CA1187595A - Method and apparatus for seismic exploration using non-linear sweeps - Google Patents
Method and apparatus for seismic exploration using non-linear sweepsInfo
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- CA1187595A CA1187595A CA000412545A CA412545A CA1187595A CA 1187595 A CA1187595 A CA 1187595A CA 000412545 A CA000412545 A CA 000412545A CA 412545 A CA412545 A CA 412545A CA 1187595 A CA1187595 A CA 1187595A
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Abstract
A B S T R A C T
The invention is for a method of seismic exploration involving the generation of a nonlinear signal of predetermined shape at the output of a sweep generator which comprises the steps of dividing the sweep time of the output signal into a plurality of preselected time intervals, calculating the rate of change of the frequency of the output signal for each time interval to realize the predetermined shape based on the range of the sweep, the sweep time, the starting frequency and the predetermined shape, and using each value in the calculating step to change the rate of change of frequency of the sweep signal, and the invention includes the apparatus specifically adapted to carry out the inventive method.
The invention is for a method of seismic exploration involving the generation of a nonlinear signal of predetermined shape at the output of a sweep generator which comprises the steps of dividing the sweep time of the output signal into a plurality of preselected time intervals, calculating the rate of change of the frequency of the output signal for each time interval to realize the predetermined shape based on the range of the sweep, the sweep time, the starting frequency and the predetermined shape, and using each value in the calculating step to change the rate of change of frequency of the sweep signal, and the invention includes the apparatus specifically adapted to carry out the inventive method.
Description
7S~i METHOD AND APPARATUS FOR SEISMIC
EXPLORATION USING NONLINEAR SWEEPS
The present invention relates to method and apparatus for seismic exploration, and, more particularly, to pro-viding a nonlinear sweep signal to a vibrator in a seismic system.
In seismic exploration, seismic waves are commonly used to probe the earth's crust as a means of determining the type and location of subsurace formations. The earth's crust can be considered a transmission medium or filter whose characteristics are to be determined by pass-ing seismic waves through that medium. In the reflection seismic method, seismic waves or impulses are generated at a point at or near the earth's surface, and the compres-sional mode of these waves is reflected from subsurface acoustic impedance boundaries and detected by arrays of seismic detectors located at the earth's crust. The seismic detectors convert the received waves into electri-cal signals which are sensed and recorded in a form which permits analysis~ Skilled interpreters can discern from such an analysis the shape and depth of subsurface refiec-tion boundaries and the likelihood of finding an accumula-tion of minerals, such as oil and gas.
Si95 Various sources of seismic energy have been utilized in the art to impart the seismic waves into the earth's crust. Such sources have included dynamite and weight-drop apparatus.
Another source of seismic energy is a vibrator which, ~hen energized, imparts relatively low level energy signals into the earth's crust. Typically, the impartation of energy with vibrator devices is for a preselected energization interval, and data are recorded during the energiæation interval and a subsequent "lis-tening 17 interval.
Often it is desirable for the vibrator to impart energies of varying frequencies into the earth's crust during the energization interval. In such instances, energy at a starting frequency is first imparted into the earth, and the frequency of energization changes over the energization interval at some rate until tl~e stopping frequency is reached at the end of the interval. The difference between the starting and stopping frequencies of the sweep generator is known as the range of the sweep, and the length of time in which the generator has to sweep through those frequencies is know~ as the sweep time.
Vibrators typically employ a sweep generator t and the output of the sweep generator is coupled to the inpu~
of the vibrating type device. The output of the sweep ge~erator dictates the manner in which the frequency of the energization signal which is imparted into the earth's crust varies as a function of time~
` 1~l~75~fi~
~3--Several methods of e~fecting the rate of change of the frequency of the sweep generator during the sweep time have been proposed. For example, in the case of a linear sweep, the frequency output of the sweep generator changes linearly over the sweep time at the rate dictated by the starting and stopping frequencies and the sweep time. Further, nonlinear sweeps have been proposed in which ~he rate o~ change of the frequency of the sweep generator varies nonlinearly between the starting and stopping frequencies over the sweep time. Examples of nonlinear sweeps have been quadratic sweeps and square root sweeps.
It is known in the seismic exploration art that the higher frequencies of energization signals are attenuated to a greater degree than lower frequency energization signals, and most authorities have concluded that the attenuation of the earth in decibels is directly propor-tional to the frequency of the energization signal.
Further, the total attenuation of any specific signal is known to ~e dependent upon the velocity, layering, thickness and attenuation coefficients of each layer traversed, as well as the frequency range.
Even though the earth attenuation is known to in-crease with increasing frequency of the energi~ation signals, linear sweeps have been extensively used in vibrators. Techniques for emphasizing the lower ampli-tude higher frequenc~ responses are well-known and have been employed to account for the attenuation applied to these higher frequency seismic signals by the earth.
Nonlinear sweep signals have been suggested but have not achieved acceptance by the industry due to poor performance.
~ ~7S915 The shortcomings of the prior art are overcome with the method and apparatus of the present invention.
In accordance with the present invention, a method of seismic exploration is provided wherein an optimal nonlinear sweep signal is presented to the source of vibrating seismic energy.
In accordance with the method of the present inven-tion, at least one array of seismic detectors is estab-lished at a location on the earth's surface and at least one vibrator is also established on the earth's surface.
The vibrator includes a sweep generator. which generates an output signal. The output signal of the sweep genera-tor is nonlinear, and preferably, varies as a function of time during the sweep time as follows:
Fo+0.43429 R [ln (2.3026CAot+1)]
C R
where:
t is the instantaneous value of time during the sweep in seconds;
Fo is the frequency at which the sweep starts in Hz;
R is the range of frequencies over which the sweep varies during the sweep time;
Ao is the initial rate of change of frequency with respect to time; and C is the logarithm to the base lO of ~he ratio of the 3, response at high frequency to the response at low frequency. C is chosen to provide compensation for any attenuation which exists to a particular impedance boundary over the range R.
In one embodiment wherein the starting frequency of the swcep is 10 ~Iz, the stopping frequency of the sweep is 110 Hz, the sweep time is 10 seconds, and the attenuation of the earth is a total of 20 db, the frequency of the sweep signal which will exactly compensate for this attenuation, varies over the sweep time as follows:
10+43.~3 ln (.9t+1).
In accordance with the present invention, a method of generating an output signal of a predetermined shape is also provided. The method comprises dividing the sweep time of the output signal into a plurality of preselected time intervals. The method also comprises calculating the rate of change of the frequency of the output signal for each time interval to reali~e said predetermined shape based on the range of the sweep, the sweep time, the starting frequency of the sweep and the predetermined shape of the output signal. The signals representative of the rate of change of the output frequency are then fed to a frequency generator, which generates the output signal.
The invention also pertains to apparatus for producing a non-linear sweep signal, comprising means for generating signals representative of the rate of change of frequency of the sweep signal for each of a plurality of time intervals of the sweep time, means for storing the rate of change signals in a memory, means for commencing the sweep signal at the starting frequency, and means for changing the frequency of the sweep signal during each time interval of the sweep at the rate stored in the memory for each time interval.
In the accompanying drawings:
Figure 1 is a pictorial diagram which illustrates component parts of a seismic exploration system.
, ~ ~
~;~7S9~ .
Figure 2 is a block diagram of some of the component parts of the vibrator depicted in Figure 1.
Figure 3 is a graphical illustration of an experimen-tal frequency response using a linear sweepO
Figure 4 is a graphical illustration of an experimen-tal frequency response using a nonlinear sweep in accor-dance with the present invention.
Figure 5 is an electrical schematic in block diagram form of apparatus for generating a nonlinear sweep signalO
Figure 6 is an electrical schematic in block diagram form of a programmer apparatus.
Figure 7 is a more detailed electrical schematic of portions of the circuitry illustrated in Figures 5 and 6.
It will be appreciated that the present invention can take many forms and embodiments. Some embodiments of the invention are described so as to give an understanding of the invention. It is not intended, however, that the embodiments described herein should in any way limit the true scope and spirit of the invention.
Referring first to Figure 1, portions of a seismic exploration system are illustrated. ~s shown, the system comprises a plurality of seismic arrays 101. Each seismic array 101 contains a plurality of seismic detectors 102.
The seismic arrays 101 are preferably located at regularly spaced intervals along the earth's surface.
The system illustrated in Figure 1 also includes recording cable 104, which is preferably a multi-pair of cable. A pair of wires 103 is ~taken out" of cable 104 and is connected to the output of each seismic array 101.
Electrical signals generated by the seismic deteetors 102 in seismic arrays 101 are conveyed via the multi pair of cable 104 to recording truck 105, where appropriate field recording of the signals takes place.
The system illustrated in Figure 1 also includes a source of seismic energy, which is shown as vibrator 1~7. Periodically, vibrator 107 imparts low `level seismic waves into the earth. The seismic arrays 101 produce electrical si~nals in response to the reflected portions of the sei~mic waves, and the electrical signals are then conveyed to recording truck 105.
Now referring to FIG. 2, there are portions of the elements of vibrator 107 of FIG. 1 are illustrated.
Vibrator 107 includes sweep generator 201, hydraulic valve 202, hydraulic piston 203 and vibrator base plate 204. The output signal from sweep generator 201 is fed to hydraulicValVe 202, which is preferably a solenoid-actuated valve. Based on the input signal received from sweep generator 201, hydraulic valve 202 controls the movement of hydraulic piston 203, which in turn drives the vibrator base plate 204~ Vibrator base plate 204 is in contact with the earth and imparts seismic energy into the earth's crust responsive to the movement of hydraulic piston 203.
In many applications where vibrator units are utilized as the source of seismic energy, energy is inputted into the earth during an energization interval of time, and seismic data are recorded during the energization interval f~
and for a listening interval following the energization interval. During the energization interval, the frequency of the input energy is often varied over a range of pre~
selected values. For example, vibrator 107 may input energy at a frequency of 10 Hz at the start of the ener-gization interval, and may change the input frequency at some preselected rate over the energization interval. The differences is known as the range of the sweep, and the length of the energization interval is known as the sweep time.
As noted above, it is well-known in the seismic industry that the higher frequencies in a seismic signal are subject to greater earth attenuation than the lower ones and that this attenuation in decibels is directly proportional to the frequency. Thus, the higher frequen-cies inputted in the earth's crust by vibrator 107 will suffer more attenuation than the lower frequency signals inputted by vibrator 107.
In accordance with the present invention, sweep generator 20~ is designed to provide a nonlinear signal to hydraulic valve 202 to provide nonlinear control of the frequencies inputted into the earth by vibrator 107, as vibrator 107 sweeps through the range of frequencies which are to be inputted at any given time. Preferably, the frequency of the output signal from sweep generator 201 varies as a function of time during the sweep as follows:
Fo+0.43429 R lln (2.3026CAot-~1)] (1) C R
where:
t is the instantaneous value of time during the sweep in seconds;
_9_ Fo is the frequency at which the sweep starts in Hz;
R is the range of frequencies over which the sweep varies during the sweep time;
Ao is the initial rate of change of frequency with respect to time; and lQ C is the logarithm to the base 10 of the ratio of the response at high frequency to the response at low frequency. We have discovered that the value of C may be chosen to provide compensa-tion for any attenuation which exists to a particular acoustic boundary over the range R.
In a specific embodiment where the frequency of the signals inputted into the earth are to vary between 10 H~
and 110 Hz over a sweep time of 10 seconds and assuming that the attenuation of the earth is a total of 20 db between 10 ~z and 110 Hz, ~he frequency of the output signal from sweep generator 201, which will exactly compensate for this attenuation, varies over the sweep time as follows:
10+43.43 ln (0.9t+1).
For example given above, experimental responses have been generated for not only the instance where the output signal of sweep generator 201 is linear, but also for the instance where the output signal from sweep generator 201 is in accordance with the generalized logarithmic expres-sion above. The experimental results for the linear sweep are shown in Figure 3, while the experimental results for the logarithmic sweep are shown in Figure 4~ It will be . .
--1 o-- .
observed that, with the logarithmic sweep, the relative frequency response in substantially flat over the range of frequencies in question. Thus, with the method of the present invention, the frequency response over the range of frequencies in the sweep is markedly improved, and compensation for earth attenuation has been obtained.
In accordance with the present invention, apparatus is also provided for generating a nonlinear signal at the output of a sweep generator. Referring to Figure 5, such apparatus includes nonlinear range generator 50t, frequency generator 502 and sine function generator 503. Nonlinear range generator 501 inputs signals representative of the rate of change which is required in the frequency of the 15 SWEEP OUT signal. The rate of change of frequency signals are provided at preselected time intervals during the sweep. Frequency generator 502 responds to the rate of change in frequency signals to produce output signals representative of the new frequency. Sine function generator 503 converts the digital output of frequency generator 502 to an analog signal of the desired frequency at its output (SWEEP O~T ) .
A significant feature of the apparatus of the present invention is that nonlinear range generator 501 provides rate of change of frequency signals to frequency generator 502. By this technique, relatively smooth transistions in the frequency of SWEEP OUT are realized. On the other hand, prior attempts to provide a nonlinear output have attempted to provide change in frequency information to the sweep generator. With this latter approach, ~mooth transitions in output frequency have not been achieved.
Referring now to Figure 6, the rate of change of frequency signals over the sweep are generated as follows.
Programmer 600 includes processor 602 and front panel control 601. The values of the starting frequency of the sweep, the range of the sweep, the sweep time, and the desired shape of the nonlinear output are programmed in front panel control 601. Processor 602 reads this pro-grammed information. Processor 602 also divides the sweep time into small intervals of time, and i~ a preferred embodiment, these intervals of time are four milliseconds.
For each four millisecond interval, processor 602 calcu-lates the rate of change of frequency required to realize the desired output of the sweep generator. The rate of change of frequency is then stored in non-volatile storage device 603, which is preferably an electrically erasable programmable read only memory (EEPROM) card. Preferably, the EEPROMs of non-volatile memory storage device 603 has a storage capacity of 4,096, eight-bit words. The con-tents of non-volatile storage device 603 thus contains the information necessary for the SWEEP OUT signal ~o move from the starting frequency to the stopping frequency in the sweep time according to the nonlinear function desired.
Preferably, the nonlinear function realized is as set forth in expression (1) above.
~5 Now referring to Figure 7, there is illust~ated a detailed implementation of the nonlinear range generator 501, frequency generator 502 and sine function generator 503 of Figure 5.
EXPLORATION USING NONLINEAR SWEEPS
The present invention relates to method and apparatus for seismic exploration, and, more particularly, to pro-viding a nonlinear sweep signal to a vibrator in a seismic system.
In seismic exploration, seismic waves are commonly used to probe the earth's crust as a means of determining the type and location of subsurace formations. The earth's crust can be considered a transmission medium or filter whose characteristics are to be determined by pass-ing seismic waves through that medium. In the reflection seismic method, seismic waves or impulses are generated at a point at or near the earth's surface, and the compres-sional mode of these waves is reflected from subsurface acoustic impedance boundaries and detected by arrays of seismic detectors located at the earth's crust. The seismic detectors convert the received waves into electri-cal signals which are sensed and recorded in a form which permits analysis~ Skilled interpreters can discern from such an analysis the shape and depth of subsurface refiec-tion boundaries and the likelihood of finding an accumula-tion of minerals, such as oil and gas.
Si95 Various sources of seismic energy have been utilized in the art to impart the seismic waves into the earth's crust. Such sources have included dynamite and weight-drop apparatus.
Another source of seismic energy is a vibrator which, ~hen energized, imparts relatively low level energy signals into the earth's crust. Typically, the impartation of energy with vibrator devices is for a preselected energization interval, and data are recorded during the energiæation interval and a subsequent "lis-tening 17 interval.
Often it is desirable for the vibrator to impart energies of varying frequencies into the earth's crust during the energization interval. In such instances, energy at a starting frequency is first imparted into the earth, and the frequency of energization changes over the energization interval at some rate until tl~e stopping frequency is reached at the end of the interval. The difference between the starting and stopping frequencies of the sweep generator is known as the range of the sweep, and the length of time in which the generator has to sweep through those frequencies is know~ as the sweep time.
Vibrators typically employ a sweep generator t and the output of the sweep generator is coupled to the inpu~
of the vibrating type device. The output of the sweep ge~erator dictates the manner in which the frequency of the energization signal which is imparted into the earth's crust varies as a function of time~
` 1~l~75~fi~
~3--Several methods of e~fecting the rate of change of the frequency of the sweep generator during the sweep time have been proposed. For example, in the case of a linear sweep, the frequency output of the sweep generator changes linearly over the sweep time at the rate dictated by the starting and stopping frequencies and the sweep time. Further, nonlinear sweeps have been proposed in which ~he rate o~ change of the frequency of the sweep generator varies nonlinearly between the starting and stopping frequencies over the sweep time. Examples of nonlinear sweeps have been quadratic sweeps and square root sweeps.
It is known in the seismic exploration art that the higher frequencies of energization signals are attenuated to a greater degree than lower frequency energization signals, and most authorities have concluded that the attenuation of the earth in decibels is directly propor-tional to the frequency of the energization signal.
Further, the total attenuation of any specific signal is known to ~e dependent upon the velocity, layering, thickness and attenuation coefficients of each layer traversed, as well as the frequency range.
Even though the earth attenuation is known to in-crease with increasing frequency of the energi~ation signals, linear sweeps have been extensively used in vibrators. Techniques for emphasizing the lower ampli-tude higher frequenc~ responses are well-known and have been employed to account for the attenuation applied to these higher frequency seismic signals by the earth.
Nonlinear sweep signals have been suggested but have not achieved acceptance by the industry due to poor performance.
~ ~7S915 The shortcomings of the prior art are overcome with the method and apparatus of the present invention.
In accordance with the present invention, a method of seismic exploration is provided wherein an optimal nonlinear sweep signal is presented to the source of vibrating seismic energy.
In accordance with the method of the present inven-tion, at least one array of seismic detectors is estab-lished at a location on the earth's surface and at least one vibrator is also established on the earth's surface.
The vibrator includes a sweep generator. which generates an output signal. The output signal of the sweep genera-tor is nonlinear, and preferably, varies as a function of time during the sweep time as follows:
Fo+0.43429 R [ln (2.3026CAot+1)]
C R
where:
t is the instantaneous value of time during the sweep in seconds;
Fo is the frequency at which the sweep starts in Hz;
R is the range of frequencies over which the sweep varies during the sweep time;
Ao is the initial rate of change of frequency with respect to time; and C is the logarithm to the base lO of ~he ratio of the 3, response at high frequency to the response at low frequency. C is chosen to provide compensation for any attenuation which exists to a particular impedance boundary over the range R.
In one embodiment wherein the starting frequency of the swcep is 10 ~Iz, the stopping frequency of the sweep is 110 Hz, the sweep time is 10 seconds, and the attenuation of the earth is a total of 20 db, the frequency of the sweep signal which will exactly compensate for this attenuation, varies over the sweep time as follows:
10+43.~3 ln (.9t+1).
In accordance with the present invention, a method of generating an output signal of a predetermined shape is also provided. The method comprises dividing the sweep time of the output signal into a plurality of preselected time intervals. The method also comprises calculating the rate of change of the frequency of the output signal for each time interval to reali~e said predetermined shape based on the range of the sweep, the sweep time, the starting frequency of the sweep and the predetermined shape of the output signal. The signals representative of the rate of change of the output frequency are then fed to a frequency generator, which generates the output signal.
The invention also pertains to apparatus for producing a non-linear sweep signal, comprising means for generating signals representative of the rate of change of frequency of the sweep signal for each of a plurality of time intervals of the sweep time, means for storing the rate of change signals in a memory, means for commencing the sweep signal at the starting frequency, and means for changing the frequency of the sweep signal during each time interval of the sweep at the rate stored in the memory for each time interval.
In the accompanying drawings:
Figure 1 is a pictorial diagram which illustrates component parts of a seismic exploration system.
, ~ ~
~;~7S9~ .
Figure 2 is a block diagram of some of the component parts of the vibrator depicted in Figure 1.
Figure 3 is a graphical illustration of an experimen-tal frequency response using a linear sweepO
Figure 4 is a graphical illustration of an experimen-tal frequency response using a nonlinear sweep in accor-dance with the present invention.
Figure 5 is an electrical schematic in block diagram form of apparatus for generating a nonlinear sweep signalO
Figure 6 is an electrical schematic in block diagram form of a programmer apparatus.
Figure 7 is a more detailed electrical schematic of portions of the circuitry illustrated in Figures 5 and 6.
It will be appreciated that the present invention can take many forms and embodiments. Some embodiments of the invention are described so as to give an understanding of the invention. It is not intended, however, that the embodiments described herein should in any way limit the true scope and spirit of the invention.
Referring first to Figure 1, portions of a seismic exploration system are illustrated. ~s shown, the system comprises a plurality of seismic arrays 101. Each seismic array 101 contains a plurality of seismic detectors 102.
The seismic arrays 101 are preferably located at regularly spaced intervals along the earth's surface.
The system illustrated in Figure 1 also includes recording cable 104, which is preferably a multi-pair of cable. A pair of wires 103 is ~taken out" of cable 104 and is connected to the output of each seismic array 101.
Electrical signals generated by the seismic deteetors 102 in seismic arrays 101 are conveyed via the multi pair of cable 104 to recording truck 105, where appropriate field recording of the signals takes place.
The system illustrated in Figure 1 also includes a source of seismic energy, which is shown as vibrator 1~7. Periodically, vibrator 107 imparts low `level seismic waves into the earth. The seismic arrays 101 produce electrical si~nals in response to the reflected portions of the sei~mic waves, and the electrical signals are then conveyed to recording truck 105.
Now referring to FIG. 2, there are portions of the elements of vibrator 107 of FIG. 1 are illustrated.
Vibrator 107 includes sweep generator 201, hydraulic valve 202, hydraulic piston 203 and vibrator base plate 204. The output signal from sweep generator 201 is fed to hydraulicValVe 202, which is preferably a solenoid-actuated valve. Based on the input signal received from sweep generator 201, hydraulic valve 202 controls the movement of hydraulic piston 203, which in turn drives the vibrator base plate 204~ Vibrator base plate 204 is in contact with the earth and imparts seismic energy into the earth's crust responsive to the movement of hydraulic piston 203.
In many applications where vibrator units are utilized as the source of seismic energy, energy is inputted into the earth during an energization interval of time, and seismic data are recorded during the energization interval f~
and for a listening interval following the energization interval. During the energization interval, the frequency of the input energy is often varied over a range of pre~
selected values. For example, vibrator 107 may input energy at a frequency of 10 Hz at the start of the ener-gization interval, and may change the input frequency at some preselected rate over the energization interval. The differences is known as the range of the sweep, and the length of the energization interval is known as the sweep time.
As noted above, it is well-known in the seismic industry that the higher frequencies in a seismic signal are subject to greater earth attenuation than the lower ones and that this attenuation in decibels is directly proportional to the frequency. Thus, the higher frequen-cies inputted in the earth's crust by vibrator 107 will suffer more attenuation than the lower frequency signals inputted by vibrator 107.
In accordance with the present invention, sweep generator 20~ is designed to provide a nonlinear signal to hydraulic valve 202 to provide nonlinear control of the frequencies inputted into the earth by vibrator 107, as vibrator 107 sweeps through the range of frequencies which are to be inputted at any given time. Preferably, the frequency of the output signal from sweep generator 201 varies as a function of time during the sweep as follows:
Fo+0.43429 R lln (2.3026CAot-~1)] (1) C R
where:
t is the instantaneous value of time during the sweep in seconds;
_9_ Fo is the frequency at which the sweep starts in Hz;
R is the range of frequencies over which the sweep varies during the sweep time;
Ao is the initial rate of change of frequency with respect to time; and lQ C is the logarithm to the base 10 of the ratio of the response at high frequency to the response at low frequency. We have discovered that the value of C may be chosen to provide compensa-tion for any attenuation which exists to a particular acoustic boundary over the range R.
In a specific embodiment where the frequency of the signals inputted into the earth are to vary between 10 H~
and 110 Hz over a sweep time of 10 seconds and assuming that the attenuation of the earth is a total of 20 db between 10 ~z and 110 Hz, ~he frequency of the output signal from sweep generator 201, which will exactly compensate for this attenuation, varies over the sweep time as follows:
10+43.43 ln (0.9t+1).
For example given above, experimental responses have been generated for not only the instance where the output signal of sweep generator 201 is linear, but also for the instance where the output signal from sweep generator 201 is in accordance with the generalized logarithmic expres-sion above. The experimental results for the linear sweep are shown in Figure 3, while the experimental results for the logarithmic sweep are shown in Figure 4~ It will be . .
--1 o-- .
observed that, with the logarithmic sweep, the relative frequency response in substantially flat over the range of frequencies in question. Thus, with the method of the present invention, the frequency response over the range of frequencies in the sweep is markedly improved, and compensation for earth attenuation has been obtained.
In accordance with the present invention, apparatus is also provided for generating a nonlinear signal at the output of a sweep generator. Referring to Figure 5, such apparatus includes nonlinear range generator 50t, frequency generator 502 and sine function generator 503. Nonlinear range generator 501 inputs signals representative of the rate of change which is required in the frequency of the 15 SWEEP OUT signal. The rate of change of frequency signals are provided at preselected time intervals during the sweep. Frequency generator 502 responds to the rate of change in frequency signals to produce output signals representative of the new frequency. Sine function generator 503 converts the digital output of frequency generator 502 to an analog signal of the desired frequency at its output (SWEEP O~T ) .
A significant feature of the apparatus of the present invention is that nonlinear range generator 501 provides rate of change of frequency signals to frequency generator 502. By this technique, relatively smooth transistions in the frequency of SWEEP OUT are realized. On the other hand, prior attempts to provide a nonlinear output have attempted to provide change in frequency information to the sweep generator. With this latter approach, ~mooth transitions in output frequency have not been achieved.
Referring now to Figure 6, the rate of change of frequency signals over the sweep are generated as follows.
Programmer 600 includes processor 602 and front panel control 601. The values of the starting frequency of the sweep, the range of the sweep, the sweep time, and the desired shape of the nonlinear output are programmed in front panel control 601. Processor 602 reads this pro-grammed information. Processor 602 also divides the sweep time into small intervals of time, and i~ a preferred embodiment, these intervals of time are four milliseconds.
For each four millisecond interval, processor 602 calcu-lates the rate of change of frequency required to realize the desired output of the sweep generator. The rate of change of frequency is then stored in non-volatile storage device 603, which is preferably an electrically erasable programmable read only memory (EEPROM) card. Preferably, the EEPROMs of non-volatile memory storage device 603 has a storage capacity of 4,096, eight-bit words. The con-tents of non-volatile storage device 603 thus contains the information necessary for the SWEEP OUT signal ~o move from the starting frequency to the stopping frequency in the sweep time according to the nonlinear function desired.
Preferably, the nonlinear function realized is as set forth in expression (1) above.
~5 Now referring to Figure 7, there is illust~ated a detailed implementation of the nonlinear range generator 501, frequency generator 502 and sine function generator 503 of Figure 5.
Claims (4)
1. A method of seismic exploration, comprising the steps of:
establishing at least one seismic array at a location on the earth's surface, each said seismic array containing seismic detectors;
establishing at least one vibrator at a location on the earth's surface, each said vibrator having the capability of sweeping through a range of frequencies during a preselected sweep time and each said vibrator including a sweep generator;
generating an output signal from the sweep gener-ator which varies as a function of time during the sweep time as follows:
where:
t is the instantaneous value of time during the sweep in seconds;
Fo is the frequency at which the sweep starts in Hz;
R is the range of frequencies over which the sweep varies during the sweep time;
Ao is the initial rate of change of frequency with respect to time; and C is the logarithm to the base 10 of the ratio of the response at high frequency to the response at low frequency, where C is chosen to provide compensation for any attenuation which exists to a particular impedance boundary over the range R.
receiving seismic data with the arrays of detectors located on the earth's surface; and recording the received seismic data.
establishing at least one seismic array at a location on the earth's surface, each said seismic array containing seismic detectors;
establishing at least one vibrator at a location on the earth's surface, each said vibrator having the capability of sweeping through a range of frequencies during a preselected sweep time and each said vibrator including a sweep generator;
generating an output signal from the sweep gener-ator which varies as a function of time during the sweep time as follows:
where:
t is the instantaneous value of time during the sweep in seconds;
Fo is the frequency at which the sweep starts in Hz;
R is the range of frequencies over which the sweep varies during the sweep time;
Ao is the initial rate of change of frequency with respect to time; and C is the logarithm to the base 10 of the ratio of the response at high frequency to the response at low frequency, where C is chosen to provide compensation for any attenuation which exists to a particular impedance boundary over the range R.
receiving seismic data with the arrays of detectors located on the earth's surface; and recording the received seismic data.
2. The method of claim 1, wherein the frequency of the sweep signal varies from 10 Hz to 110 Hz during a sweep time of 10 seconds, and wherein the output signal from the sweep generator varies as a function of time during the sweep time as follows:
fi(t) = 10 + 43.43 1n [0.9t+1], in order to compensate for an earth attenuation of 20 decibels over the frequency range of 10 to 110 HZ.
fi(t) = 10 + 43.43 1n [0.9t+1], in order to compensate for an earth attenuation of 20 decibels over the frequency range of 10 to 110 HZ.
3. A method of generating one of a plurality of possible non-linear sweep signals of predetermined time and shape at the output of a sweep generator in a seismic exploration system, comprising the steps of:
(a) dividing the sweep time of the output signal into a plurality of preselected time intervals;
(b) calculating the rate of change of the fre-quency of the output signal for each time interval to realize said predetermined shape based on the range of the sweep, the sweep time, the starting frequency and the prede-termined shape; and (c) using each said value calculated in step (b) to change the rate of change of frequency of the sweep signal.
(a) dividing the sweep time of the output signal into a plurality of preselected time intervals;
(b) calculating the rate of change of the fre-quency of the output signal for each time interval to realize said predetermined shape based on the range of the sweep, the sweep time, the starting frequency and the prede-termined shape; and (c) using each said value calculated in step (b) to change the rate of change of frequency of the sweep signal.
4. Apparatus for producing a nonlinear sweep signal, comprising:
means for generating signals representative of the rate of change of frequency of the sweep signal for each of a plurality of time intervals of the sweep time;
means for storing said rate of change signals in a memory; and means for commencing the sweep signal at the starting frequency; and means for changing the frequency of the sweep signal during each time interval of the sweep at the rate stored in said memory for each said time interval.
means for generating signals representative of the rate of change of frequency of the sweep signal for each of a plurality of time intervals of the sweep time;
means for storing said rate of change signals in a memory; and means for commencing the sweep signal at the starting frequency; and means for changing the frequency of the sweep signal during each time interval of the sweep at the rate stored in said memory for each said time interval.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US31082881A | 1981-10-13 | 1981-10-13 | |
| US31082981A | 1981-10-13 | 1981-10-13 | |
| US310,829 | 1981-10-13 | ||
| US310,828 | 1989-02-08 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1187595A true CA1187595A (en) | 1985-05-21 |
Family
ID=26977606
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000412545A Expired CA1187595A (en) | 1981-10-13 | 1982-09-30 | Method and apparatus for seismic exploration using non-linear sweeps |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA1187595A (en) |
| MX (1) | MX156722A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0257231A2 (en) * | 1986-08-25 | 1988-03-02 | Hydroacoustics Inc. | Method and apparatus for the generation and transmission of signals for echo location and other signaling purposes, particularly in geophysical exploration |
| CN112764090A (en) * | 2020-12-24 | 2021-05-07 | 中国石油化工集团有限公司 | Seismic source nonlinear scanning signal design method based on forced vibration |
-
1982
- 1982-09-30 CA CA000412545A patent/CA1187595A/en not_active Expired
- 1982-10-13 MX MX19475982A patent/MX156722A/en unknown
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0257231A2 (en) * | 1986-08-25 | 1988-03-02 | Hydroacoustics Inc. | Method and apparatus for the generation and transmission of signals for echo location and other signaling purposes, particularly in geophysical exploration |
| EP0257231A3 (en) * | 1986-08-25 | 1989-08-30 | Hydroacoustics Inc. | Method and apparatus for the generation and transmission of signals for echo location and other signaling purposes, particilarly in geophysical exploration |
| CN112764090A (en) * | 2020-12-24 | 2021-05-07 | 中国石油化工集团有限公司 | Seismic source nonlinear scanning signal design method based on forced vibration |
| CN112764090B (en) * | 2020-12-24 | 2024-04-09 | 中国石油化工集团有限公司 | Seismic source nonlinear scanning signal design method based on forced vibration |
Also Published As
| Publication number | Publication date |
|---|---|
| MX156722A (en) | 1988-09-28 |
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