Process and an apparatus for seismic geophysics with processing by focuses

The invention relates to seismic prospecting of the sub-surface. At least two adjacent focuses F and F', at least one reflecting interface H and directions such as (D.sub.1) and (D.sub.2) are selected in the sub-surface. A seismic trace, from which a portion containing the effect due to the reflecting interval is extracted, is made to correspond to each direction and to each focus. The correlation function between the two trace portions relative to two adjacent focuses and the same direction is then determined. The correlation functions obtained are finally summed for the different directions. Application to high resolution seismic prospecting.

BACKGROUND OF THE INVENTION 
The present invention relates to the seismic prospecting of the 
sub-surface. 
With this technique, a number of seismic sensors as well as one or more 
sources of artificial seismic shocks are arranged on the soil. In simple 
versions, the sensors and sources are situated in the same vertical plane. 
They can be considered as approximately aligned if the unevenness of the 
terrain is disregarded as the effects thereof can be corrected later on. 
The seismic sensors and sources are usually also distributed regularly, 
often at the same intervals. 
The various seismic sources are excited successively on the terrain. Each 
time a source is excited, the various seismic signals or "traces" received 
by each of the sensors owing to the acoustic waves produced by the shock 
are recorded selectively. Each trace therefore corresponds to a 
source-sensor pair. 
In so-called "reflection" seismics, one is interested in the reflections of 
acoustic waves from "reflection points" in the sub-surface. For this 
purpose, it is known to combine all the traces for which the source and 
the sensor are symmetrical about a given vertical, for example by adding 
all these traces. The reflections appearing in the resultant trace 
indicate the reflection points. Suitable graphic representation of the 
resultant traces associated with the various verticals permits 
geophysicists to understand better the structure of the sub-surface. This 
process of so-called "multiple coverage" reflection seismics therefore 
seeks reflection points corresponding to the same depth in the various 
traces received. 
Reflection seismics with multiple coverage will give valuable information 
about the sub-surface. However, "deaf zones" sometimes appear and result 
in uncertainties about the interpretation of the profile of certain 
strata. More generally, although reflection seismics defines the strongly 
reflecting interfaces fairly well, it does not permit the features in the 
intervals between interfaces of the sub-surface (velocity and absorbtion 
in particular) to be analysed in detail. 
SUMMARY OF THE INVENTION 
The present invention aims specifically to fill these gaps. 
The process proposed starts from the same basic stages as those of the 
prior art. Some seismic sensors are arranged on the soil with sources of 
artificial seismic shocks, the sensors and sources being situated in 
approximately the same vertical plane. The seismic sources are excited one 
by one, while the seismic signals or "traces" received as a function of 
the time by the various sensors and related to the acoustic waves induced 
in the sub-surface by each of the shocks are recorded selectively each 
time. Static and dynamic correction of these traces is then effected to 
allow for the fact that sensors and sources are not arranged strictly in 
the same horizontal. If there are i sources and j sensors, there are thus 
i sets of j traces which are therefore designated individually by 
s.sub.ij. 
Implementation of the process according to the invention also presupposes 
the availability of preliminary information on the sub-surface, in 
particular on the reflecting strata thereof. This information can 
originate from geological research and/or geophysical probes of all sorts, 
for example electrical probes. Conventional reflection seismic processing 
by multiple coverage, as defined above, is preferably carried out 
beforehand starting from the recorded and corrected traces. In other 
words, this processing involves a combination (most simply, addition) of 
traces having a common reflection point. 
The process according to the invention may comprise the following 
subsequent operations, starting from the above-mentioned i sets of j 
traces s.sub.ij : 
(a) selecting at least two adjacent points known as focuses, at least one 
reflecting interface and a predetermined direction in the sub-surface; 
(b) selecting for each focus a seismic trace which corresponds to a 
propagation path passing through this focus and orientated in the 
predetermined direction, taking the reflection at the interface into 
consideration; 
(c) extracting from each trace selected in this way a portion of trace 
containing the effect due to the reflecting interface; 
(d) quantitatively comparing the two portions of trace obtained for the two 
focuses to each other; 
(e) repeating operations a to d, changing the predetermined direction each 
time; and 
(f) averaging the results of comparisons made for paths of waves orientated 
in different directions, thereby permitting assessment of any differences 
between the two focuses with regard to their seismic properties. 
In a preferred embodiment, the two portions of traces in operation c are 
defined from the same time gate. Operation d involves determining the 
correlation function between the two portions of traces, and operation f 
involves summing the correlation functions obtained for the various wave 
path directions. 
One of skill in the art knows that the seismic properties of the 
sub-surface can be represented by various parameters. Two of these 
parameters are advantageous for carrying out the invention: time and 
energy. Thus, it is advantageous to consider at least one of the following 
parameters at the peak of the sum of the correlation functions: delay of a 
peak with regard to the time origin or peak amplitude (in relation to the 
reflected energy). 
The difference between the peak delays relative to two pairs of two 
adjacent focuses having a common focus is related to the difference in the 
propagation rates of the waves at these points, whereas the difference in 
the peak amplitudes is related to the variation in absorption of the waves 
existing when passing from one pair of focuses to the next. 
A standard correction is preferably made on the sources and sensors in the 
region of the initial seismic traces and, during operation d, the 
variations in the reflecting capacity of the interface are taken into 
consideration when passing from one focus to another. This is important, 
in particular, if the amplitude of the traces is of interest. 
In a particular embodiment, the propagation paths are defined by simple 
equations. In this case, e.sub.i designates the abscissae of the various 
sources, r.sub.j those of the various sensors, p the common interval 
between sources and between sensors, and h the depth of the reflecting 
interface which is assumed to be horizontal, while x.sub.o and y.sub.o are 
the co-ordinates of a focus. Operation b involves the search for traces 
associated with a source (e.sub.i)-sensor (r.sub.j) pair such that: 
EQU 2h x.sub.o =y.sub.o .multidot.r.sub.j +(2h-y.sub.o)e.sub.i 
The orientation of each trace is given by the value 
##EQU1## 
and the above equations are substantially satisfied with an accuracy of 
p/2. A dip correction taking into consideration the inclination of the 
reflecting interface to the horizontal is allowed for if necessary. 
Up until now, only a single reflecting interface, which is preferably a 
strongly reflecting stratum deeper than the focuses, has been considered. 
In reality, such a situation arises fairly frequently, but it is also very 
common for the sub-surface to comprise several moderately reflecting 
interfaces, some of which are above the focuses. The invention also 
applies to this situation. 
In this case, several reflecting interfaces (all those existing or only a 
proportion thereof) are selected during operation a. In operation b, one 
trace is selected for each focus, each propagation path orientation and 
each reflecting interface. And the comparison operation d involves the 
various pairs of traces which are related respectively with the two 
focuses while corresponding to the same orientation and the same 
reflecting interface. 
This process therefore involves comparison of two similar trace portions 
not only for each orientation of the wave path, but also for each of the 
reflecting interfaces retained. In practice, it is often more advantageous 
to group the portions of traces relating to various reflections and to the 
same orientation so that operation c thus makes use of trace portions 
constituting a continuous sequence of reflection effects. This operation c 
thus also involves the synthesis of a composite seismic trace connected 
with each focus and each propagation path orientation starting from 
portions extracted from the various traces relative to the various 
reflecting interfaces. 
In a particular embodiment of the process with multiple interfaces, the 
traces of all sensors r.sub.j, with j varying from 1 to n, are explored 
during operation b for each source e.sub.i and subject to a correction for 
dip. From each trace, there is taken about a time t.sub.ij a section 
defined by a time gate f.sub.i, with t.sub.ij defined by 
EQU y.sub.o (r.sub.j -e.sub.i)=(x.sub.o 
-e.sub.i).multidot.2.multidot.V.multidot.t.sub.ij.multidot.sin.alpha. 
wherein V represents the average propagation velocity in the sub-surface 
whereas x.sub.o and y.sub.o are the co-ordinates of a focus, and with 
f.sub.i defined by 
##EQU2## 
wherein p is the common interval between sources and between sensors. 
Where all the sections of traces which are temporally adjacent to each 
other are combined for each focus and each orientation 
##EQU3## 
In the process with multiple interfaces, the evaulation operations 
preferably take place in the following manner: 
the comparison operation d involves determining an upstream correlation 
function between the similar trace portions corresponding to the same 
orientation, to two focuses and to times prior to the arrival of the waves 
at each focus, and determining a downstream correlation function between 
the similar trace portions corresponding to the same orientation, to the 
two focuses and to times subsequent to the arrival of the waves at each 
focus and 
operation f involves determining for each focus, on the one hand, a first 
upstream summation which involves the upstream correlation functions for 
the various orientations and, on the other hand, a second downstream 
summation which involves the downstream correlation functions for the 
various orientations. 
If necessary, a correction is made, taking into consideration the effect of 
the disharmony between the interfaces situated above the focuses and those 
situated below. 
Fundamentally, the process according to the invention proposes a comparison 
of the seismic properties of two adjacent or focus points known as such 
because they are related to a collection of traces which are related to 
seismic wave paths which meet them in various directions. However, the 
process proposed is particularly useful when considering a fairly high 
number of focuses (for example, 100) distributed along a line or in a 
region of the sub-surface to be examined, preferably at an interval equal 
to the common interval p between the sources and between the sensors. In 
the case of a line of focuses, the line can be defined by a geological 
level to be examined, in particular reflecting interface, where the 
velocity and absorption of the seismic waves are sought. If the line of 
focuses coincides with a reflecting interface, the invention permits an 
energy balance to be made along this interface since the absorbed energy 
is equal to the incident energy reduced by the transmitted energy and the 
reflected energy. 
On the other hand, focuses which are suitably distributed in a region of 
the sub-surface corresponding to a deaf zone in conventional seismic 
reflection permit this deaf zone to be examined more fully. More 
generally, the process according to the invention provides a much better 
resolution than the processes of the prior art. 
The invention also relates to the analogical or digital apparatus intended 
for carrying out the operations characteristic of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a sub-surface comprising a single or "horizon" reflecting 
interface H which is horizontal and is situated at a depth h. The 
abscissae of the sources E.sub.i and sensors R.sub.j are designated 
e.sub.i and r.sub.j on the horizontal soil defining the zero ordinate. The 
distance between sources is the same as between the sensors and is 
designated p. If a shock is created in the region of the source E.sub.i 
the sensor R.sub.j receives the seismic trace s.sub.ij. In a known manner, 
the traces form the subject of static and dynamic correction operations so 
that a horizontal soil with a zero ordinate can be obtained by 
disregarding the uneveness of the terrain. 
The invention considers paths of seismic waves passing in various 
directions through focuses such as F. (Although the seismic waves 
propagate over a volume, it is usual to allocate a propagation axis or 
path to them). 
A wave path between E.sub.i and R.sub.j, after reflection at the reflecting 
point M of depth h, can be quantified by a parameter a which varies from 
zero to one between E.sub.i and R.sub.j. If x and y are the co-ordinates 
of a point on the path E.sub.i M R.sub.j, then: 
##EQU4## 
According to the invention, a focus F with co-ordinates x.sub.o and y.sub.o 
(unit 12, FIGS. 4 and 5) and a reflecting horizontal at depth h (unit 11) 
as well as a direction (which will change, in a plurality of possible 
values) are selected. The condition for the focus F to be situated on the 
portion E.sub.i M of a wave path is obtained by eliminating a from 
equations (I) and (III), and is written: 
EQU 2h.multidot.x.sub.o =y.sub.o .multidot.r.sub.j +(2h-y.sub.o)e.sub.i (IV) 
and the direction or orientation of this wave path is represented by the 
value 
##EQU5## 
As an alternative, each focus can be placed on a portion such as MRj of the 
wave path and the condition will thus be written: 
EQU 2h.multidot.x.sub.o =y.sub.o .multidot.e.sub.i +(2h-y.sub.o)r.sub.j (VI) 
The direction is defined by the inclination of MRj, or by reference to the 
angle .alpha. in equation (V). 
It is thus feasible to make a source sensor pair E.sub.i -R.sub.j and a 
trace s.sub.ij (unit of calculation 14 which determines, for example, 
addresses in the recording 15 of all seismic traces) correspond to a 
focus, a reflecting horizon and a predetermined direction. According to 
FIG. 4, the unit 10 defines a parameter of direction which it will be able 
to increment on command and which is transmitted to the calculation unit 
14. According to the equations given above, the calculation unit 14 can 
thus select a particular trace at 15 by making, if necessary, an 
approximation between the direction selected at 10 and that given by 
equation (V) above. 
More generally, by scanning all the traces s.sub.ij, a subset of traces 
corresponding to paths passing through the focus F in different directions 
and reflecting on the horizon H will be found. In each of the traces, 
reflection due to a reflection point such as M will be seen. 
FIG. 5 illustrates a variation on this type of system. Starting from a 
choice of focus 12 and of interface 11 the unit 24 calculates the 
addresses of this sub-assembly of traces at 15 as well as the associated 
orientations. 
In practice, the reflecting horizon is frequently not rectilinear. Each 
time that its slope to the horizontal exceeds a pre-established threshold, 
it is preferable to make a correction of dip using the unit 14 or 24 which 
causes trigonometric functions of the inclination .theta. of the horizon 
at the reflection point M or approximations of these trigonometric 
functions to intervene in the equations given above by means of their 
series expansion since the perpendicular to the horizon at point M remains 
the bisector of the angle E.sub.i M R.sub.j. 
The invention thus causes the intervention of a series of focuses placed, 
for example, along a line J. Two focuses F and F' are illustrated in FIG. 
1. As with the first one, another source-sensor pair (units 14 or 24) is 
made to correspond to the second focus F', to the horizon H and to the 
predetermined direction (D), taking into consideration the equations given 
above, for example, the pair E.sub.i+1, R.sub.j+1, with which the trace 
s.sub.i+1, j+1 is associated. 
As shown in FIGS. 2a and 2b, portions of the two selected traces s.sub.ij 
and s.sub.i+1, j+1 are extracted during a time gate f whose time is 
selected in advance with regard to its starting point and duration (sample 
memories 16 and 17). 
Next, the two trace portions obtained are compared to each other, 
advantageously by determining the correlation function between these 
portions (unit 18). The shape of such a function is illustrated in FIG. 
2c. 
The same operations are then repeated for the various focuses such as F and 
F' with the same reflecting horizon H and a different direction (D.sub.2) 
instead of (D.sub.1). For this purpose, the end of the correlation at 18 
increments the direction parameter given by the unit 10. The selection 
thus gives other source-sensor pairs: 
EQU E.sub.i+1 -M.sub.2 -R.sub.j-1 and 
EQU E.sub.i+2 -M'.sub.2 -R.sub.j-m+1 
for example, hence the traces s.sub.i+1, j-1 as well as s.sub.i+2, j-m+1. 
Two portions corresponding to the same time gate are also extracted from 
these traces. A time gate which is particular to each direction 
(connection between the unit 14 and the memories 16 and 17) can be made to 
correspond, or a general time gate which is sufficiently large to include 
the reflection whatever the orientation can be selected. And the 
correlation function between these two trace portions is determined here 
again. 
These operations are renewed for further orientations. As far as possible, 
all the orientations available are utilised for each focus by means of the 
traces s.sub.ij. A new correlation function is obtained each time. 
Finally, the unit 19 (FIGS. 4 and 5) sums the correlation functions 
obtained for the various wave path directions (FIG. 3). 
Each of the individual correlation functions comprises a peak which can be 
defined by its time and its amplitude. Similarly, the sum of the 
correlation functions will comprise a peak associated with a time and an 
amplitude. 
The applicants have observed that the time interval between the peak of the 
sum of the correlation functions and the time origin is representative of 
the travel time variation of the waves when passing from F to F'. If this 
variation is positive, the terrain is "slower" in the vicinity of the 
focus F' than the terrain in the vicinity of the focus F, slowness 
referring to the propagation of seismic waves in this case. Conversely, if 
the variation is negative, the terrain at F' is faster than at F. 
The amplitude of the peak of the sum of correlation functions is related to 
the energy carried by the seismic waves passing in the different paths 
taken into consideration. That is to say, it can be considered as 
representative of the average energy passing through the two focuses 
concerned, F and F' in this case. 
The peak amplitude of the sum of the correlations can thus be allocated to 
a point situated between the focuses F and F', for example in their 
midpoint. Repetition of this operation over a large number of focuses 
permits the relative energy curve to be constructed step by step along the 
line (J) of the focuses (FIG. 1). With a sufficiently fine sampling 
interval, information about the distribution of seismic energy along the 
line J is thus obtained without the need to evaluate the energy passing at 
each of the focuses F and F'. 
For this purpose, it is preferable to make a standard correction to the 
sources and the sensors so as to take into consideration their individual 
responses. It is advantageous also to take into consideration the 
variations in the reflecting capacity of the reflecting interface H as 
known, for example on the basis of conventional preliminary processing, in 
multiple coverage reflection seismics. 
Moreover, it will be observed that the distribution of seismic energy along 
the line of the focuses (J) is related to the variation in absorption of 
seismic waves occuring along this line (J). 
It has been stated in the foregoing that two focuses, a reflecting 
interface and a predetermined direction are chosen in order to select two 
traces, to extract two respective portions from them, to construct the 
correlation thereof and to recommence by changing the predetermined 
direction. 
Of course, it may be advantageous in practice to group the operations 
differently and, for example, giving oneself the focuses and the 
reflecting interface: 
To seek all the traces corresponding to a passage through each focus with 
reflection at the interface by associating an orientation or direction to 
each one (unit 24, FIG. 5); 
to extract from each trace a useful portion containing the "reflection" 
with the aid of a gate which may or may not depend on the orientation 
(sample memories 26 and 27 with a memory positioning for each 
orientation); 
to construct for each orientation the correlation (18) of two trace 
portions relating to two adjacent focuses (step by step if there are more 
than two focuses selected); 
to build (19) the sume of the correlation function associated with two 
adjacent focuses for each of the orientations (and for all the focuses 
initially selected). 
In this case, the operations are advantageously co-ordinated by a control 
unit 20 which may be incorporated into the calculation unit 24. The 
elements 15 to 19 as well as 26 and 27 can be analogical (magnetic tape 
memories, for example). They will preferably be digital like the magnetic 
recordings made on the terrain, and all the processing can thus be 
effected in a computer. 
Up until now, a single interface has been considered in the sub-surface, 
beneath the line or region of the focuses. Such a situation may in fact be 
encountered, at least in the form of a deep interface whose reflecting 
properties greatly surpass those of other superjacent interfaces. However, 
it is frequent to find several comparable reflecting interfaces, some of 
which are situated beneath the focuses and some above. The method in which 
the invention can be generalised to cover these cases will now be 
described. 
Overall, all or some of the existing reflecting interfaces are taken into 
consideration. All the source-sensor pairs corresponding to paths passing 
through the focus in the selected direction, with reflections at the 
various interfaces, and not only a single source-sensor pair will be made 
to correspond to each focus and each direction. FIG. 6 shows this in the 
case of a single focus F to simplify the drawing. Five reflecting 
interfaces which are all horizontal are shown. Owing to their parallelism, 
there will be a common source E.sub.i for the various possible 
reflections, whereas the sensor changes with the reflecting interface 
(R.sub.j-m, R.sub.j-n, R.sub.j+q, R.sub.j+r). 
Each of the source-sensor pairs thus defined corresponds to the same focus, 
the same direction and to one of the reflecting interfaces. As above, the 
focus can alternatively be placed on the rising portion common to the wave 
paths reflected by different reflection horizons. 
It is thus possible, as above, to determine the correlation function of two 
trace portions corresponding to two adjacent focuses for the same wave 
propagation direction and the reflections thereof at the same interface, 
then to sum the various correlation functions obtained when the 
propagation direction is changed. This results in a correlation sum for 
each pair of adjacent focuses and each interface. And the sum of 
correlations allocated to each interface, as described above with reagard 
to a single interface, can be interpreted. 
Thus, by comparing, for each pair of focuses, the data obtained at the 
level of the various interfaces it becomes possible to follow, as a 
function of the depth (successive interfaces): 
The evolution of the variations in propagation velocity between the two 
focuses; 
the evolution of the average energy passing in the vicinity of the pair of 
focuses. 
The distinction between what happens upstream of the focuses (interfaces 
situated above them) and what happens downstream of the focuses 
(interfaces deeper than them) is particularly interesting. 
For this purpose, the invention recommends a very advantageous mode of 
operation in which the reflection effects encountered at the different 
interfaces are considered in a continuous succession. A composite seismic 
trace which groups sequentially the various reflection effects, respecting 
their temporal situation and the time continuity, is preferably 
synthesized for this purpose, starting from trace portions corresponding 
to the different interfaces (same focus, same propagation direction). 
In a simple particular embodiment, the following mode of operation is 
adopted, (see FIG. 6): 
(a) As above, at least two focuses (such as F) a propagation direction (D) 
and, this time, several interfaces are selected; 
(b) a seismic trace s.sub.ij (corresponding to a source E.sub.i --sensor 
R.sub.j pair, allowing for a dip in the reflecting horizons if necessary) 
is associated with each focus, with the direction and with each interface; 
(c) in each trace, there is taken about a time t.sub.ij a portion defined 
by a time gate f.sub.1, with t.sub.ij defined by 
EQU y.sub.o (r.sub.j -e.sub.i)=(x.sub.o -e.sub.i)2.multidot.V.multidot.t.sub.ij 
.multidot.sin.alpha. (VII) 
x.sub.o and y.sub.o are the co-ordinates of focus and V represents the 
average propagation rate in the sub-surface (or the rate of the vicinity 
of the focus concerned, if it is known otherwise), whereas f.sub.i is 
defined by 
##EQU6## 
wherein p represents the interval which is assumed to be common between 
the sources and between the sensors; after which, all these trace portions 
can be combined in composite trace associated with the selected focus and 
orientation, since they are temporally adjacent to each other (FIG. 7a) 
The distinction between what happends upstream and downstream of the 
focuses can thus be made very simply, the process taking place as follows, 
with the aid of another composite trace (FIG. 7b) relating to a focus F' 
next to the first one; 
(d) An upstream correlation function (FIG. 7c) between the composite trace 
portions of two adjacent focuses (same propagation direction) situated in 
an upstream gate is determined for times smaller than y.sub.o /V as well 
as a downstream correlation function (FIG. 7d) between the same composite 
trace portions which are situated in a downstream gate for times greater 
than y.sub.o /V. The upper limit of the downstream gate can be defined 
according to the deepest reflecting stratum. 
(e) as in the case of a single reflecting interface, the process is 
recommenced for different propagation directions passing through the two 
adjacent focuses. 
(f) the upstream correlation functions (FIG. 8a) and the downstream 
correlation functions (FIG. 8b) obtained for the different propagation 
directions of the waves passing through the two adjacent focuses are 
summed separately. 
The distance between the peaks of the correlation sums reflects the 
variation in the travel time passing through the focus F or the focus F'. 
In the case of non-concordant tectonics, there may be a disparity between 
the distribution of the reflecting interfaces situated above the focuses 
and the distribution of the interfaces situated below. This disparity is 
measured over a conventional seismic section with multiple coverage for 
example. It is easy to determine from it the effect on the correlations 
and, by a time correction, to cut out this effect of the temporal interval 
between the correlation peaks. 
Moreover, as in the case of a single reflecting interface, the amplitude of 
the peaks of the correlation sums permits the evolution of the energy 
absorbed when passing from F to F' to be appreciated. 
To this end, the ratio between the peak amplitude of the upstream 
correlation sum and the peak amplitude of the downstream correlation sum 
is preferably determined for each pair of focuses. Observation of the 
evolution of this ratio along the line of the focuses, (or inside the 
region covered by the focuses) gives access to the evolution of the 
absorbed energy. 
The invention applies more particularly if the line of focuses coincides 
approximately with a reflecting interface: the sum of the upstream 
correlations corresponds to the incident energy. The sum of the downstream 
correlations corresponds to the transmitted energy. For its part, the 
reflected energy can be determined by taking the traces corresponding to 
paths having a common reflection point situated in the vicinity of the 
focus (seismic processing of the conventional multiple coverage type). And 
an energy balance can thus be made along the line of focuses since the 
absorbed energy is equal to the incident energy reduced by the transmitted 
and reflected energies. 
The invention thus permits analysis of the sub-surface with high resolution 
in the region of the selected focuses. As the skilled artisan knows, this 
analysis can obviously be refined by proceeding by successive 
approximations, the process being repeated, each time with increasing 
precision in the parameters determined during proceding stages: 
more precise corrections in the dip, the disharmony or the reflecting 
capacity of the interfaces; 
more precise definition of the wave propagation velocity or consideration 
of non-rectilinear propagations due to refraction effects, for example. 
The invention is applied in all fields where geophysical exploration is 
useful and particularly in the following case: "Enhanced recovery" is 
frequently desired in reservoirs (oil for example) by injecting water 
therein; the invention permits the evolution of a fluid during such 
operations to be followed accurately. 
For this purpose, the values finally issuing from the process according to 
the invention will form the subject of a suitable graphical representation 
along the line of the focuses or inside the region of the focuses.