Apparatus for generating a substantially oscillation-free seismic signal for use in underwater petroleum exploration, including a bag with walls that are flexible but substantially inelastic, and a pressured gas supply for rapidly expanding the bag to its fully expanded condition is described. The inelasticity of the bag permits the application of high pressure gas to rapidly expand it to full size, without requiring a venting mechanism to decrease the pressure as the bag approaches a predetermined size to avoid breaking of the bag

Yang, L. C.

English

NASA Technical Reports Server

United States Patent [19] 
yaw 
[111 4,153,134 
[451 May 8,1979 
[54] UNDERWATER SEISMIC SOURCE 
[75] Inventor: 
[73] Assignee: 
E211 Appl. No.: 
[22] Filed: 
Lien C. Yang, La Canada, Calif. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
830,458 
Sep. 6, 1977 
[51] Int. C l . 2  ............................................... GOlV 1/38 
[52] U.S. Cl. ................................. 181/120; 340/12 R 
181/1 15 
[58] Field of Search ....................... 181/115, 118, 120; 
34/12 R; 175/1; 166/299 
[561 References Cited 
U.S. PATENT DOCUMENTS 
2,069,242 2/1937 Graham ............................. 34/12  R 
3,039,559 6/1962 Ellsworth .......................... 34/12 R 
3,176,787 4/1965 Roever ............................... 34/12 R 
3,444,953 5/1969 Cholet .................................. 181/115 
3,588,801 6/1971 Leonard ............................... 181/115 
3,620,327 11/1971 Savit ..................................... 181/118 
3,658,005 4/1972 Byrne ................................. 34/12 R 
3,740,708 6/1973 Phillips ................................. 181/120 
FOREIGN PATENT DOCUMENTS 
2300346 8/1976 France ..................................... 181/120 
Primary Examiner-Howard A. Birmiel 
Attorney, Agent, or Firm-Monte F. Mott; John R. 
Manning; Wilfred Grifka 
[571 ABSTRACT 
Apparatus for generating a substantially oscillation-free 
siesmic signal for use in underwater petroleum explora- 
tion, including a bag with walls that are flexible but 
substantially inelastic, and a pressured gas supply for 
rapidly expanding the bag to its fully expanded condi- 
tion. The inelasticity of the bag permits the application 
of high pressure gas to rapidly expand it to full size, 
without requiring a venting mechanism to decrease the 
pressure as the bag approaches a predetermined size to 
avoid breaking of the bag. 
5 Claims, 6 Drawing Figures 
A- 
Y l l -  
t -2  3 3 
PRESS. 
S O U R C E  
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U.S. Patent May 8, 1979 4,153,134 
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4,133,134 
1 2 
FIG. 3 is a view taken on the fine 3-3 of FIG. 1. 
UNDERWATER SEISMIC SOURCE FIG. 4 is a graph showing the type of siesmic signal 
which can be generated by the apparatus of FIG. 1. 
FIG. 5 is a plan view of the flexible bag of FIG. 1, but ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 5 showing it folded into another configuration. 
formance of work under a NASA contract and is sub- FIG. 6 is a perspective view of a frame which can be 
ject to the provisions of Section 305 of the National utilized with the apparatus of FIG. 1. 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 USC 2457). 
BACKGROUND OF THE INVENTION 
This invention relates to siesmic signal sources. 
The exploration for underwater petroleum deposits 
can involve siesmic mapping that is conducted by gen- 
erating an underwater shock wave or other siesmic 
signal, and recording the reflections of the accoustic 
wave. A clean high amplitude pulse which is free of 
oscillations that can mimic reflections of sound waves, 
is highly desirable in many situations. One way to pro- 
duce an underwater pulse is to rapidly expand a rubber 
enclosure by punping high pressure air therein. How- 
ever, rapid expansion requires a much larger pressure 
within the enclosure than in the surrounding water, and 
such pressure difference can cause bursting of the rub- 
ber enclosure and therefore prevent its reuse. Such 
bursting can be avoided by venting the pressured gas as 
the enclosure reaches its full expansion configuration. 
However, such venting is hard to control, and typically 
results in the enclosure rapidly decreasing in size which 
results in the generation of bubbles that won burst and 
produce subsidiary oscillations. A reusable underwater 
siesmic signal apparatus which could produce a rapid 
expansion of the container without rapid decreases in 
the container size, would be of considerable value in 
underwater siesmic mapping. 
SUMMARY OF THE INVENTION 
In accordance with one embodiment of the present 
invention, an apparatus for generating underwater sies- 
mic signals is provided which can produce relatively 
oscillation-free signals. The apparatus includes a flexible 
bag and means for rapidly supplying gas to the bag to 
expand it. The bag has walls that are flexible but sub- 
stantially inelastic, so that the bag will not expand more 
than a predetermined amount and therefore will not 
break in spite of a considerable pressure difference be- 
tween gas in the bag and water outside the bag. The bag 
walls can comprise an inner layer of rubber material for 
sealing in the gas, and an outer layer of a flexible but 
substantially inelastic material such as a metal or Kavlar 
cloth. The bag can have a substantially spherical shape 
when expanded to provide large strength, and the 
sphere can be folded into a cruciform cross-section 
when contracted. In order to further strengthen a bag 
against breakage when in its fully expanded condition, a 
frame can be positioned about the bag, which is substan- 
tially open and inexpandable to engage the bag when it 
is fully inflated so as to prevent further outward move- 
ment of areas of the bag. 
The novel features of the invention are set forth with 
particularity in the appended claims. The invention will 
be best understood from the following description when 
read in conjunction with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a side elevation view of an underwater 
siesmic source, showing it in use in a body of water. 
FIG. 2 is a view taken on the line 2-2 of FIG. 1. 
_ _  
DESCRIPTION OF THE PREFERRED 
EMBODIMENTS 
FIG. 1 illustrates a siesmic signal generating appara- 
tus 10 which includes a flexible bag 12 and a pressure 
source 14 which is connected through a coupling 16 to 
the bag to inflate it. The apparatus is coupled to a buoy 
15 18 by a cable 20 that supports the apparatus at a prede- 
termined depth within the body of water in which the 
bag lies. 
In order to create a pressure pulse in the water 22 
* surrounding the bag, the valve 16 is suddenly open to 
20 transfer gas from the pressure source 14 to the bag to 
rapidly expand the bag. The rapid expansion of the bag 
causes it to push water outwardly away from the center 
of the bag, to produce a pressure pulse, with a compo- 
nent thereof directed downwardly towards the earth 
25 structure which lies under the water. The walls of the 
bag 12 are constructed, as shown in FIG. 3, of an outer 
layer 24 of a flexible but substantially inelastic material, 
such as a cloth woven of metal strands or strands of 
Kavlar, which produces a container which is flexible 
u) but substantially inelastic. The walls also include an 
inner layer 26 of rubber or the like which is useful for 
sealing the bag against the loss of gas therefrom. Al- 
though the inner layer 26 may be elastic, the outer layer 
24 will prevent expansion of the bag past a predeter- 
The fact the the bag is substantially inelastic, as com- 
pared to one constructed only with ordinary rubber 
walls, means that the bag will not break when a high 
pressure gas is applied to expand the bag. The pressure 
40 applied to the bag to expand it, must be considerably 
greater than the pressure of the surrounding water 22, in 
order to cause rapid expansion of the bag. For example, 
a pressure differential of perhaps a few hundred pounds 
per square inch may be utilized between gas inside the 
45 bag and the pressure of the surrounding water, to create 
rapid bag expansion. If the bag were elastic, then such a 
pressure differential would cause continued expansion 
of the bag and breakage. To prevent such continued 
expansion, a venting system could be utilized to vent the 
50 gas as the bag reached a predetermined size, to prevent 
further bag expansion. However, such venting is difi- 
cult to accurately control, and may result in small but 
sudden decreases in the bag size that would produce 
bubbles in the surrounding water. The bubbles would 
55 soon collapse and produce secondary oscillations that 
result in the pressure pulse not being a clean single pulse 
without secondary oscillations. The present bag 12 is 
substantially inelastic so that when deployed it will not 
expand in diameter by more than a few percent for 
60 pressure differences on the order of 100 psi, so that a 
large pressure difference necessary to expand the bag 
will not cause breakage of the bag when it is fully ex- 
panded. 
The bag 12 is constructed so that it is substantially a 
65 sphere when fully expanded. The spherical shape ena- 
bles large amplitude pressure pulses to be produced, and 
also results in a bag of maximum strength for a given 
displacement and wall thickness. When not deployed, 
10 
35 mined size. 
4,153,134 
4 3 
the bag can be folded to the configuration shown in 
FIGS. 1 and 2, wherein the bag is folded in a cruciform 
shape with four arms. This can be accomplished in a 
variety of ways, as by utilizing elastic bands 28 within 
the bag that pull the folded portions together when the 
bag does not contain high pressure gas. Another folding 
technique illustrated in FIG. 5, produces six folds 30. 
There are three folds at opposite ends of a first line 31, 
but substantially no folds at opposite ends-of a perpen- 
dicular second line 33. The expansion of the bag of FIG. 1o 
5, produces large pressure pulses in directions 32, 35 
away from the centerline of the folded bag, with the 
pulse in the direction 35 passing towards the floor of the 
body of water. 
In a complete installation, a vacuum pump 34 can be 
provided to aid in evacuation of the bag as it is slowly 15 
collapsed. In a typical operation, control cables 36, 38 
leading from a ship 40 will extend down to the pressure 
source 14 to operate it and the valve 16, and also to the 
vacuum pump 34. Where compressed air or the like is 
utilized, it is also possible to pump air from a ship down 20 
through hoses to a reservoir to repeatedly fill it. 
Where the pressure source 14 supplies compressed air 
or the like to the bag to fill it, the air supplied to the bag 
will be cooler than the surrounding water, and there- 
fore after expansion of the bag, the air will slowly heat 25 
and expand to slightly increase the pressure in the bag. 
Of course, during this time some of the air may be 
vented. Where a solid propellant or other gas generat- 
ing apparatus is utilized, the gas in the bag may be hot- 
ter than the surrounding water, and therefore the gas 3o signal, comprising: 
will cool and decrease in pressure after the bag is ex- 
panded' the rate Of pressure decease would 
be relatively slow and would not ordinarily be a prob- 
Although particular embodiments of the invention 
have been described and illustrated herein, it is recog- 
nized that modifications and variations may readily 
occur to those skilled in the art, and consequently, it is 
intended that the claims be interpreted to cover such 
modifications and equivalents. 
I claim: 
1. A method for generating an underwater seismic 
signal, comprising: 
surrounding a folded flexible bag by a substantially 
inexpandable and open frame; 
rapidly inflating said bag and rapidly expanding it 
until it presses on said frame; and 
collapsing said bag at a rate much slower than the 
rate at which it was inflated; 
said step of expanding including moving opposite 
sides of said bag rapidly towards opposite sides of 
said frame, wherein the center point of said bag 
will undergo substantially no displacement as a 
result of said expansion. 
2- A method for generating an underwater seismic 
signal, comprising: 
collapsing a flexible but inelastic bag to a cruciform 
cross-section; 
rapidly inflating said c~Ciform&' collapsed bag 
with gas to its 
the bag has 
reached its fully expanded condition, and then 
venting the gas at a rate much lower than the rate 
of inflation. 
3. Apparatus for generating an underwater seismic 
a flexible bag; and 
means for rapidly supplying gas to said bag to expand 
condition; and 
gas from said bag Only 
it; lem. A DreSSuTe D d W  aS illustrated by the graph 42 Of said bag having walls that are flexible so that the bag FIG. 4 dan be prchuced by the expandable bag,with the 
pulse having a rapid rise time and a relatively slow 
decrease without appreciable oscillations. 
FIG. 6 illustrates a cage or frame 44 that can be uti- 
lized to surround the bag 12 so as to aid in preventing 
expansion past a predetermined maximum size so as to 
prevent bursting of the bag. The cage 44 can be con- 
structed of high strength wire of a material such as steel 
or a high strength plastic. The frame is primarily open, 
with the wire of the frame constituting a very small 
percentage of the total area of the sphere formed by the 
frame, so that water rapidly displaced by the rapidly 
expanding bag is not greatly retarded by the frame. It 
may be noted that such a frame can also be utilized with 
an elastic bag to prevent its expansion to more than a 
predetermined size. 
Thus, the invention provides an apparatus for gener- 
ating an underwater siesmic signal which is relatively 
oscillation-free. The apparatus can include a relatively 
inelastic bag and a pressure source for rapidly expand- 
ing the bag to its full size by applying gas to the bag 
which is at a higher pressure than that of the surround- 
ing water. In a typical application, a bag of maximum 
diameter of a few feet may be utilized which contains 
gas at a pressure of a few hundred psi above that of the 
surronding water. A spherical bag can be utilized to 
maximize the strength of the bag for a bag of given 
value. A substantially open and inexpandable frame can 
be disposed about the bag to engage the fully inflated 
bag so as to strengthen it against further expansion, 
while allowing a bag with thinner walls to be utilized so 
as to facilitate the collapse of the bag. Where only a 
single use of the bag is sufficient, a pyrotechnic device 
such as solid rocket propellant can be provided within 
the collapsed bag, together with an igniter, to create gas 
that quickly expands the bag. 
35 can be collapsed to a small volume, but substan- 
tially inelastic so that the bag will not expand to 
more than a predetermined size; 
said bag having a cruciform cross-section when col- 
lapsed, and said bag being substantially spherical 
when expanded to said predetermined size to effec- 
tively resist breakage when rapidly inflated. 
4. Apparatus for generating an underwater seismic 
a flexible bag which can be contracted to a small 
volume and then expanded to a predetermined size 
a substantially open cage surrounding said bag and of 
said predetermined size to engage the bag when it 
is expanded, and 
means for rapidly supplying a pressure medium to 
said bag to expand it; 
said bag when collapsed having opposite sides which 
can expand apart from one another, and said cage 
extending around a majority of the surface area of 
the expanded bag, to permit bag expansion in oppo- 
site directions without obstruction to the outflow 
of water by the expanding bag. 
5. A method for generating an underwater seismic 
maintaining a bag in a collapsed state wherein it has at 
least one fold at opposite ends of a first line but 
substantially no folds at opposite ends of a second 
line perpendicular to the first, each line passing 
through the center of the bag, 
rapidly inflating said bag with gas while allowing the 
bag portions at opposite sides along said second 
line to rapidly move apart wherein the center of 
said bag will undergo substantially no displacement 
as a result of said movement. 
40 
signal, comprising: 
45 . without breaking; 
50 
55 
signal, comprising: 
60 
65 
* * * * *  


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