Abstract:
Apparatus and methods for reducing ghosts from hydrophone signals in a streamer towed underwater close to the sea surface. A multi-axis micro-electro-mechanical-system (MEMS) accelerometer with attitude sensing is used to reduce the frequency spectral notch in the response of the combined hydrophone-accelerometer system and to reduce the sensitivity to ghost-producing downward traveling acoustic waves that are sea-surface reflections of the primary upward traveling acoustic waves. Multi-axis spring load cells provide high compliance and mechanical isolation between stress members and the accelerometer system. The load cells also provide signals representing the vibration of the stress members. The signals can be used with an adaptive filter estimating the mechanical transfer function between the vibration and the motion of the accelerometer system to remove the vibration-induced noise from the accelerometer signals.

Description:
BACKGROUND 
   The invention relates generally to marine seismic prospecting and, more particularly to apparatus and methods for reducing the effects of spurious seismic reflections in hydrophones arrayed in a streamer towed behind a survey vessel. 
   In marine seismic exploration, a hydrophone array is towed behind a marine vessel near the sea surface. The hydrophones are in multiple linear hoses known as steamers. A seismic source, also towed near the sea surface, periodically emits acoustic energy. This acoustic energy, which is in the seismic frequency band, travels downward through the sea, reflects off underlying rock structures, and returns upward through the sea to the hydrophone array. Ideally the hydrophone array records the upward traveling seismic acoustic wave from the seabed. The hydrophone recordings are later processed into seismic images of the underlying rock structures. 
   Because a hydrophone has an omni-directional response, the hydrophone array also records a ghost response, which is the desired seismic acoustic wave reflected from the sea surface and arriving delayed in time and reversed in polarity. The ghost is a downward traveling seismic acoustic wave that, when added to the desired wave, blurs the recorded seismic image. A similar visual effect occurs with broadcast television using an old-fashioned “rabbit ear” antenna. 
   The ghost produces a notch in the frequency spectrum of a hydrophone record at f notch =c/2d, where c is the speed of sound and d is the streamer depth. Seismic streamers have been conventionally towed at a depth of 10 meters. At d=10 m, f notch =75 Hz. A frequency response extending beyond 100 Hz is required for high seismic image resolution. At d=4 m, f notch =188 Hz. Streamers are therefore towed at a depth of 4 meters to improve the resolution of a seismic image. But towing at 4 meters increases downtime due to adverse weather and accompanying high sea states. Furthermore, seismic image quality would actually improve at greater towing depths because there is less acoustic background noise at greater depths and because the auxiliary equipment used to measure and control the hydrophone positions works better at greater depths for a given sea state. 
   Thus, there is a need for towing a streamer at any practical depth with high seismic image resolution to increase the productivity of seismic surveying. 
   Ocean-bottom systems, in which the seismic streamer is laid on the seabed, reject ghosts by a technique known as p-z summation. In an acoustic wave, the pressure p is a scalar and the particle velocity u is a vector. A hydrophone records the seismic acoustic wave pressure p, with a +omni-directional response. A vertically oriented geophone, often implemented with an accelerometer, records the vertical component of the seismic acoustic wave particle velocity u z , with a figure-of-8 response, +lobe pointing down and −lobe pointing up, as illustrated in the beam patterns of  FIG. 10 . In p-z summation the velocity signal is scaled by the acoustic impedance pc of seawater and added to the pressure signal. This produces a compound sensor that has full response to the upward traveling wave and zero response to the downward traveling wave to reject the ghost. 
   Ocean-bottom streamers experience any roll angle from 0° to 360° and moderate pitch angles. To implement a vertically oriented geophone, ocean-bottom systems have used: (a) a gimbaled moving-coil geophone; (b) a 3-component, omni-tilt moving-coil geophone with attitude sensing and synthetic u z  computation external to the sensor; and (c) a 3-component, micro-electro-mechanical system (MEMS) accelerometer geophone with internal attitude sensing and synthetic uZ computation external to the sensor. But all these solutions have shortcomings, such as large size, mechanical reliability, and reliance on external computation. 
   SUMMARY 
   These shortcomings are overcome and these needs are satisfied by an underwater cable embodying features of the invention. In one aspect of the invention, an underwater cable comprises a stress member extending axially through the cable and an accelerometer system mounted within the cable. The accelerometer system produces output signals indicative of motion of the cable in response to acoustic signals and environmental conditions. A load cell system, connected between the stress member and the accelerometer system, is sensitive to mechanical vibrations in the stress member. The load cell system provides output signals indicative of the mechanical vibrations and has a mechanical compliance tending to isolate the accelerometer from the mechanical vibrations in the stress member. 
   In another aspect of the invention, a streamer apparatus comprises a streamer, a stress member extending along the streamer, a hydrophone system sensitive to acoustic pressure and providing pressure signals, and a deghosting and noise reduction system. The deghosting and noise reduction system includes an accelerometer system, a load cell system, and a signal processor. The accelerometer system senses the motion of the accelerometer system and provides accelerometer signals. The load cell system, which has mechanically compliant portions between first ends rigidly connected to the stress member and second ends rigidly connected to the accelerometer system, is sensitive to the vibration of the stress member and provides vibration signals. The signal processor receives the pressure signals from the hydrophone system, the accelerometer signals from the accelerometer system, and the vibration signals from the load cell system to reduce the effect of vibration noise and surface-reflected acoustic signal interference on a primary acoustic signal impinging on the streamer. 
   In yet another aspect of the invention, a streamer apparatus comprises a stress member extending along the length of the streamer in a longitudinal direction. A rigid mounting body, retained in the streamer, has first and second mutually orthogonal mounting surfaces parallel to the longitudinal direction. A multi-axis accelerometer is connected rigidly to the mounting surfaces of the rigid mounting body. A rigid block is rigidly connected directly to the stress member between the stress member and the rigid mounting body. The block includes first and second mutually orthogonal outer surfaces parallel to the longitudinal direction. First and second load cells each have first and second ends separated by a mechanically compliant portion. The first end of the first load cell is rigidly connected to the first outer surface of the rigid block, and the first end of the second load cell is rigidly connected to the second outer surface of the rigid block. The second end of the first load cell is rigidly connected to the first mounting surface of the rigid mounting body, and the second end of the second load cell is rigidly connected to the second mounting surface of the rigid mounting body. 
   In another aspect of the invention, a method for extracting a primary acoustic signal traveling upward from the seabed and impinging on an underwater cable having a stress member extending axially therealong from an acoustic signal traveling downward from the sea surface and impinging on an underwater cable and from vibration of the stress member, comprises: (a) deploying a hydrophone on the cable providing a first signal indicative of acoustic pressure; (b) deploying an accelerometer system on the cable providing output signals indicative of acoustic velocity and cable vibration induced by the motion of the cable in the water; (c) deploying a load cell system interposing a mechanical compliance between the accelerometer system and the stress member and providing output signals indicative of the vibration of the stress member relative to the accelerometer system; (d) providing a second signal, indicative of the motion of the cable, from the output signals of the accelerometer; (e) providing a third signal, indicative of the vibration of the cable due to the vibration of the stress member, from the output signals of the load cell system; and (f) combining the first, second, and third signals to provide a signal representing the primary acoustic signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These features and aspects of the invention, as well as its advantages, are better understood by reference to the following description, appended claims, and accompanying drawings, in which: 
       FIG. 1  is an isometric view, partly cut away, of a portion of a streamer embodying features of the invention; 
       FIG. 2  is a cross section of a hydrophone housing in the streamer of  FIG. 1  taken along lines  2 — 2 ; 
       FIG. 3  is an end-on view of the hydrophone housing of  FIG. 2 ; 
       FIG. 4  is an end-on view of an accelerometer housing in the streamer of  FIG. 1  with end-plates removed; 
       FIG. 5  is a cross section of a portion of the stress-member chamber of the accelerometer housing of  FIG. 4  taken along lines  5 — 5  of  FIG. 1 ; 
       FIG. 6  is a cross section of a portion of the accelerometer chamber of the accelerometer housing of  FIG. 4  taken along lines  6 — 6  of  FIG. 1 ; 
       FIG. 7  is a pictorial representation of a streamer as in  FIG. 1  towed behind a survey vessel; 
       FIG. 8  is an enlarged view of a single hydrophone of the streamer of  FIG. 7  illustrating the upward seismic wave and the downward, surface-reflected ghost seismic wave impinging on the hydrophone; 
       FIG. 9  is a block diagram of the deghosting logic used to extract the upward seismic wave from the ghost and vibration noise; and 
       FIG. 10  illustrates conventional acoustic beam patterns for a hydrophone, an accelerometer, and their combination to reduce ghost interference. 
   

   DETAILED DESCRIPTION 
   A portion of a streamer embodying features of the invention is shown cut away in  FIG. 1 . The streamer  20  includes one or more stress members  22 —two, in this example—running the length of the streamer. The stress members are conventional KEVLAR® cables that bear the towing tension. The streamer also includes an array of hydrophones  24 , typically spaced at 3.125-meter intervals along the length of the streamer. (One hydrophone section is shown in  FIG. 1 .) The hydrophone is mounted in a chamber  26  of a hydrophone housing  28 . The hydrophone housing is made of a material whose acoustic properties are compatible with those of seawater. The streamer stress members extend through the hydrophone housing, which further includes a passageway  30  for power and signal lines  32  that route power from a survey vessel to streamer instrumentation and electronic signals to and from the hydrophones and other instrumentation along the streamer. Located within about a half meter of the hydrophone is an accelerometer housing  34 . The accelerometer housing shown in this example is a rigid cylindrical body, metal or plastic, with a cruciform divider  36  defining four chambers  38 A– 38 D within the housing. The stress members extend through two of the chambers  38 A and  38 C. An accelerometer system resides in the third chamber  38 B. The fourth chamber  38 D provides a passageway for the streamer wire bundle  32 . The streamer is conventionally filled with a fluid, such as kerosene, or a solid polymer to make the streamer neutrally buoyant. 
   As shown in  FIGS. 2 and 3 , the hydrophone  24  is attached to opposite walls  40 ,  41  of the chamber  26  by tabs  42  extending outward of a ring  44  encircling the hydrophone. Receptacles (not shown) in the walls receive the tabs to mount the hydrophone in the chamber. A conventional hydrophone includes a pressure-sensitive piezo-ceramic element  46  encapsulated in a protective outer shell  48  having the acoustic characteristics of sea water. Signal wires  50  carrying hydrophone signals lead from the hydrophones to signal processing electronics in the nearby accelerometer housing. 
     FIGS. 4–6  show the accelerometer housing and its constituents. The rigid housing  34  with its cruciform structure  36  divides the interior of the housing into the four sector-shaped chambers  38 A– 38 D. The stress members  22  extend through rigid blocks  52  that extend along the length of the accelerometer housing in opposite chambers  38 A and  38 C. Each stress member runs through and is stationary relative to one of the blocks. The blocks have three orthogonal faces  54 A,  56 B, and  54 C. The cruciform structure  36  of the rigid housing forms two surfaces  56 A and  56 B opposing the lateral faces  54 A and  54 B on the block. An end plate  58  on the housing at one end of each of the stress-member chambers  38 A and  38 C forms a third rigid surface  60  that opposes the end face  54 C of the block. Each stress member and its surrounding block is suspended in its chamber by a load cell system comprising three orthogonally disposed springs  62 A– 62 C, each connecting a face  54 A– 54 C of the block to an opposed rigid surface  56 A,  56 B,  60  of the housing. The spring connection isolates the housing and the accelerometer mounted in another chamber from vibrations in the stress members. Although the springs provide a high mechanical compliance between the blocks and the housing and good isolation, the amplitude of vibrations can still be high enough to interfere with the accelerometer system. For that reason, the springs,  62 A– 62 C are designed as load cells to measure the vibration of the stress members. The load cells  62 A and  62 B attached to the lateral faces of the blocks sense the radial vibration of the stress members. The load cell  62 C attached to the end face of the block measures the in-line component of vibration. Thus, the load cell system supports the stress members in the chambers, mechanically isolates the stress members from the accelerometer system, and measures the vibration of the stress members. 
   An accelerometer system  64 , performing as a geophone, is mounted in the accelerometer housing  34  in the accelerometer chamber  38 B. The accelerometer is preferably a multi-component MEMS device. Colibrys, Inc. of Stafford, Tex., U.S.A., for example, sells MEMS accelerometers. A three-axis accelerometer is shown in this example. The three accelerometers  66 A– 66 C are orthogonally disposed in a package  68  having at least one reference surface attached directly to a surface  70  of the housing. The housing surfaces  56 A,  56 B,  60 ,  70  to which the accelerometer&#39;s reference surface is attached and to which the load cells are attached are oriented in a known relationship. In this example, all the surfaces are either parallel or perpendicular to each other. The accelerometer system, which determines the velocity of an acoustic wave impinging on it, is also sensitive to surface-reflected waves and other environmental conditions, such as streamer vibrations. The accelerometer system also includes an attitude sensor  72  that determines the direction of the earth&#39;s gravity vector. A signal processor  74  in the accelerometer uses the accelerometer and attitude signals to determine the velocity of an acoustic wave. Acoustic wave velocity signals are routed from the accelerometer system to a local signal processor and communications controller (SPC)  76  over accelerometer signal wires  78 . The SPC also receives signals from the load cell system over load cell signal wires  80 ,  81  from each chamber. And the SPC receives the hydrophone signals over hydrophone signal wires  82 . Power, control, and signal wires  84  from the streamer wire bundle  32  connect to the accelerometer system  64  and the SPC  76 . Besides processing the acoustic wave, the SPC also receives data and commands from the vessel&#39;s computer system and returns data and status. 
   The deployment of the streamer  20  is illustrated in  FIG. 7 . The streamer is an underwater cable towed behind a survey vessel  86 , typically along with other parallel streamers. A streamer may be as long as 12 km. Hydrophones  24  are spaced about every 3.125 m along its length. Located within about 0.5 m of each hydrophone is an accelerometer/load cell system as in  FIG. 1 . The streamer, which terminates in a tail buoy  88 , is maintained at a fixed depth, such as 4 m to 10 m below the sea surface  90 , or even deeper. The survey vessel also tows a seismic energy source  92  that periodically emits a burst of seismic energy  9 , which propagates into geologic structures beneath the sea floor and reflects off them as an upward traveling acoustic wave  96  that impinges on the hydrophones. The upward traveling wave W u  ( 96 ), as shown in  FIG. 8 , continues past the hydrophone and reflects at the sea-air interface to form a downward traveling acoustic wave W d . The downward traveling wave interferes with the upward traveling wave, which is the primary seismic signal of interest. Unless the interference is attenuated sufficiently, ghosts appear in the seismic data. 
   A block diagram of the signal processing logic used to suppress ghosts is shown in  FIG. 9 . The block diagram represents processing that occurs locally in the accelerometer system  64  and the local SPC  76 . The hydrophone  24 , which senses acoustic pressure, develops a pressure signal proportional to the sum of the upward traveling seismic wave W u  and the surface-reflected and phase-reversed downward traveling wave W d . The pressure signal is sent to the SPC  76  over hydrophone signal wires  82 . The accelerometer system  64  senses the particle acceleration and derives the net particle velocity resulting from the upward- and downward traveling acoustic waves W u  and W d . The particle velocity u determined by the accelerometer system also includes a noise component due to the vibration V of the stress members. A preferred version of the accelerometer system also includes the attitude sensor  72 , which allows the accelerometer processor to resolve the vertical component u z  of particle velocity, which is sent to the SPC over accelerometer signal lines  78 . Signals representing the attitude of the entire accelerometer system are also sent to the SPC over the accelerometer signal lines. The SPC scales the pressure signal from the hydrophone and the vertical velocity signal u z  from the accelerometer. The velocity signal is multiplied by the acoustic impedance pc of sea water to convert the velocity into a pressure value, where p is the density and c is the speed of sound. The combination of the scalar pressure signal p from the hydrophones and the scaled vertical particle velocity signal pcu z  from the accelerometer system yields the effective beam pattern p+pcu z  shown in  FIG. 10  which attenuates the downward traveling wave W d  and allows the streamer to be deployed at greater depths. 
   But the accelerometer reading is also affected by noise in the seismic frequency band caused by the vibration of the stress members. The vibration V affects the accelerometer output according to the transfer function H(s) between the vibration of the stress members and the velocities it induces in the accelerometer. To attenuate the vibration noise, the vibration V measured by the load cells is sent to the SPC over the load cell signal lines  80 ,  81 . From the load cell signals and the attitude sensor&#39;s signals, a synthetic vertical component of force f z  is derived. An adaptive signal processor  98 , such as a least-mean-square adaptive filter, is used to estimate the actual mechanical transfer function H(s) with enough accuracy to reduce the vibration-induced noise to the self-noise floor of the accelerometer. The estimated transfer function H(s), which is seeded with a nominal H value at start-up to improve the convergence of the filter, uses the signal processor&#39;s output estimate Ŵ u  of the primary upward traveling acoustic wave to update the adaptive filter. The noise velocity component out of the adaptive filter is scaled by the acoustic impedance pe to produce a pressure signal that is subtracted from the accelerometer and hydrophone signals to produce the noise-free estimated upward traveling wave Ŵ u . 
   Because a surface-towed streamer typically pitches less than ±6°, the in-line load cell  62 C and the in-line accelerometer  66 C shown in  FIG. 4  may be eliminated. With two-axis load cells and a two-axis accelerometer, the system has a minimum of 40 dB of ghost rejection for streamer pitches of less than the typical range of about ±6°. 
   Thus, by attenuating the surface-reflected downward traveling wave and the vibration noise, the deghosting and noise-reduction system allows streamers to be operated at greater depths unaffected by rough sea conditions at the surface. 
   Although the invention has been described in detail with respect to a preferred version, other versions are possible. For example, the accelerometer housing could include more chambers or chambers of different shapes and could be designed to accommodate a single stress member or more than two stress members. As another example, some of the processing shown performed by the SPC could be performed in the accelerometer&#39;s signal processor  74 . Furthermore, the processors could resolve the vertical components first and then scale and combine them or could first scale and combine the vector quantities and then resolve the vertical components. As yet another example, the attitude sensor need not be integrated into the accelerometer system. The accelerometers may be analog accelerometer systems that send analog signals to the SPC, or the highly-integrated digital system as described that includes a signal processor sending digital data to the SPC. So, as these few examples suggest, the scope of the claims is not meant to be limited to the preferred version described in detail.