Patent Publication Number: US-2011075514-A1

Title: Apparatus and methods for attenuating seismic noise associated with atmospheric pressure fluctuations

Description:
FIELD OF THE INVENTION 
     This invention relates to apparatus and methods for attenuating noise associated with atmospheric pressure fluctuations in a seismic signal acquired during land seismic data acquisition operations. 
     BACKGROUND TO THE INVENTION 
     Wind is considered to be one of the most common sources of noise in acquired seismic signals. Wind noise generally appears in a seismic signal when there is a significant amount of wind (surface pressure fluctuation or atmospheric events) on the surface above the seismic sensors during seismic data acquisition. Wind on the surface causes wind-ground coupled seismic waves under the surface which are recorded by the seismic sensors as noise, which in turn can lead to considerable degradation of the quality of the data. 
     Generally speaking, noise due to atmospheric events can be attributed to two distinct phenomena, namely acoustic waves generated by a point source (for example sound waves generated by noisy machinery) and fluctuating atmospheric pressure associated with a convection phenomenon (wind). The present invention is concerned mainly with the attenuation of noise associated with fluctuating atmospheric pressure using dual sensors (seismic- and pressure sensors). 
     The combination of geophones and microphones to record seismic noise for the attenuation of air-coupled waves has been reported in the literature (for example in Bass, H. E., and Bolen, L. N., Cress, D., Lundien, J., Flohr, M., 1980, Coupling of airborne sound into the earth: Frequency dependence, J. Acoust. Soc. Am., 67(5), 1502-1506; Sabatier, J. M., Bass, H. E., Bolen, L. N., and Attenborough, K., 1986a, Acoustically induced seismic waves, J. Acoust. Soc. Am., 80(2), 646-649; and Sabatier, J. M., Bass, H. E., and Elliot, G. R., 1986b, On the location of frequencies of maximum acoustic-to-seismic coupling, J. Acoust. Soc. Am., 80(4), 1200-1202). However, these works are confined mainly to the measurement of an acoustic-to-seismic transfer function. 
     The patents of Cowles, C. S., 1979, Combination Geophone-Hydrophone, U.S. Pat. No. 4,134,097 A1 and Brittan, J., and Starr, J. G., 2004, Method for Processing Dual Sensor Seismic Data to Attenuate Noise, U.S. Pat. No. 6,894,948 are further examples in the literature of descriptions of the use of dual sensors. These works refer to geophone-hydrophone combinations to remove noise of borehole and marine seismic data respectively and do not address the attenuation of wind noise. 
     The patent of Crews, G. A. and Martinez, D. R., Seismic Exploration Method and Apparatus for Cancelling Non-Uniformly Distributed Noise, U.S. Pat. No. 4,890,264 mentions that microphones can be used to detect and effectively cancel the non-uniformly distributed effects (non-coherent) of wind noise without providing any detailed description. For example, the disclosure deals with wind noise which “adversely affects the recording of seismic waves by moving cables or geophones . . . ” at Column 4, line 64. 
     It is accordingly an object of the present invention to provide improved apparatus and methods for attenuating noise associated with atmospheric pressure fluctuations in recorded seismic signals during single sensor seismic data acquisition operations. The invention also addresses the broader problem of attenuating noise recorded in seismic signals during seismic data acquisition operations and specifically deals with the attenuation of coherent noise caused by fluctuating pressure in the atmosphere above a seismic data acquisition array. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a first aspect of the invention to provide apparatus for attenuating noise associated with atmospheric pressure fluctuations in a seismic signal during seismic data acquisition, including: 
     at least a pair of sensors comprising a seismic sensor and a pressure sensor for concurrently receiving a seismic signal and a pressure signal respectively, the sensors being adapted individually to transmit the respective seismic and pressure signals to a remote recording station which is adapted to record a plurality of seismic and pressure signals; and
 
data processing means including filter means for removing, at least partly, noise associated with atmospheric pressure fluctuations in the seismic signal, wherein the filter means employs an input signal from the pressure sensor; and a model of the coupling between the atmosphere and the ground to generate a reference signal which is combined with the seismic signal to produce an output signal.
 
     The model of the coupling between the atmosphere and the ground is preferably used to predict the type of signals which may be generated by the pressure fluctuations. With this knowledge, it is possible to predict the type of seismic waves generated (phase and amplitude of the seismic signals) and to have a better spatial sampling of the signals (distribution of the geophones on the ground). 
     In a preferred form of the invention, the pressure sensor comprises a microphone, more preferably a MEMS microphone. 
     The data processing means may include additional filter means in the form of a noise cancellation filter for removing noise in the seismic- and pressure signals that are not related to atmospheric pressure fluctuations. In a preferred form of the invention the noise cancellation filter comprises an adaptive Recursive Least Squares noise cancellation filter. 
     The Recursive Least Squares (RLS) noise cancellation filter is used to attenuate non-coherent atmospheric noise in the proximity of the data acquisition operations, such as noise generated by machinery and workers generally. This type of noise would mainly be in the form of point source acoustic events. 
     In a preferred form of the invention, the data processing means includes scaling means for rescaling at least one of the seismic- and pressure signals. 
     The seismic signal would normally be scaled to match the pressure signal before the signals are combined 
     In this specification and in the appended claims, the terms “combine” or “combined” mean, insofar as they relate to seismic, pressure or reference signals, either “added to” or “subtracted from” depending on the phase shift of the signals which are combined. For example, where the phase shift between the seismic signal and the pressure signal is zero, the pressure signal is subtracted from the seismic signal to produce an attenuated output signal. Where there is a 90 degree phase shift between the signals, the pressure signal is added to the seismic signal to produce an attenuated output signal. 
     Optionally, the data processing means may include a band pass filter for passing the seismic signal and the pressure signal there-through to establish a common minimum and maximum frequency band for both sensors. 
     It is a second aspect of the invention to provide a method of real-time processing of seismic data during single sensor seismic data acquisition operations comprising the steps of: 
     receiving, at a remote recording station, a seismic signal transmitted by a seismic sensor and receiving a pressure signal concurrently transmitted by a pressure sensor;
 
employing an input signal from the pressure sensor and a model of the coupling between the atmosphere and the ground to generate a reference signal; and combining the reference signal with the seismic signal to produce an output signal.
 
     The method may include the step of passing both the seismic signal and the pressure signal through a noise cancellation filter after receipt at the remote recording station in order to remove noise in at least one of the seismic- and pressure signals which is not related to atmospheric pressure fluctuations. 
     The method may include the further step of passing the seismic signal and the pressure signal through a band pass filter to establish a common minimum and maximum frequency band for the signals. 
     The method may include the further step of rescaling at least one of the seismic signal and pressure signal after removal of noise by the noise cancellation filter. 
     It is a third aspect of the invention to provide a method of off-line processing of seismic data recorded during single sensor seismic data acquisition operations comprising the steps of: 
     receiving, at a remote recording station, a seismic signal transmitted by a seismic sensor buried in ground and receiving a pressure signal concurrently transmitted by a pressure sensor located above the ground;
 
selecting a time band from the recorded signals;
 
transforming the signal data from the seismic signal and the pressure signal in the selected time band from the time domain to the time-frequency domain;
 
applying adaptive filtering in the time-frequency domain to remove non-coherent noise in the signals;
 
deriving ratios of ground particle velocity over atmospheric pressure to obtain the inverse of the acoustic impedance of the ground;
 
deriving a time transfer function of the ground by applying the inverse of a transform operation to the inverse of the acoustic impedance of the ground;
 
estimating a particle velocity in the atmosphere by convoluting the transfer function; and
 
subtracting the particle velocity component of the seismic signal from the seismic signal to produce an output signal.
 
     In a preferred form of the invention, the transform operation is the Stockwell Transform. 
     It is a fourth aspect of the invention to provide a method of off-line processing of seismic data during single sensor seismic data acquisition operations comprising the steps of: 
     receiving, at a remote recording station, a seismic signal transmitted by a seismic sensor and receiving a pressure signal concurrently transmitted by a pressure sensor;
 
passing the seismic signal through a filter bank wherein it is decomposed into M-bands;
 
selecting bands for processing;
 
reconstructing a seismic signal from the selected bands;
 
normalizing the reconstructed seismic signal and the pressure signal;
 
applying a Recursive Least Squares algorithm to at least one of the signals to remove non-coherent noise; and
 
combining the signals to produce an output signal.
 
     The bands are preferably decimated after filtering and over-sampled and interpolated before reconstruction. 
     It is a fifth aspect of the invention to provide a seismic data recording array for comprising: 
     a plurality of seismic sensors linearly disposed and buried in the ground;
 
a plurality of pressure sensors interspersed between and exposed to the atmosphere above the seismic sensors: and
 
a remote recording station;
 
the seismic sensors and the pressure sensors being adapted individually to transmit the respective seismic- and pressure signals to the remote recording station:
 
characterised in that the distance between successive pressure sensors is substantially equal to an estimated wavelength of a pressure fluctuation waveform.
 
     The seismic data recording array may be further characterised in that the distance between successive seismic sensors is substantially half the estimated wavelength of the pressure fluctuation waveform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further aspects of this invention will now be described in more detail with reference to the following drawings, in which: 
         FIG. 1  is a schematic representation of a dual sensor pair according to the invention, located in a position suitable for seismic data acquisition; 
         FIG. 2  is a flow diagram of a method of real time data processing of seismic and pressure data according to the invention; 
         FIG. 3  is a flow diagram of a first method of off-line data processing of seismic and pressure data according to the invention; 
         FIG. 4  is a flow diagram of a second method of off-line data processing of seismic and pressure data according to the invention; 
         FIG. 5  shows the phase shift between a seismic signal and a concurrently transmitted pressure signal during seismic data acquisition operations; 
         FIG. 6  shows graphic examples illustrating the attenuation of noise associated with atmospheric pressure fluctuations during the seismic data acquisition; 
         FIG. 7  shows graphic examples of autocorrelation of a pressure signal and seismic signals and cross correlation of the pressure and seismic signals during methods of data processing according to the invention; and 
         FIG. 8  is a plan view schematic representation of one embodiment of the layout of a seismic data recording array according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , apparatus  10  according to an example of the invention shows a seismic sensor in the form of a geophone  12  and a pressure sensor in the form of a Micro Electrical-Mechanical System (MEMS) microphone  14 . The geophone  12  is buried beneath the surface  16  of the ground  18  and the microphone  14  is exposed to the atmosphere  20  above the surface  16  of the ground  18 . 
     During seismic data acquisition operations, a seismic source, such as a seismic vibrator  22  is energised which excites a series of seismic waves in a specific pattern and frequency range (sweep)  24 . Return waves  26  are reflected from geological formations (not shown) underground and are received by the geophone  12 . 
     The geophone  12 , which forms part of a recording array (not shown in this drawing), responds to the seismic waves  26  and produces a corresponding seismic signal  13 . Each geophone  12  in the array then individually transmits the signal  13  to a remote data recording station  28 . 
       FIG. 1  further shows an atmospheric pressure wave  30  travelling in direction A towards apparatus  10 . The pressure wave  30  produces a ground coupled seismic wave  32  under the surface  16  of the ground  18 , which is received by the geophone  12 . Thus, the seismic signal  13  transmitted to the remote data recording station  28  includes the signal representative of the reflected seismic wave  26  as well as the signal representative of the ground coupled wave  32 , which is regarded as noise which degrades the signal  13 . 
     The pressure wave  30  represents pressure fluctuations or wind in the atmosphere  20  above the apparatus  10 . These pressure fluctuations are traced by the microphone  14  which transmits a pressure signal  15  concurrently with the transmission of the seismic signal  13 , to the remote recording station  28 . The pressure signal  15  is representative of the ground coupled seismic signal  32 , (noise) and is used to attenuate the noise from the seismic signal  13  as is described hereunder. 
       FIG. 2  illustrates how real-time processing of the seismic and pressure signals  13 ,  15  respectively of  FIG. 1  is executed. 
     Apparatus  10  produces a seismic signal  13  and a pressure signal  15  as described with reference to  FIG. 1 . At  40 , both the seismic signal  13  and the pressure signal  15  are passed through an adaptive Recursive Least-Squares (RLS) noise cancellation filter in order to remove noise not related to atmospheric pressure fluctuations. The processed signals  13 ,  15  are then passed through a band pass filter to establish the same minimum and maximum frequency band for both signals. This step is optional and will depend on the specifications of the sensors  12 ,  14  and the data acquisition parameters selected. Due to a difference in scale of the signals  13 ,  15 , a scale factor is used to rescale the seismic signal  13  to match the pressure signal  15 . These operations are executed at  40 . From a theoretical model, it is possible to estimate the amplitude of the seismic signal  13  and the scaling factor is determined from the estimated amplitude of the seismic signal and the maximum value of the pressure signal  15 . It is also possible simply to apply an ad hoc approach by using the wind speed measured during the recording to scale the seismic signal  13 . Applicant has found that both methods can be satisfactorily applied but in real time processing, the approach based on the theoretical model is recommended. 
     The output of  40  produces an equivalent seismic signal and pressure signal at  42 . 
     The final step is to use the output pressure signal of step  42  and to combine it with the output seismic signal to remove the noise due to atmospheric pressure fluctuations from the seismic signal and to produce an attenuated seismic signal at  44 . 
       FIG. 3  is a flow diagram of a first method of off-line data processing of seismic and pressure data according to the invention. This method is used when the interaction of the pressure fluctuations with the ground manifests itself in the form of coherent events in the signals of the sensors  12 ,  14 . 
     In this drawing, the apparatus  10  is shown to transmit the seismic signal  13  and the pressure signal  15  to a remote recording station  50  (see dotted line). 
     At the recording station  50 , an appropriate time band of both signals is selected at  52  for further processing. The data from both the seismic signal  13  and the pressure signal  15  is then transformed, using the S-transform, from the time domain into the time-frequency domain at  54 . This step is completed in preparation for the next step, which is adaptive filtering of non-coherent noise in both the seismic signal  13  and the pressure signal  15  at  56 . Applicant has found that adaptive filtering of the non-coherent noise can be more conveniently done in the time-frequency domain than the time domain. After adaptive filtering, ratios of ground particle velocity over atmospheric pressure are derived at  58  to obtain the inverse of the acoustic impedance of the ground. A time transfer function of the ground is derived by applying the inverse of the Stockwell Transform to the inverse of the acoustic impedance of the ground at  60 . Thereafter, a particle velocity in the atmosphere is estimated by convoluting the transfer function at  62 . Finally, by subtracting the particle velocity component of the seismic signal from the seismic signal at  64 , an attenuated output signal is produced. 
       FIG. 4  is a flow diagram of a second method of off-line data processing of seismic and pressure data according to the invention. This method is used where the pressure fluctuation signals are random events. Applicant has found that in this case, the use of a filter bank system is the most effective. 
     In this drawing, the seismic signal  13  is shown to be passed through a filter bank  70 , where the signal  13  is decomposed into M-bands by way of a set of low-pass, band-pass and high-pass filters  72 . For a more detailed description of this known method for noise attenuation reference can be made to Ozbek, Ali, Adaptive Seismic Noise and Interference Attenuation Method, U.S. Pat. No. 6,446,008 B1 and Ozbek, Ali, Noise Filtering Method for Seismic Data, International Publication No. WO 97/25632, 17 Jul. 1997. The object of this known method is to identify (or estimate) the coherent noise in the signal, which is removed in the next step at  80 . Essentially, an extraction of spectral components of the signal  13  is effected to produce multiple output signals from the original signal. To mitigate complexity, each output signal is decimated. At this stage, the appropriate parts (bands) of the original seismic signal  13  are selected to be further processed by way of a soft threshold process at  74 . Each signal band signal is then over sampled and passed through an interpolation filter. An output signal is then reconstructed from the selected bands at  76 . 
     The reconstructed seismic signal  13  and the pressure signal  15  are then normalised at  78 . The signals are then filtered in three iterations. In the first iteration, coherent noise in the seismic signal  13  is attenuated when signals are passed through a Recursive Least Squares noise cancellation filter  80 . In a second iteration, electromagnetic noise in both the seismic and pressure signals is attenuated. Ambient noise is removed in a third iteration. The data is then denormalised at  82  to produce a filtered seismic signal  13  and a filtered pressure signal  15 . The output of this step can be used to remove pressure fluctuation noise from the seismic signal  13  having, as a reference, the pressure signal  15  data. 
     In  FIG. 5  the phase shift between a seismic signal  13  and a concurrently transmitted pressure signal  15  is shown the instant that the components of the dual sensor (seismic- and pressure-sensors) detect pressure fluctuations in the form of wind noise signals. Experiments by the Applicant predict an out of phase relationship between pressure and particle velocity. This allows a simple “subtraction” of the pressure signal from the seismic signal  13  to cancel the effect of pressure fluctuation in the seismic signal  13 . 
     In  FIG. 6 , graphic examples  6 A to  6   d  illustrate the attenuation of noise associated with atmospheric pressure fluctuations during seismic data acquisition. In particular,  FIG. 6   a  shows a power spectrum of the seismic signal  13  after having passed through the RLS noise cancellation filter.  FIG. 6   b  shows the power spectrum of the seismic signal of  FIG. 6   a  appropriately scaled.  FIG. 6   c  shows the power spectrum of the pressure signal  15  after RLS noise cancellation and after it passed through a band pass filter.  FIG. 6   d  shows the attenuation of pressure fluctuation noise on the seismic signal  13  given in dB for different center frequencies and wind speeds. 
     In  FIG. 7 , graphic examples of autocorrelation of a pressure signal and seismic signals and cross correlation of the pressure and seismic signals during methods of data processing according to the invention are shown. 
     In particular,  FIG. 7   a  shows the autocorrelation of the pressure signal  15 .  FIG. 7   b  shows the autocorrelation of seven individually recorded seismic signals, one of them being the seismic signal  13  from the geophone  12  of  FIG. 1  and the other six from geophones in the proximity of geophone  12 .  FIG. 7   c  shows the cross-correlation of the pressure signal  13  and seven seismic signals. In this drawing, a good correlation at zero time lag can be observed. It shows that both sensors  12  and  14  of  FIG. 1  are recording a similar event that can be attributed to wind noise (no seismic source was used in this instance). Some noise can be observed at lags other than zero. 
     It will be appreciated that in  FIG. 7   c , it is shown that the polarity of the cross-correlation is reversed when compared to  FIGS. 7   a  and  7   b . This is because the pressure-time series has the opposite amplitude to the particle velocity (seismic) time series as is also observed in  FIG. 5  above. 
       FIG. 8  shows the preferred layout of a seismic data recording array  100  in plan view. Geophones  12  are linearly disposed and buried in the ground in the prevailing wind direction A. Interspersed between the geophones  12  are a plurality of MEMS microphones  14 , exposed to the atmosphere  20  above the geophones  12  as shown. In this embodiment, a microphone  14  is paired with every second geophone  12 . 
     The geophones  12  and microphones  14  are spaced such that the distance between two successive geophones  12  is about half the wavelength of an estimated pressure fluctuation waveform  102 . In the present instance, the wavelength of the pressure fluctuation waveform is about 10 meters, an accordingly, the geophones  12  are spaced by 5 meters and the microphones  14  by 10 meters. The remote recording station is not shown in this drawing. 
     While the invention is described through the above exemplary embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while the preferred embodiments are described in connection with various illustrative processes, one skilled in the art will recognize that the system may be embodied using a variety of specific procedures and equipment and could be performed to evaluate widely different types of applications. Accordingly, the invention should not be viewed as limited except by the scope of the appended claims