Patent Application
20120230150
Not applicable.
Not applicable.
The invention relates generally to the field of marine geophysical surveying. More particularly, at least in some embodiments, the invention relates to methods for determining geodetic locations of positions on streamers towed at water depths which make impractical the use of geodetic position sensor signals or tail buoys.
Marine geophysical surveying techniques known in the art include seismic surveying and electromagnetic surveying. In such surveying techniques, a survey vessel or other vessel may tow one or more streamers. A streamer is essentially a long cable with sensors disposed therein at spaced apart locations along the length of the cable. Seismic streamers may include pressure responsive sensors, particle motion responsive sensors, or combinations and variations thereof generally responsive to seismic energy propagating in a body of water. Electromagnetic sensors may include, without limitation, electrodes, wire coils, magnetometers, or other devices to detect electromagnetic fields in the body of water. Such fields may be naturally occurring or may be induced by actuating an electromagnetic energy source in the water.
The foregoing surveys may be used to generate images of the distribution of acoustic and electrical properties of formations below the water bottom. Such images may be used to predict the location of hydrocarbons or other useful fluids prior to or in conjunction with drilling operations. Generating accurate images requires, among other things, accurate location of each of the sensors on each of the streamers at all times during survey operations.
In seismic surveying, systems and methods for determining geodetic position of each sensor on several seismic sensor streamers include a system and method described in U.S. Pat. No. 7,376,045 issued to Falkenberg et al. It is desirable to have a method that can also be used with electromagnetic sensor streamers for precisely determining geodetic position of each of the sensors on one or more of such electromagnetic streamers. It is also desirable to have a method that can enable location of any type of geophysical sensor streamer operated at a depth such that use of geodetic position sensor signals or tail buoys is impractical.
A method for determining geodetic position of at least one point on at least one second streamer towed by a vessel in a body of water according to some embodiments of the disclosure comprises determining geodetic positions of a plurality of first positions along at least two of a plurality of first streamers towed in the body of water. The method also comprises selecting positions from the plurality of first positions. The method also comprises determining distances between pairs of the selected positions and between each of the selected positions and the at least one point. The method also comprises determining geodetic position of the at least one point using the geodetic positions and distances.
A system for geophysical surveying according to some embodiments of the disclosure comprises a tow vessel. The system also comprises a plurality of first streamers coupled to the tow vessel, the first streamers disposed in laterally spaced apart relation to each other, at least two of the first streamers including a plurality of acoustic range detection transceivers thereon at spaced apart locations and having a tail buoy at an aft end thereof, each tail buoy including a geodetic position signal receiver. The system also comprises at least one second streamer coupled to the tow vessel, the at least one second streamer including a plurality of acoustic range detection transceivers thereon at spaced apart locations. The system also comprises computational equipment configured to determine the geodetic positions of selected positions on the at least two of the first streamers and at least one selected position on the at least one second streamer from distance measurements made by the acoustic range detection transceivers and the geodetic position signal receivers.
A method for geophysical surveying according to some embodiments of the disclosure comprises determining geodetic positions of a plurality of first positions along at least two of a plurality of first streamers towed by a vessel in a body of water at a first depth. The method also comprises selecting positions from the plurality of first positions. The method also comprises determining distances between pairs of the selected positions and between each of the selected positions and at least one point on at least one second streamer operated at a second depth in the body water. The method also comprises determining geodetic position of the at least one point using the geodetic positions and distances. The method also comprises actuating a geophysical source disposed in the body of water at selected times. The method also comprises detecting geophysical signals with a plurality of geophysical sensors disposed on at least one of the first and second streamers.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
The seismic energy source 24 may be any type of marine seismic energy source known in the art, including, without limitation, air guns or arrays thereof, marine vibrators or arrays thereof, and water guns or arrays thereof. The electromagnetic transmitter 26 may be any device known in the art for inducing a time varying electromagnetic field in a body of water, including without limitation, spaced apart electrodes and wire coils or loops. The seismic energy source 24 and/or the electromagnetic transmitter 26 may each be towed at a selected depth in the water appropriate for the type of signals to be emitted and detected therefrom.
The survey vessel 10, or another vessel (not shown) may tow a plurality of first geophysical sensor streamers 14. In the present example embodiment such geophysical sensor streamers may be seismic sensor streamers 14 towed at a first selected depth in the water. The seismic sensor streamers 14 may include a plurality of longitudinally spaced apart seismic sensors 15, which may include particle motion responsive sensors, pressure or pressure time gradient responsive sensors, and any other type of sensor known in the art for detecting seismic energy propagating through the water 11. The seismic sensor streamers 14 may include one or more depth sensors 19, which may be pressure sensors or any other device known in the art for determining depth of the seismic sensor streamers 14 in the water 11. During surveying, the seismic energy source 24 may be actuated at selected times, and seismic energy may be detected by the seismic sensors 15. The recording system 12 may make a recording of the signals detected by each of the seismic sensors 15, typically indexed with respect to the actuation time of the seismic energy source 24. It will be appreciated by those skilled in the art that the foregoing example embodiment of seismic sensor streamers 14 represents only one type of geophysical sensor streamer, and other types may be used in other embodiments. In such other embodiments, the seismic sensors 15 may be substituted and/or accompanied by any other type of geophysical sensor, including, without limitation, electromagnetic sensors. In some embodiments, the geophysical sensor streamers 14 may neither include seismic energy sensors nor electromagnetic sensors. Nonetheless, for notational ease, geophysical sensor streamers 14 may be referred to herein as seismic sensor streamers 14.
The seismic sensor streamers 14 may also include a plurality of acoustic range detection transceivers (“ARD transceivers”) 20 at spaced apart locations along the seismic sensor streamers 14. In some embodiments, one or more ARD transceivers 20 may also be located proximate the survey vessel 10. The seismic sensor streamers 14 are operated at such first depth in the water that they may be terminated at their respective aft ends by a tail buoy 16. (As used herein, “aft” refers to the trailing direction while the streamer is being towed.) Each tail buoy 16 may include a geodetic position signal receiver 18, such as a global navigation satellite system (“GNSS”) receiver. The combination of geodetic position signal receiver 18 on each of the tail buoys 16, on the survey vessel 10 (part of the recording system 12), and ARD transceivers 20 may be used to infer the geodetic position of each seismic sensor 15 on each seismic sensor streamer by determining relative position of each ARD transceiver 20 with respect to others of the ARD transceivers 20 and with respect to the geodetic position signal receiver 18. The relative positions may be determined by trilateration of a plurality of distance measurements 21 made by the ARD transceivers 20. By combining the relative positions thus determined and the geodetic positions determined by detecting geodetic position signals at the geodetic position signal receivers 18, the geodetic position at each point along each seismic sensor streamer 14 may be determined. It is thus possible to determine the geodetic position of each seismic sensor 15 along each of the seismic sensor streamers 14 at any time. An example system and method for determining such geodetic positions including examples ARD transceivers 20 and geodetic position signal receivers 18 is more fully explained in U.S. Pat. No. 7,376,045 issued to Falkenberg et al., incorporated herein by reference. In some embodiments, the distance measurements 21 and measurements from the geodetic position signal receivers 18 may be communicated to the recording system 12 for determining geodetic positions at any selected positions along the seismic sensor streamers 14 and, as will be explained further below, the geodetic position at any point along one or more electromagnetic sensor streamers.
In the present embodiment, the survey vessel 10 or another vessel (not shown) may tow one or more second geophysical sensor streamers 22 at a second depth in the water 11. In the present example embodiment, the second geophysical sensor streamer(s) 22 may be electromagnetic sensor streamer(s) 22. In other embodiments, the second geophysical sensor streamer(s) may have any type of geophysical sensor, including seismic sensors. In the present embodiment, wherein the second geophysical sensor streamer(s) 22 are electromagnetic sensor streamers, the electromagnetic sensor streamer(s) 22 may include a plurality of longitudinally spaced apart electromagnetic sensors 17. The electromagnetic sensors 17 may be any device known in the art for detecting one or more components of an electromagnetic field in the water 11. Such electromagnetic sensors 17 may include, without limitation, spaced apart electrodes, wire loops or coils, and magnetometers. During survey operations, the electromagnetic transmitter 26 may be actuated by passing electric current through the electromagnetic transmitter 26 at selected times. The electric current may include one or more transient events (e.g., switching current on, switching current off, reversing current polarity, or combinations thereof, either individual or in a predetermined sequence such as a pseudo random binary sequence). The electric current may also include one or more individual frequencies of alternating current, and may be switched on for a selected time duration. The recording system 12 may record signals detected by each of the electromagnetic sensors 17.
As will be appreciated by those skilled in the art with the benefit of this disclosure, the electromagnetic sensor streamer(s) 22 may be towed at substantially greater depth in the water 11 than the seismic sensor streamers 14. In some embodiments, for example, electromagnetic sensor streamer 22 may be towed at depths of between about 20 m and about 50 m. In other embodiments, electromagnetic sensor streamer 22 may be towed at depths of between about 50 m and about 100 m or more. Thus, it may be impractical to use geodetic position sensor signals or tail buoys to determine the geodetic position of any one or more of the electromagnetic sensors 17 along the electromagnetic sensor streamer(s) 22. In the present embodiment, therefore, the electromagnetic sensor streamer(s) 22 may include a plurality of ARD transceivers 20 at spaced apart locations along the length of the electromagnetic sensor streamer(s) 22. In the present embodiment, the ARD transceivers 20 disposed on the electromagnetic sensor streamer(s) 22 may be used to determine relative position of any point along the electromagnetic sensor streamer(s) 22 with reference to the position of any point along the seismic sensor streamers 14. Determining relative position of any point along the electromagnetic sensor streamer(s) 22 with reference to the position of any point along the seismic sensor streamers 14 may be performed as explained in the Falkenberg '045 patent. It is therefore possible to infer the geodetic position at any point along the electromagnetic sensor streamer(s) 22. It is also possible, therefore, to determine the geodetic position of any one or more of the electromagnetic sensors 17 along the electromagnetic sensor streamer(s) 22. When streamers are operated at different depths as in the present example embodiment, it is also desirable to be able to determine depth of the deeper operated streamers while minimizing the number of depth sensors required. In some embodiments, the distances 21 between a plurality of ARD transceivers 20 on the seismic sensor streamers 14 and any or all of the ARD transceivers 20 on the electromagnetic sensor streamer(s) 22 may also be used to determine depth in the water 11 at any position along the electromagnetic sensor streamer(s) 22, because the depth of the seismic sensor streamers 14 may be measured using one or more depth sensors such as shown at 19 in
The example embodiment of
It will be readily appreciated by those skilled in the art with the benefit of this disclosure that the scope of the present invention is not limited to towing and locating seismic sensor streamers and one or more electromagnetic sensor streamers as shown in and explained with reference to
A method according to the various aspects of the invention may enable determination of geodetic positions of a plurality of geophysical sensors along one or more geophysical sensor streamers without the need to measure geodetic position directly at any point along the one or more geophysical sensor streamers, e.g., electromagnetic sensor streamers operated in conjunction with seismic sensor streamers. Likewise, methods of conducting a geophysical survey may benefit from the determination of geodetic positions of a plurality of geophysical sensors along one or more geophysical sensor streamers without the need to measure geodetic position directly at any point along the one or more geophysical sensor streamers.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
