Patent Application
20110182138
Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of marine geophysical surveying.
More particularly, the invention relates to devices and methods for controlling the depth of marine geophysical sensor streamers as they are towed in a body of water.
2. Background Art
Certain types of marine geophysical surveying, such as seismic surveying include towing an energy source at a selected depth in a body of water. One or more geophysical sensor streamers are towed in the water at selected depths. The streamers are essentially long cables having geophysical sensors disposed thereon at spaced apart locations. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with the rock formations below the water bottom. Reflected energy from interfaces, generally at the boundaries between layers of rock formations, is returned toward the surface and is detected by the sensors on the one or more streamers. The detected energy is used to infer certain properties of the subsurface rock formations, such as structure, mineral composition and fluid content.
For certain types of surveying, it is important that the streamers are maintained as closely as possible to the selected depth in the water. Devices known in the art used for such purpose include “lateral force and direction” (LFD) control devices. U.S. Pat. No. 6,144,342 issued to Bertheas et al. describes a structure for LFD control devices and a method for controlling the navigation of a towed seismic streamer using “birds” affixable to the exterior of the streamer. The birds are equipped with variable-incidence wings and are rotatably fixed onto the streamer. Through a differential action, the wings allow the birds to be turned about the longitudinal axis of the streamer so that a hydrodynamic force oriented in any given direction about the longitudinal axis of the streamer is obtained. Power and control signals are transmitted between the streamer and the bird by rotary transformers. The bird is fixed to the streamer by a bore closed by a cover. The bird can be detached automatically as the streamer is raised so that the streamer can be wound freely onto a drum. The disclosed method purportedly allows the full control of the deformation, immersion depth and heading of the streamer.
In using such LFD control devices to maintain the streamer at a selected depth, sensors are required to be positioned along the streamer that generate a signal related to depth. Pressure sensors are typically used for depth measurement. Typical pressure sensors used in geophysical surveying can be calibrated to a precision of about 0.1 percent of the full scale range of the sensor. At a water depth of 1000 meters, a streamer using only pressure sensors for depth measurement can be navigated to be horizontal in the water to a precision of 1 meter. At correspondingly greater water depths, the possible error in navigation of the streamer in the vertical plane becomes proportionately larger.
What is needed is a system that can assist in navigation of a geophysical sensor streamer in the vertical plane at relatively great water depth.
A depth and level control system for a geophysical streamer according to one aspect of the invention includes a plurality of tilt sensors disposed at spaced apart locations along the streamer. The tilt sensors each have a first tilt sensing element arranged to measure tilt of the streamer along a longitudinal dimension thereof. A controller is in signal communication with each tilt sensor. A depth control device is in signal communication with each controller. The controller is programmed to operate the depth control device to cause the streamer to be level as measured by the tilt sensor.
A method for depth control of a marine geophysical streamer according to another aspect of the invention includes towing the streamer in a body of water. Tilt is measured at a plurality of spaced apart locations along the streamer in a direction along the longitudinal dimension of the streamer. The streamer is deflected in the vertical plane proximate each location of measuring tilt such that the streamer is substantially level at each location.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
An example marine geophysical survey system is shown schematically in
The system may include a survey and towing vessel 10 that moves along the surface of a body of water 11 such as a lake or the ocean. The vessel 10 includes thereon equipment, shown generally at 12 and for convenience referred to herein as a “recording system.” The recording system 12 may include devices (none shown separately) for determining geodetic position of the vessel (e.g., a global positioning system satellite signal receiver), for detecting and making a time indexed record of signals generated by each of a plurality of geophysical sensors 18 (explained further below), and for actuating an energy source 14 at selected times. The energy source 14 may be any selectively actuatable source used for subsurface geophysical surveying, including without limitation seismic air guns, water guns, vibrators or arrays of such devices, or one or more electromagnetic field transmitters.
In the present example, the geophysical sensors 18 may be disposed at spaced apart locations along a streamer cable 16. A non-limiting example of a structure for a geophysical sensor streamer cable is described in U.S. Pat. No. 7,298,672 issued to Tenghamn et al. and commonly owned with the present invention. The streamer cable 16, as explained above, includes a plurality of spaced apart geophysical sensors 18 along its length. The sensors may be, without limitation, seismic sensors such as geophones, hydrophones and accelerometers, electromagnetic field sensors such as electrodes magnetic field sensors or magnetometers. The sensors 18 may generate electrical and/or optical signals in response to detecting energy emitted from the source 14 after the energy has interacted with rock formations 13 below the water bottom 11A. The streamer cable 16 may be connected directly to the vessel 10 using a lead in line 16A to communicate power and signals between the recording unit 12 and the various electronic components in the streamer 16. The lead in line 16A may also transmit towing force from the vessel 10 to the streamer 16.
The streamer cable 16 is typically formed by connecting a plurality of streamer segments end to end as explained in U.S. Pat. No. 7,142,481 issued to Metzbower et al. and commonly owned with the present invention. The streamer segments are coupled by assembling corresponding termination plates (
The streamer 16 may also include a plurality of pressure sensors 21 disposed at spaced apart positions along the length of the streamer. The pressure sensors 21 are configured to measure pressure in the water 11, which will provide an approximate indication of the depth of the streamer 16 in the water at the position of each pressure sensor 21. In the example explained below with reference to
A tilt sensor 38 may be mounted in a gimbal bearing frame 40 to the interior of the housing 35. The tilt sensor 38 is mounted in the frame 40 so that it remains substantially vertically oriented notwithstanding twisting of the streamer 16 during operation. The tilt sensor 38 may measure tilt along only one direction, and in such examples, the direction is along the longitudinal dimension of the streamer. In other examples, the tilt sensor 38 may measure tilt along such dimension and in a direction orthogonal to the longitudinal dimension of the streamer. In one example, the tilt sensor 38 can be an electrolytic bubble level type such as one made by Spectron, Inc., Hauppage N.Y. sold under model designation SP500. The purpose for a two-axis tilt sensor will be explained below. Another example is a micro-electrical-mechanical system (MEMS) tilt sensor sold by RST Instruments, 200-2050 Hartley Avenue, Coquitlam, British Columbia, Canada. Electrical output of the tilt sensor 38 may be conducted to a first preamplifier 42, the output of which may be digitized in a first analog to digital converter (ADC) 46. Output of the first ADC 46 may be conducted to a microcontroller 50. A signal output of the microcontroller, shown as line 51, may be conducted to the LFD device 22 coupled adjacent to the tilt and depth sensor module 20. If a two-axis tilt sensor is used, output of the second signal channel of such sensor may be conducted to a second preamplifier 44, the output of which may be digitized in a second ADC 48. The output of the second ADC 48 may be conducted to the microcontroller. In the example shown in
The other longitudinal end of the housing 35 may be coupled to one end of the housing 22A of the LFD device 22 in a manner similar to the coupling of the streamer segment termination plate 30 to the opposite end of the housing 35. Such coupling may include electrical/optical connectors 31 substantially as explained above with reference to the connection between the streamer segment and the housing 35.
Functional parts of the LFD device are omitted from
As explained in the Background section herein, pressure sensors known in the art for streamer depth control have accuracy of about 0.1% of full scale. However, the resolution of such sensors can provide depth resolution on the order of 0.1 meter. In another aspect of the invention, pressure measurements made by the pressure sensor 21 in each module 20 can be communicated over a conductor in the wire harness 34 from the controller 50 to the controller in each of the other modules in the streamer. The controller 50 may include programming instructions to send a control signal to the associated LFD device to either raise or lower the streamer until the pressure measurements made by each pressure sensor are substantially equal, or differ from each other by at most a selected threshold. By matching depths, and levelling the measured tilt, the entire streamer may be maintained substantially in a straight, horizontal line.
In examples where a two-axis tilt sensor is used, the second axis signal may be used as a discriminator. If the tilt measured orthogonal to the length of the streamer is above a selected threshold, for example, the microcontroller 50 may be programmed not to generate a control signal to operate the LFD device 22, or may generate a signal to cause the LFD device 22 to generate no upward or downward force. In such cases, the tilt sensor 38 may not be oriented vertically, and measurements of tilt along the length of the streamer may be inaccurate. Thus, tilt measurements along the streamer length in such cases may be treated as the streamer being level.
A depth and level control system according to the invention may enable more precise control of depth along an entire streamer in water depths for which the accuracy of pressure measurements is insufficient.
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.
