Abstract:
A system for collecting seismic data from geologic formations underlying water. A plurality of hydrodynamic housings are deployable in the water and are permitted to descend at a fast rate through the water until the housings contact the geologic formations. A controller engaged with each housing reconfigures the housing to enhance contact between the housing and the geologic formations and to reduce impact of ocean currents acting against the housing. A marine seismic sensor is engaged with each housing to detect seismic data and to sensor orientation. Each housing can be disconnected from the associated marine seismic sensor to facilitate retrieval of the seismic sensors to the water surface.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to the collection of marine seismic data in deep water environments. 
     Marine seismic exploration requires acoustic source generators for delivering energy to subsurface geologic formations and boundaries. The acoustic energy is discharged along shot lines in the desired survey region and is reflected by the subsurface formations and boundaries. The reflected energy propagates upwardly and is detected with hydrophones or bottom cable geophones. Data from selected shot patterns are processed to construct an overall acoustic image of the subsurface geologic formations. 
     Bottom cable systems use geophones or hydrophones laid on the sea floor. Cable crews assemble each cable from multiple cable sections. Bottom cables are deployed and retrieved with the assistance of powerful linear traction engines and other machines. In shallow water up to one hundred meters deep, one or more vessels deploy the geophones and cable in the selected locations and retrieve the geophones and cables after the selected area is surveyed. 
     Bottom cables are used in relatively shallow water depths to communicate between underwater seismic equipment located on board a marine seismic vessel. Bottom cables are heavy because the sensor packs are heavy and depend upon gravity to achieve physical coupling between the geophones and ocean bottom. Additionally, such cables contain insulated wires for transmitting electrical power and signals between submarine seismic assets and the seismic vessel, and a heavy strength member is required to lift the submarine assets from the ocean bottom and to resist hydrodynamic and mechanical loads imposed on the cable during such recovery. The weight, size and reliability issues regarding bottom cables, together with the operational expense of deploying and retrieving bottom cables, effectively prevents conventional bottom cable systems from being economically deployed in deep water. 
     Conventional bottom cable systems are not useful in deep water because such systems require expensive, powerful and complex handling equipment and such systems have a large cross-section which produces significant hydrodynamic drag and correspondingly long deployment and retrieval times. Conventional bottom cable systems require significant repair costs and operational downtime due to frequent mechanical and electrical failure of cable components resulting from repeated exposure to stress loading experienced during cable retrieval from the ocean floor. 
     In addition to technical difficulties inherent in bottom cable systems, the size, weight and handling requirements of bottom cable systems are expensive and difficult to implement in deep water environments. To overcome these limitations, marine seismic streamer vessels are conventionally used in deep water to perform seismic surveys. Marine seismic vessels tow acoustic energy sources such as compressed air guns through the water. The vessels also tow one or more seismic streamer cables along the selected survey line. The streamers typically range between three and eight kilometers long, with tail buoys attached to the free streamer ends. Each streamer contains multiple hydrophones which receive the reflected energy emitted by the energy source, and the hydrophones are wired together in receiver groups regularly spaced along the streamer. To account for vessel movement, data recording and processing calculations require time and position correlations for each active component of the seismic data gathering system. 
     Variations in lateral spacing between adjacent streamers due to environmental forces and vessel course changes can adversely affect the collected data. The actual spacing between receiver groups is critical for an accurate analysis of geophysical data. Because the acoustic energy reflections propagate through the water in a towed streamer system, noise significantly distorts the reflected energy. The problems associated with undesirable noise is well known. For example, U.S. Pat. No. 4,970,696 to Crews et al. (1990) disclosed a three dimensional seismic survey system having multiple seismic receivers. Undesirable noise was characterized with uniform sampling intervals and the recorded responses were processed to remove the undesirable noise. In other systems, additional processing is required to account for additional noise and the quality of maps created from the processed data is reduced. 
     Various systems have been proposed to collect data in a marine environment and to return the data to the water surface. U.S. Pat. No. 4,007,436 to McMahon ( 1977 ) disclosed a flexible sheet for holding hydrophones. U.S. Pat. No. 4,692,906 to Neeley disclosed an ocean bottom seismometer having a weighted skirt which also provided a buoyant space for retrieving the seismometer to the water surface. U.S. Pat. No. 5,189,642 to Donoho et al. (1993) disclosed a seafloor seismic recorder having a chassis which lowered geophones into contact with the seafloor. A geophone package was embedded into the seafloor and a control package and chassis was raised above the seafloor surface with a leg extension to isolate such components from the geophone package. A ballast ring returned the geophone package to the water surface. Additionally, U.S. Pat. No. 5,696,738 to Lazauski (1997) disclosed a sensing device in contact with the seafloor. 
     Other systems have been developed to operate and to collect seismic data from multiple recorders. U.S. Pat. No. 4,281,403 to Siems (1981) disclosed a plurality of remote seismic recording units activated with local clocks in each local recording unit, together with a master clock in a central station. U.S. Pat. No. 5,623,455 to Norris (1997) disclosed remote units connected to a plurality of receivers for collecting and transmitting seismic data over a selected transmission channel. U.S. Pat. No. 5,724,241 to Wood et al. (1998) disclosed a distributed data acquisition system having a plurality of recorders for detecting and recording seismic data. The data was collected and continuously collected by data acquisition modules. 
     A need exists for an improved deep water seismic data collection system. The system should be economic to deploy and should preserve the quality of collected data. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system for collecting seismic data from geologic formations underlying water. The system comprises a plurality of housings deployable in the water, wherein each housing has a first end having a hydrodynamic profile for facilitating descent of the housing through the water and into contact with the geologic formations. A controller is engaged with each housing for reconfiguring the housing after the housing contacts the geologic formations, and one or more marine seismic sensors are engaged with each housing for detecting seismic data and for identifying the orientation of the sensor. An actuator facilitates retrieval of each marine seismic sensor to the water surface. 
     The method of the invention comprises the steps of operating a vessel in water, of deploying a plurality of housings and engaged marine seismic sensors into the water until each housing contacts the geologic formations, of operating a controller engaged with each housing to reconfigure the housing after the housing contacts the geologic formations, of operating the marine seismic sensors to detect seismic source energy reflections and to record seismic data representing such reflections, and of operating an actuator to facilitate retrieval of each marine seismic sensor to the water surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a vessel for deploying housings in water. 
     FIG. 2 illustrates housings attached to a cable descending in water. 
     FIGS. 3 and 4 illustrates housings in contact with the seafloor. 
     FIG. 5 illustrates a plan view of a housing in contact with the seafloor. 
     FIG. 6 illustrates disconnection of seismic sensors from housings for retrieval to the water surface. 
     FIG. 7 illustrates an inflatable balloon attached to each seismic sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention provides a system for positioning marine seismic sensors in deep water and for retrieving such sensors for further processing of seismic data. Referring to FIG. 1, tender barge or vessel  10  moves through water  12  to deploy and to retrieve a plurality of seismic geophones  14  deployable to the surface of geologic formations identified as seafloor  16 . Geophones  14  can be dropped into water  12  between several meters and thousands of meters in depth and can be positioned within a seismic data collection array as described below. 
     As shown in FIG. 2, each geophone  14  generally includes housing  18 , ballast weight  19 , and marine seismic sensor  20 . Ballast weight  19  can be integrated within housing  18  or can comprise a separate component. If ballast weight  19  comprises a separate component, descriptions herein for the attachment to or detachment from of housing  18  from sensor  20  are equally possible for ballast weight  19  or both. Each seismic sensor  20  detects seismic energy and can comprise different devices suitable for detecting and recording seismic source energy reflections. As a representative example, seismic sensors  20  can comprise geophones or hydrophones or combinations thereof such as an ocean bottom seismometer unit having up to four acoustic sensing components, a data recorder, battery pack, data storage mechanism, acoustic transponder, timer, acoustic modem, compass, gimbal lock, and other conventional equipment. Each seismic sensor  20  preferably includes a buoyant exterior pressure case suitable for withstanding high water pressures located deep in the ocean, and components and equipment can be located inside or outside of such pressure case. 
     In one embodiment of the invention, geophones  14  can be connected to cable  22  deployed from wheel  24  attached to vessel  10 . Although cable  22  is especially adapted to placement of geophones  14  in a substantially straight line, cable  22  is sufficiently flexible to permit placement of geophones  14  in nonlinear or alternative array configurations. As used herein, the term “plurality” means two or more. Each geophone  14  is engaged with or is attached to cable  22  at a selected position. The distances between adjacent geophones  14  can be equal or can vary to provide desired geophone  14  separations. Anchor  26  can be attached to free end  28  of cable  22  to facilitate initial placement of cable  22  within water  12  at a selected location relative to seafloor  16 . Anchors  26  can be placed at other locations to account for high current or unstable seafloor  16  conditions. 
     To deploy geophones  14  within water  12 , geophones  14  are transported with vessel  10  or other means to the seismic survey region. Geophones  14  can be stored in a sequential manner to facilitate deployment. Anchor  26  is attached to cable  22  free end  28 , and is deployed into water  12  at the desired position within the seismic survey area. 
     The deployment of geophones  14  is preferably controlled by movement of vessel  10  within water  12  in a substantially horizontal plane relative to the survey area. In other embodiments of the invention, geophones  14  can be deployed from a submarine or other subsurface vessel or from an airplane. The unique hydrodynamic shape of geophones  14  and ballast weight  19  significantly expedite the descent of geophones  14  in water  12 , thereby reducing the required deployment time. In a preferred embodiment of the invention, each geophone  14  contacts or is substantially proximate to seafloor  16  and is relatively stationary relative to seafloor  16  as shown in FIG.  3 . Following such initial contact, controller  30  engaged with housing  18  is operated to reconfigure housing  18  as illustrated in FIG.  4 . As illustrated for such an embodiment, housing  18  unfolds into one or more “petals” which hinge downwardly and flatten against seafloor  16 , as shown in FIG.  5 . By changing the configuration of housings  18 , contact with seafloor  16  and the resulting receipt of seismic data is significantly enhanced, and acoustic and mechanical effects due to ocean currents and other forces is significantly reduced. In other embodiments of the invention, controller  30  can be operated to vibrate or otherwise move housing  18  so that contact between housing  18 , ballast weight  19  and seafloor  12  is further enhanced. 
     The density and shape of geophones  14  can be selected so that geophones  14  contact seafloor  16  in a predictable and effectively coupled manner to maintain the fidelity of recorded seismic data. In other embodiments of the invention, seismic sensor  20  can be supported above seafloor  16  with a frame, support, flexible pad, inflatable bag, or other device. Although different positions of seismic sensor  20  relative to seafloor  16  are possible, the preferred embodiment places seismic sensor  20  directly in contact with seafloor  16 . 
     As each seismic sensor  20  reaches the desired location, gimbals can orient internal components such as geophones, hydrophones, data recorders, compasses, and other conventional equipment. Such orientation can be locked with existing gimbal locks. Gimbals are not necessary to the invention but are useful if seafloor  16  has a significant slope or unstable composition. Operation of seismic sensor  20  can be initiated before deployment to perform continuous recording, or it can be initiated with a preset clock or with another type of trigger or control signal (acoustic, optical, low frequency radio, or another method) transmitted through water  12  or through signals transmitted through cable  22 . If internal clock mechanisms are used within seismic sensors  20 , such clock mechanisms can be synchronized to a calibrated clock such as a GPS clock, rubidium clock, cesium clock, or other clock prior to deployment of recorders into water  12 . Although cable  22  provides the functions of positioning and retrieving multiple recorders, cable  22  can also provide digital and analog data telemetry, system control signals, and power transmission capabilities. Cable  22  may be connected directly or indirectly to a distributed or centralized data recording system (not shown). 
     The location of cable  22  and seismic sensors  20  can be mapped by acoustic positioning, by first-break positioning with a seismic source, by computer cable lay monitoring and control modeling software, or a combination of these techniques. If each geophone  14  is independently deployed to seafloor  12 , connections of cable  22  can be made to each geophone  14  with a remotely operated vehicle or with an intelligent propulsion system attached to one or more geophones  14 . Seismic sensors  20  may use active control surfaces on the hydrodynamic housing  18  to steer geophone  14  and cable  22  as they free-fall to a desired location within the survey using an acoustic or inertial navigational system. Connections between geophones  14  can be made in a serial or multi-node network. 
     After all seismic sensors  20  are positioned, a seismic event can be promulgated and recorded with sensors integrated with seismic sensors  20 . Different techniques can be implemented to conserve battery power. In one embodiment of the invention, an acoustic, optical, or low frequency radio trigger or other method can be sent to each seismic sensor  20  to initiate a recording cycle. The duration of the recording cycle can be programmed so that that a “stop” signal is not required. An initiation command can also be sent which identifies the initiation time and data to be collected, and which also includes the time for cessation of data recordation. Each seismic sensor  20  preferably records continuously and time stamps the recorded data with sufficient accuracy to correlate collected seismic data to the relevant source time. Patterns of acoustic energy source points can be planned on the surface of water  12  with standard seismic acquisition techniques. 
     After seismic shooting is completed for the selected area, seismic sensors  20  can be retrieved to the surface with cable  22  or with other techniques. Actuator  32  engaged with seismic sensor  20 , ballast weight  19 , and housing  18  is operated to separate seismic sensor  20  from ballast weight  19 , or from housing  18 , or both. Seismic sensor  20  can be retrieved to the surface of water  12  with cable  22 , as shown in FIG. 6, or with a flotation device such as balloon  34  attached to seismic sensor  20 , as shown in FIG.  7 . Actuator  32  can be operated with acoustic signals, timer clocks, or other mechanisms and can be operated simultaneously or sequentially for each seismic sensor  20  within an array. 
     Housing  18  can be formed with a substantially biodegradable material which deteriorates into innocuous residue. Ballast weight  19  can be formed with benign materials such as stone, pottery, metal or combinations thereof in a biodegradable matrix material. By separating seismic sensor  20  from ballast weight  19  or from housing  18 , the positive buoyancy of seismic sensor  20  facilitates transit to the surface of water  12  without requiring large buoyancy tanks or pods which occupy valuable deck space on board vessel  10  and which slow the descent of geophones  14  in water  12 . Actuator  32  can also jettison other external components of geophone  14  such as battery packs or other components which can also be formed with substantially biodegradable materials. 
     As seismic sensors  20  are retrieved, each seismic sensor  20  is connected to a data extraction station (not shown) and the seismic data detected can be extracted for recording and data processing. The data can be written to magnetic storage media in a common receiver, shot sequential method, or processed with other conventional data processing techniques. After the data is collected, each seismic sensor  20  can be reattached to another expendable ballast weight  19  or housing  18  or both for future deployment. 
     The invention uniquely provides a reliable, inexpensive technique for deploying and retrieving seismic data recorders in water. The invention is particularly suitable for deep water because of the deployment flexibility and the absence of seals and other operating components in the deployment and retrieval system. The ease of geophone installation and removal facilitates vessel crew work and provides the opportunity to automate such work functions. Because the system does not depend on electrical integrity within a cable, failure of a single recorder does not jeopardize data collection operations for the other deployed recorders. 
     Although the invention has been described in terms of certain preferred embodiments, it will become apparent to those of ordinary skill in the art that modifications and improvements can be made to the inventive concepts herein without departing from the scope of the invention. The embodiments shown herein are merely illustrative of the inventive concepts and should not be interpreted as limiting the scope of the invention.