CA1204494A - Seismic exploration system and an analog-to-digital converter for use therein - Google Patents
Seismic exploration system and an analog-to-digital converter for use thereinInfo
- Publication number
- CA1204494A CA1204494A CA000440326A CA440326A CA1204494A CA 1204494 A CA1204494 A CA 1204494A CA 000440326 A CA000440326 A CA 000440326A CA 440326 A CA440326 A CA 440326A CA 1204494 A CA1204494 A CA 1204494A
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- CA
- Canada
- Prior art keywords
- signal
- digital
- analog
- predicted
- signals
- 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
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
- G01V1/24—Recording seismic data
- G01V1/247—Digital recording of seismic data, e.g. in acquisition units or nodes
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M1/00—Analogue/digital conversion; Digital/analogue conversion
- H03M1/12—Analogue/digital converters
- H03M1/48—Servo-type converters
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M3/00—Conversion of analogue values to or from differential modulation
- H03M3/04—Differential modulation with several bits, e.g. differential pulse code modulation [DPCM]
Abstract
A SEISMIC EXPLORATION SYSTEM AND AN
ANALOG-TO-DIGITAL CONVERTER FOR USE THEREIN
Abstract An analog-to-digital converter especially suited for seismic data recording applications operates using the differential pulse code modulation method. A digital linear predictor 40 is used to generate a digital predicted input signal, which is converted in a digital-to-analog converter 44 to an analog signal which is directly compared to the actual input signal 42. The error signal thus generated is converted to a digital error signal for addition to the predicted signal prior to transmission to means for recording the output signals, and is used for updating of the predicted signal according to the linear predictor 40. Quantization noise is reduced by highly oversampling the input signal, and a low pass filter 46 smooths the digital output signals.
ANALOG-TO-DIGITAL CONVERTER FOR USE THEREIN
Abstract An analog-to-digital converter especially suited for seismic data recording applications operates using the differential pulse code modulation method. A digital linear predictor 40 is used to generate a digital predicted input signal, which is converted in a digital-to-analog converter 44 to an analog signal which is directly compared to the actual input signal 42. The error signal thus generated is converted to a digital error signal for addition to the predicted signal prior to transmission to means for recording the output signals, and is used for updating of the predicted signal according to the linear predictor 40. Quantization noise is reduced by highly oversampling the input signal, and a low pass filter 46 smooths the digital output signals.
Description
449~
A SEISMIC EXPLORATION SYSTEM AND AN
ANALOG-TO-DIGITAL CONVERTER FOR USE THEREIN
This invention relates to a seismic exploration system and an analog-to-digital converter or use therein.
- For many years it has been common to explore for oil, gas and other valuable minerals using seismic techniques. seismic energy is imparted to the earth by, for example, detonating a "shot" of dynamite in a hole on the earth's surface or by imparting a mechanical vibration to the earth. The wave reflects from interfaces in the earth's crust, and is detected by detectors spaced some distance from the point at which the seismic energy is imparted to the earth. The signals output by the detectors are recorded. By measuring the time taken by the waves to travel over plural paths to plural detectors, conclusions can be reached about the shape of interfaces separating varying rock layers in the earth's subsurface formation. From analyses of these interfaces, likely locations for deposits of oil, gas and other valuable minerals can be identified.
A perennial problem in the accurate measurement of the time taken by the waves is recording the signals with a sufficiently good signal-to~noise ratio to enable the received waves to be reliably distinguished from noise occurring in the earth and generated by the exploration process itself. In particular, when marine seismic exploration is performed, acoustic microphones, referred to hereinafter as "hydrophores", are trailed behind a seismic exploration vessel. The vessel includes means for imparting an acoustic wave to the ocean, which then travels through the ocean and into the sea bed. The wave is reflected from the interfaces between the rock layers forming the sea bed, back up to the hydrophores streamed behind the exploration vessel.
The return of the reflected wave is detected by the hydrophores, which typically output an analog voltage signal. Typically, the signals from the hydrophores are sampled and a digital representation of the instantaneous amplitude is stored for later analysis.
~204~4 The analysis of seismic signals is growing increasingly sophisticated and it is clearly desirable to provide as accurately recorded signals as possible. To this end, there has for some time been a need in the art to eliminate extraneous signals, or "noise", from the recorded digital samples. The term "noise" is used in geophysical research to indicate any unwanted signal; this can be clarified by noting that such unwanted signals can be generated by numerous sources.
For example, in marine seismic exploration acoustic waves generated my other vessels in the vicinity of exploration, or by turbulence generated by the streamer itself, contribute "noise". Imprecise electronic components and inaccurate digitization processes add "noise". Signal degradation also occurs during transmission of the signals from the hydrophores up the "streamer" cable to the exploration vessel for recording, effectively reducing the signal-to-noise ratio. It would obviously be desirable to improve the signal-to-noise ratio of such marine seismic explorations by any means possible so as to allow better identification of geologically significant events in the seismic record.
Those skilled in the art will recognize that not all of these classes of noise can be removed from the seismic record using a single technique. For example, the coherent noise generated by other vessels in the exploration region or by turbulence can be eliminated by mathematical filtering, but this has less effect on incoherent noise, such as that added to the seismic record by inaccurate digitization.
The present invention addresses the problem of "noise" caused by inaccurate digitization processes.
One prior art analog-to-digital conversion technique which has been employed in seismic applications uses analog devices known as galn-ranging amplifiers in the signal path. Such amplifiers are capable of amplifying both small and large signals by varying ratios so that quite small signals can be effectively digitized; that is, such gain-ranging amplifiers have good dynamic range characteristics.
However, the gain of the amplifier with respect to any particular input signal amplitude is fixed and hence if a very small signal is superimposed on a very large signal, the small signal will not be 4~94 amplified adequately to be digitized and will be lost. Accordingly, such gain-ranging amplifiers, lacking adequate "resolution", introduce substantial distortion into certain classes of input signals by randomly truncating small signal values. Furthermore, gain-ranging amplifiers are nonlinear analog devices which introduce distortion of the absolute amplitude of the input signal which is highly undesirable.
It has been determined that no presently available analog-to-digital converter, including the gain-ranging amplifier types discussed above, is available in the prior art having an adequate signal-to-noise ratio, while providing adequate resolution of signals of widely varying amplitudes, to fully utilize present day data processing capabilities. It would be desirable to accurately record signals of up to approximately 120 dub level difference for seismic analysis. The present invention is designed to provide such dynamic range in encoding analog signals, while providing accurate resolution of a small signal superimposed on larger signal values. In particular, it minimizes the "quantization noise" introduced by all analog-to-digital conversions, and avoids distortion caused by use of a nonlinear analog device such as a gain-ranging amplifier.
Accordingly, the invention resides in one aspect in a seismic exploration system comprising:
a source of seismic energy;
a detector of reflected seismic energy comprising means for outputting a detected analog signal;
means for converting the detected analog signal to a digital signal; and means for recording the digital signal;
wherein the means for conversion of the detected analog signal to a digital signal comprises:
means for predicting an expected input signal and for generating a digital representation thereof;
digital-to-analog conversion means for conversion of the digital output of the predicting means to a predicted analog signal;
12~4~94 F~1946 I
means for comparing the output of the digital-to-analog conversion means to the detected analog signal;
means for conversion of the output of the means for comparing to a digital difference signal representing the difference between the detected analog signal and the predicted analog signal;
means for adding the digital difference signal to the digital predicted value; and means for transmission of the sum of the digital difference signal and the digital predicted value to the recording means.
In a further aspect, the invention resides in an analog-to-digital converter for use in a seismic exploration system comprising:
means for generating a predicted digital input signal;
means for conversion of the predicted digital input signal to an analog representation thereof;
means for comparing the predicted analog input signal with an actual analog input signal, and for generating an analog error signal representing the difference there between;
means for converting the analog error signal to a digital error signal representative thereof;
means for summing the digital error signal and the digital predicted signal;
means for transmission of the sum of the digital error signal and the digital predicted signal to means for recording the summed digital signal; and means for updating the predicted analog input signal based on the sum of the digital error signal and the digital predicted signal.
In a preferred embodiment of the invention, over sampling is applied to the relatively low frequency seismic signals so as to provide many more samples than would strictly speaking be necessary for representation of the simple seismic waveform. Summing over the many samples provides a smoothing effect to the data and thus provides further accuracy. It also enables the predicted values to be ~04~
synthesized with greater accuracy, such that the error signals are smaller, and may be represented using smaller digital words for a given level of accuracy. Alternatively, better encoding accuracy is achieved with a word of a given length used to digitize the smaller error signals. Either approach results in reduced quantization noise.
In the accompanying drawings which illustrate one example of the invention, Figure 1 shows a marine seismic exploration system in a schematic form;
Figure 2 is a block diagram form of the analog-to-digital converter of the system; and Figure 3 is a block diagram of a hardware implementation of the circuit shown in Figure 2.
Referring to the drawings, Figure 1 shows a marine seismic exploration system in which an exploration vessel 10 tows behind it a streamer cable 12 comprising a plurality of hydrophores 14. A source of seismic energy 16 on the vessel, which may be a compressed air gun or the like, transmits seismic energy down various ray paths 18 to be reflected at the ocean bottom 20 or from an interface 22 between varying rock layers of the subset bed and reflected back upwardly along differing ray paths 24 to be received by hydrophores 14. The analog signals received by the hydrophores 14 are converted into digital signals by analog-to-digital conversion means 26 prior to being recorded on recording device 28.
The present system uses the analog-to-digital conversion technique known as differential pulse code modulation to convert thy analog signals output by the hydrophores 14 into digital signals.
According to this technique a prediction operator is used to compute a predicted signal which effectively models the anticipated seismic analog signals to be received. The actual analog signal received is then compared with the predicted signals at regular intervals; the difference between the two or the "error" signal is digitized and added to the digital predicted value for transmission.
~21~4~
The predicted value is continually updated using the past predicted values and the error value. This coding method offers substantial advantages with respect to the signal-to-noise ratio ox the conversion and the resolution which can be achieved in the process.
Thus, the dynamic range of the signals which can effectively be recorded is broadened. Differential pulse code modulation is particularly effective for seismic exploration, where the signal to be encoded is a relatively accurately predictable signal such as a low frequency sinusoid. This allows the predicted signal to match up fairly accurately with the received signal so that the error is relatively small. As noted, a relatively small error value can be accurately digitized in a relatively small number of digital bits, thus effectively reducing the quantization noise introduced in any analog-to-digital conversion. By comparison, a large error signal is less accurately digitized in a digital word of a given size; to accurately digitize a large value, e.g. a larger error signal or the actual analog value, a longer digital word must be generated all with attendant complexity and expense.
As shown in Figure 2, the differential pulse code modulation system includes a dif~erencing junction 30 at which the continuous analog input signal arriving from the hydrophores 14 along input line 42 is compared at intervals with a predicted analog signal generated by a digital-to-analog converter 44 based upon a digital predicted value output by linear predictor 40. Accordingly, the output of the differencing junction 30 is the analog difference between the actual analog signal and the predicted analog signal, that is, the analog error signal. The analog error signal is then passed to an amplifier 32 for scaling and converted back to a digital signal in an analog-to- digital converter 34. It will be appreciated that this analog- to-digital converter 34 operates on the error signal which has a much smaller dynamic range than the analog input signal. Accordingly, the analog-to-digital converter 34 need not be capable of the wide dynamic range of the entire system shown in Figure 2. For a given level of accuracy of digitization, the digital words output by converter 34 may .
4~914 thus be shorter, i.e. comprise fewer bits, than the digital words output by the digital predictor 40, which correspond to the actual input signal. Accordingly, the signal output by the converter 34 is reformatted as necessary in a formatter 36, in which the value of its least significant bit is scaled to match that of the longer word output by predictor 40, before being added thereto in junction 38 and transmitted. Successive sums of the predicted value and digitized error signal are then digitally filtered in a low pass filter 46 and sub sampled by a sampling unit 48 before becoming the output signal, which would comprise digital "words" representing the instantaneous value at the sampling time. The operation of the low pass filter and sub sampler Jill be discussed in further detain below. ye sum of the predicted value and the error signal also goes to the linear predictor 40 for updating ox the predicted value in accordance with the error signal, as noted. In this way the predicted signal is continually varied in accordance with the actual signal so as to constantly provide an updated and hence increasingly accurate prediction. This cycle is then repeated: the output of the prediction operator is transformed into an analog signal in digital-to-analog converter 44, is passed to the differencing junction 30 where the analog signal is again compared thereto; the analog difference is digitized, added to the predicted value for transmission and for use by the linear predictor to generate the next predicted value.
As discussed above, it is desirable that the error signal be as small as possible, so that for a digital horn of given length output by the analog-to-digital converter 34, the error signal can be most accurately represented. In order that this be the case, it is Obviously necessary that the predicted value output by the prediction operator equal the input signal as accurately as possible. This, in turn, requires that the sampling rate of the input signal used to update the linear prediction operator be chosen so as to ensure that an adequate number of samples are generated. In theory, any analog signal can be adequately encoded if it is accurately sampled at twice the highest frequency signal making up the analog signal, i.e., at the Nyquist ~204~94~
rate. In seismic exploration, the input analog signals are typically band limited to approximately 125 Ho. Accordingly, Nyquist's theorem would indicate that a 250 Ho sampling rate would be adequate. It will be appreciated that this is far below the abilities of modern electronics. Preferably, the sampling rate is raised to much higher than the Nyquist rate. This "over sampling" which may occur at e.g., 6û
kHz, increases correlation between samples which enables better prediction. The remaining noise can be reduced with respect to the signal by low pass filtering (smoothing). Quantization noise is thus reduced, as the least significant bit of the digitized error signal is smaller. Low pass filter 46 smooths the output signal; this in conjunction with sub sampler 48 which ensures that the output signals -which would typically comprise digital words, output at or near the Nyquist frequency - accurately represent the received analog signals.
It will be appreciated by those skilled in the art that the input signal can only be adequately represented by the predicted signal if the bandwidth of the loop is wide enough to encompass the fastest signal swings undergone by the input signal. In order that this can be so in all cases, it may be desirable in some cases to incorporate an integrator 34 and a clipper 50 in the input path. The output of integrator 34 will change with respect to the rate of change of the amplitude of the analog input signal. If a clipper 50 is used, the slope ox the output of integrator 34 can be limited to the loop's bandwidth. To ensure that the output signals still represent the integrated analog input signals accurately, a differentiator 51 should be interposed in the output signal path.
Figure 3 shows a schematic diagram of a circuit which could perform the functions shown in Figure 2. Input signal K is input to a low noise amplifier 52 which performs the function of comparison of the input signal with a predicted signal v output by an 18-bit digital-to-analog converter 54. The analog output error signal f is input to a 12-bit analog-to-digital converter 56 which outputs a Betty digital error signal. This is reformatted in a Betty output ROM 58 so as to scale the least significant bit of the 12-bit error signal to ~0449~
equal that of the 32-bit predicted input signal. As well understood by those skilled in the art, the 12-bit error signal serves as the address to the ROM 58, and the data stored in the ROM 58 corresponds to the 32-bit version of the error signal. This 32-bit error signal is one input to an adder 60, the other input of which is an 18-bit truncated version (the 14 least significant bits being lost) of the 32-bit predicted signal. The output of the adder 60 is the sum of the 32-bit error signal and the truncated 18-bit version of the 32-bit output signal. This signal is then input to a first shift register 62 and becomes one of three 32-bit inputs to a sign multiplier with accumulator 64 which performs the function of the linear predictor 4û Chilean in Figure 2. In successive clock cycles of the circuit shown in Figure 3, the 32-bit value in shift register 62 is successively passed to shift registers 66 and 68, so that each 32-bit value is input three times to the sign multiplier and accumulator 64. This is also supplied with weighting values corresponding to the linear predictor designed into the circuit by three ROMs 7û9 72, 74. The 32-bit output signal, which is the predicted value of the analog input signal, is passed to a formatter 76 where it is truncated to provide the 18-bit input to the labia digital-to-analog converter 54 and to the 32-bit adder 60. Truncation is performed because current single chip digital-to-analog converters are not available which operate on 32-bit input signals.
The output of the 32-bit adder 6û forms the output ox the circuit, which is operated upon by digital filter and sub samplers 46 and 48 shown in Figure 2 to provide digital words forming the output of the circuit.
The key hardware items of the diagram shown in Figure 3 are the digital-to-analog converter 54 and the analog-to~digital converter 56.
Adequate devices are presently available in the market. An embodiment of the invention corresponding to Figure 3 was simulator tested by computer operation. The specifications ox an 18-bit digital-to-analog converter made by Analog devices and sold under model number 1138K were used in the simulation for digital-to-analog converter 54. Similarly, the hypothetical analog-to-digital converter 56 used in the simulation 120~494 was Analog Devices' Model 110~-00~ converter, which outputs a 12-bit digital representation of the error signal.
The linear predictor 40 used in the simulation was a 1 to 3 coefficient linear predictor, envisioned to be implemented with 32-bit fixed point hardware. The coefficients can be based on a minimum mean square prediction error criterion using a band limited flat input signal model. The design of such linear predictors is fully discussed in Rabiner & Schafer, "Digital Processing of Speech Signals," Prentice Hall, (1978). In the simulated embodiment, the coder operated at a frequency of 60 kHz. This extremely high sampling rate compared to the signal bandwidth of O to 1~0 Ho resulted in an adjacent sample auto-correlation of 0.99998~. An alternative would be to use a single coefficient predictor with dithering to ensure non correlation of the error values.
The predictor output was a 32-bit prediction in the embodiment simulated, which was truncated as noted to 18 bits for input to the digital-to-analog converter 54. Similarly, the 12-bit error signal must be reformatted to 32 bits before being input to the linear predictor.
As suggested, such reformatting could be readily accomplished using ROMs, with the 12-bit error signal serving as the address, and the 32-bit input to the predictor being the data. Alternatively, a digital multiplier could perform the reformatting function; the multiplication ratio of the reformatting operation would depend on the ratio of the values of the least significant bits of the output of the 32-bit predictor 64 and the 12-bit analog-to-digital converter 56.
The low pass filter I in the simulated embodiment was a two-stage finite impulse response non-recursive digital filter as described in Rabiner and Gold, "Theory and Application of Digital Signal Processing", Prentice Hall, (1975). Filter parameters tested were as follows. First stage N = filter order = 1001, Us = sampling rate - 60 kHz, Fc = break frequency = 1200 Ho. Second stage, N = 501, Fc =
~000 Ho, Fc = 150 Ho.
The simulation results indicated that the total signal to root-mean-square quantization noise ratio equals or exceeds 126 dub for seismic band signals.
A SEISMIC EXPLORATION SYSTEM AND AN
ANALOG-TO-DIGITAL CONVERTER FOR USE THEREIN
This invention relates to a seismic exploration system and an analog-to-digital converter or use therein.
- For many years it has been common to explore for oil, gas and other valuable minerals using seismic techniques. seismic energy is imparted to the earth by, for example, detonating a "shot" of dynamite in a hole on the earth's surface or by imparting a mechanical vibration to the earth. The wave reflects from interfaces in the earth's crust, and is detected by detectors spaced some distance from the point at which the seismic energy is imparted to the earth. The signals output by the detectors are recorded. By measuring the time taken by the waves to travel over plural paths to plural detectors, conclusions can be reached about the shape of interfaces separating varying rock layers in the earth's subsurface formation. From analyses of these interfaces, likely locations for deposits of oil, gas and other valuable minerals can be identified.
A perennial problem in the accurate measurement of the time taken by the waves is recording the signals with a sufficiently good signal-to~noise ratio to enable the received waves to be reliably distinguished from noise occurring in the earth and generated by the exploration process itself. In particular, when marine seismic exploration is performed, acoustic microphones, referred to hereinafter as "hydrophores", are trailed behind a seismic exploration vessel. The vessel includes means for imparting an acoustic wave to the ocean, which then travels through the ocean and into the sea bed. The wave is reflected from the interfaces between the rock layers forming the sea bed, back up to the hydrophores streamed behind the exploration vessel.
The return of the reflected wave is detected by the hydrophores, which typically output an analog voltage signal. Typically, the signals from the hydrophores are sampled and a digital representation of the instantaneous amplitude is stored for later analysis.
~204~4 The analysis of seismic signals is growing increasingly sophisticated and it is clearly desirable to provide as accurately recorded signals as possible. To this end, there has for some time been a need in the art to eliminate extraneous signals, or "noise", from the recorded digital samples. The term "noise" is used in geophysical research to indicate any unwanted signal; this can be clarified by noting that such unwanted signals can be generated by numerous sources.
For example, in marine seismic exploration acoustic waves generated my other vessels in the vicinity of exploration, or by turbulence generated by the streamer itself, contribute "noise". Imprecise electronic components and inaccurate digitization processes add "noise". Signal degradation also occurs during transmission of the signals from the hydrophores up the "streamer" cable to the exploration vessel for recording, effectively reducing the signal-to-noise ratio. It would obviously be desirable to improve the signal-to-noise ratio of such marine seismic explorations by any means possible so as to allow better identification of geologically significant events in the seismic record.
Those skilled in the art will recognize that not all of these classes of noise can be removed from the seismic record using a single technique. For example, the coherent noise generated by other vessels in the exploration region or by turbulence can be eliminated by mathematical filtering, but this has less effect on incoherent noise, such as that added to the seismic record by inaccurate digitization.
The present invention addresses the problem of "noise" caused by inaccurate digitization processes.
One prior art analog-to-digital conversion technique which has been employed in seismic applications uses analog devices known as galn-ranging amplifiers in the signal path. Such amplifiers are capable of amplifying both small and large signals by varying ratios so that quite small signals can be effectively digitized; that is, such gain-ranging amplifiers have good dynamic range characteristics.
However, the gain of the amplifier with respect to any particular input signal amplitude is fixed and hence if a very small signal is superimposed on a very large signal, the small signal will not be 4~94 amplified adequately to be digitized and will be lost. Accordingly, such gain-ranging amplifiers, lacking adequate "resolution", introduce substantial distortion into certain classes of input signals by randomly truncating small signal values. Furthermore, gain-ranging amplifiers are nonlinear analog devices which introduce distortion of the absolute amplitude of the input signal which is highly undesirable.
It has been determined that no presently available analog-to-digital converter, including the gain-ranging amplifier types discussed above, is available in the prior art having an adequate signal-to-noise ratio, while providing adequate resolution of signals of widely varying amplitudes, to fully utilize present day data processing capabilities. It would be desirable to accurately record signals of up to approximately 120 dub level difference for seismic analysis. The present invention is designed to provide such dynamic range in encoding analog signals, while providing accurate resolution of a small signal superimposed on larger signal values. In particular, it minimizes the "quantization noise" introduced by all analog-to-digital conversions, and avoids distortion caused by use of a nonlinear analog device such as a gain-ranging amplifier.
Accordingly, the invention resides in one aspect in a seismic exploration system comprising:
a source of seismic energy;
a detector of reflected seismic energy comprising means for outputting a detected analog signal;
means for converting the detected analog signal to a digital signal; and means for recording the digital signal;
wherein the means for conversion of the detected analog signal to a digital signal comprises:
means for predicting an expected input signal and for generating a digital representation thereof;
digital-to-analog conversion means for conversion of the digital output of the predicting means to a predicted analog signal;
12~4~94 F~1946 I
means for comparing the output of the digital-to-analog conversion means to the detected analog signal;
means for conversion of the output of the means for comparing to a digital difference signal representing the difference between the detected analog signal and the predicted analog signal;
means for adding the digital difference signal to the digital predicted value; and means for transmission of the sum of the digital difference signal and the digital predicted value to the recording means.
In a further aspect, the invention resides in an analog-to-digital converter for use in a seismic exploration system comprising:
means for generating a predicted digital input signal;
means for conversion of the predicted digital input signal to an analog representation thereof;
means for comparing the predicted analog input signal with an actual analog input signal, and for generating an analog error signal representing the difference there between;
means for converting the analog error signal to a digital error signal representative thereof;
means for summing the digital error signal and the digital predicted signal;
means for transmission of the sum of the digital error signal and the digital predicted signal to means for recording the summed digital signal; and means for updating the predicted analog input signal based on the sum of the digital error signal and the digital predicted signal.
In a preferred embodiment of the invention, over sampling is applied to the relatively low frequency seismic signals so as to provide many more samples than would strictly speaking be necessary for representation of the simple seismic waveform. Summing over the many samples provides a smoothing effect to the data and thus provides further accuracy. It also enables the predicted values to be ~04~
synthesized with greater accuracy, such that the error signals are smaller, and may be represented using smaller digital words for a given level of accuracy. Alternatively, better encoding accuracy is achieved with a word of a given length used to digitize the smaller error signals. Either approach results in reduced quantization noise.
In the accompanying drawings which illustrate one example of the invention, Figure 1 shows a marine seismic exploration system in a schematic form;
Figure 2 is a block diagram form of the analog-to-digital converter of the system; and Figure 3 is a block diagram of a hardware implementation of the circuit shown in Figure 2.
Referring to the drawings, Figure 1 shows a marine seismic exploration system in which an exploration vessel 10 tows behind it a streamer cable 12 comprising a plurality of hydrophores 14. A source of seismic energy 16 on the vessel, which may be a compressed air gun or the like, transmits seismic energy down various ray paths 18 to be reflected at the ocean bottom 20 or from an interface 22 between varying rock layers of the subset bed and reflected back upwardly along differing ray paths 24 to be received by hydrophores 14. The analog signals received by the hydrophores 14 are converted into digital signals by analog-to-digital conversion means 26 prior to being recorded on recording device 28.
The present system uses the analog-to-digital conversion technique known as differential pulse code modulation to convert thy analog signals output by the hydrophores 14 into digital signals.
According to this technique a prediction operator is used to compute a predicted signal which effectively models the anticipated seismic analog signals to be received. The actual analog signal received is then compared with the predicted signals at regular intervals; the difference between the two or the "error" signal is digitized and added to the digital predicted value for transmission.
~21~4~
The predicted value is continually updated using the past predicted values and the error value. This coding method offers substantial advantages with respect to the signal-to-noise ratio ox the conversion and the resolution which can be achieved in the process.
Thus, the dynamic range of the signals which can effectively be recorded is broadened. Differential pulse code modulation is particularly effective for seismic exploration, where the signal to be encoded is a relatively accurately predictable signal such as a low frequency sinusoid. This allows the predicted signal to match up fairly accurately with the received signal so that the error is relatively small. As noted, a relatively small error value can be accurately digitized in a relatively small number of digital bits, thus effectively reducing the quantization noise introduced in any analog-to-digital conversion. By comparison, a large error signal is less accurately digitized in a digital word of a given size; to accurately digitize a large value, e.g. a larger error signal or the actual analog value, a longer digital word must be generated all with attendant complexity and expense.
As shown in Figure 2, the differential pulse code modulation system includes a dif~erencing junction 30 at which the continuous analog input signal arriving from the hydrophores 14 along input line 42 is compared at intervals with a predicted analog signal generated by a digital-to-analog converter 44 based upon a digital predicted value output by linear predictor 40. Accordingly, the output of the differencing junction 30 is the analog difference between the actual analog signal and the predicted analog signal, that is, the analog error signal. The analog error signal is then passed to an amplifier 32 for scaling and converted back to a digital signal in an analog-to- digital converter 34. It will be appreciated that this analog- to-digital converter 34 operates on the error signal which has a much smaller dynamic range than the analog input signal. Accordingly, the analog-to-digital converter 34 need not be capable of the wide dynamic range of the entire system shown in Figure 2. For a given level of accuracy of digitization, the digital words output by converter 34 may .
4~914 thus be shorter, i.e. comprise fewer bits, than the digital words output by the digital predictor 40, which correspond to the actual input signal. Accordingly, the signal output by the converter 34 is reformatted as necessary in a formatter 36, in which the value of its least significant bit is scaled to match that of the longer word output by predictor 40, before being added thereto in junction 38 and transmitted. Successive sums of the predicted value and digitized error signal are then digitally filtered in a low pass filter 46 and sub sampled by a sampling unit 48 before becoming the output signal, which would comprise digital "words" representing the instantaneous value at the sampling time. The operation of the low pass filter and sub sampler Jill be discussed in further detain below. ye sum of the predicted value and the error signal also goes to the linear predictor 40 for updating ox the predicted value in accordance with the error signal, as noted. In this way the predicted signal is continually varied in accordance with the actual signal so as to constantly provide an updated and hence increasingly accurate prediction. This cycle is then repeated: the output of the prediction operator is transformed into an analog signal in digital-to-analog converter 44, is passed to the differencing junction 30 where the analog signal is again compared thereto; the analog difference is digitized, added to the predicted value for transmission and for use by the linear predictor to generate the next predicted value.
As discussed above, it is desirable that the error signal be as small as possible, so that for a digital horn of given length output by the analog-to-digital converter 34, the error signal can be most accurately represented. In order that this be the case, it is Obviously necessary that the predicted value output by the prediction operator equal the input signal as accurately as possible. This, in turn, requires that the sampling rate of the input signal used to update the linear prediction operator be chosen so as to ensure that an adequate number of samples are generated. In theory, any analog signal can be adequately encoded if it is accurately sampled at twice the highest frequency signal making up the analog signal, i.e., at the Nyquist ~204~94~
rate. In seismic exploration, the input analog signals are typically band limited to approximately 125 Ho. Accordingly, Nyquist's theorem would indicate that a 250 Ho sampling rate would be adequate. It will be appreciated that this is far below the abilities of modern electronics. Preferably, the sampling rate is raised to much higher than the Nyquist rate. This "over sampling" which may occur at e.g., 6û
kHz, increases correlation between samples which enables better prediction. The remaining noise can be reduced with respect to the signal by low pass filtering (smoothing). Quantization noise is thus reduced, as the least significant bit of the digitized error signal is smaller. Low pass filter 46 smooths the output signal; this in conjunction with sub sampler 48 which ensures that the output signals -which would typically comprise digital words, output at or near the Nyquist frequency - accurately represent the received analog signals.
It will be appreciated by those skilled in the art that the input signal can only be adequately represented by the predicted signal if the bandwidth of the loop is wide enough to encompass the fastest signal swings undergone by the input signal. In order that this can be so in all cases, it may be desirable in some cases to incorporate an integrator 34 and a clipper 50 in the input path. The output of integrator 34 will change with respect to the rate of change of the amplitude of the analog input signal. If a clipper 50 is used, the slope ox the output of integrator 34 can be limited to the loop's bandwidth. To ensure that the output signals still represent the integrated analog input signals accurately, a differentiator 51 should be interposed in the output signal path.
Figure 3 shows a schematic diagram of a circuit which could perform the functions shown in Figure 2. Input signal K is input to a low noise amplifier 52 which performs the function of comparison of the input signal with a predicted signal v output by an 18-bit digital-to-analog converter 54. The analog output error signal f is input to a 12-bit analog-to-digital converter 56 which outputs a Betty digital error signal. This is reformatted in a Betty output ROM 58 so as to scale the least significant bit of the 12-bit error signal to ~0449~
equal that of the 32-bit predicted input signal. As well understood by those skilled in the art, the 12-bit error signal serves as the address to the ROM 58, and the data stored in the ROM 58 corresponds to the 32-bit version of the error signal. This 32-bit error signal is one input to an adder 60, the other input of which is an 18-bit truncated version (the 14 least significant bits being lost) of the 32-bit predicted signal. The output of the adder 60 is the sum of the 32-bit error signal and the truncated 18-bit version of the 32-bit output signal. This signal is then input to a first shift register 62 and becomes one of three 32-bit inputs to a sign multiplier with accumulator 64 which performs the function of the linear predictor 4û Chilean in Figure 2. In successive clock cycles of the circuit shown in Figure 3, the 32-bit value in shift register 62 is successively passed to shift registers 66 and 68, so that each 32-bit value is input three times to the sign multiplier and accumulator 64. This is also supplied with weighting values corresponding to the linear predictor designed into the circuit by three ROMs 7û9 72, 74. The 32-bit output signal, which is the predicted value of the analog input signal, is passed to a formatter 76 where it is truncated to provide the 18-bit input to the labia digital-to-analog converter 54 and to the 32-bit adder 60. Truncation is performed because current single chip digital-to-analog converters are not available which operate on 32-bit input signals.
The output of the 32-bit adder 6û forms the output ox the circuit, which is operated upon by digital filter and sub samplers 46 and 48 shown in Figure 2 to provide digital words forming the output of the circuit.
The key hardware items of the diagram shown in Figure 3 are the digital-to-analog converter 54 and the analog-to~digital converter 56.
Adequate devices are presently available in the market. An embodiment of the invention corresponding to Figure 3 was simulator tested by computer operation. The specifications ox an 18-bit digital-to-analog converter made by Analog devices and sold under model number 1138K were used in the simulation for digital-to-analog converter 54. Similarly, the hypothetical analog-to-digital converter 56 used in the simulation 120~494 was Analog Devices' Model 110~-00~ converter, which outputs a 12-bit digital representation of the error signal.
The linear predictor 40 used in the simulation was a 1 to 3 coefficient linear predictor, envisioned to be implemented with 32-bit fixed point hardware. The coefficients can be based on a minimum mean square prediction error criterion using a band limited flat input signal model. The design of such linear predictors is fully discussed in Rabiner & Schafer, "Digital Processing of Speech Signals," Prentice Hall, (1978). In the simulated embodiment, the coder operated at a frequency of 60 kHz. This extremely high sampling rate compared to the signal bandwidth of O to 1~0 Ho resulted in an adjacent sample auto-correlation of 0.99998~. An alternative would be to use a single coefficient predictor with dithering to ensure non correlation of the error values.
The predictor output was a 32-bit prediction in the embodiment simulated, which was truncated as noted to 18 bits for input to the digital-to-analog converter 54. Similarly, the 12-bit error signal must be reformatted to 32 bits before being input to the linear predictor.
As suggested, such reformatting could be readily accomplished using ROMs, with the 12-bit error signal serving as the address, and the 32-bit input to the predictor being the data. Alternatively, a digital multiplier could perform the reformatting function; the multiplication ratio of the reformatting operation would depend on the ratio of the values of the least significant bits of the output of the 32-bit predictor 64 and the 12-bit analog-to-digital converter 56.
The low pass filter I in the simulated embodiment was a two-stage finite impulse response non-recursive digital filter as described in Rabiner and Gold, "Theory and Application of Digital Signal Processing", Prentice Hall, (1975). Filter parameters tested were as follows. First stage N = filter order = 1001, Us = sampling rate - 60 kHz, Fc = break frequency = 1200 Ho. Second stage, N = 501, Fc =
~000 Ho, Fc = 150 Ho.
The simulation results indicated that the total signal to root-mean-square quantization noise ratio equals or exceeds 126 dub for seismic band signals.
Claims (10)
1. A seismic exploration system comprising:
a source of seismic energy;
a detector of reflected seismic energy, comprising means for outputting a detected analog signal;
means for converting the detected analog signal to a digital signal; and means for recording the digital signal;
wherein the means for conversion of the detected analog signal to a digital signal comprises:
means for predicting an expected input signal and for generating a digital representation thereof;
digital-to-analog conversion means for conversion of the digital output of the predicting means to a predicted analog signal;
means for comparing the output of the digital-to-analog conversion means to the detected analog signal;
means for conversion of the output of the means for comparing to a digital difference signal representing the difference between the detected analog signal and the predicted analog signal;
means for adding the digital difference signal to the digital predicted value; and means for transmission of the sum of the digital difference signal and the digital predicted value to the recording means.
a source of seismic energy;
a detector of reflected seismic energy, comprising means for outputting a detected analog signal;
means for converting the detected analog signal to a digital signal; and means for recording the digital signal;
wherein the means for conversion of the detected analog signal to a digital signal comprises:
means for predicting an expected input signal and for generating a digital representation thereof;
digital-to-analog conversion means for conversion of the digital output of the predicting means to a predicted analog signal;
means for comparing the output of the digital-to-analog conversion means to the detected analog signal;
means for conversion of the output of the means for comparing to a digital difference signal representing the difference between the detected analog signal and the predicted analog signal;
means for adding the digital difference signal to the digital predicted value; and means for transmission of the sum of the digital difference signal and the digital predicted value to the recording means.
2. The system of claim 1 further comprising low pass filter means for filtering the output summed signals.
3. The system of claim 1 further comprising means for clipping the amplitude of the analog signals input to the means for comparing, means for integrating the clipped analog signal prior to being input to the means for comparing, and means for differentiating the output summed signals.
4. The system of claim 1 wherein the detected analog input signals are sampled at a frequency greater than the maximum frequency of the seismic input signals.
5. A analog-to-digital converter for use in a seismic exploration system comprising:
means for generating a predicted digital input signal;
means for conversion of the predicted digital input signal to an analog representation thereof;
means for comparing the predicted analog input signal with an actual analog input signal, and for generating an analog error signal representing the difference therebetween;
means for converting the analog error signal to a digital error signal representative thereof;
means for summing the digital error signal and the digital predicted signal;
means for transmission of the sum of the digital error signal and the digital predicted signal to means for recording the summed digital signal; and means for updating the predicted analog input signal based on the sum of the digital error signal and the digital predicted signal.
means for generating a predicted digital input signal;
means for conversion of the predicted digital input signal to an analog representation thereof;
means for comparing the predicted analog input signal with an actual analog input signal, and for generating an analog error signal representing the difference therebetween;
means for converting the analog error signal to a digital error signal representative thereof;
means for summing the digital error signal and the digital predicted signal;
means for transmission of the sum of the digital error signal and the digital predicted signal to means for recording the summed digital signal; and means for updating the predicted analog input signal based on the sum of the digital error signal and the digital predicted signal.
6. The analog-to-digital converter of claim 5 wherein the means for generating a predicted analog signal comprises means for outputting a digital predicted signal and digital to-analog conversion means for conversion of the digital predicted signal to analog form prior to comparison with the actual analog input signal.
7. The converter of claim 5 wherein the analog input signal is a continuous analog signal and the comparison is performed at a frequency greater than the maximum frequency of the signal measured, and the converter further comprises low pass filter means for smoothing the digital output signals generated thereby.
8. The converter of claim 5 further comprising means for limiting the rate of change of the analog input signal.
9. The converter of claim 8 wherein the means for limiting the rate of change of the input signals comprises clipper means for limiting the amplitude of input signals and integrator means for providing analog input signals to the means for comparison with the predicted analog signals.
10. the converter of claim 9 wherein differentiation means are provided in the path of the summed digital signals output by the analog-to-digital converter.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US44349182A | 1982-11-22 | 1982-11-22 | |
| US443,491 | 1982-11-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1204494A true CA1204494A (en) | 1986-05-13 |
Family
ID=23760992
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000440326A Expired CA1204494A (en) | 1982-11-22 | 1983-11-03 | Seismic exploration system and an analog-to-digital converter for use therein |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA1204494A (en) |
| GB (1) | GB2130829B (en) |
| NO (1) | NO834212L (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS6354827A (en) * | 1986-08-26 | 1988-03-09 | Sony Corp | Differential pcm signal generating circuit |
| GB2202702A (en) * | 1987-03-27 | 1988-09-28 | Philips Electronic Associated | Analogue to digital converter |
| US5266952A (en) * | 1992-03-30 | 1993-11-30 | Hughes Aircraft Company | Feed forward predictive analog-to-digital converter |
| JP3268624B2 (en) * | 1995-09-29 | 2002-03-25 | 株式会社ケンウッド | Quantization distortion reduction device |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4308524A (en) * | 1979-06-05 | 1981-12-29 | Harrison Systems, Inc. | Fast high resolution predictive analog-to-digital converter with error correction |
| NL8005950A (en) * | 1980-10-30 | 1982-05-17 | Philips Nv | DIFFERENTIAL PULSE CODE MODULATION TRANSFER SYSTEM. |
-
1983
- 1983-11-03 CA CA000440326A patent/CA1204494A/en not_active Expired
- 1983-11-03 GB GB08329414A patent/GB2130829B/en not_active Expired
- 1983-11-16 NO NO834212A patent/NO834212L/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| GB2130829B (en) | 1986-11-19 |
| NO834212L (en) | 1984-05-23 |
| GB2130829A (en) | 1984-06-06 |
| GB8329414D0 (en) | 1983-12-07 |
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