Patent Application
20120020184
Subterranean surveying for determining the content of a subterranean structure can be performed in a marine environment. In performing such marine subterranean surveying, sensors (such as seismic sensors or electromagnetic sensors) can be towed by a structure (sometimes referred to as a streamer) through a body of water. Alternatively, sensors can be arranged on a cable placed on a sea floor.
Source signals, such as seismic signals or electromagnetic signals, are generated by one or more signal sources for propagation into the subterranean structure. The propagated signals are reflected from or otherwise affected by the subterranean structure, where the reflected or affected signals are detected by the sensors on the streamer or cable.
In a survey arrangement, positions of various components of a survey spread, including the streamer or cable, can be difficult to accurately ascertain.
In general, according to an embodiment, a method includes providing a distributed optical acoustic sensor along a structure in a body of water, and using the optical acoustic sensor to detect acoustic waves generated by at least one acoustic source for positioning at least one object in relation to the structure.
Other or alternative features will become apparent from the following description, from the drawings, and from the claims.
Some embodiments of the invention are described with respect to the following figures:
Traditionally, a marine survey arrangement for surveying the content of a subterranean structure involves towing one or more streamers through a body of water, where each streamer has sensors for detecting signals reflected from or affected by the subterranean structure. Alternatively, sensors can be deployed on a cable that is positioned on a bottom surface of a body of water (e.g., a sea floor). Elements of interest in the subterranean structure include hydrocarbon reservoirs, fresh water aquifers, gas injection zones, and so forth.
For seismic surveying, the sensors that are part of the streamer or cable are seismic sensors, such as hydrophones, accelerometers, and so forth. For electromagnetic (EM) surveying, the sensors can be EM receivers.
In a marine environment, a structure in the body of water can be subjected to various forces (caused by water currents, movement of marine vessels, and other factors) that can make determination of exact positions of the components of the survey arrangement difficult. In one example conventional arrangement, a streamer is provided with acoustic pingers that are arranged along the length of the streamer. The acoustic pingers are able to emit relatively high-frequency pings that are substantially above the maximum frequency of interest for seismic applications (which are typically in the kilohertz range). During a seismic survey, the acoustic pingers are activated regularly, and the high-frequency acoustic signals are picked up by designated seismic sensors (e.g., hydrophones) along the streamer or in other structures that are part of the seismic survey spread. A “survey spread” refers to equipment used for performing the marine subterranean survey, where the equipment can include the streamer or cable carrying sensors, as well as other equipment such as one or more source arrays (that carry signal sources), navigation equipment for navigating components of the survey spread, and so forth.
The time of arrival of an acoustic signal at a designated seismic sensors is determined. The travel time of the acoustic signal between an acoustic pinger and the receiving seismic sensor can be determined. The travel time data can be used to solve for positions of various portions of the seismic survey spread, since the velocity of sound in water can be determined by various techniques, and points in the spread such as the front and/or tail (or other location) of any spread can be determined using a global positioning system (GPS) receiver.
A survey spread can have multiple streamers, where each of the streamers can have acoustic pingers. Positioning a particular streamer can be accomplished by receiving signals from acoustic pingers on streamers that are the two sides of the particular streamer.
Using the foregoing technique for positioning a marine survey spread can be somewhat complicated, since the same recording elements are used for recording both seismic data and acoustic pings. In addition, the recording of high-frequency acoustic pings on hydrophones may not be possible due to relatively high bandwidth requirements for communications.
In accordance with some embodiments, instead of using traditional acoustic sensors such as hydrophones for detecting acoustic waves generated by one or more acoustic sources for positioning a marine survey spread, a distributed optical acoustic sensor is used instead. The “distributed optical acoustic sensor” refers to a sensor that extends along some predefined length with respect to a structure that is located in a body of water. In some embodiments, the distributed optical acoustic sensor includes one or more optical fibers.
An optical source is used to generate optical signals that are emitted into an optical fiber in the distributed optical acoustic sensor, with backscattered light responsive to the emitted optical signals being detected by an optical receiver. Certain parts of the optical fiber may be affected by acoustic waves, such as acoustic waves generated by the acoustic pingers that are part of a streamer, or by other acoustic sources. The acoustic waves cause strain to be applied on portions of the optical fiber, which affect the backscattered optical signals that are reflected back to the optical receiver.
Analysis of the received backscattered optical signals allows for positioning of one or more objects of interest in relation to a structure carrying the distributed optical acoustic sensor. For example, the one or more objects of interest can include one or more portions of a structure that carries survey sensors. Such a structure can include a streamer towed through a body of water, or a seabed cable positioned on the sea floor.
Alternatively, the one or more objects of interest can also include external objects that may intrude upon the marine survey spread. For example, the external object that may intrude upon the marine survey spread may be a marine vessel or a large fish or mammal (or other living being). A marine vessel or large living being may cause damage to portions of the marine survey spread, such that it would be useful to detect possible collision between the marine survey spread and the external object.
Positioning of one or more objects of interest using some embodiments can also be applied in the context of passive acoustic monitoring. Passive acoustic monitoring is used for protecting marine living beings from injury caused by survey activities. Passive acoustic monitoring using some embodiments of the inventions can be used to determine whether a marine living being is nearby, such that survey activities can be slowed down or even stopped to protect such marine living beings. Some countries have passed legislation that mandate steps to ensure that marine living beings are not injured or damaged.
The distributed optical acoustic sensor can be employed in a marine survey arrangement that performs either a seismic survey or an electromagnetic survey. Alternatively, the distributed optical acoustic sensor can be used in other marine contexts in which it may be useful to position portions of equipment in a body of water.
As yet other alternatives, acoustic pingers or other acoustic sources can be mounted elsewhere, such as on the marine vessel 100, on a platform, on a buoy, in an aircraft that is in the air, and so forth.
The marine vessel 100 also has a control system 110 that is electrically coupled to the streamer 102. The control system 110 can receive signals collected by the survey sensors 106. Also, the control system 110 can control activation of the acoustic pingers 108.
In accordance with some embodiments, a distributed optical acoustic sensor 112 (shown as a dashed line) is arranged along the length of (or part of the length of) the streamer 102. The distributed optical acoustic sensor 112 can be externally attached or otherwise mounted to the streamer 102, or alternatively, the distributed optical acoustic sensor 112 can be provided inside the external housing of the streamer 102. The distributed optical acoustic sensor 112 can be attached to the streamer 102 using an adhesive or some other attachment mechanism.
In some embodiments, the distributed optical acoustic sensor 112 can include one (or multiple) optical fibers that extend along the length of the distributed optical acoustic sensor 112. The control system 110 includes an optical source to emit optical signals into the optical fiber of the distributed optical acoustic sensor 112. The control system 110 also includes a receiver to receive backscattered optical signals from the optical fiber, where the backscattered signals are in response to the optical signals emitted by the optical source. The control system 110 can also include a processor to analyze the backscattered signals for the purpose of positioning one or more objects of interest in relation to the streamer 102, where the objects of interest can be one or more portions of the streamer 102, or an external object that may collide with the streamer 102.
When trying to position an external object such as another marine vessel or a large living being, the external object may provide the acoustic source, such as in terms of noise produced by the external object when moving through the body of water 104.
In some implementations, the optical fiber (or multiple optical fibers) of the distributed optical acoustic sensor 112 can be generally encased in a protective layer. For example, the optical fiber may be disposed within a control line strapped to the outside of the streamer 102. Alternatively, the protective layer can be the streamer housing itself if the distributed optical acoustic sensor 102 is located inside the streamer housing.
In some embodiments, monitoring of acoustic waves by the distributed optical acoustic sensor 112 can be based on coherent Rayleigh backscatter in which a pulse of coherent light is launched into the optical fiber and returned (backscattered) light is analyzed. When the optical fiber is disturbed by an acoustic wave, the modulation of the backscattered optical signal is varied in the vicinity of the disturbance.
In some embodiments, rather than employ a fully distributed optical sensing fiber, an array of discrete reflectors can be used instead by inserting such discrete reflectors into the optical fiber. For example, the reflectors may be Bragg reflectors.
The pulses emitted by the optical source 302 are launched into the optical fiber through a directional coupler 306, which separates outgoing and returning optical signals and directs the returning (backscattered) signals to an optical receiver 308. The directional coupler 306 may be a beam splitter, a fiber-optic coupler, a circulator, or some other optical device.
The backscattered optical signals returned from the optical fiber of the distributed optical acoustic sensor in response to interrogating pulses may be detected and converted to an electrical signal at the receiver 308. This electrical signal may be acquired by a signal acquisition module 310 (e.g., an analog-to-digital converter) and then transferred as data representing the backscattered signals to a signal processing module 312. The signal processing module 312 can include a processor such as a microprocessor, microcontroller, digital signal processor, computer, and so forth. The signal processing module 312 analyzes the waveforms received to determine, at each location along the optical fiber, where the signal is changing. The signal processing module 312 is able to interpret the change in terms of acoustic waves modulating the backscatter return of the optical fiber.
When an optical fiber portion is disturbed by acoustic waves, the optical fiber portion is strained by the acoustic waves. A strain on the optical fiber portion changes the relative position between the scattering centers by simple elongation of the optical fiber portion. The strain also changes the refractive index of the glass of the optical fiber portion. Both these effects alter the relative phase of the light scattered from each scattering center.
In alternative implementations, the optical fiber can be manufactured with optical gratings or other types of reflectors that can cause backscatter of light whose characteristics are affected by presence of acoustic signals.
At least one acoustic source can be activated (at 404), where the at least one acoustic source can include acoustic pingers, and/or some other acoustic source(s). For implementations to detect intrusion of an external object such as a marine vessel or a living being, the acoustic source can be the external object itself.
The interrogation system 300 (
The control system 500 includes an acoustic generation control module 502 to cause activation of one or more acoustic sources, such as the pingers 108 or 205 of
The positioning of portions of a survey spread or of an external object or of any other equipment can be accomplished based on analysis by software, such as software that is in the signal processing module 312 of the interrogation system 300.
Instructions of the software can be loaded for execution on a processor, which can include one or more microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), programmable integrated circuits, programmable gate arrays, or other control or computing devices. As used here, a “processor” can refer to a single component or to plural components (e.g., one CPU or multiple CPUs, or one computer or multiple computers).
Data and instructions (of the software) are stored in respective storage devices, which are implemented as one or more computer-readable or computer-usable storage media. The storage media include different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy and removable disks; other magnetic media including tape; optical media such as compact disks (CDs) or digital video disks (DVDs); or other types of storage devices. Note that the instructions of the software discussed above can be provided on one computer-readable or computer-usable storage medium, or alternatively, can be provided on multiple computer-readable or computer-usable storage media distributed in a large system having possibly plural nodes. Such computer-readable or computer-usable storage medium or media is (are) considered to be part of an article (or article of manufacture). An article or article of manufacture can refer to any manufactured single component or multiple components.
In the foregoing description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details. While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.
