Patent Publication Number: US-6665619-B2

Title: Noise estimator for seismic exploration

Description:
This application is a continuation application of U.S. patent application Ser. No. 09/340,274 entitled “NOISE ESTIMATOR FOR SEISMIC EXPLORATION,” filed on Jun. 25, 1999 now U.S. Pat. No. 6,366,857. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of seismic exploration and, more specifically, to a method and apparatus for noise estimation in seismic surveying. 
     BACKGROUND 
     In seismic surveying, acoustic energy waves are transmitted into the earth in order to map subterranean geological structures by sensing returned acoustic energy waves reflected from those geological structures. Land seismic surveys are commonly performed using vibroseis trucks that provide the source of the acoustic energy. The vibroseis trucks generate (“vibrate”) the acoustic energy waves at predetermined vibrator points (“VP”) that are usually marked with stakes that have been placed by surveyors. During operations, the vibroseis trucks navigate from point to point using these survey stakes. 
     The acoustic energy wave, known as a chirp sweep, vibrated by the trucks is swept in frequency over a period of time. A typical chirp may sweep from 5 to 150 Hertz (Hz) and last for 15 seconds. The subterranean geological layers create changes in the chirp due to refractions, reflections, and diffractions at the boundaries of changes in acoustic impedance of each subterranean layer. Some of these altered acoustic energy waves, known as echoes, return to the earth&#39;s surface to be sensed by seismic detectors. The arrival time of the echoes at the seismic detectors depends mainly on the depth of the subterranean layers reflecting the chirp. A listening time window is used to capture the return echoes down to the depth of interest. The echoes are compressed by correlating with the chirp sweep. The arrival times of the compressed echoes are used to generate imaging data of the subterranean structure. 
     One problem with seismic surveying operations is that the presence of ambient noise during the listening time window may drown out the echoes to be sensed by the seismic detectors. Ambient noise may be generated by sources in the area being surveyed such as wind tugging on grass or vehicular traffic. The imaging data process requires a minimum signal-to-noise ratio (SNR) for the data to be of sufficient quality for surveying use. If the ambient noise level is too high, then surveying operations may have to be halted until the ambient noise level falls to an acceptable SNR level. As such, in order to obtain an in-field estimate of the data quality, a passive measurement of the ambient noise level is made. 
     FIG. 1A illustrates a prior art sequential sweep operation that uses a broad band energy detector to measure the total noise energy across the entire frequency band of interest. The broad band noise estimation occurs in between the end of a prior listening region and the start of a new chirp sweep. One problem with such a system is that in order to obtain an accurate noise estimate, a dead time (e.g., 2 seconds shown) when no sweeps occur is necessary for broad band energy detection. The deadtime for noise estimation adds to the overall cycle time of seismic surveying operations. 
     For example, as illustrated in FIG. 1A, a single chirp sweep (15 seconds), plus listening time (5 seconds), plus noise estimation (2 seconds) may take 22 seconds. If 1000 VPs are made in a day, then approximately 33 minutes are used in a non-productive mode listening for noise. Assuming a 12 hour working day, this translates to approximately 5% of the available work time used for noise estimation. 
     FIG. 1B illustrates a prior art slip sweep operation that also uses a broad band energy detector to measure noise energy. In slip sweep operations, multiple groups of vibroseis trucks are used in which a group starts sweeping without waiting for the other groups&#39; sweep to be completed. The sweeps of the different groups occur in non-overlapping frequency-time windows such that no two groups are sweeping in the same frequency at the same time. For example, sweep  2  from a second group is started at time, ts, as soon as the start frequency, fs, of the previous sweep  1 &#39;s echo region has completed. 
     As with sequential sweep operations, a slip sweep operation using a broad band noise estimator requires a dead time (e.g., 2 seconds shown) when no sweeps occur for broad band energy detection in order to obtain an accurate noise estimate. The noise estimate occurs in between the end of the last echo region of a vibroseis group&#39;s sweeps and the start of a new chirp sweep in a new vibroseis group&#39;s sweeps. One problem with using a broad band noise estimator is that the operations of all vibroseis groups must stop in order to obtain the noise estimate, thereby adding to the overall cycle time of seismic surveying operations. 
     SUMMARY OF THE INVENTION 
     The present invention pertains to a method and apparatus for noise estimation. The method including producing a first chirp signal having a varying frequency over a first time period, sampling noise energy at a frequency different than the first chirp signal frequency during the first time period, and generating a noise estimate based on the noise energy sampled. 
     Additional features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
     FIG. 1A illustrates a prior art sequential sweep operation that uses a broad band energy detector to estimate noise energy. 
     FIG. 1B illustrates a prior art slip sweep operation that uses a broad band energy detector to estimate noise energy. 
     FIG. 2A is a flow diagram illustrating one embodiment of a noise estimation scheme. 
     FIG. 2B is a flow diagram illustrating another embodiment of a noise estimation scheme. 
     FIG. 2C illustrates a frequency-time spectrum of vibroseis operation according to one embodiment of a noise estimation scheme. 
     FIG. 3 illustrates a frequency-time spectrum of vibroseis operation according to another embodiment of a noise estimation scheme. 
     FIG. 4 illustrates one embodiment of a noise estimation scheme in slip sweep operations. 
     FIG. 5A illustrates a system utilizing one embodiment of a noise estimation scheme. 
     FIG. 5B illustrates one embodiment of a noise estimator. 
     FIG. 6 illustrates another embodiment of a noise estimator. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth such as examples of specific times, frequencies, process steps, components, etc. in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details need not be employed to practice the present invention. In other instances, well known circuits or methods have not been described in detail in order to avoid unnecessarily obscuring the present invention. 
     A noise estimation system and method wherein noise is sampled approximately concurrent with chirp signal sweeps are described. In one embodiment, the noise estimation circuit and method described herein may be implemented with a seismic surveying operation. When used with a seismic surveying operation, the cycle time of seismic surveying operations may be reduced by providing for noise estimation without adding deadtime to the surveying operations. It should be noted, however, that the present invention is described in relation to a seismic surveying only for illustrative purposes and is not meant to be limited to noise estimates in seismic surveying as the present invention may be applied to other frequency variant signal environments. 
     In a typical surveying operation, a group of vibroseis trucks proceeds to a vibrator point (VP) and each truck lowers a pad used to generate a chirp sweep. The trucks sweep chirp signals, wait for the end of the listening period, and sweep again. This procedure is repeated until the requisite number of sweeps for the VP is completed. The vibroseis trucks in the group then lift their pads, move to the next point, and repeat the sequence at the new VP. In order to obtain an in-field estimate of the data quality, a passive measurement of the ambient noise level is made. If the ambient noise level is too high, then surveying operations may have to be halted until the ambient noise level falls to an acceptable SNR level. 
     FIG. 2A is a flow diagram illustrating one embodiment of a noise estimation scheme. The noise estimation scheme described herein may be used to perform noise measurement in a time interval that is approximately concurrent with a time interval that a chirp signal is transmitted. A chirp signal having a swept frequency during a first time period is produced, step  201 . Noise energy is sampled during the same time period but at a different frequency than the chirp sweep at any given point in the time period, step  203 . 
     In one embodiment, a desired signal is monitored during a listening period. The chirp sweep and listening period occur in the same time period as the noise measurement but at frequencies that are offset from the noise measurement frequency. As such, noise energy is sampled during the same time period but at a different frequency than the chirp sweep at any given point in the time period. In this manner, the noise measurements may be performed while the chirp sweep and listening periods are proceeding. A noise estimate may be generated based on the noise measurements, step  204 . 
     FIG. 2B is a flow diagram illustrating another embodiment of a noise estimation scheme. In one embodiment, a chirp signal having an increasing frequency in a time period is generated, step  206 . The frequency is increased within a frequency range having a start frequency and an end frequency. Reflected seismic signals are listened for in the same frequency range as the chirp signal transmission, step  207 . The listening time period for the reflected signals is approximately concurrent with the chirp signal generation time period, but with an end time that is offset from the end time of the chirp signal generation by a predetermined amount. A subsequent predetermined time after the start time of the listening period, noise energy measurement is initiated beginning at the same start frequency as the chirp signal, step  208 . 
     In this manner, the noise energy measurements are made in the same frequency range and approximately concurrent with the chirp signal sweep; however, during any given point in time, noise is measured in a frequency range different than the frequency of the chirp signal and the frequencies of the echoes. A noise estimate is then generated based on the sampled noise energy, step  209 . It should be noted that the frequencies and times used in the embodiments described herein are for illustrative purposes only, and that other frequencies and times may be used. 
     FIG. 2C illustrates a frequency-time spectrum of vibroseis operation according to one embodiment of a noise estimation scheme. Chirp signal  210  is an acoustic energy wave that is linearly increased in frequency from f 0  to f 1  over a period of time from t 0  to t 3 . In one embodiment, for example, chirp signal  210  has a frequency that is linearly increased from 5 to 150 Hertz (Hz) during a 15 second time period. Echoes returning to the surface will be delayed by the travel time through the subterranean layers. The earliest received echo will be from the shallowest layer and will have a frequency content corresponding to the lowest chirp frequency f 0 . The listening time may start immediately after the start t 0  of chirp signal  210  with echoes being received out to a time t 1  corresponding to the travel time of the acoustic waves to and from the deepest layers of interest. The listening time window has a predetermined time interval Te that the seismic detectors will listen for a given frequency resulting in echo region  220 . In one embodiment, a broad band filter is used to screen the reflected waves in echo region  220  between f 0  and f 1  until time t 4 . 
     Passive noise estimation  230  is performed using a sliding time-variant narrow band filter in a frequency-time domain  230  that is delayed with respect to chirp signal  210  and echo region  220 . In one embodiment, the narrow band filter is a bandpass filter. In another embodiment, the narrow band filter is a combination of a high pass filter and a low pass filter. 
     The noise measurement begins at a time just after the end of the listening time period t 1  for frequency f 0  and lasts for a predetermined time Tn, until time t 2 . The frequency at which noise energy is measured is delayed with respect to echo region  220  and linearly increases at a rate equal to or less than the rate of the chirp sweep so that noise measurement in one frequency band may be performed while a chirp is sweeping in a different frequency band. In this manner, noise estimation may be performed concurrently with sweep operations because the frequency-time domain of noise measurement region  230  is outside echo region  220 . Furthermore, a new chirp signal  250  may be started as soon as the listening period of the first chirp signal  210  ends, at time t 4 . The new chirp signal  250  may begin at time t 4  even though noise measurement is still in progress, because the time-variant narrow band filter has been swept to evaluate noise in a frequency range f 2  to f 1  while new chirp signal  250  is being swept at the frequency range f 0  to f 4  in the same time period t 4  to t 5 . 
     Using the sliding time-variant narrow band filter described herein eliminates the deadtime required when using a broad band noise estimator functioning outside of a sweep and listening time period. By performing noise estimation during a sweep and listening period, the time between consecutive sweeps (cycle time  260 ) may be reduced because a new sweep may begin immediately after the end of a prior sweep. Therefore, the total cycle time of surveying operations may be reduced. 
     For example, if chirp signal  210  has a sweep time of 15 seconds (t 3 =15 seconds), a listening sweep time of 5 seconds (t 4 −t 3 =5 seconds), and a noise estimation sweep time of 2 seconds (t 5 −t 4 =2 seconds), then cycle time  260  is only 20 seconds (t 4 =20 seconds). This is approximately a 9% reduction in cycle time from the 22 second cycle time of operations using the prior art noise estimation technique shown in FIG.  1 A. 
     It should be noted that although FIG. 2C illustrates chirp signal  210  and echo region  220  with a linearly increasing frequency over time, the chirp signal and listening region may also be linearly decreased with the noise region  230  correspondingly varied. In an alternative embodiment, the frequencies of the chirp signal  310 , echo region  320 , and noise region  330  may be varied over time in other manners, for example, exponentially as illustrated in FIG.  3 . It should also be noted that noise estimation scheme described herein may also be used in slip sweep operations as shown in FIG.  4 . 
     FIG. 4 illustrates one embodiment of noise estimation in slip sweep operations. In slip sweep operations, multiple groups of vibroseis trucks sweep in non-overlapping frequency-time windows such that different groups are not sweeping in the same frequency at the same time. Noise measurement in noise region  430  is performed using a sliding time variant narrow band filter in a frequency-time domain that is delayed with respect to chirp signal  410  and echo region  420 . In one embodiment, the narrow band filter may be a bandpass filter. In another embodiment, the narrow band filter may be a combination of a high pass filter and a low pass filter. 
     The noise measurement for frequency f 40  of noise region  430  begins at a time just after the end of the echo region (listening time period) t 41  and lasts for a predetermined time Tn 4 , until time t 42 . The frequency for which noise measurement is performed may be delayed in time with respect to the frequencies received in echo region  420  and may be linearly increased at a rate approximately equal to the rate of chirp  410 . This allows noise measurement to be performed concurrently with the listening period defined by echo region  420 . The overlapping duration of the echo region and the noise measurement is possible because the noise for any given frequency is measured at time delayed from the receipt of reflected seismic waves for the same frequency. In another embodiment, the frequency at which noise measurement is performed may be linearly increased at a rate less than the rate of chirp  410 . 
     A chirp signal  450  for a subsequent vibroseis truck group is started at frequency f 40  at t 42  as soon as the time period Tn 4  for noise measurement at frequency f 40  has ended. A time variant sliding narrow band filter may be used to separate chirp signals  410  and  450 . In this manner, noise estimation may be performed because the frequency-time domain of noise measurement region  430  is outside of chirp signal  410 , echo region  420 , chirp signal  450 , and the echo region  460  for chirp signal  450 . It should be noted that the frequency of the chirp signal sweeps and the corresponding noise measurements in slip sweep operations may also be non-linearly varied with time, for example, exponentially. 
     FIG. 5A illustrates a system utilizing one embodiment of a noise estimation scheme. The system includes a chirp signal generator  501 , and a signal detector and a noise estimation circuit  502 . The chirp signal generator  501  has a control input  504  that is used to generate a frequency swept signal over time. In one embodiment, timing control  505  can be used to synchronize the operation of chirp signal generator  501  with signal detector and noise estimation circuit  502 . The detector  502  includes input circuitry to detect acoustic energy and generate internal signals having frequency components to be used by a frequency filter. 
     FIG. 5B illustrates one embodiment of a noise estimator. The noise estimator  500  is used to measure noise concurrently with sweeping operations and generate a noise estimate. The noise estimate may be used to determine if sweeping operations should be halted until the ambient noise falls to allow for clear reception of echoes. In one embodiment, the noise measurements include reflections from subterranean layers deeper than those of interest. 
     In one embodiment, noise estimator  500  includes a geophone  510 , an amplifier  520 , a mixer  540 , a voltage controlled oscillator (VCO)  550 , a bandpass filter  570 , a rectifier  580 , and an integrator (∫dt)  590 . It should be noted that a geophone, an amplifier, a mixer, a VCO, a bandpass filter, a rectifier, and an integrator are well known in the art; accordingly, a detailed description of their internal components and their operation is not provided herein. 
     The noise estimator  500  calculates an estimate of the noise energy based on the noise measurements made throughout the frequency range of operation. Acoustic energy waves of noise are received at geophone  510  and converted into an analog electrical signal  511  that characterizes the magnitude and frequency content of the acoustic energy waves. The electrical signal  511  is amplified by amplifier  520  and transmitted to mixer  540 . The output  531  of amplifier  520  is combined with VCO  550  output signal  551  by mixer  540 . VCO  550  is used to slide signal  531  so that noise estimation can be performed at different frequencies while keeping the band range of bandpass filter  570  fixed. Bandpass filter  570  is used to screen out frequencies, in a particular time domain, for which noise measurement is not desired. In one embodiment, bandpass filter  570  may remove unwanted negative signal images from previous lower frequencies that have been shifted up into a higher frequency range. 
     In one embodiment, for example, the chirp signal is swept in frequency from 10 Hz to 150 Hz while the frequency of VCO  550  is decreased from 390 Hz to 250 Hz. By mixing the amplified sensor signal  531  with output signal  551  of VCO  550 , the output  541  of mixer  540  will have approximately a constant frequency of 400 Hz. The mixer difference frequencies between the chirp signal and the VCO range from 380 Hz to 100 Hz. As such, bandpass filter  570  may be selected to have a bandpass frequency centered at 400 Hz with approximately a +/−20 Hz bandwidth in order to filter out these mixer difference frequencies. 
     In an alternative embodiment, the signal representing the acoustic energy waves of noise is fixed and the band range of bandpass filter  570  is adjusted based on a sliding time. The VCO  550 , mixer  540 , and filter  570  are replaced with a tunable digital bandpass filter having taps that are changeable based on a run time. 
     FIG. 6 illustrates another embodiment of a noise estimator. In this embodiment, noise estimator  600  includes a geophone  610 , an amplifier  620 , a digitizer  630 , a fast fourier transform circuit (FFT)  645 , a complex conjugate multiplier circuit  665 , a multiplexer  675 , and a digital summation circuit  690 . It should be noted that a geophone, an amplifier, a digitizer, a fast fourier transform circuit, a complex conjugate multiplier circuit, a multiplexer, and a digital summation circuit are well known in the art; accordingly, a detailed description of their internal components and their operation is not provided herein. 
     The digital signal characterizing the magnitude and frequency content of the noise energy is applied to FFT  645 . FFT  645  functions as a bank of narrow band filters and generates an output  646  in the form of a complex number. The magnitude of output  646  is squared  665  to generate an estimate of the power spectral density. A multiplexer  675  is used to select from among the different frequency bins  667  of the power spectral density. The output of the multiplexer  675  is applied to digital summation circuit  690  to generate the noise estimate  691 . 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.