WO1989010572A1

WO1989010572A1 - Acoustic data transmission through a drill string

Info

Publication number: WO1989010572A1
Application number: PCT/US1989/001641
Authority: WO
Inventors: Douglas Schaeffer Drumheller
Original assignee: United States Department Of Energy
Priority date: 1988-04-21
Filing date: 1989-04-21
Publication date: 1989-11-02
Also published as: EP0408667A1; DE68912584D1; EP0408667B1; JPH03501408A; EP0408667A4

Abstract

Acoustical signals are transmitted through a section of drill string (30) by cancelling upward moving acoustical noise and by preconditioning the data in recognition of the comb filter characteristics of the drill string. Spaced sensors (52, 54) provide a delayed signal to summer (46) which is combined with a signal from suitable circuitry (28) and then applied to sensors (42, 44) for transmitting an uphole signal having no downward moving noise.

Description

ACOUSTIC DATA TRANSMISSION THROUGH A DRILL STRING

BACKGROUND OF THE INVENTION

This invention relates generally to a sj'stem for transmitting data along a drill string, and more particularly to a system for transmitting data through a drill string by mod¬ ulation of intermediate-frequency acoustic carrier waves.

Deep wells of the type commonly used for petroleum or geothermal exploration are typically less than 30 cm (12 inches) in diameter and on the order of 2 km (1.5 miles) long. These wells are drilled using drill strings assembled from relatively light sections (either 30 or 45 feet long) of drill pipe that are connected end-to-end by tool joints, additional sections being added to the uphole end as the hole deepens. The downhole end of the drill string typically includes a drill collar, a dead weight assembled from sections of relatively hcavj^τ lengths of uniform diameter collar pipe having an overall length on the order of 300 meters (1000 feet). A drill bit is attached to the downhole end of the drill collar, the weight of the collar causing the bit to bite into the earth as the drill string is rotated from the surface. Sometimes, downhole mud motors or turbines are used to turn the bit. Drilling mud or air is pumped from the surface to the drill bit through an axial hole in the drill string. This fluid removes the cuttings from the hole, provides a hydrostatic head which controls the formation gases, and sometimes provides cooling for the bit.

Communication between downhole sensors of parameters such as pressure or tem¬ perature and the surface has long been desirable. Various methods that have been tried for this communication include electromagnetic radiation through the ground forma¬ tion, electrical transmission through an insulated conductor, pressure pulse propagation through the drilling mud, and acoustic wave propagation through the metal drill string.

Each of these methods has disadvantages associated with signal attenuation, ambient noise, high temperatures, and compatibility with standard drilling procedures.

The most commercially successful of these methods has been the transmission of information by pressure pulse in the drilling mud. However, attenuation mechanisms in the mud limit the transmission rate to about 2 to 4 bits per second.

This invention is directed towards the acoustical transmission of data through the met l drill string. The history of such efforts is recorded in columns 2 - 4 of U.S. Patent No. 4,293,93G, issued Oct. 6, 1981, of Cox and Chaney. As reported therein, the first efforts were in the late 1940's by Sun Oil Company, which organization concluded there was too much attenuation in the drill string for the technology at that time. Another company came to the same conclusion during this period.

U.S. Patent No. 3,252,225, issued May 24, I960, of E. Hixon concluded that the length of the drill pipes and joints had an effect on the transmission of energy up the drill string. Hixon determined that the wavelength of the transmitted data should be at least twice the length of a section of pipe.

In 196S Sun Oil tried again, using repeaters spaced along ihe drill string and trans¬ mitting in the best frequency range, one with attenuation of only 10 dB/1000 feet. A paper by Thomas Barnes et al., "Passbands for Acoustic Transmission in an Idealized Drill String", Journal of Acoustical Society of America, Vol. 51, No. 5, 1972, pages

1606-1608, was consulted for an explanation of the field-test results, which were not totally consistent with the theory. Eventually, Sun went back to random searching for the best frequencies for transmission, an unsuccessful procedure.

The aforementioned Cox and Chaney patent concluded from their interpretation of the measured data obtained from a field test in a petroleum well that the Barnes model must be in error, because the center of the passbands measured by Cox and Chaney did not agree with the predicted passbands of Barnes et al. The patent uses acoustic repeaters along the drill string to ensure transmission of a particular frequency for a particular length of drill pipe to the. surface. U. S. Patent No. 4,314,365, issued February 2, 1982, of C. Pεtersen et al. discloses a system similar to Hixon for transmitting acoustic frequencies between 290 Hz and 400 Hz down a drill string.

U. S. Patent No. 4,390,975, issue June 28, 1983, of E. Shawhan, noted that rin ¬ s' ing in the drill string could cause a binary "zero" to be mistaken as a "one". This patent transmitted data, and then a delay, to allow the transients to ring down before transmitting subsequent data.

U. S. Patent No. 4,562,559, issued December 31, 1985, of H. E. Sharp et al., uncov¬ ered the existence of "fine structure" within the passbands; e.g., "such fine structure 10 is in the nature of a comb with transmission voids or gaps occurring between teeth representing transmission bands, both within the overall passbands." Sharp attributed this structure to "differences in pipe length, conditions of tool joints, and the like." The patent proposed a complicated phase shifted wave with a broader frequency spectrum to bridge these gaps. 15 The present invention is based upon a more thorough consideration of the underlying theory of acoustical transmission through a drill string. For the first time, the work of Barnes et al. has been analyzed as a banded structure of the type discussed by L_ Brillouin, Wave Propagation in Periodic Structures, McGraw-Hill Book Co., New York, 1946. The theoretical results have also been correlated to extensive laboratory 0ι experiments on scale models of the drill string, and the original data tape obtained from Cox and Chaney's field-test has been reanalyzed. This analysis shows that Cox and Chaney's measurements contain data which is in excellent agreement with the theoretical predictions; that Sharp misinterpreted the cause of the fine structure; and that the ringing and the frequency limitations cited by Shawhan and Hixon are easily _5- overcome by signal processing.

Figure 1 shows some of the results of the new analysis of the data recorded by Cox and Chaney. This figure is a plot of the power amplitude versus frequency of the transmitted signal. The theoretical boundaries between the passbands and the stopbands are shown by the vertical dotted lines. If this figure is compared to Figure 1 . Cox and Chaney's patent, significant and obvious differences can be noted. These are attributable to error in Cox and Chaney's analysis.

Furthermore, this Figure 1 also shows the "fine structure" of Sharp et al. From the new analysis we now know that this fine structure is caused by echos bouncing between

5. opposite ends of the drill string, the number of peaks being correlated to the number of sections of drill pipe. A theoretical calculation of this field test was used to produce

Figure 2. All of the phenomena important to the tratpsmission of data in the drill string is represented in this calculation. These theoretical results accurately predict the location of the passbands and the fine structure produced by the echo phenomena.

SUMMARY OF THE INVENTION

10 It is. an object of this invention to provide apparatus and method for transmitting data along a drill string by use of a modulated continuous acoustical carrier wave (waves) whicli is (are) centered within one (several) of the passbands of the drill string.

It is a further object of this invention to provide a method for transmission at carrier frequencies which are on the order of several hundreds to several thousands of Hertz 15- in order to minimize the interference by the noise which is generated by the drilling process.

It is an additional object of this invention to provide a system for suppressing the transmission of noise within the transmission band or bands.

It is another -object of this invention to provide a system for suppressing echos from 20 the ends of the drill string.

It is still another object of this invention to provide a system for preconditioning acoustical data for transmission through a passband having characteristics determined by the parameters of the drill string.

Additional objects, advantages, and novel features of the invention will become

25. apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the inven- tion may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention may comprise transmitting means for coupling data to a drill string near a first end of said drill string for acoustical transmission to a second end of said drill string; anti- noise means near the first end of said drill string for preventing acoustical noise from the first end from being transmitted through the drill string to the second end; and receiving means near the second end for receiving the acoustically transmitted data. In addition, the invention may further comprise a method comprising the steps of preconditioning the data to counteract distortions caused by the drill string, the distortions corresponding to the effects of multiple passbands and stopbands having characteristics dependent upon the properties of the drill string; applying the precon¬ ditioned data to a first end of the drill string; and detecting the data at a second end of the drill string.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form part of the spec¬ ification, illustrate an embodiment of the present invention and, together with the description, serve to explain the principles of the invention.

• Fig. 1 shows the measured frequency response within two passbands of the Cox- and-Chaney drill string.

• Fig. 2 shows the calculated frequency response within two passbands of the Cox- and-Chaney drill string.

• Fig. 3 shows a drill string.

• Fig. 4 shows dispersion curves for a uniform string (dashed line) and a typical drill string (solid line). • Fig. 5 shows the transmission arrangement at a first end of a drill string.

DETAILED DESCRIPTION

As shown in Figure 3, this invention involves the transmission of acoustical data along a drill string 10 which consists of a plurality of lengths of constant diameter drill pipe 15 fastened end-to-end at thicker diameter joint portions 18 by means of screw threads as is well known in this art. Lower end 12 of drill string 10 may include a length of constant diameter drill collar to provide downward force to drill bit 22. A constant diameter mud channel 24 extends axially through each component of drill string 10 to provide a path for drilling mud to be pumped from the surface at upper end 14 through holes in drill bit 22 as is well known in this art. The upper end 14 of drill string 10 is terminated in conventional structure such as a derrick, rotary pinion, and kelly, represented by box 25, to permit additional lengths of drill pipe to be added to the string, and the string to be rotated for drilling. Details of this conventional string structure may be found in the aforementioned patent of E. Hixon.

Although the disclosure is directed towards transmitting data from the lower end to the upper end, it is to be understood that the teachings of this invention apply to data transmission in either direction.

The theory upon which this invention is based begins with the derivation the fol¬ lowing Equation 1, which equation is in the form of a classical wave equation:

where impedance z = pac, and total axial force F(x, t) = — cz - where p is density, a is .urea, and c is speed of sound over a cross-section of a slender, elastic, rod, u is the displacement, x is the position , m is the Lagrangian mass coordinate, and t is the time.

The existence of frequency bands which block propagation of acoustic energy is demonstrated for an idealized drill string where each piece of drill pipe consists of a tube of length d-*, mass density _{l 5} cross-sectional area α-* , speed of sound c*_., and mass ri; and a tool joint of length -_"₂, mass density _2. cross-sectional area α₂, speed of sound c₂, and mass r₂. A procedure demonstrated at page 180 of Brillouin has been used with the Floquet theorem to generate the following eigenvalue problem:

where

(3)

Here k is the wave number, i

= ω/z_ξ, and / is the frequency being transmitted. This equation is seen to be similar to Equation IS of Barnes et al., except the present examination shows Barnes' "W" to be kd. i Brillouin shows that frequencies which yield real solutions for k are banded and separated by frequency bands which yield complex solutions for k. He calls these two types of regions passbands and stopbands. The attenuation in the stopbands is generally quite large. Within each of the passbands the value of the phase velocity ω/k depends upon the value of ω. The drill string functions as an acoustic comb filter, _' and frequencies which propagate in the passbands are dispersed. Thus, signals which have broad frequency spectra are severely distorted by passage through a drill string. However, signal processing techniques can be- used to remove this distortion.

It is to be understood that the "comb filter" referenced above refers to the gross structure in the frequency spectrum which is produced by the stopbands and the pass- bands, where each tooth of the comb is an individual passband. In contrast, Sharp's reference to a comb refers to a fine structure which exists within each passband.

Figure 4 shows a plot of the characteristic determinate of Equation 2 using values for p_ξ, a._ζ, C_ξ, and d_ς representative of actual drill pipe parameters. The straight dotted line represents the solution for a unifor drill string, e.g., one where the diameter of the joints is equal to the diameter of the pipe. The velocity of propagation for a given frequency is represented by the phase velocity. For the uniform drill string, this ratio is constant and equal to the bar velocity of steel. When waves containing multiple frequency components travel through a uniform drill string (or drill collar 20), they do not distort as all frequency components remain in the same relative position.

A different result occurs when the plot of Fig. 4 is curved, as each frequency then travels at a different speed. The solid lines of Fig. 4 represent the solution to Equation 2 for a realistic drill string where the area of the drill pipe is 2450 mm² (4 in²) and the area of a tool joint is 12,900 mm² (20 in²). In this situation, the phase velocity within each passband is curved, meaning that distortion exists.

Furthermore, the gaps represent stopbands. This analysis predicts the same values for the boundaries between the stopbands and the passbands as that of Barnes et al.; however, it also shows the characteristics of wave propagation within each of the passbands- Barnes et al. did not predict the distortion resulting from the effects of the passbands .

Calculations using a smaller diameter tool joint, representative of the reduction in diameter that occurs from wear, shows the stopbands to be narrower. This change is to be expected, because the worn joints bring the string geometry closer to the uniform geometry that produced the straight, dotted, line of Fig. 4.

Further calculations show that strings comprised of random length pipes will have significantly narrowed passbands. This result corresponds with, and for the first time explains, observations made by others.

Since the transmission of acoustical data through the drill string involves sending waves with complex transient shapes through strings of finite length, transient waλ'e analysis has been used to predict the performance of the drill string. Fig. 2 shows the third and fourth passbands of a fast Fourier transform of the waveform which results from a signal which represents, to a rough approximation, the hammer blow used in the Cox and Chaney field test. This signal has a relatively narrow frequency content which only stimulates the third and fourth passband of the drill string. Ten sections of drill pipe were used in this field test, and the ends of the drill string produced nearly perfect reflection of the acoustic waves which resulted from the hammer blows.

This figure shows the "fine structure" of Sharp et al. to be caused by standing wave resonances within the drill string. The number of spikes in each passband correlates with the number of sections of pipe in the drill string, as explained in greater detail in the Appendix.

The analysis suggests the following technique for processing data signals and com¬ pensating for the effects of the stopbands and dispersion. First, transmit information continuously (as opposed to a broad-band pulse mode) and only within the passbands and away from the edges of the stopbands. Second, compensate for dispersion by mul¬ tiplying each frequency component by exp(— ikL), where L is the transmission length in the drill pipe section 18 of the drill string. Where a large amount of acoustical noise is present, such as would be caused by a drill bit or drill mud, it is preferable to transform the data signal before transmission, resulting in an undispersed signal at the receiver position.

The foregoing analysis is based on the assumption that echos are suppressed at each end of the drill string. This is necessary to eliminate the spikes or fine structure within each of the passbands. It is common knowledge that signal processing is effective when echo strength is 20 dB below the the signal level. Each time the acoustic wave interacts with the intersection of the drill pipe and the drill collar 80, the signal weakens by 6 dB. Also, from the analysis of Cox and Chaney's field test, the signal attenuates about 2 dB/1000 feet. Therefore, an echo which is generated by a reflection of the data signal at the top of the drill string 14 will lose 6 + 41, dB as it travels back down the drill string to 80 and then returns to the receiver. Thus, if the drill pipe section has a length of 3500 feet or more, the echos from the receiving end of the string will be naturally attenuated to an acceptable level.

For shorter drill strings, additional echo suppression will be required. This can be accomplished with a device called a terminating transducer. This device has an acoustical impedance which matches the acoustical impedance of the drill string and an acoustical loss factor which is sufficient to make up the required 20 dB of echo suppression.

Because attenuation in the drill string is low, the energy velocity and group velocity are approximately equal. Therefore, the characteristic impedance of the drill string is

I(> the- force F divided by velocity ^•■?--. This value is the eigenvalue part of Equation 2, a complex number with a real part called the viscous component and an imaginary part called the elastic component. Ideally, the terminating transducers must have a stiffness equal to the elastic component and a damping coefficient equal to the viscous component. Practically, the response need only make up the difference between 20 dB

15 and the natural attenuation of the drill string.

The characteristic impedance is a function of frequency and position, the position dependence being periodic in accordance with the period of the drill string. Calculations show that tool joints are not a good location for a termination because the impedance is a sensitive function of position. For the fourth passband, a location 1/3 or 2/3 along

2D the pipe is better.

The design of termination transducers is a conventional problem to those of ordinary skill in that art provided with the impedance data from Equation 2. This device, for example, could consist of a ring of polarized PZT ceramic elements and an electronic circuit whose reactive and resistive components are adjusted to tune the transducer to

25 the characteristic impedance of the drill string and provide the necessary acoustic loss factor.

Echo suppression is a more critical problem at the downhole end of the drill string where echos travel freely up and down the drill collar section and confuse the transmis¬ sion of data. At this location, it is useful to use noise cancellation techniques both to suppress echos and to prevent the noise of the drill bit or drilling mud from interfering with the desired data signal uphole. A noise cancellation technique for use with this invention is disclosed hereinafter.

Fig. 5 shows a section 30 of dri¹! collar 20 located relatively close to downhole end

5 12 of drill string 10 and containing apparatus for transmitting a data signal towards the other end of the drill string while suppressing the transmission of acoustical noise up the drill string. In particular, this apparatus includes a transmitter 40 for transmitting data uphole, but not downhole, a sensor 50 for detecting acoustical noise from downhole and applying it to transmitter 40 to cancel the uphole transmission of the noise, and a

10 sensor 60 for providing adaptive control to transmitter 40 and sensor 50 to minimize uphole transmission of noise.

Transmitter 40 includes a pair of spaced transducers 42, 44 for converting an electrical input signal into acoustical energy in drill collar 30. Each transducer may be a magnetostrictive ring element with a winding of insulated conducting wire. These

15 transducers are spaced apart a distance b equal to one quarter wavelength of the center frequency of the passband selected for transmission. A data signal from source 28 is applied directly to uphole transducer 44, preferably through a summing circuit 46. The data signal is also applied to transducer 42 through a delay circuit 47 and an inverting circuit 48. Delay circuit 47 has a delay value equal to distance b divided by

20) the speed of sound in drill collar 30 at transmitter 40.

The operation of this transmitter may be understood from the following explanation. Each of transducers 42, 44 provide an acoustical signal ₂, F₄ that travels both uphole and downhole. Accordingly the resulting upward and downward waves from both transducers are:

φ_u{t, τ) = F₂(t - x/c) -4- ₄(f - (x - b)/c)) where X > b

(^β) φ_d(t, x) = F₂(t + x/c) -4- F_A(t + (x - b)/c)) where X < 0

25> where x is the uphole distance from transducer 42 and c is the speed of sound. For no downward wave, φ_d(t, x = 0, or

F₂(t) = -F_A(i - b/c) (7)

and φ_u(t, x) = -F₂{t - (x + b)/c) + F₂(t - {x - b)/c) (8)

If tiie acoustical signal _F₂ has the form A os(ωt), then Equation 8 solves to

φ_u(τ) = —2Asm(ωb/c) sm(ωτ) (9)

where r — (t — x/c). 5. Accordingly, with a quarter wavelength spacing for waves at the center of the trans¬ mission passband, transmitter 40 transmits an uphole signal having approximately twice the amplitude A of the applied signal, and no downhole signal.

Noise sensor 50 includes a pair of spaced sensors 52, 54 which operate in a similar manner to provide an indication of acoustic energy moving uphole, and no indication of

10' energy moving. downhole. The output of sensor 52, which sensor may be an accelerom- eter or strain gauge, is an electrical signal that is summed in summing circuit 56 with the output of similar sensor 54, which output is delayed by dela}- circuit 57 and and inverted by inverting circuit 58. If the delay of circuit 57 is equal to the spacing b divided by the speed of sound c, downward moving energy is first detected by sensor

15 54. and delayed, and later detected by downhole sensor 52. The inverted electrical signal from 54 arrives at summing circuit 56 at the same time as the output of sensor 52, providing a net output of zero for downward moving noise. Upward moving noise of the form Asinu. (£ — x/c) yields an output from summing circuit 56 of:

φ(t) = 2Asin(π//2/₀) cosω(t - b/c) (10) where o is the center frequency of the passband.

In the description which follows it is to be understood that all electrical signals are filtered so that the frequency content is limited to the passband or bands which are used for data transmission. Sensor 50 is spaced from transmitter 40 by distance a. Accordingly, noise that is sensed at sensor 50 arrives at transmitter 40 a time a/c later.

If the output of sensors 50 is delayed by delay circuit 59 for an interval of a/c and applied to transmitter 40 through summing circuit 46, the output of transmitter 40 can be shown to cancel the upward moving noise to within an error e = — (sin( _> fe/c))² - 1. For a bandwidth-to-center frequency ratio of 150 Hz/650 Hz, the error is zero at the center of the transmission band and is only .03 at the band edges, a result showing 30 db noise cancellation.

Further control of upward moving noise is provided by adaptive control 70, a con¬ ventional control circuit that has an input from a second pair of sensors 62, 64. These sensors, identical to sensors 52, 54, also have corresponding delay circuit 67 and in- verter 68 to provide an output indicative of an upward moving wave and no output in response to a downward moving wave. The upward moving wave at control sensors 60 is a mixture of the noise and data that passed transmitter 40. Accordingly, by delaying the data signal in delay circuit 72 and adding the result to the output of sensors 60 with summing circuit 74, an error signal is produced which indicates the effectiveness of noise cancelation. This signal is fed into an adaptive control circuit 70 which controls conventional circuitry 75 to adjust voltage amplitudes or phases of the signals being applied to any of sensors 52 and 62 or transmitters 42, 44 to minimize the amount of noise being transmitted upward towards the surface.

For a conventional steel drill collar, the spacing _ between sensors or transmitters in the third passband would be about 30 cm (78 inches) or about 21 cm (53 inches) in the fourth passband.

The operation of the invention is as follows: The circuitry of Fig. 5 is mounted on a drill collar, including suitable circuitry 28 for generating data representative of a down- hole parameter. Power supplies, such as batteries or mud-driven electrical generators, and other supportive circuitry known to those of ordinary skill in the art, would also be incorporated into drill collar 30. The drill bit and mud create acoustic noise that travels in both directions through drill string 10. Downward noise is not sensed by the sensors; however, upward noise, including echos from the bottom of the drill collar, are sensed by sensor circuit 50 and applied to transmitter circuit 40, yielding a greatly reduced upward noise component. Primarily the data travels to the connection 80 (Fig. 3) between drill collar 30 and the lowest drill joint 18, where a significant reflec¬ tion of the data occurs because of the mismatch in acoustic impedance between these elements. Further echos occur at the tool joints 18 between each section of drill pipe 15. These echos move downward through drill collar 30 where they pass the circuitry of Fig. 5 undetected, and become noise that is canceled out when they echo off the bottom of the drill collar. The signal that reaches the top is detected by a receiver such as an accelerometer. If necessary because of low attenuation within the drill string, an acoustically impedance matched transducer 80 may be used to terminate the signal and provide an accurate representation of the data transmitted from below.

As stated above, the data from circuit 28 may be precompensated by multiplying each frequency component of the signal by exp(— ikL) to adjust for the distortion caused by the passbands of the drill string. Such compensation may be accomplished by any manner known to those of ordinary skill in the art with a device such as an analog-to- digital signal processing circuit.

This invention recognizes and solves the problems noted by many previous workers in the field of transmitting data along a drill string. As a result, quality transmission on continuous acoustic carrier waves without extensive downhole circuitry, and without the use of impractical repeater circuits and transducers along the drill string, is possible at frequencies on the order of several hundred to several thousand Hertz. These frequencies are high in relation to the ambient drilling noise (about 1 to 10 Hz), and therefore allow transmission relatively free of this noise. Also the bandwidths of the passbands allow data rates far in excess of present mud pulse systems. Also it is recognized that this method will work in drilling situations where air is used instead of mud. The particular sizes and equipment discussed above are cited merely to illustrate a particular embodiment of this invention. It is contemplated that the use of the invention may involve components having different sizes and shape? as long as the principle set forth in the claims is followed. It is intended that the scope of the invention be defined by the claims appended hereto. A more detailed explanation of the calculations behind this invention, and results of scale model tests and evaluations of field data, are provided in the Appendix attached to this disclosure.

Claims

I claim:

1. A method for transmitting data through a drill string comprising the steps of:

• preconditioning said data to counteract distortions caused by said drill string, said distortions corresponding to the effects of a comb filter having charac¬ teristics dependent upon the properties of said drill string;

• applying said preconditioned data to a first end of said drill string; and

• detecting said data at a second end of said drill string.

2. The method of claim 1 wherein said preconditioned data is a first electrical signal, said method further comprising:

• converting said first electrical signal to a first acoustical signal for application to said first end of said drill string; and

• converting said detected acoustical data to a detected electrical signal at said second end of said drill string.

3. The method of claim 2 wherein said drill string has low attenuation passbands and high attenuation stopbands of acoustical signals, the frequencies of said first acoustical signal being in the passbands of said drill string.

4. The method of claim 3 wherein said preconditioning comprises multiplying each frequency component of said first electrical signal by exp(— ikL) where L is the transmission length of said drill string and k is the wave number in said drill string at the frequency of each component.

5. The method of claim 1 further comprising the step of suppressing acoustical echos from each end of said drill string.

6. The method of claim 5 wherein said step of suppressing acoustical echos at the receiver comprises matching the acoustical impedance of said transducers to the acoustical impedance of said drill string and providing a sufficient loss factor to terminate the signal.

_*

7. The method of claim 5 wherein said step of suppressing acoustical echos at the transmitter comprises applying echo-cancellation energy to said drill string in a position adjacent to said transducers.

8. The method of claim 5 wherein said step of apply energy comprises:

• providing an output indicative of acoustical noise traveling from said first end towards the location on said drill string where said data is applied, and providing no indication of acoustical noise traveling from said location

5 towards said first end; and

• applying a delayed output to said drill string to cancel noise traveling from said location towards said second end.

9. Apparatus for transmitting data on a continuous carrier wave through a drill string comprising a plurality of drill pipe sections connected end-to-end by tool joints, the length and cross-sectional area of the pipe sections being different from the length and cross-sectional area of the tool joints, said apparatus comprising:

5 • transmitting means for coupling data to said drill string near a first end of said drill string for acoustical transmission to a second end of said drill string;

• anti-noise means near said first end of said drill string for preventing acousti¬ cal noise from said first end from being transmitted through said drill string

10 to said second end; and

• receiving means near said second end for receiving said acoustically trans¬ mitted data.

10. The apparatus of claim 9 wherein said anti-noise means comprises: • first noise-receiving means for providing a first output indicative of acoustical noise traveling from said first end toward said transmitter means, and for providing no indication of acoustical noise traveling from, said transmitter means towards said first end; and

• noise-cancelling means for applying a delayed output from said noise-receiving means to said transmitting means to cancel noise traveling from said trans¬ mitting means towards said second end of said drill string.

11. The apparatus of claim 10 wherein said anti-noise means further comprises:

• second noise-receiving means for providing a second output indicative of acoustical noise and data traveling from said transmitter means towards said second end, and for providing no indication of acoustical noise traveling from said second end towards said transmitter means; and

• adaptive control means for comparing said second output with said data, and adjusting at least one of said first and second outputs or said transmitted data to minimize the transmission of noise towards said second end.

12. The apparatus of claim 9 wherein said transmitting means comprises:

• a first and a second acoustical transmitter spaced along said drill string a distance equal to an odd multiple of a quarter wavelength of said carrier wave, said first transmitter being closer to said first end than said second * transmitter;

• second signal applying means for applying said data signal to said second transmitter; and

• first signal applying means for applying a delayed, inverted, data signal to said first transmitter, the delay being equal to the transmission time of the 0 transmitted signal from said first transmitter to said second transmitter, whereby said data signal is transmitted only toward said second end.

13. The apparatus of claim 9 wherein said first noise-receiving means comprises:

• a first and a second acoustical receiver spaced along said drill string a dis¬ tance equal to an odd multiple of a quarter wavelength of the carrier wave, said first receiver being between said first end and said second receiver, said second receiver being between said first receiver and said transmitting means;

• means for summing a noise signal from said first receiver and a delayed, inverted noise signal from said second receiver to produce a noise-cancelling signal, the delay being equal to the transmission time of the received noise signal from said first receiver to said second receiver; and

» the delay of said noise-cancelling means being equal to the transmission time of said noise from said second receiver to said transmitting means.

14. The apparatus of claim 11 wherein said second noise-receiving means comprises:

• third and fourth acoustical receivers spaced along said drill string a distance equal to an odd multiple of a quarter wavelength of said carrier wave, said receivers being between said transmitter means and said second end, said third receiver being between said fourth receiver and said transmitter means;

• means for summing a signal from said first receiver and a delayed, inverted noise signal from said second receiver to produce a noise-cancelling signal, the delay being equal to the transmission time of the received noise signal from said first receiver to said second receiver.

15. The apparatus of claim 12 wherein each of said acoustical transmitters comprise:

• a transducer for converting an electrical signal into an acoustical signal for application to said drill string; and

• said receiving means comprises output transducer means for converting said received acoustical data to a detected electrical signal at said second end of said drill string.

16. The apparatus of claim 9 wherein said drill string further comprises a drill collar at said first end of said drill string, said transmitting means and anti-noise means being affixed to said drill collar.

17. The apparatus of claim 9 further including means for preconditioning sεdd data to counteract distortions caused by said drill string, said distortions corresponding to the effects of a comb filter having characteristics dependent upon the properties of said drill string.

IS. The apparatus of claim 9 wherein the acoustical impedance of said receiving means is matched to the acoustical impedance of said drill string at said second end, thereby preventing the generation of echos from said second end towards said first end of said drift string.