Suspended structure for coupling seismic sources to the walls of a borehole

A suspended structure for coupling a seismic source to the walls of a borehole, in particular for underground prospecting purposes, the structure comprising top (14) and bottom (16) elongate plug elements having transverse dimensions substantially equal to the inside dimensions of the borehole (10), defining therebetween a substantially closed space (32), and suspension elements (30, 50) connecting the bottom plug element to the top plug element without isolating said space from the surrounding walls of the borehole, a seismic source (34) suitable for acting in said space, with the top and bottom plug elements opposing said action in the longitudinal direction of the borehole so that said action is applied essentially against the walls (10) of the borehole level with said space. Applicable to underground prospecting together with pick-up means.

The present invention relates generally to seismic sources for prospecting 
underground, and relates in particular to a suspended structure for 
coupling seismic sources to the walls of a borehole. 
BACKGROUND OF THE INVENTION 
In some seismic prospection methods, a seismic source is disposed in a 
borehole and the source is excited at given depths in order to produce 
well-determined acoustic waves in the adjacent underground formations, and 
a set of sensors is used, either spread over the surface of the ground or 
else disposed in other adjacent boreholes, in order to pick up acoustic 
information for analysis purposes after it has propagated through and been 
reflected from geological formations. 
In the prior art, when a seismic source is deployed in a borehole, no 
special precautions have been taken with respect to the source concerning 
the way in which it is coupled to the geological formations situated at 
the same depth. 
In particular, regardless of the type of source used, the acoustic wave(s) 
developed by exciting the source act both sideways towards said geological 
formations and upwardly and downwardly along the borehole in the form of 
compression waves and/or tube waves. This upwardly and downwardly 
propoagating acoustic energy sets up interference noise phenomena in the 
adjacent geological formations, and this noise is picked up by the sound 
sensors for providing the signals to be analyzed. 
Further, for economic reasons, it is desirable to optimize the quantity of 
acoustic energy which is applied to the geological formations concerned. 
Thus, since a generally very large fraction of the acoustic energy 
propagates vertically along the borehole without substantial effect on the 
geological formation, it is necessary in practice, to repeat sound 
emission a large numer of times, and this is disadvantageous. 
The object of the present invention is to mitigate these drawbacks and to 
provide a structure for confining the acoustic energy produced by the 
source, when excited, against the walls of the borehole at the depth of 
said source. 
SUMMARY OF THE INVENTION 
To this end, the present invention provides a suspended structure for 
coupling a seismic source to the walls of a borehole, in particular for 
underground prospecting purposes, the structure comprising top and bottom 
elongate plug elements having transverse dimensions substantially equal to 
the inside dimensions of the borehole, defining therebetween a 
substantially closed space, and suspension elements connecting the bottom 
plug element to the top plug element without isolating said space from the 
surrounding walls of the borehole, a seismic source suitable for acting in 
said space, with the top and bottom plug elements opposing said action in 
the longitudinal direction of the borehole so that said action is applied 
essentially against the walls of the borehole level with said space. 
Thus, by virtue of their inertia and of their length, the plug elements 
confine the acoustic energy generated by exciting the source to said space 
and prevent it from propagating along the borehole, thereby increasing the 
fraction of useful sound and eliminating a large portion of interfering 
noise. 
Preferably, the top and bottom plug elements are metal cylinders and 
include means for absorbing acoustic energy. 
Thus, those acoustic waves which are emitted vertically from the source are 
further attenuated by the damping effect of the absorption means, with the 
absorption means also contributing to attenuating resonance waves which 
may appear in a borehole.

MORE DETAILED DESCRIPTION 
With reference initially to FIG. 1, a borehole intended for use with a 
seismic source at various depths is designated by reference 10, and the 
geological formation in which the borehole is made is referenced 12. 
A suspended structure in accordance with a first embodiment of the 
invention comprises, going downwardly, two cylindrical elements or boxes, 
namely a top box 14 and a bottom box 16. The bottom end of the top box 14 
is closed by a solid circular plate 18 while its top end is closed by a 
perforated plate such as a grating or the like, which is likewise circular 
and which is referenced 20. The diameter of the cylinders is chosen so 
that they plug the borehole as much as possible at their respective depths 
while not hindering vertical maneuvers of the suspended structure in the 
borehole. 
The bottom end of a suspension cable 22 is attached to the top cylinder 14, 
preferably via its bottom closure plate by means not shown. 
In a similar manner, the bottom cylinder 16 is closed at its top end by a 
solid circular plate 24 and at its bottom end by a grating 26 (not visible 
in the figure) analogous to the grating 20. 
Both cylinders 14 and 16 are filled with a foam of plastic material, as 
shown at 28 for the bottom cylinder. 
Suspension elements, for example three metal cables 30 in the present 
example, extend between the bottom end of the top cylinder 14 and the top 
end of the cylinder 16, with the suspension elements being attached at 
regular intervals around the peripheries thereof. The method of connecting 
these cables 30 to the cylinders is not described since it may be any 
method considered appropriate the person skilled in the art. The cylinder 
16 is thus suspended from the cylinder 14 which is in turn suspended from 
the cable 32, thereby defining a suspended structure for use in seismic 
source firing operations for the purposes of underground prospecting. 
The two cylinders 14 and 16 are thus vertically separated by a 
predetermined height and they plug the borehole so as to define an 
inbetween working space 32. The sides of the space are delimited by the 
walls of the borehole 10, while the top and bottom of the space are 
delimited by the solid plates 18 and 24 of the top and bottom cylinders. 
The presence of the cables between the space 32 and the walls of the 
borehole can be considered negligible, as shown in detail below. 
A seismic source is capable of being excited so as to operate within the 
space 32. In the present example, the primary source consists in a 
primacord fuse 34 which is conventionally constituted by a core of 
explosive material with a predetermined mass per unit length, and 
surrounded by a sheath. A firing cap 36 is provided at the top end of the 
fuse 34 and may be set off at a determined instant from the surface by 
means of an electric cable 38 running along the suspension cable 22. 
When the suspended structure in accordance with the invention is used to 
set up successive seismic shocks at different depths down a borehole, it 
is advantageous to use a mechanism suitable for paying out primacord fuse 
from a continuous roll. A predetermined length of fuse is thus paid out 
after each explosion under appropriate remote-control from the surface, in 
preparation for the next excitation. Such a mechanism may be received in 
one or other of the two cylinders 14 and 16. 
When the seismic source 34 is excited, as described above, it creates a 
three-dimensional acoustic shock wave in the space 32. The vertical 
components of this wave encounter the solid surfaces (the closure plates 
18 and 24) of the top and bottom cylinders 14 and 16, which surfaces 
oppose propagation of said waves along the vertical space constituted by 
the borehole by virtue of their inertia and by the absorption effect of 
the foam 28 contained in each of the cylinders. 
The gratings 20 and 26 serve to hold the blocks of absorbent foam 28 in 
place while still allowing communication between the inside volume of each 
of the cylinders 14 and 16 and the borehole beyond the cage so as to 
enable the foam to provide an acoustic damping effect on the tube waves 
which might otherwise appear in said borehole. 
Consequently, the major portion of the acoustic energy created by exciting 
the source is confined in a relatively uniform manner to the side walls of 
the working space, i.e. to the adjacent geological formation. This 
improves the directivity of the seismic source, and thus improves its 
coupling with the geological formation surrounding it, while avoiding the 
interference phenomena which could be set up by the wave propagating 
longitudinally along the borehole. It may be observed here that the 
suspension cables 30 which are of small transverse dimensions constitute 
substantially no obstacle to said sideways action of the seismic source. 
In the present embodiment, the suspended cage structure is made of steel. 
The pltes 18 and 24 and the gratings 20 and 26 may be fixed to the ends of 
the cylinders by welding. 
FIG. 2 shows a variant embodiment of the invention. In this figure the 
elements or parts which are similar to those of FIG. 1 are designated by 
the same reference numerals. 
The suspended structure in this variant comprises a top cylinder 14 and a 
bottom cylinder 16 which define a closed space 32 therebetween. In this 
case the two cylinders are interconnected by rigid bars 50, made of steel, 
for example. 
As can be seen, the top and bottom walls of each cylinder 14 and 16 are 
conical in shape having an angle at the apex of about 120.degree.. 
In the same manner as described above, the outer walls 20' and 26' of the 
cylinders are made of gratings or similar structures, whereas the inner 
walls 18 and 24 vertically delimit the working space 32 and are solid so 
that they close the borehole at their own depths. 
As in the previous embodiment, (but not illustrated) the cylinders are 
filled with absorbent foam. 
In this embodiment the suspended structure includes a seismic source 
referenced 52 which extends axially and which is suitable for acting over 
360.degree. in the radial direction. This source may be selected from a 
variety of possible sources mentioned below. 
The conical shape of the end surfaces of the cylinders has a first purpose 
of facilitating vertical displacement of the suspended structure along the 
borehole, and in particular of facilitaing the flow of mud present in the 
borehole between the cylinders and the walls of the borehole. 
In addition, the conical shape is intended to reduce the mechanical force 
exerted by the application of the shock wave to the cylinders when the 
seismic source is excited. The slope of the surfaces exposed to the 
working space contributes to reflecting a portion of the received acoustic 
energy sideways towards the adjacent geological formation. In combination 
with the above-mentioned absorption effect obtained by the foam, this 
technique minimizes the transmission of acoustic energy in the 
longitudinal direction along the borehole. 
Further, when using a seismic source which makes use of high-pressure gas 
charges, the above-mentioned conical shape encourages the removal of the 
bubbles formed in the mud upwardly along the borehole, with said removal 
being necessary in order to obtain a satisfactory seismic shock next time 
the source is used. 
It should be observed that the use of rigid bars 50 in this embodiment 
instead of the suspension cables 30 shown in FIG. 1 has the advantage of 
allowing implosion-type seismic sources to be used without tending to move 
the two cylinders towards each other in an undesirable manner under the 
effect of a sudden drop in pressure created by exciting this type of 
source. 
Naturally, the present invention is not limited to the embodiments 
described and shown, but includes any variant or modification within the 
spirit of the invention. 
In particular, the portion via which the bottom box is suspended from the 
top box could be constituted by any type of open structure, such as bars, 
cables, or gratings, in any appropriate number and density. 
Further, any known type of conventional source may be used in the suspended 
structure, the source may be implosive or explosive and preferably extends 
axially to operate over 360.degree. in the radial direction. In 
particular, the following types of source may be used: a water gun, a gun 
for setting of a burst of explosive charges, a sparker, or a 
"mini-sleeve," i.e. an expanding air sleeve-type source. The top and/or 
bottom cylinder(s) could then be specially arranged to receive all or part 
of the mechanism of the source, provided, of course, that the source is 
designed to act in the space 32 between said cylinders. 
The damping of the acoustic energy which tends to propagate in the vertical 
direction along the borehole may be further improved, while retaining or 
omitting the damping by absorbent foam as described above, by ensuring 
that the top and bottom cylinders have very high inertia. They may 
therefore be at least partially made of solid metal. 
In general, the two cylinders should have substantially identical inertia 
in order to prevent the entire suspended structure from having a tendency 
to move upwards or downwards each time the source is excited. 
Further, the structure may be fitted with any other type of device for 
damping or absorbing the lost energy which is applied to the inside ends 
of the two cylinders. 
For example as, shown in FIGS. 3 and 4, vertical fluting 60 or helical 
grooves 62 may be provided in the outside surfaces of the cylinders in 
order to evacuate air bubbles created when the source is excited, and this 
may be done without compromising the coupling to the walls of the 
borehole. 
Another solution may consist in providing a valve member inside each 
cylinder in the vicinity of its end facing the working space, said valve 
member responding to the pressure set up by the source being excited to 
move away from its seat and establish fluid communication with the 
borehole beyond the structure via an appropriate internal passage. For 
example, as shown in FIG. 5, return means such as spring 56 is provided to 
return valve members 54 against it's seat 58. 
The valve members are preferably made of metal and have high inertia. 
Further, regardless of the absorption means provided, the two cylinders 
should behave substantially symmetrically when the source is excited in 
order to limit the algebraic sum of the vertical forces applied to the 
structure to a reasonable value. 
Finally, it may be observed that improving the coupling between the seismic 
source and the surrounding underground formations makes it possible to use 
seismic signals having very much simpler waveforms, which is advantageous 
both from the technical point of view and from the economic point of view.