Patent Application
20120250456
Scientists and engineers often employ geophysical surveys for exploration, archeological studies, and engineering projects. Geophysical surveys can provide information about underground structures, including formation boundaries, rock types, and the presence or absence of fluid reservoirs. Such information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbons and ores. Oil companies in particular often invest in extensive seismic and electromagnetic surveys to select sites for exploratory oil wells.
Seismic and electromagnetic surveys can be performed on land or in water. Marine surveys usually employ sensors below the water's surface, e.g., in the form of long cables or “streamers” towed behind a ship, or cables resting on the ocean floor. A typical streamer includes sensors positioned at spaced intervals along its length. Several streamers are often positioned in parallel over a survey region.
For seismic surveys, an underwater seismic wave source, such as an air gun, produces pressure waves that travel through the water and into the underlying earth. When such waves encounter changes in acoustic impedance (e.g., at boundaries between strata), some of the wave energy is reflected. The seismic sensors in the streamer(s) detect the seismic reflections and produce output signals. The sensor output signals are recorded, and later interpreted to infer structure of, fluid content of, and/or composition of rock formations in the earth's subsurface.
Similarly, for electromagnetic surveys, a underwater electrodes generate current flows in the water and the subsurface formations. Such current flows cause voltage drops to build and decay across subsurface formations and interfaces, thereby producing electric fields that can be sensed by antennas or electrodes in an underwater streamer. The sensor output signals are recorded, and later interpreted to infer structure of, fluid content of, and/or composition of rock formations in the earth's subsurface.
Conventional marine geophysical survey streamers may include hundreds, or even thousands, of sensors that are concurrently recording and communicating high resolution digital data to the ship and drawing power from the ship as they operate. The wiring that is typically employed to provide power and support communication may become a limiting factor as attempts are made to provide ever-longer streamers with improved performance. Though the use of more wiring can be offset by increasing the diameter of the streamer cable (so as to maintain a neutral buoyancy), the increased diameter tends to cause increased drag, to cause streamers to occupy substantially more room on the ship, and to make handling more difficult.
A better understanding of the various disclosed system and method embodiments can be obtained when the following detailed description is considered in conjunction with the drawings, in which:
While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description are not intended to limit the disclosure, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.
The issues identified in the background are at least in part addressed by the disclosed systems and methods for energy harvesting in a geophysical survey streamer. At least one embodiment of a geophysical survey system includes one or more streamers having sensors, and at least one energy harvesting device that converts vibratory motion of the streamers into electrical power. As the streamer is towed through a body of water, it can experience vibratory motion from a number of sources including, e.g., vortex shedding, drag fluctuation, breathing waves, and various flow noise sources including turbulent boundary layers. The energy harvesting device can take various forms including a mass-spring system and a piezoelectric transducer. To increase conversion efficiency, the device may be designed with an adjustable resonance frequency. The design of the streamer electronics may incorporate the energy harvesting power source in a variety of ways, so as to reduce the amount of wiring mass that would otherwise be required along the length of the streamer.
To assist the reader's understanding of the disclosed systems and methods, we first describe an environment for their use and operation. Accordingly,
The streamers 24A-24D are towed via a harness that produces a desired arrangement of the streamers 24A-24D. The harness includes multiple interconnected cables, and a pair of controllable paravanes 30A and 30B connected to opposite sides of the harness. As the ship 12 tows the harness through the water 14, the paravanes 30A and 30B pull the sides of the harness in opposite directions, transverse to a direction of travel of the ship 12. Depth-controllers may also be provided along the length of the streamer to keep the streamer array largely horizontal.
The seismic source 20 produces acoustic waves 32 under the control of the data recording and control system 18, e.g., at regular intervals or at selected locations. The seismic source 20 may be or include, for example, an air gun, a vibratory source, or another form of seismic energy generator. The acoustic waves 32 travel through the water 14 and into a subsurface 36 below a bottom surface 34. When the acoustic waves 32 encounter changes in acoustic impedance (e.g., at boundaries between strata), some of the wave energy is reflected. In
Sensor units of the sensor array 22, housed in the streamer sections 26 of the streamers 24A-24D, detect these seismic reflections and produce output signals. The output signals produced by the sensor units are recorded by the data recording and control system 18 aboard the ship 12. The recorded signals are later interpreted to infer structure of, fluid content of, and/or composition of rock formations in the subsurface 36.
There are often thousands of detectors in a given sensor array 22. A modular construction, e.g., with substantially identical and interchangeable sections 26, greatly simplifies handling, maintenance, and repair. If a problem develops with one of the streamer sections 26, the problematic streamer section 26 can be replaced by any other spare streamer section 26. The wiring that is typically employed to provide power and support communication may become a limiting factor as attempts are made to provide ever-longer streamers with improved performance. Accordingly, streamers 24 may be modified to employ energy harvesters so as to reduce wiring requirements.
Energy harvesting systems convert ambient energies such as vibration, temperature, light, etc. into usable electrical energy using energy conversion materials or structures to drive electronics, which often store the electrical energy in addition to performing other functions. See e.g., Chandrakasan, Amirtharajah, Goodman, Rabiner, “Trends in low power digital signal processing”, Proceedings of IEEE International Symposium on Circuits and Systems, 1998, 4:604-607. Three types of harvesting energy mechanisms are common: electromagnetic, electrostatic, and piezoelectric. Using the techniques taught in this disclosure, any of these three types can be employed to harvest energy from the vibrations of a towed seismic streamer.
However, resonance frequency coupling may not be suitable for all environments, particularly those having irregular vibration patterns and large displacements. Such vibration characteristics are not expected for towed seismic streamers, but should that turn out to be the case, there do exist energy harvesting device embodiments which are designed to operate in a non-resonance mode or with a high degree of vibration damping to provide a broadband response. See, e.g. Mitcheson, Miao, Stark, Yeatman, Holmes, and Green, “MEMS electrostatic micropower generator for low frequency operation”, Sensors Actuators A, 115:523-9, 2004. Such designs offer the further advantages that frequency tuning is largely unnecessary and that they enable simultaneous conversion of energy at multiple frequencies.
These status measurements are supplied to a power management circuit 514 in the sensor node which uses these measurements to determine the operating parameters of the sensor node electronics and thereby manage their power requirements. A power switching circuit 512 operates under control of the power management circuit 514 to deliver power to those portions of the sensor node electronics 511 that the power management circuit 514 selects based on the amount of stored energy and the rate at which additional energy is being harvested. With the built-in power management algorithm, the power management circuit 514 makes decision to either turn on or off the power switching 512 and control and optimize the functions of the regulator 508.
Alternative streamer embodiments, rather than having a single sensor node per energy harvesting module as shown in
Other contemplated harvesting device embodiments are MEMS (micro-electromechanical systems) devices having cantilever beams that oscillate in response to vibrations of the systems. The oscillations can be converted into electrical energy with piezoelectric materials, with electrostatic (i.e., capacitive) coupling, or with electromagnetic (i.e., inductive) coupling. Such devices can be obtained in the form of an integrated chip, enabling very compact implementations of energy harvesting modules. With such modules, it becomes possible to provide an energy harvesting device for each sensor, thereby enabling the creation of a self-contained sensor module. When the embodiment of
While specific system and method embodiments have been described above, it should be understood that they are illustrative and not intended to limit the disclosure or the claims to the specific embodiments described and illustrated. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the streamers may be electromagnetic survey streamers rather than seismic survey streamers. The streamers can receive power from the ship as well as from the energy harvesting modules, with the harvesters operating to reduce the required current draw from the ship. Some segments of a given streamer may employ harvesters (e.g., those segments farthest from the ship) while others do not. Other energy harvesting techniques (e.g., stretching electroactive polymers) can be employed besides those described herein. It is intended that the following claims be interpreted to embrace all such variations and modifications.
The present application relates to co-pending U.S. application Ser. No. ______ (Atty Dkt PGS-10-35), titled “Systems and Methods for Wireless Communication in a Geophysical Survey Streamer” and filed by inventors William T. Rickert, Jr. and S. Rune Tenghanm on the same day as the present application.
