Mounting and control means for full waveform seismic source

A full waveform seismic source is described which is adapted to propel a heavy mass powerfully downward against the surface of an earth-contacting target plate. Means are provided for rotating the barrel of the actuator about two mutually perpendicular horizontal axes so that energy can be delivered to the earth along a plurality of preselected slant paths. In this way, any desired combination of compressional and shear waves, both SH and SV type, may be generated at a point on the earth's surface. The target is contoured so that any slant path followed by the accelerating mass always impacts the target normal to its surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to the field of seismic exploration and 
more particularly to apparatus for the generation of combined 
compressional and shear waves. 
2. Description of the Prior Art 
Injection of seismic signals into the earth by means of apparatus which 
utilizes compressed gas to propel an impact mass against a rigid earth 
contacting target as described in detailed in U.S. Pat. Nos. 4,284,164 and 
4,284,165, both issued Aug. 18, 1981 and entitled "Acoustic Pulse 
Generator". A significant advantage of such devices is their ability to 
produce rapidly repeated seismic signals of variable carefully controlled 
energy content. Devices of these character are therefore well suited to 
the production of combined compressional and shear waves. 
Recent advances in the ability of seismic data acquisition and processing 
equipment to handle large amounts of data have stimulated greater interest 
in the development of versatile high seismic sources adapted to produce 
both compressional and shear waves. By recording the full waveform of a 
seismic wave, including P, SH and SV waves, one can get far more detailed 
information concerning rock properties and depth. With the aid of the 
invention to be described, devices of the type disclosed in the above 
referenced patents constitute efficient means for producing such combined 
compressional and shear waves. 
The actuator assembly for seismic sources such as described in the above 
referenced patents typically includes an impact mass weighing several 
thousands of pounds which must be propelled downwardly by compressed gas 
or the like against a rigid target resting on the earth beneath it. In 
order to generate a desired combination of P, SH and SV waves, it is 
necessary to be able to operate the source so that this heavy impact mass 
will travel along a selected slant path to the target. The term "full 
waveform seismic sourcep" as used in this application means a device which 
can be manipulated in position so that this slant path may be readily 
varied through 360 degress of aximuth at 90 degree intervals. Such a slant 
path needs to be achieved with precision, that is to say, within a degree 
or so of a prescribed inclination and azimuth. Furthermore, the mechanism 
whereby this can be accomplished, practically speaking, must be 
transportable by means of a highly mobil vehicle from place to place along 
an extended line of survey. Achieving the kind of accuracy and 
maneuverability which is required for these purposes in a vehicle mounted 
system presents a challenge. 
It is therefore a general object of this invention to provide a seismic 
source with improved means for mounting and controlling such source which 
delivers energy to a selected point on the earth along any of a plurality 
of selected slant paths. 
It is a more particular object of this invention to provide such a seismic 
source wherein a heavy impact mass is accelerated downwardly along any 
such slant paths to impact a rigid earth-contacting target. 
Other and further objects and advantages of the invention will becme 
apparent from a consideration of the detailed description and drawings to 
follow taken in conjunction with the appended claims. 
SUMMARY OF THE INVENTION 
In accordance with the preferred embodiment of this invention, a seismic 
source is described which can be controlled so as to deliver energy to a 
point on the earth's surface along a plurality of preset vector paths 
inclined in any direction from the vertical. Broadly speaking, the source 
comprises a hollow cylindrical housing, an impact mass slideably 
interfitted with the housing for limited movement along the longitudinal 
housing axis, means for supporting the housing and the mass above the 
earth, means for tilting the housing axis selectively about either of two 
mutually perpendicular horizontal pivot axes so that the housing axis is 
aligned with a preset slant path, and means within the housing for 
applying thrust to the impact mass causing it to accelerate downwardly to 
impact the earth.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference now generally to FIG. 1, there is illustrated a motorized 
transport vehicle 10 upon which is mounted a seismic source 12 consisting 
generally of actuator assembly 14 and a target 15 which may be lowered to 
rest on the earth. Target 15 comprises generally a base plate 16 and an 
upper striker plate 17 having a concave upper surface. 
Actuator assembly 14 comprises a hollow cylindrical barrel 18, the closed 
upper end 19 of which is provided with a plurality of ports (not shown) 
communicating with a like number of external compressed air containers 20. 
Barrel 18 contains a piston 22 which extends downwardly from the open 
lower end 23 of barrel 18 and terminates in a heavy impact mass 24. 
In a manner detailed thoroughly in U.S. Pat. Nos. 4,284,164 and 4,284,165, 
when source 12 is to be operated, mass 24 is retracted upwardly until 
piston 22 blocks or seals off the above referenced ports in barrel 18. 
Four upstanding lift cylinders 26, two of which are visible in FIG. 1, are 
secured adjacent the four corners of centrally located fixed vehicle frame 
29. Working within each cylinder 26 is a piston (not shown) with a 
downwardly projecting rod 31 welded to one of adapters 32 attached 
respectively adjacent the four corners of cross beam frame 33 which is 
secured to the top of base plate 16 by means of a plurality of shock 
isolators 34. Extension of rods 31 provides downward thrust against cross 
beam frame 33 and base plate 16. In reaction, cylinders 26 provide upward 
thrust against vehicle frame 29 which raises vehicle 10 off its spring 
suspension until one or both sets of tires 35 clear the ground. This 
brings the weight of vehicle 10 partially or fully to bear on target 15. 
Extending downwardly through frame 29 parallel to cylinders 26 are a like 
number of sleeve guides 36 adapted to receive guide shafts 37, the lower 
ends of which are rigidly attached to cross beam frame 33. Shafts 37 move 
integrally with rods 31 upwardly and downwardly. This prevents lateral 
loads from causing buckling or failure of lift cylinders 26 particularly 
in their extended state. Details of operation of elements such as lift 
cylinders 26 and sleeve guides 36 are set forth more particularly in U.S. 
Pat. No. 4,402,381 issued to Tom P. Airhart, Sept. 6, 1983. 
When a firing signal is given, mass 24 is released by operating suitable 
trigger means (not shown). The resultant downward gravity displacement of 
piston 22 exposes the side ports in barrel 18 allowing entry of compressed 
gas therein from chambers 20 so as to exert a powerful downward 
accelerating force upon piston 22. In this way, impact mass 24 is driven 
against striker plate 17 of target 15 to generate a seismic pulse of 
interest. Actuator assembly 14 is positioned at or close to the center of 
mass of vehicle 10 so as to eliminate as much as possible the unbalancing 
effect of powerful recoil forces in operation. 
The succeeding description is particularly concerned with the manner in 
which actuator assembly 14 is mounted and manipulated to enable an 
operator to rapidly tilt the axis of barrel 18 from a vertical path 40 to 
any of a plurality of inclined paths without moving vehicle 10. For the 
sake of simplicity, many features of vehicle 10 and its related equipment 
associated with the operation of source 12 have been omitted where they 
are either conventional or well within the skill of the art and their 
description is unnecessary to an understanding of this invention. 
Referring now particularly to FIG. 2, actuator assembly 14 is shown 
extending vertically as mounted on horizontal axles 44 and 45 rotatable 
within a surrounding rectangular yoke 48. Yoke 48 is provided with a 
further pair of horizontal axles 50 and 51 extending at right angles to 
axles 44 and 45 for rotation within fixed vehicle frame 29. Frame 29 is 
rigidly interconnected with longitudinal chassis members 53. It is 
apparent that this arrangement provides actuator assembly 14 with two 
degrees of rotational freedom. That is to say, actuator assembly 14 may be 
tilted fore and aft with respect to vehicle 10 by rotation of axles 44 and 
45 and from side to side by rotation of axles 50 and 51. The allowable 
tilt in any of these four directions depends upon the design of vehicle 10 
and source 12, a maximum of 30 degrees variation from the vertical in any 
rotational sense being found sufficient for much important geophysical 
work. 
FIG. 3 illustrates in plan the means by which the tilt motion of actuator 
assembly 14 and yoke 48 are accomplishd. Extending along opposite sides of 
actuator 14 and mounted on yoke 48 are two pairs of opposing or oppositely 
directed hydraulic cylinders. One such pair consist of cylinders 60 and 
62, the other of cylinders 64 and 66. The construction and operation of 
cylinders 60 and 62 is exactly the same as that of cylinders 64 and 66 and 
thus only the former will be described in detail. Cylinder 60 is pivotally 
mounted within bracket 70 fixed to one corner of yoke 48 for rotation 
about the axis of pivot 72. Similarly, cylinder 62 rotates about the axis 
of pivot pin 76 situated within mounting bracket 78 located at a further 
corner of yoke 48. Piston 80 of hydraulic cylinder 60 terminates in a rod 
clevis 82 which surrounds horizontally extending tilt pin 84 by rigidly 
attached to thrust bearing wing 85 mounted on the sidewall of barrel 18. 
In like manner, piston 86 of cylinder 62 of cylinder 62 terminates in a 
rod clevis 88 which also surrounds pin 84 and dovetails within the end of 
clevis 82. The outward end of pin 84 is held in place by mounting frame 89 
extending laterally from wing 85. 
As best seen in FIG. 4, tilt pin 84 is vertically offset above axle 45 so 
that the axes of cylinders 60 and 62 slant upwardly in opposite directions 
to engage pin 84. By means well known in the art, suitable electrical 
control signals are provided to operate cylinders 60 and 62 in opposition 
or "push-pull" in either direction. For example, if rod 80 is retracted 
rod 86 is extended at the same time and pin 84 is thrust in a direction to 
cause actuator assembly 14 to tilt or rotate with respect to axles 44 and 
45 in a counterclockwise sense or forwardly with respect to vehicle 10. In 
order to equalize the rotational forces generated in the manner described 
on both sides of actuator assembly 14 and to avoid undesirable linear 
forces cylinders 64 and 66 are operated in tandem with cylinders 60 and 62 
on the opposite side of actuator assembly 14. Thus, horizontal tilt pin 90 
fixed to thrust bearing wing 92 and supported by frame 94 is engaged by 
cylinders 64 and 66 so as to enable application of a torque to actuator 
assembly 14 balancing the tilting force described above. 
With further reference to FIG. 4, a cam 95 is fastened over the end of axle 
45 which extends through the wall of yoke 48. Rotation of barrel 18 on 
axles 44 and 45 causes cam 95 to depress or permit linear extension of cam 
roller 96 extending from linear variable displacement transducer 97 
mounted on a vertical sidewall of yoke 48. By means well known in the art, 
this linear displacement may be converted to a variation in electrical 
signal which is proportional to the angular rotation of actuator assembly 
14 from some reference axial position such as vertical path 40. By 
comparison with a preset position this signal enables a suitable control 
device to stop the motion of hydraulic cylinders 60, 62, 64 and 66 when 
the axis of actuator assembly 14 reaches any desired slant position such 
as, for example path 98 or path 99. 
The manner in which yoke 48 may be tilted or rotated on axles 50 and 51 
from side to side is best understood with further reference to FIG. 3 and 
to the detail of FIG. 6. As shown in FIG. 6, a pair of hydraulic cylinders 
100 and 102 are rotatable respectively about parallel horizontal axes on 
pivots 103 and 105 carried by bracket 108 and 110 depending from fixed 
frame 29. Another pair of such oppositely directed hydraulic cylinders (as 
shown in phantom in FIG. 3) are disposed on the opposite or front side of 
yoke 48. They are constructed and operated in an identical manner to 
cylinders 100 and 102 and therefore will not be described in great detail. 
Piston 112 of cylinder 100 terminates in knuckle 114 which is rotatable 
about pin 116 fixed to bracket 118 depending from yoke 48. Similarly, 
piston 120 of cylinder 102 terminates in knuckle 122 rotatable about pin 
124 in dependent bracket 126. 
In fashion similar to that described to effect tilt of actuator assembly 
14, an opposite or push-pull operation of hydraulic cylinders 100 and 102 
in tandem with the parallel pair of hydraulic cylinders 104 and 106 
produces a tilt or rotation of yoke 48 in either sense about axles 50 and 
51. Cam 130 bolted to fixed lower frame extension 132 integral with frame 
29 is followed by cam roller 133. Therefore, the angular tilt of yoke 48 
from a reference position such as path 135 is read by linear variable 
displacement transducer 136. The amount and sense of rotation or tilt of 
yoke 48 from side to side with respect to vehicle 10 as, for example, to 
slant paths 137 or 138 can be monitored, controlled and indicated. 
FIG. 5 provides a sectional view showing the position of attachment to 
barrel 18 of thrust bearing wings 85 and 92. When source 12 is fired 
driving mass 24 downward against target 15 powerful recoil forces exerted 
upwardly against barrel 18 are transmitted to vehicle 10 by means of wings 
85 and 92. An advantage of the design of this invention is that axles 44 
and 45 are brought as close together as possible while still providing 
sufficient clearance for travel of mass 24. This contributes to smooth and 
efficient operation of the mechanism for tilting actuator assembly 14, 
which may weigh many thousands of pounds. For similar reasons the fore and 
aft clearance between the yoke 48 and vehicle frame 29 is reduced to a 
minimum. 
Details of the electrical and hydraulic lines required for operation of the 
apparatus of this invention as described above have been omitted for 
simplicity. Clearly a central controller (not shown) can be utilized to 
provide electrical signals which are converted into appropriate action of 
the various hydraulic cylinders. Such controller may further be used to 
monitor the precise slant path or inclinication of actuator assembly 14 as 
described. Preset positions of actuator assembly 14 can be stored in a 
microprocesor (not shown) for continuous comparison with the output of 
linear variable displacement transducers 97 and 136. In addition, of 
course, such a controller can perform related functions such as to operate 
lift cylinders 26, to raise and lower target plate 16, to deliver a firing 
signal as desired, and to operate means for retraction of mass 24 and 
piston 22 to their uppermost positions preparatory to the next shot. 
In summary what has been described is a novel system and apparatus for 
injecting seismic energy into the earth along slant paths in any 
direction. One of the advantages of the system of this invention is 
inherent in the power and reliability of hydraulic cylinders as a means 
for moving heavy masses and to control the very large force moments 
created without overshoot. Experiments indicate that the actuator assembly 
14 may be shifted from a vertical orientation to any desired preset slant 
path within one to two seconds. 
The particular configuration and mode of operation of the seismic source 
described herein as well as other features of the mounting and control 
system are illustrative only and many modifications will occur to those 
skilled in the art without departing from the scope of this invention as 
more particularly set forth in the appended claims.