Open-ended seismic source

An open-ended seismic source for use in a liquid medium such as water is disclosed. The source includes a source of explosive gas, a firing chamber connected to the source, a capacitor discharge ignition system connected to the firing chamber for igniting the gas in the firing chamber, a barrel connected to the firing chamber, defining an explosion chamber and having an outlet at its lower end, a plurality of baffle plates fixed in the barrel near the outlet for permitting the exploded gas to enter the medium and for retarding the entry of the medium into the barrel, and a reaction plate attached to the barrel and disposed below the outlet so that the exploded gas entering the medium impinges on the plate and the recoil of the barrel is reduced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to improvements in seismic energy sources used to 
create pulses in a liquid medium such as water. More particularly, it 
relates primarily to improvements in such seismic sources which create 
pulses in the medium by the detonation of explosive gas in an explosion 
chamber which communicates directly with the medium. 
2. Description of the Prior Art 
In prospecting in subsea areas and in other earth formations underlying 
bodies of water, it is desirable to provide a source of energy for 
introducing pulses or waves into the water. These waves propagate through 
the water, enter the underlying formation, are reflected in part by 
discontinuities in the formation, and subsequently propagate back through 
the water and are detected by geophones and other measuring devices at or 
near the water's surface. The characteristics of the reflected waves are 
compared with the characteristics of the waves at or near their 
introduction into the water. This comparison reveals valuable information 
about the structure of the underlying formation and the probability of the 
presence of petroleum accumulations in the formation. 
It has become common to use seismic sources known as gas guns to induce 
pulses of desired characteristics in the liquid medium. These guns operate 
by abruptly introducing under high pressure bubbles of compressed air or 
other gaseous material into the liquid medium, thereby generating a pulse 
in the medium, or by abruptly expanding a flexible member in contact with 
the medium, thereby generating a pulse in the medium. An example of the 
former type of gun is shown in U.S. Pat. No. 3,397,755 (1968) to Loper; an 
example of the latter type of gun is shown in U.S. Pat. No. 3,658,149 
(1972) to Neal et al. 
This invention is directed to guns of the former type, and in particular, 
primarily to guns which achieve the abrupt introduction of high pressure 
gas into the medium through the detonation of explosive gas in an 
explosion chamber which communicates with the medium. U.S. Pat. No. 
3,397,755 illustrates a subsea seismic source in which a valve is 
interposed between the chamber of the gun and the liquid medium and the 
release of the gas to the medium is controlled by the valve. It is also 
well known in the art to make and use open ended gas guns in which no 
valve is interposed between the chamber and the medium, but instead the 
chamber communicates directly with the medium through a section of open 
pipe or other outlet which remains open continually. Such open ended gas 
guns have certain advantages of simplicity of construction, operation and 
maintenance over gas guns with valves which control the release of the gas 
into the medium. However, an open ended gas gun can have the disadvantage 
that in operation, as it is moved through the liquid medium, eddy currents 
are created in the medium, so that the medium enters the explosion chamber 
and interferes with the introduction and detonation of the explosive gas. 
A further disadvantage of gas guns, whether open ended or not, is that the 
detonation or release of the gas and subsequent abrupt introduction of the 
gas into the medium can create a recoil of the barrel of the gun. This 
recoil, depending on the mass of the barrel, the magnitude of the 
detonation and other factors, may be damaging to the equipment and 
dangerous. Leonard proposes in U.S. Pat. Nos. 3,588,801 (1971) and 
3,951,231 (1976) eliminating or reducing the recoil by closing the end of 
the barrel and providing a series of side ports to permit the discharge of 
the gas into the medium. While this technique may reduce or eliminate the 
recoil, it also greatly reduces the efficiency of the transfer of energy 
to the medium. 
SUMMARY 
In order to permit the gas to pass freely from a chamber in the gun through 
an outlet in the gun into the medium, but also to retard the entry of the 
medium into the chamber, baffle plates are fixed near the outlet. These 
baffle plates are aligned so that they permit the efficient discharge of 
the gas into the medium, but retard the entry of the medium into the 
chamber. 
In order to reduce the recoil of the gun upon release of the gas into the 
medium and simultaneously to permit the efficient entry of the gas into 
the medium, a reaction plate is fixed to the gun adjacent to the outlet 
through which the gas passes, so that gas passing through the outlet 
impinges upon the reaction plate. The plate is fixed to the gun by 
relatively thin rods which provide relatively slight interference with the 
passage of the gas into the liquid medium. Because the gas diffuses as it 
leaves the outlet, the surface area of the face of the reaction plate is 
designed to exceed the cross-sectional area of the outlet. Thus, the 
reaction plate reduces the recoil to acceptable levels while maintaining 
the efficient transfer of the energy of the detonation to the liquid 
medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a simplified representation of an open-ended gas gun embodying 
the invention. 
The gun includes a tank 10 of oxygen and a tank 11 of propane, with 
pressure regulators 12 and 13 respectively, connected by conduits 14 and 
16 respectively to a mixing block 18. Mixing block 18 defines a mixing 
chamber 19 for blending the gases to form an explosive mixture. Valves 20 
and 22 are disposed in conduits 14 and 16 respectively to meter the oxygen 
and propane as desired. Any suitable valves may be used; in practice the 
V5 Series two-way normally closed solenoid valves by Skinner Precision 
Industries, Inc. of New Britain, Conn. have been acceptable. 
Mixing block 18 is connected by conduit 24 to firing block 26 defining a 
firing chamber 27. A back-fire preventer 28 may be disposed in conduit 24 
for the purpose of preventing the fire front from spreading to the mixing 
block 18 when the gas is ignited in the firing chamber 27. 
A spark plug 30 extends through the wall of firing block 26 into firing 
chamber 27 and is connected to a capacitor discharge system 32. At desired 
time intervals discharge system 32 causes spark plug 30 to fire and ignite 
the gas in firing chamber 27. 
Conduit 34 communicates between firing chamber 27 and the gun barrel 36. In 
the preferred embodiment gun barrel 36 comprises an elongated cylindrical 
member or pipe 38 having a cap 40 closing its upper end, defining an 
outlet 42 at its lower end and defining an explosion chamber 44 between 
cap 40 and outlet 42. However, it will be appreciated that the barrel need 
not be formed of a pipe and cap and in fact need not be cylindrical, so 
long as it defines a suitable explosion chamber for the gases and provides 
a suitable outlet for the gases between the chamber and the medium. In 
FIG. 1 a single passage 46 in cap 40 for receiving the lower end of 
conduit 34 is shown. If desired, a plurality of passages (not shown) could 
be formed in cap 40 to provide alternate means for receiving conduit 34 
and thereby facilitate the connection of conduit 34 to cap 40. Naturally 
those passageways would be blocked off when not in use. 
Valve 48 is mounted on cap 40 and serves to vent into the water the 
residual gases left in chamber 44 after an explosion and thus may be said 
to be a means for venting residual gas from the explosion chamber. The 
details of the operation of valve 48 will be discussed below. However, 
briefly the valve is controlled by a tank 50 of compressed air connected 
to valve 48 by conduit 52 and by valve 54 disposed in conduit 52 between 
the tank 50 and valve 48. 
Barrel 36 is designed to be placed underwater so that its longitudinal axis 
is substantially vertical and to towed through the water so that its line 
of travel through the water is substantially perpendicular to its 
longitudinal axis. In such event eddy currents tend to form in the water 
around outlet 42 and water tends to enter the barrel through outlet 42. 
This entry of water into the barrel is undesirable because it interferes 
with the introduction of gas into chamber 44 and with the detonation of 
that gas. In order to retard such entry of water, a baffle indicated 
generally at 56 is fixed in the lower end of the barrel. In the preferred 
embodiment, this baffle comprises a set of baffle plates 58 and a set of 
baffle plates 60 welded into place in pipe 38. All baffle plates are fixed 
so that their planes are substantially parallel to the flow of gas through 
the outlet. Further, the planes of one set of plates are perpendicular to 
the planes of the other set of plates, so that the baffle will be 
effective as the barrel is rotated about its central longitudinal axis. It 
will be appreciated that the baffle need not comprise a plurality of 
mutually perpendicular plates to come within the spirit of the invention. 
In fact, any suitable baffle which permits the flow of gas through outlet 
42 and which retards the flow of water into the barrel will serve the 
purpose. For example, the baffle could comprise one or more pipes of 
diameters smaller than the diameter of pipe 38 and fixed in the lower end 
of pipe 38 (either concentrically or not), so that their longitudinal axes 
are parallel with that of pipe 38. Further, while the baffle is shown in 
the preferred embodiment to be wholly within the barrel, clearly it could 
project from the barrel or be disposed outside of the barrel and still 
come within the spirit of the invention. 
When the gas in chamber 44 is detonated and enters the water, the barrel 36 
will recoil in accordance with Newton's third law of motion. This recoil 
is undesirable, because it can be dangerous and can damage the equipment. 
In order to attenuate the recoil, a reaction plate 62 having a face 64 is 
disposed outside the barrel adjacent to outlet 42 and is attached to the 
barrel by rods 66 which extend from the lower end of the barrel downwardly 
to the reaction plate. Reaction plate 62 is spaced from outlet 42 so that 
the gas escaping through the outlet 42 upon detonation enters the water 
and impinges upon face 64, whereby the recoil of the barrel is reduced. It 
is preferable, but not essential, that the plane of plate 62 be 
substantially perpendicular to the central longitudinal axis of the barrel 
and that said axis pass through the approximate center of said plate 62. 
The upper ends of rods 66 are fixed to the lower outside surface of barrel 
36 by welding or other suitable means and the lower ends of rods 66 are 
fixed by similar means to plate 62. The longitudinal axes of rods 66 are 
substantially parallel to each other and to the logitudinal axis of barrel 
38. While the preferred embodiment of the invention includes the rods as 
just described, it will be appreciated that many other means for fixing 
the reaction plate to the barrel would fall within the spirit of the 
invention. However, it is desirable to keep such means relatively thin, so 
that there will be efficient entry of the exploded gas into the water and 
thus efficient creation of a seismic pulse in the water. Preferably the 
length of rods 66 is set so that the reaction plate is close enough to 
outlet 42 effectively to reduce the recoil, but far enough away to permit 
the efficient entry of the gas into the water. In the preferred 
embodiment, the barrel is about 21 inches (53 cm.) long and about 8 inches 
(20 cm. ) in diameter; six rods, each about 1 inch (2.5 cm.) in diameter 
are provided; and the reaction plate is spaced about 7 inches (18 cm.) 
from the outlet. However, clearly these parameters may be changed without 
departing from the spirit of the invention. 
As the gas escapes through outlet 42 upon detonation, it will diffuse 
before it impinges upon face 64 of reaction plate 62. For this reason, the 
surface area of face 64 (or of the horizontal cross-section of plate 62) 
is greater than the cross-sectional area of outlet 42. In the preferred 
embodiment, the reaction plate is about 12 inches (31 cm.) in diameter, so 
that the horizontal cross-sectional area of the plate is slightly greater 
than twice that of the outlet. However, clearly outlets and plates of 
other dimensions could fall within the scope of the invention. 
Attention is now directed to the details of valve 48, which is provided for 
the purpose of venting the residual gas from chamber 44 between explosions 
and which for clarity is shown in FIG. 1 in somewhat enlarged relationship 
with respect to barrel 36. In order to provide a means for attaching valve 
48 to cap 40, an upstanding cylindrical member 70 with annular flange 72 
at its top extends through and is attached to cap 40 by suitable means 
such as welding. Cylindrical member 70 provides a vertical passageway 74 
between chamber 44 and valve 48. Valve 48 comprises a generally 
cylindrical body 76 which defines a central, longitudinal, cylindrical 
passageway therethrough. This passageway has a lower inlet 77, a lower 
passageway section 78a, a middle passageway section 78b, and an upper 
passageway section 78c. Passageway 74 has a greater diameter than 
passageway section 78a and passageway section 78b has a greater diameter 
than passageway section 78a and passageway section 78c has a greater 
diameter than passageway section 78b, in order to permit the proper 
cooperation of certain moveable components which will be described below. 
Ports 80 communicate between lower passageway section 78a and the water 
and thus provide paths for the residual gas to enter the water. Annular 
flange 82 is secured to the bottom end of valve body 76 and is designed to 
match flange 72 and to be secured thereto by bolts 84 so that valve 48 is 
mounted on cap 40 with passageway 74 aligned with and in communication 
with the longitudinal passageway through valve body 76. For the purpose of 
alternately opening and closing the inlet 77, which has a circular 
cross-section, a rod 86 having a circular plate 88 attached to its lower 
end is slideably disposed in the longitudinal passageway though valve body 
76. Plate 88 has a larger diameter than inlet 77. When rod 86 and plate 88 
are in their lower or open position (shown in FIG. 1), gas may flow 
through passageway 74, inlet 77, passageway 78a and ports 80 into the 
water. When rod 86 and plate 88 are in their upper or closed position, 
plate 88 blocks the passage of gas through inlet 77. To enhance the seal 
between plate 88 and valve body 76 an O-ring is placed around inlet 77. To 
maintain rod 86 in proper alignment within the valve body 76, an annular 
guide sleeve 89 is fixed in passageway section 78a so that rod 86 slides 
through sleeve 89. In order to urge rod 86 and plate 88 into the closed 
position, a coil spring 90 is placed around rod 86 in passageway section 
78b. This spring 90 presses up on annular plate 92 which is disposed 
around the upper end of rod 86 between the spring 90 and a nut 93 fixed to 
the top of rod 86. To provide a means for overcoming the upward pressure 
of spring 90 when it is desired to move rod 86 and plate 88 to their open 
position, a piston 94 is slideably disposed in passageway section 78c. 
Piston 94 is not attached to rod 86, but can press down on the top of rod 
86. To improve the seal between piston 94 and valve body 76, O-rings are 
provided around piston 94. Normally spring 90 will urge rod 86 and plate 
88 upwardly to close inlet 77 so that no gas can escape through cap 40. 
After an explosion, valve 54 is actuated to permit compressed air from 
tank 50 to flow through conduit 52 and impinge on the top of piston 94, 
moving rod 86 and plate 88 downwardly as spring 90 compresses, so that 
residual gases can flow from chamber 44, through passageway 74, inlet 77, 
passageway section 78a, and ports 80 into the water. Normally the 
hydrostatic pressure of the water will be sufficient to move the residual 
gases through valve 48 when it is opened. Valve 54 can be any suitable 
type. In practice the V5 Series three way valves by Skinner Precision 
Industries, Inc. of New Britain, Conn. have been found to be acceptable. 
While the preferred embodiment of the invention includes the valve 48 as 
described, it will be appreciated that many equivalent means for venting 
residual gases from chamber 44 could be used within the spirit of the 
invention. 
In operation, one or more gun barrels 36 and ancillary equipment are 
disposed below the water surface and moved through the water at a desired 
depth and velocity. The barrels may be mounted on an underwater sled or 
held by other means well known in the art. Generally the barrels will be 
held so that their longitudinal axes are substantially vertical and, as 
the barrels move through the water, eddy currents will form around the 
barrels, but the entry of water into chamber 44 will be inhibited or 
retarded by baffles 56. When a seismic pulse is desired, gases from tanks 
10 and 12 will be metered through valves 20 and 22 to obtain a desired 
stoichiometric mixture in mixing block 18 and will then pass through 
conduit 24 and backfire preventer 28 to firing chamber 27 and through 
conduit 34 to explosion chamber 44. Valve 48 will be in the closed 
position. When desired, a trigger pulse will be sent to ignition system 
32, resulting in the firing of spark plug 30 and detonation of the gas in 
firing chamber 27. The ensuing flame front will be blocked by backfire 
preventer 28, but will travel down conduit 34 and will detonate the gas in 
chamber 44. Thus, firing chamber 27, spark plug 30, capacitor discharge 
system 32 and conduit 34 may be said to constitute means for detonating 
the explosive gas in chamber 44. In fact, viewed broadly the explosion 
mechanism and related equipment may be said to constitute a means for 
abruptly increasing the pressure of the gas in the chamber. This gas will 
escape through outlet 42, impinge on plate 62, thus reducing the recoil, 
and enter the surrounding water through rods 66, thus creating a seismic 
pulse. A signal is then sent to valve 54 and air under pressure in tank 50 
is transmitted through conduit 52 to open valve 48 and thus permit the 
residual gases to escape to the water through ports 80. 
From the above, it can be seen that the invention accomplishes its objects. 
The advantages of an efficient open-ended gas gun are achieved without the 
disadvantage of entry of the water into the explosion chamber. The recoil 
of the barrel is reduced and yet the gas passes efficiently into the water 
between rods 66 and creates the desired pulse. While the preferred 
embodiment involves mixing a plurality of different gases in a mixing 
block, it can be appreciated that the gases could be mixed in the 
explosion chamber or that a single explosive gas could be used. Further, 
while the preferred embodiment involves a firing chamber separate and 
distinct from the explosion chamber, the ignition of the gas could be 
accomplished in the explosion chamber. Further, the invention could be 
practiced by the use of non-explosive gas abruptly introduced under high 
pressure into the gun barrel. Thus, the foregoing disclosure and 
description of the invention are illustrative and explanatory thereof, and 
various changes in the size, shape and materials, as well as in the 
details of the illustrated construction may be made within the scope of 
the appended claims without departing from the spirit of the invention.