Patent Application
20170168174
Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for extending a low-frequency content of seismic waves generated by a marine seismic source array.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of the earth's subsurface, information that is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, a seismic survey system 100, as illustrated in
Positioning devices (birds) 128 are attached along the streamer and controlled by a controller 126 for adjusting a position of the streamer according to a survey plan.
Source array 130 has plural source elements 136, which typically are air guns. The source elements are attached to a float 137 to travel at desired depths below the water surface 104. During operation, vessel 102 follows a predetermined path T while source elements 136 emit seismic waves 140. These waves bounce off the ocean bottom 142 and other layer interfaces below the ocean bottom 142 and propagate as reflected/refracted waves 144 that are recorded by sensors 122. The positions of both the source element 136 and recording sensor 122 are estimated based on GPS systems 124 and recorded together with the seismic data in a storage device 127 onboard the vessel. Controller 126 has access to the seismic data and may be used to achieve quality control or even full processing of this data. Controller 126 may be also connected to the vessel's navigation system and other elements of the seismic survey system, e.g., birds 128.
A source element may be vibratory or impulsive (e.g., an air gun). A vibratory source element is described in U.S. Pat. No. 8,830,794 (herein the 794 patent), and an impulsive source element is described in U.S. Pat. No. 4,472,794, the entire contents of which are incorporated herein by reference.
Presently, the air gun is the most commonly used source for marine seismic acquisition. Neither the air gun nor the existing vibratory source elements are effective in the low-frequency range of the spectrum, mainly from 1 to 10 Hz. However, there is increased interest in acquiring low-frequency seismic data in the marine environment that is useful for imaging prospective hydrocarbon reservoirs. For example, the low-frequency energy range is useful in seismic exploration because it provides better depth penetration of the seismic signal, which is extremely valuable for imaging in complex geological settings, such as sub-salt, basalt or even dense carbonate. The successes of advanced techniques, such as seismic inversion, require energy in the low-frequency range. Current air guns do not produce much useful acoustic energy below about 6 Hz.
Marine vibrators were expected to fix this air gun problem. Some marine vibrators recently introduced employ either flextensional shells or pistons that are in effect stroke-limited at low frequency. For an acoustic monopole (a model used to describe a seismic source element) in a liquid medium, the generated acoustic pressure is directly related to the volumetric acceleration of the liquid (considering that the acoustic source operates in a free-field). Thus, in a free-field, to maintain the same acoustic output at 2 Hz as at 4 Hz, the same shell or piston acceleration for both frequencies is required. This means that for a vibrating shell or piston to achieve the same acoustic output at 2 Hz as at 4 Hz, it requires four times the shell or piston displacement (assuming that for sinusoidal motion the displacement is directly proportional to the acceleration times a constant of proportionality that is the inverse of the frequency expressed in radians/s squared). This requirement is burdensome and makes it impractical for use in emitting adequate low frequency signals using existing vibratory sources.
In most cases, marine seismic source elements are towed at modest depths, so the generated acoustic waves are not the same as in a free-field due to the ghost reflection at the sea/air surface. The sea/air interface has an acoustic reflection coefficient very close to −1. It is well-known that this produces spectral notches and peaks in the down-going output source spectrum. This filtering effect of the surface ghost, in effect, is convolved with the marine source element output. For a monopole source element operating at depth z, where the speed of sound is c, there will be a first notch at zero Hz, a 6 dB spectral peak at c/(4z), a spectral notch at c/(2z) and so on.
Thus, there is a need for a source element that can generate the low-frequency range of seismic data.
According to one embodiment, there is a low-frequency source element for generating seismic waves in water. The low-frequency source element includes a hydraulic reservoir configured to hold a given fluid volume; a compliant chamber configured to hold a given gas volume and to accommodate volume changes of the given fluid volume; a rotational kinetic energy actuator configured to impart rotational kinetic energy to the fluid volume; and a flow modulator device configured to transform part of the rotational kinetic energy of the fluid volume into translational energy to generate an acoustic signal.
According to another embodiment, there is a method for generating low-frequency seismic signals. The method includes a step of deploying a source element in water; a step of towing the source element with a vessel; and a step of generating low-frequency seismic signals by transforming, with a flow modulator device, part of a rotational kinetic energy of a fluid volume hold by a hydraulic reservoir into translational energy.
According to still another embodiment, there is a low-frequency source element for generating seismic waves in water. The low-frequency source element includes a working chamber; a compliant chamber in a same housing as the working chamber; a rotational kinetic energy actuator configured to impart rotational kinetic energy to a fluid volume inside the working chamber; and a flow modulator device configured to transform part of the rotational kinetic energy of the fluid volume into translational energy to generate an acoustic signal. The housing houses the working chamber, the compliant chamber, the rotational kinetic energy actuator and the flow modulator.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:
The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a source element configured to generate low-frequency acoustic energy in a marine environment. However, the embodiments to be discussed next are not limited to a marine source element; they may be applied to source arrays (i.e., to a collection of source elements) or even to land and transition zone sources.
Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.
According to an embodiment, a source element that generates low frequencies is not constrained by a stroke limitation and can be towed behind a seismic vessel and operated at a given depth. Such a source element includes a hydraulic reservoir configured to receive a given fluid volume, a compliant chamber configured to hold a given gas volume, an actuator configured to impart kinetic energy to the fluid volume, and a flow modulator device configured to modulate the fluid volume's flow to generate an acoustic signal.
According to another embodiment, a source element includes a hydraulic reservoir configured to hold a given fluid volume, a compliant chamber configured to hold a given gas volume, a rotational kinetic energy actuator configured to impart rotational kinetic energy to the fluid volume, and a flow modulator device configured to transform part of the rotational kinetic energy of the fluid volume into translational energy to generate an acoustic signal.
The low range of frequencies is mainly considered to extend from 1 Hz up to about 10 Hz. Above 10 Hz, existing air gun sources provide adequate source strength. Thus, the present embodiments introduce source elements that focus on the low-frequency range. However, these source elements are also capable of generating middle- and high-frequency ranges.
Prior to discussing various embodiments of the novel source elements that generate acoustic energy in the low-frequency range, some nomenclature is introduced for classification purposes. It is known that existing vibratory source elements have a piston or flextensional shell that is physically displaced over a length L for generating a frequency f. All these vibratory source elements share the fact that the displacement length L imposes a low frequency constraint on how much sound pressure can be generated. Because sound pressure is directly proportional to volumetric acceleration for a monopole, then a piston vibratory source will have a maximum achievable sound pressure output level that is inverse proportional with the square of the generated frequency f if the piston is driven to its limits at low frequency. In other words, the maximum achievable sound pressure falls rapidly as the frequency decreases. Most of the embodiments to be discussed next will describe a source element that modulates a fluid flow. These source elements are called herein fluid flow-modulated source elements, to distinguish them from traditional vibratory and impulsive source elements. Modulating a fluid flow is akin to changing the fluid volumetric velocity of a monopole. Fluid flow-modulated devices tend to be subject to flow rate constraints as opposed to displacement constraints, so that the maximum achievable sound pressure output level is inverse proportional with the frequency when operated under a flow rate constraint. In summary, for operation at low frequency, conventional sources have outputs that fall as the inverse square of frequency while fluid flow-modulated source elements have outputs that fall only as the inverse of frequency. When there is no reason for confusion, these novel fluid flow-modulated source elements are simply called source elements.
The fluid flow-modulated source elements can be classified as active and passive source elements. For example, a fluid flow-modulated source element that achieves a net (i.e., non-zero) fluid flow is considered to be an active source element, while a source element that achieves a zero average fluid flow is considered to be passive source element. In one embodiment, the net fluid flow is constant. For the net fluid flow source elements, a net amount of water or other medium passes through a body of the source element, while for a zero average fluid flow source element, no net amount of water or other medium passes through the body of the source element. In some embodiments, the zero average fluid flow source elements fully contain the water or other medium inside their bodies and do not fluidly communicate with the environment. Another distinguishing characteristic is that passive fluid flow-modulated source elements can provide sound pressure output while in a stationary position, while active fluid flow-modulated source elements have to be towed through the water to provide sound pressure and the towing velocity can affect their output.
In one embodiment, the fluid flow-modulated source elements are towed devices that operate at moderate depth (e.g., 10 to 100 m) and act as an acoustic monopole suitable for use over the frequency range of 1-5 Hz. Other frequencies are possible, and the descriptions/concepts presented herein can be applied to adapt the fluid flow-modulated source elements to operate over other frequency ranges. The low-frequency acoustic monopole source can be realized in different ways. Active source elements achieve fluid flow by being towed through water by a vessel, and the input fluid flow is later modulated by a flow-modulator mechanism to create a dynamic pressure fluctuation at its outlet. The passive source elements may use an oscillatory pumping action created by, for example, a propeller, impeller, pump or other means to effectively dynamically inject and/or extract a volume of water.
Both passive and active source elements 300, as illustrated in
Source element 300 also includes a flow modulator 330 that can be a propeller, flow diverter, flow restrictor, orifice or other means that allow the rate of fluid flow passing the flow modulator to be varied. In one application, the flow modulator fluidly communicates with environment 308. Trapped gas 322 acts as a compliant volume, increasing (decreasing) in size as liquid 306 is removed from (injected into) the hydraulic reservoir compartment. The effect of the trapped gas 322 is to increase the effective compliance of the liquid supply volume contained in reservoir 304. Source element 300 is configured to be towed under the water surface 340, at a given depth H, by vessel 342. In one application, source element 300 may be attached to a float 344.
The various net fluid flow-modulated source elements are now discussed with reference to
As source element 400 is towed along direction 429, ambient fluid 407 enters throat or inlet 403, which may or may not have a funnel shape to act as a fluid ram to increase the inlet pressure. The resulting fluid flow 401 enters throat 403 and moves down along a passage 404 inside source element 400. At the exit point 406 of passage 404, which may be a hollow cylinder, a flow restrictor 408 is located. In one embodiment, flow restrictor 408 includes two slotted plates 415 and 417. Plate 415 is rigidly attached to housing 402, while plate 417 rides inside a track so that it can slide vertically or horizontally with respect to plate 415. Plate 417 is also attached to an actuator 419 through linkage mechanism 427. A controller 431, located on the vessel or on the source element or distributed both on the vessel and the source element, is in communication with actuator 419 through communication link 433, which can be an electrical connection, so that actuator 419 causes plate 417 to slide relative to plate 415. For example, controller 431 may cause plate 417 to move vertically up and down to follow, for example, a sinusoidal motion 421. Motion pattern 421 may be loaded by the source operator into controller 431. Other motion patterns may be used.
As plate 417 slides relative to plate 415, fluid flow 401 experiences a variable restriction so that, for example, when the slots of the two plates are misaligned, i.e., the flow is restricted, a back pressure will be created due to the momentum of the flow traveling axially. When the plate's slots are aligned, the restriction is removed and there will be a pressure drop inside passage 404. In order to absorb this back pressure or water hammer effect, a compliant compartment 413 is created. Compliant compartment 413 in this embodiment is a circumferential rigid-walled compartment that houses one or more gas-filled bladders (for example an inner tube) 425. Bladder 425 may directly interact with the fluid 409 inside passage 404 through circumferential screen 423. Circumferential screen 423 may be attached to walls 411 that define passage 404. Circumferential screen 423 may also be used to maintain gas-filled bladder 425 inside compartment 413. When properly inflated for the operating depth H, gas-filled bladder 425 only partially fills compartment 413. In one application, the gas volume is about 40-60% of the total compliant compartment volume. The volume of fluid 410 flowing into and out of compartment 413 is the supply and or replacement volume needed to make up for the deviation in exit flow rate created by the variable restriction caused by plates 415 and 417.
The fluid volume 409 in passage 404 will tend to act like a fluid mass. Because the gas-filled bladder 425 is more compliant to the pressure perturbations in passage 404 than the inertia presented by the slug of fluid 409, compartment 413 effectively acts like a hydraulic accumulator that tends to reduce pressure disturbances within passage 404. The net result is that at outlet 406 and to the right of screen 417 in
For bladders 425 to function properly over the desired frequency range of 1-5 Hz, their volume as well as the volume of compartment 413 needs to be sized appropriately and properly inflated. Also, it is necessary that the axial length of passage 404 from throat 403 to flow restrictor 408 be sufficient to contain the slug of water 409 whose mass in combination with the spring rate of gas-filled bladders 425 has a resonance that is at or below the operating frequency.
If passage 404's length is inadequate, then the pressure perturbations at the throat due to the change in exit restriction will tend to make the device behave more like a dipole than a monopole acoustic source, which is undesirable for this application.
In one application, transducers 420, such as flow measuring devices, pressure transducers or other sensors, could be used to provide feedback or monitoring signals to controller 431 or send information via a telemetry system (not shown) to the recording system located on the seismic vessel for later use in seismic data processing.
Instead of sliding slotted plates as illustrated in
Further, other mechanisms may be used instead of circumferential air bladder 425 to create an effective hydraulic accumulator as shown in
A passive source element 700 (i.e., a source element that achieves zero net fluid flow) is now discussed with regard to
An interior chamber 704 of housing 702 is flooded with a fluid 711 that is in communication with the ambient 710 (e.g., seawater). Propeller 719, impeller, or another device is driven by a rotary actuator (e.g., motor) 715 for generating a fluid flow 709. Rotary actuator 715 may be attached to shell 702 via some ribs 721 or other means that support actuator 715, but do not unduly restrict the exchange of fluid between interior chamber 704 and the ambient. Propeller 719 is connected to hub 723, which may contain devices to vary the pitch of the propeller vanes. Actuator 715 connects to hub 723 via a rotating shaft 717. Actuator 715 can be a torque motor operating over a fixed angular range of travel; it can be a brushless servomotor with a permanent magnet rotor (for example, a neodymium iron boron magnet) and a field winding that is in communication through link 733 with controller 731, which may contain a power amplifier. Link 733 can be a two-way link; for example, actuator 715 may be equipped with feedback devices like a tachometer useful for measuring shaft velocity or a shaft encoder to measure shaft position and whose signals could be received by controller 731 to aid with control. Additional transducers 741, for example, flow sensors or pressure sensors, could be located in outlet region 739 to monitor output signals that could also be used as feedback signals for controller 731. In one application, actuator 715 may be a pneumatic motor or a hydraulic motor, in which case controller 731 might contain servovalves, and link 733 may contain hydraulic or pneumatic lines as well as electrical cables.
Shell 702 may contain accumulator compartments 713 that are circumferential or other compartments 735 that house one or more gas-filled bladders 725. These compartments function as hydraulic accumulators, and the gas-filled bladders expand or contract in volume as water is pumped in and out of interior chamber 704 due to the action of propeller 719. At a given depth, when the propeller is stationary, the accumulator compartment volume is about half water and half gas. The liquid in compartments 713 and 735 is in fluid communication through a screen 737 with liquid 711 that is in the interior chamber 704 of shell 702. The hydraulic accumulators act as a soft spring so that the liquid can more easily be pumped in and out of the housing's interior chamber 704 as propeller velocity changes.
If actuator 715 is a servomotor, it can be variable-speed and reversible. The volume of liquid that propeller 719 injects into or extracts from the surrounding ambient 710 over an interval of time is a function of the propeller's pitch, shaft velocity and the pressure gradient between interior chamber 704 and outlet region 739.
Propeller 719 can have a fixed pitch, variable pitch and/or reversible pitch. Several operational options are possible for such a system:
A screen or guard 742 may be located at outlet region 739 to allow for the free exchange of fluid between acoustic outlet region 739 and the propagating medium 710 to help filter out any debris or to prevent marine life from entering the source element. Such a guard could also be included in the sources discussed in the previous embodiments.
For the case where the source element is configured so that the same amount of fluid just flows back and forth (i.e., zero net fluid flow source element), it may be possible to cover outlet region 739 with a membrane or rubber boot as now discussed with regard to
Other passive source elements are possible that allow for partitioning of pitch control and propeller rotation mechanisms, as are now discussed with regard to
A sealed actuator 918 (e.g., a motor) suitable for underwater operation is powered and controlled by controller 919. Controller 919 may be located on housing 901 (inside or outside it), or on the vessel towing the source, or it may be distributed on the vessel and source element 900. Actuator 918 may have a shaft 922 that drives a paddle wheel 920, for example, at constant rotations per minute (rpm). Paddle wheel 920 imparts rotational kinetic energy to internal fluid 925. In one application, a variable speed motor may be used. Actuator 918 is attached to enclosure 901 through a structural support device 916 that may be, for example, a system of radial ribs covered by a screen or grate that blocks large debris but allows for the exchange of fluid between the ambient liquid media 924 and the internal fluid 925 of working chamber 911. Note that
With this mechanism in place, the source element's operation is now described. Paddle wheel 920 spins the fluid near it, imparting rotational kinetic energy to internal fluid volume 925. Controller 919 controls actuator 918 to generate the necessary spinning. Vanes 910 can be rotated by corresponding torque-producing devices 912 to transform part of the rotational kinetic energy into translational energy so that fluid flow 931 moves (translates) either to the left as shown in
Similar to the other active flow-modulated source elements discussed above, as the translational fluid flow is directed to the left as shown in
Variations of source element 900 are now discussed with regard to
Each vane shaft 1023 is connected to a central hub 1019, which can contain vane shaft bearings to allow for the free rotation of vanes 1021 by vane drivers 1027. Additional spoke elements 1017 are used to transmit the torque generated by motor 1011 to assembly 1010 (the spokes are not shown in the end view of 1010). Assembly 1010 acts as a variable-pitch propeller. Electrical connections to vane drivers 1027 can be made, for example, by running wires down the spoke elements 1017, to hub 1019, which in an embodiment might be equipped with slip rings to make an electrical connection to the vane/pitch controller. Motor 1011 could be operated at a constant rpm, for example, 300 rpm, with vane drivers being 1.5 kW servomotors or gearmotors. By changing the pitch of vanes 1021, water can be directed into or ejected from enclosure 1001 as in other embodiments.
In another embodiment, motor 1011 could be offset from the center of the housing, instead of being located at the center of the fluid outlet. For example, motor 1011 may be located on the exterior of enclosure 1001, and a suitable drive motor coupling system, for example, chain drive, pulley system, gears, flexible shaft or other means, could be used to connect drive motor 1011, in which case, assembly 1010 might be equipped with a sprocket, gear, pulley or other compatible device.
An alternative source element 1100 is now discussed with regard to
Another source element 1150 is illustrated in the embodiment depicted in
Those skilled in the art would understand that other configurations of the source elements discussed above are within the scope of the invention as long as low frequencies are generated by modulating a fluid flow to produce a fluid volumetric velocity with a resultant sound pressure signal emission. As also discussed in the previous embodiments, there is no directly driven piston or shell that moves the fluid flow, unlike traditional source elements. Fluid flow may be achieved by towing the source element with opposite ends open to the ambient so that the fluid flow passes through the source element or, by moving in an oscillatory manner, a volume of water is housed by the source element so that the moving volume of water impinges on the flexible end or directly on the ambient to generate acoustic waves. For the latter case, rotational kinetic energy may be imparted to the volume of water by the action of paddles or other devices. Thus, for this case, after rotational kinetic energy is imparted to the water volume, the water volume is made to move along a longitudinal axis by transforming part of the rotational kinetic energy into translational energy. This translational energy is impinging on a flexible cover or directly on the ambient medium to generate the acoustic waves. The rotational kinetic energy is “stored” in the water volume and then “discharged” into the ambient at desired instants to generate the desired low frequency. Those skilled in the art would appreciate that a volume of about a couple of cubic meters of water may be used inside the source element for achieving the low frequencies.
Passive and active source elements as discussed above illustrate some of the possible embodiments. Many other embodiments may be imagined that will not depart from the principles of the passive and/or active source elements discussed above. These principles of passive and active source elements may be illustrated in block diagram style as now discussed with regard to
With regard to
The block diagram for a passive source element is now discussed with regard to
The desired low frequency is achieved by using an actuator 1312 that transforms the rotational kinetic energy into translational energy. Controller 1314 may control actuator 1312 and rotational kinetic energy actuator 1306. This controller may also receive information from one or more sensors distributed throughout the source element. Similar to source element 1200, source element 1300 may use a GPS system 1316 for receiving location information and correlating the instant when to shoot the source element with the seismic survey pre-plot.
If the flow modulator 1206 or 1308 is a variable pitch propeller,
Comparator 1403 compares feedback signal 1417 to the desired signal 1423 to form an error signal 1425 that is an input to a motor's controller 1405. In one application, feedback signal 1417 is an estimate of the far-field signal formed by combining measured signals 1411, 1421 and 1423 using feedback processor 1409. In one embodiment, feedback signal 1417 is an estimate of the total volumetric fluid acceleration in region 739 in
Motor controller 1405, in addition to signal 1425, may receive feedback signals 1433c from sensors 1419 located in proximity to motor 715, shaft 717 and/or devices to control pitch located within hub 723. Motor controller 1405 combines input 1425 with feedback sensor signals 1433c to produce amplifier drive signal(s) 1427. Drive signals 1427 may in fact be two signals, one to adjust propeller speed and the other for changing the pitch setting. Power amplifier 1407 could be a one or more electrical power amplifiers, since motor 715 could be an electrical motor, but may instead contain servovalves. For example, if motor 715 and pitch control device 723 were hydraulically or pneumatically driven, then power amplifier 1407 might contain both electronic devices for generating electrical signals 1433a for the electronic parts and suitable signals 1433b for hydraulic or pneumatic valves.
From an operational point of view of a low-frequency source element, the arrangement of the towed source element to form a source array is of little consequence so long as its overall dimension is small compared to the wavelength of the sound being radiated. For frequencies in the range of 1 to 5 Hz, corresponding wavelengths are in the range of about 1,500 m down to 300 m. Thus, a source element and/or source array that has an effective acoustic diameter less than 100 m in size is omni-directional, having no directivity when operated in a free-field. The only directivity will be due to the surface reflection. Note that a source element is a single element as discussed above with the embodiments. A source array is understood to include at least one sub-array. One sub-array is understood to include plural source elements.
For the passive monopole (i.e., source element shown in
One advantage of the embodiments discussed herein is that there is no stroke limitation on source elements' output, as is the case for a piston actuator source element, if the compliant component and interior fluid volume are sufficient. For low-frequency operation, stroke limits are typically what constrain the output of the traditional source element. The propeller-driven source element may be limited at higher frequencies by the rotational inertia of the propeller assembly, power/torque requirements and the acoustic radiation mass that will tend to create an increasing inertial load on the propeller as frequency increases. Ultimately, the source element may be limited by the motor current and/or voltage ratings. Also, as propeller velocity is increased, the risk of fluid cavitation increases, and this can lead to erosion of material in the propeller mechanism.
To better understand the peak fluid volume Vpk that is cycled to achieve a certain peak acoustic pressure, refer to equation (1). In this equation, P(r,ω) is the peak pressure in Pascals at a point r, for a monopole source oscillating at frequency ω (radians/s) operating in a free-field with a sinusoidal peak volumetric displacement of V(m3). If the density of the fluid medium is D, typically about 1,020 kg/m3 for seawater, equation (1) is given by:
To achieve a free-field peak sound pressure level of 4,000 Pa referred to 1 m, in seawater, it requires a peak volume V as shown in equation (2):
To achieve this sound pressure in the free-field for a frequency of 1 Hz, or about 6.28 radians/s, it requires a peak volumetric displacement of 1.25 m3, while to achieve the same sound pressure at 4 Hz, it would only require a peak volumetric displacement of about 0.078 m3. The corresponding peak fluid flow rates would be about 8 m3/s for 1 Hz and about 2 m3/s for 4 Hz. Thus, for the example of an active monopole source, it would need to have an internal liquid volume in excess of these liquid ejection volumes. Furthermore, the gas bladder volumes would ideally need to be greater, and preferably at least twice as big as the peak volumes to be ejected to keep the pressure fluctuations low within the closed shell. For logistical and practical reasons, it is anticipated that a low-frequency source array might be towed behind the seismic vessel, so the total volumetric displacement required to realize a desired sound pressure level would be shared among a plurality of source elements.
For example, if the required total peak volumetric displacement is 1.25 m3 at 1-Hz and a source array including, for example, five source elements is used, each source element would require a peak volumetric displacement of 0.25 m3. The fluid volumetric flow that can be created by a propeller is related to the effective propeller diameter, propeller pitch, motor shaft speed and pressure gradient. Under some simplifying assumptions, for example, comparing 1 Hz and 4 Hz operation of a source element with a fixed-pitch propeller trying to achieve a sound pressure of 4,000 Pa-m at 1 Hz, a peak rotational velocity for 1 Hz would be four times as great as would be required for the same propeller to achieve the same peak sound pressure of 4,000 Pa-m at 4 Hz.
The generated seismic pressure signals are not limited to sinusoidal waveforms. The generated sound pressure waveforms can be a chirp signal with linear or nonlinear frequency versus time profiles, pseudorandom signals or other types. For a monopole source (like the passive or active source elements discussed above), no matter what the low-frequency signal type, the far-field acoustic signal will be proportional to the time derivative of the volumetric fluid flow; that is, the fluid volumetric acceleration. Thus, a measurement of the fluid flow (displacement, velocity or acceleration) should provide a useful signal for use as either a feedback control signal or source signature signal for later processing steps.
A device equipped with a large revolving propeller would tend to act like a gyroscope. To mitigate these effects, in one embodiment it is possible to employ a plurality of source elements and connect them to a common frame. The propellers could be oriented so that their precession forces tend to cancel one another during towing, if a travel direction change is made. In another application, the frame could carry devices that have propellers whose pitch is reversed so that some propellers are designed to increase flow while being turned faster in a clockwise direction, while others have propellers designed to increase flow while being turned faster in a counter-clockwise direction. For this case, the source elements might be paired on a common frame to cancel the gyroscope effect. Other reasons for pairing units would be to help counteract reactive force created by outlet flow.
Another way to balance reactive forces created by a single monopole source element, like the one illustrated in
Other means may also be used to modulate fluid flow, for example, based on fluidics. In a fluidic amplifier, a small flow is used to control a larger flow, as disclosed, for example, in U.S. Pat. No. 4,000,757 (the entire content of which is included herein by reference), which illustrates a bistable fluidic amplifier. For one of the embodiments discussed above, it is possible to direct, as illustrated in
The second outlet port 1540B as well as the leakage 1542 are connected to chamber 1522, which contains the compliant chamber 1524. Compliant chamber 1524 is configured to damp any fluid pulsations so that the water released is basically a steady flow of water. Outlet ports modulated flows 1512 and 1514 may be about 180 degrees out of phase if intended to generate a sinusoidal output. Thus, there is a need to have chamber 1522, which acts as a muffle, to achieve a net acoustic output.
In one embodiment, it is possible to use both outlet flows 1512 and 1514 instead of just one for acoustic output. For this embodiment, if a resonant chamber is added to phase shift one of the outlet flows so that at least, over a narrow band of frequencies, the resultant two modulated flows are additive (as long as the relative phase of the two flows are less than 90 degrees apart when combined) it should be possible to obtain a larger resultant amplitude.
Returning to the active and passive sources discussed above, a few observations about the compliant chamber are now discussed. The compliant chamber serves several useful purposes. First, because it is elastic, it allows the reservoir of liquid between the flow modulation device (propeller, sliding gate, variable orifice, etc.) and the elastic trapped gas volume to source or sink fluid flow transients created by the fluid flow modulator. Second, it can be used to create a resonance to improve the source element's overall efficiency. Third, for the active case, the compliant chamber is housed within a rigid outer shell, so the pressure fluctuations internal to the source element are acoustically isolated from the surrounding medium, thereby enabling the source element to act like an acoustic monopole. For the active case, the compliant chamber in combination with the internal fluid mass tend to stabilize, dampen and/or mitigate pressure fluctuations near the inlet due to pressure perturbations created by the action of the fluid modulation device, so that the acoustic energy radiated at the inlet is much less than the acoustic energy radiated near the outlet.
In the various embodiments previously described, a moveable barrier is used to prevent mixing between the fluid and gas volumes. A moveable barrier is needed particularly if the orientation of the source gas volume with respect to the gravity vector is unfavorable, variable, or if the source element undergoes vibrations which will tend to promote mixing of gas and fluid. The moveable barrier in the various embodiments described above can be a sliding piston, membrane, bladder, corrugated membrane, diaphragm, bellows or other suitable means. In practice, this moveable barrier will have some mass, may introduce some friction effect, or introduce some added stiffness effect. However, for the purposes of discussion, these effects will be assumed to be negligible.
The effective gas spring of the compliant chamber in combination with the total mass load create a second order resonant system. For example, using the embodiment shown in
Assuming that the water is not compressible, that the compliant chamber gas behaves like an ideal gas, and that the gas chamber volume is large compared to the peak flow volume, the effective spring rate Ke in Newtons/meter of the source is given by:
where A0 is the outlet area; Pg is the gas absolute pressure (measured in Pa) at the operating depth; and Vg is the total gas volume at operating depth (measured in m3). The term is the polytropic index. If the expansion and contraction of the gas is an isothermal process then, γ=1. If the process is adiabatic (i.e., no energy loss) and the gas is diatomic, then γ=1.4. In practice, the process will neither be isothermal nor adiabatic, but somewhere in between. For this example, it can be assumed that γ=1.2.
In a typical low-frequency source design, it is desirable for the source element to have a resonance that falls within its intended operational frequency range. Thus, for a low-frequency source element designed to cover the range of low frequencies of 1 to 3 Hz, operating at a depth of 50 m, and having an outlet diameter of about 2 m (A0=3.14 m2), it can be assumed the resonant frequency Fr to be about 1.4 Hz. Assuming that peak fluid displacement will be about 0.5 m3, the radiation mass corresponding to an acoustic piston (the effective trapped fluid mass being accelerated back and forth near the outlet will act like a piston of the same area) will be about 2,300 kg, and if the volume of the liquid inside the enclosure is assumed to be about 6.9 m3, there will be another 6,900 kg of mass. When this mass is combined with the radiation mass, it would result in Me=9,200 kg. Substituting this value in equation (3) results in a spring rate of about Ke=7.1×105 N/m. At a depth of 50 m, Pg is about 9×105 Pa. Equation (4) then requires that the gas volume needs to be about Vg=10 m3. If the peak flow volume is about 0.5 m3, the gas volume and the liquid volume are sufficient.
A ring-shaped bladder with a rectangular shaped cross-section having the following dimensions will be suitable: outer diameter of 2.6 m, inner diameter of 1.6 m and axial length of 3 m. These calculations have been presented for purposes of exemplifying one possible implementation of one of the embodiments and for giving the reader a sense of the dimensions, pressures and forces involved with such a low-frequency source element. The assumption of an ideal diatomic gas was used to simplify the calculations. If a monatomic gas were used, such as argon, then the polytropic index should be adjusted. The calculations assume a gas compression and relaxation process that lie somewhere between isothermal and adiabatic. Thermal analysis/simulations could be performed to better estimate the behavior of the gas under compression cycling. In addition, total liquid mass inside the enclosure will generally be larger than the effective mass. For example, not all of the liquid will be accelerated by the action of the fluid modulator due to stagnant areas within the enclosure volume, such as near corners or other flow restrictions. Fluid flow analysis/simulations could be used to estimate more accurately the effective fluid mass for a candidate source element design. The viscous damping effect of the liquid as it flows over surfaces around corners in the interior of the enclosure will also tend to dampen the resonance peak near Fr.
As can be observed from the above discussion, equations (3) and (4) are coupled, which is to say that for a selected resonant frequency, the selection of an effective mass impacts the required gas volume and vice versa. As a practical matter, if the coupled mass is reduced, the gas volumes become even larger. Thus, it is possible to end up with an even larger source element to deploy and tow. In addition, various limits may be imposed by the flow modulator driver or other devices that may constrain performance over some frequency interval within the desired operating frequency range. For example, the amount of torque and/or force that can be delivered by the flow modulator driver mechanism is finite. As the frequency increases, the amount of current and/or voltage required to operate the flow modulator, in general, increases above the resonant frequency. Also, as the amount of effective liquid mass that has to be accelerated back and forth increases, this will compound the problem and tend to load down further the flow modulator.
Therefore, in an embodiment, various calculations to optimize overall performance subject to certain factors are made. For example, these factors may include operating frequency range, resonant frequency, logistical concerns, deployment concerns, towing concerns, equipment constraints or other constraints. More specifically, in one embodiment it is anticipated that a calculation can be made to select/balance the sizes of the liquid coupled mass and/or gas volume so that overall performance (signal output, efficiency, overall size, tow force, etc.) is optimized subject to these constraints. Furthermore, it is recognized that the dimensions of the source, for example, acoustic opening size, length of the device, weight of the device (in and/or out of the water), can all be used as key variables in this selection process.
According to an embodiment, a method for generating low-frequency seismic signals is now discussed with regard to
The above embodiments were discussed without specifying the type of seismic receivers used to record seismic data. In this sense, it is known in the art to use, for a marine seismic survey, streamers towed by one or more vessels, and the streamers include seismic receivers. The streamers may be horizontal, slanted or have a curved profile as disclosed, for example, in U.S. Pat. Nos. 8,456,951 and 8,451,682, the entire contents of which are incorporated herein by reference.
Also, the above embodiments were discussed in a marine environment in which a depth of the ocean bottom is at least 100 m relative to the water surface. For depths less than 100 m, which is called a transition zone area, the above sources may be modified as now discussed to address the shallow water conditions. Note that transition zone areas may include estuaries, lagoons, shoreline, tidal zone, marsh or wetlands, etc. Some of the transition zone sources to be discussed next, are more appropriate for even smaller depths, e.g., less than 3 m because they have small sizes, around 0.5 to 3 m.
One or more transition zone sources 1720 may be attached to the ocean floor 1710, as illustrated in
An actuation device 1820 is located inside passage 1816 and configured to translate a fluid mass 1822 up and down along a longitudinal axis X of the housing. Actuation device 1820 may be a motor having one or more blades 1824 attached to a hub 1826. Hub 1826 may be configured to change an orientation (pitch) of the propeller's blades 1824 to control the fluid flow. Actuation device may be attached with a bracket 1828 to an interior of housing 1802. By changing the orientation of the blades or the rotation direction of the motor, the fluid volume 1822 is moved up and down along axis X. Thus, the fluid acts as a reaction mass that imparts a force to the housing. Because the housing is coupled to the ocean bottom via the auger base 1812, sound waves 1830 may be generated into the water and/or directly into the ocean bottom. Note that in this case, the fluid volume 1822 inside passage 1816 does not communicate with the ambient of the source.
According to a variation of this embodiment,
According to another embodiment, a flow modulated seismic source as discussed above may be adapted to be used in a borehole. Borehole sources are commonly used in conducting reverse vertical seismic profiles. These surveys are useful in determining the acoustic properties of the geological formation surrounding a borehole. Typically, a small source that is attached to a wireline cable is lowered into a borehole that is often fluid filled. The source is usually raised and/or lowered to a number of different depths and activated with a shot record collected at each level. A receiver spread that typically uses geophones is located at the surface. When the source is activated, the seismic signal propagates from the source through the borehole walls and into the surrounding geologic formation and a portion of the radiated energy propagates through the strata to the surface where the seismic signal is measured.
Because each stratum typically has different acoustic properties, the seismic signal ray path is altered (reflected or refracted) as it propagates through the rock layers and impinges on interfaces. This changes the arrival time of the signals. When the source is moved, its radiated energy travels a different path to the receivers to provide additional information. In addition to imaging information, the acquired data set can provide information that is useful for tying conventional seismic surveys to well logs that can be used in reservoir modeling. Borehole sources can be impulsive, for example airguns, or vibratory, for example piezoelectric bender types or even small hydraulic vibrators that couple directly to the casing. There is always a concern about the use of impulsive sources in boreholes, because they release all of their output at once. This creates large peak stresses in the borehole that can lead to casing cement failures.
Vibratory sources do not produce much low-frequency energy (energy below 10 Hz) due to stroke limitations—this is especially true if the energy is coupled via the borehole fluid. Hence there is a need for a fluid-coupled borehole vibratory source capable of producing low to moderate seismic frequency signals.
According to an embodiment illustrated in
Borehole 1901 is assumed to be fluid filled at least for depths that lie below the water table. Source 1907 is a flow modulated source to be described later, that is connected to a wireline cable 1904 that is suspended from a boom 1903 and connected to a wench mechanism 1905 located on or near service truck 1902. Service truck 1902 may contain equipment to both control/operate source 1907 and to control its operating depth. In addition, geophones or other receivers 1910 are arranged at the earth's surface, near borehole 1901. Source 1907 typically is lowered to a starting depth and emits a seismic signal over a pre-determined frequency band. Some of the radiated seismic signals travel as a P-wave and follows ray path 1916 when traveling from source 1907 to receiver 1910 where they are recorded. The seismic signal travels like a wave through the rock layers with part of its energy refracted at a layer interface where the two layers may have different acoustic impedance, for example, interface 1914, and passing on through the adjacent layer and being refracted and transmitted until a portion of the signal is recorded by receiver 1910. Also, at each interface, it is possible that some energy may be reflected or undergoes mode conversion, for example going from P-wave to S-wave. The travel time for the seismic signal to traverse ray path 1916 is dependent upon the velocity in the various rock layers and the distance in each rock layer. Snell's law determines the seismic wave front refraction angle for each interface. Other effects can occur at interfaces, as mode conversion from pressure to shear waves, and reflected signals are possible.
The process is repeated at various source depths, usually at regular intervals between endpoint levels 1908-1909. Signals to operate and monitor source 1907 are carried in wireline cable 1904 where they connect to a source control and acquisition management system located at or near vehicle 1902. Remote operation is possible via telemetry links, for example, via a radio link to a remote site. The received signals are also recorded using a data acquisition system that is also interfaced to the acquisition management system located at or near service truck 1902 or it could be linked to some remote site using wireless transmission means.
The acquired data set can be processed to either image the rock formations near the well or to determine the velocity of sound through the various rock formations at seismic frequencies, which typically are much lower in frequency than are utilized in a sonic log that utilizes ultrasonic signals. Velocity measurements of the rock layers made using a reverse vertical seismic profile are useful in forming well ties to reconcile conventional seismic data with well logs.
Simultaneously or at a different time, a cross-hole tomography survey could be conducted with source 1907 operated as stated above at various levels. At the same time, a portion of the seismic energy produced by source 1907 propagates horizontally, through the rock formation, to borehole 1912, which is equipped with receivers (geophones, accelerometers or hydrophones) at different levels. For example, some energy follows ray path 1915. The received signals travel through wireline cable and are recorded using equipment located in service truck 1913, which is also equipped with a boom and wench useful for raising and lowering the receiver string. Data acquired through cross-hole tomography can be used to measure the transit time and/or amplitude of P- and/or S-waves from a source in one borehole to geophones in another borehole, which is useful for determining the velocity of seismic signals in the various rock layers. The amplitude information can be used to measure acoustic absorption in the various rock layers.
A flow-modulated source 1907 for use in the borehole can be of two types. The first type is called active and the second type passive and they can be used to perform either reverse vertical seismic surveys or cross-hole tomography surveys using one of the methods described above. A passive source employs a propeller or vane to create a fluid flow. The vane, propeller or impeller act on the liquid to create the flow disturbance. An active source is a source that relies on its own linear motion to create a net static flow, which is modulated by an orifice or other device to create pressure disturbances. The linear motion of the source in the borehole can be induced by the action of gravity, by a force applied through a wireline cable, or in some cases, the source may be pulled and/or pushed by a special borehole tractor to be described later. Both active and passive sources share three common features: 1) a compliant chamber which may be gas-filled; 2) means to generate a fluid flow; and 3) means to modulate the flow rate.
According to an embodiment illustrated in
The interior of housing 2002 also includes a compliant compartment 2022 (e.g., gas-filled bladder, diaphragm, or bellows) that can freely expand or contract as the fluid pressure inside the housing changes. The gas-filled compliant compartment may be connected to the surface through a hose (not shown; or it may be connected to a gas pressure regulator system, also not shown) so that the compartment's volume can be adjusted to compensate for operating depth or in other ways described later.
A screen 2024 may be used to keep the compliant compartment from expanding too much if there is a rapid pressure drop due to a change in operating level. When the passive borehole source is at the desired depth level, the motor 2012 is commanded by the source controller 2020 to activate propeller 2010. Propeller 2010 acts like a pump to generate a volumetric flow that is related to its rpm, direction and blade pitch. The propeller hub 2018 may contain devices to vary the propeller's pitch and/or reverse pitch. In other words, in one embodiment, the propeller is controlled either by changing its pitch or the motor is reversed to generate fluid flows in upward and downward directions along axis X. The propeller's speed, direction and pitch are controlled by the source controller to produce a seismic signal having a desired frequency spectrum and/or follow a target signal.
Passive borehole source 2000 acts like a monopole acoustic source. As the propeller generates an outward flow of fluid (i.e., along the negative direction of direction X), the fluid is removed from the interior of the housing and the compliant chamber expands. Because the housing is rigid and closed at one end, the drop in pressure in the interior of the housing is not in communication with the surrounding medium, and thus, a net pressure increase is produced near the outlet 2026 of the housing. Similarly, when the propeller directs the fluid flow inward (i.e., along the positive direction of the X axis), the internal chamber pressure rises, causing the compliant compartment to collapse. The rise in internal pressure inside the housing is isolated from the medium.
In one application, a reaction mass 2028 may be attached to the passive borehole source 2000 to reduce any axial vibration of the source. In another embodiment, two passive borehole sources 2000A and 2000B may be connected back to back, as shown in
Returning to
In fluid filled boreholes, tube waves can sometimes create problems. Tube waves are acoustic waves that propagate up and down in the column of borehole fluid 2034 and usually occur at about 3 Hz. To mitigate this problem, packers that help prevent the propagation of tube waves can be used to isolate a portion of the borehole. For example, the passive borehole source 2000 could be equipped with packers, one packer 2040 being located at some distance below the source and another packer 2042 located somewhere above it, to provide a trapped fluid volume in which the source can operate, as illustrated in
As noted above,
In one embodiment, a number of passive borehole sources 2000 like the one in
According to another embodiment, a passive borehole source is now discussed with regard to
Housing 2213 also accommodates an actuating device 2215, e.g., servomotor, which may be attached to the housing with a bracket (not shown). Actuating device 2215 drives moving plate 2220, e.g., rotates it. Moving plate 2220 works in conjunction with a fixed orifice plate 2222 that is attached to housing 2213 to form a variable orifice mechanism. The variable orifice mechanism acts as the flow modulator for the passive borehole source 2200.
Optionally, one or more dynamic seals 2211, for example a lip, bushing or a segmented roller seal, or other seal compatible with the borehole environment that can at least partially or substantially block the axial flow of liquid past it while source 2201 is moving, are attached to housing 2213 and slide, glide or roll against the casing 2203. Seal 2211 forces the ambient fluid 2217 to enter through ports 2218 inside housing 2213 as source 2201 is pulled up by force 2205 through the borehole via cable 2207, which is attached to housing 2213. In one application, seal 2211 may contain a mechanism (e.g., a pump that inflates the seal) to radially activate or retract it; when activated, the seal contacts casing 2203 and when retracted, seal 2211 does not substantially contact casing 2203.
In an embodiment, source 2200 is lowered to a certain depth via cable 2207 with the seal 2211 retracted. Once at the desired depth, seal 2211 is activated to press against casing 2203. Then, upon receiving a command from the data acquisition management system, force 2205 is applied to cable 2207 and source 2200 is pulled toward the surface at a rate of, for example, about 1 m/s. Other speeds are possible. In one application, consider that the source is pulled for 30 s.
In the 30 s interval, source 2200 will travel about 30 m. While source 2200 is moving, fluid 2217 enters inside housing 2213 through ports 2218 and moves through the source to exit as volume 2219 at the source's outlet 2221. The movement of the fluid 2217 through the source is illustrated by arrows 2216. At the same time, actuating device 2215 moves plate 2220, for example rotates it, (could be a sliding plate instead of rotating), at a variable but controlled rate. The speed that moving plate 2220 moves with respect to orifice plate 2222 will determine the frequency imparted to the borehole fluid 2217 that moves through the source and consequently into the surrounding rock formation.
Actuating device's speed can be varied as a function of time to generate a swept sine wave emission signal. Other signal types are possible. For example, the rpm of actuating device 2215 could change so that at the start of the sweep a frequency of 3 Hz is emitted with the rpm increasing monotonically at a rate chosen so that at, for example, 30 s, the rpm is sufficient to produce a 100 Hz signal. The rate of change in rpm could be linear or nonlinear. In another embodiment, the actuating device's speed is varied in a pseudorandom fashion. Sensors (not shown), for example, a hydrophone, could be located in the fluid near the outlet region 2221 to measure the source output. The output signal 2224 can be sent to a recording system at the surface via cable 2207. The hydrophone signal could be sent as an analog signal, or digitized and then sent back to the surface via cable 2207.
In another embodiment, source 2200 could be equipped with a weight, sufficient in size, to produce a gravity force that would cause source 2200 to descend while the seals are activated. In this case, source 2200 could be deployed at a starting depth. Tension 2205 in cable 2207 could be reduced and source 2200 would descend in the hole due to the gravity of the weight. For this case, the flow inlet would be through the orifice plate 2222 with the flow outlet at the top of the source. A pressure fluctuation would still be created in region 2219.
In this embodiment, if repeated sweeps are required over a particular borehole depth interval, some signals could be recorded with the source moving up through the depth interval and other signals could be recorded with the source moving down through the same or a different or overlapping depth interval.
Source 2200 discussed with regard to
Tractor 2401 is shown in
Most of the above discussed sources include a compliant compartment. The compliant compartment discussed in this disclosure serves several useful purposes. First, because it is elastic, it allows the reservoir of fluid between the flow modulation device (propeller, sliding gate, variable orifice, etc.) and the elastic trapped gas volume, to source or sink fluid flow transients created by the fluid flow modulator. Second, the compliant compartment can be used to create a resonance to improve the overall efficiency of the source. Third, for the active source, the compliant compartment is housed within a rigid outer shell, so the internal pressure fluctuations internal to the source are acoustically isolated from the surrounding medium, thereby enabling the source to act like an acoustic monopole. For the passive source, the compliant compartment in combination with the internal fluid mass tend to stabilize/dampen/or mitigate pressure fluctuations near the inlet, which are due to the pressure perturbations created by the action of the fluid modulation device, so that the acoustic energy radiated at the inlet is much less than the acoustic energy radiated near the outlet.
In the various embodiments previously described, a moveable barrier is used to prevent mixing between the fluid and gas volumes. A moveable barrier is needed in particular if the orientation of the source gas volume with respect to the gravity vector is unfavorable, variable or if the source undergoes vibrations which will tend to promote mixing of gas and fluid. The moveable barrier in the various embodiments described above can also be a sliding piston, membrane, bladder, corrugated membrane, diaphragm, bellows, or other suitable means. In practice, this moveable barrier will have some mass, may introduce some friction effect, or introduce some added stiffness effect. However, for purposes of this application these effects are assumed to be negligible.
A greatly simplified explanation of the resonance effect noted above is as follows. The gas volume in the compliant chamber in combination with the borehole effective liquid stiffness create a combined spring constant Ke, which in combination with the fluid mass load create a second order resonant system. For example, using the embodiment shown in
The resonant frequency (Fr) in Hz of a second order spring-mass system is given by equation:
F
r=(Ke/Me)1/2/(2π), (5)
where it is assumed that the compliant compartment gas behaves like an ideal gas, and that the gas chamber volume is large compared to the peak flow volume. Under these assumptions, the effective gas spring rate Kg of the source is given by:
K
g
=γA
o
2
P
g
/V
g, and (6)
the combined spring constant Ke is given by:
Ke=1/[(1/Kg)+(1/Kb)] (7).
The terms in equations (5) to (7) are defined as follows: Ao is the outlet area; Pg is the gas absolute pressure (Pa) at the operating depth; and Vg is the total gas volume at operating depth (m3). The term y is the polytropic index of the gas. If the expansion and contraction of the gas is an isothermal process then γ=1. If the process is adiabatic (no energy loss) and the gas is diatomic, then γ=1.4. In practice, the process will neither be isothermal nor adiabatic, but somewhere in between. For this example it is assumed that γ=1.2.
Because the hydrostatic pressure can vary greatly with depth, (which is about 11 bar absolute pressure at 100 m depth and about 301 bar at 3000 m depth), if a single bladder with a fixed air mass will shrink in volume with depth, its pressure will increase with depth. At the same time, the volume of fluid inside the source may increase. This creates two issues: it changes the maximum amount of peak flow that can be achieved, and it may shift the source resonant frequency. For example, one result might be that Kg will increase by a factor of about 27 causing an increase in Ke. For this situation Me will increase slightly, so the net result would be that Fr would increase, which may be undesirable.
In an embodiment, gas compartment 2505 is filled with pressurized gas suitable for operation at a certain depth, for example 200 bars (20 MPa) and gas compartment 2509 is filled with gas at 10 bars (1 MPa). Other charge pressures are possible. At the surface, not shown, both moveable partition devices 2501 and 2507 may stretch until they contact the walls of the containment device 2503 or 2511. Once the source is deployed, the hydrostatic pressure rises and
At the modest depth of 150 m, the hydrostatic pressure is still insufficient to cause moveable partition device 2501 to change its volume and the moveable partition device is still constrained by containment device 2503. Thus, gas compartment 2505 does not respond to dynamic pressure fluctuations in the fluid in its vicinity. At about 150 m, the compliant chamber will have a pressure of about Pg=1.5 MPa and a volume Vg=V1.
It will be appreciated that by selecting the proper size gas compartments, number of separate gas compartments, charge pressures, and volumetric constraints, an effective compliance constant Ke can be maintained to within a certain range of values over a wide operating depth range. This will allow the operator to keep the effective resonant frequency of the source within a desired range of frequencies.
If greater control over the resonant frequency is required, an adjustable compliant compartment system can be used. For example, according to an embodiment illustrated in
In addition, metering device 2609 may be in communication with other external devices, for example, other control/management equipment 2619 that may be located also in the borehole or at the surface. Umbilical 2613 interconnects control equipment 2619, metering device 2609 and other sensors 2607 and may provide power, and/or a communication link. Sensor 2607 may be, for example, a device suitable for measuring/estimating volume 2603. Control equipment 2609 may contain means to vent the gas into the borehole. In this embodiment, at modest depths, gas volume 2603 is nominally at the hydrostatic pressure corresponding to the operating depth. Gas volume 2603 can be increased or reduced through the action of metering device 2609, which can supply or vent gas, upon receiving a command signal, either generated internally based upon some internal program or from control equipment 2619.
Thus, for example, in one operational embodiment, at a modest depth, gas volume 2603 is small. When the source is at a larger depth, gas is introduced by metering device 2609 to increase the gas volume 2603 and barrier 2601 moves to a new position 2605, thus increasing the gas volume is a way to offset the increase in pressure to maintain a more constant compliance value. Other means are possible. In a different embodiment, gas volume 2603 can be adjusted, optionally, in combination with positioning of at least one packer, to tune the source to excite a particular resonance mode in the borehole. The excitation resonance mode may be axial and/or radial in direction and may be a fundamental or higher order resonance mode.
Seismic data generated by the seismic sources discussed above and acquired with the streamers also noted above may be processed in a corresponding processing device for generating a final image of the surveyed subsurface as discussed now with regard to
An example of a representative processing device capable of carrying out operations in accordance with the embodiments discussed above is illustrated in
The exemplary processing device 2800 suitable for performing the activities described in the exemplary embodiments may include server 2801. Such a server 2801 may include a central processor unit (CPU) 2802 coupled to a random access memory (RAM) 2804 and/or to a read-only memory (ROM) 2806. The ROM 2806 may also be other types of storage media to store programs, such as programmable ROM (PROM), erasable PROM (EPROM), etc. Processor 2802 may communicate with other internal and external components through input/output (I/O) circuitry 2808 and bussing 2810 to provide control signals and the like. For example, processor 2802 may communicate with the various elements of each source element. Processor 2802 carries out a variety of functions as are known in the art, as dictated by software and/or firmware instructions.
Server 2801 may also include one or more data storage devices, including disk drives 2812, CD-ROM drives 2814, and other hardware capable of reading and/or storing information, such as a DVD, etc. In one embodiment, software for carrying out the above-discussed steps may be stored and distributed on a CD-ROM 2816, removable media 2818 or other form of media capable of storing information. The storage media may be inserted into, and read by, devices such as the CD-ROM drive 2814, disk drive 2812, etc. Server 2801 may be coupled to a display 2820, which may be any type of known display or presentation screen, such as LCD, plasma displays, cathode ray tubes (CRT), etc. A user input interface 2822 is provided, including one or more user interface mechanisms such as a mouse, keyboard, microphone, touch pad, touch screen, voice-recognition system, etc.
Server 2801 may be coupled to other computing devices, such as the equipment of a vessel, via a network. The server may be part of a larger network configuration as in a global area network (GAN) such as the Internet 2828, which allows ultimate connection to various landline and/or mobile client/watcher devices.
As also will be appreciated by one skilled in the art, the exemplary embodiments may be embodied in a wireless communication device, a telecommunication network, as a method or in a computer program product. Accordingly, the exemplary embodiments may take the form of an entirely hardware embodiment or an embodiment combining hardware and software aspects. Further, the exemplary embodiments may take the form of a computer program product stored on a computer-readable storage medium having computer-readable instructions embodied in the medium. Any suitable computer-readable medium may be utilized, including hard disks, CD-ROMs, digital versatile discs (DVD), optical storage devices or magnetic storage devices such a floppy disk or magnetic tape. Other non-limiting examples of computer-readable media include flash-type memories or other known types of memories.
This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. For greater clarity, the figures used to help describe the invention are simplified to illustrate key features. For example, figures are not to scale and certain elements may be disproportionate in size and/or location. Furthermore, it is anticipated that the shape of various components may be different when reduced to practice, for example, to improve their hydrodynamic properties, to reduce towing force or for other reasons. Also, elements like flow diverters and/or vanes and/or wings that may be located on the source element housing structure to improve efficiency, operability, handling, utility or to promote certain preferred directions of fluid flow are not shown. These and other means can be incorporated into any embodiment to further improve the overall performance and/or function of the invention. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims. Those skilled in the art would appreciate that features from any embodiments may be combined to generate a new embodiment.
The disclosed embodiments provide a method and source element capable of boosting an energy generated in the 0.1 to 10 Hz range. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.
Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.
This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2015/000538 | 3/27/2015 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62025153 | Jul 2014 | US | |
| 61977281 | Apr 2014 | US |
