U.S. patent application number 12/618906 was filed with the patent office on 2010-06-03 for downhole timing recovery and signal detection.
This patent application is currently assigned to XACT DOWNHOLE TELEMETRY INC.. Invention is credited to John G McRory.
Application Number | 20100135117 12/618906 |
Document ID | / |
Family ID | 42212025 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100135117 |
Kind Code |
A1 |
McRory; John G |
June 3, 2010 |
Downhole Timing Recovery and Signal Detection
Abstract
The present invention relates to telemetry apparatus and
methods, and more particularly to acoustic telemetry apparatus and
methods used in the oil and gas industry. More specifically, the
invention relates to a method for enhancing a received signal
transmitted by acoustic telemetry through a drill string by
modifying the received signal by a multiplication of the received
signal with a second waveform.
Inventors: |
McRory; John G; (Calgary,
CA) |
Correspondence
Address: |
DAVIDSON BERQUIST JACKSON & GOWDEY, LLP;ATTN: BRIAN SIRITZKY, Ph.D.
4300 WILSON BLVD., 7TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
XACT DOWNHOLE TELEMETRY
INC.
Calgary
CA
|
Family ID: |
42212025 |
Appl. No.: |
12/618906 |
Filed: |
November 16, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61118501 |
Nov 28, 2008 |
|
|
|
Current U.S.
Class: |
367/82 |
Current CPC
Class: |
E21B 47/16 20130101 |
Class at
Publication: |
367/82 |
International
Class: |
E21B 47/16 20060101
E21B047/16 |
Claims
1. A method for enhancing a received signal transmitted by acoustic
telemetry through a drill string comprising the step of modifying
the received signal by a multiplication of the received signal with
a second waveform.
2. The method of claim 1 where the received acoustic signal is a
linear frequency chirp.
3. The method of claim 2, where the received acoustic signal is a
linear frequency upchirp and the second waveform is a linear
frequency downchirp.
4. The method of claim 2, where the received acoustic signal is a
linear frequency downchirp and the second waveform is a linear
frequency upchirp.
5. The method of claim 2 wherein an autocorrelation function of the
received signal is calculated and the autocorrelation function is
optimized to compensate for limited chirp bandwidth.
6. The method of claim 5 wherein the autocorrelation function is
optimized to remove dispersion effects.
7. A method of enhancing a received chirp signal within a receiver
wherein the received chirp signal has been transmitted by acoustic
telemetry through a drill string, comprising the step of: applying
a non-constant frequency local oscillator signal to the received
chirp signal to selectively shift component frequencies of the
received chirp signal by spreading the received chirp in the
frequency domain while maintaining baud rate in order to create a
processed signal having an increased time-bandwidth.
8. The method as in claim 7 wherein the non-constant frequency
local oscillator is a linear chirp waveform.
9. The method as in claim 7 wherein step a) includes adjusting an
autocorrelation waveform to a desired form during down-conversion
in the receiver.
10. The method as in claim 7 wherein the local oscillator signal is
a down chirp opposite in frequency span to the received chirp
signal.
11. The method as in claim 8 wherein the local oscillator signal is
a down chirp opposite in frequency span to the received chirp
signal.
12. The method as in claim 9 wherein the local oscillator signal is
a down chirp opposite in frequency span to the received chirp
signal.
13. The method as in claim 10 wherein the frequency span of the
down chirp is chosen to obtain a desired correlation waveform.
14. The method as in claim 7 wherein the processed signal is
subjected to multiple frequency sweeps and the time-bandwidth is
increased with each frequency sweep and wherein the frequency
sweeps are limited to twice the received chirp's frequency
sweep.
15. The method as in claim 8 wherein the processed signal is
subjected to multiple frequency sweeps and the time-bandwidth is
increased with each frequency sweep and wherein the frequency
sweeps are limited to twice the received chirp's frequency
sweep.
16. The method as in claim 9 wherein the processed signal is
subjected to multiple frequency sweeps and the time-bandwidth is
increased with each frequency sweep and wherein the frequency
sweeps are limited to twice the received chirp's frequency
sweep.
17. The method as in claim 13 wherein the processed signal is
subjected to multiple frequency sweeps and the time-bandwidth is
increased with each frequency sweep and wherein the frequency
sweeps are limited to twice the received chirp's frequency sweep.
Description
RELATED APPLICATIONS
[0001] This application is related to and claims priority from U.S.
Provisional Patent Application No. 61/118,501, filed Nov. 28, 2008,
the entire contents of which are incorporated herein by reference
for all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to telemetry apparatus and
methods, and more particularly to acoustic telemetry apparatus and
methods used in the oil and gas industry. More specifically, the
invention relates to a method for enhancing a received signal
transmitted by acoustic telemetry through a drill string by
modifying the received signal by a multiplication of the received
signal with a second waveform.
BACKGROUND OF THE INVENTION
[0003] As is known, acoustic telemetry is a method of communication
in the well drilling and production industry. In a typical drilling
environment, acoustic carrier waves from a downhole acoustic
telemetry device are modulated in order to carry information via
the drillpipe to the surface. Upon arrival at the surface, the
waves are detected, decoded and displayed in order that drillers,
geologists and others helping steer or control the well are
provided with drilling and formation data.
[0004] In a typical acoustic telemetry drilling or production
environment acoustic waves are produced and travel predominantly
along the metal wall of the tubing(s) associated with the downhole
section required to drill the well. The acoustic energy is usually
detected by sensitive accelerometers and sometimes by relatively
less sensitive strain gauges. Care needs to be taken about the
positioning and coupling of such devices to the tubing in order
that the maximum signal energy can be obtained in order to optimize
the detection system's signal to noise ratio (SNR).
[0005] The theory of acoustic telemetry as applied to communication
along drillstrings has a long history, and a comprehensive
theoretical understanding has been achieved and backed up by
accurate measurements. It is now generally recognized that the
nearly regular periodic structure of drillpipe sections and the
inter-section connections imposes a passband/stopband structure on
the frequency response, similar to that of a comb filter. A typical
passband/stopband frequency response for drill pipe sections is
shown in FIG. 1 where multiple ranges of frequencies cannot be
transmitted along the drill pipe sections.
[0006] Further still, other factors including significant surface
and downhole noise from drilling, dispersion, phase non-linearity
and frequency dependent attenuation as well as passband effects all
contribute to the challenge of effectively enabling acoustic
telemetry through the drillpipe.
[0007] Thus, as a result of the passband/stopband structure, the
acoustic signal transmitted up the drillpipe to the surface must be
limited in frequency bandwidth such that its frequencies fall
within one or simultaneously two or more of the frequency passbands
within the passband/stopband structure.
[0008] However, the main challenge in effective transmission up the
drillpipe is the frequency dependant destructive interference of
the signals within these passbands. This interference arises from
both the effect of the regular structure of the drill string
(Drumheller), and signal reflections local to the acoustic
transmitter. These reflections arise from the different acoustic
impedances offered to the signal by the various parts of the bottom
hole assembly. The combination of the fine structure of the
passband with the destructive interference of the transmitted
signal by local reflections can result in narrow notches within the
passband with attenuations of 10 dB or greater.
[0009] Several different digital modulation schemes have been
proposed for this transmission, including Binary Phase Shift Keying
(BPSK) and Phase Shift Keying (PSK) however one of the most
effective is the use of a sinusoidal linear frequency chirp falling
within the passband.
[0010] A linear chirp is a constant amplitude signal that begins at
one frequency and then over a period of time sweeps to another in a
linear fashion. From its starting point, the frequency can either
increase forming an "up-chirp", or decrease forming a "down-chirp".
The use of linear chirps in both radar and sonar applications is
well established. In these applications the chirp offers both an
improvement in signal to noise ratio and improved ranging
resolution through pulse compression resulting from the use of the
cross-correlation of the received signal with a reference chirp
waveform in the receiver. A detailed explanation of the
cross-correlation and its application in digital receivers can be
found in Digital Communications, third edition by John G. Proakis
(p. 65, pp. 235-238).
[0011] In the cases of sonar and radar applications, the
cross-correlation operation is used to detect a short waveform in a
longer data record by sliding the reference waveform through the
received data one point at a time and carrying out a correlation
calculation at each data point. The resulting output data record
contains the autocorrelation waveform of the transmitted chirp in
which most of the transmitted energy has been concentrated into a
main lobe that has a much higher peak value and shorter duration
than the transmitted chirp, leading to better range resolution and
an improved SNR.
[0012] These aspects of the chirp's auto-correlation also work to
advantage in downhole acoustic telemetry. The increase in the
signal to noise ratio improves the system performance, while the
narrower waveform peak aids in timing recovery and signal
detection. An additional advantage is that the chirp effectively
fills the available frequency band while sweeping through the
notches in the passband.
[0013] However, these aspects are also an inverse function of the
time-bandwidth product, or the product of the duration and the
frequency span of the chirp. If the duration of the chirp is
decreased by increasing the baud rate, or if the frequency span of
the chirp is reduced, then the main lobe of the auto-correlation
function widens, reducing the improvement in SNR and hence the
improvement in signal detection.
[0014] The bandpass filter properties of the downhole acoustic
channel limit the frequency span available for the chirp placing a
limit on the time bandwidth product. Also, the market need for
increasing baud rates requires a reduction in the chirp duration
for each increment in baud rate, placing a further constraint on
the time bandwidth product. This combination of constraints forms a
boundary on the available performance of the acoustic telemetry
system by limiting the shape of the auto-correlation waveform,
making it more difficult to demodulate and detect the telemetry
signal at higher data rates.
[0015] As a result, there has been a need for a system and method
in which the above problems are addressed. In particular, there has
been a need for a system and method in which detection of the
acoustic signal at the surface and system synchronization of the
acoustic signal is improved for improved timing recovery.
SUMMARY OF THE INVENTION
[0016] In accordance with the invention, a method that improves the
detection and synchronization of an acoustic signal and that
increases the operating baud rate by modifying the time-bandwidth
product of the transmitted linear acoustic chirp at the receiver is
provided.
[0017] More specifically, the invention provides a method for
enhancing a received signal transmitted by acoustic telemetry
through a drill string by modifying the received signal by a
multiplication of the received signal with a second waveform.
[0018] In various embodiments, the received acoustic signal is a
linear frequency chirp and/or the received acoustic signal is a
linear frequency upchirp and the second waveform is a linear
frequency downchirp.
[0019] In a more specific embodiment, the received acoustic signal
is a linear frequency downchirp and the second waveform is a linear
frequency upchirp.
[0020] In another embodiment, an autocorrelation function of the
received signal is calculated and the autocorrelation function is
optimized to compensate for limited chirp bandwidth. In another
embodiment, the autocorrelation function is optimized to remove
dispersion effects.
[0021] In another aspect, the invention provides a method of
enhancing a received chirp signal within a receiver wherein the
received chirp signal has been transmitted by acoustic telemetry
through a drill string, comprising the step of: a) applying a
non-constant frequency local oscillator signal to the received
chirp signal to selectively shift component frequencies of the
received chirp signal by spreading the received chirp in the
frequency domain while maintaining baud rate in order to create a
processed signal having an increased time-bandwidth.
[0022] In another embodiment, the non-constant frequency local
oscillator is a linear chirp waveform.
[0023] In yet another embodiment, step a) includes adjusting an
autocorrelation waveform to a desired form during down-conversion
in the receiver.
[0024] In another embodiment, the local oscillator signal is a down
chirp opposite in frequency span to the received chirp signal.
[0025] In a still further embodiment, the frequency span of the
down chirp is chosen to obtain a desired correlation waveform.
[0026] In yet another embodiment, the processed signal is subjected
to multiple frequency sweeps and the time-bandwidth is increased
with each frequency sweep and wherein the frequency sweeps are
limited to twice the received chirp's frequency sweep.
BRIEF DESCRIPTION OF THE FIGURES
[0027] The invention is described with reference to the
accompanying figures in which:
[0028] FIG. 1 is a graph showing frequency response (signal
strength vs. frequency) in a typical drill pipe section;
[0029] FIG. 2 shows a typical heterodyne receiver structure that
can be used to shift the frequency of a bandpass linear chirp
signal to base band through the use of a local oscillator (LO);
[0030] FIG. 3 is a graph showing a typical linear chirp
autocorrelation;
[0031] FIG. 4 is a graph showing autocorrelation as a function of
baud rate; and
[0032] FIG. 5 is a graph showing autocorrelation for a 20 baud rate
with different local oscillator.
DETAILED DESCRIPTION
Overview
[0033] With reference to the figures, systems and methods of
improved acoustic telemetry are described.
[0034] As noted above, the unique transmission characteristics of
the down-hole environment within the available acoustic channels of
a typical drill pipe string create an extremely difficult digital
communications environment given that the frequency channel is a
narrow, highly dispersive band-pass system with substantial levels
of echoes, reverberation and attenuation.
[0035] As noted above, one digital communication scheme that has
been used with success in this environment is a BPSK or PSK signal
modulated on a linear chirp that is centered on one of the
passbands of the channel. A working system using this approach is
described in detail by Camwell and Neff in "Field Test Results of
an Acoustic MWD System".
[0036] The receiver in this system employs a correlation
demodulator as described by Proakis pp: 234-238. In this case the
demodulator correlates the received data stream with a reference
wave form that has been derived from an ideal linear chirp. In the
simplest case for a binary phase shift keying a single reference
waveform is used, and the output of the correlation operation in
the presence of a received chirp is either a positive or negative
correlation peak. If a chirp is not present, then the output of the
correlation operation is simply the correlation between the
reference waveform and the in-band channel noise. Since the
reference waveform is derived from an ideal linear chirp then the
correlation between the reference and the received signal is
effectively the autocorrelation waveform of the chirp.
[0037] However, the transmission channel reduces the effectiveness
of the correlation receiver by limiting and distorting the
autocorrelation waveform. In accordance with the invention, a
method is described by which the autocorrelation function of the
received signal at surface may be modified to compensate for the
limited chirp bandwidth as well as to remove the effects of
dispersion, and thereby optimize the performance of the
receiver.
[0038] A common heterodyne receiver structure such as shown in FIG.
2 can be used to shift the frequency of the bandpass linear chirp
signal to base band through the use of a local oscillator (LO). The
LO waveform may comprise a simple sinusoid, a complex sinusoid or
perhaps a more complex signal. The receiver as shown in FIG. 2 may
be implemented in hardware, software or a combination thereof.
[0039] More specifically, a finite linear chirp modulating a
carrier frequency f.sub.o, of a duration T, and a chirp frequency
rate a can be represented by
f(t)=exp(j2.pi.(f.sub.0t+0.5.alpha.t.sup.2)), 0.ltoreq.t.ltoreq.T
(1)
[0040] The instantaneous frequency of the chirp is the first time
derivative of the phase in (1), 2.pi.(f.sub.o+.alpha.t). Thus, the
frequency bandwidth of the chirp is .alpha.T over the chirp
duration T, and that the time-bandwidth product of the chirp is
.alpha.T.sup.2.
[0041] The autocorrelation function for the linear chirp can be
written as
acf ( .tau. ) = .PHI. ( .tau. ) ( T - .tau. ) sin ( .pi.
.alpha..tau. ( T - .tau. ) ) ( .pi..alpha..tau. ( t - .tau. ) ) ( 2
) ##EQU00001##
[0042] Where in (2) .PHI. is the carrier harmonic that is modulated
by both a triangle pulse (T-|.tau.|), and a sinc function of time.
Examination of (2) shows that the autocorrelation function can be
completely characterized by the frequency chirp rate .alpha. and
the chirp duration T, and that the first root of the sinc function
occurs at approximately at .tau.=1/.alpha.T so the width of the
main lobe of the waveform is an inverse function of the
time-bandwidth product as determined by the baud rate and frequency
sweep.
[0043] In most linear chirp applications, the time/bandwidth
product of the chirp is chosen such that its autocorrelation
waveform displays a narrow peak with well controlled sidelobes. As
an example, the autocorrelation waveform for .alpha.T.sup.2=40 is
shown in FIG. 3. The well defined peak combined with the high peak
to average ratio make the chirp an ideal waveform for ranging
applications.
[0044] However, the downhole acoustic channel limits the frequency
span of the chirp to tens of Hz. In the case of the third passband
of the down-hole channel as shown in FIG. 1, the frequency passband
encompasses 550 to 720 Hz. Assuming a symbol rate of T=0.2, 0.1 or
0.05 sec for 5, 10 or 20 baud, with a chirp frequency span of 40
Hz, results in a time/bandwidth product of 1.6, 0.4 and 0.1
respectively.
[0045] FIG. 4 shows the evolution of the baseband autocorrelation
waveform for a linear chirp with a frequency span of 640 to 680 Hz
at different baud rates at a normalized sample rate (equal number
of samples per chirp). In this case a fixed frequency
downconversion and lowpass filtering is used to generate the
baseband chirp. The figure shows the increase in the width of the
main lobe of the autocorrelation as the time-frequency product
reduces with increasing baud rate until the limiting triangle is
reached. As the time/bandwidth product drops, the peak to average
ratio of the waveform is reduced, thereby also reducing the
receiver's immunity to noise in the timing recovery and symbol
detection. The difficulty arising in the downhole channel is that
since the frequency span is limited by the channel, the
autocorrelation waveform is determined by the baud rate. However as
the baud rate increases, the autocorrelation waveform will reduce
to a simple triangle pulse, thereby losing the advantages of the
linear chirp carrier.
[0046] In accordance with the invention, the autocorrelation
waveform is restored to its desired form is by using a non-constant
frequency local oscillator in the downconversion stage of the
receiver. More specifically, the linear chirp waveform is used for
the local oscillator in order to selectively shift the component
frequencies of the received chirp, thereby increasing the
time-bandwidth product by spreading the received chirp in the
frequency domain while maintaining the baud rate.
[0047] In order to achieve the frequency spreading operation, the
local oscillator chirp must be opposite in its frequency span to
received chirp. For example, for a received linear up-chirp of 640
to 680 Hz, the corresponding LO chirp must be a down-chirp with the
same time period as the received chirp. The frequency span of the
downchirp is chosen to obtain the desired correlation waveform.
[0048] As an example, FIG. 5 shows the effect of four different
local oscillator signals on a 20 baud received signal using a
640-680 Hz linear chirp. Examination of the figure shows that that
the time bandwidth product is increased with each increase in the
frequency sweep of the local oscillator down chirp, and that
diminishing returns limit the maximum LO sweep to twice the chirp's
frequency sweep.
[0049] Although the present invention has been described and
illustrated with respect to preferred embodiments and preferred
uses thereof, it is not to be so limited since modifications and
changes can be made therein which are within the full, intended
scope of the invention as understood by those skilled in the
art.
REFERENCES
[0050] Thomas G. Barnes and Bill R Kirkwood, "Passbands for
Acoustic Transmission in an Idealized Drill String", The Journal of
the Acoustical Society of America, Vol. 51, Number 5, 1972, pp.
1606-1608. [0051] Douglas S Drumheller, "Acoustical properties of
drill strings", The Journal of the Acoustical Society of America,
Vol. 85, Number 3, March 1989, pp. 1048-1064. [0052] John C.
Burgess, "Chirp design for acoustical system identification", The
Journal of the Acoustical Society of America, Vol. 91, Number 3,
March, 1992, pp. 1525-1530. [0053] Coert Olmsted, Alaska SAR
Facility, Scientific SAR User's Guide, July, 1993, pp. 5-6. [0054]
Douglas S Drumheller, "Attenuation of sound waves in drill
strings", The Journal of the Acoustical Society of America, Vol.
94, Number 4, October 1993, pp. 2387-2396. [0055] Douglas S.
Drumheller, "The propagation of sound waves in drill strings", The
Journal of the Acoustical Society of America, Vol. 97, Number 4,
October 1995, pp. 2116-2125. [0056] J. M. Neff and P. L. Camwell,
"Field-Test Results of an Acoustic MWD System", Proceedings of the
SPE/IADC Drilling Conference, February, 2007. [0057] John G.
Proakis, Digital Communications, Third Edition, McGraw-Hill Inc.
1995.
* * * * *