Patent Application
20140301163
The present application relates to marine seismic exploration, and more particularly to marine seismic acoustic positioning of seismic exploration equipment with respect to temperature gradients across depths.
Marine seismic exploration can involve the use of seismic streamers that are towed by vessels. The streamers can be elongated tubular structures that support various seismic sensors therein. They also can include various electronics for communications relating to the seismic signals detected. A source such as an air gun or vibrator is used to generate a source signal. The source signal propagates though the water into the seafloor formation where it is reflected. The seismic sensors in the streamers detect that reflection and use the reflected signals, in conjunction with timing and position information to generate a plot indicating attributes of the underlying surface formation.
In connection with the towed streamers are floats that are generally connected near the front of the streamer. This can be by way of a cable, cord, rope, chain or other connecting device. The floats can also be connected at the tail end of the streamers. The floats could also be connected at a mid-point of a streamer. The floats are buoyant and can hold the streamers at a depth. The floats can also have connected therewith acoustic positioning devices (e.g., sources, receivers, and/or transducers). Acoustic positioning devices can also be connected with a vessel. The acoustic positioning devices can also be connected at various portions of the streamers. The acoustic positioning devices can emit acoustic signals that are detected by sensors on the streamer and are used to determine the position of the sensors on the streamer. Also, the acoustic positioning devices can receive acoustic positioning signals. The positioning devices can also be a combination of emitter and receiver of acoustic positioning signals. Based on the time the signal is emitted and the time the signal is received by a sensor, a distance can be calculated. Based on distances, the position of the sensor can be calculated with triangulation. This is possible when the position of the acoustic positioning device(s) is known. A global positioning system (GPS) can be connected with a positioning devices (or associated support structure like a vessel or float) to provide location of the positioning devices. The positioning devices are normally close to the surface of the water when connected with a float or a vessel. Acoustic positioning devices can be located on the streamers.
There are various aspects of the ocean environment in which these surveys are conducted that interfere or disrupt the acoustic signals transit thereby causing issues for the data integrity and the position determination of the sensors and in turn the streamer. Accordingly the present application provides a number of embodiments that address some of those issues.
The following is a summary of various embodiments of the present application. It is not meant in any way to unduly limit any present or future scope of claims related to this application.
A method of acoustic positioning determination includes detecting a temperature gradient profile across a depth of water, determining a level of a thermal boundary between an upper temperature level and a lower temperature level, and determining a distance from the thermal boundary to position the acoustic positioning device and positioning the acoustic positioning device at that location.
The following brief description of the figures is meant to help one skilled in the art better understand the various embodiments described herein. They are not meant in any way to unduly limit any present or future related claims in connection with this application.
The following description concerns a number of embodiments and is meant to provide an understanding of the embodiments to one skilled in the art. The description is not in any way meant to limit the scope of any present or subsequent related claims.
In a marine seismic survey, a vessel tows seismic streamers. One or more seismic streamers can be towed. The streamers can be directly connected with a tow vessel or have an intermediary connector. For the purposes of the application, it will be understood that this intermediary connector is possible but that the streamer plus the intermediary will be simply referred to as a streamer. The intermediary can include a series of connectors that make up the front portion of the spread, for example deflectors that force the front part of the spread into a wide configuration. The intermediary is much shorter in comparison to a streamer and does not generally poses seismic sensors and acoustic survey equipment as the streamer does. An embodiment of a towed spread could be four, six, eight or more streamers spaced laterally from one another. A seismic source is provided to generate an acoustic signal. The source can be an air gun, or could be a vibrator. The source can be interconnected with the spread or the vessel towing the seismic streamer spread. Also, the source could be separate from the spread and connected with a vessel different than the tow vessel. There can be one or more acoustic sources.
The depth of the seismic streamer spread can be controlled. One way to control the seismic streamer spread is with steering devices that connects with the streamer. One such steering device is a fin steering device known as the Q-fin. This is provided commercially by WesternGeco. There are other commercial steering devices on the market for steering streamers. The steering can be lateral and/or vertical. The shape of the streamer can also be controlled by the steering. In the vertical reference, one part of the streamer can be at one depth and another part of the streamer can be at another depth. The shape of the streamer can be an “S” shape or can be a single curve. Another option is to use buoyancy to control the depth that the streamer is towed.
As part of a seismic survey the position of the streamer is determined. One way to determine the position of the streamer is to determine position of various sensors along the streamer. Based on determination of the various points on the streamer, the shape of the streamer can be determined.
One way to make these position determinations is by emitting acoustic positioning signals from a positioning deice and detecting those signals with sensors on the streamer. The sensors that receive the signal can be on the streamer. The reverse can be used too, where the signal is produced on the streamer and is received by the positioning device. From determining the travel time of the positioning acoustic signals from a known position, the distance between the positioning device and the position on the streamer can be determined. Two or more positioning acoustic devices can be used. With two or more acoustic positioning devices the position of a sensor (or source of acoustic signal) on the streamer can be triangulated.
The float can have a GPS connected thereto and associated electronics. Other devices can be used in place of the GPS, including optical and other wireless range/positioning devices. Also the float can communicate to receive signals to command action such as transmission of the acoustic positioning signal.
Plural floats can be coordinated to provide two or more acoustic positioning signals. Also, a positioning signal from an acoustic positioning device connected with the tow vessel can be used. The acoustic positioning signals are detected by the streamer sensors and are used to determine the position of the streamer with respect to the floats and/or vessel where the position of the floats and/or vessel is known by way of a device such as a GPS. In turn, the shape of the streamer can be determined based on the position of various points (sensors) on the streamer.
Issues with acoustic positioning can develop due to temperature boundary gradients in the water. In the summer time the upper levels of the water can be warmer than the lower levels of the water. Conversely, in the winter months the upper levels of the water can be colder than the lower levels of the water. A temperature gradient can develop that affects the speed of travel for the acoustic position signal depending on depth. This boundary will also refract the sound. If the difference in travel time/speed and/or refraction is not accounted for, the accuracy of the determined position and shape of the streamer can be negatively affected because the distance/position calculations will be inaccurate. At certain depths, this issue can be magnified or worsened compare to other depths. Thus, it can be beneficial to position the acoustic positioning device and the sensors on the streamer at a position spaced from the certain troublesome depths. For example, the depth to be avoided could be close to or across the sharpest point of the temperature gradient.
One way to address that issue is by locating the temperature gradient. This can be done in various ways such as lowering a temperature sensor in the water and detecting the temperature. Once a temperature gradient is determined, the acoustic positioning device and the streamer can be positioned at a depth that is sufficiently far above or below the temperature gradient, to reduce the impact the gradient has on the travel of the acoustic positioning signal, thus reducing any negative effect on the seismic survey.
According to an embodiment, a float includes an acoustic positioning device that is adjustable with respect to depth. One way to adjust that depth is by connecting the positioning device to an extendable/retractable support member that is connected with the float. This extendable/retractable support member can be telescopic and can extend downward from the float. The positioning device can also be connected with a cord or other member connecting the float to the streamer. The positioning device can travel and be located long the cord or member at various positions to change the depth of the source. Further, the depth of the streamer can be changed by way of a steering device such as steering fins (e.g., the Q-fin that is commercially provided by WesternGeco).
Where the acoustic positioning device is on a tow vessel (or other vessel), a similarly adjustable support member can be used to adjust the depth at which the positioning device is located. In any event, there are number of different way to adjustably locate the positioning device at a desired depth.
Now looking at the figures,
According to various embodiments, the depth of the temperature gradient 8 can be determined. One way is to take temperature measurements at different depths. Another is to reference prior collected date or records to determine a likely depth for the temperature gradient.
The streamer 6 can be positioned at a determined depth that is adequately above or below the temperature gradient 8. Also, the acoustic positioning device 2 can be positioned at an adequate distance above or below the temperature gradient 8. Normally, the acoustic positioning device 2 will be positioned on the same side of the temperature gradient 8 as the streamer 6.
As shown in
Another source of interference and issues with the acoustic positioning signal is the surface of the water 4. Waves and other surface actions such as reflections and variances can disrupt the continuity of the travel from the positioning device 2 to the sensors on the streamer 6. If the positioning device 2 is too close to the surface 4, the signal can be reflected and otherwise have the velocity or travel path disrupted. This is detrimental to the accurate determination of the streamer 6 position. Accordingly, the depth of the positioning device 2 can be far enough from the surface of the water 4, and the depth of the streamer 6 can be sufficiently deep so as to reduce the interference caused by the surface of the water.
It should be understood that the sensors on the streamer 6 that are used for the preceding described positioning can be incorporated within the structure of the streamer 6, can be attached externally to the streamer 6, can be incorporated with the steering device 11, and can be connected to the streamer in numerous envisioned ways.
These actions are covered by Snell's law explaining refraction, which is represented by the following equation:
The preceding description is meant to provide assistance to one skilled in the art in understanding the various embodiments described herein. It is not meant in any way to unduly limit any present or future claims associated with this application.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2012/059641 | 10/11/2012 | WO | 00 | 4/10/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61545969 | Oct 2011 | US |
