Abstract:
A method of marine seismic surveying, whereby a seismic streamer is towed in a body of water while deploying three (3) sensors in close proximity to each other. In one embodiment of the invention, the first sensor generates a first signal indicative of pressure of the water, the second sensor generates a second signal indicative of vertical movement of water, and the third sensor generates a third signal indicative of vertical movement of the streamer relative to the water. A seismic signal is generated in the water and the resulting signal is detected with each of the three (3) sensors simultaneously during time intervals following the generation of the seismic signal. The second signal is combined with the third signal to generate a fourth signal that measures vertical movement of the water with noise due to streamer vibration attenuated. The first signal is combined with the fourth signal to give a signal that attenuates the ghost due to the water surface. In a second embodiment of the invention, the third sensor is sensitive to the pressure in the water and the vertical motion of the sensor. By combining the output of the three sensors, a substantially noise-free and ghost-free seismic signal is obtained.

Description:
BACKGROUND OF THE INVENTION 
     1. Background of the Invention 
     The present invention relates generally to marine seismic surveying and, more particularly, to a method and apparatus for reducing the signal noise from vertical movement in a dual sensor towed streamer cable caused by vibrations in the stress members of the streamer. 
     2. Description of Related Art 
     Seismic surveying is a method for exploring subterranean formation layers in the earth. An acoustic source generates seismic waves, which insonify the formation layers. Differences in acoustic impedance of adjacent formation layers cause a portion of the seismic waves to reflect from the interfaces between the formation layers. Acoustic impedance varies across formation layers since it is the product of seismic wave velocity and rock density. Seismic sensors detect the seismic waves reflected upward from the formation interfaces and record wave amplitude versus time of arrival as electrical signals for later analysis regarding the locations of the formation interfaces. 
     Marine seismic surveying is seismic surveying for formation layers in parts of the earth located beneath bodies of water. An acoustic source placed in the water, such as an airgun, generates the seismic waves which insonify the subterranean formation layers. Seismic sensors, typically arrayed at intervals along a streamer cable towed in the water behind a vessel, detect the reflected seismic waves. Marine seismic surveying typically uses pressure sensors, such as hydrophones, to detect changes in water pressure caused by seismic compression and rarefaction waves propagating through the water. The pressure sensors detect the primary pressure waves traveling upward in the water after reflection from the formation interfaces in the earth below the water. The pressure sensors also detect secondary pressure waves traveling downward in the water after a portion of the primary waves traveling upward reflect down from the water surface above. The air-water interface at the water surface has a large contrast in acoustic impedance which causes a large downward reflection. Secondary reflections are unwanted ghost waves, a type of noise in the seismic signal. 
     The water-earth interface at the water bottom may also have a large contrast in acoustic impedance. Thus, the downward-traveling secondary reflections from the water surface may reflect back upward again from the water bottom. Thus secondary reflections may continue to reverberate through the water column from surface to bottom and back. Water column reverberation is a serious source of signal noise obscuring the primary reflections carrying the sought-after information concerning the subsurface formation layers. 
     FIG. 1 shows a diagrammatic view of marine seismic surveying employing a seismic streamer cable, generally designated as  100 . A ship  102  tows a seismic streamer  104  through a body of water  106 . The seismic streamer  104  contains a plurality of sensors  108 . Subterranean substrata, such as  110  and  112 , to be explored, are located in the earth  114  beneath the body of water  106 . Interfaces, such as  116 , separate the substrata. An acoustic source  118 , such as an air gun, creates seismic waves in the water  106 . A portion of the seismic waves travel downward along ray paths  120  through the water  106  toward the earth  114 . A portion of the downward-traveling seismic waves reflect upward from an interface, such as interface  116  between substrata  110  and  112 . The reflected, upward-traveling seismic waves are primary reflections from the formation layers. The primary reflections travel upward along ray paths  122 , a portion of which intersect the towed streamer  104 . Sensors  108  deployed in the towed streamer  104  detect the primary reflections. The primary reflections travel past the towed streamer  104  and continue along ray paths  122  upward toward the air-water interface  124  at the surface of the body of water  106 . A portion of the seismic waves comprising the primary reflections reflect downward from the air-water interface  124 . The twice-reflected, downward-traveling seismic waves are secondary reflections from the water surface. The secondary reflections travel downward along ray paths  126 , a portion of which intersect the towed streamer  104 . The sensors  108  deployed in the towed streamer  104  detect the secondary reflections from the air-water interface  124 . 
     The towed streamer  104  contains a plurality of sensors  108 . Towed streamers  104  typically carry pressure sensors, such as hydrophones, which will be described below in FIG.  2 . Dual sensor towed streamers  104  carry pairs of pressure sensors and motion sensors, such as geophones or accelerometers. The present invention adds a third sensor, a noise reference sensor, which will be described below in FIG.  3 . The third sensor is a variant of the prior art pressure sensor. 
     FIG. 2 shows a diagram of a pressure sensor  200 , an acceleration-canceling hydrophone, typically used in a towed streamer. The pressure sensor  200  typically comprises a housing  202  having a first end and a second end, a first element  204  mounted at the first end of the housing  202 , and a second element  206  mounted at the second end of the housing  202 . The first element  204  is mounted parallel to the second element  206 . The housing  202  is typically made of brass and the first and second elements  204 ,  206  are typically made of piezoelectric crystal. A first pair of electric wires  208 ,  210  attaches to the opposing faces of the first element  204  and a second pair of electric wires  212 ,  214  attaches to the opposing faces of the second element  206 . The arrows in FIG. 2 show the relative polarities of the connections. 
     FIGS. 3 a - 3   d  show conceptual diagrams of an acceleration canceling hydrophone  300  subject to accelerations and passing seismic waves. The electric wires  308 ,  310 ,  312  and  314  are connected so that flexures of the elements  304 ,  306  such as shown in FIGS. 3 a  and  3   b  generate output voltages which add, resulting in a nonzero signal. The pressure manifestation of a compression seismic wave propagating past the pressure sensor  300  causes the flexure of the elements  304 ,  306  shown in FIG. 3 a , while the pressure manifestation of a rarefaction seismic wave propagating past the pressure sensor  300  causes the flexure of the elements  304 ,  306  shown in FIG. 3 b . This flexures of the elements  304 ,  306  as shown in FIGS. 3 c  and  3   d  generate output voltages which cancel, resulting in a substantially zero signal. Upward motion of the pressure sensor  300  through a fluid causes the flexure of the elements  304  and  306  shown in FIG. 3 d . The above-described con figuration accomplishes the acceleration canceling property of the typical marine hydrophone  300 . The polarity indications on FIGS.  3 ( a )- 3 ( d ) show the relative polarities of signals generated for the flexure of elements  304 ,  306  as shown. 
     Marine seismic surveying also uses motion sensors, detecting particle velocity or acceleration, in addition to pressure sensors. Motion sensors typically used in marine seismic surveying are geophones and accelerometers. Motion sensors detect the vertical velocity or acceleration of water particles accompanying seismic waves propagating past the sensors. Thus motion sensors detect primary and secondary reflections, just as pressure sensors do. Proper combination of the signals from pressure sensors and motion sensors can lead to a reduction of secondary reflections from the water surface in the seismic signal. The air-water interface causes a reverse in polarity in the downward reflected pressure wave, since the acoustic impedance of the water exceeds the acoustic impedance of the air. Thus pressure sensors detect a reverse in phase polarity for the secondary reflections from the water surface. The air-water interface does not cause a reverse in polarity in the vertical motion wave. Thus motion sensors do not sense a reverse in phase polarity for the secondary reflections from the water surface. Pressure and motion detectors sense upward traveling primary reflections with the same polarity while sensing downward traveling secondary reflections with opposite polarity. Therefore, combining the signals from pressure and motion detectors enhances the desired primary reflections and reduces the undesired secondary reflections from the water surface. 
     Combining the signals from pressure sensors with the signals from motion sensors in a dual sensor towed streamer in an effort to reduce the effects of secondary reflections from the water surface is well known in the prior art. Berni discloses methods for the combination of signals from different sensors in three patents. U.S. Pat. No. 4,345,473 teaches combining the signal from a vertical component accelerometer with the signal from a hydrophone to cancel surface reflected waves in marine surveys. U.S. Pat. No. 4,437,175 teaches combining the signal of a hydrophone with the signal of an accelerometer which has passed through a integrator and a high-pass filter. U.S. Pat. No. 4,520,467 teaches combining the filtered signals of a motion sensor and a pressure sensor in proportion to the signal-to-noise ratios of the sensors. All three Berni patents address the problem of reducing unwanted secondary reflections from the water surface by means of dual sensors in a towed streamer, but merely mention the accompanying problem of insulating or filtering out the noise generated by the movement of the streamer itself. Removing the undesired secondary reflections from the desired primary reflections is not effective if streamer vibration noise still remains to obscure the seismic signal. Signal noise caused by streamer vibration is one of the problems encountered in employing dual sensor towed streamers to solve the problem of secondary reflections from the water surface. 
     One of the particular problems with previous attempts to implement a dual sensor towed streamer has been the high level of undesired vertical motion caused by vibrations in the stress members of the towed streamer. A towed streamer is typically ballasted to be neutrally buoyant. Thus a geophone deployed in the streamer generates output signals proportional to the vertical particle velocity of the seismic reflection waves in the water. Unfortunately, the geophone also generates signals proportional to the vertical velocity of the streamer itself, as caused by vibrations of the stress members of the streamer. The geophone is detecting and recording the primary and secondary reflection waves needed to combine with the hydrophone signal to reduce the effect of secondary reflections from the water surface. However, the geophone is also detecting and recording further noise in the form of streamer motion. The vibrations of the streamer stress members add obscuring noise to the geophone data. Thus, the mere combination of a pressure wave signal from a hydrophone with a particle motion signal from a geophone may severely degrade the signal-to-noise ratio in the seismic frequency band. 
     Pavey (U.S. Pat. No. 3,282,293) discloses a device in which an attempt is made to have a velocity sensor that is sensitive only to vibrations of the cable and not to the velocity of motion of the water. The effectiveness of this device is greatly reduced due to the use of a copper coil having a significant inertial mass as part of the sensor. 
     The present invention is directed toward improving the effectiveness of the dual sensor towed streamer by reducing the signal noise coming from vertical motion caused by vibrations in the stress members of the streamer. 
     SUMMARY OF THE INVENTION 
     The present invention is a method for reducing the signal noise from vertical movement in a dual sensor towed streamer cable caused by vibrations in the stress members of the streamer. A seismic streamer is towed in a body of water while deploying three (3) sensors in close proximity to each other. The first sensor generates a first signal indicative of pressure of the water, the second sensor generates a second signal indicative of vertical movement of water particles and vertical motion of the streamer in the water, and the third sensor generates a third signal indicative of vertical movement of the streamer relative to the water, i.e., streamer noise. A seismic signal is generated in the water and the resulting signals are recorded from each of the three (3) sensors simultaneously during time intervals following the generation of the seismic signal. The second signal is combined with the third signal to generate a fourth signal that is a measurement of vertical movement with the streamer noise attenuated. The first signal is combined with the fourth signal to give a ghost free seismic reflection signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the benefits and advantages of the present invention may be obtained from the appended detailed description and drawing figures, wherein: 
     A better understanding of the benefits and advantages of the present invention may be obtained from the appended detailed description and drawing figures, wherein: 
     FIG. 1 is a diagrammatic view of marine seismic surveying utilizing a ship towing a seismic streamer through water. 
     FIG. 2 is a conceptual diagram of an acceleration-canceling hydrophone. 
     FIGS. 3 a - 3   d  are conceptual diagrams of an acceleration canceling hydrophone subject to accelerations and passing seismic waves. 
     FIG. 4 is a view of the preferred embodiment of the noise reference sensor. 
     FIG. 5 is a schematic block diagram of the system for reducing the effect of secondary reflections from the water surface using the preferred embodiment of the noise reference sensor. 
     FIG. 5 a  is a schematic block diagram of the method of reducing the effect of secondary reflections from the water surface using the preferred embodiment of the noise reference sensor 
     FIG. 5 b  (Prior Art) shows an adaptive noise cancellation system. 
     FIG. 6 is a view of an alternative embodiment of the noise reference sensor. 
     FIG. 7 is a schematic block diagram of the system for reducing the effect of secondary reflections from the water surface using the alternative embodiment of the noise reference sensor. 
     FIG. 7 a  is a schematic block diagram of the method of reducing the effect of secondary reflections film the water surface using the alternative embodiment of the noise reference sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4 shows a preferred embodiment of the third sensor  400  of this invention, the noise reference sensor. The polarity signs in FIG. 4 show the connection of the leads. The third sensor  400  is a variation of the above-described acceleration cancelling hydrophone described in connection with FIGS. 2 and 3, wherein the elements are made with two flexible sheets  404 ,  406 , preferably of polyvinylidene fluoride (“PVDF”) film, mounted parallel to each other at the opposite ends of the housing  402 , in place of the two piezoelectric crystals. Other piezoelectric material such as piezoelectric crystals or ceramics may be substituted for the PVDF film, its PVDF film is preferred because of its low mass. Due to the small mass of the PVDF film, its response to motion rive to inertial is negligible and it responds primarily to the relative motion of the cable to the water. The third sensor  400  is formed by reversing a par of wires,  408 ,  410  or  412 ,  414  on one of the two sheets  404 ,  406  of PVDF film relative to their arrangement in FIG.  3 . This has the effect of turning a hydrophone into an accelerometer. Now flexures as shown in FIGS. 3 e  and  3   d  generate output voltages which add, giving a nonzero signal. Thus the third sensor  400  is sensitive only to motion relative to the surrounding fluid. Flexures as shown in FIGS. 3 a  and  3   b  generate output voltages which cancel, giving a substantially zero signal. Thus the third sensor  400  is not sensitive to seismic waves propagating past the third sensor  400 . The streamer containing the sensors is ballasted to be nearly neutrally buoyant. Thus the skin, internal parts and internal fluids of the seismic streamer move in harmony with the water molecules around the streamer as an seismic reflection wave propagates past. The motion sensor and its gimbal support both move in harmony with the surrounding water molecules. If the third sensor  400  is vertically oriented by a gimbal support, preferably the same one containing the motion sensor, the third sensor  400  experiences the same motion as the motion sensor. 
     Flexures as shown in FIGS. 3 a  and  3   b , caused by seismic wave pressure, now generate output voltages which cancel, giving a substantially zero signal for the noise reference sensor  400 . The noise reference sensor  400  is insensitive to the pressure manifestations of seismic waves propagating past the sensor. There may be some extremely slight bending of the PVDF sheets  404 ,  406  due to their mass and acceleration relative to inertial space. But, because of the very small mass of the PVDF sheets  404 ,  406 , this sensitivity to motion will be negligible. The noise reference sensor  400  also moves with the geophone case, relative to inertial space, in response to the particle velocity of a seismic reflection wave. However, in this situation, the upper and lower PVDF sheets  404 ,  406  move in synchronization with the fluid molecules above and below the sensor. The lack of differential movement between the sensor and the fluids around it means there is no flexure of the upper and lower PVDF sheets  404 ,  406  caused by the particle motion manifestation of the seismic wave. Thus, no output voltage is generated. Therefore, the noise reference sensor  400  is substantially insensitive to seismic waves. It is substantially sensitive to vertical vibrations of the streamer caused by its being towed through the water. It is substantially sensitive to vertical vibrations of the streamer caused by its being towed through the water. 
     FIG. 5 shows a schematic block diagram of the method of the present invention for enhancing the effectiveness of a dual sensor towed streamer for reducing the effect of secondary reflections from the water surface on the seismic signal, generally designated as  500 . This method demonstrates the use of dual sensors, a pressure sensor  502  and a motion sensor  504 , in combination with the preferred embodiment of the third sensor  506 , the noise reference sensor, to reduce the noise from the vibrations of the stress members of the streamer. The third sensor  506  generates a third signal  516 , a noise reference signal, which is proportional to the vertical movement of the steamer. The third signal  516  is input to an adaptive filter  508 . B. Widrow et al.,  Adaptive Signal Processing , Prentice-Hall. Inc., 1985, describes adaptive filters. The adaptive filter  508  primarily integrates the third signal  516  (an acceleration signal) to make it equivalent to the particle velocity in the output signal  514  of the motion sensor  504  caused only by the vibrations of the streamer&#39;s stress members. The adaptive filter  508  also adjusts the third signal  516  for the low frequency cutoff of the motion sensor  504  and for the damping coefficient differences between the third sensor  506  and the motion sensor  504 . The operation of the adaptive noise canceller is discussed in Widrow. In brief, the noise canceling system of Widow as shown in FIG. 5 b  attempts to produce a system output s+n 0 −y, that is a best fit in a least squares sense to the signal s. The signal s corresponds to the output  514  of the motion sensor in FIG. 5 while the noise n corresponds to the output  516  of the noise sensor in FIG.  5 . This objective is accomplished by feeding the system output back to the adaptive filter and adjusting the filter output through an adaptive algorithm to minimize the total system output power. As noted in Widrow, the only assumption made is that s, n 0 , and y are statistically stationary with zero mean, that s is uncorrelated with n 0  and y, and that the noise n 1  is uncorrelated with n 0 . The method of Widrow is only an example of an adaptive noise cancellation algorithm and any other suitable method could also be used. 
     The adaptive filter  508  outputs a fourth signal  518 , a filtered noise reference signal. The fourth signal  518  contains the portion of the vertical motion recorded in the second signal  514  contributed by the vertical motion of the streamer itself rather than the seismic wave propagating past. The fourth signal  518  is input to the negative input of a summing junction  520 , while the second signal  514 , from the motion sensor  504 , is input to the positive terminal of the summing junction  520 . The output from the summing junction  520  is a fifth signal  522 , an enhanced motion signal. The fifth signal  522  has the streamer motion subtracted out, leaving just the water particle motion from the seismic wave. Because the third signal  516  contains just noise and none of the desired seismic reflection signals contained in the second signal  514 , these desired reflections are enhanced in the fifth signal  522 . The fifth signal  522  is scaled  523  and input to a positive terminal of a summing junction  524 , while the first signal  512 , from the pressure sensor  502 , is input to another positive terminal of the summing junction  524 . The scale factor relating the pressure and motion signals would be known to those versed in the art. It is discussed in U.S. Pat. No. 4,979,150. The output from the summing junction  524  reduces the undesired secondary reflections from the air-water interface in the first signal  512  and enhances the desired primary reflections from the formation layers in the earth, thus yielding the desired ghost-free signal  526 . 
     FIG. 5 a  is a flow chart showing the method of the preferred embodiment of the present invention using the steps indicated in FIG.  5 . Seismic signals generally designated as  550  are generated. Signals are recorded with a pressure sensor  552  (first signal), a vertical motion sensor  554  (second signal), and a noise reference sensor  506  (first signal). The third signal is adaptively filtered  558  and combined  570  with the second signal. 
     The adaptive filter  558  outputs a fourth signal, a filtered noise reference signal. The fourth signal contains the portion of the vertical motion recorded in the second signal contributed by the vertical motion of the streamer itself rather than the seismic wave propagating past. The fourth signal is combined  570  with the second signal to give a fifth signal an enhanced motion signal. The fifth signal has the streamer motion subtracted out, leaving just the water particle motion from the seismic wave. Because the third signal contains just noise and none of the desired seismic reflection signals contained in the second signal, these desired reflections are enhanced in the fifth signal. The fifth signal is scaled  573  and combined  574  with the first signal from the pressure sensor  552 , to give the desired ghost-free signal. 
     FIG. 6 shows an alternative embodiment of the noise reference sensor  600 . This alternative embodiment uses only a single sheet  604  of polyvinylidene fluoride film rather than two (2) sheets and a single pair of wires  608 ,  610  rather than two (2) pairs. The alternative noise reference sensor  600  will still be quite sensitive to acceleration relative to the surrounding fluid when the acceleration is in a direction normal to the plane of the PVDF sheet  604 . The alternative noise reference filter  600  will also be quite insensitive to acceleration if the fluid above and below the PVDF sheet  604  moves in harmony, because the PVDF sheet  604  has very low mass. This low mass gives the PVDF sheet  604  low sensitivity to inertial acceleration caused by particle velocity due to a passing seismic wave. However, because there is only one PVDF sheet  604 , the alternate embodiment of the noise reference sensor  600  will be sensitive to the pressure manifestation of the seismic reflections. This sensitivity must be corrected for when using the alternative noise reference sensor  600  in a dual sensor towed streamer. 
     FIG. 7 shows a schematic block diagram of the method of the present invention using dual sensors, a pressure sensor  702  and a motion sensor  704 , in combination with the alternative embodiment of the noise reference sensor  600 . The transfer function  710  between the pressure signal  712  sensed by the pressure sensor  702  and the third signal  716  sensed by the noise reference sensor  706  can be measured by bringing the seismic streamer to zero towing velocity long enough to record one seismic reflection record. Under this condition, the only output from the alternative noise reference sensor  706  will be due to the pressure manifestation of the passing seismic wave. This transfer function  710  can then be used to correct the pressure signal  712  generated by the pressure sensor  702  for optimum subtraction from the noise reference signal  716  under normal towing conditions. Applying the transfer function  710  to the pressure signal  712  yields the corrected pressure signal  728 . The corrected pressure signal  728  can then be subtracted from the noise reference signal  716  of the alternative noise reference sensor  706  in summing junction  730 . This subtracting yields the corrected noise reference signal  732 . The corrected noise reference signal  732  is now input to an adaptive filter  708 . The filtered output signal  718  from the adaptive filter  708 , is then subtracted from the motion signal  714  in summing junction  720 . The difference signal  722  is scaled  723  and added to the pressure sensor signal  712  at the summer  724 . This combining yields the desired final output signal, the ghost-free signal  726 . 
     FIG. 7 a  is a flow chart showing the implementation of the steps indicated in FIG.  7 . Seismic signals are generated  750  and signals recorded using a pressure sensor  752  (first signal), a motion sensor  754  (second signal), and a noise reference sensor  756  (third signal). A transfer function  760  between the first signal sensed and the third signal is measured by bringing the seismic streamer to zero towing velocity long enough to record one seismic reflection record. Under this condition, the only output from the alternative noise reference sensor  756  will be due to the pressure manifestation of the passing seismic wave. This transfer function  760  can then be used to correct the first signal generated by the pressure sensor  752  for optimum subtraction from the noise reference signal  756  under normal towing conditions. The transfer function  710  is applied to the first signal to give a corrected pressure signal. The corrected pressure signal is combined  780  with the noise third signal of the noise reference sensor  756 . This combination yields the corrected noise reference signal. The corrected noise reference signal is now input to an adaptive filter  758 . The filtered output signal from the adaptive filter  708 , is then combined with  770  the second signal motion signal  714  The output of  770  is scaled  773  and combined  774  with the first signal. This combining yields the desired final output signal, the ghost-free signal. 
     Those versed in the art would recognize that embodiments of the invention depicted in the flow charts of FIGS. 5 and 7 may be implemented by suitable electronic circuits or in a digital computer. Both types of implementation are intended to be within the scope of the invention. 
     The sensors signals—the noise reference signal, the motion signal, and the pressure signal—can be transmitted to the ship, where the appropriate integrating, filtering, scaling and combination can be performed by computer or other suitable electronic devices. 
     The present has been described with a certain degree of specificity. Variations will occur to those skilled in the art which are within the scope of this invention. while the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.