Method for sorting acoustic signals

The invention relates to a method for sorting the acoustic signals emitted by a mobile, which can be submerged, and which will be designated under the name of target, by eliminating parasitic acoustic signals, radiated in particular by surface vessels at sea, and which interfere with the detection of acoustic signals radiated by the target.

The detection of the signals radiated by the target is generally carried out by the use of a set of acoustic sensors located in the same horizontal plane and arranged in a line.

The method applies to all fields using acoustic passive listening sensors or acoustic buoys, fixed or mobile listening lines made up of hydrophones or groups of hydrophones, etc., as well as to all types of known acoustic signals (spectral lines, narrow bands, wide bands, transient signals, etc.).

The method according to the invention requires the following two conditions to be fulfilled: prior knowledge of the detection domain of the target for each of the N acoustic sensors of the set as a function of the type of acoustic signal processed and permanent knowledge of the geographical position of the N acoustic sensors of the assembly.

The presence of numerous parasitic signals of energy greater than the energy of the target and due to intense maritime traffic, makes the detection of the signals of the target more and more difficult and complicates the task of the operator in charge of the detection. .

The aim of the invention is to significantly reduce the number of unwanted parasitic signals which unnecessarily disturb the observation of the useful signals to be detected, with a view to reducing the workload of the operator.

The subject of the invention is a method for sorting an acoustic signal (S) or a spectral line (F) of maximum energy E, emitted by a target, which can be submerged at sea, from among a multitude of acoustic signals. originating from any acoustic noise generators (W), radiating acoustic signals of the same type and with an energy greater than the energy E, using:

-a set of N acoustic sensors, configured identically in standby mode, which can be submerged, intended to detect the presence of the signal (S) or of a desired spectral line (F), the set of N sensors defining a number any standby cells with two adjacent sensors, each sensor being associated with one or more detection / target domains, which are defined as the geographical areas within which the presence of the target is deemed to be detectable and outside which the target is deemed to be non-detectable, a system for locating the sensors,

a set of elementary geographical areas determined by the intersection of all the detection / target areas of the N sensors, each elementary geographical area being defined as a geographical area within which any acoustic noise generator is always detectable by the same sensors acoustic,

a set of setpoint areas selected from among the set of previous elementary geographic areas, each setpoint area relating to a single watch cell being defined as an elementary geographic area within which the target can be detectable by at least one of the acoustic sensors of the watch cell,

-a maximum number Nmax of sensors relating to one or more reference zone (s) of the same watch cell and capable of simultaneously detecting the target, characterized by the fact that the acoustic signal (S) or the line (F) is detected by a number Nd of acoustic sensors from all of the N sensors, and analyzed by the series of the following three consecutive steps:

1) if the number Nd of sensors detecting the signal (S) or the line (F) is greater than the number Nmax, the signal (S) or the line (F) is eliminated,: otherwise, the signal (S) or the line (F) is kept for localization of the acoustic noise generator (W) radiating the signal (S) or the line (F);

2) definition of the location conditions of the noise generator (W); if these conditions are met, the noise generator is located by the following step 3, otherwise the signal (S) or the line (F) is retained;

3) localization of the buzzer (W) and determination in relation to the reference zones:

if the noise generator (W) is located geographically within one of the reference zones, the signal (S) or the line (F) is kept, otherwise, the signal (S) or the line (F) is eliminated .

The principle of the method, applied to a set of acoustic sensors which can be submerged, consists in determining the geographical zones within which any noise generator radiating an acoustic signal will be declared as being able to be a target or not.

In the first case, the detected signal is kept as it is, and subjected to the observation of the operator in charge of the detection. In the second case, the signal is eliminated and not subjected to the observation of the operator.

When the localization of the noise generator is necessary, the first step of the process can be omitted,

The present invention finds its application in reducing the rate of false alarms generated by high energy acoustic signals radiated by surface noise makers at sea.

Other characteristics and advantages of the present invention will become apparent on reading the following description of a preferred embodiment, given by way of illustration and not limiting, and from the following figures among which:

- Figure 1 shows a set of N passive listening acoustic buoys (A, I, J, B) with N = 4;

FIG. 2 represents the functional diagram of the implementation of the method;

FIG. 3 represents the detection / target domains of the N buoys;

FIG. 4 represents the elementary geographic zones for detecting a parasitic noisemaker;

FIG. 5 represents the residual surface of false alarms by application of the method according to the invention;

FIGS. 6a and 6b represent the principle of determining major detection / target domains;

FIG. 7 represents the case of several detection / target domains for two different types of buoys.

In the example shown in FIG. 1, the acoustic signals from each buoy are transmitted by VHF to a reception unit which then sends them to an analysis system.

The type of analysis proposed, by way of nonlimiting example, consists in carrying out the spectral analysis of the signals received by each of the N buoys in a frequency range Bw (of resolution Df) and for which the analysis results are required. available to the process.

The method constantly knows the position of the N buoys of the set by virtue of an annex system for locating the buoys.

According to the rules of the art in the field, it is possible to ..... geographically locate an acoustic noise generator from the moment when an acoustic signal emitted by the noise generator is detected by at least n buoys of the set to water, with n = 3 for omnidirectional buoys, n = 2 for other types of buoys (Difar, BARRA ...), and that the N buoys of the set are themselves located by a system of localization of buoys (GPS, Gonio VHF, etc ...).

In addition to the necessary detection of the acoustic signal by n buoys, the classical rules require for reasons of precision that the Signal / Noise (S / N) ratio on the n buoys is greater than a minimum value left to the discretion of the man of art.

To simplify the description, and without restricting the generality of the method, it will be assumed that the minimum value of the required S / N ratio is equal to 0 db (limit threshold for detection of the acoustic signal).

Consequently, the only condition required for locating a noisemaker is limited to the detection by at least n buoys of the acoustic signal emitted by the noisemaker.

All the buoys except the omnidirectional type, for which the notion of orientation is not necessary, are fitted with a compass which makes it possible to permanently know the orientation of a reference axis specific to the buoy in relation to to magnetic north.

The main types of signals (S) that can be envisaged are as follows: spectral lines, narrow bands, wide bands, transients.

Each type of buoy is also associated with a detection / target area which can be defined as the geographical area within which the presence of the target is deemed to be detectable and outside which the target is deemed to be undetectable (by the type of buoy concerned).

The detection / target range of each type of buoy for a type of acoustic signal (S) of maximum energy E emitted by the target is determined from the estimate of the maximum detection range of the type of buoy. concerned, which takes into account both the following parameters among others:

.type of acoustic signal (S) to be processed,. characteristics of the type of buoy concerned,. environmental conditions of the medium,. characteristics of the target to be detected, in particular of the maximum radiated energy E,. uncertainties relating to influential parameters, realistic safety margin intended to overcome any contingencies.

According to the previous definitions, we can consider that the delimiting perimeter of a detection / target domain, relating to a buoy X, is the geographical location where a target radiating a given type of acoustic signal (S) and energy maximum E, causes, at buoy X, an S / N ratio equal to the detection threshold (0 db, in the general case).

Consequently, for a given buoy X and depending on the knowledge of the detection / target domain of the type of buoy X, the orientation of the reference axis specific to the X buoy, the geographical position of the buoy X, it is possible to delimit the geographical arrangement of the detection / target domain relating to buoy X, in any system of geographical axes.

In the case of several potential targets to be detected and capable of emitting the same type of acoustic signal (S) but of different energies (E) specific to each of the potential targets such as, for example: target A (Ea), target B (Eb) ............ target Q (Eq) leading to the existence of particular respective detection / target domains Da, Db, ..., Dq, it is possible to determine, for a given type of buoy, the unique detection / target domain resulting from the 15 meeting of all the particular detection / target areas (Da, Db, ... Dq) relating to the type of acoustic signal (S).

Returning to the case of a single target to be tracked, FIG. 1 shows the delimitation of the N (4 in number) detection / target domains of buoys A, I, J, B, respectively designated by (Da, Di , Dj, Db). It is assumed that each of the N 20 buoys in the set has only one detection / target domain for a given type of acoustic signal (S).

It is customary that, taking into account the low immersion of the acoustic sensors of the N buoys of the set, compared to the distance from the noise makers (including the target), the detection / target range for a given type of buoy returns to a flat surface contained in the horizontal plane of the acoustic sensors.

The principle of the method according to the invention can be applied to all types of passive buoys as well as to all types of acoustic signals (S) encountered (spectral lines, narrow bands, wide bands, transients, etc.) subject to fulfilling the 2 following conditions: knowledge of all the detection / target domains of the N buoys of the set and permanent knowledge of the geographical position of the N buoys of the set.

On the assumption that the two preceding conditions are met, the method can then determine so-called reference geographic zones, defined further on ...... ...... - ------------------------------------------------- ---------------------- Then, over time and depending on the number of buoys detecting the acoustic signal emitted by the buzzer, and / or the position of the noise generator radiating the acoustic signal, the method according to the invention is able to decide whether the noise generator can be a target, in which case the detected acoustic signal is then kept as it is, and subjected to the observation of the operator .

If the buzzer cannot be a target, then the detected acoustic signal is eliminated and not subject to operator observation.

To illustrate the operation of the method, we rely on the principle diagram of Figure 1, assuming that the acoustic signal considered is a spectral line (F), analyzed in a range of standby (Bw, Df), on each of the N buoys in the set.

It is accepted, according to current practice, that the line (F) always remains identifiable despite its “dopplerization”, due to the displacement of the mobile radiating it.

It is also accepted, as is customary in standby mode, that all the N watch ranges (Bw, Df), processing the acoustic signals of the N buoys, are configured in a strictly identical manner (band Bw, resolution Df, central frequency FO, constant of integration ...).

We also rely on the functional application diagram of the method shown in Figure 2, the rules will be applied to any set of buoys as shown in Figure 3.

If we consider the representation of the detection / target domains (for N = 4 buoys A, I, J, B) according to figure 3, we can see that we are in the presence of 4 20 different types of buoy and therefore detection / target domains of different shapes and sizes. The 4 corresponding detection / target domains, the respective limits of which Da, Di, Dj, Db are indicated in FIG. 3, determine elementary geographical zones whose designation adopted is as follows:

Zone 1a Zone where the target can be detected by 1 buoy: buoy A

Zone 1i Zone where the target can be detected by 1 buoy: buoy I

Zone 1j Zone where the target can be detected by 1 buoy: buoy J

Zone 2ai Zone where the target can be detected by 2 buoys: buoys A and I

Zone 2ij Zone where the target is detectable by 2 buoys; buoys I and J

Zone 2jb Zone where the target is detectable by 2 buoys: buoys J and B

An elementary geographical area is defined as a geographical area within which any noise generator, in this case the target, is always detectable by the same buoy (s), regardless of the position of the buoy. -35 -— noisemaker-inside the-zone .----------------------------------- ----------------------------------- We can also consider that any set of N buoys can always break down into a set of elementary watch cells, each elementary watch cell being itself made up of 2 neighboring buoys.

The number of elementary watch cells of a set of N buoys is variable and depends essentially on the geographical distribution of the buoys.

In the general case, as shown in FIG. 3, usage requires that (N -1) adjacent standby cells be determined.

By conforming to this usage, we are therefore in the presence in the particular case of FIG. 3, of the following 3 elementary standby cells:

. watch cell (Al). watch cell (lJ). monitoring unit (JB)

By considering the standby cell (lJ) as the basic cell, it will gradually be possible to extend the principle of the process, applied to the cell (lJ), to other elementary standby cells.

By focusing only on the cell (lJ), we can see that the meeting of the 2 detection / target domains (Di and Dj, highlighted in figure 3) constitutes in a way a sanctuary in which the target is detectable by the buoy ( I) and / or by the buoy (J).

Given that, for the example considered, only cell (lJ) interests us, we can exclude from the previous geographical areas areas 1a and 1b, areas where the target is not detectable either by buoy I or by buoy J .

Consequently, the 5 interesting geographical areas, which will henceforth be called locker areas, and which will be identified in italics, are as follows:

Setpoint zone 1 i

Setpoint zone 1 day

Lockout zone 2 ai

Setpoint zone 2 ij

Setpoint zone 2 jb

A setpoint area can be defined as an elementary geographic area in which the target is deemed to be detectable by at least one of the 2 buoys (I or J).

Observation of the above setpoint areas allows us to deduce that the maximum number Nmax of buoys in contact, that is to say of buoys capable of simultaneously detecting the signal from the target, in the sanctuary ”lJ, is equal to 2

-—35- - (Nmax = 2) .--------------------------------------- ---------------------------------------- For the purposes of the discussion we admit, for example, that the localization of any acoustic noise generator can be carried out from the detection of the line (F) emitted by the noise generator, by at least n = 2 buoys of the set.

In accordance with the functional diagram according to FIG. 2, it is possible to follow all the stages of the course of the proposed process, which necessarily ends with the conservation or elimination of the line (F), namely:

Step 1: Taking into account the basic conditions: detection / target areas of the N buoys in the set and geographic positions of the N buoys in the set,

Step 2: Determination of the setpoint zones and of Nmax, with storage of the setpoint parameters thus determined, and updating, over time, of the setpoint parameters,

Step 3: Check that the detection of the line (F) concerns only the cell (lJ) (detection by I and / or J otherwise another watch cell concerned), 15

Step 4: Counting of the number of buoys (Nd) detecting the line (F), on all the N buoys,

Step 5: if the number Nd of sensors detecting the line (F) is greater than the number Nmax, the line (F) is eliminated,: otherwise, the line (F) is kept for localization of the acoustic noise generator (W) radiating it according to the following steps.

Step 6: definition of the location conditions for the noise generator

If the conditions for locating the noise generator are met, the localization of the noise generator is carried out by the following step 7. Otherwise the line (F) is kept.

Step 7: localization of the buzzer and determination in relation to the reference zones

If the noisemaker is in one of the reference zones, the line (F) is kept as being able to come from a target.

If the buzzer is outside any setpoint zone, the line (F) is eliminated.

-----------3₅_-------- Ee- method-according to the invention-takes-place-deliberately -in-the-case - where — one— tries to reduce as much as possible the volume of calculation linked to the location of the noise maker.

If, for various reasons, there is a need to systematically locate the noise generator when possible, the first noise elimination test

I line from step 5 would not be necessary and it would then be sufficient to go directly from i step 4 to step 6.

In order to clarify the operation of the method, the rules detailed above will be applied to the diagram of FIG. 3, assuming, for example, the sole presence of a parasitic noisemaker (W) radiating a line (F) of energy (Wdb), greater than the energy (Edb) of the target, at the frequency (F).

Assume that Wdb = Edb + 6db (approximately) which results, for example, in doubling the detection range of buzzer W of each of the N buoys in the set.

We can therefore consider that for any buoy X of the set, the new detection / noisemaker domain (W) is deduced from the detection / target domain relating to the buoy X by a homothety of ratio K = 2 and having for center homothety, the geographical position of buoy X.

FIG. 4 is a schematic representation of the new detection domains of a parasitic noisemaker (W) with an energy Wdb approximately 6 db greater than the energy Edb of the target. These domains are designated by D'a, D'i, D'j, D'b, and obtained by homothety of ratio K = 2, from respectively, the detection / target domains (Da, Di, Dj, Db) , shown in figure 3.

In FIG. 4, the new detection / noisemaker domains (W) have been highlighted and the initial detection / target domains in thin lines have been retained, as a reminder, from which the setpoint parameters were determined and used by the operator. process.

The intersection of the new detection / noisemaker domains (W) such as D'a, 25 D'i, D'j, D'b determines the existence of new elementary geographical areas which are designated in a similar way to that used previously.

To facilitate understanding, the new zones are designated by standard characters (eg 3aij) while the designation of the setpoint zones always remains in italics (eg: 2iJ).

Assuming that steps 1, 2, 3 of the course of the process have been carried out, the following steps will be approached, based on the diagram in FIG. 4.

It should be remembered, for the record, that the setpoint parameters known and used by the method are the following: zones (2ai, 2ij, 2jb), zones (H, 1j), Nmax = 2.

-------- 35 ----------------------------------------- -------------------------------------------------- --------------------- Step 4 consists of counting the number of buoys (Nd) detecting the line (F) on all of the N buoys.

According to figure 4, there are:

. 1 zone where Nd = 3 (3aij). 3 zones where Nd = 2 (2ai, 2ij, 2jb). 2 zones where Nd = 1 (1i, 1j)

Step 5 consists of eliminating the areas where Nd> Nmax

From the detail of the areas above, we see that if the noise generator (W) is in the area (3aij), the line (F) will be eliminated (Nd = 3> Nmax = 2)

In step 6, the conditions relating to the location of the noise generator are defined.

From the detail of the previous areas, we can deduce that:

.if the noisemaker (W) is in one of the zones (1i or 1j) the line (F) will be kept (location impossible: Nd = 1 <n = 2). if the noisemaker (W) is in l 'one of the zones (2ai, 2ij, 2jb), it is necessary to locate the noise generator (W).

In the case not encountered in the arrangement of figure 4 where the noise generator (W) would be in a zone foreign to the target zones (for example: zone 2ab), the line (F) would be, systematically, eliminated (zone not known of the process).

Step 7 locates the noise maker.

According to the method of the invention, it can be deduced that:

. if for a given elementary geographical area, the designation of this area (in standard characters) is identical to the designation of this same area (in italics), the area considered is part of the setpoint areas and, therefore, the line (F) will be kept.

. otherwise, the zone considered does not form part of the setpoint zones and, therefore, the line (F) will be eliminated.

Examination of FIG. 4 shows that the application of the method results in the conservation of the line (F) if the noise generator (W) is in one of the following zones;

. Zone 2ai / 2ai Setpoint zone

----- 35. Zone 2ij 12 ij. Zone-2jb / 2/5. Zone 1d / 1d. Zone 1 i, 1j

Setpoint area

- Setpoint zone ------------------------------ Setpoint zone (location impossible) Location zones impossible

With regard to the objective of reducing false alarms, it should be noted that the parasitic buzzer (W) being by definition a source of false alarms (FA), the fact:

. keeping the line (F) amounts to keeping a false alarm. eliminating the line (F) amounts to eliminating a false alarm.

FIG. 5 shows the delimitation of the false alarm zones, resulting from the application of the method to the particular case of FIG. 4 and where the hatched false alarm zones correspond to the zones where the line (F) is kept.

Assuming that the parasitic noisemaker (W) can occupy, in a homogeneous and uniform way, the totality of the union of the domains D'i and D'j and by calling:

. St the total surface of the union of the domains D'i and D'j. Sfa the sum of the areas of false alarms (hatched areas in figure 5), we can estimate the% reduction in false alarms according to the following relation:

% reduction (FA) = (1 - Sfa / St) * 100

An evaluation of the surfaces of FIG. 5 made it possible to estimate, in an approximate manner, that the reduction of FA, in the case studied (FIGS. 3, 4, 5) was of the order of 70%.

It can be predicted that if the energy of the parasitic noisemaker (W) (always greater than the maximum energy E of the target) increases, the detection / noisemaker domains (W) will correlatively increase, in a substantially equal homothety ratio at :

Κ = 10^Λ [(Wdb-Edb) / 20], which will tend to involve the presence of many elementary geographical areas which will cause a net increase:

. or the number of buoys (Nd), detecting the line (F), greater than Nmax,. either zones foreign to the setpoint zones,. or identification zones that comply with the locker zones, but not geographically located at the position of the locker zones.

This will lead to a rapid decrease in the rate of false

----3 ₅.

alarms ”and^_to a-rapid ~ decrease in the-% of-volume-of-calculation-of-location-according to the increase in energy (W) of the parasitic noisemaker.

Generally, the shape and / or the dimension of any detection / target domain (D) relating to a given type of acoustic sensor, depend on the frequency of the line (F).

In the frequent case where the type of acoustic signal (S) radiated by the target consists of a certain number (Nx) of spectral lines and that one wishes to apply the method in standby mode, simultaneously, on Nf spectral lines ( with Nf <or = Nx), it will be necessary to take into account the variation of the detection / target range (D) as a function of the frequency (F).

A practical way is to determine the unique detection / target domain D ', by considering the global domain resulting from the union of the Nf detection / target domains (D, F), relating to the domain (D) and to the Nf spectral lines selected.

The new bounding domain D ′ thus defines the detection / target domain relating to the Nf spectral lines to be monitored simultaneously.

The basic rules of the method are then applied to all of the N major detection / target domains thus obtained.

In FIGS. 6a and 6b, there is an illustration for a sensor I of the principle of determining the detection domain / increasing target (D ') relating to any detection / target domain (D), in the case where the acoustic signal (S) is made up of Nf = 3 spectral lines (Fa, Fb, Fc) and where it is desired to simultaneously monitor all of the Nf (= 3) spectral lines.

Each of the spectral lines (Fa, Fb, Fc) respectively determines particular detection / target domains (D, F) such as:

Detection area / target (D, Fa) Detection area / target (D, Fb) Detection area / target (D, Fc)

FIG. 6a represents the very frequent case where the upper domain D ', obtained by the union of the 3 domains [(D, Fa), (D, Fb), (D, Fc)], is identical to one of the domains composites (eg: D, Fc).

FIG. 6b represents the exceptional case where the upper bound domain D ', obtained by the union of the 3 domains [(D, Fa), (D, Fb), (D, Fc)], is different from any one of the 3 composite domains.

----- Dance the^_In reality, most “common buoy types have more than one ---- detection / target domains referred to as lanes.

These pathways can be formed:

. either from the intrinsic characteristics of the type of buoy concerned (eg cardioid pathways of a Difar type buoy). either from a group of acoustic sensors (e.g .: tracks of a BARRA type buoy or a network of buoys)

In general, it can be considered that each path constitutes a particular detection / target domain.

The tracks ”of the same type of buoy have detection / target domains which may be different or identical and have different angular orientations.

By way of example, FIG. 7 shows the basic cell (lJ), consisting of 2 different types of multi-lane buoy.

Each of the buoys (I and J) being, for example, defined by 3 distinct channels, of different detection / target domains and angular orientation.

According to the designation principle adopted, we can write according to figure 7:

<td>Buoy I</td><td>Way</td><td>detection / target domain</td><td>Dia</td>

<td></td><td>Route Ib</td><td>detection / target domain</td><td>Dib</td>

<td></td><td>See it</td><td>detection / target domain</td><td>Die</td>

<td>Buoy J</td><td>Jd Way</td><td>detection / target domain</td><td>Djd</td>

<td></td><td>Way I</td><td>detection / target domain</td><td>Dje</td>

<td></td><td>Jg channel</td><td>detection / target domain</td><td>Djg</td>

In accordance with the basic rules of the method, the intersection of the detection / target domains of the different pathways creates target zones, the meeting of which constitutes the detection / target sanctuary of the cell (lJ).

We can note, as an indication, the delimitation of the corresponding instruction zones (figure 7):

zones (3-way intersection) (I a I b I c) target detectable, at the same time, by the (I b I c J e) target detectable, at the same time, by the (J d J e J g ) target detectable both by

Ways la, Ib, le

Ib, le, I lanes

Jd, Je, Jg zones (with 2 intersection lanes)

—....... 2 (balb) --------- target detectable, in the past, by the -------------- Voies-la, -1b

<td>2 (I b I c) 2 (I b J e) 2 (I c J e)</td><td>target detectable both by the Ib routes, the target detectable, both, by Pathways Ib, I detectable target, both, by the channels, I</td>

<td>2 (I cjg)</td><td>target detectable both by</td><td>Track it, Jg</td>

<td>2 (I b J d)</td><td>target detectable both by</td><td>Routes Ib, Jd</td>

<td>2 (J e J g)</td><td>target detectable both by</td><td>Channels I, Jg</td>

<td>2 (J d J e)</td><td>target detectable both by</td><td>Jd, I lanes</td>

<td>5 zones (1 way)</td><td></td><td></td>

<td>1 (I a)</td><td>detectable target, only, by the</td><td>Channel I a</td>

<td>1 (I c)</td><td>detectable target, only, by the</td><td>Channel I c</td>

<td>1 (J g)</td><td>detectable target, only, by the</td><td>Channel J g</td>

<td>1 (J d)</td><td>detectable target, only, by the</td><td>Channel J d</td>

<td>1 (J θ)</td><td>detectable target, only, by the</td><td>Way J e</td>

Depending on the determination of the previous setpoint zones, it can be deduced that Nmax = 3.

By initializing steps 1 and 2 of the process with the new setpoint parameters, determined above, and taking into account the position of buoys I and J, normal operation of the process can then be allowed to proceed, in accordance with the description above. performed.

Let, by way of nonlimiting example, be Q targets capable of emitting the same type of acoustic signal (S) composed, for each of the Q targets, of a set of Zq spectral lines of different respective energies (Eq), and designated as follows:

Target A defined by Zam lines of respective energies: Ea1, Ea2, ... Eam,

Target B defined by Zbk lines of respective energies: Eb1, Eb2, .... Ebk,

Target Q defined by Zqp lines of respective energies: Eq1, Eq2 ..... Eqp, and let Nz be the total number of lines to be monitored simultaneously, with Nz = Zam + Zbk + ... + Zqp. There are therefore Nz particular detection / target domains, relating to any one channel V of a buoy X of the set.

According to the principle described above, the single upper detection / target domain, relating to channel V of buoy X, can be obtained by combining the Nz particular detection / target domains defined above.

In general, one can easily imagine that the principle of the 35 ...... process, applied to the^-cell^- base (l-jy, ~ can extend, -from · close to, to all the other standby cells of the set.

In an identical manner, the principle of the method, applied to the line (F) of the watch range (Bw, Df), can initially be extended to all the lines detected in the watch range (Bw , Df), and secondly, to extend to all the lines detected in the other watch ranges, if they exist.

The method according to the invention can benefit from the conventional methods of implementation according to the rules of the art such as, for example:

-location of a noise generator (W) from buoys detecting the line (F) with the maximum S / N ratio (improvement of the location accuracy);

- data fusion such as, for example, possibility of extension, of the elimination of the line (F) to all the watch ranges detecting the line (F), in the case where the line has been previously eliminated by the method in any range (Bw, Df);

- correlation of the acoustic location of a noise generator (W) with the surface situation (Radar, for example) in order to remove any doubt about the identification of a noise generator (surface building or not);

- elimination, by associativity, of any type of signal (S) whose geographical position of the emission source coincides with the geographical position of the noise generator (W) radiating the line (F) 'and declared "non-target" following the application of the method according to the invention.

Considering that the differential drift of the buoys is a slow phenomenon (a few meters / minute) with a low periodicity of updating the reference zones, that the calculation volume is low (high percentage of elimination of AF without calculation of location ), that the elimination of a line (F) in any range (Bw, Df) can lead to the elimination without calculation of this line (F), on all the watch ranges concerned, it can be deduced from this that the computing power dedicated to the implementation of the method should be very marginal, compared to the computing power of conventional acoustic detection equipment.

It can also be noted that the application of the method according to the invention is based only on the consideration of the acoustic signals, which gives it total autonomy; in particular, the knowledge of the geographical location of the buildings of surface, while useful, is not necessary.

In conclusion, the application of the method according to the invention makes it possible to reduce, very significantly, the rate of false alarms, to release the operator from any manual intervention by the complete automation of the operation, to adapt easily. various 'configurations' of sets of buoys, in standby mode. ------------ In any case, the application of the process simplifies, in a very significant way, the problems associated with the processing of false alarms and has a very favorable impact on the cost of construction of acoustic buoy detection equipment.

In addition, taking into account the fact that the method can be applied to any type of acoustic listening sensor as well as to any type of acoustic signal subject to the required conditions, it is possible to envisage its application to any listening device. passive fixed or mobile, operated by current or future signal processing.

The method according to the invention can also be transposed to the field of aerial acoustic detection.