Multiple frequency range hydraulic actuator

A multiple frequency range hydraulic actuator includes a low frequency cylinder with a low frequency actuator piston within. The low frequency piston operates as a high frequency cylinder having a high frequency piston within. The low frequency piston, and high frequency piston operate in conjunction to generate low frequency movement. The high frequency piston operates independently to generate high frequency movement.

CROSS REFERENCE TO RELATED APPLICATION 
This application is related to the concurrently filed U.S. Pat. application 
Ser. No. 265,428 entitled "Compound Hydraulic Seismic Source Vibrator". 
BACKGROUND OF THE INVENTION AND RELATED ART 
The present invention relates to hydraulic actuators which may be used in 
seismic pulse generation and more particularly to hydraulic actuators 
which are capable of generating acoustic pulses having several different 
frequency ranges. 
In present seismic exploration, acoustic pulses are generated by seismic 
sources, propagate through the earths crust, are reflected by subsurface 
interfaces and detected upon the return to the surface. In marine 
exploration, seismic sources have taken the form of explosive charges and 
airguns. However, both of these types of seismic sources have had 
deleterious effects on marine life. As a result, a hydraulic vibrator had 
been developed. The hydraulic vibrator used in marine exploration is 
similar to that used in land based seismic exploration This type of 
seismic source has been found to have less deleterious effects on marine 
ecosystems. 
In seismic pulse generation, it is beneficial to be able to generate pulses 
over a wide frequency range. In this regard, the use of hydraulic 
vibrators includes a problem in the range of frequencies generated. In 
general, a hydraulic vibrator system includes a hydraulic power plant, a 
hydraulic cylinder, hydraulic circuitry and structural members designed to 
operate over a range of frequencies. Stroke and flow requirements for low 
frequency operation necessarily are exclusive of high frequency operation 
due to their size and mass. Similarly, stroke and flow design requirements 
concommitant with high frequency propagation exclude the applicability of 
these vibrator systems from use in low frequency systems. 
PRIOR ART 
An example of an early type hydraulic vibrator system is described in U.S. 
Pat. No. 3,392,369 titled "Fluid-Actuated Dual Piston Underwater Sound 
Generator" issued to J. A. Dickie et al. In the patent, two similarly 
sized sound radiating pistons are driven by hydraulic actuators in unison. 
The pistons are arranged as a pair of oppositely outwardly facing elements 
on opposite sides of the stationary housing and are sealed to the housing 
by flexible rubber gaskets. The actuator is adapted to move each piston in 
the direction opposite to that of the other at any particular time. As the 
pistons move out changing the external volume of the transducer, the 
internal space is filled with a gas under pressure. The apparatus 
described in this patent is designed to operate at low frequencies so that 
the sound waves which are generated under water have low attenuation. 
U.S. Pat. Nos. 3,329,930 and 3,394,775, both entitled "Marine Vibration 
Transducer" issued to J. R. Cole et al. also describe hydraulic seismic 
source generators. The 3,329,930 patent relates to a vibrational 
transducer that is driven at a controlled rate, two-part vibration by 
driving a piston vertically, reciprocally against the water medium. In 
this patent, a single piston is used in conjunction with a single 
actuator. The 3,394,775 patent, which is a continuation in part of the 
3,329,930 patent, introduces a vibrational transducer unit which consists 
of two pistons attached to a cylinder and a piston rod. A flexible rubber 
cylinder or boot is slipped over these two pistons and securely fastened 
to each so that air which is trapped between the pistons cannot escape 
into the water nor can water flow into the air chamber. The reciprocating 
piston imparts a pressure wave into the water while the innerhousing areas 
within the rubber enclosure are isolated and maintained at a predetermined 
air pressure such that maximum coupling of vibrational energy into the 
water medium is provided. 
U.S. Pat. No. 3,482,646 titled "Marine Vibrator Devices" issued to G. L. 
Brown et al. is a single piston, single actuator type of assembly similar 
to that of the 3,392,369 patent. A pair of shell-like housing members are 
disposed generally in parallel and are flexibly sealed between the 
respective outer peripheries to define an interior air space. A drive 
means is contained within the air space and connected to the respective 
housing members to impart reciprocal movement to one housing member with 
respect to the other. 
Additional hydraulic seismic source generating systems are described in 
U.S. Pat. No. 4,103,280, titled "Device for Emitting Acoustic Waves in a 
Liquid Medium" issued to Jacques Cholet et al., U.S. Pat. No. 4,211,301 
titled "Marine Seismic Transducer" issued to J. F. Mifsud, U.S. Pat. No. 
4,294,328 titled "Device for Emitting Acoustic Waves in a Liquid Medium by 
Implosion" issued to Jacques Cholet et al. and U.S. Pat. No. 4,578,784 
titled "Tunable Marine Seismic Source" issued to J. F. Mifsud. 
However, as stated previously, all of the foregoing hydraulic vibrator 
systems share a common problem. That is, none of the foregoing systems are 
capable of operating over a wide range of frequencies but in general, are 
limited to acoustic pulse generation in the low frequency range. 
SUMMARY OF THE INVENTION 
The present invention provides a dual mode hydraulic actuator capable of 
activating a radiating surface at a multiple inconsistent frequency 
ranges. A large low frequency piston having a large mass and a long strobe 
distance is located within a large outer cylinder to generate low 
frequency movement. A high frequency piston is located within the low 
frequency piston which acts as the high frequency cylinder. The high 
frequency piston has a small mass and a short stroke which generates high 
frequency movement. Thus the present invention is capable of generating 
two inconsistent frequency ranges, a low frequency range which requires 
large mass and long stroke distance, and a high frequency range which 
requires small mass and short stroke distance.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following description identifies an apparatus by which both low 
frequency and high frequency acoustic pulses may be generated in 
subsurface environments. A compound marine vibrator is described in which 
a woofer/tweeter type of arrangement is configured to permit generation of 
both low frequency and high frequency acoustic pulses. 
Referring now to FIG. 1, a first embodiment of the present invention is 
illustrated as hydraulic vibrator 8 which includes an upper housing 10, a 
low frequency radiating surface 12, and a high frequency radiating surface 
14. Low frequency radiating surface 12 is connected to upper housing 
portion 10 through flexible gasket 16 and low frequency piston rod 18. Low 
frequency piston rod 18 is connected to low frequency piston (not shown) 
within low frequency hydraulic cylinder 20. Low frequency hydraulic 
cylinder 20 is mounted to upper housing 10 on a cross piece 22 and on a 
cap 24 at its top ledge 26. Cap 24 is mounted on upper housing 10 at its 
central uppermost portion. Support brackets 28 are provided connecting 
upper housing 10 with cross pieces 22 to provide stability for low 
frequency hydraulic cylinder 20. Additional support brackets 30 are 
connected to a support disk 32 which is mounted on low frequency piston 
rod 18. Support brackets 30 are connected to low frequency radiating 
surface 12 to transmit the force generated through low frequency piston 
rod 18 directly to low frequency radiating surface 12. 
High frequency hydraulic cylinder 34 is mounted on support disk 32 which 
has additional support members 36 mounted to low frequency radiating 
surface 12. High frequency piston rod 38 is connected directly to high 
frequency radiating surface 14 through mounting disk 40. High frequency 
radiating surface 14 is connected to low frequency radiating surface 12 
through flexible gasket 42. 
In operation, when low frequency acoustic pulses are to be generated, 
hydraulic cylinder 20 is actuated which drives low frequency piston rod 
18. Movement of low frequency piston rod 18 forces low frequency radiating 
surface 12 along with support disk 32, high frequency hydraulic cylinder 
34, mounting disk 40, and high frequency radiating surface 14 to move in 
unison to generate low frequency acoustic pulses. During high frequency 
operation, low frequency piston rod 18 is maintained in a stable position, 
holding support disk 32 fixed. Accordingly, high frequency hydraulic 
cylinder 34 is also held fixed allowing high frequency piston rod 38 to 
move independent of support disk 32 and low frequency radiating surface 
12. In operation, high frequency piston rod 38 moves, moving mounting disk 
40 which is connected to high frequency radiating surface 14 causing the 
generation of high frequency acoustic pulses. Low frequency radiating 
surface 12, due to its connection to support disk 32, high frequency 
hydraulic cylinder 34, and high frequency radiating surface 14, is only 
capable of generating low frequency acoustic pulses due to the mass 
involved. On the other hand, high frequency radiating surface can be moved 
rapidly to generate high frequency acoustic pulses by the action of high 
frequency piston rod 38 due to its size and its construction for movement 
independent of the operation of low frequency radiating surface 12. 
In this regard, hydraulic vibrator 8 provides an acoustic pulse generator 
capable of generating both low frequency acoustic pulses and, because of 
its unique configuration, high frequency acoustic pulses also. 
Referring now to FIG. 2, a second embodiment of the present invention is 
illustrated having similar components identified with the same numerals as 
those in FIG. 1. In the embodiment of FIG. 2, two low frequency hydraulic 
cylinders 20A and 20B are illustrated as individually being connected to 
low frequency radiating surface 12. 
Cylinders 20A and 20B are mounted to upper housing 10 through brackets 44A 
and 44B, respectively. Brackets 44A and 44B are further supported by cross 
piece 46. In operation, low frequency hydraulic cylinders 20A and 20B are 
actuated simultaneously causing low frequency piston rods 18A and 18B to 
move low frequency radiating surface 12 in unison. Low frequency piston 
rods 18A and 18B are connected directly to low frequency radiating surface 
12 through mounting disks 46A and 46B. When low frequency hydraulic 
cylinders 20A and 20B are not actuated, they maintain the position of low 
frequency radiating surface 12 in a fixed position with respect to upper 
housing 10. Accordingly, high frequency radiating surface 14 may be moved 
by high frequency piston rod 38 through the actuation of high frequency 
hydraulic cylinder 34 independently of low frequency radiating surface 12. 
This is due to the fact that high frequency hydraulic cylinder 34 is 
mounted on support members 36 and support brackets 30, both of which are 
secured to low frequency radiating surface 12. 
As with the operation of the embodiment illustrated in FIG. 1, hydraulic 
vibrator 8A as illustrated in FIG. 2 may generate low frequency acoustic 
pulses through the operation of low frequency hydraulic cylinders 20A and 
20B in unison, forcing the motion of low frequency radiating surface 12, 
support brackets 30, support members 36, high frequency hydraulic cylinder 
34, high frequency radiating surface 14 and mounting disk 40. When high 
frequency acoustic pulses are desired, actuation of high frequency 
hydraulic cylinder 34 permits motion of high frequency radiating surface 
14 independent of low frequency radiating surface 12. 
In operation, for both the embodiments of FIG. 1 and FIG. 2, any movement 
of the low frequency hydraulic cylinder piston rod 18 in FIG. 1 or 18A and 
18B in FIG. 2 is transmitted directly to the low frequency radiating 
surface 12, the high frequency hydraulic cylinder 34 and the high 
frequency radiating surface 14 only. Any movement of the high frequency 
hydraulic cylinder piston rod 38 is transmitted directly to the high 
frequency radiating surface 14. Thus, the low frequency hydraulic system 
is optimized to drive the low frequency hydraulic cylinder over a range of 
frequencies from very low frequency, long stroke, up to intermediate 
frequencies. The high frequency hydraulic system is optimized to drive the 
high frequency hydraulic cylinder over a range of frequencies from 
intermediate frequencies up to very high frequencies. The stroke of the 
high frequency hydraulic cylinder is relatively short, to minimize the 
volume of hydraulic oil between a hydraulic servovalve and the face of the 
hydraulic cylinder ram. Also, the structural mass is very small so that 
the system can be driven at very high frequencies. The outer housing of 
low frequency hydraulic cylinder 20 and that of hydraulic cylinder 20A and 
20B in FIG. 2 is attached to the upper housing 10 of vibrators 8 and 8A, 
respectively, in order to minimize the mass that the low frequency 
cylinder is required to move. Also, the outer housing of high frequency 
hydraulic cylinder 34 rather than high frequency piston rod 38 is attached 
to low frequency radiating surface 12 in order to minimize the mass that 
the high frequency cylinder is required to move. 
The upper housing of the compound vibrator is relatively heavy as compared 
with the radiating surfaces in order to maximize the amount of energy that 
is radiated in a downward direction. Vibrators 8 and 8A are also larger in 
diameter than conventional marine vibrators so that a large amount of 
energy can be output at low frequencies. This, combined with superior high 
frequency performance, results in a fewer number of vibrators being 
required for a given total energy output, as compared with conventional 
marine vibrators. The number of power plants, amount of handling 
equipment, and number of persons required to operate the equipment can 
also be less. Compound marine vibrators 8 and 8A may be operated in 
numerous modes. For example, marine vibrators 8 and 8A may be operated by 
actuating only the low frequency hydraulic cylinder 20 or 20A and 20B. The 
vibrator thus functions as a conventional marine vibrator. High frequency 
radiating surface 14 would not move with respect to low frequency surface 
12. Thus, the frequency bandwidth would be limited to low to mid-range 
frequencies. 
A second mode in which marine vibrators 8 or 8A may be operated is one in 
which the vibrator could start a sweep at very low frequencies with the 
high frequency hydraulic cylinder 34 fixed or with the same sweep as the 
low frequency hydraulic cylinder 20. As intermediate frequencies are 
reached, the low frequency sweep system could be stopped and the high 
frequency system would continue to the desired level. 
A third mode in which the marine vibrator system may be operated is one in 
which the low frequency and high frequency systems could be actuated 
simultaneously with two different sweeps. For example, the low frequency 
system could sweep through a frequency range of 3 to 50 Hz at the same 
time that the high frequency system was sweeping with a frequency range of 
50 to 150 Hz. 
Finally, a fourth mode in which marine vibrators 8 or 8A could be operated 
is one in which the vibrator functions as a conventional marine vibrator 
by actuating only the high frequency system. 
Hydraulic and electrical control circuitry required to produce and control 
the vibrator sweeps are not illustrated since the actual controls are 
conventional and are considered to be standard for the industry and 
understood by one skilled in the art. 
Referring now to FIG. 3, a compound hydraulic actuator is illustrated. This 
actuator may be used in the embodiment of FIG. 1. Reference surfaces and 
similar portions of the actuator have been identified with the same 
numbers as they appear in FIG. 1. The top portion of a main cylinder 
housing 52 is attached to the upper housing 10 of a hydraulic vibrator or 
the like. Main cylinder housing 52 has a lip 54 which may be attached to 
upper housing 10 through the use of bolts 56 or by some other method known 
in the art such as welding, etc. Illustrated as a portion of main cylinder 
housing 52 is hydraulic servo control 58 with inlet/outlet passages 60 and 
62. Passages 60 and 62 feed to open areas 64 and 66, respectively, between 
main cylinder housing 52 and a low frequency piston actuator 68. Low 
frequency actuator piston 68 may be connected to low frequency radiating 
surface 12 or to any similar device which is to be actuated at low 
frequencies. Piston 68 is fixed to surface 12 by bracket 70 through bolts 
72. The top portion of low frequency actuator piston 68 includes high 
frequency servo control 74 which includes inlet outlet passages 76 and 78 
that feed open areas 80 and 82 located between low frequency actuator 
piston 68 and high frequency actuator piston 84. High frequency piston 84 
may be connected to a high frequency radiating surface 14 or the like by 
any method known in the art. 
In operation, low frequency actuator piston 68 and high frequency actuator 
piston 84 may be connected to any surface which requires vibrating or back 
forth motion of the frequencies these two pistons are designed to 
generate. For low frequency operation, servo control 58 forces hydraulic 
fluid through passage 60 into open area 64 to force low frequency actuator 
piston 68 towards its full downward position. When this has been 
accomplished, servo control 58 reverses, permitting fluid to exit open 
area 64 through passage 60 while forcing fluid into open area 66 through 
passage 62. Since the combined length of open areas 64 and 66 comprise a 
long stroke distance, piston 68 will move surfaces 12 and 14 at a low 
frequency rate. 
When actuation of surface 14 at a high frequency is desired, servo control 
74 is used to force fluid through passage 76 into open area 80 forcing 
high frequency actuator piston 84 to its fully outwardly extended 
position. Upon reaching its fully extended position, hydraulic fluid is 
then forced into open area 82 through passage 78 while hydraulic fluid 
occupying open area 80 is permitted to exit through passage 76. Thus, an 
in and out motion is provided through high frequency actuator piston 84 to 
vibrate surface 14 at a high frequency. The high frequency is accomplished 
through two aspects. First, high frequency actuator piston 84 together 
with surface 14 constitute a low mass. Second, the stroke length of high 
frequency actuator piston 84 is short, that is, the total length of open 
areas 80 and 82, when high frequency actuator piston 84 is centered as 
illustrated in FIG. 3 is relatively short when compared to the stroke 
length of low frequency piston 68. 
Thus, a single cylinder assembly may be used to generate both low 
frequencies such as 5 to 50 Hz and high frequencies such as 50 to 150 Hz 
through the use of the design of the present invention. 
Although the present invention was illustrated by way of a preferred 
embodiment, it is understood that the present invention should not be 
limited to the described embodiments but only limited by the following 
claim elements and their equivalents.