SEISMIC SOUND SOURCE

Abstract: The volumetric capacity of the firing chamber (15) of an air gun is discretely quantized by mounting unit-­ volume inserts (44) interiorly of the firing chamber (15).

Description: This invention provides a means for adjusting the volumetric capacity of a pressurized-fluid seismic sound source, without changing the external dimensions thereof. Pressurized-fluid seismic sound sources are extensively used in marine seismic exploration for liquid and gaseous hydrocarbons. To a lesser extent they are used for special types of land surveys. The fluid used in such sources may be a liquid under pressure, such as water, or a compressed gas such as air. By way of example, and convenience, but not by way of limitation, the invention is described in terms of air guns. In its simplest form, an air gun consists of a firing chamber for containing a volume of air compressed from 3448 KPa (500 psi) to more than 41369 KPa (6000 psi). The firing chamber is provided with one or more air exhaust ports that may be closed by a sliding, differential-area valve. The valve may be a piston, a sliding shuttle, or an internal or external sliding sleeve valve. In operation, the valve is held closed by control-air pressure in a valve-control chamber. The area of the valve face that is exposed to the control-air pressure is greater than the area of the opposite valve face that is exposed to the air pressure in the firing chamber. The greater force applied by the control-air pressure holds the valve closed. Upon command, a solenoid valve bleeds off the control-air pressure. When the balance of forces is upset across the two faces of the valve, the air pressure in the firing chamber causes the valve to abruptly open. Some of the air in the firing chamber is impulsively released through the exhaust ports into the water to create a shock wave which then degenerates into an acoustic wave. The air released from the air gun creates a pulsating air bubble in the water. The periodicity of the bubble depends upon the static water head, the air pressure in the firing chamber and the volume of air released. The pulsating air bubble wreaks havoc with the resulting seismic signals because each expansion of the bubble acts like a new seismic source. Although computer-aided data processing can de-bubble the recorded seismic signals, it is preferable to suppress or eliminate the bubble effects ab initio in the field. In accordance with modern practice, arrays of air guns are employed to maximize the primary-to-bubble amplitude ratio. In the array, the respective air guns are characterized by firing chambers having different volumes. Since the periodicity of the bubble is a function of the volume of air released, by properly tuning the individual guns of the array, the bubble pulses generated by the guns tend to cancel by destructive interference. The firing-chamber volumes of the guns in a single array may range from a few cubic centimetres to several thousand cubic centimetres. In an array of 15 to 20 guns, the combined volumetric capacity may be as much as 89140 cm³ (5000 in³). It is preferable, of course, that, regardless of the volumetric capacity of the respective firing chambers, the basic operating parts of the air guns of an array be interchangeable and of uniform size, e.g. valve, control-air chamber, triggering solenoids and the like. To change firing-chamber volume, it should only be necessary to change the firing chamber itself. Some air-gun designs are readily adaptable to that desideratum. See U.S. Patent 4,114,723, as an example of one type of air gun. In that patent, the firing chamber consists of an elongated, cylindrical casing, capped at one end. The other end is screwed to a plug that includes the internal sleeve valve and the valve-control assembly. Although not specifically taught in that patent, it is a simple matter to change the air-gun volume by replacing an existing elongated casing by one of a different length. The dimensions of the operating parts of the gun remain unchanged. Since the mechanical operating parts of the gun are interchangeable regardless of gun volume, the spare-parts inventory is minimal. Another style of air gun has recently become popular which is exemplified by U.S. Patent 4,623,033. That air gun employs an externally mounted sliding sleeve valve or shuttle and a partially externally-mounted valve-control chamber. The firing-chamber portion of that air gun includes a supporting center post that is integral with the body of the gun. An external sleeve surrounds and encloses the center post assembly. It is not possible with the '033 gun to merely change the dimensions of the firing chamber to change its volume. It is necessary to physically re-scale the entire gun assembly including the external shuttle valve, the valve-control mechanism and the sleeve. For that reason a large stock of complete guns, all of different physical sizes, along with sets of differently-sized spare parts, must be maintained. That requirement poses a horrendous logistics problem. GB-A-2141824 and GB-A-1090363 use very complex and expensive piston arrangements to alter air gun volume remotely. EP-A-0112696 discloses an air gun in which, to change volume, the entire air chamber is replaced. US-A-3653460 has an internal, movable, partition whereby the relative volumes of two chambers to provide two pressure pulses. There thus remains a need for a standard-sized air gun assembly, the firing-chamber volume of which is readily adjustable, without affecting the interchangeability of the mechanical operating parts. In accordance with one aspect of this invention, there is provided a seismic sound source comprising a firing chamber having fixed dimensions, characterised in that the chamber has a plurality of means by which a selectable number of inserts can be removably mounted internally of the firing chamber to determine its effective volume. Thus, in one example, a plurality of unit-volume inserts may be removably mounted internally of the firing chamber in preselected combinations, thereby to adjust the volume of the contained pressurized fluid in discrete steps in accordance with the number of inserts mounted therein. In accordance with a further aspect of this invention, there is provided a seismic sound source comprising: a body member; a firing chamber in fluid communication with said body member, the firing chamber having fixed dimensions, for containing a quantity of pressurized fluid, the firing chamber having an external valve member for periodically releasing some of the contained pressurized fluid; a plurality of solid inserts, each having a preselected unit volume, removably mounted internally of said firing chamber in preselectable combinations thereby to adjust the initial volume of the firing chamber in discrete volumetric decrements in accordance with the number and volume of the internally-mounted solid inserts; a center-post member integral with said body member and said firing chamber; and means for anchoring said unit-volume solid inserts to said center-post member. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a cross section of one type of air gun to which this invention may be beneficially applied; Figure 2 is a cross section along line 2-2'; and Figure 3 is an isometric view of a typical unit-volume insert. Figure 1 is a cross sectional-view of the air gun described in the '033 patent as being one type of gun to which the invention may be applied. That gun and its operation are disclosed in the above reference, but for completeness of disclosure, its construction will be briefly reviewed. Basically, the gun consists of a body 10, a flanged center post 12, and a solenoid type fire control unit 14. For purposes of this disclosure, the firing chamber 15 is formed between a sleeve 16, the lower face 18 of body 10 and center post 12 and is of fixed dimensions. A shuttle valve 20 is held gainst the lip of sleeve 16 by pressurized air fed through conduit 22 to a control chamber 24. Air is also admitted to firing chamber 15 through conduit 26. To fire the gun, the control unit 14 opens line 28 to admit pressurized air to trigger chamber 30. Since the lower face 32 of shuttle 20 has a greater area than upper face 34, shuttle 20 moves to the left, away from sleeve 16, releasing a volume of pressurized air from firing chamber 15. Referring now to Figures 1 and 2, centerpost 12 is provided with flanges such as 36 and 38 which may include bolt holes such as 40 and 42. A plurality of solid inserts such as 44, 44', 44'', 44''' are provided. Four are shown but additional inserts may be supplied. Each insert is characterized by a preselected unit volume. For example, assuming use of the English system of measurement (although not limited thereto), each insert of a set might have a volume of 82cm³ (5 in³) for a total of 328 cm³ (20 in³) for a four-insert set. Further, different sets of inserts may be provided, the unit-volume inserts of each set having, for example, preselected unit volumes of 82, 123 and 164 cm³ (5, 7.5 and 10 in³) respectively. If the initial fixed volumetric capacity of firing chamber 15 is 1311 cm³ (80 in³), then by use of preselected combinations of the unit-volume inserts the volumetric capacity of firing chamber 15 can be quantized in discrete, 82 cm³ (5 in³) steps from 1230 cm³ (75 in³) to 656 cm³ (40 in³) without altering any of the original physical dimensions of the air gun assembly. As shown in Figure 2, the inserts are removably mounted internally of firing chamber 15. They are anchored in place to flanges 36 and 38 by suitable bolts 46 and nuts 48 of any convenient size. Preferably the inserts may be stainless steel blocks approximately 1.9 cm (0.75 in) thick, 3.8 cm (1.5 in) wide, and of a length sufficient to provide the required unit volume as shown in Figure 3. In a preferred embodiment of this invention, the combined volume of a plurality of inserts mounted internally of the air gun is less, by a preselected quantum, than the initial volumetric capacity of firing chamber 15 which has fixed dimensions. For example, in a 983 cm³ (60-in³) gun, if the combined insert volume were 656 cm³ (40 in³), then the preselected quantum might be 328 cm³ (20 in³). It follows that the volume of the contained pressurized air varies inversely in discrete proportion to the number of unit-volume inserts mounted in the firing chamber. One embodiment has been described in terms of a specific air gun by way of example but not by way of limitation. The invention may be applied to any type of pressurized-fluid seismic sound source that has an internal structure suitable for receiving unit-volume inserts.

Claims: A seismic sound source comprising a firing chamber (15) having fixed dimensions, characterised in that the chamber has a plurality of means (46,42) by which a selectable number of inserts (44) can be removably mounted internally of the firing chamber (15) to determine its effective volume. A source according to claim 1 in combination with an insert (44) releasably attached in the chamber. A source according to claim 2, and having a plurality of inserts (44) including a set of inserts of which each insert has substantially the same volume as each other. A source according to claim 4, wherein the plurality includes a second set of inserts of which each insert has substantially the same volume as each other and different from the insert volume of the first-mentioned set. A source according to any one of the preceding claims, wherein the firing chamber (15) includes a centre post (12) relative to which the inserts are securable. A source according to claim 5, wherein the mounting means are provided on webs (38) integrally extending from the centre post (12). A seismic sound source comprising: a body member (10; a firing chamber (15) in fluid communication with said body member, the firing chamber having fixed dimensions, for containing a quantity of pressurized fluid, the firing chamber having an external valve member (20) for periodically releasing some of the contained pressurized fluid, characterised by a plurality of solid inserts (44), each having a preselected unit volume, removably mounted internally of said firing chamber (15) in preselectable combinations thereby to adjust the initial volume of the firing chamber in discrete volumetric decrements in accordance with the number and volume of the internally-mounted solid inserts; a center-post member (12) integral with said body member (10) and said firing chamber (15); and means (44) for anchoring said unit-volume solid inserts to said center-post member. A source according to claim 7 wherein the firing chamber (15) is characterized by an initial fixed volumetric capacity and the combined volume of the plurality of unit-volume solid inserts (44) is less, by a preselected quantum, than said initial fixed volumetric capacity. A source according to claim 8, wherein the volume of the contained pressurized fluid varies in inverse discrete proportion to the number of unit-volume solid inserts mounted internally of said firing chamber.

Inventors: HAUGLAND, TOR-ARVID
Applicants: WESTERN ATLAS INTERNATIONAL, INC.
Application year: 1988
Main classification: G01V1
IPC codes: G01V1