Abstract:
An embodiment of a method for conducting a bathymetric survey across a seismic spread includes the steps of emitting a signal from a pinger positioned within the seismic spread, measuring the arrival time of a bottom reflection of the signal at a receiver positioned within the seismic spread, measuring the arrival time of a direct arrival of the signal at the receiver, and determining the water depth utilizing the measured arrival times of the signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates in general to marine seismic surveys and more specifically to systems and methods for creating a bathymetric survey across a seismic spread by utilizing the components of the seismic spread. 
   BACKGROUND 
   In marine seismic surveys it is desired to have accurate information about the depth of the water across the seismic spread. The depth information is important so as to avoid entering bottom reflections in the positioning solutions and to remove waterborne multiples in the seismic processing. Heretofore, water depth is commonly obtained along the seismic survey track by an echo sounder mounted on the hull of the vessel. The depth information obtained in this manner is of little value because it only represents a limited area and does not cover the seismic spread. More accurate depth information for generation of charts may be obtained by making multiple passes with a survey vessel utilizing multiple echo sounders or side scan sonar. This method of obtaining a bathymetric chart of an area is expensive and time consuming. 
   Therefore, it is a desire to provide a system and method of obtaining water depth information across the seismic survey that addresses drawbacks of the prior art methods. It is a still further desire to provide a method of obtaining water depth information across a seismic spread utilizing an acoustic ranging system in a seismic spread. 
   SUMMARY OF THE INVENTION 
   Accordingly, methods for obtaining water depth information across a seismic spread are provided. An embodiment of a method for conducting a bathymetric survey across a seismic spread includes the steps of emitting a signal from a pinger (acoustic transducer) positioned within the seismic spread, measuring the arrival time of a bottom reflection of the signal at a receiver positioned within the seismic spread, measuring the arrival time of a direct arrival of the signal at the receiver, and determining the water depth utilizing the measured arrival times of the signal. 
   Another embodiment of a method for conducting a bathymetric survey across a seismic spread including the steps of emitting a signal from a pinger positioned within the seismic spread, measuring the arrival time of a bottom reflection of the signal at a receiver positioned within the seismic spread, measuring the arrival time of a direct arrival of the signal at the receiver, and determining the water depth according to the equation: 
           H   =       Z   s     +       1   c     ⁢         T     R   ,   i     2     -     T     D   ,   i     2                   
wherein “H” is the water depth, “c” is the speed of sound in the water column, Z s  is the depth of streamer  14  in the water, T R,i  is measured arrival time of bottom reflection and T D,i  is the measured arrival time of direct arrival.
 
   Another method for conducting a bathymetric survey across a seismic spread includes the steps of towing a seismic spread from a vessel, the seismic spread comprising a plurality of streamers and an acoustic ranging system including a plurality of pingers and receivers connected to the streamers; emitting signals from the pingers; measuring the arrival time of a bottom reflection of the signals at the receivers; measuring the arrival time of a direct arrival of the signals at the receivers; positioning the streamers; and determining the water depth utilizing the measured arrival times of the signals. 
   The foregoing has outlined the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and aspects of the present invention will be best understood with reference to the following detailed description of a specific embodiment of the invention, when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a top view of an embodiment of a seismic spread of the present invention; 
       FIG. 2  is a side view of the seismic spread of  FIG. 1  illustrating the depth surveying system of the present invention; and 
       FIG. 3  is a side view of an embodiment of a geometry of the present invention for surveying the water depth across a seismic spread. 
   

   DETAILED DESCRIPTION 
   Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     FIG. 1  is a top view an embodiment of the depth surveying system of the present invention, generally denoted by the numeral  10 . System  10  may be an acoustic ranging system such as described in U.S. Pat. No. 5,668,775. System  10  includes a vessel  12  towing one or more streamers  14 . Streamers  14  extend longitudinally from vessel  12  and are spaced from one another laterally to form a seismic spread  16  for conducting a seismic survey. It is not uncommon for seismic spread  16  to extend 300 to 1200 meters laterally, denoted  18 , and to extend longitudinally 3 to 12 kilometers, denoted  20 . 
   Seismic spread  16  includes an acoustic ranging system for navigation and positioning purposes. The acoustic ranging system includes a plurality of transmitters referred to as pingers  22  and receivers  24 . The acoustic ranging system measures the range between pingers  22  and receivers  24 . The range is the travel time of a direct arrival  28  ( FIG. 2 ) of a signal  26  transmitted from a pinger  22  and received by a receiver  24 . In some situations, particularly for long ranges in shallow water, the direct signal arrival  28  is weaker than the bottom reflection  30 . It is desirable to have bathymetric information across seismic spread  16  to avoid entering bottom reflections in the positioning solutions as well as to remove multiples in the seismic processing. 
     FIG. 2  is a side view of system  10  shown in  FIG. 1 , illustrating a bathymetric survey being conducted across seismic spread  16  ( FIG. 1 ). Streamer  14  is towed behind vessel  12  at a depth Z s  below the surface  34  of the sea. Proximate the distal ends of streamer  14  are global positioning systems  36 . Birds  40 , carrying instruments to provide dynamic information regarding the position of streamer  14 , may also be connected along streamer  14 . 
   Currently, bathymetric information is typically provided via an echo sounder or side scan sonar  38  from vessel  12 . This depth information is of limited value since it only covers a limited area around vessel  12  and does not incorporate the area across seismic spread  16 . Independent surveys may be conducted across an area in which the seismic survey is going to be provided, however, these surveys are expensive and time consuming. The current invention provides a method and system for conducting an accurate bathymetric survey of the area covered by spread  16  proximate the time the seismic survey is conducted. 
   With reference to  FIGS. 1 and 2 , pingers  22  emit signals  26  in all directions. Signal  26  along each streamer  14  and across the plurality of streamers  14  are used for positioning streamers  14 . In the present invention signal  28  energy propagating close to vertical, illustrated as reflected signal  30 , is utilized to provide bathymetric information across the seismic spread. For each pinger  22 , one can utilize data from multiple receivers  24  or from one receiver  24  to determine the water depth H. The depth Z s  that streamer  14  is below surface  34  is known from depth sensors. The depth sensors may be included in birds  40 . 
     FIG. 3  illustrates an embodiment of a geometry of the invention for determination of the water depth across spread  16 . Water depth H is provided by equation: 
           H   =       Z   s     +       1   c     ⁢         T     R   ,   i     2     -     T     D   ,   i     2                   
wherein “c” is the speed of sound in the water column, Z s  is depth of streamer  14  below surface  34 , T R,i  is the arrival time of bottom reflection  30  at receiver  24   a  from pinger  22   a  and T D,i  is the arrival time of direct arrival  28  at receiver  24   a  from pinger  22   a . The index “i” represents that the measurement may be taken for several offsets or to average the measurements. Further, utilization of measurements of different offsets may be used to estimate the speed of sound in water column H.
 
   Dipping seafloor inline may be addressed by comparing ranges between the distal ends of streamer  14  or by analysis of data from ping to ping. Cross-line dip may be determined by analysis of the relationship between pingers  22  and receivers  24  on neighboring streamers  14 . 
   From the foregoing detailed description of specific embodiments of the invention, it should be apparent that a system and method for obtaining a depth survey across a seismic spread that is novel has been disclosed. Although specific embodiments of the invention have been disclosed herein in some detail, this has been done solely for the purposes of describing various features and aspects of the invention, and is not intended to be limiting with respect to the scope of the invention. It is contemplated that various substitutions, alterations, and/or modifications, including but not limited to those implementation variations which may have been suggested herein, may be made to the disclosed embodiments without departing from the spirit and scope of the invention as defined by the appended claims which follow.