OA20888A - Detection system for detecting discontinuity interfaces and/or anomalies in pore pressures in geological formations. - Google Patents

Abstract

Detection system (100) comprising: a drill bit (200) in which one or more electro-acoustic transducers (10, 11, 12) configured to operate as a transmitter and/or receiver, are integrated; analogue driving electronic circuits (110) and analogue receiving electronic circuits (111); a processing and control unit (120) associated with the analogue driving (110) and receiving (111) electronic circuits, the processing and control unit (120) being associated with a data storage unit (121) and is powered electrically by an electrical supply system (122), the processing and control unit (120) being configured for generating driving signals to be sent to the electro-acoustic transducer acting as a transmitter (10) by means of the analogue driving electronic circuits (110), for acquiring the signals received from the electroacoustic transducer acting as a receiver (20) by means of the analogue receiving electronic circuits (111) and for processing the received signals in such a way as to determine the presence of discontinuity interfaces and/or anomalies in pore pressures in geological formations; characterised in that each of said electro-acoustic transducers (10, 11, 12) is adapted to be in contact with a pressurised fluid and is of the type comprising: - a tubular body (20), which extends in length along a longitudinal direction X, said tubular body (20) comprising a first end portion (21) and a second end portion (22), opposed to each other longitudinally, the tubular body (20) having internally a first chamber (23), which ends with the first end portion (21) and a second chamber (24) on one side adjacent and in fluid communication with the first chamber (23) and, on the other side ending with the second end portion (22), the first end portion (21) being closed towards the outside by means of a membrane (26) applied to the tubular body (20), the second end portion (22) having one or more openings (27) that put it into fluid communication towards the outside, the first chamber (23) containing in its walls a plurality of electrical windings (25) arranged in succession between them in the longitudinal direction X, the second chamber (24) being filled with a liquid; a movable element (30) housed in the first chamber (23), the movable element (30) comprising a plurality of permanent magnets (31) packed and arranged one above the other with alternate magnetisation in the longitudinal direction X, and separated one from the other by discs of ferromagnetic material, the movable element (31) being supported at the longitudinal ends by springs (40), the movable element (30) also being connected to said membrane (26); a movable piston (45) positioned and slidable in the second end portion (22).

Description

DETECTION SYSTEM FOR DETECTING DISCONTINUITY INTERFACES AND/OR ANOMALIES IN PORE PRESSURES IN GEOLOGICAL FORMATIONS

The présent invention relates to a détection System for detecting discontinuity interfaces and/or anomalies in pore pressures in geological formations.

These may be due to the passage from rock formations of different densifies (e.g. clayey layers, réservoir rocks that contain liquîd and/or gascons hydrocarbons, sait dômes, basalts) or to karst phenomena or faults which if not signalled can lead to the formation loss of the drilling mud with ail the conséquences of the case such as an increase m costs to restore the quantity of mud necessary to continue drilling or such as problems of instability of the walls of the well that could arise when the hydraulic thrust of the mud that is lacking due to the formation loss is reduced.

It is therefore essential to detect rock formations contaîning overpressure fluîds before the drilling head or drill bit reaches the area itself.

The possibility of identifying the position of a discontinuity interface and/or an anomaly in the pore pressures allows in fact to adopt a sériés of préventive measures in order to prevent a blowout situation from being triggered, that is, a dangerous event of uncontrolled leakage from the extraction well (blowout) of the tluids, in overpressure with respect to the hydraulic thrust of the mud, by adopting appropriate countenneasures.

The solution currently used is to estimate the pressures in advance with respect to the drilling operations, by means of seismîc reflectometry methods in order to identify the trends of these pressures at resolutions of the order of about ten meters.

Subsequently, the estimâted trends are possibly recalibrated as a function of the depth of the well, with local measurements in the well (sound speed, resistivity, gamma ray, density and so on) carried ont during drilling. In this way, there is also an improvement in the latéral resolution of the pressure trends.

The mathematical models used today for Processing the measurements and the définition of the trends of the pore pressures of the formations are estimation models that do not allow to predict anomalous pressures, possibly présent in the formations yet to be drihed, in particular if the lithology of these formations shows rapid variations with respect to the formation on the side of the drill bit and/or the causes of the geo-pressures lie in phenomena not attributable to subcompaction.

In order to detect discontinuity interfaces and/or anomalies in the pore pressures in geological

-2formations, it is known to apply the geophony by using voice coil type electromagnetic actuators as a receiver which can hâve a movable coil and a fixed magnetic core or vice versa a fixed coil and a magnetic movable equipment.

US20180100387A1 illustrâtes the réalisation of electromagnetic transducers by exploiting different types of permanent magnets in a movable part for geophonic measurement.

It is also known to use capacitive type transducers, such as the so-called MEMS, or of the optical or piezoelectric type; the latter can reach maximum operating frequencies comprised between about 3 and 5 MHz.

In any case, the transducers used in known détection systems are characterized by relatively high dimensions and energy consumption high enough to require the connection to an electrical power supply which increases the complexîty of installation at high depths; finally, the abovementioned devices are not designed to operate at the high pressures that are typical of the working area at the bottom of the well.

The object of the présent invention is to overcome the above-mentioned drawbacks and in particular to devise a détection System for detecting discontinuity interfaces and/or anomalies in pore pressures in geological formations which is capable of carrying out measurements directly ahead of the drill bit in order to make a prédiction of anomalous pressures at a few meters of depth în the formations yet to be drilled in front of the drill bit.

This and other aims according to the présent invention are obtained by realising a détection System for detecting discontinuity interfaces and/or anomalies in the pore pressures in geological formations as recîted în claim 1.

Further characteristics of the détection System for detecting discontinuity interfaces and/or anomalies in pore pressures in geological formations are the subject of the dépendent claims.

The characteristics and advantages of a détection System for detecting discontinuity interfaces and/or anomalies în pore pressures in geological formations according to the présent invention will be more évident from the following description, which is to be understood as exemplifying and not limiting, with reference to the appended schematic drawings, wherein:

- figure 1 shows a potentîal application scénario of a détection System according to the présent invention;

- figure 2 shows a block diagram of an embodiment of the détection system according to the présent invention;

- figures 3a and 3b are respectîvely a perspective view and a plan view of a preferential but not limiting embodiment of a détection System according to the présent invention;

- figure 4 is a schematic view of another embodiment of a detail of the détection System according to the présent invention;

- figure 5 is a plan view of another embodiment of the détection System according to the présent invention;

- figure 6a is a section view of an electro-acoustic transducer included in the détection System according to the présent invention;

- figure 6b is a view of a detail of the transducer of figure 6a;

- figure 7 is a schematic perspective view of an electrical winding présent in the electro-acoustic transducer of figure 6a.

With référencé to the figures, a détection System for detecting discontinuity interfaces and/or anomalies in pore pressures in geological formations indicated as a whole with 100 is shown.

The détection system 100 for detecting discontinuity interfaces and/or anomalies in pore pressures in geological formations comprises a drill bit 200 for drilling the underground in which, according to the présent invention, one or more electro-acoustic transducers 10, 11, 12 are integrated. If only one electro-acoustic transducer 12 is présent, it is intended to operate selectîvely as a transmitter or as a receiver of acoustic waves in the frequency range 450-5000 Hz, preferably 500-3000Hz; in this case the electro-acoustic transducer is defined as bîfiinctional. If at least two electro-acoustic transducers 10, 11 are présent, they are intended to operate one as a transmitter 10 and the other as a receiver 11 of acoustic waves in the frequency range 450-5000 Hz, preferably 500-3000Hz.

In the présent discussion, the working band will refer to the frequency range 450-5000 Hz, or more preferably the frequency range 5OO-3OOOHz.

In this way it is possible to identify discontinuities in the acoustic response of the acoustic transducers 10, 11, 12 due to the characteristics of the rocks (e.g. karst phenomena, faults, alternation of rock formations, clayey layers, réservoir rocks that contaîn liquid and/or gaseous hydrocarbons, sait dômes, basalts) or overpressures of the fluids of the rock formations.

The détection system 100 then comprises analogue driving electronic circuits 110 configured to control the electro-acoustic transducer that acts as a transmitter and analogue receiving electronic circuits 111 for amplifying and processing the signal received from the electro-acoustic transducer acting as a receiver. The détection system 100 also comprises a processing and control unît 120, for example a microprocessor, associated with the analogue driving 110 and receiving 111 electronic circuits for managing the détection process. The processing and control unit 120 is associated with a data storage unit 121 and is powered electrically by an electrical

-4supply System 122, for example comprising a battery System. The processing and control unit 120 is also provided with an interface module 123 towards the bottom hole assembly or BHA. For example, this interface module 123 comprises electrical/electronic circuits suitable for communicating with and possibly receiving power supply from the BHA.

In the présent description, reference will be made by way of example to the application scénario of figure 1 in which the drill bit 200 is close to two geological formations 300, 301 placed in succession along the advancement direction of the drill bit and separated by a discontinuity interface 302 placed at a distance d from the drill bit 200.

Each of the electro-acoustic transducers 10, 11, 12 is axial-symmetri cal and comprises a main tabulai body 20 preferably of a cylindrical shape and preferably of ferromagnetic material which extends in length along a longitudinal direction X; said main tubular body 20 comprises a first end portion 21 and a second end portion 22 opposed to each other longitudînally.

Furthermore, the main tubular body 20 has întemally a first chamber 23 which ends with the first end portion 21 and a second chamber 24 on one side adjacent and in fluid communication with the first chamber 23 and on the other side ending with the second end portion 22.

The compartiment defined întemally by the chambers 23, 24 can be of any shape, preferably cylindrical.

The first end portion 21 is closed towards the outside by means of a membrane 26 applied to the main tubular body 20.

Said membrane 26 is preferably made of harmonie steei.

The second end portion 22 has one or more openings 27 that put it into fluid communication towards the outside of the main tubular body 20.

The first chamber 23 contains in its walls a plurality of electrical windings 25 arranged in succession between them in the longitudinal direction X.

The electric windings 25 are preferably made by means of métal rings, preferably of copper separated by an insulating layer, for example an insulating film. This embodiment of the electric windings 25 is particularly advantageous since it uses the electro-acoustic transducer as an acoustîc signal transmitter.

The electro-acoustic transducer 10 also comprises a movable element 30 housed in the first chamber 23; said movable element 30 advantageously comprises a plurality of permanent magnets 31, preferably but not necessarily cylindrical, packed one above the other. In particular, the permanent magnets 31 are arranged with altemate magnétisation in the longitudinal direction X, are stacked, separated one from the other by dises 32 of ferromagnetic material and held

-5together by a pin 33 Crossing them for example centrally as shown in figure 1.

The permanent magnets 31 are preferably of Samarium-Cobalt.

The movable element 31 is supported at the longitudinal ends by springs 40, preferably by a pair of pre-Ioaded Belleville springs 40 as illustrated in figure 1. Each of these springs 40 is constrained on one side to the movable element 31 and on the other side to the internai wails of the first chamber 23.

The movable element 30 is also advantageously connected to the membrane 26, preferably by means of an extension element 27 coupled on one side to an end of the movable element 30 and on the other side to the membrane 26.

The electro-acoustic transducer 10 further comprises a movable piston 45 positioned in the second end portion 22.

The second end portion 22 is preferably coupled to a bush 28 which extends towards the inside of the second chamber 24 for a stretch of its length in such a way as to restrict the inner passage. In this case the movable piston 45 is positioned in the restricted inner passage.

The second chamber 24 is filled with a liquid, preferably oil.

When the electrical windings 25 are electrically powered with a signal to be transmitted, the interaction between the variable magnetic field generated by the electrical windings 25 and the permanent magnets 31 of the movable element 30 induces an oscillating translation of the movable element 30 which acts on the membrane 26 causing it to vîbrate and thus causing acoustic waves in the fluid surrounding the electro-acoustic transducer 10 în contact with the membrane 26 itself. The displacements of the movable element 31 cause a pressure variation inside the second chamber 24. These pressure variations are compensated by the movement of the movable piston 45 which is free to move depending on the pressure différence that can temporarily arise between the environment outside the electro-acoustic transducer and the second chamber 24. The movable piston 45 in fact reduces or încreases the volume of the second chamber 24 in which oil is contained, obtaining static pressure compensation.

This pressure compensation achieved by the piston advantageously allows using the electroacoustic transducer 10 in critical environments at high pressures up to about 700 bar.

The movable piston 45 and the second chamber 24 are sized to allow pressure compensation when acoustic signais are transmitted and received in the entire frequency range specified above, i.e. 450-5000 Hz, preferably 500-3000 Hz.

In parti cul ar, the second chamber 24 is sized in such a way that the System composed of the movable element 30, of the liquid contained inside the second chamber 24 and of the movable

-6piston 45, has an overall dynamic behaviour such that it guarantees the balance of the internai and extemal pressure, keeping the différence between the two pressure values close to zéro outside the entire frequency range 450-5000 Hz against a peak-to-peak displacement of the movable element 30 by a few tens of micrometers.

This behaviour is determined by the transfer fonction which is determined between the displacement of the movable element 30 and the pressure différence between the inside and outside of the electro-acoustic transducer 10. The transfer fonction dépends on the volume of the second chamber 24, on the section of the same chamber, on the mass and diameter of the movable piston 45 and on the elasticity modulus of the liquîd that fills the second chamber 24, normally indicated as the bulk module.

The length of the second chamber 24 is determined as a fonction of the internai section of the electro-acoustic transducer 10, that is of the internai section of the first chamber 23, as a fonction of the mass, of the diameter of the movable piston 45 and of the bulk module of the liquid that fills the second chamber 24.

Since this last parameter varies according to the type of liquîd used, the pressure and the température, the sizing must be developed considering the most critical expected conditions. The sizing îs carried out on the basis of a dynamic model of the System described by the following équations:

nip + βρ.χ + 0_mÀ + (Α_)Γ1 + kp'fx — F + P^Ap — P_eS[A_ltl liŸi + /ΛΤι + ^TTi = ^- f’esMi b'i = Ino + Λ,,.ν + Λ» rfl', , . . .

d r Vj \ dt ) where F is the force generated by the transducer, x is the displacement of the movable element 30, yl is the displacement of the movable piston 45, Pi is the pressure of the second chamber 24, Pest is the extemal pressure, Ap is the area of the cross section of the movable element 30, Al is the area of the cross section of the movable piston 45, Am is the area of the cross section of the membrane 26, VI-V10 is the volume variation of the second chamber 24 due to the displacement of the equipment and movable pistons Jol is the oil compressibility modulus, dm, LI 1 and »

-1are the damping coefficients of the membrane 26, of the movable piston 45 and of the movable element 30, respectively , mp and ml are the masses of the movable element 30 and of the movable piston 45, respectively, km, kp and kl are the stiffnesses of the membrane 26, of the movable element 30 and of the movable piston 45, respectively.

By way of example, în order to work at a température of 200°C and at a pressure of 700 bar, the following configuration has been identified:

• membrane diameter 26 = 9.6 mm;

• diameter of the second chamber 24 = 8 mm;

• length of the second chamber 24 = 25.5 mm;

* section of the movable piston 45 = 6 mm;

• mass of the movable piston 45 = 0.9 g;

• oil elasticity modulus 1 < □ < 2.5 GPa.

Furthermore, again by way of example, in order to maximize the transmitted power and the sensitivity of the electro-acoustic transducer 10 in the 500-3000 Hz band, the équivalent stiffnesses of the pairs of Belleville springs must be:

• 3.5 kN/mm for an electro-acoustic transducer intended to be used as a transmitter;

• 0.4 kN/mm for an electro-acoustic transducer intended to be used as a receiver.

An electro-acoustic transducer 10 intended to be used as a transmitter is designed to operate for example in a steady régime in the bands specîfied above, guaranteeing an acoustic power of approximately effective 20 mW.

An electro-acoustic transducer 10 intended to be used as a receiver is preferably designed to guarantee a transduction sensitivity of 20 Vs/m.

Preferably, the drill bit 200 is of the PDC type (Polycrystalline Diamond Composite) as the one illustrated in figures 3a and 3b. The PDC type drill bit 200 has a plurality of ridges 201 provided with diamond cutting éléments 202 and a central portion where there are holes 203 for the passage of the drilling mud.

The one or more electro-acoustic transducers 10, 11, 12 are housed in spécial compartiments made in the drill bit 200; this entai 1 s a sériés of constraînts on the sîze of the electro-acoustic transducers 10, 11, 12 which must bave a diameter in the order of a few centimeters.

In particular, if the drill bit 200 is of the PDC type, the housing compartiments for the electroacoustic transducers 10, 11, 12 are made in the space among the ridges avoiding the centrai portion; the space among the ridges can hâve for example a diameter comprised between 0.5” and 1”.

-8In a possible embodiment of the présent invention, a pair of electro-acoustic transducers 10, 11 configured to operate as a receiver and transmitter are housed in two separate compartments of the drill bit 200. In the embodiment illustrated in figures 3a and 3b the housing compartments of the electro-acoustic transducers are positioned at a distance not greater than 7 cm among the ridges 201 with the diamond cutting éléments 202 so that the flow of the drilling mud towards theholes 203 is allowed.

In an alternative embodiment illustrated in figure 4, a pair of electro-acoustic transducers 10, 11 configured to operate as a receiver and transmitter are housed in a single compartment, preferably with a diameter not greater than 7 cm.

In a further alternative embodiment, the drill bit 200 bouses one or more bifunctional electroacoustic transducers 12 associated with analogue driving and receiving electronic circuits so as to operate altemately as a transmitter and receiver. If a pair of bifunctional electro-acoustic transducers 12 is housed in the drill bit 200, the overall reliability of the détection System is increased.

The détection System 100 according to the présent invention implements a détection method for detecting discontinuity interfaces and/or anomalies in pore pressures in geological formations.

This détection method comprises two détection phases; the first détection phase provides for an initial phase in which the electro-acoustic transducer 10 acting as a TX transmitter generates a first acoustic wave to detect the possible presence of any discontinuity in the formation, of the lîthological type and/or due to pressure conditions in the pores.

In this phase, therefore, the electro-acoustic transducer 10 is driven by the processing and control unit 120 and by the respective analogue driving electronic circuit 110 by means of a driving signal which can be, for example:

• an impulse with a fondamental frequency centred in the working band (for example 3 sinusoid cycles);

• a frequency-modulated continuons sinusoid (at least 100 cycles), for example by a linear ramp (linear chirp).

Following transmission, a reflected signal of this first acoustic wave is received, which signal is generated by at least one discontinuity interface due to the passage from a first to a second different geological formation, arranged in succession along the émission direction of the electro-acoustic transducer 10-transmitter.

After receiving the reflected signal, the processing and control unît 120 calculâtes the back and forth travel time of the compression stress wave which is the fastest and therefore the first to

-9reach the electro-acoustic transducer 11 acting as a receiver, in the following exemplary ways:

• by looking for the peak of the cross-correlation between the transmitted and received signais;

or • by identifyîng the beat frequency of the multiplication between the transmitted and received signais.

The choice for the frequency-modulated continuons sinusoid as a driving signal of the electroacoustic transducer 10 acting as a transmitter has the following advantages:

• digital processing which, in a manner known per se in the State of the art, involves a multiplication between the transmitted driving signal and the received signal, a low-pass filtering to remove the spectral components centred at the “sum” frequencies and an AC coupling to reduce the possible crosstalk comportent, can be implemented analogically by the analogue receiving electronic circuit 111 ; thereby:

o in case of “deafening” of the electro-acoustic transducer 11 acting as a receiver (crosstalk), that is, in the case in which the electro-acoustic transducer-transmîtter is very close to the transducer-electro-acoustic receiver and the latter receives together with the signal reflected by the formation also the signal transmitted by the transmitter, there is no impainnent in the resolution of the useful signal before the digital acquisition performed by the processing and control unit since samplîng is carried ont after the analogue signal conditioning;

o at equal depth of the discontînuity interface, the signal/noise ratio at the receiver is greater since the useful signal band is lower and consequently the noise power is lower;

• at equal transmitted energy, the transmission power is lower and therefore the required driving voltage is lower.

However, this choice for continuons wave driving signal is not compatible with the embodiment of figure 5, since the transmission and réception intervals are almost coïncident.

After determining the back and forth travel time of the wave, the processing and control unit 120 calculâtes the distance d between the drill bit 200 and the discontînuity interface starting from the back and forth travel time of the wave and the compressions! rate in the layer being drilled. The value of the compressional rate can be already known and obtained by the surface seismics and can possibly be confirmed and refined by the sonie logs of the conventional logging tool while drilling LWD. If however it is not available, it is possible to estimate the rate by repeating the measurement described above after having drîlled at least a distance equal to the resolution of

-10the measurement.

The resolution of the measurement dépends on the band B of the transmitted signal and the propagation speed of the compressional wave in the geological formation cP I :

being B in the order of a few kHz and cPl in the range 3-6 km/s, the best resolution is in the order of the meter.

After the first détection phase, the second détection phase begins, which provides for the electroacoustic transducer 10 acting as a transmitter to generate a second acoustic wave to discriminate whether the anomaly detected in the first détection phase is due to a lithologîcal change or to an abnormal pore pressure. The possible presence of an abnormal pressure in the second formation entaîls a particular attenuating/dispersive effect which can be mapped, for example, in the following transfer fonctions:

• electrical impédance Z(f) of the electro-acoustic transducer 10 acting as a transmitter • frequency response H(f) of the System consisting of the chain of the following subsystems: electro-acoustic transducer 10 acting as a transmitter - geological formation - electro-acoustic transducer 20 acting as a receiver.

In this second détection phase, therefore, the electro-acoustic transducer-transmitter 10 is driven by the processing and control unit 120 and by the respective analogue driving electronic circuit 110 by means of a driving signal which can be, for example:

- a set of discrète tones (duration at least 100 cycles to reach steady State) whose frequencies cover the working band with a sufficîently small pitch F (for example 10 Hz); or

- Gaussian white noise (duration of a few seconds), suîtably filtered in the working band (bandpass filtering).

Following transmission, a reflected signal of this second acoustic wave is received, which signal is generated by at least one discontinuity interface due to the passage from a first to a second different geological formation, arranged in succession along the émission direction of the electro-acoustic transducer 10-transmitter.

After réception, the processing and control unit 120 calculâtes the above-mentioned transfer functions and estimâtes the pore pressure on the basis of these transfer functions using at least two types of approach known per se în the State of the art:

- an approach based on a suitable physical model that links the pore pressure in the second

-11formation to the transfer fonctions described above;

- a pattern récognition approach based on a “supervised” classification algorithm for the récognition of “signatures” in the transfer fonctions îndîcated above due to the pore pressure in formation 2, such as for example some peak frequencies, both in the module and in the phase.

Since the second détection phase is based on a continuons wave (non-impulsive) measurement scheme, it is not compatible with the embodiment of figure 5, since the transmission and réception întervals are almost coincident.

From the description made, the characteristics of the détection System object of the présent invention are clear, as are the relative advantages.

Finally, it is clear that the détection System thus conceîved is susceptible of numéro us modifications and variations, ail of which are within the scope of the invention; moreover, ail the details can be replaced by technically équivalent éléments. In practice, the materials used, as well as the dimensions thereof, can be of any type according to the technîcal requirements.

Claims (10)

1 ) Détection system (100) for detecting discontinuity interfaces and/or anomalies in pore pressures in geological formations comprising:

- a drill bit (200) for drillîng the subsoil in which one or more electro-acoustic transducers (10, 11, 12) configured to operate as a transmitter and/or receiver, are integrated;

- analogue driving electronic circuits (110) configured to control the electro-acoustic transducer, which acts as a transmitter (10) and analogue receiving electronic circuits (111) for amplifying and processing the signal received from the electro-acoustic transducer actîng as a receiver (11);

- a processing and control unit (120) associated with the analogue driving (110) and receiving (111) electronic circuits, saîd processing and control unit (120) being associated with a data storage unit (121) and is powered electricaliy by an electrical supply system (122), said processing and control unit (120) being configured for generating driving signais to be sent to the electro-acoustic transducer acting as a transmitter (10) by means of said analogue driving electronic circuits (110), for acquiring the signais received from the electro-acoustic transducer acting as a receiver (20) by means of said analogue receiving electronic circuits (111) and for processing said received signais in such a way as to déterminé the presence of discontinuity interfaces and/or anomalies in pore pressures in geological formations;

characterised in that each of said electro-acoustic transducers (10, 11, 12) is adapted to be in contact with a pressurised fluid and is of the type comprising:

- a tubular body (20), which extends in length along a longitudinal direction X, said tubular body (20) comprising a first end portion (21) and a second end portion (22), opposed to each other longitudinally, said tubular body (20) having intemally a first chamber (23), which ends with the first end portion (21) and a second chamber (24) on one side adjacent and in fluid communication with said first chamber (23) and, on the other side ending with said second end portion (22), said first end portion (21) being closed towards the outside by means of a membrane (26) applied to said tubular body (20), said second end portion (22) having one or more openings (27) that put it into fluid communication towards the outside of said tubular body (20), said first chamber (23) containing in its walls a plurality of electrical windings (25) arranged in succession between them in the longitudinal direction X, said second chamber (24) being filled with a liquid;

- a movable element (30) housed in said first chamber (23), said movable element (30) comprising a plurality of permanent magnets (31 ) packed and arranged one above the other with altemate magnétisation in the longitudinal direction X, and separated one from the other by dises

-13of ferromagnetic material, said movable element (31) being supported at the longitudinal ends by springs (40), said movable element (30) also being connected to said membrane (26);

- a movable piston (45) positioned and slidable in the second end portion (22).

2) Détection System (100) according to claim 1, wherein said electric windings (25) are made of métal rings separated by an insulating layer.

3) Détection System (100) according to claim 1 or 2, wherein said movable element (30) is connected to said membrane (26) by means of an extension element (27) coupled on one side to an end of the movable element (30) and on the other side to the membrane (26).

4) Détection System (100) according to one of the preceding daims, wherein said springs (40) are a pair of pre-loaded Belleville springs (40).

5) Détection System ( 100) according to one of the preceding claims, wherein said second end portion (22) is coupled to a bush (28), which extends towards the inside of the second chamber (24) for a stretch of its length in such a way as to restrict the inner passage, said movable piston (45) being positioned in the restricted inner passage.

6) Détection System (100) according to one of the preceding daims, wherein said movable piston (45) and said second chamber (24) are sized to allow pressure compensation when acoustic signais are transmîtted or received in the frequency range 450-5000 Hz, preferably in the frequency range 500-3000 Hz.

7) Détection System (100) according to one of the preceding daims, wherein a pair of said electro-acoustic transducers (10, 11) configured to operate as a receiver and transmitter, are housed in two separate compartments ofthe drill bit (200).

8) Détection System (100) according to one of daims 1-6, wherein a pair of electro-acoustic transducers (10, 11) configured to operate as a receiver and transmitter, are housed in a single compartment of the drill bit (200).

9) Détection System (100) according to one of daims 1-6, wherein said one or more electroacoustic transducers are of the bifunctional type (12), that is they are configured to operate altematdy as a transmitter and receiver.

10) Détection System (100) according to claim 9, wherein a pair of said bifunctional electroacoustic transducers (12) are housed in one or more compartments of the drill bit (200).