pulses d and e and in turn vary the
amplitude of voltage V.sub.1. The change in V.sub.1, an error
signal, will be effective to rebalance the system and bring the
motor back to its proper speed and selected phase.
The operation of the transmitting system in effecting phase
shifting of the acoustic signal will now be described. The rate or
frequency of signal a applied to the multiplexer 80 from counter 87
is one pulse per 11 seconds. Upon the application of the signal
pulse a, the multiplexer steps from one channel to the next and
applies a selected analog value to the coder 82. The frequency of
the signal b to the coder is also one pulse per 11 seconds. This
signal is delayed slightly with respect to signal a in order to
accommodate the stepping time of the multiplexer and the conversion
time of the coder. The coder converts the analog value to a 10-bit
binary word representative of the applied analog value. The
frequency of the signal c applied to the encoder 84 is one pulse
per second and this signal is also shifted in phase with respect to
signal b to provide a suitable time delay. The encoder responds to
each applied pulse to generate a command signal j, either a "1" or
a "0." The motor control unit 85 responds to the occurrence of each
"1" command to reverse the phase of the acoustic signal and to the
occurrence of each "0" command to continue the acoustic signal in
the same phase.
The bit interval A is a "1" bit. As a result, the encoder 84
responds to generate pulse j.sub.1 which is applied to waveform
generator 100 which produces a square wave voltage k.sub.1. The
summing amplifier 98 output now increases and is effective by way
of supply 90 to increase the speed of motor 56 to advance the phase
of the acoustic signal. The motor speed and the attendant acoustic
signal are illustrated waveforms l and m, respectively. As shown,
the motor speed increases substantially linearly as the voltage
output from amplifier 98 increases. When the voltage k.sub.1 is
suddenly decreased, the motor decelerates until it returns to the
normal speed equivalent to the carrier frequency of 25 hertz. At
this point, the signal is approximately 180 degrees out of phase
with respect to the previous phase state and the motor control unit
will operate in response to the feedback signal from pulse
generator 58a to achieve an exact phase locked condition.
The amplitude of square waves k is made large compared with the
maximum output from integrator 94 and this nullifies the effect of
the feedback control provided by pulses e, enabling the motor 56 to
speed up and change phase. The generator 100, for purpose of this
description, may be considered a part of the motor control circuit
85 though illustrated outside the dashed lines. The output of
generator 100, though shown and described as a square wave, may be
of any form suitable to change the speed of motor 56.
When the bit value as shown during interval B is a "0," the command
signal from the encoder is simply the absence of a pulse and the
acoustic generator continues to operate to produce an acoustic
signal having the same phase state as in the previous bit interval
A. The bit value during interval C is a "1" so the encoder 84 again
responds to the pulse c to generate a pulse j.sub.2 which is
received at generator 100. An output k.sub.2 is generated and the
motor 56 is again accelerated and decelerated as indicated in order
to reverse the phase of the acoustic signal.
The format in which the data is framed for transmission from the
downhole sonde to the surface for detection is illustrated in FIG.
5. Beginning each frame is a sync word, followed by data words "1"
through "N." In the embodiment being described, there are 15 data
words: N = 15. Examples are illustrated in the various data words
of the value of the parity bit as being either 0, as in data word
"1," or a one, as in data word "2," in order that an odd number of
one bits comprise each data word. Illustrated also is the
occurrence of pulses a and b at the onset of each word.
Pulse g, however, occurs only once for each frame and is utilized
to control the time occurrence of each sync word and also the
opening and closing of necessary gates in order to apply the sync
word to the encoder 84. The sync-word control pulse g is generated
by the counter 87 and applied to the sync-word generator 74.
Concurrently it is applied to a gate generator 77 which may be of
the monostable, multivibrator type which outputs a gating waveform
h as well as a gating waveform h', not illustrated but which is the
complement of the waveform h. The waveform h is applied to the gate
76 to open it while the waveform h' is applied to the gate 75 to
close it. With gate 76 open and gate 75 closed, the output from the
sync-word generator is applied directly to the encoder 84 and the
output from the A-D converter 82 is blocked from the encoder. After
a time interval, predetermined to be one-word length in duration,
the gate generator returns to its stable state and waveform h moves
in a negative-going direction to close gate 76; and its complement
waveform h', moving in a positive-going direction, is effective to
open gate 75.
Another feature of the present system is the use of two sync words,
one the complement of the other. These sync words are generated by
the sync-word generator alternately. In other words, the first sync
word would be used for the odd-numbered frames of data whereas its
complement would appear at the beginning of the even-numbered
frames. The sync word is illustrated in FIG. 5 in frame 1, and its
complement is illustrated beginning at frame 2.
The length of each bit may be varied by adjusting the counter 87,
for example, by turning the knob 87a, in order to increase or to
decrease the number of cycles representing each bit. Where the
signal-to-noise ratio is high, such as may be encountered during
the shallow portion of the drilling program, the bit length may be
one second or less. As the hole deepens, the signal-to-noise ratio
will decrease and it will be desirable to increase the number of
cycles representing each bit or, in other words, increase the bit
length. A change in the bit length will, of course, necessitate
change in the rates of the various pulses, for example, the pulses
a, b, c, etc., all in accordance with the relationships earlier set
forth.
As noted previously, the transmitted acoustic wave signal is
received at the surface by a transducer 38 which functions to
convert the acoustic signal to a form suitable for demodulation.
Transducer 38 may be of any appropriate type which responds to the
acoustic signal to generate an output signal which is
representative of the phase of the acoustic signal. By way of
example, transducer 38 may be a piezoelectric crystal which
generates an a-c signal of the same frequency as the acoustic
signal.
The output signal from the transducer 38 is applied to a receiving
system which generates from the output signal a control signal
which remains in synchronism with one phase state of the received
acoustic or output signal. The received signal is demodulated by
comparing it with the control signal to detect phase shifts in the
received signal and appropriate readout functions are generated
which correspond with different phase relationships between the
received signal and the control signal.
Referring now to FIG. 6, a preferred embodiment of the receiving
system employed in the present invention is illustrated in block
diagram. The waveforms associated with the operation of the system
are illustrated in FIGS. 7 and 8 with the waveforms and the points
at which they appear in FIG. 6 designated by common reference
characters. In order to simplify the description, the exemplary
transmission mode and parameters specified above with respect to
the transmitting system will be assumed here. Thus, the carrier
frequency of the received acoustic signal is 25 hertz and binary
nonreturn to zero keying is employed. However, it will be
recognized that the receiver may be employed with other suitable
transmission modes such as those noted previously.
The received signal is applied from the output of transducer 38 to
a low noise amplifier 110 to avoid introducing noise distortion to
the signal. The higher gain signal from amplifier 110 is applied to
a filter 112, whose band pass is the reciprocal of the bit length
utilized in order to attenuate noise outside the frequency of
interest. Where the bit length is one (1) second, the band pass
would be one cycle, for example, 24.5 to 25.5 hertz. From filter
112 the signal is fed through an automatic gain control amplifier
114 in order to eliminate wide amplitude excursions in the signal
and to provide a signal of relatively uniform amplitude p.
The signal p from amplifier 114 is applied both to a synchronous
phase detector 116 and to a phase-lock loop 117 in which a control
signal z for detector 116 is derived as described hereinafter.
Phase detector 116 functions to compare the phase of signal p with
the control signal z to produce a signal r representative of the
phase relationship between the two signals. By way of example,
phase detector 116 may be a switching-type unit in which the
control signal z functions to switch the signal p between positive
and negative inputs of the detector. During the positive-going
excursions of the control signal the output signal r of the phase
detector is reversed in polarity with respect to signal p. During
negative excursions of the control signal z, the output signal from
detector 116 is the same polarity with respect to the signal p.
Thus, when the transducer signal p and the control signal z are out
of phase, as indicated in FIG. 7 by segments appearing in portion
labeled Phase I, the output r from the detector 116 is positive.
When the signal p and the control signal z are in phase with
respect to one another as shown by segments appearing in portion
labeled Phase II, the output from unit 116 is a negative half-wave
signal.
In the phase-lock loop 117, the control signal z for detector 116
is originated by a voltage-controlled oscillator 120. The
oscillator 120 is initially adjusted by application of appropriate
voltage values to produce pulses t at a repetition rate four times
that of the frequency of the received acoustic signal p. In the
embodiment disclosed the pulses t have a repetition rate of 100
hertz.
Feedback control for oscillator 120 is derived from the transducer
output signal p. This signal is applied to a frequency multiplier
in which the frequency is doubled to produce a 50-hertz signal s.
By doubling the frequency, the phase state of signal s remains the
same regardless of phase shifts which may occur in the transducer
signal p.
Signal t from oscillator 120 is applied to a square wave generator
121, which may be a bistable multivibrator, to produce pulse train
u having a frequency equal to that of signal s. Signals u and s are
applied to a synchronous phase detector 126 which operates in the
manner described above with reference to phase detector 116. The
output from detector 126 is the wave train v whose average d-c
component will be zero when the loop 117 is in phase lock. Under
such condition the pulses u are 90.degree. out of phase with signal
s.
The signal v is utilized to control the frequency and phase of VCO
120. It is applied by way of amplifier 130 and filter 132 to VCO
120. The amplifier 130 has a preadjusted gain set in accordance
with parameters of loop 117 to provide the necessary control from
signal v. The filter 132 has a long time constant such that
substantially only the d-c component of v, the error signal w, is
fed VCO 120. This feedback arrangement assures that the output u
from square wave generator 121 will be maintained in a
predetermined phase relationship with signal s, that is, it will be
90.degree. out of phase with signal s.
The operation of the feedback control is illustrated in FIG. 7 by
an initial error condition and a change in the phase of signal p.
Assuming that the signals s and u are other than in the 90.degree.
phase relationship, a negative component will be developed as shown
by waveform w. The error is greatly exaggerated in the drawings for
purpose of illustration. The negative signal w when applied to VCO
120 corrects the output such that signals s and u are brought into
the desired phase relationship. When the downhole generator changes
phase, an error signal is again developed, first moving in a
negative direction and then in a positive direction. However, about
the time the change in phase of signal p is completed, signals s
and u are again in the desired phase relationship.
The wavetrain or signals u are applied to a pulse generator 135
which responds only to the leading edge of signals u to produce
pulses x. The repetition rate of pulses x is at twice the frequency
of signals p, in this instance, 50 per second.
Remembering that the purpose of the circuitry is to produce signals
z whose phase state will be constant and be one of the phase states
of the signal p, it will be necessary to shift the phase of signals
or pulses x. The actual phase shift introduced will be such that
the shifted pulse will have a time occurrence corresponding with
the zero crossing of signal p. The shift is introduced by delay
circuit 136 whose output is a series of pulses y.
The pulses y are applied to square wave generator 138, a bistable
multivibrator, to produce the square wave pulses z. The train of
pulses z are applied to the phase detector 116 and in the manner
described above there are produced the summation signals r, the
detected output, wherein a phase of one state of signal p will be
represented by positive-going signals r and a phase of another
state of signal p will be represented by negative-going signals
r.
It will be recalled that the amplifier 130 is of fixed gain.
Therefore in order to assure operation of the loop 117, the
amplitude of signal p applied to the loop is held substantially
constant. The amplitude control is provided by amplifier 114 and an
associated automatic gain control circuit. The detected signals r
are full wave rectified by rectifier 140 and filtered by low pass
filter 142 to produce a representation of the amplitude of signal
p. The function, which may be monitored by meter 144, is applied to
a differential amplifier 146 which compares the value of the
function with the value of a reference voltage V.sub.R from source
148. Changes in the value of the function about the value of
V.sub.R are fed to the amplifier 114 to control its gain and to
hold the amplitude of signal p substantially constant. The
amplitude or level at which signal p is held is determined by
setting the amplitude or value of voltage V.sub.R as by adjusting
knob 149 on source 148.
At this stage useful data is developed in the form of signals r.
This signal need but be decoded applying the principle that a
change in phase (polarity) is a "1" bit and a lack of change in
phase (polarity) is a "0" bit. The signal r may be recorded in
visible form on a chart recorder (not shown) and decoded by an
operator. However, automatic machine decoding is preferred
especially into a format that will be machine readable. To this
end, a bit lock or sync system is utilized.
As a first step in acquiring bit sync and detection, the signal z,
at a frequency f.sub.s (the frequency of the signal p), is applied
to a divider 150 which produces a pulse train aa at a repetition
rate equal to the bit rate. In the present system the bit period is
one (1) second and the duration of each of the pulses aa is one
half a period or one-half second. The onset of pulses aa must, as
will be apparent, coincide with the beginning of each bit period.
Because the pulses z are independent of bit period and the onset,
provision is made to adjust the onset of pulses aa to coincide with
the onset of each bit period. The adjustment is provided by
adjustable delay 152 whose output is a train of pulses bb shown in
FIG. 8 to be 180.degree. out of phase with pulses aa and having
onsets coinciding with the beginning of bit periods shown as signal
r.
With the occurrence of pulses bb properly adjusted the bit
detection may be accomplished by starting the integration of the
signal r at a time corresponding with the onset of each pulse bb
and terminating the integration at a time corresponding with the
end of each bit period. The sampled value of each integration will
be indicative of the value of the bit signal r and thus each bit
will be properly identified.
In carrying out the integration-detection, the pulse train bb is
utilized to trigger pulse generator 154 which responds to the reset
or positive-going portion of each pulse bb to produce a train of
short duration pulses cc. The pulses cc are applied to integrator
156, also referred to as the in-phase integrator, which is reset in
response to the trailing edge of each pulse cc. A sample and hold
circuit 158 responds to the leading edge of each pulse cc to sample
the output of the integrator 156. The output of integrator 156 is
the integrated value ee of the signal r applied to the input of the
integrator. The output of the sample and hold circuit 158 is the
waveform ff.
A "1" bit is represented by each zero crossing of the waveform ff.
If, as utilized herein, a zero-crossing detector is utilized to
generate the "1" and the "0" bits, the waveform ff should be
modified such that it has uniform maximum and minimum values,
otherwise there may be false tracking by the zero-crossing
detector. Accordingly, the waveform ff is applied to a bit-polarity
detector 160, a saturation amplifier that is driven to saturation
for each positive or negative value of the waveform ff. The result
is the waveform gg which is applied to a bit-value generator 162, a
zero-crossing detector whose output is the pulse train mm comprised
of "1" bits, represented by the presence of pulses, and "0" bits,
represented by the absence of pulses.
The pulse train mm is applied to a utilization device 163 which may
be a recorder or a digital computer either hardwired or of the
general purpose programmable type. In either event it is desirable
to provide information as to when to read the information in pulse
train mm, that is, information as to time at which to look for a
"0" bit or a "1" bit. This information is provided by pulses from
the "read" generator 164. The "read" generator is a unit to which
is applied the pulse train cc. The pulses are delayed a
predetermined time to occur during the expected time occurrence of
the "bit" pulses mm and the result is the pulse train nn.
All necessary information is provided by the pulse trains mm and nn
to decode the data and provide measurement of downhole conditions.
By utilizing a programmable computer, appropriate software may be
provided to identify the sync words, announcing the start of each
frame and to collate the data from each of the downhole data
channels.
The amount of delay to be introduced to the pulses aa in order that
the integrator 156 integrate signal r representing one bit at a
time and avoiding overlap into preceding or succeeding bit
information is determined by an arrangement utilizing an
out-of-phase integrator 170. The integrator 170 has applied to its
input the bit information as represented by waveform r and has an
output when the system is properly adjusted as represented by the
initial portion of wavetrain hh. As illustrated, the integrator 170
begins its operation at a time delayed approximately 90.degree.
from that of the in-phase integrator 156. The integrator 170 is set
to integrate over a period of one bit length; therefore, its output
at the end of an integrating period should be zero. Accordingly,
the delay unit 152 is adjusted until the output from the integrator
170 is zero at the end of each period of integration. The
adjustment is carried out by applying the pulses bb to the input of
a pulse generator 172 which responds to the negative-going
excursions or the trailing portion of the pulses bb to produce a
series of short duration pulses dd. The integrator 170 is reset by
the trailing portion or the negative-going portion of each pulse dd
and begins the integration of the input waveform r to produce
wavetrain hh. The wavetrain hh together with the bit information,
the wavetrain gg, is supplied to a synchronous phase detector 174
operating in the same manner as the phase detectors 116 and 126 to
produce an output represented by the wavetrain ii. The wavetrain ii
is applied by way of normally open gate 176 to the input of a
sample and hold circuit 178 which responds to the leading edge of
the pulses dd to momentarily sample and then to hold the output or
signal ii at a time corresponding with the end of the period of
integration by integrator 170. With the system properly adjusted
the sample and hold circuit 178 will read zero each time the
wavetrain ii is sampled and its output can be represented as shown
by the signal ll. If the system is not tracking properly, an error
signal will result at the output of the sample and hold circuit 178
which will be detected by meter 180. The meter will indicate the
amount of error and the direction of error which can be corrected
by adjusting the delay unit 152 as by moving knob 152a.
It will become readily apparent from the examination of the
waveform r that there are conditions under which the output from
the integrator 170 will be other than zero at the time it is to be
sampled even though the system is properly adjusted. This condition
exists when zero bits are being transmitted and received. Such
condition is illustrated by the waveform hh attaining full negative
value whenever a zero bit or a succession of zero bits are
transmitted and received. An operator making the adjustments by
observing the meter 180 will learn that large excursions of the
meter are to be ignored as merely representing the occurrence of a
zero bit. As a matter of practice, the maximum error to be expected
or to be read by the meter 180 will be approximately one half the
full integrated value of a received bit. Any indication by the
meter 180 of a value less than one half the integrated value of a
bit may be treated as an error and anything larger than the
one-half value is to be ignored.
The above logic in interpreting the output of a meter 180 may be
represented by hardware and the adjustment made automatically. This
is accomplished by providing a discriminator 182 having applied to
one input a reference voltage V.sub.O having an amplitude equal to
one half the integrated bit value. The wavetrain ii is applied to
the other input of the discriminator. Whenever the amplitude of the
signal or wavetrain ii exceeds the half value, the discriminator
will apply a control pulse which will close gate 176. The end
result will be the wavetrain kk where an operation of the
discriminator to close the gate is represented by portions of the
wavetrain kk appearing between the segments bounded by small arrows
where the signal kk has been reduced to zero. With the
discriminator operating as above described, the sample and hold
circuit 178 will see a zero signal at each sampling interval unless
the system is in fact out of adjustment. Let us assume that the
train of pulses dd has been delayed an amount .delta.. In this
event the operation of sample and hold circuit 178 will be delayed
and it will see an output from the integrator. This output,
identified by reference character 190, will be applied by way of a
low pass filter 192 to the delay unit 152 to change impedances
therein to effect the necessary correction in the amount of delay
being applied to the pulse train aa. If the error is in the other
direction as illustrated by advance in one of the pulses d by an
amount -.delta., the sample and hold circuit 178 again will see an
output from the integrator 170. This output, represented by the
negative-going waveform 194, will likewise be applied by way of the
filter 192 to effect the necessary corrections in the delay unit
152.
The above description of automatic feedback to acquire and maintain
bit phase sync utilizes an analog feedback control signal which is
amplitude proportional to the phase error .delta. and is bipolar in
accordance with the direction of the phase error. Other feedback
mechanisms may be used. For instance, a purely digital scheme of
either inserting extra pulses or deleting pulses at the input of
divider 150 is effective in shifting the phase of the bit
integrators. Delay 152 is used in this modification as an initial
adjustment of the circuits to get exact phasing.
* * * * *