Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Reexamination Certificate
1999-11-04
2003-07-08
Lefkowitz, Edward (Department: 2862)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
C702S014000
Reexamination Certificate
active
06590831
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method and apparatus for coordinating the operation of multiple remotely operated or autonomous marine vessels engaged in marine seismic data acquisition comprising a vessel management system and in particular to a real-time feed back and positioning method and apparatus that will to provide a recommendation for optimal midpoint coverage.
2. Description of the Related Art
The prior art discloses a wide variety of marine seismic systems with one or more streamers and/or one or more seismic sources, some of which include a main or host vessel and other unmanned remote control vessels, or apparatuses such as floats, paravanes, or buoyant members which are connected to the host vessel by lines, streamer cables or tethers. Considerable power is required for a host vessel to tow such existing seismic streamer systems and cables interconnecting sensing devices with a tow vessel. A typical host vessel is able to tow a plurality of associated vessels or apparatuses and can carry a plurality of undeployed seismic streamers and associated apparatuses.
With known cable tow systems, the location and spacing of system components is illustrated by the type, size, and length of cables used and by the characteristics of tow vessels and other devices of the systems. Changing the configuration of an array of prior art components, e.g., streamers can be a complex, time-consuming operation.
U.S. Pat. No. 5,724,241, entitled “Distributed Seismic Data-Gathering System,” by Wood, et al., describes a distributed seismic data acquisition system of a plurality of Autonomous Data Acquisition Modules (ADAMs) to each of which are interconnected a subplurality of data-collection channels. Each data collection channel is composed of an array of seismic sensors for continuously measuring seismic signals. The ADAMs includes a GPS satellite receiver for providing geographic coordinates and a system clock. Measured seismic signals are quantized and continuously downloaded to respective interconnected ADAMs from the data-collection channels. The system includes both field-testing capability as well as means for transmitting the results of self-tests.
During a typical marine seismic survey a seismic vessel traverses programmed tracks towing arrays of seismic sources and seismic streamer cables. A seismic streamer cable normally contains a plurality of hydrophones that convert seismic pressure waves, initiated by the sources and reflected from the subsurface geological formations, into electrical signals, which are recorded on a marine seismic data acquisition system located on the vessel. Due to the increasing use of marine three-dimensional (3-D) seismic data, multi-cable marine surveys are now commonplace. During a typical marine 3-D seismic survey, a vessel may tow as many as ten or more streamer cables, with cables ranging in length from three to eight or more kilometers. As reported by Gadallah in “Reservoir Seismology” 1994, pp. 209-237, the goal of a normal marine 3-D seismic survey is to use these arrays of seismic sources and streamer cables to record a highly sampled grid of “bins” of subsurface seismic coverage.
A natural consequence of towing such streamer cable configurations in a marine environment is that currents, wind, and wave action will deflect the streamer cables from their intended paths. Streamer cable drift presents a continuing problem for marine seismic surveys. See, for example, U.S. Pat. No. 5,532,975. The ability to control the position and shape of the streamer cables is desirable for preventing the entanglement of the streamer cables and for avoiding collisions with offshore hazards such as marine drilling rigs and platforms. It is also desirable to have the ability to control the position and shape of the streamer cables during marine 3-D seismic surveys because the 3-D seismic binning process acquires subsurface seismic coverage by combining seismic data from different lines. The need for ability to control the position and shape of the streamer cables is taught by Franklyn K. Levin in “Short Note: The Effect of Binning on Data from a Feathered Streamer,” Geophysics, Vol. 49, No. 8, pp. 1386-138,7.
Streamer positioning devices are well known in the art. Apparatuses, such as those disclosed in U.S. Pat. Nos. 5,532,975; 4,729,333; and 4,463,701, have been devised for attachment to the front end of streamer cables for the purpose of maintaining them at a lateral offset to the pathway of the towing vessel. Steerable tail buoys, as described in U.S. Pat. No. 4,890,568, have also been designed for controlling the position of the tail end of seismic streamer cables. The prior art also discloses streamer positioning devices that may be attached externally to the streamer cables. For example, devices to control the lateral positioning of streamer cables by using the camber adjustable hydrofoils or angle wings are disclosed in (U.S. Pat. Nos. 4,033,278 and 5,443,027. U.S. Pat. No. 3,931,608 describes an apparatus, typically known as a “bird”, to control the vertical positioning of streamer cables with diving planes and a present depth control means.
The use of streamer positioning devices comes at the price of introducing increased noise onto the seismic streamer and hence into the hydrophones. The areas of greatest noise are from those hydrophones adjacent externally attached streamer-positioning devices, such as depth controlling birds. This problem has been described by Schoenberger and Misfud, “Hydrophone Streamer Noise,” GEOPHYSICS, Vol. 39, No. 6, pp. 782-784. It is well known in the art that noise limits the resolution of a seismic survey. Consequently, a maximum allowable hydrophone noise level is typically established for each marine seismic surveying project. When this noise level is exceeded, seismic acquisition is usually suspended, resulting in lost time and additional cost. Data acquired under such conditions may need to be reacquired.
Location sensing devices and methods for determining the positions of the seismic sources and seismic streamer cables are also well known in the art. For example, both a Global Positioning System, as described in U.S. Pat. No. 4,809,005, and a network of acoustic elements, as described in U.S. Pat. No. 4,912,682 may be deployed on the vessel, streamer cables, and tail buoy. These devices and methods may then be used to determine the real time positioning of the seismic sources and seismic streamer cables by computing a network solution to a Kalman filter, as disclosed by U.S. Pat. No. 5,353,223.
As known to those familiar with the art of marine seismic surveying, during a typical seismic survey a human operator monitors the survey's operational conditions, such as the extent of the subsurface seismic coverage, the adequacy of the separations between streamer cables, and the proximity of the streamer cables to obstructive hazards. When these conditions indicate the need to reposition the streamer cables, the operator may manually issue commands to the various individual streamer positioning devices in order to adjust the position and shape of the streamer cable, or order the helmsman or vessel remote control to redirect the vessel, or suspend data acquisition.
A typical three-dimensional marine geophysical survey is performed by transiting a pre-defined grid of parallel lines in order to cover a desired survey area at a required minimum multiplicity for common midpoint coverage. During each pass over the grid a spread of seismic sources and receivers is used to produce the desired subsurface common midpoint coverage. Because the seismic spread is perturbed due to errors in the towing vessel's motion, tidal streams, ocean current, river estuaries, etc. sub-optimal midpoint coverage is obtained. To mitigate the loss of coverage an operator will attempt to maneuver the towing vessels to his or her interpretation of the best geometry. Such manual maneuvering is by nature a labor-intensive process and highly subject to operator bias, error and
Ambs Loran D.
Bennett Colin M.
Zajac Mark
Figatner David S.
Lefkowitz Edward
Madan Mossman & Sriram P.C.
Taylor Victor J.
Westerngeco L.L.C.
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