Device for avoiding multiple bounces against a target of a mass _falling thereon along a rectilinear trajectory

Multiple shocks of a vertically falling mass against a target member coupled with the earth are prevented by holding the mass and retaining it above the target member after its first bounce. This retention is accomplished by means of deformable elements laterally pressed against the mass body by jack rods actuated in response to a control signal produced by a sensor at the moment of the first impact of the mass.

BACKGROUND OF INVENTION 
The present invention has an object to provide a device for avoiding 
multiple shocks, against a target member, of a weighty mass falling 
thereon along a rectilinear trajectory. These multiple shocks result 
because of the bounces of the mass after the first impact. 
More precisely, the invention concerns a device for avoiding multiple 
shocks of a mass associated with guide means and falling along a 
rectilinear direction towards a target member couple with the earth, as a 
result of the bounces of the mass after the first impact. 
This device may be used, for example, in combination with an apparatus for 
generating in the earth longitudinal acoustic waves. Such an apparatus is 
described in French Pat. No. 2,398,316. It essentially comprises a target 
member secured at a first end of a guide tube forming the guide means and 
arranged so as to come in contact with the earth when the tube is in a 
substantially vertical position. A mass is provided adapted to impinge on 
the target member, and its fall is guided by the guide tube and means for 
moving said mass to an upper position at the second end of the tube and to 
maintain it intermittently in said position until the selected triggering 
times. 
This apparatus has the disadvantage of not being provided with means for 
controlling the free motion of the mass after the first impact, the latter 
usually effecting multiple bounces of decreasing amplitude, thereby 
generating multiple shocks on the target member. Multiple secondary 
impulses are generated in addition to the primary impulse resulting from 
the first impact, with a disturbing effect on the recordings and result in 
a lower resolution power of the seismic apparatus. 
SUMMARY OF THE INVENTION 
The device according to this invention makes it possible to avoid these 
disadvantages by preventing multiple shocks of the mass after the first 
impact. For this purpose, it comprises at least one deformable element 
secured to the guide means, and control means adapted to press the 
deformable element against the lateral wall of the mass after its bounce 
and to secure the latter in position. The control means comprises, for 
example, at least one movable member and driving means connected to the 
guide means for displacing the movable member in the direction of the 
lateral wall of the mass. 
The movable member preferably consists of the rod of a hydraulic jack which 
is adapted to press intermittently against a push-plate secured to a 
deformable element. The control means comprises sensor means adapted to 
generate a signal in response to the impact of the weight against the 
target member, the driving means being actuated by said signal. 
With such an arrangement, the mass is maintained laterally at the moment of 
the first bounce and can no longer fall and impinge the target member.

DETAILED DISCUSSION OF THE INVENTION 
The device diagrammatically shown in FIGS. 1 and 2 comprises a guide tube 1 
of a diameter larger than that of a cylindrical mass 2 placed inside. 
Four slides 3, opposite by pairs, are secured along generatrices of the 
guide tube (FIG. 4). The spacing between the slides is selected so that 
the mass 2 bears thereon when it moves inside the guide tube 1. At a first 
end of the latter is secured a first plate 4 whose central portion has an 
opening 5 of a cross-section larger than that of the mass 2. This first 
plate is secured to a second plate 6 through deformable fixation means 
formed of bolts 7 co-operating with springs 8. The second plate 6 is 
rigidly secured to the target member 9 at its periphery. The latter is 
provided with a striking plate 10 for the mass and a coupling surface 11 
with the earth. The striking plate 10 is made of a dampening material 
(e.g. polyurethane) which protects the target member and optionally 
enables to modify the shape of the seismic signal transmitted to the 
earth. 
Protruding elements 12 (teeth or pins, for example) may be optionally 
secured to the base of the target-member in order to improve its coupling 
with the earth but, when it must be used on the roads, the target-member 
is not provided with asperities at its base in order to avoid damaging the 
road ways. 
Two elongate deformable elements, each consisting of a strip 13 of elastic 
material, for example, are placed between the slides 3 and parallel 
thereto over two opposite parts of the internal wall of the guide tube, 
and secured thereto at their both ends. To the medium part of each strip 
is secured a push-plate 14 on which presses intermittently a movable 
member associated with driving means, said movable member consisting of 
the rod 15 of a hydraulic jack 16 which will be described in more detail 
with reference to FIG. 3. The strips 13 are placed at the lower part of 
the guide tube 1 so as to face the lateral wall of the mass 2 when the 
latter bounces after its impact against the target member 9. Means (not 
shown) are provided to move the mass to the second end of the tube 
opposite to that at which is secured the target-member. This means may 
consist, as described in the prior above cited French patent, of an 
operating system adapted to rock the guide tube in such a manner that its 
second end will be below its first end, so as to drop the mass by gravity 
toward its second end where it is intermittently secured in position by 
means of an electromagnet. 
This means may also consist for example of a cable wound on a winch 
co-operating with driving means, this cable being secured to the mass. 
Each of the control jacks 16 (FIG. 3) comprises a body 17 provided with a 
base plate 18, fixed to a flange 19, rigidly secured to the external wall 
of the guide tube 1. Openings are provided in the wall of the tube, of 
flange 19 and of the base plate 18 of each jack body 17 for providing a 
passage for the rod 15 thereof. 
The head 20 of the rods 5 is provided with sealing joints 21 whose section 
is adapted to the internal section of the jack body 17. A return spring 
22, bearing on the head 20 on the one hand, and on a wall of the base 
plate 18 on the other hand, pushes the rod 15 towards the inside of the 
jack body 17 (retracted position). The end of the rod 15 penetrating 
inside the guide tube presses against one of the push plates 14. 
The end of the jack body 17 opposite to the base plate 18 is provided with 
an opening communicating with a hydraulic circuit. 
This hydraulic circuit comprises (FIG. 4) two pipes 23 opening at a first 
end inside bodies 17 of the jacks 16 and communicating at their second end 
with a common pipe 24 connected to the output of an electro-valve 25 of 
known type. 
The two inputs of said electro-valve respectively communicate through pipes 
26 and 27 with a pressurized liquid accumulator tank 28 and a low pressure 
(atmospheric pressure) liquid tank 29. A pump 30, fed by tank 29, fills 
the accumlator tank 28 through a flap valve 31. The electro-valve 25 
comprises a two-position drawer 32 whereby pipe 24 can be connected to any 
one of pipes 26 and 27. In a first position of the drawer 32, shown in 
FIG. 4 (rest position) the valve interconnects pipes 24 and 27 and a 
pressure equal to the atmospheric pressure is applied to the bodies 17 of 
jacks 16. In a second position, shown in FIG. 5, after displacement of the 
drawer 32 the valve interconnects pipes 24 and 26 and the high pressure 
prevailing in the accumulator tank 28 is applied to jacks 16. The drawer 
32 of the valve 25 is displaceable by the action of electro-magnetic means 
energized through a control box 33, which is actuated by an appropriate 
signal TB. This signal is generated by sensor means, consisting for 
example of a geophone placed on the target-member 9 or on the ground at 
the immediate vicinity of the target-member. It is adapted to detect the 
seismic impulse transmitted when the mass impinges the target-member, said 
impulse defining the initial reference time. The jacks 16 and the whole 
hydraulic circuit from the motor means which, in combination with the 
movable members adapted to push the deformable elements and with the 
sensor means, form the control means. 
The device operates as follows: 
Once the guide tube has been set up to a substantially vertical position, 
so that the target-member 9 is in contact with the earth and the mass 2 
displaced up to the second end of said tube, the triggering is effected at 
the selected time. The mass 2 falls under the action of its own weight, 
towards the first end of the guide tube 1, guided by the slides 3. At the 
end of its stroke it impinges the target-member 9 and, as a result of the 
shock, bounces in the reverse direction. The valve 32 is then in its rest 
position wherein the pipes 23 and 24 communicate with pipe 27 and with the 
low pressure tank 29. The movable member (14, 15) of each jack 16 is in 
retracted position (right-hand side half cross-section of FIG. 3) and the 
push-plate 14 is pressed against the inner wall of the guide tube 1. The 
deformable strips 13 are in released position and their spacing is larger 
than the diameter of the mass 2. 
The sensor means, upon detection of the transmitted impulse defining the 
instant of the shock, generates a signal TB which actuates the control box 
33. The electro-magnetic means of a valve 25 displaces the drawer 32 
towards its second position (shown in FIG. 5) wherein the pipe 23 and 24 
are in communication with the pipe 26 fed with liquid under pressure from 
the accumulator tank 28. 
The rods 15 of jacks 16 push the push plates 14 secured to the deformable 
strips towards the inside of the guide tube 1 (left hand-side half 
cross-section of FIG. 3). The central portions of the deformable strips 13 
come closer to each other sufficiently to press with force against the 
lateral walls of the mass 2 and thus to secure it in position after its 
bounce (position shown in FIG. 2). The mass cannot fall again and produce 
parasitic seismic impulses. In addition, the flexibility of the deformable 
strips 13 enables a progressive braking and secures the mass in position 
thus avoiding generation of parasitic seismic impulses as in the case of 
an abrupt locking. This effect is completed by coupling the tube with the 
target-member through the assembly of suspension springs 8. It can be 
observed that the mechanical stresses at the locking time are not 
transmitted to the rods 15 of jacks 16, the latter being not secured to 
the push-plates 14. 
The means for moving the mass towards the second end of the guide tube are 
then operated and the drawer 32 of the electrovalve is brought back to its 
first position. The fluid in the bodies 17 of jacks 16 and in pipes 23 and 
24 is brought to the low pressure prevailing in the liquid tank 29 and the 
rods 15 of the jacks are pulled back to their retracted position by the 
return springs 22. The deformable strips 13 move apart from each other, 
thus releasing the mass 2. 
According to the embodiment of FIG. 6, the device comprises three jacks 
placed at 120.degree. from one another and in the same diametral plane of 
the guide tube, at its lower end, in the manner shown in FIG. 3. 
The rods of the jacks 16 are adapted to press against the push-plates 14 
secured to the median portions of three deformable strips 13, also placed 
along the generatices of the tube 1 and secured to the wall thereof at the 
vicinity of their ends, and are adapted to push the three strips towards 
the inside of the tube. The mass 2 is guided, in its motion inside the 
guide tube 1, by three slides 3 placed in the interval between the 
hydraulic jacks 16, at 120.degree. from one another, and secured to the 
inner wall of the guide tube over its entire length. The three pipes 23 
issued from the jacks 16 are connected to a single pipe 24 fed by a 
hydraulic system (not shown) identical to that of the embodiment of FIG. 
4. 
The embodiment of FIG. 6 operates similarly to that described above. 
It would not be outside the scope of the invention to replace the jack 
bodies as well as the hydraulic circuit by any other driving means such as 
electro-magnetic means.