BACKGROUND OF THE INVENTION

The present invention relates to the field of seismic exploration. More particularly, the invention relates to a method and apparatus for seismic exploration, and most particularly to marine seismic exploration utilizing ocean bottom seismometer systems.

Seismic exploration generally utilizes a seismic energy source to generate an acoustic signal that propagates into the earth and is partially reflected by subsurface seismic reflectors (i.e., interfaces between subsurface lithologic or fluid layers characterized by different elastic properties). The reflected signals (known as “seismic reflections”) are detected and recorded by seismic receivers located at or near the surface of the earth, thereby generating a seismic survey of the subsurface. The recorded signals, or seismic energy data, can then be processed to yield information relating to the lithologic subsurface formations, identifying such features, as, for example, lithologic subsurface formation boundaries.

Typically, the seismic receivers are laid out in an array, wherein the array of seismic receivers consist of a single string of receivers distributed along a line in order to record data from the seismic cross-section below the line of receivers. For data over a larger area and for three-dimensional representations of a formation, multiple strings of receivers may be set out side-by-side, such that a grid of receivers is formed. Often, the receivers within an array are remotely located or spread apart. In land seismic surveys for example, hundreds to thousands of receivers, called geophones, may be deployed in a spatially diverse manner, such as atypical grid configuration where each string extends for 1600 meters with detectors spaced every 50 meters and the successive strings are spaced 500 meters apart. In marine surveys, a towed streamer having receivers, called hydrophones, attached thereto may trail up to 12,000 meters behind the tow vessel.

Generally, several receivers are connected in a parallel-series combination on a single twisted pair of wires to form a single receiver group or channel. During the data collection process, the output from each channel is digitized and recorded for subsequent analysis. In turn, the groups of receivers are usually connected to cables used to communicate with the receivers and transport the collected data to recorders located at a central location. More specifically, when such surveys are conducted on land, cable telemetry for data transmission is used for detector units required to be interconnected by cables. Other systems use wireless methods for data transmission so that the individual detector units are not connected to each other. Still other systems temporarily store the data until the data is extracted.

While the fundamental process for detection and recording of seismic reflections is the same on land and in marine environments, marine environments present unique problems due to the body of water overlaying the earth's surface, most notably the high pressure of deep water activities and the corrosive environment of salt water activities. In addition, even simple deployment and retrieval is complicated since operations must be conducted off the deck of a seismic exploration vessel, where external elements such as wave action, weather and limited space can greatly effect the operation.

In one common method of marine seismic exploration, seismic operations are conducted at the surface of the water body. Marine vessels tow streamers in which are embedded hydrophones for detecting energy reflected back up through the water column. The streamers are typically comprised of hydrophone strings, other electrical conductors, and material for providing near neutral buoyancy. The streamers are made to float near the water's surface. The same or other similar marine vessels tow acoustic energy sources, such as air guns, to discharge energy pulses which travel downwardly into subsurface geologic formations underlying the water.

Systems placed on the ocean bottom floor have also been in use for many years. These devices are typically referred to as “OBC” (Ocean Bottom Cabling) or “OBS” (Ocean Bottom Seismometer) systems. The prior art has centered on three main groups of ocean bottom apparatus to measure seismic signals at the seafloor. The first type of apparatus is an OBC system, similar to the towed streamer, which consists of a wire cable that contains geophones and/or hydrophones and which is laid on the ocean floor, where the detector units are interconnected with cable telemetry. Typically, a seismic vessel will deploy the cable off the bow or stern of the vessel and retrieve the cable at the opposite end of the vessel. OBC systems such as this can have drawbacks that arise from the physical configuration of the cable. For example, when three-dimensional geophones are employed, because the cable and geophones are not rigidly coupled to the sediment on the ocean floor, horizontal motion other than that due to the sediment, such as for example, ocean bottom currents, can cause erroneous signals. In this same vein, because of its elongated structure, OBC systems tend to have satisfactory coupling only along the major axis of the cable when attempting to record shear wave data. In addition, three ships are required to conduct such operations since, in addition to a seismic energy source vessel, a specially equipped vessel is necessary for cable deployment and a separate vessel is needed for recording. The recording vessel is usually stationary attached to the cable while the deployment vessel is generally in constant motion along the receiver line deploying and retrieving cable. Because the recording vessel is in constant physical contact with the cable, the effort required to maintain the vessel's position, wave action and ocean currents can generate great tension within the cable, increasing the likelihood of a broken cable or failed equipment, as well as the introduction of signal interference into the cable. Finally, such cable systems have a high capital investment and are generally costly to operate.

A second type of recording system is an OBS system in which a sensor package and electronics package is anchored to the sea floor. The device digitizes the signals and typically uses a wire cable to transmit data to a radio unit attached to the anchored cable and floating on the water surface. The floating transmitter unit then transmits the data to a surface vessel where the seismic data are recorded. Multiple units are typically deployed in a seismic survey.

A third type of seismic recording device is an OBS system known as Seafloor Seismic Recorders (SSR's). These devices contain the sensors and electronics in sealed packages, and record signals on the seafloor. Data are retrieved by retrieving the device from the seafloor. Such devices are typically re-usable. The focus of the present invention is on SSR type of OBS systems.

SSR type OBS systems generally include one or more geophone and/or hydrophone sensors, a power source, a seismic data recorder, a crystal oscillator clock, a control circuit, and, in instances when gimbaled geophones are used and shear data are recorded, a compass or gimbal. Except to the extent power is provided from an outside source via a cable, the power source is generally a battery package. To the extent prior art OBS systems have utilized on-board batteries, as opposed to external cabling, to supply power, the prior art batteries have been lead-acid, alkaline or non-rechargeable batteries. All of the OBS systems of the prior art generally require that the individual units be opened up for various maintenance, quality control and data extraction activities. For example, data extraction from prior art units require the units be physically opened or disassembled to extract data. Likewise, the unit must be opened up to replace spent batteries.

With respect to the timing function of the OBS system, synchronization between the timing of the sensor data and the firing of the seismic energy source or shot is critical in order to match a seismic source event with a reflection event. In the past, various crystal oscillator clocks have been used in OBS systems for this function. The clocks are relatively inexpensive and accurate. One drawback to such prior art clocks, however, is that the clock crystals are subject to gravitational and temperature effects. These gravitational and temperature effects can cause a frequency shift in the oscillator frequency, thereby resulting in errors in the seismic data. In addition, since the crystals are subject to gravitational effects, orientation of the OBS system can effect operation of the clock. Since the clock is typically secured within the OBS package so as to be correctly oriented when the OBS system is properly oriented on the ocean floor, any misorientation of the OBS system on the ocean floor can result in clock inaccuracies. Finally, such clocks often are characterized by drift and time shifts due to temperature changes and aging, which again, can cause inaccuracies in the recorded seismic data. While it may be possible that mathematical corrections could be made to the data to account for temperature aging and time shifts, there is no prior art device that corrects for gravitational effects on the crystal clock. At most, the prior art only corrects for effects of temperature on the crystal clocks.

More modern OBS systems may also include a mechanical device to correct for tilt, namely a gimbal. A gimbal is a device that permits free angular movement in one or more directions and is used to determine orientation of the OBS system on the ocean floor. Orientation data generated by the gimbal can then be used to adjust the seismic data recorded by the geophones. To the extent the prior art utilizes gimbals, they are most often incorporated as part of the geophone itself, which are referred to as “gimbaled geophones,” One drawback to these mechanical gimbals of the prior art is the limited angular orientation permitted by the devices. For example, at least one of the prior art devices permit a gimbal roll of 360° but is limited in gimbal pitch to 30°. For this device, in order for such prior art gimbals to function properly, the OBS system itself must settle on the ocean floor in substantially the desired position. To the extent the OBS system is not oriented at least substantially in the horizontal, such as settling on its side or upside down, the mechanical gimbal of the prior art may not function properly. Other gimbaled devices of a mechanical nature are not limited by 30°, however, in such mechanically gimbaled devices, mechanical dampening in the device can deteriorate the fidelity of the recorded signal. Finally, gimballing of a geophone is expensive and requires more space than a non-gimballed geophone. For OBS systems that utilize multiple geophones, it may be impractical to gimbal the geophones due to size and space requirements.

As with orientation, the location of OBS system on the ocean floor is necessary to properly interpret seismic data recorded by the system. The accuracy of the processed data depends in part on the accuracy of the location information used to process the data. Since conventional location devices such as GPS will not operate in the water environments, traditional prior art methods for establishing the location of the OBS systems on the ocean floor include sonar. For example, with a sonar system, the OBS device may be “pinged” to determine its location. In any event, the accuracy of the processed data is directly dependent on the precision with which the location of the OBS system is determined. Thus, it is highly desirable to utilize methods and devices that will produce dependable location information. In this same vein, it is highly desirable to ensure that the planned positioning of the OBS device on the ocean floor is achieved.

With respect to operation of the aforementioned OBS systems, the prior art systems generally require some externally generated control command in order to initiate and acquire data for each shot. Thus the seismic receiver units must be either physically connected to the central control recording station or “connectable” by wireless techniques. As mentioned above, those skilled in the art will understand that certain environments can present extreme challenges for conventional methods of connecting and controlling the detectors, such as congested or deep marine areas, rugged mountain areas and jungles. Difficulties may also arise in instances where the receiver array is periodically moved to cover a larger area.

Whatever the case, each type of connection, whether via a physical cable or through wireless techniques, has its own drawbacks. In cable telemetry systems, large arrays or long streamers result in large quantities of electrically conductive cabling that are expensive and difficult to handle, deploy or otherwise manipulate. In instances where ocean bottom cabling is used, the corrosive environment and high pressures often require costly cable armoring in water depths over 500 feet. Furthermore, conventional cabling also requires a physical connection between the cable and the sensor unit. Since it is generally not practical to hard wire sensors on a cable, the more conventional technique is to attach cabling to sensors using external connections between the cable and the sensor. This point of the connection between the cable and the sensor is particularly vulnerable to damage, especially in corrosive, high pressure marine environments. Of course, with systems that are physically cabled together, it is much easier to provide power to the sensors, to synchronize sensors with the shot time and with each other and to otherwise control the sensors.

It should be noted that whether for cabled or wireless systems, where external cabling is required to connect the sensor package of the equipment with the recording and/or radio telemetry packages of the unit, many of the aforementioned drawbacks exist. Specifically, the OBS systems of the prior art are comprised of separate sensing and recording/radio telemetry units or packages mounted on a carriage. The separate units have external connectors that are cabled together, presenting many of the same problems as cabling from the central control on the surface of the water. The primary reason for the separation between the sensing units, i.e., the geophone package, and the remainder of the electronics is the need to ensure that the geophones are effectively coupled to the ocean floor.

In cases where either wireless technology is utilized or operation of sensors is through pre-programming, control of the sensors becomes more difficult. For example, ensuring that recording is synchronized with the shot timing is crucial since the individual sensors are not wired together as described above. Hence the need for accurate on-board clocks as mentioned above. In this regard, activating each unit for sensing and recording at the appropriate time must coincide with the shot. Ensuring that the units are sufficiently powered has also heretofore been a concern. Many prior art patents have focused on techniques and mechanism s for powering up sensors during data acquisition and recording and powering down the sensors during dormant periods.

Various attempts have been made to address some of the above-mentioned drawbacks. For example, a seafloor seismic recorder is described in U.S. Pat. No. 5,189,642. This patent discloses an elongated, upright chassis formed of spaced apart, horizontal ring plates connected by vertical leg members. Each leg member is formed of nested tubes that can slide relative to one another and that are secured to one another by a clamp mechanism. Releasably attached to the lower plate is a ballast ring. Also attached to the lower plate is the geophone package. Attached to the upper plate is a foam buoy. A control package extends down from the upper plate. The control package houses a power source, a seismic data recorder, a compass and a control circuit. An external hard wire electrically connects the control package with the geophone package. The system does not utilize any hard-wired communications link to the surface monitoring station but utilizes acoustical or preprogrammed means for controlling the unit. When released into the water, the ballast ring is supposed to provide sufficient mass to maintain the system upright and couple the geophones to the ocean floor upon settling. To minimize the likelihood of geophone noise produced by wave or water current motion acting against the buoy and control package, once the system is coupled to the ocean bottom, the clamp mechanism on each leg is released, allowing the control package and buoy to slide upward on the nested legs, isolating the geophones from the other parts of the system. Once seismic recording is complete, the ballast ring is then released from the chassis, and the system rises to the water surface under the positive buoyancy of the ballast. Acoustic transducers, a radio beacon and strobe light are provided to permit the system to be located and retrieved.

Another marine seismic data recording system is taught in U.S. Pat. No. 6,024,344. This patent teaches a method for deploying and positioning seismic data recorders in deep water. From a surface vessel, data recorders are attached to a semi-rigid wire which is deployed into the water. Due to the rigid nature of the wire, it functions to define a fixed interval between recorders as the recorders and wire sink to the seafloor. The wire also provides electrical communication fat power or signals between adjacent recorders and between recorders and the vessel. Once the recorders are in place, they are activated either by way of a preset clock or by utilizing a control signal transmitted through the water or through the wire. Upon completion of data gathering, the wire and recorders are retrieved. Deployment is accomplished utilizing a cable engine positioned on the surface vessel. As shown in FIG. 1 of the '344 patent, deployment occurs over the stern of the vessel as it moves in a direction away from the wire and recorders. This patent also teaches the need to store the recorders in a sequential manner to facilitate deployment and to track the seafloor location of the OBS system during data collection.

GeoPro offers a self-contained, i.e., cable-less, OBS system comprised of a 430 mm diameter glass sphere in which is enclosed all electrical components for the system, including batteries, a radio beacon, a seismic data recording unit, an acoustic release system, a deep sea hydrophone and three gimbal mounted geophones. The sphere is mounted on a weighted skid that counteracts the buoyancy of the sphere and anchors the OBS system to the sea bed. The geophones are positioned in the bottom of the sphere adjacent the skid. To recover the OBS system upon completion of data collection, an acoustical command signal is transmitted to the sphere and detected by the deep sea hydrophone. The signal activates the acoustic release system which causes the sphere to separate from the weighted skid, which remains on the sea floor. Under positive buoyancy of the sphere, the free-floating system rises to the ocean surface, where the radio beacon transmits a signal for locating and retrieving the sphere. One drawback to this particular design is that the geophones are not coupled directly to the ocean floor. Rather, any seismic signal recorded by the geophones must pass through the skid and the bottom of the sphere, and in so doing, are subject to noise and other, distortions described above. It should be noted that this packaging design is representative of many of the cylinder and sphere shapes utilized in the prior art since it is well known that such shapes are more effective in withstanding the high pressures likely to be found in ocean environments.

K.U.M. and SEND offer a cable-less OBS system comprising a frame having a rod at the top and forming a tripod at the bottom. A foam flotation device is attached to the rod. An anchor is fixed to the lower portion of the tripod and secures the frame to the sea floor. Pressure cylinders mounted on the tripod portion of the frame contain seismic recorders, batteries and a release system. A hydrophone is attached to the frame in order to receive command signals from the ocean surface and activate the release system. Also attached to the frame is a pivotally mounted crane arm to which is releasably attached a geophone unit. During deployment, the crane arm is initially maintained in a vertical position with the geophone unit attached to the free end of the arm. When the frame contacts the sea floor, the crane arm pivots out from the frame and releases the geophone unit onto the sea floor approximately 1 meter from the frame system. A hard wire permits electrical communication between the geophone unit and the recorders. The geophone unit itself is an approximately 250 mm diameter, non-symmetrical disk which is flat on one side and domed on the opposite side. The flat side of the geophone unit is grooved and contacts the sea floor when released by the crane arm. Upon completion of data gathering, an acoustic signal activates the release system, which causes the anchor to be detached from the frame system. The foam flotation device causes the frame system and geophone to rise to the ocean surface where the system can be located using the radio beacon and retrieved.

SeaBed Geophysical markets a cable-less OBS system under the name CASE. This system is comprised of a control unit, i.e., electronics package, and a node unit or geophone package connected to each other by a cable. Both the control unit and the node unit are carried on an elongated frame. The control unit is comprised of a tubular body which contains batteries, a clock, a recording unit and a transponder/modem for hydro-acoustic communication with the surface. The node unit is comprised of geophones, a hydrophone, a tilt meter and a replaceable skirt, wherein the skirt forms a downwardly open cylinder under the geophone unit. The node unit is detachable from the elongated frame and control unit, but remains in communication with the control unit via external cabling. The use of a tubular body such as this is very representative of prior art designs because the system packaging must be designed to withstand the high pressures to which the device is exposed. During deployment, the entire unit is dropped to the sea floor, where a remotely operated vehicle (separate from the OBS system) is used to detach the node unit from the frame and plant the node unit into the seafloor, pushing the open-ended skirt into the seafloor sediment. The elongated frame includes a ring to which a deployment and retrieval cable can be attached. The communication transducer and modem are utilized control the system and transmit seismic data to the surface.

Each of the referenced prior art devices embodies one or more of the drawbacks of the prior art. For example, the OBS system of U.S. Pat. No. 5,189,642, as well as the devices of GeoPro and K.U.M./SEND are upright systems that each have a relatively tall, vertical profile. As such, seismic data collected by these systems is subject to noise arising from water movement acting against the devices. In addition, it has been observed that shear motion caused by movement of the ocean floor under such a tall profile OBS system can cause rocking motion of the OBS system, particularly as the motion translates from the bottom to the top of the unit, further deteriorating fidelity of the recorded data. Furthermore, these prior art devices are all asymmetrical, such that they can be positioned in only a single orientation. Typically this is achieved by heavily weighting one end of the OBS carriage. However, such a device likely must pass through hundreds of feet of water and contact an often rugged, uneven ocean floor that may be scattered with debris. All of these factors can result in mis-orientation of the system as it settles on the ocean floor, thereby effecting operation of the system. For example, to the extent such a prior art OBS system settles on its side, the geophones will not couple with the ocean floor at all, rendering the device unusable. In addition, incorrect orientation could interfere with the system's release mechanism, jeopardizing recovery of the system.

The tall profile of these prior art systems is also undesirable because such units lend themselves to becoming entangled in fishing lines, shrimping nets, various types of cables or other debris that might be present in the vicinity of the seismic recording activity.

On the other hand, prior art systems that have a smaller profile, such as ocean bottom cables, tend to have poor coupling ability or require external assistance in placement utilizing expensive equipment such as ROVs. For example, the elongated shape of ocean bottom cables results in “good” coupling in only a single orientation, namely along the major axis of the cable. Furthermore, even along the maj or axis, because of the small surface area of actual contact between the cable and the ocean floor, coupling can be compromised due to a rugged ocean bottom or other obstacles on or near the ocean floor.

Another drawback to these prior art systems is the need to activate and deactivate the units for recording and operation. This generally requires a control signal from the surface vessel, typically either transmitted acoustically or through a cable extending from the surface to the unit. External control of any type is undesirable since it requires signal transmission and additional components in the system. While acoustical transmission can be used for some data transmission, it is generally not reliable to use for synchronization purposes due to unknown travel path variations. Of course, any type of control signal cabling for transmission of electrical signals is undesirable because it adds a level of complexity to the handling and control of the unit and requires external connectors or couplings. Such cabling and connectors are particularly susceptible to leakage and failure in the high pressure, corrosive environment of deep ocean seismic exploration.

A similar problem exists with units that utilize external electrical wiring to interconnect distributed elements of the unit, such as is taught in U.S. Pat. No. 5,189,642 and similar devices where the geophone package is separate from the electronics package. Furthermore, to the extent the electronics of a system are distributed, the likelihood of malfunction of the system increases.

Many of the prior art systems also use radio telemetry rather than recording data on-board the unit, to collect the data. Such systems, of course, have limitations imposed by the characteristics of radio transmission, such as radio spectrum license restrictions, range limitations, line-of-sight obstructions, antenna limitations, data rate limitations, power restrictions, etc.

Those OBS units that utilize flotation devices for retrieval are undesirable because the typical decoupler device adds additional expense and complexity to the units, and generally must be activated in order to release the systems to the surface. In addition, such systems typically discard part of the unit, namely the weighted anchor or skid, leaving it as debris on the ocean floor. During deployment, since they are free-floating, such systems are difficult to position in a desired location on the ocean floor. Notwithstanding the above-mentioned possibility of malfunction due to misorientation, during retrieval, the free-floating systems are often difficult to locate and have been known to be lost-at-sea, despite the presence of radio signals and beacons. Likewise, in rough seas, the units prove unwieldy to snare and lift on board, often colliding with the boom or vessel hull and potentially damaging the system.

In this same vein, handling of the units, both during deployment and retrieval, has proven difficult. To the extent a rigid or semi-rigid cable system is utilized to fix distances and position individual recorder units, such cables are inflexible, extremely heavy and difficult to manipulate. Such cables do not lend themselves to corrections during deployment. For example, as explained above, a desired grid layout identifies specific positions for individual units along a line. If a deployment vessel drifts or otherwise causes a cable being laid to be positioned off of the desired line, the vessel at the surface must reposition to cause the cable to get back on line. However, because of the rigid nature of the cable, the mispositioned portion of the cable will result in all of the remaining units on the cable to be mispositioned along the desired line.

Furthermore, current procedures utilized in the prior art to retrieve cables tends to place undue stress on the cables. Specifically, the widely accepted method for retrieval of a cable line from the ocean floor is to either back down over a line or drive the boat down the line retrieving the cable over the bow of the vessel. This is undesirable because the speed of the vessel and the speed of the cable winch must be carefully regulated so as not to overtension or pull the cable. Such regulation is often difficult because of the various external factors acting on the vessel, such as wind, wave action and water current. Failure to control tensioning or pulling of the cable will have the effect of dragging the entire length of the line, as well as the units attached thereto, subjecting the entire line and all of the units to damage. An additional drawback to this method is that if the vessel is moving too fast, it will cause slack in the cable and the cable will float under the vessel, where it can become entangled in the vessel's propellers.

Finally, nowhere in the prior art is there described a back-deck system for handling the above-described OBS units, whether it be storage of the units or deploying and retrieving the units. As the size of deep water seismic recorder arrays become larger, the need for a system for efficiently storing, tracking, servicing and handling the thousands of recorder units comprising such an array becomes more significant. Additional surface vessels are costly, as are the personnel necessary to man such vessels. The presence of additional personnel and vessels also increases the likelihood of accident or injury, especially in open-sea environments where weather can quickly deteriorate.

Thus, it would be desirable to provide a seismic data collection system that does not require external communication/power cabling, either from the surface or on the seismic data collection unit itself, nor any type of external control signal for operation. In other words, the unit should operate on a “drop and forget” basis. Likewise, the device should be easily serviced without the need to open the device to perform activities such as data extraction, quality control and power replenishment. The device should also be designed to withstand the corrosive, high pressure environment common in deep water marine applications. The unit should be configured to minimize the effects of noise arising from ocean currents, and maximize coupling between the device and the ocean floor. In this same vein, the device should be designed to properly orient itself for maximum coupling as the device contacts the ocean floor, without the assistance of external equipment such as ROVs, and minimize the likelihood of misorientation. Likewise, the device should be less susceptible to snaring or entrapment by shrimping nets, fishing lines and the like.

The device should include a timing mechanism that is not susceptible to orientation. Similarly, orientation should not effect gimballing of the geophones.

The device should be easily deployable, yet able to be placed at a certain location with a high degree of confidence. Likewise, the device should be easily retrievable without the need for flotation devices or release mechanisms, nor should parts of the unit be left in the ocean during retrieval. Further, there should be a device and retrieval procedures that minimize potentially damaging tension in the cable connecting the seismic units.

There should also be provided a system for readily handling the hundreds or thousands of recorder units that comprise an array for deployment in ocean environments. Such a system should be able to deploy, retrieve, track, maintain and store individual recorder units while minimizing manpower and the need for additional surface vessels. The system should likewise minimize potential damage to the individual units during such activity. Likewise, it would be desirable to include safety devices in the system to minimize harm to personnel handling the recorder units.

SUMMARY OF THE INVENTION

The present invention provides a system for collecting seismic data in marine environments by deploying multiple, continuous operating, wireless, self-contained ocean bottom sensor units or pods, each characterized by a symmetrical, low profile casing, and a unique external bumper to promote ocean bottom coupling and prevent entrapment in fishing nets. The pods are attached to one another utilizing a flexible, non-rigid, non-conducting cable that is used to control deployment of the pods through the water. The pods are deployed and retrieved from the uniquely configured deck of marine vessel, wherein the deck is provided with a conveyor system and a handling system to attach and detach individual pods from the non-rigid cable. In one embodiment, as part of the deck configuration, the individual pods are randomly stored in juke box fashion in slotted racks. When seated within the slot of a rack, the seismic data previously recorded by the pod can be retrieved and the pod can be charged, tested, re-synchronized, and operation can be re-initiated without the need to open the pod. In another embodiment, the individual pods are stored in stacked, rotating carousels that permit seismic data previously recorded by the pods to be retrieved and the pods to be charged, tested, re-synchronized, and operation can be re-initiated without the need to open the pod. During deployment and retrieval, the non-rigid cable and pods attached thereto are handled so as to minimize the likelihood of tension developing within the deployed line by virtue of movement of the surface vessel. This includes a uniquely configured non-rigid cable system designed to automatically shear apart if a certain level of tension is reached in the cable.

More specifically, each individual sensor unit is comprised of a disk-shaped, water tight case formed of two parallel, circular plates joined around their peripheries by a shallow wall, thereby forming a package which is symmetrical about the axis of the plates and has a very low height profile relative to the diameter of the plates, much in the shape of a wheel. The case is internally supported to protect the integrity of the case from external pressure effects and to provide rigid mechanical coupling between the unit case and the geophones. In one embodiment of the invention, the unit is configured so that it will effectively couple with the ocean floor and collect seismic data whichever plate side it settles on, obviating many of the orientation problems of the prior art. The plates may include ridges, projections or grooves to enhance coupling with the ocean floor.

Disposed around the shallow wall of the unit in one embodiment is a bumper having a cross section shape designed to urge the unit to settle onto one of the plate sides of the package, thereby resulting in a high degree of coupling between the unit and the ocean floor. In at least one embodiment, a bumper is provided and designed to prevent the unit from becoming entangled or snared in shrimping nets or fishing lines.

The unit utilizes several different devices for connecting to a cable. In one embodiment, each unit includes an over-center latching mechanism to permit the units to be attached to a cable. In another embodiment, an attachment bracket is located off-center on the side of the case. In still yet another embodiment, an attachment bracket is centrally located on one of the unit's circular plates forming the case.

The unit is self-contained such that all of the electronics are disposed within the case, including a multi-directional geophone package, a seismic data recording device, a power source and a clock.

In one embodiment of the invention, the clock is a rubidium clock. The rubidium clock is much less susceptible to temperature or gravitational effects or orientation of the unit on the ocean floor.

In another embodiment, the unit includes a crystal clock and a tilt meter. Gravitational effects on the crystal clock are preferably corrected on-board the unit in real time utilizing tilt meter data.

The power source is preferably rechargeable batteries that can operate in a sealed environment, such as lithium ion batteries.

Units incorporating a tilt meter may also utilize the tilt meter data to perform various functions other than crystal clock correction. For example, one aspect of the invention utilizes tilt meter data for mathematical gimballing. Specifically, in the invention, gimballing of the geophones is accomplished mathematically using tilt meter data, and as such, is not subject to the orientation of the unit as arc mechanical gimbals.

Of course, tilt meter data may also be used to determine the position of a unit on the ocean floor as is the common use of such data in the prior art. However, unlike the prior art devices, one aspect of the invention is to obtain and utilize tilt meter data in a time continuous fashion. Prior art units typically only determine a unit's position once at the beginning of seismic recording. Yet it has been observed that the position of a unit may change over the course of deployment as the unit is subject to external forces such as water currents, shrimp lines and the like. Thus, in the invention, tilt meter data is measured as a function of time. This is performed multiple times during operation so that seismic data can be corrected as necessary.

With respect to corrections for tilt, timing or similar data that could effect the accuracy of the collected seismic data, all of the prior art devices make such corrections at a processing center. None of the prior art devices make such corrections on-board the unit while it is deployed or even on board the deployment vessel. Thus, one method of the invention is to make such corrections on-board the unit while it is deployed.

The unit may also include a compass, a hydrophone, an acoustical location transducer and/or one or more accelerometers. Compass data may be used to provide frame of reference data for each individual unit relative to the frame of reference for the overall survey. In one embodiment of the invention, sensors such as accelerometers are used to track the position of the unit as it descends through a water column and settles on the ocean floor. Specifically, such sensors provide inertial navigation data and record x, y and z position information as the unit is passing through the water column. This position information, along with initial position and velocity information, is used to determine the eventual location of the unit.

In another aspect of the invention, the unit is activated while on-board the seismic vessel and deactivated once pulled from the ocean, such that it is continuously acquiring data from before the time of deployment to after the time of retrieval. Likewise in one embodiment, the unit begins recording data prior to deployment in the water. Systems that are activated and begin recording before deployment in the water are thereby stabilized prior to the time when signal detection is desired. This minimizes the likelihood that an altered state in electronics operation will disrupt signal detection and recording.

In another aspect of the invention, the seismic data recording device includes wrap around memory and continuously records, even when not in use. This obviates the need for initiation or start instructions, ensures that the unit is stabilized at the desired recording times, and serves to back-up data from prior recordings until such time as the prior data is written over. As long as the clock is synchronized, such a recording device is ready for deployment at any time. Furthermore, routine operations such as data collection, quality control tests and battery charging can take place without interrupting recording. In the case of a continuously recording unit such as this, the unit can be used on land or in a marine environment.

Use of a non-rigid cable is an additional aspect of the invention. While rope may have been used in the very early prior art as a tow line for surface floating seismic devices, heretofore, to the extent OBS systems have been connected to one another, the prior art has utilized only rigid or semi-rigid wire cable. One of the reasons wire cable has been desirable for the prior art OBS systems is the need to electrically interconnect the systems. In the current invention, however, flexible, non-rigid cable is utilized since the pods, as described above, operate independently and do not require external communications or connections. The non-rigid cable of the invention is preferably formed of a synthetic fiber material, such as polyester, and is encased in a protective overmold, such as a polyurethane casing. In one embodiment, the non-rigid cable is formed of a twelve stranded braided polyester core. The overmold is ribbed or grooved to reduce drag in the water.

The non-rigid cable of the invention is also useful in a unique deployment method for the pods. Specifically, the non-rigid cable has only a slightly negative buoyancy. When attached between two pods each having a negative buoyancy much greater than the cable, as the two jointed pods sink down through a water column, the drag on the non-rigid cable is much greater than the drag on the units and thus acts as a parachute or brake, slowing the descent of the pods and maintaining the pods in an upright position. This is particularly desirable in units that must be placed in a particular orientation, such as those units having non-symmetrical bumper configurations, because the cable, when attached to a centrally mounted connector on the top plate, functions to maintain the orientation of the unit as it passes down through the water column and settles on the ocean floor. Furthermore, since the cable of the invention is non-rigid, there is slack in the cable between adjacent pods. A vessel operator can utilize this slack to make corrections in the drop location while deploying the pods.

Likewise, the non-rigid cable enhances a unique retrieval method of the invention, wherein the cable is retrieved over the stern of the vessel as the vessel “drives down” the cable. In so doing, the drag on the cable created by the water causes the cable to parachute or billow out behind the vessel, minimizing excessive tension on the cable and ensuring that the cable is less likely to become entangled in the vessel's propellers.

On the deck of the seismic vessel, in one embodiment of the invention, a storage system includes a rack having multiple rows and columns of slots is disposed for receipt of the individual units. Each slot includes a communications portal such that when a unit is seated within the slot, the unit interfaces with a master control station via the communications portal. Through the portal, information recorded on the unit can be downloaded, the unit batteries can be recharged, quality control checks on the unit can be conducted, recording can be re-initiated and the unit can be reactivated. In another embodiment of the invention, a storage system includes stacked, u-shaped carousels. Each carousel includes rollers to permit the recording units to be moved along the path of the carousel in conveyor type fashion until the units are positioned adjacent a communications portal. Whichever storage system is utilized, the storage systems may be configured to have the dimensions of a standard 8′×20′×8′ shipping container so that the storage systems and any seismic units stored therein, can be easily transported utilizing standard container ships.

Each unit may include a unique identification means, such as a radio frequency identification (RFID) tag or similar identification indicia to permit tracking of the individual units as they are handled on the deck. Likewise, as mentioned above, each unit may include an acoustical location transducer or accelerometers to determine a unit's location on the ocean floor. Since the individual units are self-contained, the location information, in association with the identification indicia allows the units to be randomly inserted into the storage rack, but permits data from multiple units to be retrieved and sequentially ordered according to the previous location of the unit on the ocean floor. Thus, the need to keep units in sequential order is obviated. Units that might have been adjacent one another on a receiver line need not be stored next to one another in the racks.

In addition, the overall deployment and retrieval system for the units is substantially automated on the deck. The deck configuration includes a conveyor system running adjacent the racks and extending to the edge of the deck adjacent the water. A robotic arm is positioned for moving the units between the storage rack and the conveyor belt. In one embodiment, a cable engine and cable spool/container are positioned to pay out non-rigid cable so as to run adjacent the conveyor system and over the side of the vessel. As units are placed on the conveyor system for attachment to the non-rigid cable, the speed of the conveyor is adjusted to match the speed of the cable, permitting attachment of the units on-the-fly. Furthermore, those skilled in the art will understand that the payout speed of line is not constant since movement of the vessel through the water is not constant, even under calm seas and low wind conditions. As such, in order to prevent too much tension from developing in the line, which can result in damage to the line and dragging of the units, and to permit accurate placement of the units on the ocean floor, the speed of the line as it is paid out into the water is constantly adjusted to compensate for the erratic and unpredictable movement of the vessel on the water. Thus, the speed of the conveyor carrying the units for attachment to the line must be continually adjusted.

In another embodiment of the invention, the conveyor intersects with the cable being paid out by the cable engine. At the intersection, a seismic unit is attached to the cable and the attached unit is subsequently released into the water. A cable grabber downstream from the attachment station is used to securely clamp the cable prior to attachment of a unit, thereby removing upstream line tension during attachment of the unit to the cable. The cable grabber may include a release system requiring an operator to use both hands in order to open the grabber, thereby minimizing danger to the operator when the unit is released and the upstream cable is again placed under tension.

With respect to tension in the cable, the cable is sectioned and the cable sections are attached to one another utilizing a uniquely designed, break-away connector. The connector is comprised of first and second fittings that nest into each other. A shear pin is inserted through the nested fittings to secure the fitting together. Each fitting is attached to the end of a cable section such that when the fittings are secured together, the cable sections form a longer length of cable. If the tension in the cable become greater than the shear limit of the shear pin, the shear pin with break away and the cable will separate.

Furthermore, while one embodiment of the invention utilizes a clamping mechanism that permits units to be clamped directly on a length of cable, another embodiment of the invention utilizes a sleeve attached to the cable. The clamping mechanism secures to the sleeve which is bounded by overmolded shoulders. Rather than attaching shoulders between adjacent lengths of cable as is common in the prior art, the sleeve of the invention can be clamped or placed around a length of cable and secured in place without cutting the cable. In the embodiment, the sleeve is secured to the cable by inserting pins through the sleeve and cable in the x and y planes perpendicular to the axis of the cable. Shoulders are molded over the pins at the ends of each sleeve. While the overmolding on opposite ends of the sleeve can be used to define an attachment area along the sleeve, the sleeve may include flared ends that further define such attachment area.

