Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This lnvention relates to marine seismic sources
and more particularly to such sources in which a slug of
liquid is propelled into surrounding water by an expanding
volume of pressurized gas.
In seismic surveying in a body of water, air guns
submerged in the water are a common means for generating
seismic energy to investigate geological conditions and
formations in the earth below or adjacent to the body of
water. For this purpose, one or more of such air guns are
submerged in the water; compressed air, or other gas or
gases under pressure, is fed to the submerged guns and
temporarily stored therein. At the desired instant, the
air guns, the seismic energy sources,are actuated, i.e.,
fired, and pressurized gas, usually highly compressed air,
is abruptly released into the surrounding water. In this
manner powerful acoustical waves are generated, and the
waves are capable of penetrating deeply into the subsurface
material of the earth to be reflected and refracted therein
by the various strata and formations. The reflected or
refracted acoustical waves are sensed and recorded to provide
information and data about the geological conditions and
formations.
In order to avoid the generation of strong
secondary impulses, such as can be created by the oscillating
bubble of di harged alr from an individu~1 air gun, seismic
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energy sources known as "water guns" have recently been
used. One such water gun is discussed in an article by P.
Newman in the Oil and Gas Journal, August 7, 1978, Pages
138-150. In that water gun, water is pushed from the gun
by a piston propelled by expanding pressurized gas. After
such a water gun is fired, the expanded pressurized gas is
vented into the surrounding water over a relatively long
period of time such that the discharged air does not generate
any significant seismic signal which would interfere with
the seismic signal generated by the propelled water. The
water gun is then reset, and it is recharged by refilling
the gun with pressurized air.
Both air guns and known water guns require that
the gun be recharged with pressurized air (or other pres-
surized gas) after each firing (shot) of the gun, since
the source of the energy for propelling the air or water
is pressurized air which is recharged into the gun from a
compressed air supply on shipboard. With each shot, or
firing of the gun, a typical air gun requires the discharge
of a relatively large number of cubic inches of air pres-
surized to for example 2,000 to 4,000 pounds per square
inch. The size of the firing chambers of air guns of -
various sizes may range in volume upwards to several
thousand cubic inches. When an array of, for example, ten
to forty air guns is fired once every few seconds, the
volume of air which must be compressed by the high pressure
air compressors on shipboard is quite large. For example,
a typical ar y of thirty air guns in use today may require
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that approximately one thousand cubic feet of air at atmos-
pheric pressure be compressed per minute of operation during
a seismic survey. To provide such a great volume of high
pressure air, large air compressors are required. Such com-
pressors occupy a substantial portion of the very limited
amount of the valuable space available on board ship, and
they require considerable amounts of energy to power them.
Even water guns require that a substantial amount
of air be compressed to high pressures on board ship. In
such seismic sources, the expanded pressurized gas must be
vented into the water in order to reset the water gun. Thus,
substantially the same large volume of compressed air is
consumed as in the case of air guns of similar size. The
above-identified articIe by P. Newman is entitled Water Gun
Fills Marine Seismic Gap, and it compares the operation of
a water gun with an air gun. The author states: "Operation
is very similar to that of a standard air gun, in fact
identical electrical firing circuits and compressed air
supplies are commonly used." This statement confirms that
substantially the same large volume of compressed air is
consumed by water guns as by air guns of similar size.
'~ SUMMARY OF PREFERRED EMBODIME~T
;'
, An object of the illustrative embodiment of the
present invention is to provide a marine seismic energy
source, whi may be called a "bydro gun", in which expandin
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pressurized gas powerfully propels a slug of water into
the surrounding water, for generating a seismic energy
signal, but in which the pressurized gas, usually compressed
air, is retained in the hydro gun.
Among the advantages of a hydro gun seismic
surveying method and system embodying the present invention
is that a large volume of pressurized air or other pres-
surized gas is not required to continue firing an array
of such seismic energy sources. Therefore, the need for
large compressor capability on board ship is obviated.
Among the further advantages of a hydro gun
seismic surveying method and system embodying this invention
is that each seismic energy source is energized by recharg-
ing with water fed thereto using a high pressure water
pump on shipboard. Water desirably has insignificant
compressibility, and for all intents and purposes herein
water can be deemed to be "incompressible". Consequently,
the volume of water required to be pumped by such a high
pressure water feed pump is small, for it is only equal to
the sum of the volumes of water discharged by the respective
hydro guns. This advantageous situation with hydro guns is
entirely different from the case of recharging with com-
pressed air wherein approximately 200 cubic inches of air at
atmospheric pressure must be compressed and supplied for each
one cubic inch of air which is ultimately discharged or ventec
into the water at, say, 3,000 pounds per square inch (psi).
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Therefore, a relatively small and compact high pressure
water pump on shipboard serves as the energizing source
in contradistinction to the large bulky compressed air
supplies now used for recharging air guns or water guns.
Significant space and weight are saved on shipboard.
From another aspect, a hydro gun surveying method
and system embodying the present invention achieves the
desirable seismic energy signal characteristics of using
water guns while avoiding the large consumption of compressed
air as required by water guns.
In accordance with the illustrative embodiment
of the invention in one of its aspects, a liquid-charge
chamber in a hydro gun is held closed by the force of
gas pressure against an operating piston of a reciprocatable
shuttle. An incompressible liquid, e.g. water, is pumped by
a small powerful high pressure pump into the charge chamber -
and pushes a piston follower against retained pressurized
gas in a gas-propulsion chamber; the piston follower moves
against the gas pressure, thereby compressing that gas.
This hydro gun seismic energy source is then fired hy
suddenly applying this gas pressure to the reverse surface
of the operating piston, so that the shuttle becomes
suddenly free to accelerate under the force of the expanding
compressed gas acting on the follower piston. A slug of
liquid is thereby violently propelled from the liquid-charge
chamber into the surrounding body of water; however, the
propelling gas is retained in the hydro gun.
151)~'~0
In accordance with another aspect of the illus-
trative embodiment of the invention, an equalization passage
is provided between the operating chamber and the gas-
propulsion chamber in order to assure a sufficiently high
closing pressure on the front surface of the operating
piston of the shuttle as the gas retained in the gas-
propulsion chamber is compressed by recharging of high
pressure water into the liquid-charge chamber.
According to yet another aspect of the illus-
trative embodiment of this invention, the portion of the
shuttle adjacent the liquid-charge chamber includes a
pseudoconical or cusped surface that redirects the axially
flowing liquid discharge out radially through the discharge
ports.
In accordance with yet another aspect of the
illustrative embodiment of the invention, the follower
piston is a cup-shaped piston member which assures that a
sufficient volume of pressurized gas is retained immediately
adjacent to the piston follower for producing a powerful
firing action of the hydro gun after the gas is compressed
in the gas-propulsion chamber.
In accordance with a further aspect of the
illustrative embodiment of this invention, the piston
follower has a reduced diameter top portion directed toward
gas discharge ports, and the liquid-charge chamber tapers
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into a reduc diameter throat in the region adjacent to
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the discharge ports. Consequently, when this seismic energy
source is fired, water outside of the reduced diameter
portion of the follower piston becomes trapped to damp the
high speed forward driving movement of the piston.
In the use of a hydro gun array, there will be a
small air compressor on shipboard for the purpose of re-
supplying the small amount of air which is released from the
firing vent orifice of each gun during the brief instant
when the guns are fired.
As used herein the term "pressurized gas" is
intended to be interpreted sufficiently broadly to include
compressed air which is shown as being used in the pre-
, ferred embodiment.
It is also possible ~to string or submerge oneor more arrays of hydro guns embodying the present invention
in marshland, swamp or mud areas which are infused with
sufficient water that hydro guns can be used effectively. -
~i Accordingly, the terms "water-, or "body of water" and the
terms "towing vessel", "ship", as well as "marine seismic
surveying", as used in the specification and claims are
intended to be interpreted sufficiently broadly to include
these marginal applications of the present invention.
:~', . . 1,. BRIEF DESCRIPTION OF THE DRAWINGS
' The foregoing and other objects, features, aspects
,;i and advantages of the invention will be apparent from the
following mo particular description of a preferred
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embodiment of the invention, as illustrated in the accompany-
ing drawings in which like reference characters refer to
the same parts throughout the different views. The drawings
are not necessarily to scale, emphasis instead being placed
upon illustrating the principles of the invention.
FIG. 1 is a pictorial illustration of a seismic
survey vessel towing an array of hydro guns, two guns being
shown;
FIG. 2 is a sectional elevational view of a
single hydro gun embodying the present invention and shown
on enlarged scale as compared with FIG. l;
FIG. 3 is a cross-sectional view of the hydro gun
of FIG. 2 taken along line 3-3 looking downwardly;
FIG. 4 is an elevational sectional view similar
to FIG. 2, taken along line 4-4 in FIG. 3, but showing the .
piston follower moved into the charged gun position as the
result of recharging the.hydro gun by high pressure water
pumped into the liquid-charge chamber;
FIG. 5 is an elevational sectional view taken
along line 5-5 in FIG. 3 and showing a slug of water being .
discharged by means of trapped pressurized gas expanding
upwardly pushing the piston follower upwardly; .
FIG. 6 graphically demonstrates the operating .
results of a water gun embodying the present invention.
,~, .
.~ DESCRIPTION OF A PREFERRED E~BODI~NT
. As shown in FIG. 1, a survey vessel 20 on a body
¦ o~ water 19 >ws a streamer array 21 of b~dro guns, including
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guns 22 and 23, by a tow line 24 extending from the vessel
20. It is to be understood, for example, that this array 21
may include two to thirty or more of the hydro guns 22, 23
etc. The tow line 27 interconnects buoys 26, there being one
buoy for supporting each hydro gun. Other arrangements as
known in the art for towing a multiplicity of marine seismic
energy sources may be used for towing these hydro guns, for
example, streamer apparatus may be used similar to that which
is shown in Patent No. 4,038,630, issued July 26, 1977.
Sea water 31 is drawn through an intake line 30
into the intake 29 of a high pressure water pump 34 having its
outlet 25 connected to the high pressure water feed line 32.
The sea water is pumped at high pressure into each hydro gun
through the recharging feed line 32 by the high pressure
water pump 34. A single feed line 32 is shown extending alon
the array and is connected by in-feed branch lines 33 to
the respective hydro guns in the array, and each hydro gun is
automatically recharged with high pressure water 35 (FIG. 2)
after each firing. The water enters the interior of the hydr
gun through an inlet port 37. The high pressure water pump
34 is a positive displacement pump having a pressure relief
valve as will be understood, con~ected to the outlet of the
pump 34 for assuring that the pressure of the water in the .
high pressure recharging line 32 does not exceed a predeter-
mined limit. For example, the hydro guns may be arranged
to be charged with water up to a pressure of approximately
3,000 psi, in which case the pressure relief valve is set at
a level of approximately 3,000 psi; however, greater or
lesser operating pressures may be used, as may be desired
by the survey team.
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For initially supplying each hydro gun with its
retained pressurized air and for making up any small amounts
of pressurized air which becomes released during firing
through the small firing vent orifice (not shown) discussed
below, there is a make-up air source 36. This air source 36
is connected to the respective hydro guns by a plurality of
make-up supply lines 38 extending from the vessel 20. The
make-up air source 36 includes a relatively small and compact
high pressure air compressor for providing a low volume of
pressurixed gas at the desired high pressure.
There is a control console 40 including shut-off
valves 39 and pressure-indicating meters 41 for the respect-
ive make-up lines 38. The pressure of the compressed gas
occurring in any particular one of the hydro guns during an
operating cycle can be determined by shutting off the
respective valve 39 for isolating the particular line 38
extending from the source 36 to that par~icular gun. After
this particular line 38 has been isolated f rom the source,
the meter 41 for that isolated line will indicate the gas
pressure in the hydro gun during an operating cycle. Also,
the shut-off valves 39 can bè used for shutting off the
supply of make-up air from any hydro gun which malfunctions
or misfires or leaks.
The hydro guns 22, 23, etc. are triggered so as
to be fired by triggering arrangements similar to the known
triggering arrangements used for air guns by electrical
trigger impulses transmitted through an electrode cable 83
(FIG. 2) to the solenoid-controlled valve 72 from a central
on-board electronic control system.
11 390
. The hydro guns in the array 21 are shown as being
: identical, and one of those guns, gun 22, is shown in section
in FIG. 2. The hydro gun 22 includes three sections, the
operating chamber section -42, the discharge port section
44, and a firing chamber section 46. These sections are
united to form a single unit by~removable clamps 48 and 50.
The firing chamber 65 in firing chamber section
46 is divided into an upper liquid-charge chamber 66 and a
lower gas-propulsion chamber 70 by a cup-shaped piston
: follower 68. The liquid-charge chamber 66 is closed by.
action of a reciprocatable shuttle 52, which includes a
; central shaft 54, an upper operating piston element 56, and
5 ~ a lower piston element 60. The upper operating piston
~ element 56 is secured onto the shaft 54 by a lock nut 58, ~ . :
i~ ~ and this upper piston 56 rides in an operating chamber.~B .
i~ : and controls discharge of the hydro gun. The lower piston
J:~ element 60 normally rests against a seat 64 to close the
. liquid-charge chamber 66. As will be described subsequently,
the shuttle 52 is actuated by a solenoid-controlled valve .
72 mounted to the tcp of the hydro gun.
. Make-up pressurized gas which has been fed through
the line 38 is stored and is retained in the operating . .
chamber 62. The gas also has access through an equalization
passage 74 to be stored and retained in the gas-propulsion
chamber 70. High pressure recharging water is pumped into
i the liquid-charge chamber 66 through the branch line 33 and
~ serves to compress further the pressurized gas, as will be
. explained. -
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115~1390 l l
In operation, the operating piston 56 of shuttle
52 initially rests against a seat 76 and is held there by
the pressurized gas in the operating chamber 62, such gas
pressing downwardly on the front ~upper) face of the
operating piston 56. With. the shuttle 52 in this lowermost
position, the lower piston element 60 also initially rests
against the seat 64 to close the liquid-charge chamber 66.
Then, high pressure water 35 is pumped into the liquid-
charge chamber 66 through the line 33, this water being
noncompressible and being driven by the pump 34 which is
capable of feeding the water 35 at a higher pressure than
the initial pressure of the pressurized air in the gas-
propulsion chamber 7n. The entering water 35 pushes the
piston follower 68 downwardly against the force of the
pressurized gas in gas-propulsion chamber 70, thereby
further compressing the pressurized air (or other pressurized
gas) therein. The follower 68 moves steadily downwardly
as high pressure water 35 is introduced into the liquid-
charge chamber 66 until the follower 68 reaches a lowermost .
position as shown in FIG. 4.
. As the follower piston 68 is driven steadilydownwardly by the entry of high pressure water 35 into the
liquid-charge chamber 66, the pressurized gas which is
retained and stored in the gas-propulsion chamber 70
becomes steadily compressed to a higher-and-higher pressure.
As this pressure in the gas-propulsion chamber 70 becomes
steadily increased, some o~ the gas flows upwardly through
the pressure-equalization passage 74 which communicates at
115~390
75 with the lowest region in the propuls'on chamber 70 and
communicates at 77 with the top of the operating chamber 62.
Thereby, the pressure in the operating chamber 62 also
steadily increases to remain equalized with that in the
propulsion chamber 70. Consequently, the operating piston
56 remains held firmly seated against its seat 76 as the
pressures in chambers 70 and 62 are both increasing.
Considering the dynamic action occurring during
recharging by the high-pressure water, it is to be noted that
the area of the upwardly facing surfaces of the piston
follower 68 is equal to the area of its downwardly facing
surfaces. Thus, the piston follower 68 moves within theliquid _
charge chamber 66 to maintain substantially equal fluid
pressures in the liquid-charge chamber 66 and in the gas-
propulsion chamber 70 as the follower piston 68 is driven
downwardly by the entry of the recharging water 35. The
liquid-charge chamber 66 becomes increased in volume, and
the gas-propulsion chamber 70 becomes reduced in volume
as the follower piston moves downwardly in a stroke which
may be called its ~nergizing stroke. The effect
is like hydraulically cocking an enormously powerful spring,
with the trapped gas in the chamber 70 actins like the
powerful spring. The gas-propulsion cha~ber 70 is equalized
in pressure with the operating chamber 62 through the
equalization passage 74. Thus, the upwardly acting pressure
(force per square inch) on the lower surface of the piston
element 60 of shuttle 52 is equal to the do~mwardly acting
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pressure on the upper face of the operating piston 56.
However, the diameter and hence the active area (exposed
surface) of the operating piston 56 is somewhat greater than
the diameter and hence the active area (exposed surface) of
the lower piston element 64; thus, the do~mward force on the
shuttle 52 exceeds the upward force. Consequently, the
shuttle is held in its seated position, and the liquid-charge
chamber 66 thus remains closed even as liquid is pumped into
it and as the piston follower 68 moves downwardly to compress
the pressurized gas in the gas-propulsion chamber 7~.
The piston follower 68 is in the shape of an
inverted cup so that, even when in its lowermost position
as shown in FIG. 4, there is a sufficient volume of com-
pressed gas below the piston follower for powerfully pro-
pelling the slug of water from the liquid-charge chamber
when the hydro gun is fired. O-rings 71 provide the desired
sliding seal between the liquid-charge chamber 66 and the
gas-propulsion chamber. These O-rings 71 encircle the
skirt portion 67 of the piston follower, near the top and
bottom thereof, respectively. The skirt 67 is reduced in
thickness and weight by a broad, shallow channel 69 located
between these O-rings. A lubricant may be retained in this
broad channel 69 to act as a semi-permanent lubrication
source for the piston follower 68.
In order to provide a shock-absorbing damping
effect for the piston follower 68 at the end of its li
expulsion stroke as will be explained further below, there
is a centrally located and axially protruding piston top
portion 73 (FIG. 4) on the piston follower. The diameter
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of the cylindrical side wall 79 of this reduced diameter
piston top 73 is sized to fit with clearance within the
throat region 86 of the liquid-charge containing chamber
66. A tapered region 81 of the side wall 79 is contiguous
with the top surface of the piston top 73.
Once the piston follower 68 has moved to its lower
most position as shown in FIG. 4, the hydro gun remains
static until a trigger signal is transmitted through an
electrical cable 83 to the solenoid valve 72. At that
time, pressurized gas from the operating chamber 62 is
free to flow through the solenoid valve 72 and through a
trigger passage 78 to the region below the lower face of
the operating piston 56. The upward force applied to the
operating piston 56 by the pressurized gas flowing through
this trigger passage 78 causes the resultant forces on the
shuttle to act upwardly so that the shuttle 52 jumps suddenly
upwardly. The by-pass channels 82 permit the operating
piston 56 to travel upwardly~at great velocity.
The lower p1ston element 60 is initially seated
on the seat 64 located a substantial distance below the
liquid discharge ports 80 so that the operating piston 56
reaches by-pass channels 82 before the liquid-charge chamber
66 becomes opened to the liquid discharge ports 80. Con-
*equently, the shuttle can accelerate rapidly upwardly to
suddenly open the liquid-charge chamber 66 to the liquid
discharge ports 80. The compressed pressurized gas in the
gas-propulsion chamber 70 acting like a powerful spring
expand rapidly, forcing the piston follower 68 upwardly to
abruptly push the slug of water 35 which was previously
stored in the liquid-charge chamber 66 out through discharge
ports 80, as shown in FIG. 5.
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The reduced diameter top portion 73 of the pistonfollower 68 moves upwardly into the throat portion 86 of the
liquid-charlge chamber 66. This traps a quantity of water in
the annular region 88 of the liquid-charge chamber 66 sur-
rounding the side wall 79 of the piston top 73 above the ex-
ternal shoulder 87 of the piston follower and below the
internal shoulder 89 at the upper end of the cylindrical
surface 65. This trapped water 88 acts as a shock-absorber t
damp the upward movement of the piston follower 68 to reduce
the mechanical shock to the water gun as the piston follower
68 is smoothly brought to a halt, by the trapped water 88
being ejected upwardly as shown at 85 between the slopping
surface 81 on the piston top 73 and the converging neck 84
below the throat 86 of the chamber 66. The external shoulder
87 on the piston follower acts as a stop and comes into
contact with the internal shoulder 89 when the piston followe
is in its uppermoslt position, as shown in FIG. 2. .
It will be noted, primarily in FIG. 5, that the .
lower surface of the lower piston element 60 of the shuttle
52 has a downwardly pointed pseudoconical shape or cusp 90
which is concentrically positioned for redirecting the axial-
ly flowing mass of liquid from the liquid-charge chamber
66 radially outwardly through the three liquid dis- .
charge ports 80. This cusp surface 90 reduces turbulent
losses in the liquid flow and thereby provides for increased
velocity in the sudden ejection of the slugs of water 35
outwardly through the ports 80. The momentum of the high
velocity of each slug of water 35 (FIG. 5) carries it
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outwardly, leaving a void in the ambient body of water 19
(FIG. 1) near the respective liquid discharge port 80. The
surrounding water 19, being under hydrostatic pressure, then
flows inwardly to fill that void. A powerful seismic
impulse is generated when the void collapses, and it is
a strong and discrete acoustical impulse.
The gas in the chamber 70, which acted like a
powerful spring in driving the follower piston 68 upwardly
in what may be called the ejection stroke, is confined
behind this follower piston and cannot escape through the
ports 80.
After the hydro gun has fired as shown in FIG. 5,
the liquid pressure acting upwardly on the lower piston 60
of the shuttle 52 is reduced to ambient hydrostatic pressure.
The confined gas in the operating chamber 62 is still at a
high pressurel and thus the shuttle is returned to its
initial position with the operating piston 56 returning down
onto its seat 76. The hydro gun is thus reset and may be
reenergized by pumping high pressure water through line 33
(FIG. 2) into the liquid-charge chamber 66.
It will be noted that, except for the small amount
of airl as mentioned beforel which is released out during
firing through the small firing vent orificel there is no
loss of pressurized gas from each hydro gun during firing or
recharging of the gun. The firing vent orifice (not shown)
communicates with the trigger passage 78 to bring the pres-
sure beneath the operating piston 56 back to ambient hydro-
static pressure when the operating piston is reseated after
each firing cycle of the hydro gun 22. Ideally, even with a
long sequence of firings of the hydro gun, only a comparative-
ly small amount of pressurized gas need be provided to the
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operating chamber 62 or to the gas-propulsion chamber 70
once the gun has received its initial charge of pressurized
gas, such a minimal amount of make-up gas being provided
through lines 38.
Turning attention to the liquid-charge chamber 66
and the ga~-propulsion chamber 70, it is to be noted that
both of these chambers are cylindrical; they are axially
aligned, and they have the same diameter. Each chamber
forms an axial extension of the other chamber so that the
piston follower 68 can slide smoothly with minimal friction
between them, thereby increasing the volume of one while
decreasing the volume of the other, and vice versa.
The results of using the above-described hydro gun
i seismic energy source are demonstrated by the plot 92 shown
in FIG. 6. This plot 92 is a copy of a photographic print
made from an oscillograph readout produced by firing a hydro
gun having a liquid-charge-containing chamber 66 of 60 cubic
inches volume when measured with the follower piston 68 in
its lowermost position as shown in FIG. 4. The gas pressure
in the propulsion chamber 70 was approximately 2,200 psi
(gage pressure) measured when the follower piston 68 is in
its initial uppermost position as shown in FIG. 2. The
pressure variation at 93 is caused by the discharge of the
¦ water 35 into the surrounding body of water as shown in FIG.
¦ S. The spike-like peak 94 of the seismic signal plot 92 has
a peak amplitude of 4 barmeters. The measurements were made
using a hydrophone positioned 45 feet directly below the
hydro gun, i.e. at a depth of 75 feet below the surface, in
~¦ the so-caLled 'far field" region in the ~cdy of w~lter s r~o~ding
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the hydro gun. The horizontal time scale for the plot in
FIG. 6 is 10 milliseconds per centimeter. The hydro gun
was fired at a dep~h of 30 feet below the surface of
the body of water. It is interesting to see the desirably
abrupt rise and sharp peak of the seismic signal pulse 94
and to note that there is almost a complete absence of any
secondary signals. The negative-going pulse 96 which was
received by the hydrophone approximately 10 milliseconds
after the original signal 94 is the result of reflection
of the original signal pulse from the surface of the body
of water. Such a reflection occurs with a 180 phase shift,
thereby producing a negative-going pulse 96 as seen.
With only a low volume of make-up pressurized gas
needed for continued firing of the hydro gun, the bulky, lar
multi-stage high pressure air compressors necessary to
energize past air guns and water guns are eliminated.
Instead, because water is substantially incompressible, a
relatively small high pressure pump can provide the
relatively small volume of water 35 at high pressure to
recharge and energize the water guns at each firing.
It is to be noted that in FIGS. 1, 2 and 4, the
water in-feed line 33 is shown connected to the firing
chamber section 46 of the hydro gun 22 or 23 at a point which
is located generally near the central portion of the gun. It
is my presently preferred mode of practicing the invention
to connect the line 33 to the top of the hydro gun near the
make-up air line 38, because the line 33 is then farther
away from the discharge ports 80. Then,there is a second
drilled passage (not shown) similar to the passage 74. This
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second passage extends down from the line 33 to the inlet
port 37 where the high pressure water enters the liquid-
charge chamber 66.
Although a single, common high pressure
water line 32 is shown in FIG. 1, for recharging all of the
hydro guns, it is to be understood that an individual line
may extend from the vessel 20 to each hydro gun. In that
case a shut off valve may be associated with each of the
individual water feed lines for shutting off the supply to
any particular hydro gun in the array, if desired.
While the invention has been particularly shown
and described with reference to a preferred embodiment
thereof, it will be understood by those skilled in the art
that various changes in form and details may be made therein
without departing from the spirit and scope of the invention
as defined by the appended claims.
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