Abstract:
A technique includes obtaining different sets of data, which are provided by seismic sensors that share a tow line in common. Each data set is associated with a different spatial sampling interval. The technique includes processing the different sets of data to generate a signal that is indicative of a seismic event that is detected by the set of towed seismic sensors. The processing includes using the different spatial sampling intervals to at least partially eliminate vibration noise from the signal.

Description:
BACKGROUND 
       [0001]    The invention generally relates to removing vibration noise from seismic data that is obtained from towed seismic sensors. 
         [0002]    Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves which propagate into the geological formations creating pressure changes and vibrations along their way. Changes in elastic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (hydrophones), others to particle motion (geophones), and industrial surveys may deploy only one type of sensors or both. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits. 
         [0003]    Some surveys are known as “marine” surveys because they are conducted in marine environments. However, “marine” surveys may be conducted not only in saltwater environments, but also in fresh and brackish waters. In a first type of marine survey, called a “towed-array” survey, an array of seismic sensor-containing streamers and sources is towed behind a survey vessel. In a second type of marine survey, an array of seismic cables, each of which includes multiple sensors, is laid on the ocean floor, or sea bottom; and a source is towed behind a survey vessel. 
         [0004]    The data that is recorded from the towed streamers may be contaminated with vibration noise. The vibration noise typically has a relatively slow apparent velocity along the streamer, and the vibration noise inside the signal cone may be reduced by increasing the density (and number) of the sensors along the streamer. However, it may be impractical and/or relatively costly to reduce the vibration noise to the desired level by merely increasing the number of sensors. 
       SUMMARY 
       [0005]    In an embodiment of the invention, a technique includes obtaining different sets of data, which are provided by towed seismic sensors that share a tow line in common. Each data set is associated with a different spatial sampling interval. The technique includes processing the different sets of data to generate a signal that is indicative of a seismic event that is detected by the set of towed seismic sensors. The processing includes using the different spatial sampling intervals to at least partially eliminate vibration noise from the signal. 
         [0006]    In another embodiment of the invention, a system includes an interface and a processor. The interface receives different sets of data, which are provided by seismic sensors that share a tow line in common while in tow, and each data set is associated with different spatial sampling intervals. The processor generates a signal that is indicative of a seismic event that is detected by the set of seismic sensors, and the processor uses the different spatial sampling intervals to at least partially eliminate vibration noise from the signal. 
         [0007]    In another embodiment of the invention, an article includes a computer accessible storage medium to store instructions that when executed by a processor-based system cause the processor-based system to obtain different sets of data, which are provided by seismic sensors that share a tow line in common. Each data set is associated with a different spatial sampling interval. The instructions when executed by the processor-based system cause the system to process the different sets of data to generate a signal that is indicative of a seismic event that is detected by the set of towed seismic sensors and use the different spatial sampling intervals to at least partially eliminate vibration noise from the signal. 
         [0008]    In yet another embodiment of the invention, a system includes a streamer and first and second sets of seismic sensors, both of which are located on the streamer. Adjacent sensors of the first set are separated by a first distance, and adjacent sensors of the second set are separated by a second distance. Neither the first distance nor the second distance is a multiple of the other of the first and second distances. 
         [0009]    Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0010]      FIG. 1  is a schematic diagram of a marine seismic acquisition system according to an embodiment of the invention. 
           [0011]      FIG. 2  is a plot in frequency-wave number (f-k) space of exemplary vibration noise that is present in a signal that is recorded from a towed streamer. 
           [0012]      FIG. 3  is a plot in f-k space of an exemplary signal that is recorded from a towed streamer. 
           [0013]      FIG. 4  is a flow diagram depicting a technique to remove vibration noise from a signal that is recorded from a towed streamer according to an embodiment of the invention. 
           [0014]      FIGS. 5 and 6  are plots in f-k space of exemplary signals recorded using different spatial sampling intervals according to an embodiment of the invention. 
           [0015]      FIGS. 7 and 8  are plots in f-k space of the signals in  FIGS. 5 and 6 , respectively, after filtering to remove velocity noise according to an embodiment of the invention. 
           [0016]      FIGS. 9 and 10  are plots in f-k space of the signals in  FIGS. 7 and 8 , respectively, after frequency band filtering according to an embodiment of the invention. 
           [0017]      FIG. 11  is a schematic diagram of a seismic data processing system according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  depicts an embodiment  10  of a marine seismic data acquisition system in accordance with some embodiments of the invention. In the system  10 , a survey vessel  20  tows one or more seismic streamers  30  (one exemplary streamer  30  being depicted in  FIG. 1 ) behind the vessel  20 . The seismic streamers  30  may be several thousand meters long and may contain various support cables (not shown), as well as wiring and/or circuitry (not shown) that may be used to support communication along the streamers  30 . 
         [0019]    Each seismic streamer  30  contains seismic sensors, which record seismic signals. In accordance with some embodiments of the invention, the seismic sensors are multi-component seismic sensors  58 , each of which is capable of detecting a pressure wavefield and at least one component of a particle motion that is associated with acoustic signals that are proximate to the multi-component seismic sensor  58 . Examples of particle motions include one or more components of a particle displacement, one or more components (inline (x), crossline (y) and depth (z) components, for example) of a particle velocity and one or more components of a particle acceleration. 
         [0020]    Depending on the particular embodiment of the invention, the multi-component seismic sensor  58  may include one or more hydrophones, geophones, particle displacement sensors, particle velocity sensors, accelerometers, or combinations thereof. 
         [0021]    For example, in accordance with some embodiments of the invention, a particular multi-component seismic sensor  58  may include a hydrophone  55  for measuring pressure and three orthogonally-aligned accelerometers  50  to measure three corresponding orthogonal components of particle velocity and/or acceleration near the seismic sensor  58 . It is noted that the multi-component seismic sensor  58  may be implemented as a single device (as depicted in  FIG. 1 ) or may be implemented as a plurality of devices, depending on the particular embodiment of the invention. 
         [0022]    The marine seismic data acquisition system  10  includes one or more seismic sources  40  (one exemplary source  40  being depicted in  FIG. 1 ), such as air guns and the like. In some embodiments of the invention, the seismic sources  40  may be coupled to, or towed by, the survey vessel  20 . Alternatively, in other embodiments of the invention, the seismic sources  40  may operate independently of the survey vessel  20 , in that the sources  40  may be coupled to other vessels or buoys, as just a few examples. 
         [0023]    As the seismic streamers  30  are towed behind the survey vessel  20 , acoustic signals  42  (an exemplary acoustic signal  42  being depicted in  FIG. 1 ), often referred to as “shots,” are produced by the seismic sources  40  and are directed down through a water column  44  into strata  62  and  68  beneath a water bottom surface  24 . The acoustic signals  42  are reflected from the various subterranean geological formations, such as an exemplary formation  65  that is depicted in  FIG. 1 . 
         [0024]    The incident acoustic signals  42  that are generated by the sources  40  produce corresponding reflected acoustic signals, or pressure waves  60 , which are sensed by the multi-component seismic sensors  58 . It is noted that the pressure waves that are received and sensed by the seismic sensors  58  may be primary pressure waves that propagate to the sensors  58  without reflection, as well as secondary pressure waves that are produced by reflections of the pressure waves  60 , such as pressure waves that are reflected from an air-water boundary  31 . 
         [0025]    In accordance with some embodiments of the invention, the seismic sensors  58  generate signals (digital signals, for example), called “traces,” which indicate the detected pressure waves. The traces are recorded and may be at least partially processed by a signal processing unit  23  that is deployed on the survey vessel  20 , in accordance with some embodiments of the invention. For example, a particular multi-component seismic sensor  58  may provide a trace, which corresponds to a measure of a pressure wavefield by its hydrophone  55  and may provide one or more traces, which correspond to one or more components of particle motion, which are measured by its accelerometers  50 . 
         [0026]    The goal of the seismic acquisition is to build up an image of a survey area for purposes of identifying subterranean geological formations, such as the exemplary geological formation  65 . Subsequent analysis of the representation may reveal probable locations of hydrocarbon deposits in the subterranean geological formations. Depending on the particular embodiment of the invention, portions of the analysis of the representation may be performed on the seismic survey vessel  20 , such as by the signal processing unit  23 . In accordance with other embodiments of the invention, the representation may be processed by a seismic data processing system (such as an exemplary seismic data processing system  600  that is depicted in  FIG. 11  and further described below) that may be, for example, located on land or on the vessel  20 . Thus, many variations are possible and are within the scope of the appended claims. 
         [0027]    The seismic streamers  30  may contain, in accordance with some embodiments of the invention, geophones, which may be particularly sensitive to vibration noise. As a result, the seismic streamers  30  may introduce vibration noise into the seismic data. For example,  FIG. 2  is a plot  100  in frequency-wave number (f-k) space of exemplary vibration noise  104 , which may be present in a signal that is recorded from a streamer  30 .  FIG. 3  generally depicts an f-k space plot  106  of a recorded signal that contains content  110  that represents the detected seismic event, as well as the vibration noise  104 . For a sufficiently small spatial sampling interval (i.e., the uniform distance between the sensors of the streamer  30 , which provide the data set), the content  110  is concentrated within the signal cone (about wavenumber zero) and is distinguishable from the vibration noise  104 . However, achieving a spatial sampling interval that results in sufficient elimination of the vibration noise  104  from the signal cone may require a large number of closely-spaced sensors, an arrangement that may be quite costly and technically challenging. 
         [0028]    Instead of reducing vibration noise in the recorded signal by relying solely on a small spatial sampling interval, an approach in accordance with embodiments of the invention described herein uses multiple spatial sampling intervals to achieve the same result. More specifically, in accordance with some embodiments of the invention, the streamer has sensors that are organized to have two different spacing intervals. In other words, the streamer includes a first set of sensors, which are spaced apart pursuant to a first spacing distance and a second set of sensors, which are spaced apart by a second spacing distance that is different than the first distance. Although each of the recorded signals may contain vibration noise that invades the signal cone, noise contamination occurs at different frequencies for the two data sets. Therefore, the two data sets may be frequency filtered to remove the corresponding signal content that falls within the contaminated frequency bands. Because the filtered out frequency bands do not overlap, the two frequency filtered data sets may be combined to generate a single full bandwidth data set, which represents a recorded seismic signal that contains very little, if any, vibration noise in the signal cone. 
         [0029]    As a more specific example, in accordance with some embodiments of the invention, a technique  150  that is depicted in  FIG. 4  may be used to remove vibration noise. Pursuant to the technique  150 , two sets of data, which are recorded from the same streamer are obtained; and each set of data is associated with a different spatial sampling interval, as depicted in block  152 . It is noted that each spatial sampling interval may be too large for purposes of sufficiently eliminating vibration noise from the corresponding data set. Thus, the signal that corresponds to each data set may have vibration noise that is aliased into the signal cone. Additionally, it is noted that in accordance with embodiments of the invention, the spatial sampling intervals are not multiples of each other for purposes of ensuring that the vibration noise is not aliased into the same frequency band(s). 
         [0030]    Pursuant to the technique  150 , wavenumber filtering may first be applied to the data sets to filter out (block  154 ) vibration noise. It is noted that wavenumber filtering is one type of filtering, although filtering may be used to remove vibration noise. For example, in accordance with other embodiments of the invention, the filtering that is applied may be more complex than just truncation in a frequency band. For example, the filtering may involve a weighted sum, which is dependent on the noise levels, for example. As a more specific example, the filtering applied in block  154  may be the same type of filtering discussed in U.S. Pat. No. 6,446,008, entitled “ADAPTIVE SEISMIC NOISE AND INTERFERENCE ATTENUATION METHOD,” which issued on Sep. 3, 2002. Next, pursuant to the technique  150 , the data sets are filtered (block  158 ) to reject the corresponding content in the frequency bands in which vibration noise is present. The two sets of frequency filtered data are then combined (block  160 ) to generate a full bandwidth data set, which represents a signal that is significantly free of vibration noise in the signal cone. 
         [0031]    The technique  150  is merely provided as an example of a possible embodiment of the invention. It is noted, however, that many variations may be made to the technique that fall within the scope of the appended claims. For example, in accordance with other embodiments of the invention, block  158  may be performed before block  154 . 
         [0032]    Vibration noise may not be constant along the streamer  30  because of differences in tension, and the vibration noise may change with time in one position, such as a change due to a corresponding change in towing speed, for example. The spatial aliasing frequency for vibration noise will therefore be variable. However, such variation does not impact the technique  150 , as a change in vibration velocity merely stretches the f-k plot along the frequency axis. The stretching is similar for both data sets; and therefore, the aliasing still occurs at different frequencies for the two data sets. 
         [0033]    As a more specific example,  FIGS. 5-10  depict application of the technique  150  to data sets that are associated with 90 centimeter (cm) and 150 cm spatial sampling intervals along the same towed streamer.  FIGS. 5 ,  7  and  9  depict processing of the 90 cm interval data set (before combination with the 150 cm interval data set); and  FIGS. 6 ,  8  and  10  depict processing of the 150 cm data set (before combination with the 90 cm data set). 
         [0034]    In this regard,  FIG. 5  depicts an f-k plot  200 , which contains a signal cone  204  that is centered about wave number zero. As shown in  FIG. 5 , vibration noise is aliased into the cone  204 , such as at reference numeral  210 . For the 150 cm interval data set, an f-k plot  208  ( FIG. 6 ) reveals that vibration noise is also aliased into the cone  204  but at different frequencies than the frequencies at which the vibration noise is aliased into the signal cone  204  for the plot  200 . Thus, as depicted in  FIG. 6 , the vibration noise is aliased into the cone  204  at reference numerals  212  and  214 . 
         [0035]      FIGS. 7 and 8  depict the two data sets after wave number filtering. In this regard, the wave number filtering removes seismic data associated with slower waves. Thus, an f-k plot  220  ( FIG. 7 ) shows the result of the wave number filtering for the 90 cm interval data set, which results in signal content that outside of a wave number band  230  being removed. Similarly, an f-k plot  250  ( FIG. 8 ) shows the result of the wave number filtering for the 150 cm interval data set, which results in signal content that outside of a wave number band  231  being removed. 
         [0036]    Frequency band rejection filters are next applied to the two data sets to remove the content from frequency bands in which the vibration noise is aliased into the signal cone  204 . For example,  FIG. 9  depicts the application of a frequency band rejection filter to the 90 cm interval data set to remove the content from a frequency band  282 , which corresponds to frequencies (such as at reference numeral  210  in  FIGS. 5 and 7 ) in which the vibration noise is aliased into the signal cone  204 . For the 150 cm interval data set, two frequency band rejection filters are applied to reject a frequency band  312 , which corresponds to the vibration noise at reference numeral  212  (see  FIGS. 6 and 8 ) and a frequency band  314 , which corresponds to the frequencies at reference numeral  214  (see  FIGS. 6 and 8 ). 
         [0037]    As can be seen from a comparison of  FIGS. 9 and 10 , as a result of the frequency filtering, the two frequency filtered data sets may be combined to produce a data set, which corresponds to a full bandwidth signal, which is significantly free of vibration noise. Thus, with the combination, signal content from the non-frequency filtered bands  317  and  319  (see  FIG. 9 ) of the 90 cm sampling interval data set are combined with signal content from the non-frequency filtered band  321  (see  FIG. 10 ) of the 150 cm sampling interval data set to generate the full bandwidth composite data set that is substantially free of vibration noise. 
         [0038]    Specific spatial sampling intervals of 90 cm and 150 cm are set forth herein for purposes of example. However, it is noted that other sampling intervals may be used in other embodiments of the invention. For example, in other embodiments of the invention, sensor spacing interval pairs of 140 cm and 250 cm; 113 cm and 210 cm; or 113 cm and 312.5 cm may be used, depending on the particular embodiment of the invention. Other spacing interval pairs may be preferable for optimal noise and sensor number reduction. Thus, many variations are possible and are within the scope of the appended claims. 
         [0039]    It is noted that the seismic sensors may take on numerous forms, depending on the particular embodiment of the invention. Thus, although the seismic sensors are described above as being geophones, which may be particularly sensitive to vibration noise, the techniques and systems that are described herein may likewise be applied to sensors other than geophones. For example, depending on the particular embodiment of the invention, the seismic sensors may be multicomponent sensors, moving coiled geophones, microelectromechanical sensors (MEMs), accelerometers, piezo accelerometers or any combination thereof. Thus, many variations are possible and are within the scope of the appended claims. 
         [0040]    Referring to  FIG. 11 , in accordance with some embodiments of the invention, a seismic data processing system  600  may perform the technique  150  and variations thereof to generate a data set from which vibration noise has been filtered. In accordance with some embodiments of the invention, the system  600  may include a processor  602 , such as one or more microprocessors or microcontrollers. The processor  602  may be coupled to a communication interface  630  for purposes of receiving the seismic data (such as the data sets that correspond to the different spatial sampling intervals). As examples, the communication interface  630  may be a USB serial bus interface, a network networked interface, a removable media (such as a flash card, CD-ROM, etc.) interface, or a magnetic storage interface (an IDE or SCSI interface, as just a few examples). Thus, the communication interface  630  may take on numerous forms, depending on the particular embodiment of the invention. 
         [0041]    The communication interface  630  may be coupled to a memory  610  of the computer  600 , which may, for example, store the various data sets involved with the technique as indicated at reference numeral  620 , in accordance with some embodiments of the invention. Additionally, the memory  610  may store at least one application program  614 , which is executed by the processor  602  for purposes of performing the technique  150 . The memory  610  and communication interface  630  may be coupled together by at least one bus  640  and may be coupled by a series of interconnected buses and bridges, depending on the particular embodiment of the invention. 
         [0042]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.