Device for generating acoustic waves in a fluid by implosion

Device for generating acoustic waves by implosion in a fluid medium comprising fluid-operated means to move a piston, having one end face in contact with said fluid, from a retracted position in the enclosure to a spaced apart position of maximum extension towards the exterior of the enclosure or vice versa and intermittent locking means for maintaining the piston at its spaced apart position, said fluid-operated means being actuated by two sources of fluid at different pressures, one of which is said fluid medium and the other a source of lower pressure.

This invention relates to a device for generating acoustic waves in a fluid 
by implosion. It is particularly well adapted to generate seismic waves 
for underwater seismic prospection. 
It is already known to generate acoustic waves by implosion in a fluid by 
means of a device comprising a cylinder and a piston having a wall in 
permanent contact with the fluid. The piston is adapted to slide into the 
cylinder, while insulating the interior thereof from a liquid medium, from 
a position of maximum penetration into the cylinder (retracted position) 
to a position of maximum outward extension therefrom (spaced apart 
position). It also comprises a moving member adapted to intermittently 
contact the piston to displace it up to the spaced apart position. 
Pressure means are used to apply unequal pressures on the two opposite 
walls of the moving member. Moreover, intermittent locking means are used 
to lock the piston at its the spaced apart position. The pressure means 
consists of a hydraulic system comprising a high pressure oil source, a 
low pressure oil receptacle and an oil distributor for creating a pressure 
difference between the two sides of the moving member so as to make it 
slide in such a direction that it drives the piston towards the exterior 
of the cylinder and then in the opposite direction to retract it when the 
piston is maintained in its spaced apart position. Such a device is 
described in U.S. Pat. No. 3,997,022. The device requires the use of 
powerful motor means for generating a pressurized fluid at a pressure 
which is the higher as it is operated at a greater depth at which the 
hydrostatic pressure opposing the displacement of the piston towards its 
spaced apart position is the greater. The motor means are preferably 
located on a surface ship, the pressurized oil feeding pipes having 
possibly a very great length, particularly when the transmission device is 
towed while submerged at a relatively great distance from the ship. 
The device of the present invention makes it possible to avoid the use of 
motor means for generating a pressurized fluid and also the need of 
feeding pipes of great length. 
It is note-worthy in that the surface area of at least one of the two 
opposite walls of the movable member is greater than that of the piston 
wall in permanent contact with the fluid and in that the pressure means 
comprises at least two fluid sources at different pressures, the pressure 
of the first fluid source being that of the surrounding fluid and the 
pressure of the second source being lower than that of the first source. 
The intermittent locking means comprises a fluid control system for 
applying unequal pressures on valve means integral with the piston. The 
pressure means and the fluid control system comprise a first pressure 
source consisting either of the surrounding fluid or of an auxiliary fluid 
in pressure equilibrium therewith or the combination of both, and a second 
pressure source constituted by a reservoir wherein prevails a pressure 
lower than that of the surrounding fluid or the auxiliary fluid. 
With this arrangement, the pressure means may be located in the vicinity of 
the device, irrespective of its submersion depth or its distance from a 
towing ship.

The device comprises (FIGS. 1, 2 and 3) an enclosure 1 provided with a 
first chamber or recess 2. A plate 3, provided with an opening 4, is fixed 
to the wall of the enclosure. Preferably, the plate 3 is screwed into the 
wall of the enclosure and may thus be adjustably positioned. Through the 
opening 4 of plate 3 may slide, substantially tightly, a piston 5, also 
permanently guided through an opening of an internal partition 6 of 
enclosure 1. Piston 5 comprises a terminal wall 7 subjected to the 
pressure of the external medium. A second end wall 8, opposite to wall 7, 
comprises preferably a recess which is filled with water, at least 
partially. 
The enclosure also comprises a second partition 9, more remote than the 
first partition 6 from the plate 3, and provided with an opening 10 
through which may slide, substantially tightly, a protruding element 11 of 
a moving member or push-member 12. The latter is provided with a head 13 
having a diameter larger than that of the protruding element 11 and a 
surface area larger than the surface area of the wall 7 of the piston in 
contact with the external medium. The head 13 of the push-member may slide 
substantially tightly in a second cylindrical chamber 19 delimited by the 
external wall of enclosure 1 and by the internal partition 9. The 
push-member comprises an inner cavity 14. A channel 15 is provided through 
the wall of the protruding element 11 of the push-member and establishes 
communication between cavity 14 and the chamber part 19 between the head 
13 and the partition 9. An opening 16 is also provided through the head 13 
of the push-member. A tube 17, integral with the terminal wall 45 of the 
enclosure slides tightly in the opening 16 and opens into cavity 14 of the 
protruding element 11. 
An opening 18 is also provided through the external wall 45 of chamber 19. 
The piston part internal to the first chamber or cavity 2 is integral, at 
its periphery, with a valve 20 comprising a first contact wall 21 facing 
plate 3 and a second wall 46 opposite to the first one, its surface area 
being larger than that of the end wall 7 of the piston which is 
permanently in contact with the external medium. The plate 3 comprises an 
annular seat 22 of a diameter smaller than that of the external edge of 
valve 20 and delimiting an internal recess 23. A hole 24 is perforated 
through plate 3 and opens into the inner recess 23. An opening 25 is also 
arranged in the external wall of cavity 2 facing the wall 46 of valve 20. 
In addition, a channel 26 passes through the wall of the enclosure and 
opens inside a third chamber or cavity 31 delimited by the partitions 6 
and 9, the protruding element 11 of the push-member and the terminal wall 
8 of the piston. The sizes of the different parts of the device are such 
that the displacement of the push-member is at least equal to the maximum 
stroke of the piston, irrespective of the selected position of plate 3 
with respect to the wall of the first chamber. On the other hand, the 
push-member is so arranged that, on at least one portion of its stroke, it 
comes in abutment against the wall 8 of the piston and drives it in 
translation to its spaced apart position. 
The lateral wall of the first chamber 2 may comprise, towards its end 
opposite to plate 3, a narrow part 47 producing a pressure drop and a 
laminating of the fluid. The piston is thus progressively braked in the 
vicinity of its retracted position. 
The pressure means used to actuate the push-member comprises two lines 27 
and 28 connected at one end thereof respectively to the outlets of a 
distributor D1 and at their second end respectively to the inlet of tube 
17 and to the opening 18. An inlet of distributor D1 is open to the 
external medium. The other is connected through a line 29 to a closed 
reservoir 30, isolated from the external medium, where prevails a pressure 
lower than that of the external medium. Cavity 31 communicates with 
reservoir 30 through a draining system comprising a pipe 44 connected, on 
the one hand, to channel 26 and, on the other hand, to line 29 through a 
check valve 32. The excess of liquid contained in cavity 31, which might 
infiltrate from the vicinal cavity of the enclosure, may thus be drained 
towards the low pressure reservoir 30. 
The distributor D1 is for example a two-way slide distributor. In position 
I, it establishes communication of lines 27 and 28 respectively with the 
external fluid and with reservoir 30. In position II, conversely, it 
establishes communication of channels 27 and 28 respectively with 
reservoir 30 and with the external medium. 
The locking means of piston 5 comprises a fluid control system. The latter 
comprises two pipes 33 and 34 connected, through a first end thereof, 
respectively with openings 24 and 25 on both sides of the valve 20 and, 
through a second end, respectively to the two outputs of a second two-way 
distributor D2. A first input of distributor D2 is open to the external 
medium. A second input communicates, through a pipe 35, with the low 
pressure reservoir 30. Distributor D2 is, for example, identical to 
distributor D1. In a first position (I), it establishes communication of 
pipes 33 and 34 respectively with reservoir 30 and with the external 
medium. In a second position (II) it reverses the direction of the 
connections and establishes communication of pipes 33 and 34 respectively 
with the external medium and with the low pressure reservoir 30. 
Pipes 29 and 35 open into the low pressure reservoir 30 and discharge into 
a vessel 36 the external fluid conveyed through distributors D1 and D2. A 
pump 37 sucks the fluid at the lower pressure contained in vessel 36 and 
discharges it to the external medium. 
In the recess of wall 8 there can be arranged a cushion (not shown) made of 
elastic material whose damping effect will be added to that of the water 
layer contained therein. 
The device is operated in the following manner: 
The piston 5 and the push-member being both in retracted position (FIG. 3), 
distributor D1 is operated to position II. There is thus created a 
pressure difference between the two opposite faces of the push-member in 
such a direction that its protruding element 11 comes in abutment against 
piston 5 and drives it in translation, up to a position where the face 21 
of valve 20 abuts against the seat 22 and where piston 5 is at its spaced 
apart position (FIG. 1). The distributor D2 is then actuated to position 
I. The pressure difference between the two opposite walls of the valve has 
the effect of maintaining piston 5 at its spaced apart position. 
The distributor D1 is again actuated and placed to position I, which has 
the effect of establishing between the opposite faces of the push-member a 
pressure difference which tends to drive it away from piston 5, always in 
locking position, towards its retracted position (FIG. 2). The device is 
ready for being triggered. A direction reversal of distributor D2 
(position II) has the effect of releasing the piston which abruptly 
penetrates the cylinder by the effect of the pressure difference between 
the pressure of the external medium and the lower pressure prevailing in 
chamber 31 (FIG. 3). At the end of the stroke, the piston 5 is braked by 
the water layer contained in the recess of surface 8 and/or by the layer 
of elastic material arranged therein. 
In the embodiment of FIG. 4 the high pressure fluid source no longer 
consists directly of the surrounding medium but of a deformable reservoir 
38 subjected to the pressure of the surrounding medium and containing an 
auxiliary fluid, oil for example. This fluid source of higher pressure is 
connected to distributors D1 and D2 through pipes 39 and 40. 
Pump 37, arranged in reservoir 30, discharges the fluid at lower pressure 
from vessel 36 to the deformable reservoir 38 through pipe 41. 
In the embodiment of FIG. 5, the pressure means and the fluid control 
system are mixed. The inlets of distributor D1 communicate respectively 
with reservoir 30 through pipe 29 and with the external medium. On the 
contrary, the inlets of the distributor D2 communicate respectively with a 
deformable reservoir 38 containing a fluid in pressure equilibrium with 
the external medium and with a low pressure reservoir 42. The latter also 
comprises a pump 43 to discharge the low pressure auxiliary fluid issued 
from distributor D2 towards the deformable reservoir at higher pressure 
through pipe 48. 
The operation of the device according to the embodiments of FIGS. 4 and 5 
is identical to that described above since the only change consists in the 
nature of the motive fluid and/or the auxiliary fluid used for locking the 
valve. 
The device may be operated alone or in combination with other identical 
devices. The carrying vehicle may for example be towed, when immersed, 
from a surface ship. Reservoirs 30, 38 and 42 depending on the case, will 
then be incorporated into the vehicle.