Seismic transducer unit for marshy terrains

The transducer unit includes a rigid hollow casing. The casing defines through its cylindrical wall a plurality of sound-transmitting windows. A hydrophone having an elastomer core is removably positioned in the bore of the casing opposite to the windows. A pressure transducer element is embedded inside the core. The core is slightly compressed so as to form plugs for the windows.

BACKGROUND OF THE INVENTION 
Seismic pressure transducers adapted for marshy terrains are known and 
widely used. One such pressure transducer unit comprises a hollow 
cylindrical perforated casing defining an inner cavity. An air-filled bag 
or bladder containing a pressure transducer element is mounted inside the 
cavity. The bag is made from a sound-transmitting material such as rubber. 
When the transducer unit is submerged in shallow water or marshy ground, 
the water fills the cavity through the perforations of the casing. Seismic 
pressure waves from the liquid are transmitted to the pressure transducer 
element through the wall of the bag and the air therein. 
Such known seismic transducer units have certain drawbacks chief among 
which are: the wall of the rubber bladder has to be relatively thin in 
order not to unduly attenuate the arriving seismic pressure waves. But, a 
thin-walled bladder is susceptible of becoming easily damaged by sharp 
objects piercing through the perforations of the casing. When the bladder 
becomes ruptured, the air therefrom escapes, water enters the inner volume 
of the bladder and damages the transducer element. Another serious problem 
with such prior art transducer units is that mud enters the cavity of the 
casing and forms a "cake" around the bladder, thereby preventing effective 
acoustic coupling between the external fluid medium and the transducer 
element. 
In U.S. Pat. No. 3,932,834, assigned to the same assignee, is shown an 
improved transducer unit which comprises a casing defining a cavity and 
having sound-transmitting windows for acoustically coupling the cavity 
with a fluid medium surrounding the casing. At least one pressure 
transducer element is mounted inside the cavity. A core completely fills 
the cavity and supports the transducer element in place. The core 
transmits the pressure wave energy from the fluid medium to the transducer 
element. 
This invention is an improvement over the transducer unit described in said 
patent. 
SUMMARY OF THE INVENTION 
This invention relates to a seismic transducer unit for converting acoustic 
energy in a fluid medium into electric energy. The transducer unit 
includes a rigid hollow casing having a conical end cap to facilitate 
positioning the casing in the ground. The casing defines through its 
cylindrical wall a plurality of soundtransmitting windows. A hydrophone 
having an elastomer core is removably positioned in the bore of the casing 
opposite to the windows. A pressure transducer element is embedded inside 
the core. The core is slightly compressed so as to form plugs for the 
windows.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In one embodiment, the transducer unit 10 comprised a hollow cylindrical 
casing 12 made of a rigid material which can be metal or plastic. Casing 
12 has a cylindrical bore 14 communicating with a plurality of angularly 
spaced openings or windows 16. Each window can be a cylindrical opening. 
Removably mounted inside bore 14 is a hydrophone 15 having an elastomer 
core 17 in which is embedded a pressure transducer element 18. The 
elastomer core 17 is preferably symmetrically positioned relative to a 
transverse plane passing through windows 16. Casing 12 has an upper end 
cap 20 defining a center bore 22 for receiving an electric cable 24 having 
at least a pair of electric conductors 26-26'. 
While in the embodiment shown it is desired to connect the output terminals 
27-27' from the hydrophone to the electric conductors 26-26' through a 
transformer 30, the transformer can be eliminated. 
An anchor 29, preferably of the type shown in U.S. Pat. No. 3,931,453, 
assigned to the same assignee, will prevent tension from becoming 
transmitted by cable 24 to hydrophone 15 or transformer 30. A water seal 
28 prevents water from entering through end cap 20. A threaded conical 
bottom end cap 32 is provided to allow the transducer unit 10 to more 
easily penetrate into marshy ground. 
Hydrophone 15 is made by totally encapsulating the pressure transducer 
element 18 in the elastomer core 17 which serves as a 
pressure-transmitting support for the transducer element 18 and as a plug 
16' to seal windows 16 against moisture penetration into bore 14. 
The mold which receives the encapsulating elastomer is provided with top 
and bottom circular grooves such that the encapsulant forms O-rings 19 
which improves the sealing function of the core. The elastomer material 
should be sufficiently flexible, relatively incompressible and have a 
hardness in the approximate range of 40 to 90 on the Shore A scale. It 
should have good thermal stability between -50.degree. F and +200.degree. 
F. Typical of sound-transmitting materials that might be employed are 
natural rubber, synthetic rubber, silicon rubber, urethanes, flexible 
epoxys, etc. Silicon rubber can be employed having a hardness of 45 on the 
Shore A Scale, a tensile strength of 400 psi., and an elongation of 180%. 
The material should be water-impervious to prevent water penetration into 
cavity 14. Silicon rubber can be purchased as a two-part liquid. Prior to 
pouring into the mold's cavity, the liquids are mixed together and the 
entrapped air is removed as by using vacuum techniques. Thereafter, the 
liquid mixture is cured using well-known methods. 
A preferred type material for the encapsulant is SILGAN type H-621 
manufactured by Stauffer Wacker Silicons with a tear strength of about 100 
lbs. per inch. The hardness of the material is about 60 on the Shore A 
scale. 
The transducer element 18 comprises a spacer ring 34 having two counter 
bores 35 at each end adapted to receive circular conductive substrate 
discs 36, 36' made of brass or beryllium copper. The discs serve as 
supports as well as electrodes for crystals 37, 37', respectively. The 
separation between the circular discs is on the order of 0.050 and 0.150 
mils so as to allow the discs to flex in response to impinging pressure 
waves P. The discs 36, 36' are connected in parallel by wire 41 and the 
crystals are connected in parallel by wire 43. Wire 40 connects the 
substrates 36, 36' to output terminal 27', and wire 42 connects the 
crystals 37, 37' to output terminal 27. 
It is desired for the core 17 to become slightly compressed in order to 
improve the coupling to the transducer element 18 and to form the radially 
and outwardly-extending plugs 16' which partially fill and completely seal 
the windows 16. This is accomplished by sandwiching the core 17 between 
top and bottom discs 46, 48 and exerting a compressive force between the 
discs with the threaded end cap 32. The convex, radially and outwardly 
projecting plugs 16' become exposed through the windows 16 for contact 
with the surrounding fluid medium. There is thus established a smooth 
profile to the outside of casing 12, and at the same time neither mud nor 
water can penetrate into bore 14. The diameter of core 17 is such that 
even if the core material were cut or scratched through the windows 16, 
the operation of the crystals would not be adversely impaired. The 
hydrophone would still continue to convert acoustic energy, transmitted 
from the surrounding medium to the crystals through the solid core 17, 
into electric energy. 
Although this invention has been described with reference to a presently 
preferred embodiment, it will be apparent to those skilled in the art that 
the advantages of this invention can be embodied in other structural 
forms. Among the advantages derived from the present invention are the 
following: the elastomer core 17 when slightly compressed between the 
discs 46, 48 greatly enhances the acoustic energy transmission qualities 
between the ambient fluid medium and the crystal assembly 18, with a 
negligible loss of signal level in the elastomer itself; the elastomer, 
being under compression, establishes good contact between the elastomer 
and the crystals; the elastomer when compressed provides good seals 16' 
against the windows 16 and the sealing function is adequate even without 
the O-rings 19 which provide redundant protection; the compression on the 
core can be adjustably controlled by the threaded end cap 32; and release 
of compression by unscrewing the end cap 32 makes it easy to remove from 
or install a hydrophone 15 in the casing 12. 
Accordingly, the transducer unit 10 of this invention is easily serviceable 
in the field since all the internal components are removable which is a 
considerable advantage over the transducer unit described in said U.S. 
Pat. No. 3,932,834. Also, the hydrophone 15 can be constructed from 
relatively inexpensive materials by relatively unskilled workers and the 
assembly of the entire transducer unit 10 can be made following relatively 
simple procedures. 
The tear strength of the elastomer is selected such that it can be removed 
from the mold without damage to the O-rings 19 and to resist splitting, 
cutting or tearing as the core is installed in the casing and subjected to 
normal abuse. 
The core is installed inside the casing and becomes compressed in a 
longitudinal direction. This causes the elastomer to expand in a radial 
direction forcing the O-rings 19 to establish a better seal against the 
inner wall of bore 14. In the event that the seal provided by plugs 16' 
around one or more of the windows 16 should fail, then the top and bottom 
O-rings 19 will prevent moisture from passing beyond the O-rings, causing 
damage to the electrical connections. 
It is often necessary to replace the transducer's leader cable 24 since in 
typical field use, the leader cable becomes cut and/or its insulation 
eaten up by rodents. To replace the leader's wires 26, 26', it is 
necessary to disassemble the parts of the transducer unit 10. In 
accordance with this invention, the hydrophone 15 can be easily removed 
from and inserted into the casing 12 without sacrifice to the transducer 
characteristics of the unit 10.