CA1206089A - Method and apparatus for signal recovery in a logging while drilling system - Google Patents
Method and apparatus for signal recovery in a logging while drilling systemInfo
- Publication number
- CA1206089A CA1206089A CA000427966A CA427966A CA1206089A CA 1206089 A CA1206089 A CA 1206089A CA 000427966 A CA000427966 A CA 000427966A CA 427966 A CA427966 A CA 427966A CA 1206089 A CA1206089 A CA 1206089A
- Authority
- CA
- Canada
- Prior art keywords
- signal
- pressure
- signals
- electrical
- downhole
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH DRILLING; MINING
- E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B47/00—Survey of boreholes or wells
- E21B47/12—Means for transmitting measuring-signals or control signals from the well to the surface, or from the surface to the well, e.g. for logging while drilling
- E21B47/14—Means for transmitting measuring-signals or control signals from the well to the surface, or from the surface to the well, e.g. for logging while drilling using acoustic waves
- E21B47/18—Means for transmitting measuring-signals or control signals from the well to the surface, or from the surface to the well, e.g. for logging while drilling using acoustic waves through the well fluid, e.g. mud pressure pulse telemetry
Abstract
METHOD AND APPARATUS FOR SIGNAL RECOVERY
IN A LOGGING WHILE DRILLING SYSTEM
Abstract of the Disclosure A method and an apparatus are provided for recovery of data in a well logging while drilling system wherein a data signal originating from a downhole apparatus is transmitted to the earth's surface through drilling mud.
The method includes measuring pressure pulsations in a mud flow line at the earth's surface in two spaced apart locations then combining the data signals to cancel downhole propagating pressure waves such as surface generated interference noise. The combined data signal is then processed by convolution filters, and then passed through a detector to determine the presence or absence of a valid data signal. In the apparatus these data signals are passed through the drilling mud by pressure fluctuations originating at a downhole apparatus. Two closely spaced pressure sensors measure mud pressure pulsations in a mud line at a surface location. One of the measured pressure pulsation signals is time shifted an amount equal to the travel time of the group propagation acoustic velocity in the mud between the two pressure sensors. One of the two signals has its polarity reversed and the two signals are combined to cancel any downward propagating energy from the combined pressure data signal. The combined pressure data signal is then processed by each of n matched filters in a baseband PCM system to determine the presence or absence of one of n symbols in the residual data signal. Each output of the filtered data signals is then input to a "Largest of"
Bayesian detector to identify either the presence of a symbol or absence of the data signal.
IN A LOGGING WHILE DRILLING SYSTEM
Abstract of the Disclosure A method and an apparatus are provided for recovery of data in a well logging while drilling system wherein a data signal originating from a downhole apparatus is transmitted to the earth's surface through drilling mud.
The method includes measuring pressure pulsations in a mud flow line at the earth's surface in two spaced apart locations then combining the data signals to cancel downhole propagating pressure waves such as surface generated interference noise. The combined data signal is then processed by convolution filters, and then passed through a detector to determine the presence or absence of a valid data signal. In the apparatus these data signals are passed through the drilling mud by pressure fluctuations originating at a downhole apparatus. Two closely spaced pressure sensors measure mud pressure pulsations in a mud line at a surface location. One of the measured pressure pulsation signals is time shifted an amount equal to the travel time of the group propagation acoustic velocity in the mud between the two pressure sensors. One of the two signals has its polarity reversed and the two signals are combined to cancel any downward propagating energy from the combined pressure data signal. The combined pressure data signal is then processed by each of n matched filters in a baseband PCM system to determine the presence or absence of one of n symbols in the residual data signal. Each output of the filtered data signals is then input to a "Largest of"
Bayesian detector to identify either the presence of a symbol or absence of the data signal.
Description
METHOD AND APPARATUS FOR SIGNAL REC~ERY
IN A LOGGING WHILE DRILLING SYSTEM
Technical Field _ This invention is related to measurement while drilling csystems fox earth boreholes. More specifically it is related to the telem~try of data signals in such systems S and the recep~ion, detection and processing of these signals at the earth's surface.
Background of the Invention The desirability and usefulness of a measurement while drilling system that will measure downhole well drilling parameters such as physical and geological characteri~tics and transmit the!m to the earth's surface while the well is being drilled has been re~ognized. In such measurement while drilling sy~te!ms one of the major problems i~ the telemetry of data ~rom the downhole sensors and associated transmit.ting appaxatus to a receiving system at the earth's surface. Several telemetxy and data transmission systems have be~n developed with each having its particular strong pointsO
This invention is directed to a telemetry system that transfers the information by mean~ of pressure pulsation~ of the drilling ~luid or mud that is normally associated with rotary drilling operationsO The pressure pulsations that are representative of the data signal of a particular parameter are generated by a downhole apparatus neax ~he drilling bit and the. pressure pulsati~ns pass upward through mud in the drill string to a signal detector at the earth's surface. In this sy~tem the pressure pulses
IN A LOGGING WHILE DRILLING SYSTEM
Technical Field _ This invention is related to measurement while drilling csystems fox earth boreholes. More specifically it is related to the telem~try of data signals in such systems S and the recep~ion, detection and processing of these signals at the earth's surface.
Background of the Invention The desirability and usefulness of a measurement while drilling system that will measure downhole well drilling parameters such as physical and geological characteri~tics and transmit the!m to the earth's surface while the well is being drilled has been re~ognized. In such measurement while drilling sy~te!ms one of the major problems i~ the telemetry of data ~rom the downhole sensors and associated transmit.ting appaxatus to a receiving system at the earth's surface. Several telemetxy and data transmission systems have be~n developed with each having its particular strong pointsO
This invention is directed to a telemetry system that transfers the information by mean~ of pressure pulsation~ of the drilling ~luid or mud that is normally associated with rotary drilling operationsO The pressure pulsations that are representative of the data signal of a particular parameter are generated by a downhole apparatus neax ~he drilling bit and the. pressure pulsati~ns pass upward through mud in the drill string to a signal detector at the earth's surface. In this sy~tem the pressure pulses
-2 are passed upward through the interior of the clrill string via the medium of the circulating drilling mud.
Pressure pulse transmission through the drilling mud in the interior of a drill stxing encounters certain difficulties due to extraneous vibrations, shocks and pressure pulses that are placed in this fluid system by operation of the drilling e~uipment. Some of these pressure pulses and noises are of a magnitude at least as great as the transmitted pressure pulse from the downhole transmitting equii~ment. Also these pressure pulses are reflected within the drill string at certain locations that present a significant impedance change in the pressure pulse wave guide provided by the drill string. These reflected pressure pulses are also ~assed through the drill string to further complicate the signal identification. All of this pressure pulsation and pressure ~ulsation noise generation and reflection within the drill string crea~es a noise environ-ment in which the data signal is traveling and from which it must be recovered.
In prior signal recovering devices they rely on mechanical receiving devices in the drilling fluid flow path and other associated mechanical devices in the signal recovery system to extract the data siynals. These mechani-cal devices ~resent some problem~s in signal recovery due to ~heir basic nature. One problem is that resonances of the transmitted pressure pulse wave~ can be a source of inter~erence within the mechanical devices that are extract-ing and processing the data. Anotner difficulty with thase mechanical devices is their inabilit~ to separate upwardly and downwardly traveling waves within the drilling fluid medium. A result of this dif~iculty in separating signal~
i.s that reflected pressure waves can be mistaken for real data and additional data processing is needed to extract valid data from the available data.
Summary of the Invention The pre~ent invention provides a method and apparatus for receiving and processing telemetry data that passes through the drill string of a borehole drilling rig ~rom a measurement while drilling apparatus in the d~ill string. The receiving apparatus is designed to eliminate or substantially reduce the contam:inating noise that is supex-posed on the pressure measurement signal as it is passed through the mud stream in the drilling string. The signal receiving apparatus includes a ~air of relatively closely spaced detectors in a mud flow conduit between the mud circulating pump and the drill string. Additionally the receiving apparatus is constructed to extract the intelli-gence data from the signal by time shifting the signal from one of the detectors relative to the other then c4mbining the signals tnereby canceling energy propagating in one direction while passing energy propagation in the other direction. This novel technique passes the enexgy, but in a form that is not in its original form, but a time diEference form of the transmitted energy. Further processing of apparatus extracts the intelligence carrying data without reconstructing the oxiginally transmitted signal.
One object of this invention is to provide a method ana apparatus of signal reco~ery for data in a logging while drilling system overcoming the a.orementioned disadvantages of the prior art devices.
Still, one other object of this invention is to provide a signal recover~I apparatus for use with a logging while drilling system that uses drilling mud pressure pulses as the data transmission medium and ha a pair of relatively closely spaced pressure sensors in the mud flow conduit to recover the signal and pass it to processing equipment for removal of interferring no.is~ from the signal and ex~rac~ion of intelligence carrying data~
Still, another object o~ this invention is to pxovide signal prvcessing equipment for this type of measuxement while drilling apparatus that will extxact the intelligence carrying data from the mud pressure pulse signal without resorting to utilizing pressur~ sensors or signal detector~ that are spaced a very long distance apart on a uniform diameter flow conduit.
Yet, another obj~ct of this invention is to provide a signal processing apparatus f~1 this type of measurement ~hile drilling that does not requi.re regeneration of the t~ansmitted data signal in order to es~tract the intelligence carrying data there~rom; that will also process the data to reduce the influence o-f noise thereon; and tha~ will identify the actual data from the noise signals by means of actively filtering the signal.
Therefore, in accordance with a firct aspect of the present inventlon, there is provided a method of substantially reducing uphole noise from a downhole signal in a logging-while-drilling system where the downhole signal is in the form of a pulse modulated pressure waves being transmitted ln the drilling flui.d of the system. The method comprises ~a) measuring the fluid pressure in the drillin~ fluid at a first point and at a second point respectively, and converting both pressure measurements to corresponding electrical signals indicative of the measured pressures, the first and second points being spaced from each other along the drilling fluid flow path between a source of drilling flui~ and a downhole portion of a well in which the downhole signal is originating;
(b~ time shi.fting one of the electrical pressure measurement signals by an amount corresponding with the propogation time o~ the pulse modulated pressure wave ~ithin the drilling fluid flow path from one of the points to the other; (c) combining the time shifted to the electrical pressure measurement signal with the other electrical pressure measurement signal to produce a composite electrical measurement signal in order to substantially remove the downward propagatin~ energy therefrom;
and (d) filtering the composite electrical pressure measurement cw/! 4 ,, ~
:'` ' signal in a matched filter to identify actual data in the electrical signa]s from noise present in the composite electrical measurement signal to produce a filtered e].ectrical pressure measurement signal and fur-ther processing the filtered electrical measurement signal by detecting within the signal the presence of data that is within a predetermined range of being within a set of permissible values to produce a recovered data signal that has one value if no signal is detected, a first set of values if the filtered electrical measurement signal exceeds a threshold ~alue, a second set of values if the filtered electrical measurement signal exceeds another threshold value.
In a further aspect of the present invention, there is provided in a logging-while drilling system wherein a downhole signal :representative of a measured downhole parameter is transmitted to the earth surface in the form of a pressure pulse in the dril].ing fluid of the syst:em, there is provided in accordance with the present invention an apparatus for substantially reducing the influence of pressure pulsation interferriny noise on the downhole signal. The apparatus comprises (a) conduit means for conducting drilling fluid from a source of dr.illing flu.id at the earth surface to the well in which the downhole signal is originating; (b) a first transducer means at a first point on the conduit means for measuring the fluid pressure in the conduit means at the first point and for converting the pressure into a corresponding first electrical pressure m~asurement signal; (c) a second transducer means at a second point on the conduit means, spaced from the first point for measuring the fluid pressure in the cw/,~ 4a -6~
condult means at the second point and for convertiny the pressure into a corresponding second electrical pressure signal measurement; (d) means for time shifting the second electrical pressure measurement signal by an amount corresponding with the pressure wave propogation travel time in the drilling fluid from one of the transducer means to the other transducer means; (e) means for generating a composite electrical measurement signal representative of the difference between the first and the second electrical pressure measurement signals; (f) means for filtering the composite electrical pressure measurement signal to identify valid pressure measurement signals in the composite signals to derive a filtered signal; and (g) means for processing the filtered signal including determining the presence of a signal that is between prede-termined limits and to differentiate these signals as being valid whereupon a transmission of the valid signal is made or a transmission of no signal is made, and for adaptively determming detector synchronization with the transmitted data.
Various objects, advantages, and features of this invention will become apparent to tho~e skilled in the art from the following discussion, taken in conjunction with the accompanying drawings, in which:
Brief Description of the Drawln~s Fig. 1 is a schematic and pictorial representation of a well drilling rig having a measurement while drilling system .incorporating this invention wherein the system uses pulsations of the mud column through the drill string as the medium for transmission of the telemetry and data s:ignal;
cw/~ - 4b -,~',s', ~6~
Fig. 2 is a schematlc block diagram of a portion of the data receiving portion of thls system illustrating the cooperative relationship of the elements of the present invention;
Fig. 3 is a graph illustrating the output signals of the. received signal processor illustrated in Fig~ 2.
The following is a discussion and description of preferred specific embodiments of the method and apparatus for signal recovery of this invention, such being made with reEerence to the drawings whereupon the same reference numerals are used to indicate the same or similar parts and/or structure.
It is to be understood that such discussion and description is not to ~nduly limit the scope of the invention.
Detailed Description of the Preferred Embodiments The apparatus and method of this invention can be used with a borehole measurement while drilling system as incorporated in a drilling rig such as, that illustrated in Fig. 1. As shown, the measurement whi.le drilling system is used with a conventional rotary type well drilling rlg wherein a drill striny 10 comprised or a plurality of segments of dri].ling pipe with a drilling bit 12 at the bot-tom end thereoE is rotated to drill a borehole 14 through cw/~ 4c . .~
-5~
the earth formations 16. The measurement while drilling a~paratus includes downhole equipment including a sensor package 18 in the lower portion of drill string 10. Sensor package 18 can contain a plurality of devices adapted to measure geophysical conditions within the borehole and the surrounding formations. For example, sensor package 18 can contain an orientation device to sense the direction and inclination of the borehole at that location or it can contain devlces to measure temperature, pressure, weight as applied to the bit or any of a variety of other parameters that may be desired.
Information or data that is generated by any of the element or elements in sensor package 18 is communicated within th~ downhole equipment to a transmitter 20 in the lower portion of the drill string. Transmitter 20 is adapted to encode this data into pressure pulsations of the drilling fluid or mud that is contained with drill string 10. These pressure pulsations can be either positive pressuxe pulsations or negative pressure pulsation~ of the mud in the drill string. Positive pressure pulsations are preferred for use with this invention however it will function with baseband pulse code modulation and, with some modifications to the detection filtexing exponen~ial modulation. Pressure pulsations introduced into the drill ~5 string mud column tr~vel upward from transmitter 20 toward the earth' 5 surface. As these pressure pulsations travel upward through the drill string in the interior of the drill string it functions as a wave guide to contain and direct the pressure pulsations. Reflections of these pressure pulsations occur within the dril] string at locations in the drill string's interior and which represent a significant impedance change in the wave guide formed by the drill string~ Major influences on these reflections are the junctures in the fluid path at the swivel connection ~2, the gooseneck 24, and its connection with the standpipe 26 as well as other fluid couplings and the like in the mud flow conduit between mud pump 28 and swivel 22. Pressure pulsations traveling u~ward through the mud flow stream in the drill strin~i a~e detected at a pair of pxessure sensors 30 and 32 that are mounted at a selected location within the condui.t 34 from mud pump 28 to gooseneck 24.
Pressure sensors 30 and 32 are operably connected with conduit 34 to directly sense the fluid pressure therein or depending upon the character of the specific pressure sensors utilized they ~rovide a signal indicative of the fluid within conduit 34. It is to be noted that pressure sensors 30 and 32 can be of a mechanical design that is fluidically connected with the interior o conduit 34 to provide access to the mud in order to provide the pressure signal data necessary for extracting the intelligence from the encod~d pressure pulsations in the mud flow stream~
Tl~ese pressur~ sensors or pressure transducers can be of any other configuration internal to the conduit or external that provides an electrical outpuk signal representative of the fluid pressure in the mud filled conduit.
Placement of pressure sensors 30 and 32 i.s selected with them being relatively closely spaced a.nd along a segmen~ of flow line conduit 34 that is substantially without internal obstructions and is of a su~stantiall~
uniform cross sectional axea so that fluid flow is undisturbed between the segment of a conduit. Spacing between pressure sensors 30 and 32 can be between about tllrae (3) feet (about 0.9 meters) and about one hundred (100~
feet (about 30.5 meters). It has been found that a preferred spacing of pressure sensors 30 and 32 is between about five (5j feet (about 1.5 met~rs~ and about forty (40) feet (about 12~4 meter~ ressure sensors 30 and 32 are electrically connected to a receiver 36. ~eceiver 36 functions to receive data from pressure sensors 30 and 32 then process this received signal data in order to transform it into data usable in other portions of this system.
A data pxocessor and display apparatus 40 is operably connected to the output of receiver 36. The data processor and display appaxatus 40 is adapted to process the received and processed data thereby extracting the intelligence informati.on carried therein and display thi~
information and provide or its storage if desirad.
Figure;2 c;hows in a schematic representation the _7~ ~2~
app~r~tus for recovering the mud pressure pulse data from the mud flow stream and the associated apparatus for processing the received signals. Pressure transducers 30 and 32 are spac2d apar-t a distance d on conduit 34 to ex-tract the pressure pulse data from the mud stream. Considering the conduit segment 34 as shown, the mud flow is from left to right or in the opposite relation from pump 28 -to the well as shown in Fig. l. The intelligence carrying transmitted signal ST is moving from the right to the left whil~ the pump noise signal Sp is shown moving in the opposite direc-tion on the left side of the figure moving toward the right.
The output from transducer 30 passes through a time delay and polarity reversal circuit 42 and then to a summing circuit 44. The output from the other pressure transducer 32 is connected directly to summing circuit 44. Summing circuit 44 combines its two inputs to produce a single composite output signal representative of the combined receiv~d signals~ This composite signal is a signal representative of the time difference residual of the combined rece,ived signals. This composite signal is then passed to the processor portion of the apparatus, indicated generally at 40. Initially the co:mposite signal enters a set of n detection filters that function to maximize the abi].ity to detect a symbol (wavelet in PCM) from all other symbols and to do so in the presence of accompanying noise.
~ter this, the resultant or filtered signals are passed to a threshold detector where they are examined for the presence of an actual signal ~symbol ) that is sought, then the resultant output is provided as binary coded bit stream.
This output i.s representative of the data produced by the downhole measurement wllile drilling equipment and this data is appro~riately coded to be indicative of ~eophysical parameters and other data sensed by the downhole apparatus, Returning to the schematic of Figure 2 for discussion in further detail. The conduit 34 is for purposes of this discussion a segment of pipe, tubing or other flow conduit in *he mud line or flow path between the mud circulating pump 28 and swivel 22. Prefe.rably conduit 34 is a segment of unlform internal diame-ter material -that ~2~
is subs-tantially rigi.d and mounted ~ith the drilling ri~
structure in a secure position so that flow conditions of the mud at and be-tween the two pressure sensors wil]. no-t be substantially di-Eferent. It is desirable that flow conditions at and between the pressure sensors be substantially the same so that the monitor~d.pressure signal at one of the pressure sens~rs will be substantially the same as that monitored at the other pressure sensor and any modification of the flow conditions between the pressure sensors can be neglected.
Pressure sensors 30 and 32 are spaced from each other along conduit 34 at a distance d. Pressure sensors 30 and 32 can be ally commercially available type of pressure transducer that will with.in a predetermined accuracy measure pressure or the presence of the pressure wave in the mud at the appropriate point in the conduit and convert this to a corresponding electrical signal that can be expressed as a function with a reference to time. Also the pressure sensors could be any type which wi].l provide an electrical signal corresponding to and representative of fluid pressure or the pulse pressure wave in the mud at that location in conduit 34. The out.puts from pressure sensors 30 and 32 are pre~erably ma-tched or adjusted so the relative output signal magnitudes of the two pressure transducers are t~e same for similar sta~ic and dynamic measurements. Apparatus and circuitry for this matcllin~ is not illustrated in Figure 2 because it is not essential to understaning of this invention but basically a technical adaptation necessary to implement the invention.
6~
Referri,ng to Fig. 2, pressure sensors 30 and 32 function to measure the fluid pressure in conduit 34 as it is ef:Eected by the pressure pulses carried in the mud.
Using pressure sensor 32 as a reference point tlle signal from it is designated as yl(t). At any point in time,-t, the signal at pressure sensor 32 is equal to the pressure of the transmitted signal, the pump generated signal, their multiple reflections, and a noise pressure signal. The noise pressure signal is a background noise factor including miscellaneour, press.ure fluctuations that appear as uncorrelated fluctuations at the output of the pressure sensors. The presence of a reflected pump signal is recognized however it has been determined that this signal is o~ a substantially insigni~icant value when considered in view of the relative magnitudes and effects of the other p~essure signals involved. In the following it is assumed that two (2), reflectors are present in the system. One reflector being downstream of the pressure sensors and the other being upstream of the pressure sensors. These reflections occur as the pressure waves pass from one impedance zone of the flow path to anothex. These impedance changes occur at obstructions in the waveguide formed by drill string 10 and the conduits connecting it to mud circulating pump 28. Also these impedance changes occur at junctions of the flexible conduit of gooseneck 24 with swivel 22 and standpipe 26. In the following a change in impedance is referred to as change from one impedance medium to another. The relationship of the pxessure at any pressure sensor placed along the conduit may be expressed as:
Yn~t) = ST~t) -~ Sp(t) -~ Nn(t) + rO ~1 rO ~lS(t+T_l) 0,+1 5p(t+T~ -- where Yn(t) is the extracted pressure pulse signal frsm the mud flow line conduit 34, ST(t) is the attenuated and dispersed transmitted signal coming from the downhole transmitter, S ~t~ is the pump generated signal, Nn(t) is the noise ob~erved by the nth pressure transducer, ando r0 +1 is the first reflection coefficient down-stream of the ~ransducers r0 1 is the first reflection coefficient upstreamof the tran~ducers The signal Y1(t) at pressure sensor 32 will differ from that of the signal Y2(t) at pressure transducar 30 with respect to time because of ~he distance d saparating the two pressllre sensors and the associated delay in pressure wave propaga~ioIl time. The actual time differential involved and depends upon the propogation time between the separate - pressur~ sensors which is a function of the velocity of the 2~
pressure wave within conduit 34.
The signal Yl(t) at pressure transducer 32 can be expressed as:
Y (t) = S (t) ~ S (t) -~ r . S (t~2T 1) + rO -1 ST(t + 2(t +T~
rO 1 ^ rO +1 Sp(t+2(t'~T l+T~
~ r ~ rO ~1 ST(t+2( + ... + Nl(t) And the signal Y2(t) at pressure transducer 30 can be expressed as:
Y (t) = S (t ~ t') ~ ST(t - t ) + rO,~l p +1) r0~-lsT(t+t +2~t' + T 1)) + rO,-1 rO ~1 o Sp(t+t' + 2(t'+T 1 +
~1)) r0,-lro,~lST~t~~' + 2(t'+T 1 +
+1)) + ~ N2(t) where~
r. . i5 the reflection coefficient of wave moving 1~]
from medium i of impedence Zi into another medium j of impedence Zj.
t' is the propogation time ~etween transducers 30 and 32, the locations of observing Y2(t) and Yl(t) res]?ectively.
T 1 is the propogation time from transducer 30, (Y2~t)1, to t]he boundary between mediums -1 an~ 0.
T~l is the propogatio:n time ~rom transducer 32, (Yl(t)), the boundary between medium~
0 and ~1.
In order to timewise align the signals coming from pressure sensors 3~ and 32 it is necessary to account for the time shift taking place as a pressure wave moves betwen the pressure sensors. The propogation time between the pressure sensors t' can be used to shift the signal coming from either pressure sensor in order that one reference point for well to 3S surface traveling pressure signals can he established. In this system Y2~t) is delayed by time t' throu~h time delay element 42 as shown in Pig. ~.
The sign change is necessary so that noise canceling will tak~ place when the signals are combined by summing circuit 44. Y2(t) as it is del.ayed by -time t' can be expressed as follows:
Y2(t-t') = Sp(t) ~- ST(t-2t ) -~ rO ~lSp(t-2t -~2T~l) + rO -1 S~(t~-2(t' -~ T 1)) -~ rO -1 rO+l Sp (t--2(t'+T l-T+l)) -~ rO 1 rO ~1 Sl,(t-2t +2(t -~T_l+T~
-~ ...+ N2 (t-t') Summin~ circuit 44 combines the two signals Yl(t) and Y2(t--t') into a composite signal Zt~t). This composite signal Z~(t) is representative of the combined received signal of the mud pressure sensors representative of positive or upwardly traveling energy. This composite signal is the resultant output o~ receiver 36 and is ready for operations of the data processor apparatus 40. This composite signal z~(~) may be expressed as:
z~(t) Yl(t) - Y2(t-t') = ST(~) ~ ST(t-2t') rO ~l[Sp(t -2T~l) - Sp(t-2t'+2T~l)]
(~,-1 o,~l[ST(t~-2(t'~T l~T~
S (t-2t'+2(k'-T ~T ))]
-~ ...+ Nl(t) - N2 (t-t') From this it can be observed that Z~t) is comprised of the time difference o:E the primary passage of the transmitted signal; and the re:Elected time difference of ~he transmitted signal, the pump s:ignal, and the noise. By considering the reflection coef.icients to be so small as to render the reflection coe~ficient terms neglectable then the expression of Z~(t) can be simplied to:
Z~(tl = ST(t) - ST(t 2t') ~ N1~t) - N2(t-tl) The expression can be further simplified by generalizing the noise component in -terms of Gaussian noise representation. If Nl(t) and N2(t) are assumed to have a Gaussian distribution wherein they are ~lcorrelated and have an essentially equal variance. With t.his assumption the noise term would be represented as N*(t) with that term being a normalized Gaussian noise fac-tor. Therefore, the composite signal can be represented as:
Z~(t) = ST~t) - ST(t-2t'~ - N*(t) The composite signal Z+(t) that has been generated is next fed into separate matched filters 46 and 48. The matched filters are sometimes referred to as correlation filters wherein a sample signal is convolved with a desired signal ~time-reversed~ that is to be found in the sample signal. Each of the matched filtPrs 46 and 48 function similarly with the differences being in the input of the known functions A(t) and B(t) respectively for each of n = 2 symbols as shown hPre. Functions A(t)and B(t) are chosen to have a magnitude such that output signals, gO(t) and gl(t), from the matched filters are normalized. In matched filter 46 composite signal Z~(t) is convolved with signal A(t).
The signal gl(t) resulting from matched filter 46 is then pro~ided as one input to a threshold and detector circuit 50.
The other matched filter 48 receives as one input the composite signal Z+(t) and convolves it with th~ known signal B(t). The output signal gO(t) of matched filter 48 i5 then supplied as another input to detector circuit 50.
Detector cixcuit 50 performs several functions on the signals it receives. The separate input signals gO(t) and gl(t) are introduced into detector circuit 50 for evaluation in comparison. Each of the signals arriving at detector circuit 50 are compared to a threshold value L to determine as a function of time whether or not a desired signal is present in that given received signal gO(t) or gl(t). Consldering the input signal gl(t) if it is greater than L then it is probable that the desired symbol is present in this input 5ignal. However if gl(t) is less than L it is probable that the desired signal is not present. If neither one of g~(t) or gl(t) are greater than L, then it is proba'~le that then neither o tne sougiht after symbols are present in the receiv~d signal. In the event that both gl(t) and gO(t~ are greater than Ll or L2 respectively then it is assumed that the larger of these two is most probably the sought after signal. Because the r~presentation A(t) and B(t) are normalied functions then the mag~itudes of gO(t) and gl(t) provide meaningful criteria upon which to detect the larger of the responses~ thereby indicating the most probably signal.
When a symbol is detected/ the maximum o~ the -14~
signal represents the most likely syncllronization point of the detection process relative to the incoming data symbols in gO~t) and gl(t~. Detector circuit 50 includes circuitry by which it adaptably tracks the incoming data signals to maintain syncllronization. This adapti~e tracking of the incoming signals utilizes the time location of the ~elected peak in the incoming data signa~ as ~ell as its magnitude to synchronize the next timewise location for observing the next expected symbol. In this tracking process, the signal strength or amplitude is weighted in a factor having an effect on detecting the succeeding 5ymbol.
The output from detector circuit 50 is identified as X~t) and illustrated graphically in Fig. 3. The output of detector circuit 50 is supplied to data processor and display 40 shown in Fig. 1 for further manipulation and for presentation in data representative for the downhole measurements taken in the earth borehole. In other words the filtered and processed data displayed in Fig. 3 is representative of the intelligence carrying information originally derived by the measuring equipment and formatted for use by the transmitter of th~e downhole measurement while drilling equipment. This data can be decoded to extract its intelligence carrying information by the data processor and in turn provide a human intelligible output from this system.
In practiciny this invention, several important features are to be noted including the noise cancellation in processing that extracts the intelligence carrying portion of the downhole created signal without the necessity for
Pressure pulse transmission through the drilling mud in the interior of a drill stxing encounters certain difficulties due to extraneous vibrations, shocks and pressure pulses that are placed in this fluid system by operation of the drilling e~uipment. Some of these pressure pulses and noises are of a magnitude at least as great as the transmitted pressure pulse from the downhole transmitting equii~ment. Also these pressure pulses are reflected within the drill string at certain locations that present a significant impedance change in the pressure pulse wave guide provided by the drill string. These reflected pressure pulses are also ~assed through the drill string to further complicate the signal identification. All of this pressure pulsation and pressure ~ulsation noise generation and reflection within the drill string crea~es a noise environ-ment in which the data signal is traveling and from which it must be recovered.
In prior signal recovering devices they rely on mechanical receiving devices in the drilling fluid flow path and other associated mechanical devices in the signal recovery system to extract the data siynals. These mechani-cal devices ~resent some problem~s in signal recovery due to ~heir basic nature. One problem is that resonances of the transmitted pressure pulse wave~ can be a source of inter~erence within the mechanical devices that are extract-ing and processing the data. Anotner difficulty with thase mechanical devices is their inabilit~ to separate upwardly and downwardly traveling waves within the drilling fluid medium. A result of this dif~iculty in separating signal~
i.s that reflected pressure waves can be mistaken for real data and additional data processing is needed to extract valid data from the available data.
Summary of the Invention The pre~ent invention provides a method and apparatus for receiving and processing telemetry data that passes through the drill string of a borehole drilling rig ~rom a measurement while drilling apparatus in the d~ill string. The receiving apparatus is designed to eliminate or substantially reduce the contam:inating noise that is supex-posed on the pressure measurement signal as it is passed through the mud stream in the drilling string. The signal receiving apparatus includes a ~air of relatively closely spaced detectors in a mud flow conduit between the mud circulating pump and the drill string. Additionally the receiving apparatus is constructed to extract the intelli-gence data from the signal by time shifting the signal from one of the detectors relative to the other then c4mbining the signals tnereby canceling energy propagating in one direction while passing energy propagation in the other direction. This novel technique passes the enexgy, but in a form that is not in its original form, but a time diEference form of the transmitted energy. Further processing of apparatus extracts the intelligence carrying data without reconstructing the oxiginally transmitted signal.
One object of this invention is to provide a method ana apparatus of signal reco~ery for data in a logging while drilling system overcoming the a.orementioned disadvantages of the prior art devices.
Still, one other object of this invention is to provide a signal recover~I apparatus for use with a logging while drilling system that uses drilling mud pressure pulses as the data transmission medium and ha a pair of relatively closely spaced pressure sensors in the mud flow conduit to recover the signal and pass it to processing equipment for removal of interferring no.is~ from the signal and ex~rac~ion of intelligence carrying data~
Still, another object o~ this invention is to pxovide signal prvcessing equipment for this type of measuxement while drilling apparatus that will extxact the intelligence carrying data from the mud pressure pulse signal without resorting to utilizing pressur~ sensors or signal detector~ that are spaced a very long distance apart on a uniform diameter flow conduit.
Yet, another obj~ct of this invention is to provide a signal processing apparatus f~1 this type of measurement ~hile drilling that does not requi.re regeneration of the t~ansmitted data signal in order to es~tract the intelligence carrying data there~rom; that will also process the data to reduce the influence o-f noise thereon; and tha~ will identify the actual data from the noise signals by means of actively filtering the signal.
Therefore, in accordance with a firct aspect of the present inventlon, there is provided a method of substantially reducing uphole noise from a downhole signal in a logging-while-drilling system where the downhole signal is in the form of a pulse modulated pressure waves being transmitted ln the drilling flui.d of the system. The method comprises ~a) measuring the fluid pressure in the drillin~ fluid at a first point and at a second point respectively, and converting both pressure measurements to corresponding electrical signals indicative of the measured pressures, the first and second points being spaced from each other along the drilling fluid flow path between a source of drilling flui~ and a downhole portion of a well in which the downhole signal is originating;
(b~ time shi.fting one of the electrical pressure measurement signals by an amount corresponding with the propogation time o~ the pulse modulated pressure wave ~ithin the drilling fluid flow path from one of the points to the other; (c) combining the time shifted to the electrical pressure measurement signal with the other electrical pressure measurement signal to produce a composite electrical measurement signal in order to substantially remove the downward propagatin~ energy therefrom;
and (d) filtering the composite electrical pressure measurement cw/! 4 ,, ~
:'` ' signal in a matched filter to identify actual data in the electrical signa]s from noise present in the composite electrical measurement signal to produce a filtered e].ectrical pressure measurement signal and fur-ther processing the filtered electrical measurement signal by detecting within the signal the presence of data that is within a predetermined range of being within a set of permissible values to produce a recovered data signal that has one value if no signal is detected, a first set of values if the filtered electrical measurement signal exceeds a threshold ~alue, a second set of values if the filtered electrical measurement signal exceeds another threshold value.
In a further aspect of the present invention, there is provided in a logging-while drilling system wherein a downhole signal :representative of a measured downhole parameter is transmitted to the earth surface in the form of a pressure pulse in the dril].ing fluid of the syst:em, there is provided in accordance with the present invention an apparatus for substantially reducing the influence of pressure pulsation interferriny noise on the downhole signal. The apparatus comprises (a) conduit means for conducting drilling fluid from a source of dr.illing flu.id at the earth surface to the well in which the downhole signal is originating; (b) a first transducer means at a first point on the conduit means for measuring the fluid pressure in the conduit means at the first point and for converting the pressure into a corresponding first electrical pressure m~asurement signal; (c) a second transducer means at a second point on the conduit means, spaced from the first point for measuring the fluid pressure in the cw/,~ 4a -6~
condult means at the second point and for convertiny the pressure into a corresponding second electrical pressure signal measurement; (d) means for time shifting the second electrical pressure measurement signal by an amount corresponding with the pressure wave propogation travel time in the drilling fluid from one of the transducer means to the other transducer means; (e) means for generating a composite electrical measurement signal representative of the difference between the first and the second electrical pressure measurement signals; (f) means for filtering the composite electrical pressure measurement signal to identify valid pressure measurement signals in the composite signals to derive a filtered signal; and (g) means for processing the filtered signal including determining the presence of a signal that is between prede-termined limits and to differentiate these signals as being valid whereupon a transmission of the valid signal is made or a transmission of no signal is made, and for adaptively determming detector synchronization with the transmitted data.
Various objects, advantages, and features of this invention will become apparent to tho~e skilled in the art from the following discussion, taken in conjunction with the accompanying drawings, in which:
Brief Description of the Drawln~s Fig. 1 is a schematic and pictorial representation of a well drilling rig having a measurement while drilling system .incorporating this invention wherein the system uses pulsations of the mud column through the drill string as the medium for transmission of the telemetry and data s:ignal;
cw/~ - 4b -,~',s', ~6~
Fig. 2 is a schematlc block diagram of a portion of the data receiving portion of thls system illustrating the cooperative relationship of the elements of the present invention;
Fig. 3 is a graph illustrating the output signals of the. received signal processor illustrated in Fig~ 2.
The following is a discussion and description of preferred specific embodiments of the method and apparatus for signal recovery of this invention, such being made with reEerence to the drawings whereupon the same reference numerals are used to indicate the same or similar parts and/or structure.
It is to be understood that such discussion and description is not to ~nduly limit the scope of the invention.
Detailed Description of the Preferred Embodiments The apparatus and method of this invention can be used with a borehole measurement while drilling system as incorporated in a drilling rig such as, that illustrated in Fig. 1. As shown, the measurement whi.le drilling system is used with a conventional rotary type well drilling rlg wherein a drill striny 10 comprised or a plurality of segments of dri].ling pipe with a drilling bit 12 at the bot-tom end thereoE is rotated to drill a borehole 14 through cw/~ 4c . .~
-5~
the earth formations 16. The measurement while drilling a~paratus includes downhole equipment including a sensor package 18 in the lower portion of drill string 10. Sensor package 18 can contain a plurality of devices adapted to measure geophysical conditions within the borehole and the surrounding formations. For example, sensor package 18 can contain an orientation device to sense the direction and inclination of the borehole at that location or it can contain devlces to measure temperature, pressure, weight as applied to the bit or any of a variety of other parameters that may be desired.
Information or data that is generated by any of the element or elements in sensor package 18 is communicated within th~ downhole equipment to a transmitter 20 in the lower portion of the drill string. Transmitter 20 is adapted to encode this data into pressure pulsations of the drilling fluid or mud that is contained with drill string 10. These pressure pulsations can be either positive pressuxe pulsations or negative pressure pulsation~ of the mud in the drill string. Positive pressure pulsations are preferred for use with this invention however it will function with baseband pulse code modulation and, with some modifications to the detection filtexing exponen~ial modulation. Pressure pulsations introduced into the drill ~5 string mud column tr~vel upward from transmitter 20 toward the earth' 5 surface. As these pressure pulsations travel upward through the drill string in the interior of the drill string it functions as a wave guide to contain and direct the pressure pulsations. Reflections of these pressure pulsations occur within the dril] string at locations in the drill string's interior and which represent a significant impedance change in the wave guide formed by the drill string~ Major influences on these reflections are the junctures in the fluid path at the swivel connection ~2, the gooseneck 24, and its connection with the standpipe 26 as well as other fluid couplings and the like in the mud flow conduit between mud pump 28 and swivel 22. Pressure pulsations traveling u~ward through the mud flow stream in the drill strin~i a~e detected at a pair of pxessure sensors 30 and 32 that are mounted at a selected location within the condui.t 34 from mud pump 28 to gooseneck 24.
Pressure sensors 30 and 32 are operably connected with conduit 34 to directly sense the fluid pressure therein or depending upon the character of the specific pressure sensors utilized they ~rovide a signal indicative of the fluid within conduit 34. It is to be noted that pressure sensors 30 and 32 can be of a mechanical design that is fluidically connected with the interior o conduit 34 to provide access to the mud in order to provide the pressure signal data necessary for extracting the intelligence from the encod~d pressure pulsations in the mud flow stream~
Tl~ese pressur~ sensors or pressure transducers can be of any other configuration internal to the conduit or external that provides an electrical outpuk signal representative of the fluid pressure in the mud filled conduit.
Placement of pressure sensors 30 and 32 i.s selected with them being relatively closely spaced a.nd along a segmen~ of flow line conduit 34 that is substantially without internal obstructions and is of a su~stantiall~
uniform cross sectional axea so that fluid flow is undisturbed between the segment of a conduit. Spacing between pressure sensors 30 and 32 can be between about tllrae (3) feet (about 0.9 meters) and about one hundred (100~
feet (about 30.5 meters). It has been found that a preferred spacing of pressure sensors 30 and 32 is between about five (5j feet (about 1.5 met~rs~ and about forty (40) feet (about 12~4 meter~ ressure sensors 30 and 32 are electrically connected to a receiver 36. ~eceiver 36 functions to receive data from pressure sensors 30 and 32 then process this received signal data in order to transform it into data usable in other portions of this system.
A data pxocessor and display apparatus 40 is operably connected to the output of receiver 36. The data processor and display appaxatus 40 is adapted to process the received and processed data thereby extracting the intelligence informati.on carried therein and display thi~
information and provide or its storage if desirad.
Figure;2 c;hows in a schematic representation the _7~ ~2~
app~r~tus for recovering the mud pressure pulse data from the mud flow stream and the associated apparatus for processing the received signals. Pressure transducers 30 and 32 are spac2d apar-t a distance d on conduit 34 to ex-tract the pressure pulse data from the mud stream. Considering the conduit segment 34 as shown, the mud flow is from left to right or in the opposite relation from pump 28 -to the well as shown in Fig. l. The intelligence carrying transmitted signal ST is moving from the right to the left whil~ the pump noise signal Sp is shown moving in the opposite direc-tion on the left side of the figure moving toward the right.
The output from transducer 30 passes through a time delay and polarity reversal circuit 42 and then to a summing circuit 44. The output from the other pressure transducer 32 is connected directly to summing circuit 44. Summing circuit 44 combines its two inputs to produce a single composite output signal representative of the combined receiv~d signals~ This composite signal is a signal representative of the time difference residual of the combined rece,ived signals. This composite signal is then passed to the processor portion of the apparatus, indicated generally at 40. Initially the co:mposite signal enters a set of n detection filters that function to maximize the abi].ity to detect a symbol (wavelet in PCM) from all other symbols and to do so in the presence of accompanying noise.
~ter this, the resultant or filtered signals are passed to a threshold detector where they are examined for the presence of an actual signal ~symbol ) that is sought, then the resultant output is provided as binary coded bit stream.
This output i.s representative of the data produced by the downhole measurement wllile drilling equipment and this data is appro~riately coded to be indicative of ~eophysical parameters and other data sensed by the downhole apparatus, Returning to the schematic of Figure 2 for discussion in further detail. The conduit 34 is for purposes of this discussion a segment of pipe, tubing or other flow conduit in *he mud line or flow path between the mud circulating pump 28 and swivel 22. Prefe.rably conduit 34 is a segment of unlform internal diame-ter material -that ~2~
is subs-tantially rigi.d and mounted ~ith the drilling ri~
structure in a secure position so that flow conditions of the mud at and be-tween the two pressure sensors wil]. no-t be substantially di-Eferent. It is desirable that flow conditions at and between the pressure sensors be substantially the same so that the monitor~d.pressure signal at one of the pressure sens~rs will be substantially the same as that monitored at the other pressure sensor and any modification of the flow conditions between the pressure sensors can be neglected.
Pressure sensors 30 and 32 are spaced from each other along conduit 34 at a distance d. Pressure sensors 30 and 32 can be ally commercially available type of pressure transducer that will with.in a predetermined accuracy measure pressure or the presence of the pressure wave in the mud at the appropriate point in the conduit and convert this to a corresponding electrical signal that can be expressed as a function with a reference to time. Also the pressure sensors could be any type which wi].l provide an electrical signal corresponding to and representative of fluid pressure or the pulse pressure wave in the mud at that location in conduit 34. The out.puts from pressure sensors 30 and 32 are pre~erably ma-tched or adjusted so the relative output signal magnitudes of the two pressure transducers are t~e same for similar sta~ic and dynamic measurements. Apparatus and circuitry for this matcllin~ is not illustrated in Figure 2 because it is not essential to understaning of this invention but basically a technical adaptation necessary to implement the invention.
6~
Referri,ng to Fig. 2, pressure sensors 30 and 32 function to measure the fluid pressure in conduit 34 as it is ef:Eected by the pressure pulses carried in the mud.
Using pressure sensor 32 as a reference point tlle signal from it is designated as yl(t). At any point in time,-t, the signal at pressure sensor 32 is equal to the pressure of the transmitted signal, the pump generated signal, their multiple reflections, and a noise pressure signal. The noise pressure signal is a background noise factor including miscellaneour, press.ure fluctuations that appear as uncorrelated fluctuations at the output of the pressure sensors. The presence of a reflected pump signal is recognized however it has been determined that this signal is o~ a substantially insigni~icant value when considered in view of the relative magnitudes and effects of the other p~essure signals involved. In the following it is assumed that two (2), reflectors are present in the system. One reflector being downstream of the pressure sensors and the other being upstream of the pressure sensors. These reflections occur as the pressure waves pass from one impedance zone of the flow path to anothex. These impedance changes occur at obstructions in the waveguide formed by drill string 10 and the conduits connecting it to mud circulating pump 28. Also these impedance changes occur at junctions of the flexible conduit of gooseneck 24 with swivel 22 and standpipe 26. In the following a change in impedance is referred to as change from one impedance medium to another. The relationship of the pxessure at any pressure sensor placed along the conduit may be expressed as:
Yn~t) = ST~t) -~ Sp(t) -~ Nn(t) + rO ~1 rO ~lS(t+T_l) 0,+1 5p(t+T~ -- where Yn(t) is the extracted pressure pulse signal frsm the mud flow line conduit 34, ST(t) is the attenuated and dispersed transmitted signal coming from the downhole transmitter, S ~t~ is the pump generated signal, Nn(t) is the noise ob~erved by the nth pressure transducer, ando r0 +1 is the first reflection coefficient down-stream of the ~ransducers r0 1 is the first reflection coefficient upstreamof the tran~ducers The signal Y1(t) at pressure sensor 32 will differ from that of the signal Y2(t) at pressure transducar 30 with respect to time because of ~he distance d saparating the two pressllre sensors and the associated delay in pressure wave propaga~ioIl time. The actual time differential involved and depends upon the propogation time between the separate - pressur~ sensors which is a function of the velocity of the 2~
pressure wave within conduit 34.
The signal Yl(t) at pressure transducer 32 can be expressed as:
Y (t) = S (t) ~ S (t) -~ r . S (t~2T 1) + rO -1 ST(t + 2(t +T~
rO 1 ^ rO +1 Sp(t+2(t'~T l+T~
~ r ~ rO ~1 ST(t+2( + ... + Nl(t) And the signal Y2(t) at pressure transducer 30 can be expressed as:
Y (t) = S (t ~ t') ~ ST(t - t ) + rO,~l p +1) r0~-lsT(t+t +2~t' + T 1)) + rO,-1 rO ~1 o Sp(t+t' + 2(t'+T 1 +
~1)) r0,-lro,~lST~t~~' + 2(t'+T 1 +
+1)) + ~ N2(t) where~
r. . i5 the reflection coefficient of wave moving 1~]
from medium i of impedence Zi into another medium j of impedence Zj.
t' is the propogation time ~etween transducers 30 and 32, the locations of observing Y2(t) and Yl(t) res]?ectively.
T 1 is the propogation time from transducer 30, (Y2~t)1, to t]he boundary between mediums -1 an~ 0.
T~l is the propogatio:n time ~rom transducer 32, (Yl(t)), the boundary between medium~
0 and ~1.
In order to timewise align the signals coming from pressure sensors 3~ and 32 it is necessary to account for the time shift taking place as a pressure wave moves betwen the pressure sensors. The propogation time between the pressure sensors t' can be used to shift the signal coming from either pressure sensor in order that one reference point for well to 3S surface traveling pressure signals can he established. In this system Y2~t) is delayed by time t' throu~h time delay element 42 as shown in Pig. ~.
The sign change is necessary so that noise canceling will tak~ place when the signals are combined by summing circuit 44. Y2(t) as it is del.ayed by -time t' can be expressed as follows:
Y2(t-t') = Sp(t) ~- ST(t-2t ) -~ rO ~lSp(t-2t -~2T~l) + rO -1 S~(t~-2(t' -~ T 1)) -~ rO -1 rO+l Sp (t--2(t'+T l-T+l)) -~ rO 1 rO ~1 Sl,(t-2t +2(t -~T_l+T~
-~ ...+ N2 (t-t') Summin~ circuit 44 combines the two signals Yl(t) and Y2(t--t') into a composite signal Zt~t). This composite signal Z~(t) is representative of the combined received signal of the mud pressure sensors representative of positive or upwardly traveling energy. This composite signal is the resultant output o~ receiver 36 and is ready for operations of the data processor apparatus 40. This composite signal z~(~) may be expressed as:
z~(t) Yl(t) - Y2(t-t') = ST(~) ~ ST(t-2t') rO ~l[Sp(t -2T~l) - Sp(t-2t'+2T~l)]
(~,-1 o,~l[ST(t~-2(t'~T l~T~
S (t-2t'+2(k'-T ~T ))]
-~ ...+ Nl(t) - N2 (t-t') From this it can be observed that Z~t) is comprised of the time difference o:E the primary passage of the transmitted signal; and the re:Elected time difference of ~he transmitted signal, the pump s:ignal, and the noise. By considering the reflection coef.icients to be so small as to render the reflection coe~ficient terms neglectable then the expression of Z~(t) can be simplied to:
Z~(tl = ST(t) - ST(t 2t') ~ N1~t) - N2(t-tl) The expression can be further simplified by generalizing the noise component in -terms of Gaussian noise representation. If Nl(t) and N2(t) are assumed to have a Gaussian distribution wherein they are ~lcorrelated and have an essentially equal variance. With t.his assumption the noise term would be represented as N*(t) with that term being a normalized Gaussian noise fac-tor. Therefore, the composite signal can be represented as:
Z~(t) = ST~t) - ST(t-2t'~ - N*(t) The composite signal Z+(t) that has been generated is next fed into separate matched filters 46 and 48. The matched filters are sometimes referred to as correlation filters wherein a sample signal is convolved with a desired signal ~time-reversed~ that is to be found in the sample signal. Each of the matched filtPrs 46 and 48 function similarly with the differences being in the input of the known functions A(t) and B(t) respectively for each of n = 2 symbols as shown hPre. Functions A(t)and B(t) are chosen to have a magnitude such that output signals, gO(t) and gl(t), from the matched filters are normalized. In matched filter 46 composite signal Z~(t) is convolved with signal A(t).
The signal gl(t) resulting from matched filter 46 is then pro~ided as one input to a threshold and detector circuit 50.
The other matched filter 48 receives as one input the composite signal Z+(t) and convolves it with th~ known signal B(t). The output signal gO(t) of matched filter 48 i5 then supplied as another input to detector circuit 50.
Detector cixcuit 50 performs several functions on the signals it receives. The separate input signals gO(t) and gl(t) are introduced into detector circuit 50 for evaluation in comparison. Each of the signals arriving at detector circuit 50 are compared to a threshold value L to determine as a function of time whether or not a desired signal is present in that given received signal gO(t) or gl(t). Consldering the input signal gl(t) if it is greater than L then it is probable that the desired symbol is present in this input 5ignal. However if gl(t) is less than L it is probable that the desired signal is not present. If neither one of g~(t) or gl(t) are greater than L, then it is proba'~le that then neither o tne sougiht after symbols are present in the receiv~d signal. In the event that both gl(t) and gO(t~ are greater than Ll or L2 respectively then it is assumed that the larger of these two is most probably the sought after signal. Because the r~presentation A(t) and B(t) are normalied functions then the mag~itudes of gO(t) and gl(t) provide meaningful criteria upon which to detect the larger of the responses~ thereby indicating the most probably signal.
When a symbol is detected/ the maximum o~ the -14~
signal represents the most likely syncllronization point of the detection process relative to the incoming data symbols in gO~t) and gl(t~. Detector circuit 50 includes circuitry by which it adaptably tracks the incoming data signals to maintain syncllronization. This adapti~e tracking of the incoming signals utilizes the time location of the ~elected peak in the incoming data signa~ as ~ell as its magnitude to synchronize the next timewise location for observing the next expected symbol. In this tracking process, the signal strength or amplitude is weighted in a factor having an effect on detecting the succeeding 5ymbol.
The output from detector circuit 50 is identified as X~t) and illustrated graphically in Fig. 3. The output of detector circuit 50 is supplied to data processor and display 40 shown in Fig. 1 for further manipulation and for presentation in data representative for the downhole measurements taken in the earth borehole. In other words the filtered and processed data displayed in Fig. 3 is representative of the intelligence carrying information originally derived by the measuring equipment and formatted for use by the transmitter of th~e downhole measurement while drilling equipment. This data can be decoded to extract its intelligence carrying information by the data processor and in turn provide a human intelligible output from this system.
In practiciny this invention, several important features are to be noted including the noise cancellation in processing that extracts the intelligence carrying portion of the downhole created signal without the necessity for
3~ recreating the transmitted into the modulated form that it had when leaving the downhole transmitter. Another important feature is that by u~ing this technique, it is possible to plac~ the pressure sensors or transducers substantially closer together than is indicated in prior art in data transmission systems where the original signal i~ a phase modulated carrier to wave length considerations. The receiver and received signal processor portion of this apparatus prepares a composite signal with respect to the real time Eorm of the signals that are expected from the -~5.-pressure pulses within the mud carrying conduit~ The received signal processor portion of this apparatus filters the composite data signal Z+(t~ so that an output signal X(t) is maximized when there is truly a data signal to be extxacted, thereby minimizing the noise that is present within the measured pressure signal within which the data is communicated. From receiver 36, as shown in Fig~ 2, the output signal X(t) can be utilized by additional data processing equipment ~not shown) to extract the intelligence information carried by this data signal and from that reproduce representations of the measurements made by the downhole equipment.
Claims (14)
1. A method of substantially reducing uphole noise from a downhole signal in a logging-while-drilling system where said downhole signal is in the form of a pulse modulated pressure waves being transmitted in the drilling fluid of said system, said method comprising:
a) measuring the fluid pressure in the drilling fluid at a first point and at a second point respectively, and converting both pressure measurements to corresponding electrical signals indicative of the measured pressures, said first and second points being spaced from each other along the drilling fluid flow path between a source of drilling fluid and a downhole portion of a well in which said downhole signal is originating;
b) time shifting one of said electrical pressure measurement signals by an amount corresponding with the propogation time of the pulse modulated pressure wave within said drilling fluid flow path from one of said points to the other;
c) combining said time shifted to the electrical pressure measurement signal with the other said electrical pressure measurement signal to produce a composite electrical measurement signal in order to substantially remove the downward propagating energy therefrom, and d) filtering said composite electrical pressure measurement signal in a matched filter to identify actual data in the electrical signals from noise present in the composite electrical measurement signal to produce a filtered electrical pressure measurement signal and further processing said filtered electrical measurement signal by detecting within said signal the presence of data that is within a predetermined range of being within a set of permissible values to produce a recovered data signal that has one value if no signal is detected, a first set of values if the filtered electrical measurement signal exceeds a threshold value, a second set of values if the filtered electrical measurement signal exceeds another threshold value.
a) measuring the fluid pressure in the drilling fluid at a first point and at a second point respectively, and converting both pressure measurements to corresponding electrical signals indicative of the measured pressures, said first and second points being spaced from each other along the drilling fluid flow path between a source of drilling fluid and a downhole portion of a well in which said downhole signal is originating;
b) time shifting one of said electrical pressure measurement signals by an amount corresponding with the propogation time of the pulse modulated pressure wave within said drilling fluid flow path from one of said points to the other;
c) combining said time shifted to the electrical pressure measurement signal with the other said electrical pressure measurement signal to produce a composite electrical measurement signal in order to substantially remove the downward propagating energy therefrom, and d) filtering said composite electrical pressure measurement signal in a matched filter to identify actual data in the electrical signals from noise present in the composite electrical measurement signal to produce a filtered electrical pressure measurement signal and further processing said filtered electrical measurement signal by detecting within said signal the presence of data that is within a predetermined range of being within a set of permissible values to produce a recovered data signal that has one value if no signal is detected, a first set of values if the filtered electrical measurement signal exceeds a threshold value, a second set of values if the filtered electrical measurement signal exceeds another threshold value.
2. The method of claim 1 wherein the step of combining said time shifted electrical pressure measurement signal with the other of said electrical pressure measure-ment signals additionally includes;
generating a composite signal representative of the difference between said time shifted electrical pressure measurement signal and said other electrical pressure measurement signal.
generating a composite signal representative of the difference between said time shifted electrical pressure measurement signal and said other electrical pressure measurement signal.
3. The method of claim 1 wherein said step of combining said time shifted electrical pressure measurement signal with the other said electrical pressure measurement signal additionally includes:
a) reversing the polarity of said time shifted electrical measurement signal; and b) adding said polarity reversed and time shifted electrical pressure measurement signal to said other pressure measurement signal thereby producing a composite signal.
a) reversing the polarity of said time shifted electrical measurement signal; and b) adding said polarity reversed and time shifted electrical pressure measurement signal to said other pressure measurement signal thereby producing a composite signal.
4. The method of claim 3 wherein said filtering additionally includes:
convolving said composite signal with a time reverse signal representative of each of the transmitted pulse symbols that is sought to be recovered thereby generating a filtered signal.
convolving said composite signal with a time reverse signal representative of each of the transmitted pulse symbols that is sought to be recovered thereby generating a filtered signal.
5. The method of claim 4 wherein said processing additionally includes:
a) testing each said filtered signal for a threshold determination of the filtered signal being between predterminable values and passing signals that are determined to exceed the predetermined threshold of permissible signal values;
b) discriminating the filtered signals to determine those which are the largest of the signal values within a predetermined time interval from those signals which are not the largest or within the predetermined time interval and thereby producing a recovered data signal that is represen-tative of the transmitted downhole signal; and c) synchronizing said measuring the fluid pressure with the timewise occurrence of said discriminating the filtered signals to predict the time of taking said measurements of said fluid pressure.
a) testing each said filtered signal for a threshold determination of the filtered signal being between predterminable values and passing signals that are determined to exceed the predetermined threshold of permissible signal values;
b) discriminating the filtered signals to determine those which are the largest of the signal values within a predetermined time interval from those signals which are not the largest or within the predetermined time interval and thereby producing a recovered data signal that is represen-tative of the transmitted downhole signal; and c) synchronizing said measuring the fluid pressure with the timewise occurrence of said discriminating the filtered signals to predict the time of taking said measurements of said fluid pressure.
6. The method of claim 1, wherein:
a) filtering of the composite filtered signals is done in time synchronization with the time occurrences of the maximums of selected pressure measurement filtered signals exceeding said threshold values; and b) said synchronization is adjusted timewise to compensate for time variations in said maximums of pressure measurement signals in order to maintain the taking or said measurements in synchronization with expected occurrences of selected pressure pulses.
a) filtering of the composite filtered signals is done in time synchronization with the time occurrences of the maximums of selected pressure measurement filtered signals exceeding said threshold values; and b) said synchronization is adjusted timewise to compensate for time variations in said maximums of pressure measurement signals in order to maintain the taking or said measurements in synchronization with expected occurrences of selected pressure pulses.
7. In a logging-while-drilling system wherein a downhole signal representative of a measured downhole parameter is transmitted to the earth surface in the form of a pressure pulse in the drilling fluid of the system, an apparatus for substantially reducing the influence of pressure pulsation interferring noise on the downhole signal, comprising:
a) conduit means for conducting drilling fluid from a source of drilling fluid at the earth surface to the well in which the downhole signal is originating;
b) a first transducer means at a first point on said conduit means for measuring the fluid pressure in said conduit means at said first point and for converting said pressure into a corresponding first electrical pressure measurement signal;
c) a second transducer means at a second point on said conduit means, spaced from said first point for measuring said fluid pressure in said conduit means at said second point and for converting said pressure into a corresponding second electrical pressure signal measurement;
d) means for time shifting said second electrical pressure measurement signal by an amount corresponding with the pressure wave propogation travel time in said drilling fluid from one of said transducer means to the other said transducer means;
e) means for generating a composite electrical measurement signal representative of the difference between said first and said second electrical pressure measurement signals;
f) means for filtering said composite electrical pressure measurement signal to identify valid pressure measurement signals in said composite signals to derive a filtered signal; and g) means for processing said filtered signal including determining the presence of a signal that is between predetermined limits and to differentiate these signals as being valid whereupon a transmission of the valid signal is made or a transmission of no signal is made, and for adaptively determining detector synchronization with the transmitted data.
a) conduit means for conducting drilling fluid from a source of drilling fluid at the earth surface to the well in which the downhole signal is originating;
b) a first transducer means at a first point on said conduit means for measuring the fluid pressure in said conduit means at said first point and for converting said pressure into a corresponding first electrical pressure measurement signal;
c) a second transducer means at a second point on said conduit means, spaced from said first point for measuring said fluid pressure in said conduit means at said second point and for converting said pressure into a corresponding second electrical pressure signal measurement;
d) means for time shifting said second electrical pressure measurement signal by an amount corresponding with the pressure wave propogation travel time in said drilling fluid from one of said transducer means to the other said transducer means;
e) means for generating a composite electrical measurement signal representative of the difference between said first and said second electrical pressure measurement signals;
f) means for filtering said composite electrical pressure measurement signal to identify valid pressure measurement signals in said composite signals to derive a filtered signal; and g) means for processing said filtered signal including determining the presence of a signal that is between predetermined limits and to differentiate these signals as being valid whereupon a transmission of the valid signal is made or a transmission of no signal is made, and for adaptively determining detector synchronization with the transmitted data.
8. The logging-while-drilling system of claim 7, additionally including a means for synchronizing signals from said first and said second transducer means with said means for processing said filtered signal in order to coordinate processing of said filtered signal to identify valid pressure measurement signals, said means for synchronizing has means to timewise adjust the synchronization of the signals in order to compensate for time variations in pressure pulsations in the downhole signal.
9. The apparatus of claim 7, wherein:
the spacing of said first and second transducer means is no closer than about approximately three feet and no greater than about approxi-mately one hundred feet.
the spacing of said first and second transducer means is no closer than about approximately three feet and no greater than about approxi-mately one hundred feet.
10. The apparatus of claim 7, wherein the spacing of said first and second transducer means is no closer than about five feet and no greater than approximately about thirty feet.
11. The apparatus of claim 7, wherein said filter means includes a convolving means to convolve said composite signal with a signal indicative of a signal that is sought to be recovered.
12. The apparatus of claim 11, wherein:
said processing means includes a threshold detector means to determine the presence of filtered signals between established limits, and a discriminator operable to determine whether the signal passed by the processor means should be the largest signal of those passing the threshold detector or no signal.
said processing means includes a threshold detector means to determine the presence of filtered signals between established limits, and a discriminator operable to determine whether the signal passed by the processor means should be the largest signal of those passing the threshold detector or no signal.
13. The apparatus of claim 12, wherein:
said processing means includes a synchronization means operable with said processing means said first and second transducer means and said means for generating a composite signal to optimize the detection of these signals sought to be recovered by synchronizing said means for filtering said composite signal with said discriminator.
said processing means includes a synchronization means operable with said processing means said first and second transducer means and said means for generating a composite signal to optimize the detection of these signals sought to be recovered by synchronizing said means for filtering said composite signal with said discriminator.
14. In a logging-while-drilling system wherein a downhole signal representative of a measured downhole parameter is transmitted to the earth surface in the form of a pulse modulated pressure wave in the drilling fluid of the system, an apparatus for substantially reducing the influence of interferring noise from the downhole signal, comprising:
a) conduit means for conducting drilling fluid from a source of drilling fluid at the earth surface to a drill string of the well in which the downhole signal is originating;
b) a first transducer means at a first point on said conduit means for measuring the fluid pressure at said first point in said conduit means and for convering said pressure measure-ment into a corresponding first electrical pressure measurement signal;
c) a second transducer means at a second point on said conduit means, spaced from said first point for measuring said fluid pressure at said second point in said conduit means and for converting said pressure measurement into a corresponding second electrical pressure signal measurement;
d) means for time shifting said second electrical pressure measurement signal by an amount corresponding with the pressure wave propogation travel time in said drilling fluid from one of said transducer means to the other said transducer means;
e) means for generating a composite electrical measurement signal representative of the difference between said first and said second electrical pressure measurement signals;
f) a filter means having a plurality of matched filter segments with one matched filter segment adapted to receive said composite signal and convolve it with a signal indicative of one signal that is sought to be recovered, and another of said matched filter segments being adapted to receive said composite signal and convolve it with another signal indicative of another signal sought to be recovered thereby producing a plurality of filtered output signals;
g) means for processing said plurality of said filtered output signals separately including means for determining whether the amplitude of each of the signals individually exceeds predetermined limits and whether each of the signals individually occurs within predetermined time limits and means for discriminating between the filtered output signals to provide an output signal representative of the measured downhole parameter; and h) means for synchronizing said predetermined time limits with expected occurrences of the pulse modulated pressure waves in the drilling fluid being detected by said first and second transducer means.
a) conduit means for conducting drilling fluid from a source of drilling fluid at the earth surface to a drill string of the well in which the downhole signal is originating;
b) a first transducer means at a first point on said conduit means for measuring the fluid pressure at said first point in said conduit means and for convering said pressure measure-ment into a corresponding first electrical pressure measurement signal;
c) a second transducer means at a second point on said conduit means, spaced from said first point for measuring said fluid pressure at said second point in said conduit means and for converting said pressure measurement into a corresponding second electrical pressure signal measurement;
d) means for time shifting said second electrical pressure measurement signal by an amount corresponding with the pressure wave propogation travel time in said drilling fluid from one of said transducer means to the other said transducer means;
e) means for generating a composite electrical measurement signal representative of the difference between said first and said second electrical pressure measurement signals;
f) a filter means having a plurality of matched filter segments with one matched filter segment adapted to receive said composite signal and convolve it with a signal indicative of one signal that is sought to be recovered, and another of said matched filter segments being adapted to receive said composite signal and convolve it with another signal indicative of another signal sought to be recovered thereby producing a plurality of filtered output signals;
g) means for processing said plurality of said filtered output signals separately including means for determining whether the amplitude of each of the signals individually exceeds predetermined limits and whether each of the signals individually occurs within predetermined time limits and means for discriminating between the filtered output signals to provide an output signal representative of the measured downhole parameter; and h) means for synchronizing said predetermined time limits with expected occurrences of the pulse modulated pressure waves in the drilling fluid being detected by said first and second transducer means.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US38707682A | 1982-06-10 | 1982-06-10 | |
| US387,076 | 1982-06-10 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1206089A true CA1206089A (en) | 1986-06-17 |
Family
ID=23528356
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000427966A Expired CA1206089A (en) | 1982-06-10 | 1983-05-11 | Method and apparatus for signal recovery in a logging while drilling system |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1206089A (en) |
| DE (1) | DE3321138A1 (en) |
| FR (1) | FR2528491A1 (en) |
| GB (1) | GB2121572B (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5375098A (en) * | 1992-08-21 | 1994-12-20 | Schlumberger Technology Corporation | Logging while drilling tools, systems, and methods capable of transmitting data at a plurality of different frequencies |
| US5969638A (en) * | 1998-01-27 | 1999-10-19 | Halliburton Energy Services, Inc. | Multiple transducer MWD surface signal processing |
| US8004421B2 (en) | 2006-05-10 | 2011-08-23 | Schlumberger Technology Corporation | Wellbore telemetry and noise cancellation systems and method for the same |
| US8629782B2 (en) | 2006-05-10 | 2014-01-14 | Schlumberger Technology Corporation | System and method for using dual telemetry |
| US8880349B2 (en) | 2010-06-21 | 2014-11-04 | Halliburton Energy Services, Inc. | Mud pulse telemetry |
| CN106850479B (en) * | 2017-02-15 | 2020-02-14 | 中国石油大学(华东) | Drilling fluid continuous wave frequency continuous phase smooth coding modulation and demodulation system and method |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3742443A (en) * | 1970-07-27 | 1973-06-26 | Mobil Oil Corp | Apparatus for improving signal-to-noise ratio in logging-while-drilling system |
| US3747059A (en) * | 1970-12-18 | 1973-07-17 | Schlumberger Technology Corp | Electronic noise filter with means for compensating for hose reflection |
| US3716830A (en) * | 1970-12-18 | 1973-02-13 | D Garcia | Electronic noise filter with hose reflection suppression |
| NO790496L (en) * | 1978-02-27 | 1979-08-28 | Schlumberger Technology Corp | METHOD AND DEVICE FOR DEMODULATING SIGNALS IN A BURGING LOGGING SYSTEM |
-
1983
- 1983-05-11 CA CA000427966A patent/CA1206089A/en not_active Expired
- 1983-05-18 GB GB08313695A patent/GB2121572B/en not_active Expired
- 1983-06-08 FR FR8309500A patent/FR2528491A1/en not_active Withdrawn
- 1983-06-10 DE DE19833321138 patent/DE3321138A1/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| FR2528491A1 (en) | 1983-12-16 |
| DE3321138A1 (en) | 1983-12-15 |
| GB2121572A (en) | 1983-12-21 |
| GB8313695D0 (en) | 1983-06-22 |
| GB2121572B (en) | 1985-12-04 |
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