Note: Descriptions are shown in the official language in which they were submitted.
This application is directed to a seismic energy source and di-
visional applications S.N. 381,210, S.N, 381,211, S.N. 381,212,
S.N. 3al,213, S.N. 381,214, S.N. 381,215, all filed 6 July,
1981 are also directed to seisl~c energy sources.
This invention relates to improvements in seismic energy sourc~s,
and in particular, to a broad band vibratory seismic energy source.
In the practice of expoloration seismoloqy ~or the location of
subsurface petroleum accumulations-~ it is necessary to provide a source
of energy for inducing propagating elastic waves in the area of the earth
to be explored. These elastic waves propagate down into the upper crust-
al material of the earth, are reflected from impedance discontinuities
located therein, and are subse~uently detected by geophones or seismom
eters located at the surface of the earth. The records produced by the
geophones or seismometers contain much valuable information about the
crustal structure of the earth and may be used to ascertain the existence
of petroleum accumulations, It has become common in many cases to use,
as the source of propagating elastic waves, a hydraulically operated
vibratory source more simply referred to as a vibrator.
In a typical embodiment, a vibrator comprises a double ended
piston rigidly affixed to a coaxial piston rod. The piston is located in
reciprocating relationship in a cylinder formed within a heavy reaction
mass. Means are included for alternately introducing hydraulic fluid
under high pressure to opposite ends of the cylinder, thereby imparting a
reciprocating motion to the piston relative to the reaction mass. The
piston rod extending from the reaction mass is rigidly coupled to a base-
plate which is maintained in intimate contact with -the earth material.
Since the inertia of the reaction mass tends to resist displacement of
the reaction mass rela-tive to the earth, the motion of the piston is
coupled through the piston rod and baseplate to impart vibratory seismic
3a energy în the earth.
Typically, the vibrators are transported by truck, and it is also
known to prevent decoupling of the baseplate from the ground by applyinga
~"
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"~
1 portion of the truck's ~eight to the baseplate during operation, The
weight of the truck is frequently applied`to the ~aseplate t~rough one or
more spring members, each having a large compliance, with the result that
a static bias force is imposed on the baseplate, while the dynamic forces
of the baseplate are decoupled from the truck itself.
Conventional vibrators are capable of effective operation over a
relatively small range of low frequencies 3 typically 5 to 70 hertz. In the
past, these relatively low frequency vibrators have proven to be useful
seismic energy sources. As existing oil reserves become depleted, however,
1~ it becomes necessary to search deeper and with increased resolution ~o
locate additional reserves. A broad band vibra~or (BBV) capable of opera-
tion over a band of frequencies wider than those previously achievable with
known vibrators is useful in providing greater resolution and meaningful
interpretation at greater depth. In order to operate the vibrator so as to
provide significant output force levels at high frequencies, it is
necessary to minimize the weight of the baseplate and of other structural
elements rigidly affixed to the baseplate. In this way, the inertial force
which must be overcome solely to move the baseplate weight is minimized.
Further, it is necessary to provide sufficient force acting on the piston
to overcome the inertial force of the baseplate structure and still induce
significant energy in the earth.
The present invention provides various improvements in a BBV,
In one aspect the invention provides a seismic energy source comprising in
combination: a) a reaction mass having located therein a cylindrical
aperture~ b) an actuator rod comprlsing a piston and plston rod, the piston
being locat:ed ln the cylindrLcal aperture, and c) means for introducing a
fluid under high pressure Lnto the cylindrical aperture to exert a force
on the piston, d) the actuator rod having an internal bore with a diameter
that varies as a func~ion of position along the rod.
In a second aspect the invention provides a vibratory seismic
energy source having a double ended piston reciprocably mounted in the
cylinder of a reaction mass and a piston rod extending from ~pposite ends
_ 3 _
Z59~4
1 o~ the piston to project from the reacti.o~ mass and further comprisin~ a
pair of relatively soft bushings lining the bore of the reaction mass to
provide mechanical support for the portions of the piston rod located within
the reaction mass and for portions of the piston, the travel of the piston
within the cylinder being limited so that each end of the piston remains
within one of the bushings at all times.
In another aspect the invention provides a vibratory seismic energy
source having a piston reciprocably mounte~ within the cylinder of a
reacLion mass and at least one port in the wall of the cylinder for
admitting high pressure ~luid to exert a force acting on the piston, the
piston being disposed in the cylinder such that when a piston over travel
condition occurs, the plston substantially restricts the flow of fluid
through the at least one port thereby trapping a volume of fluid between the
face of the pistorl and the cylinder, whereby a braking force is generated.
In still another aspect the invention provides a seismic energy
source comprising in combination: a) force generating means for providing
a force to be imparted to the earth and b) an aluminum baseplate for coupling
the force to the earth, the baseplate defining a grid of internal cavities,
each cavity enclosed by four plane surfaces parallel to the general plane oE
the baseplate and by two plane surfaces parallel to the general plane of the
baseplate intersecting the four perpendicular planes at the top and bottom
thereof respectively.
In another aspect the invention provides a vibratory seismic
energy source comprising: a) a reaction mass having a hydraulic cylinder
~ormed there:ln, b) an actuator rod includin~ a piston reciprocably locatcd
within the hydraulic cylinder, c) a baseplate, d) a frame me~ber coupled to
an end of the actuator rod, an(l e) a plurali.ty o~ stilt legs e~tending ~rom
the frame member to the baseplate.
-3a-
Sg~
A further aspect of the in~ention is a vibratory seismic energy
source transported by a vehicle and including a baseplate for coupling
seismic energy to the ground, at least one structural member adapted to
support at least a portion of the weight of the vehicle thereon, means for
coupling substantially vertical forces fro~ the structural member to the
baseplate9 and stabilizing means for limitlng translation of the structural
member relative to the baseplate. The stabili~ing means may comprise a
-3b-
'i '
~lZ59~
1 center link pîvotally coupled to the b~seplate, a first stabilizing rod
rotatably coupled at one end to a point on the center link above the level
of its pivot and at the other end to a first point of the structural member,
and a second stabilizing rod rotatably coupled at one end to a point on the
center link below the level of its pivot and at the other end to a second
point of the structural member. Alternatively the center link can be
pivotally coupled to the structural member and the stabilizing rods to
the baseplate.
Finally, there is another aspect of the invention which is a
vibratory seismic energy source adapted to be transported on a vehicle,
including a vertically disposed piston and cylinder assembly, and a piston
rod extending downward from the piston and connected at its lower end to
a baseplate, a rigid plate interposed between the piston and cylinder
assembly and the baseplate, and adapted to perrnit motion of the piston rod
in relation to the rigid plate, elastic means supporting the rigid plate
from the baseplate and means for imposing at least a portion of the weight
of the vehicle on the rigid plate.
Preferred characterlstics of the seismic energy source will be
discussed in the following description:
The ground contacting surface of the baseplate in the preferred
embodiment has an area substantially less than that of conventional vibrators
in the 20,000 to 30,000 pound force range and has a square shape in contrast
with the typical rectangular shape of other vibrators. The baseplate area
of the BBV is less than that of existing vibrators, even though the peak
actuator force of the latter may be less than half that of the BBV.
The plston rod of the BBV is coaxial with the major axis of the
piston, and extends from both sides thereof. The lower segment of the
piston rod is rigidly coupled to the center of the baseplate, while the
upper segrnent of the piston rod is coupled through a structure consisting
of four st:ilts to the four corners of the baseplate. Thus, the baseplate
is supported both at its center and at each of its four corners. This
type of support, coupled with the unique two dimensional I-beam structure
-- 4 --
:,
- 1~l2~ 4
1 of the aluminum baseplate, provides a very light baseplate structure
capable of withstanding the high force levels generatred by the actuator
of the BBV.
Most known vibrators are carried by a vehicle having a sol~rce of
motor power located near the front end of the vehicle and a drive line
extending therefrom to the rear end of the vehicle so as to engage a
differential assembly and ultimately to power the rear wheels of the
vehicle. Typically, the drive line extends through a portion of the
vibrator itself, and it is necessary to configure the vibrator such that
there is sufficient clearance for the drive line, whether the vibrator be
in its raised or lowered position. In the case of the BBV, the drive line
is eliminated and in the absence of a need to provide clearance for a drive
line, the vibrator dimensions, particularly its vertical extent, can be
substantially reduced. With the reduced dimensions, the stresses imposed
on certain structural members, particularly the stilt legs, are less severe
and ie becomes possible to make these members lighter.
- 4a -
~L~259~)~
1 In view of the reduced dimensions of the baseplate, previously
2 known means for imposing the weight of the vehicle on the baseplate itself
3 become impractical. Accordingly, in an embodiment of the invention, a
4 unique one piece hold-down plate is provided to couple the weight of the
vehicle through air bags to the baseplate of the vibrator. Further in this
6 connection, and again in view of the reduced baseplate dimensions, the use
7 of radius rods as a means for providing lateral stability between the hold-
8 down plate and the baseplate becomes difficult. In an embodiment of the
9 invention, a system of linkages sometimes referred to as a Watt's linkage
and adapted to the reduced baseplate dimensions, provides excellent lateral
11 stability.
12 In the BBV, the reaction mass is a multiple piece structure
13 wherein the plurality of subassemblies are bolted together so as to provide
14 a unitary reaction mass. This economical means of fabrication results in
a reaction mass which provldes the necessary hlgh mass within the confines
16 of the stilt legs themselves. The stilt legs slant inward as they e~tend
17 upward from the baseplate, so as to provide effective resistance to hori-
18 zontal stress, as well as to the vertical stresses generated by the vibra-
19 tor itself.
The actuator piston and piston rod assembly of the BBV is pro-
21 vided with a hollow tapered bore, designed to result ~n constant stress as
22 a function of length along the rod and to minimize the mass of the piston
23 and rod assembly itself. ~urther, the piston and piston rod assembly
2l~ cooperates with the cylinder of the reaction mass 90 as to provide an
internal braking mechanism for limiting piston over-travel.
26 Other ob~ects and features of the invention will become obvious
27 from a consideration of the following detailed description when taken in
28 connection with the accompanying drawings wherein:
29 FIGURE 1 is a perspective view of a BBV.
FIGURE 2 is a cross sectional schematic view of por~ions of the
31 vibrator.
--5--
: ,
~ZS9~ `
1 ~IGURE 3 is a perspective view (partlally cut away) of a base-
2 plate.
3 FIGURE 4 illustrates the vibrator mounted in a truck.
4 FIGURE 5 is a perspective view illustrating stabilizing means for
a vibrator.
6 FIGURE 6 illustrates an alternative configuration for the sta-
7 bilizing means.
8 FIGURE 7 is a partial sectional view of the cylinder of a prior
9 art vibrator and of the BBV.
FIGURE 8 is a sectional view of a vibrator actuator rod.
i. , .
11 FIGURE 1 is a perspective view of a vibrator, portions of which
12 are shown in cross sectlon in FIGURE Z. Referring to FIGUR~S 1 and 2, a
13 baseplate 17 is driven by a hydraulic drive mechanism comprising a driving
14 piston 103 reciprocably mounted within a cylindrical bore 101, and a piston
rod 104 carrying piston 103 and extending from both the top and bottom of
16 the cylinder housing lOla. The lower end of piston rod 104 is rigidly
17 affixed to the center of the baseplate and the top is rigidly affixed to
18 the center of frame member 106. Stilt legs 108a, 108b, 108c, and 108d (the
19 latter not visible in FIGURE 1) extend in slanting relationship from the
baseplate to join the corners of frame member 106 to the corners of the
21 baseplate,
22 It i6 an ob~ect in constructing the BBV to reduce the mass moving
23 with the ground while the vibrator is generating a seismic wave. This
24 "baseplate weight" consists of the actual baseplate which couples the
actuator force to the ground and all components, structures, and members
i 26 rigidly attached to the baseplate. A minimum baseplate weight is of
27 paramount importance in high frequency operation. With a fixed peak
28 actuator force available, the only force which may be coupled into the
29 ground is the actuator force minus the force used to move the baseplate
weight. The less force required to move the baseplate, the more there is
~ 31 available to be imparted to the ground.
:
,
.
~5~
1 The equa~ion for the force magnitude required to move a mass at a
2 specific frequency, assuming sinusoidal motion, is:
3 F=~W/g)A(2 ~f) sin (2 ~ft)
4 where:
F= force,
6 ~= "baseplate weight",
7 A= mass displatement (1/2 of peak-to-peak),
8 g= acceleration of gravity,
9 5 driving frequency, and
t= time.
11 With a displacement of A, the force required increases with
12 frequency as a function of the frequency squared. This illustrates the
13 need for a min~mum baseplate weight W for high frequency operation.
14 To achieve the ob~ective of minimum baseplate weight while re-
taining structural rigidity, the ground coupling plate (baseplate) is
16 constructed from two aluminum sections, 17a and 17b, the internal surfaces
17 of which have a honeycomb appearance. One surface of each of the plate
18 sections may be milled out as lllustrated in FIGURE 3. In a preferred
19 embodiment, the baseplate may be square, having dimensions of four feet on
each side. Each of the baseplate sections may be approxlmately 3-1/2"
21 thick. As shown in FIGURE 3, the internal sides of both sections of the
22 baseplate are milled out in a 16 x 16 grid, of 3" each. Individual ~ection~
23 may be milled ou~ to a depth af 2.85 inches, with .25 inches left between
2~ each milled out section. It should be empha~ized that the dimensions can
vary, depending on the force and rigidity requirements of a particular
26 vibratory structure.
27 Certain portions of the baseplate sections, such as those desig-
28 nated by numeral 29, are left unmilled in order to provide locations for
29 bolting the two ~ections together. In the top portion of the baseplate,
holes, such as those designated by numeral 31 are drilled, and bolts, such
59~
1 as bolt 32, are inserted through ~hese holes to secure the two sections of
2 the baseplate together. Corresponding holesj such as those designated by
3 numeral 33, are drilled into the bottom section of the baseplate. Screw
4 threaded receptacle means are inserted in the bottom section of the base-
plate to receive the bolts inserted through the holes in the top section.
6 In general, the holes utilized for securing the two sections of the base-
7 plate together are positioned so that the connecting bolts also affix
8 additional vibrator structure to the baseplate.
9 It is known in the seismic vibrator art to fabrlcate the gene-
10 rally rectangular ba~eplate of a plurality of parallel steel l-beams. The ~ -~
11 longitudinal axes of the I-beams are located parallel to the maJor axis of
12 the rectangle and adJacent I-beams are oriented so the edges of their upper ~ ~
13 and lower flanges are abutting (the webs of the I-beams lie in vertical ~ ;
14 planes). The I-beam flanges are welded together so as to provide a unitary
structure which may be further reinforced by top and bottom reinforcing
16 plates. It will be appreciated that this type of structure provides great
:
17 resistance to stress exerted along the ma~or axis of the rectangle. While
18 the resistance to stress may be expected to be less along the minor axis of
19 the rectangle, the use of steel I-beams resulted in a baseplate with suffi-
cient stress tolerance for the force levels employed in prior art vibra-
21 tors. ~ ;
22 In the case of the BBV, the use of a steel baseplate structure
23 would impose a severe weight penalty on the vibrating mechanism. Alterna-
24 tively, an aluminum baseplate structure comprlsed of a plurality of one-
dimensional I-beams might not have sufficient stress resistance for the
26 forces generated by the BBV. It will be appreciated that the two-piece
27 aluminum baseplate structure of the present invention, when bolted together
28 as set forth above, comprises a unique two-dimensional I-beam structure.
29 As a result of this unique structure, the baseplate may be maintained
w-thin acceptable weight limitations and yet be capable of withstanding the
~" '
:. .
, ~ .
i~ZS~Q~
1 large forces generated by the BBV. In this description and in the accom-
2 panying claims the term "two-dimensional I-beam structure" is considered to
3 include structure such as that illustrated in FIGURE 3 as well as similar
4 structures such as one having a plurality of I-beams radiating from the
baseplate center. Further, in the structure of FIGURE 3, it is not neces-
6 sary that the I-beam webs be coplanar from cell to cell of the structure.
7 The center of the baseplate is rigidly connected to the lower end
8 of piston rod 104. Piston rod 104 may have an annular flanged portion at
9 the lower end with a plurality of holes therein. The same bolts that function
to secure the center portions of the baseplate sections together affix the
11 piston rod to the baseplate.
12 As stated earlier, the top of piston rod 104 is connected to the
13 center of frame member 106. Connected at each corner of this frame member
14 is an inclined stilt leg which is fixedly connected to a corner of the
baseplate. When vibrations are induced by controlled hydraulic -Eluid flow
16 into and from cylinder 101, motion generated in the piston is transmitted
17 to the center and to each of the four corners of the baseplate. This
18 configuration is a very rigid vibratory structure which praduces a uniform
19 movement of the entire baseplate closely corresponding to the motion of the piston~
21 The stil~ legs 108 are subjected to a complex form of loading.
22 The loading consists of vertical and horizontal forces. The vertical load-
23 ing is due to the simple vibratory operational mode of the piston 103. The24 piston rod 104 is o adequate strength to withstand the vertical loading.
The horizontal loading is due to ground rocking and ground
26 resonances. Because the rigidity of the stilt legs 108, when loaded hori-
27 zontally, is many times greater than the rigidity of piston rod 104 as a
28 cantilever, onl~ the stilt legs provide significant resistance to hori-
29 zontal loading. In doing so, the horizontal loading appears primarily as
tension or compression in the stilt legs 108. The stilt legs provide
_g _
;
1 greater strength when loaded in elther compression or ~ension than when
2 sub~ected to bending forces. The use of inclined stilt legs is superior
3 because of the efficient structural use of its members. The efficiency of
4 the inclined stilt legs permits the stil~ leg dimensions to be reduced
below those which would be necessary if previously known stilt leg con-
6 figuration (i.e., vertical legs) were employed. It will be seen, there-
7 fore, that the unique stilt leg configuration further contributes to mini-
8 mization of the base*late weight. The baseplate weight is reduced even
9 further by the elimination of the lower cross member, commonly used in
previous vibrators.
11 For the purpose of imparting vibratory movement to the piston
12 103, there is provided a manifold and servo valve member 19. The servo
13 valve follows the electrical control signal fed to it through conducting
14 leads l9a. Hydrauli actua~ing power for the servo valve is supplied
through line 43 from an ex~ernal hydraulic power source, including pump 42
16 and reservoir 48 (see FIGURE 2~. The servo valve controls the flow of
17 hydraulic fluid into and from cylinder 101 through port means 64 and 66
1~ within the walls of cylinder housing lOla above and below piston 103 to
19 generate piston motion corresponding to the electrical input control sig-
nal. Hydraulic supply mechanisms are well known ln the art and need not be
21 discussed in greater detail here. One such mechanism is disclosed in U. S.
22 Patent No. 3,929,206.
23 The hydraulic vibra~or generates a force against the ground by
24 pushi~g against a reaction ma~s conlprising the mass oE cylinder housing
lOla, plus additional mass affi~ed to the cylinder housing. This addition-
26 al mass includes manifold and servo valve member 19, which in the preferred27 embodiment is mounted on the forward portion of the vibrator. The addi-
28 tional mass further includes rear balance weight lOlb and side weights lOlc29 and lOld. Thus, it will be seen that the reaction mass is a "multi-piece
bolt together" mass consisting of a center section lOla which houses the
--10--
;
1 actuator, two side weights lOlc and lOld, ~ rear balance weight lOlb, and
2 the manifold and servo valve member 19. The mass pleces are shaped so that
3 the stilt legs pass through but do not touch the mass shapes.
4 The weight of the hydraulic cylinder housing and mass attached
thereto is decoupled from the baseplate mass by air spring 37 which is
attached to the top of frame member 106. Two vertically extending members,
7 130a and 130b, connect the top of the reaction mass to a frame member 134
~ affixed to the top of the air spring 37. Air spring 37 is an isolation
g spring and also sets the average position of the hydraulic pis~on I03 in
the center of the cylinder to ensure more linear operation of the hydraulic
ll vibrator. Air spring 37 is iDflated to a desired pressure through a con-
12 ventlonal fill valve shown as valve 140. Two arcuate sections 136a and 136b
13 tthe latter not shown) are affixed to the bottom of fra~e member 134 on
14 opposite sides thereof) and a layer of elastic material 138 is attached to
the lower edges thereof. This elastic layer serves as a buffer when the
16 up-stroke of the hydraulic piston is too large. A comparable structure
17 (not shown) functions as a buffer when the down-stroke is too large.
18 As is common practice in the art, the body of the vibrator is
l9 located between the frame members of the truck. In the usual design of
hydraulic vibrators, the baseplate extends outwardly from the vibrator, and
21 vertical gulde rods and hydraulic lift cylinders extend from the truck
22 frame to a "footpiece" above outwardly extending portions of the baseplate.
23 The weight of the vibrator transport truck is applied to the baseplate
2~ through the "footpiece" and spring isolation means to assist ln holding the
baseplate on the ground. In the present vibrator, in addition to milling
26 out portlons of the baseplate, the dimensions of the baseplate have also
27 been reduced in order to reduce the moving mass. Be&ause of ~he reduced
28 size, the baseplate does not extend from beneath the vibratory body suffi-
29 ciently to permit applying the truck weight to the baseplate in the con-
ventional manner. A unique one-piece hold-down plate 50 per~its the use of
--11--
1 the small baseplate. The weight of the truck is transferred to the hold-
2 down plate hydraulic lift cylinders 5 and 7 (see FIGURES 1 and 4). The
3 hold-down plate extends beneath the reaction ma6s and rests upon four air
4 bags 33a, 33b, 33c, and 33d (~he latter two not shown~in FIGURE 1), which
are affixed between the hold-down plate and the baseplate. The air bags
6 may preferably be spaced at regular intervals around the baseplate to
7 couple the weight of the truck to the baseplate evenly. The vibrator
~ piston rod extends through the center portion of the hold-down plate which
9 has a cut out section for that purposeO In addition to permitting the use
of a small baseplate, the hold-down plate also provides a means for dis-
11 tributing the air springs so as to couple the weight of the transport truck
12 to the baseplate in a more uniform manner about ~he surface of the base-
13 plate. In the preferred embodiment, air bags 33a, 33b, 33c, and 33d are
14 pneumatically isolated. It i8 possible, however, for the air bags to be
pneumatically coupled without departing from the spirit and scope of the
16 invention.
17 Hydraulic lift cylinders 5 and 7 (see FIGURES 1 and 4) control
18 the vertical position of the vibrator relative to the truck. The cylinder
19 housings of lift cylinders 5 and 7 are afflxed to the truck frame and the
piston rods thereof are affixed to the hold-down plate. When hydraul1c
21 fluid is pumped into the upper portion of the lift cylinders 5 and 7, the
22 pistons are forced down relative to the cylinders and the vibrator is
23 lowered to the ground. After the baseplate is lowered to the earth's
24 surface, if additional hydraulic fluid is pumped into the upper portions of
lift cylinderg 5 and 7, the truck will be liEted oEf the ground and its
26 weight will bear on the hold~down plate. The air springs which inter-
27 comlect the hold-down plate with the baseplate transmit the weight of the
28 truck to the baseplate. The truck is lowered back to the ground and the
29 baseplate lifted off the ground by pumping hydraulic fluid into the lower
1 portions of the lift cylinders 5 and 7. As the basepla~e is lifted off the
2 ground, it is suspended from the hold-down plate by means of a plurality of
3 chains 8. Guide rods 6a and 6b (see FIGURES 1 and 4) slide through the
4 cylindrical bores of guide frames 9a and 9b (9a not shown) which are
rigidly affixed to opposite sides of the transport truck. These guide rods
6 are normally interconnected so that they move up and down in unison.
7 Techniques for performing this function are well known in the art and need
8 not be discussed here, one such technique being shown in the aformentioned
9 U. S. Patent No. 3,929,206.
Because air springs 33a, 33b, 33c, and 33d have little resistance
11 to lateral stress, a linkage mechanism is used to maintain the baseplate in
12 vertical alignment with the vibrator and to apply the weight of the truck
13 substantially to the center of the baseplate. This horizontal stabiliza-
14 tion is required to not interfere with or datract from, to any appreciable
extent, the desired vertical motion of the baseplate. Further, the hori-
16 zontal stabilization mechanism should impose no appreciable horiæontal
17 motions to the truck. It is known in the ar~ to use for this purpose, a
18 multiplicity of radius rods, one end of each radius rod being pivotally
19 attached to the baseplate, and the other end being pivotally attached to a
part of the lift mechanism of the truck.
21 This radius rod type of stabilization system provides good hori-
22 zontal control between the baseplate and the hold-down plate of the vehicle
23 lift system. The radius rod system, however, does impose undesirable
24 horizontal motion to the hold-down plate as the baseplate ~oves vertically.
The undesirable horizontal motions become more severe as the radius rod
26 length is decreased, as it would have to be lf this type of stabilization
27 system were used on the BBV. It is common practice in the design of vi-
28 brators to use pairs of radius rods mounted so that they are in opposition.
29 The tendency for horizontal displacement as the vibrator moves verticaIly
is taken up by rubber bushings in the eye ends of the radius rods. Deflec-
31 tion of the rubber bushings, however, takes energy from the vibrating
32 baseplate.
-13-
~ZS904
1 With reference to FIGURES 1 and 5, a linkage mechanism designated
2 by numeral 150 is used to maintain the baseplate of the BB~ in vertical
3 alignment with the truck. The linkage mechanism is comprised of equal ~ -
4 length rods 152a and 152b and rotating center link 154. An end of each ~;
equal length rod is connected to a mounting assembly 156 rigidly affixed to
6 the corners of the baseplate. The other end of each equal length rod ls
7 connected to center link 154. Center link 154 is connected in the center ;
8 thereof in a rotating manner to plate 158 which extends do~nwardly from the
9 hold-down plate in substantially vertical allgnment with the edge of the `~
hold-down plate. Center link 154 rotates freely about this center con-
11 nection and the two equal length rods rotate freely about a connection to
12 opposing ends of center link 154. The ends of the equal length rods con-
13 nected to mounting assembly 156 are also rotationally free. One of these -
14 linkage mec~anisms 154 is affixed to each side of the vibrator baseplate.
This mechanism permits the baseplate to move up and down relative to the
16 hold-down plate while maintaining vertical alignment. When the baseplate
17 moves down in relation to the hold-down plate the center link 154 will
18 rotate in a counterclockwise direction. When the baseplate moves up rela-
19 tive to the hold-down plate, center link 154 rotates in a clockwise direc-
tion. In a preferred emhodiment, the ends of rods 152a and 152b may be
21 connected by a ball joint to the mounting assembly connected at the corners
22 of the baseplate, to permlt an additional degree of freedom. This linkage
23 mechanism permits the baseplate to tilt relative to the hold-down plate.
24 Therefore, if the vibrator is on ground which is not in parallel alignment
with the plane of the truck, the baseplate can tilt so as to rest evenly on
26 the ground.
27 In the preferred embodiment, the center link and its pivot are
28 attached to the hold-down plate, while the extremities of rods 152a and
29 152b are pivotally attached to supports on the baseplate. Alternatlvely,
the center link and yivot may be attached to the baseplate, while the
-14-
1~2S9~4
1 extremities of the rods can be pivotally attached to supports on the hold-
2 down plate. The arrangement of the preferred embodiment, ho~ever, has a
3 distinct advantage when the vibrator is operating on sloping ground. With
4 reference to FIGURE 6a, which illustrates the arrangement of the preferred
embodiment, the baseplate is driven by the vibrator along a line perpen-
6 dicular to the baseplate and ground. The baseplate is also guided along
7 this same line by the constraint imposed by the linkage. F~GURE 6b illus-
8 trates the arrangement of the alternative embodiment wherein the baseplate
9 motion is again along a line perpendicular to the baseplate and the ground,
while the linkage tends to constrain the motion to a vertical line. Thus,
11 the stabilizing linkage in this case will impose horizontal forces between
12 the baseplate and the hold-down plate.
. ~ .
13 In the preferred embodiment, stabilizing rods 152a and 152b have
14 eqoal length and center link 154 is symmetrically disposed about its pivot
point. It is also within the contemplation of the invention to use sta-
16 bilizing rods of unequal length and a non-symmetrical center link.
17 In the preferred embodiment, the length of each of stabilizing
18 rods 152a and 152b is 19 inches, while the distance between the points at
19 which the stabilizing rods are coupled to center link 154 is 5.625 inches.
As an example of the effectiveness of the stabilizing means, if the base-
21 plate moves vertically with respect to the hold-down plate by a distance of
22 2 inches, there will be a relative horizontal displacement between the
23 baseplate and hold-down plate of 0.00021 inches. If a radius rod suspen-
2~ sion were used in this case (with a rod length of 38 inches), for a ver-
tical displacement of 2 Inches there would result a horlzontal displacement
26 of 0.0527 inches, Thus, the stabilizing linkage employed in the BBV re-
27 duce9 the horizontal displacement by a factor of 250.
23 From the foregoing, it will be seen that the unique stabilizing
29 sy9tem provides several important advantages. Vertical translation of the
baseplate results in negligible horizontal motion imparted to the support-
31 ing truck by the stabilization system. Further, the space required for the
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.
1 stabilization system is reduced. Finally, the energy absorbed by the ~ -
2 stabilizatiGn system is reduced, thereby increasing the net seismic energy
3 into the ground. ~ 3
4 ~IGURE 7a is a sectional view of the cylinder area of the re-
action mass from a prior art vibrator. FIGURE 7b is a sectional view of
6 the corresponding portion of an embodiment of the present invention. All
7 vibrators are provided with an over-travel limit system which serves to
8 prevent the piston from traveling beyond its nomlnal stroke to the point
9 where the piston impacts an end of the cylinder. Most vibrator actuators
have sprlngs or externally mounted hydraullc shock absorbers as part of the
~1 over-travel limit system. That type of over-travel limit system has
12 several features which render it undesirable for use in the present vibra-
~13 tor. Tha first problem relates to the effective oil volume in the cylinder
14 of the actuator. A frequency is reached at which the volume of the oil
wlthin the cyllnder acting as a spring resonates in con~unction with the
16 load mass. This oil column resonance places an upper limit on the range of
17 frequencies over which the vibrator can operate. The frequency of oil
18 column resonance varies inversely with the square root of effective volume
19 of oil within the cylinder. Accordingly, it will be seen that in the BBV
it is essential to minimi~e this effective oil volume.
21 As the piston moves from the center af the cylinder to the limit
22 of lts stroke, it sweeps a volume of oil from the plston equal to the
23 product of the piston area and its Iength of stroke. If the plston travel,
24 however, exceeds its nominal stroke, it will sweep an additional volume of
oil equal to the product of the piston area and the length of over travel.
26 'rhus, thi9 addit:Lonal oil volume, for which no benefit i8 received, is
27 proportional to the distance that the piston over travels before the over-
28 travel limit system stops 3.t. ~le amount of over travel with prior art
29 externally mounted limit systems is large and results in a~ e~fective
cylinder oil volume that is undesirable for the present invention.
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1~259~
1 Additionally, the use of external shock absorbers reduces the re-
2 liability of an over-travel limit system. For example, if the plunger
3 fails to reset due to a broken return spring or if not enough time has
4 elapsed for a normal resetting before the over travel condition repeats, no
shoc~ absorber action will occur. In an embodiment of the present inven-
6 tion, there is provided an interna~ over-travel limit system which is reset
7 if the actuator is in the working stroke. While an internal over-travel
8 limit system is known in the prior art, the internal limit system of the
9 present invention has several features which render it more advantageous
for use in a high frequency vibrator.
11 The essential features of the prior art internal limit system are
12 illustrated in FIGURE 7a. There is sho~n in sectional view a portion of a
13 reaction mass 200 including the cylinder region of the reaction mass. An
14 axial hollowed out portion of circular cross section extends through the
reaction mass. This hollowed out portion is lined over part of its length
16 by bronze bushings 202 and Z04 which serve to support in sliding relation-
17 ship, the piston rod 206 of a double ended piston. The cylinder area of
18 reaction mass 200 is lined by a sleeve 208 of a metal such as cast iron.
19 Piston 2].0, including a plurality of piston r-lngs, is reciprocably located
within the confines of the cylinder. Clearance is provided between the
21 walls of the piston and the inner surface of sleeve 208 so that only the
22 piston rings are in contact with sleeve 208. Ports 212 and 214 communicate
23 with the manifold (not shown) so as to admit oil under high pressure al-
24 ternately to opposlte sides of the piston. Port 212 opens lnto an annular
passage 216 which extends around the outer circumference of bushing 202.
26 Passage 216 in turn communicates with a plurality of holes 218 located in
27 and about the circumference of bushing 202. Thusl it will be seen that
28 high pressure oil from the manifold is admitted through port 212, passage
29 216 and holes 218 into a portion of the cylinder formed by the inner surface
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1 of bushing 202 and a narrowed portion of piston rod 206. The oil so ad-
2 mitted may flow into the main body of the cylinder so as to generate a
3 force acting against one side of the piston. In a similar manner, oil from
; 4 the manifold flows through port 214, channel 220 and holes 222 to the
opposite end of the cylinder.
6 The internal braking action provided by the illustrated structure
7 may be appreciated from the following brief operational discussion. ~et it
; 8 be assumed that oil is admitted under high pressure through port 214 to the
9 right side of the cylinder. This causes the piston 210 and piston rod 206
to move to the left and oil from the left side of the cylinder is exhausted
11 through port 212 to a reservoir of low pressure oil. This continues until
12 shoulder 224 of the piston rod reaches and begins to enter bushing 202. At
13 this point in time, a volume of oil is trapped in region 226 of the cylin-
14 der. This trapped volume of oil imparts a braking action to the piston and
prevents it from impacting the end of bushing 202. The piston braking is
16 accomplished by the trapped volume which escapes slowly through the small
17 clearance provided between the portion of piston rod 206 which is not
18 narrowed and the inner surface of bushing 202. Because of the high pressure
lSf occurring in region 226 during braking action, it is necessary to provide
an 0-ring 228 for preventing the escape of oil be~ween bushing 202 and
21 reaction mass 200. A corresponding 0-ri~g 230 is provided at the other end
22 of the cylinder. In the structure illustrated, it will be noted that the
23 piston and piston rod are unsupported over the entire reglon between
24 shoulder 232 and shoulder 234. In the prior art vibrator illustrated, this
is a length of approximately 16.5 inches.
26 The internal hydraulic braking action provided by an embodiment
27 of the present invention may be illustrated with the aid of FIGURE 7b which
28 shows a sectional view of a portion of the reaction mass lOla. Reaction
29 mass lOla is provided with an axial bore extending through its entire
length.
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1 The bore is lined at its ends by bushings 252 and 254 which
2 provide bearing surfaces for the piston rod 104 of a double ended piston
3 103. In the preferred e~bodiment bushings 252 and 254 are made of bronze.
4 Other materials, however, may be used for the bushings. ~ne such suitable
material is a polymide manufactured under the trade mark ~ESPEL by Dupont.
6 ~ushings 252 and 254 also serve to support piston 103 in the areas indi-
7 cated by reference designators 260 and 262. In the region of the piston
8 rings, the surface of the bore is lined by a sleeve 264 of a metal such as
, :,
9 cast iron.
10 Ports 64 and 66 communicate with the manifold (not shown) to ~ ;
11 serve as input and exhaust ports for oil en~ering and leaving the cylinder.
12 Port 64 opens into an annular passage 270 which extends around the outer
13 circumference of bushing 252. Passage 270 in turn communicates with a
14 plurality of slots 272 formed in bushing 252 whereby oil is admitted to and
exhausted from one slde of the cylinder. In a similar manner, port 66
16 cooperates with annular passage 274 and slots 276 to provide a path for the
17 oil to the opposite end of the cylinder.
18 During operation of the vibrator, as high pressure oil is ad-
19 mitted through port 66 so as to exert a force against the piston driving it
to the le~t, oil is forced out through port 64 to a low pressure reservoir.
21 During normal operation, prior to the time when piston 103 passes the
22 plurality of ports 272, the flow of oil will be reversed so as to admit
23 high pressure oil to port 64 and allo~ oil to leave the cylinder through
24 part 66 to the low pressure reservoir. Under abnormal conditions, however,
the piston may exceed its designated stroke and travel sufficien~ly far to
26 the left to close off slots 272, thereby trapping a volume of oil in the
27 end of the cylinder. This trapped oil i9 bled back to the slots through
2~ the radial clearance between piston 103 and bushing 252. The concentricity
--19--
1 and radial clearances are selected to obtain a desired shock absorber
2 action. The equation used to predict the shock absorber response is:
3 F = 6U~(r2 - rO2)
D RC3 (1 ~ 3E )
4 where
FD = Retarding force,
6 u = Oil viscosity,
7 L = Length of engagement
8 L' = Relative velocity between the rod and
rod bushing,
ro = Small rod diameter,
11 R = Rod bushing cavity radius,
12 ~ r = Piston radius (plunger),
13 C = R - r = radial clearance,
14 e = eccentricity, piston relative to cavity, and
E = C
.
16 The internal shock absorber lllustrated in FIGURE 7b is superior
17 to the prior art arrangement of FIGURE 7a in several respects. First,
18 piston 103 is always engaged in bushings 252 and 254 at regions 260 and 262
19 respectively. Accordingly 9 there is no mechanical "plunger" insertlon into
the bushing. This enhances the reliability of the mechanism.
21 Secondly, since mechanlcal support for the pis~on is provided by
22 bushings 252 and 254 in regions 260 and 262, respectively, the longest
23 unsupported length of the actuator rod structure is that portion of the
24 piston that is enclosed wi~.hin llner 264. In the preferred embodiment,
this unsupported portion of the piston extends only over a length of 9.2
26 inches. By reducing the unsupported length of the piston, the stresses
-20-
.
~zs~
1 exerted on the actuator rod are relativPly reduced. As will be discussed
2 below in connection with FIGURE 8, this permits the piston rod assembly to
3 be hollow, thereby further reducing the weight of the baseplate and asso-
4 ciated elements.
~hen the over travel condition occurs, in the preferred embodi-
6 ment the oil is trapped between the piston 103, the piston rod 104, and
7 either of bushings 252 and 254. As a result, it is not necessary to pro-
8 vide 0-rings corresponding to 0-rings 228 and 230 in FIGURE 7a. These O-
9 rings are required in the prior art structure since, there~ the braking oil
volume is trapped ahead of bushings 202 and 204. Elimination of the 0-ring
11 reduces the actuator cost. Further, it will be notéd that piston rod 104
12 does not require the additional machining operations required to produce
13 the reduced diameter section of pis-ton rod 206 in the prior ar~ structure.14 Finally, in the preferred embodiment, there is no oil volume corresponding
to that volume of oil located between the reduced diameter section of
16 piston rod 206 and bushings 202 and 204 in the prlor art structure. As
17 mentioned previously, this reduced oil volume is advantageous in high
18 frequency operation of the vibrator.
19 FIGURE 8 is a sectional view showing the configuration of the
actuator rod 280. The rod is hollow so as to reduce the baseplate weight
21 of the vibrator. The tapered bore of the rod is specifically designed such22 that the stresses resulting from forces applied transverse to actuator rod
23 280 are approximately constant as a function of distance along the rod. As24 a result, no portion of ~he rod i8 "over-deslgned" (and correspondingly
over-weight) relative to another portion of the rod.
26 The diameter of the sleeve in which the piston 103 runs, in the
27 preferred embodiment, is nine inches, while the diameter of the piston rod
28 is seven inches. As a result the effective piston area at either end of
-21-
~12Si~
1 the piston is 25.13 square inches. Hydraulic fluid is supplied to the BBV
2 at a pressure of 3000 psi. Accordingly, it will be seen that the peak
3 force acting on the piston of the BBV is 75,390 pounds, far in excess of
4 that used in other vibrators.
There has been disclosed a new seismic vibrator, suitable for
6 operation over a broad band of frequencies. I~hereas the preferred embodi-
7 ment of the invention has been disclosed, there may be suggested to those
8 skilled in the art certain minor modifications which do not depart from the
9 spirit and scope of the invention as set forth in the appended claims.
~ .
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