GB2394046A

GB2394046A - Pressure profile measurement of a seismic survey airgun using a pressure senor located within the airgun firing chamber

Info

Publication number: GB2394046A
Application number: GB0223676A
Authority: GB
Inventors: Jon-Fredrik Hopperstad
Original assignee: Westerngeco Seismic Holdings Ltd
Current assignee: Westerngeco Seismic Holdings Ltd
Priority date: 2002-10-11
Filing date: 2002-10-11
Publication date: 2004-04-14
Anticipated expiration: 2022-10-11
Also published as: GB0223676D0; GB2394046B

Abstract

The apparatus comprises a seismic airgun having a firing chamber 210 and at least one port 255 for releasing high pressure air from the firing chamber to create a seismic signal, and a pressure sensor 265 positioned within the firing chamber near to the release port 210. The method comprises positioning a pressure sensor within a firing chamber of the airgun, executing a firing cycle using the firing chamber, and measuring the pressure profile of the firing chamber during the firing cycle with the pressure sensor.

Description

APPARATUS FOR MEASURING THE PRESSURE PROFII,E OF SEISMIC

AIRGUNS

5 BACKGROUND OF THE INVENTION

I FIELD OF THE INVENTION

This invention relates generally to seismic airguns, and, more particularly, to measuring the pressure profile of seismic airguns.

10 2. DESCRIPTION OF THE RELATED ART

Subsurface hydrocarbon accumulations are increasingly found in geologically complex areas. The ability to conduct accurate seismic surveys may help improve the discovery rates and even the production of such accumulations. Seismic surveying is a method of simulating a geological subsurface formation with, e.g., electrical, magnetic, 15 and/or acoustic signals to acquire seismic data about the formation. From this data, one can hopefully tell whether the formation contains hydrocarbon deposits and, if so, where.

In marine seismic surveying, an acoustic array containing acoustic sensors and sources typically is deployed. In one variation, an array of marine seismic streamers is 20 towed behind seismic survey vessel. Each streamer typically is several thousand meters long and contains a large number of hydrophores and associated electronic equipment distributed along its length. The seismic survey vessel also tows one or more seismic sources, typically airguns. 25 Acoustic signals, or "shots," produced by the airguns are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected

signals are received by the hydrophores in the array, digitized, and transmitted to the seismic survey vessel, where they are recorded. The recorded signals are at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed. The representation may be read or interpreted to discover and locate hydrocarbon 5 deposits. The quality and reliability of seismic data are controlled by a number of factors, but are ultimately constrained by the nature of the seismic source. In the early development of marine seismic surveying, chemical explosives were typically used as a seismic source.

10 Chemical explosives, however, were dangerous to use as well as destructive on the environment. Beginning in the 1960's, new seismic sources were developed that were safer to use and had a lesser environmental impact. One of the new seismic sources developed was the airgun. In recent times, seismic airguns have become the preferred seismic source in marine seismic surveying. These airguns release high pressure air, typically 2,000 or 2,500 15 psi into the water to create the desired acoustic signal.

Conventional airguns typically comprise an annular housing that contains means for discharging compressed air through exhaust ports in the housing. In one embodiment of a conventional airgun, compressed air is stored within the housing in a firing chamber, or 20 reservoir, and is sealed from the exhaust ports by a shuttle. The shuttle, when moved from its rest position, permits a rapid outflow of air from the firing chamber through the exhaust ports into the surrounding water. The rapid outflow of air pushes the water away from the airgun and creates a bubble.

The water around the bubble suddenly becomes highly compressed, which creates acoustic waves that radiate outwardly. The bubble continues to expand and eventually reaches a point where the gas pressure equals the hydrostatic pressure. At this point, the bubble continues to expand past equilibrium and the hydrostatic pressure causes the bubble to 5 collapse. The bubble continues to collapse and eventually reaches a point where the gas pressure exceeds the hydrostatic pressure and begins to expand again. The continuous expansion and collapse of the bubble causes oscillations that generate acoustic waves. After a few oscillations the bubble reaches the water surface and releases its air, or the oscillations die out.

The series of acoustic waves radiating from the bubble is referred to as an acoustic pressure field. The amplitude and spectral content of the acoustic pressure field is influenced

by various parameters, such as airgun depth, pressure in the airgun reservoir, volume of the airgun reservoir, the airgun release mechanism, and the synchronization of the airguns in an 15 airgun array. The synchronization of the airguns will maximize the construction of the primary pressure pulse by adding up the primary pressure pulses of each guns. Previous attempts to synchronize airguns in an array have not yielded optimal results.

For example, U.S. Patent No. 4,210,222 to Chelminski et al. ("Chelminski et al.") 20 describes an airgun with a pressure sensor located in the housing of a solenoid. The sensor detects a pressure increase resulting from rapid motion of the shuttle during firing of a bubble. This method detects air release indirectly, and does not take into account any time delay between the initial release of high pressure air from the airgun and the pressure increase in the solenoid. Experiments have shown that the time delay differs among airgun types and

chamber volumes. Chelminski et al. assumes a fixed time delay similar for all airguns, which is not optimal.

U.S. Patent No. 5,450,374 to Harrison ("Harrison") describes a method and apparatus 5 for detecting the firing of an airgun. In Harrison, permanent magnets are placed circurnferentially on the end of the airgun shuttle. A timing coil is used to detect rapid motion of the shuttle during the firing stage. The timing coil described in Harrison indirectly measures shuttle motion by estimation. Harrison synchronizes airguns at the time of the maximum stroke of previous firings. The flaw with such a method is that maximum acoustic l O output does not necessarily occur at maximum shuttle stroke.

U.S. Patent No. 4,402,382 to Mollere ("Moliere") describes an apparatus for determining the firing instant of an airgun. Moliere uses a coil/magnet system for detecting the pressure change in the airgun firing chamber via a magnetic switch. The magnetic switch 15 detects only an initial pressure change in the airgun reservoir as air is released. The initial pressure change, however, does not necessarily correspond to maximum acoustic output.

! The monopole pressure field from the airgun bubble is referred to as a notional source

signature. An overall source array response can be obtained from the individual notional 20 source signatures. The plane wave response of the array in a given direction is referred to as the farfield signature. The farfield signature is used in seismic processing to deconvolve the

source output from the reflection data. As is well known to those in the art, several methods have been reported to estimate the source signature, and they can be distinguished in three categories: (1) mathematical models of the bubble oscillation; (2) measurements of the

acoustic pressure field; and (3) a combination of (1) and (2). Previous attempts to estimate

the source signature have not yielded optimal results.

U.S. Patent No. 6,081,765 to Ziolkowski ("Ziolkowski") describes a method for 5 determining a farfield signature of an airgun array by mounting pressure sensors on the

exterior of individual airguns. The pressure sensor first measures the pressure of the water outside the bubble and then measures the pressure inside the released air bubble. The measurements are used in an equation of motion for radial flow about the center of an oscillating bubble to calculate the acoustic monopole pressure field. Because the pressure

10 sensors are placed outside the airguns, Ziolkowski requires that the initial value for the particle velocity of the water at the pressure sensor to be known or determined though separate experiment. Furthermore, Ziolkowski requires the initial volume of the bubble be known or determined through separate experimentation.

15 The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.

SUMMARY OF THE INVENTION

The present invention provides an improved apparatus for measuring the pressure 20 profile of a seismic airgun. The apparatus comprises a seismic airgun having a firing chamber and at least one port for releasing high pressure air from the firing chamber to create a seismic signal, and a pressure sensor positioned within the firing chamber. The present invention further provides an improved method for measuring the pressure profile of a seismic airgun. The method comprises positioning a pressure sensor within a firing chamber

of the airgun, executing a firing cycle using the firing chamber, and measuring the pressure profile of the firing chamber during the firing cycle with the pressure sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

5 The invention may be understood by reference to the following description taken in

conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: Figure I illustrates a plan view of a seismic survey vessel towing a source array and 10 an acoustic array, in accordance with one embodiment of the present invention; Figure 2A illustrates an airgun of Figure I, in accordance with one embodiment of the present invention; 15 Figure 2B illustrates a shuttle of the airgun in Figure 2A, in accordance with one embodiment of the present invention; Figure 3 illustrates graphically the relationship between air pressure in the firing chamber and the time it takes for air to be released from the airgun of Figures 2A, in 20 accordance with one embodiment of the present invention; Figure 4 illustrates graphically the relationship between estimated shuttle displacement and the time it takes for air to be released from the airgun of Figures 2A, in accordance with one embodiment of the present invention;

I 7 Figure 5 illustrates graphically the relationship between the pressure change in the firing chamber and time it takes for air to be released from the airgun of Figures 2A, in accordance with one embodiment of the present invention; and Figure 6 illustrates graphically the air flow from the firing chamber to the ports.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are 10 herein described in detail. It should be understood, however, that the description herein of

specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

15 DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will

of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, 20 such as compliance with systemrelated and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

Figure I conceptually illustrates a seismic survey system lOO comprising a towed receiver array 101 and a towed source array 102, in accordance with one embodiment of the present invention. A seismic survey vessel 105 tows a seismic streamer 110 by way of a first tow cable l l S. The streamer 110 may comprise a tail buoy 120. The tail buoy 120 typically S identifies the end of the streamer 110. The streamer 110 is additionally provided with one or more leveling devices or "birds" 125 that regulate the depth of the streamer 110 within the water. The seismic survey vessel 105, by way of a second tow cable 130, also tows one or more airguns 135, which generate an acoustic wave (not shown) in the water that generally travels in a downward direction towards the sea bed (also not shown). The acoustic wave 10 reflects from various structures (also not shown) within the sea bed, and the reflected wave (also not shown) is detected by one or more acoustic receivers 140, such as hydrophores, in the streamer 110.

As discussed in greater detail below, the airgun 135 is equipped with a pressure 15 sensor (not shown in Figure 1) for estimating a source signature and detecting a time when air is released. As is well known in the art, upon receipt of the reflected wave, the acoustic receiver 140 typically generates analog signals. The analog signals may be converted to digital signals by analog-to-digital converters (not shown) in the streamer 110 and transmitted along the streamer 110 and the tow cable 120 to the seismic survey vessel 105. The seismic 20 survey vessel 105 may comprise digital signal devices (not shown) for recording and processing the digital signals. The seismic survey vessel 105 may further comprise a computing device 132 for processing any of a variety of information, such as estimating the signature of the airgun 135 or synchronizing the firing of a number of airguns 135.

For the sake of simplicity, Figure 1 illustrates two towed arrays 101, 102 comprising two tow cables 115, 130 and one streamer 110 attached to the first tow cable 115. However, any number of arrays may contain any number of streamers, in accordance with conventional practice. The two towed arrays 101, 102 may further comprise devices not shown in Figure 5 1, in accordance with conventional practice, such as a towed buoy. Furthermore, it should be appreciated that the acoustic source 135 and the acoustic receiver 140 may be towed by the same cable. In other embodiments, the acoustic sources 135 may be placed on a mobile or semi- mobile unit (not shown) positioned some distance away from the seismic survey vessel 105. It should also be appreciated that, in one embodiment, the seismic streamer 110 may be 10 an ocean-bottom cable ("OBC"). OBCs may be deployed on the seafloor to record and relay data to the seismic survey vessel 105. OBCs generally enable surveying in areas where towed streamers 110 are unusable or disadvantageous, such as in areas of obstructions and shallow water inaccessible to ships.

15 Figure 2A illustrates a schematic of a airgun 135 of Figure 1, in accordance with one embodiment of the present invention. The airgun 135 is similar to the airgun disclosed in United States Letters Patent 5,572, 486, entitled "Seismic Airgun Arrangement", and issued November 5, 1996, to Landro et al., but modified to implement the present invention. Note, however, that the invention is not so limited and may be applied to virtually any airgun 20 known to the art. For instance, the airgun disclosed in U.S. Letters Patent 4,210,222, entitled "Air Gun Monitoring Method and Apparatus," issued July 1, 1980, to Bolt Associates, Inc. assignees of Chelminksi et al. may be employed in alternative embodiments.

10 The airgun 135 comprises two high pressure air chambers: an operating chamber 205 and a firing chamber 210. In its charging state, the high pressure air chambers 205, 210 are sealed by a shuttle 215. As is more clearly illustrated in Figure 2B, the shuttle 215 comprises an operating piston 220 and a firing piston 225 mounted on a common shank 230. Referring 5 again to Figure 2A, the shuttle 215 is shown in a first, or sealed, position. High pressure air 235, at typically 2,000 or 2,500 psi, may be supplied to the operating chamber 205 from an exterior source (not shown), such as a compressor onboard the seismic survey vessel 105 of Figure 1 via an air intake 240.

10 It should be noted that a number of arrows in Figure 2A leading from the air intake 240 and ending in the firing chamber 210 illustrates the flow of high pressure air 235 flowing into the airgun 135. As high pressure air 235 enters the operating chamber 205, an orifice 245 in the shuttle 215 allows air to flow into the firing chamber 210 through a channel 212.

The shuttle 215 of the airgun 135 remains in a charging position, thus sealing the two 15 chambers 205, 210, because the area of the operating piston 220 is larger than that of the firing piston 225.

The airgun 135 is actuated by sending an electrical pulse to the solenoid valve 250, which releases the high pressure air 235 to the lower side of the operating piston 220. The 20 high pressure air 235 forces the shuttle 215 to move from the charging position to a second, or firing, position. This firing position is indicated in Figure 2A with broken lines, and the high pressure air 235 in the firing chamber 210 is rapidly released through the ports 255, as illustrated by air release arrows 260. In the illustrated embodiment, the airgun 135 comprises four ports. In an alternate embodiment, the airgun 135 may comprise a 360-degree port (not

shown), typically known as a sleeve, where the high pressure air 235 is expelled. Such an airgun is commonly referred to as a "sleeve gun." Other embodiments of airgun ports known to one skilled in the art may be used in the present invention. The high pressure air 235 released from the ports 255 forms a bubble, which creates a pressure wave used as a seismic 5 signal. The shuttle 215 is forced back to the charging position illustrated in Figure 2A by high pressure air 235 supplied into the operating chamber 205.

As illustrated in Figure 2A, the pressure sensor 265 is located inside the firing chamber 210 near the port 255. The pressure sensor 265 should generally be placed near the 10 port 255. In one embodiment, placing the pressure sensor near the port 255 allows the pressure sensor 265 to measure the local pressure in the port throttle and optimize the measurement of the air flow. In addition, because the sensor is placed within the firing chamber, the measured air pressure will be substantially free of external influences, i.e the pressure field of neighboring bubbles. The pressure sensor 265 should also substantially

15 conform to the profile (i.e., the interior) 270 of the firing chamber 210 when positioned therein so as to minimize the interference with the release of the high pressure 235 air from the firing chamber 210. For example, the pressure sensor 265 may be flush with the profile 270 of the firing chamber 210. In other examples, the pressure sensor 265 may be recessed or at least partially shielded from the profile 270. In one embodiment, the pressure sensor 20 265 measures the air pressure inside the firing chamber 210 during a firing cycle (discussed below). As previously described, when the airgun 135 is in the charging state (i.e., the ports 255 are sealed as illustrated in Figure 2A), high pressure air 235 is supplied to the firing chamber 210, thereby increasing the air pressure inside the firing chamber 210. When the airgun 135 is in the firing state (i.e., the ports 255 are exposed as illustrated in Figure 2C) ,

high pressure air 235 is released from the firing chamber 210, thereby causing a sudden drop in air pressure. This drop in air pressure is sensed by the pressure sensor 265 and the firing of the airgun 135 can be determined from its output. As described in greater detail below, the pressure sensor 265 may be used to synchronize the firing of airguns 135 in an array. The 5 pressure sensor 265 may also be used to derive the flow through the ports 255.

Figure 3 illustrates graphically the relationship between air pressure in the firing chamber 210 and the time it takes for air to be released from the airgun 135 in the illustrated embodiment. Figure 3 illustrates this relationship from the firing state to the charging state.

10 The series of steps from releasing of air from the firing chamber 210 to charging high pressure air 235 back into the firing chamber 210 is referred to as the firing cycle. The firing cycle may further include the initial charging of high pressure 235 into the firing chamber 210 prior to releasing the air. Between approximately O milliseconds ("me") and 8 ms, the shuttle 215 begins to move forward but has not moved forward far enough to open the ports 15 255 for air release. As the shuttle 215 moves from the charging position to the firing position, the volume in the firing chamber 210 increases, thus reducing the air pressure.

Between approximately 8 ms and 16 me, the shuttle 215 moves far enough forward such that the ports 255 open and air is suddenly released into a bubble. This sudden release of air from the firing chamber 210 causes a sharp drop in pressure, as illustrated in Figure 3. From 16 ms 20 and on, the shuttle 215 recoils and is returned to its original charging position and the pressure inside the firing chamber 210 begins to rise.

For airguns 135 in which the volume increase of the firing chamber 210 is a one dimensional function of the shuttle position, the volume increase caused by the movement of

the shuttle 215, and thus the shuttle displacement, can be calculated by assuming an adiabatic process in the following shuttle displacement equation: = Ash UP: l where Po,Vo'P,Ash'X denotes initial pressure, initial volume, pressure in the firing chamber 5 210 measured by the pressure sensor 265, cross-sectional area of the shuttle 215, and the displacement of the shuttle 215, respectively. The airgun 135 starts to release air when x > xc, where xc is the offset to the edge of the ports 255. The value XC is generally a known value for each airgun type.

]0 Accordingly, differently sized airguns 135 in a towed source array 102 can be properly synchronized at the initial release of air (i.e., at the sharp drop in pressure in the firing chamber 210). It should be appreciated that the synchronization at the initial release of air, among other things, may be performed by the computer or other computing device 132 in Figure 1. Synchronization may also be performed by a computer in a remote location. Other 15 methods known in conventional practice of synchronizing at the initial release of air may be used as well.

It should be appreciated that the shuttle displacement equation can also be used to extrapolate the entire shuttle motion. By differentiating the shuttle displacement equation, 20 the velocity of the shuttle 215 when the ports 255 open and close can be determined. The velocity data of the shuttle 215 can be used to estimate the shuttle stroke. Figure 4 illustrates an estimation of the shuttle stroke of an airgun 135. The shuttle displacement is shown as a solid line in Figure 4 and is calculated by inputting the pressure measurements from Figure 3

into the shuttle displacement equation. The velocity estimates calculated by differentiating the shuttle displacement equation is used to extrapolate the shuttle displacement and is shown as dashed lines in Figure 4.

5 In an alternative embodiment of the present invention, the airguns 135 in a towed acoustic array 102 can be synchronized at the maximum acoustic output. It is well known that the maximum acoustic output from an airgun is proportional to the volume acceleration of the bubble. It is thought that the maximum volume acceleration of the bubble is closely related to the maximum pressure decay in the firing chamber 210. Accordingly, the 10 maximum acoustic output from an airgun generally occurs when the maximum pressure decay of the in the firing chamber 2 l 0 occurs.

Figure 5 illustrates graphically the first order derivative of the pressure measurement in Figure 3, also known as a pressure profile. The pressure profile shows the pressure decay 15 in the firing chamber 210 throughout the shuttle stroke. The maximum decay is visible as a sharp peak 510. The maximum decay can be determined from a variety of airguns 135 at each firing. The airguns 135 in the towed array 102 can be timed such that the airguns 135 are fired simultaneously at the maximum decay of each airgun 135. Accordingly, the airguns 135 in the towed array 102 can be synchronized at the maximum acoustic output. It should 20 be appreciated that the synchronization at the maximum acoustic output, may be performed by the computer or other computing device 132 in Figure 1. Synchronization may also be performed by a computer at a remote location. Other methods known in conventional practice of synchronizing at the maximum acoustic output may be used as well.

In yet another embodiment of the present invention, the pressure sensor 265 can be used to characterize the flow of air from the firing chamber 210 into the bubble. The characterization of the flow of air can be used to set initial conditions for models of bubble oscillation (hereinafter referred to as "initial conditions"). The initial conditions can, in turn, 5 be used to determine an acoustic signature. As mentioned, the acoustic signature is generally the product of volume acceleration of the bubble and density of the water. It should be appreciated that the acoustic signature can be determined by the computer or other computing device 132 in Figure 1. The acoustic signature may also be determined by a computer at a remote location. Other methods known in conventional practice of determining the acoustic 10 signature may be used as well. The initial conditions include, but are not limited to, the pressure inside the firing chamber 210, pressure of the bubble both assumed and derived from a model, density of the air inside the firing chamber 210 both assumed and derived from a model, and flow area. The initial conditions are derived from measurements collected by the pressure sensor 265. As is well known in conventional practice, a variety of methods may be 15 used to determine the volume acceleration of the bubble from the initial conditions.

Also as mentioned, the pressure sensor 265 is preferably placed near one of the ports 250. Placing the pressure sensor 265 near one of the ports 255 allows it to measure local pressure of the port 255 throttling. A variety of methods for modeling throttling effect of the 20 ports 255 is well known in conventional practice. Figure 6 illustrates one such model, which assumes a smooth transition from the firing chamber 210 to the ports 255.

This concludes the detailed description. The particular embodiments disclosed above

are illustrative only, as the invention may be modified and practiced in different but

equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations 5 are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

Claims

1 1. An apparatus for measuring the pressure profile of a seismic airgun, 2 comprising: 3 a seismic airgun having a firing chamber and at least one port for releasing high pressure 4 air from the firing chamber to create a seismic signal; and 5 a pressure sensor positioned within the firing chamber near the port for measuring the air 6 pressure in the firing chamber during a firing cycle, 1

2. The apparatus of claim 1, wherein the seismic airgun comprises a plurality of 2 ports. 1

3. The apparatus of claim 1, wherein the pressure sensor is adapted to 2 substantially conform to the profile of the firing chamber when positioned therein so as to 3 minimize the interference with the release of the high pressure air from the firing 4 chamber. 1

4. The apparatus of claim 1, further comprising means for synchronizing the 2 airgun using the measured air pressure profile.

1

5. The apparatus of claim 4, wherein the means for synchronizing the airgun 2 comprises: 3 means for determining an initial release of air from the firing chamber; and 4 means for synchronizing the airgun at the determined initial release of air.

1

6. The apparatus of claim 4, wherein the means for synchronizing the airgun 2 comprises: 3 means for determining a maximum pressure decay of air from the firing 4 chamber; and

5 means for synchronizing the airgun at the determined maximum pressure 6 decay. 1

7. The apparatus of claim 3, further comprising a seismic survey vessel for 2 deploying the seismic airgun.

1

8. The apparatus of claim 4, further comprising a source array adapted to hold at 2 least one seismic airgun.

1

9. The apparatus of claim 1, further comprising a plurality of pressure sensors.

1

10. A method of measuring the pressure profile of a seismic airgun, compnsmg 2 the steps of: 3 positioning a pressure sensor within a firing chamber of the airgun near a port, wherein 4 the firing chamber has at least one port; 5 executing a firing cycle; and 6 measuring the pressure profile of the firing chamber during the firing cycle with the 7 pressure sensor.

1

11. The method of claim 10, wherein the firing cycle comprises: 2 discharging the high pressure air through the port; and 3 charging the firing chamber with high pressure air.

1

12. The method of claim 10, further comprising initially charging the firing 2 chamber with high pressure air prior to discharging the high pressure air.

Page 18 of 19