Downhole piezoelectric acoustic transducer

1. A sensor system for use in a wellbore, the sensor system comprising:

a piezoelectric transducer for generating an acoustic wave in a fluid medium of the wellbore by repeatedly bending between two positions in response to an actuation signal, the piezoelectric transducer comprising exactly four piezoelectric layers stacked on top of one another, each layer of the four piezoelectric layers coupled to an adjacent layer via a bonding material and comprising:

a piezoelectric material,

a top electrode coupled to a top surface of the piezoelectric material, and

a bottom electrode coupled to a bottom surface of the piezoelectric material;

wherein a middle two of the four piezoelectric layers are polarized in one direction and an outer two of the four piezoelectric layers are polarized in an opposite direction; and

a hydrophone for detecting a reflection or a refraction of the acoustic wave off an object in the wellbore and transmitting an associated signal to a processing device.

2. The sensor system of claim 1 , wherein the piezoelectric transducer comprises a circular cross-sectional shape, and wherein the bonding material comprises an epoxy.

3. The sensor system of claim 2 , wherein each of the four piezoelectric layers is at least 500 micrometers thick, and wherein the actuation signal comprises a frequency of between 5 kHz and 30 kHz for generating the acoustic wave with a corresponding frequency.

4. The sensor system of claim 1 , further comprising:

the processing device in communication with the piezoelectric transducer and the hydrophone; and

a memory device in which instructions executable by the processing device are stored for causing the processing device to:

receive the signal from the hydrophone; and

determine a distance between the sensor system and the object or a sound velocity that is characteristic of the object based on the signal.

5. The sensor system of claim 4 , wherein a material is positioned between at least two of the four piezoelectric layers for tuning a response characteristic of the piezoelectric transducer.

6. The sensor system of claim 5 , wherein the sensor system is positioned in a well tool comprising a logging-while-drilling tool or a wireline tool.

7. A well tool usable in a wellbore, the well tool comprising:

a piezoelectric transducer for generating an acoustic wave in a fluid medium positioned in the wellbore, the piezoelectric transducer comprising exactly four piezoelectric layers stacked on top of one another, each layer of the four piezoelectric layers coupled to an adjacent layer via a bonding material and comprising:

a piezoelectric material,

a top electrode coupled to a top surface of the piezoelectric material, and

a bottom electrode coupled to a bottom surface of the piezoelectric material;

wherein a middle two of the four piezoelectric layers are polarized in one direction and an outer two of the four piezoelectric layers are polarized in an opposite direction.

8. The well tool of claim 7 , wherein the piezoelectric transducer comprises a circular shape and the bonding material comprises an epoxy.

9. The well tool of claim 7 , wherein the piezoelectric transducer generates the acoustic wave by repeatedly bending between two positions in response to an actuation signal.

10. The well tool of claim 9 , wherein the actuation signal comprises a frequency of between 5 kHz and 30 kHz for generating the acoustic wave with a corresponding frequency.

11. The well tool of claim 7 , wherein each of the four piezoelectric layers is at least 500 micrometers thick.

12. The well tool of claim 7 , further comprising:

an acoustic sensor for detecting a reflection or a refraction of the acoustic wave off an object in the wellbore and transmitting an associated signal;

a processing device in communication with the piezoelectric transducer and the acoustic sensor; and

a memory device in which instructions executable by the processing device are stored for causing the processing device to:

receive the signal from the acoustic sensor; and

determine a distance between the well tool and the object or a sound velocity that is characteristic of the object based on the signal.

13. The well tool of claim 7 , wherein a material is positioned between at least two of the four piezoelectric layers for tuning a response characteristic of the piezoelectric transducer.

14. The well tool of claim 7 , wherein the well tool comprises a logging-while-drilling tool or a wireline tool.

15. A method comprising:

positioning a piezoelectric transducer coupled to a well tool in a wellbore, the piezoelectric transducer including exactly four piezoelectric layers stacked on top of one another, wherein a middle two of the four piezoelectric layers are polarized in one direction and an outer two of the four piezoelectric layers are polarized in an opposite direction; and

transmitting an actuation signal to the piezoelectric transducer for actuating the piezoelectric transducer, the actuation signal causing the piezoelectric transducer to generate an acoustic wave in a fluid medium of the wellbore by repeatedly bending between two positions.

16. The method of claim 15 , further comprising:

receiving, via a hydrophone, a reflection or a refraction of the acoustic wave off an object in the wellbore.

17. The method of claim 16 , further comprising:

determining a distance between the piezoelectric transducer and the object based on the reflection or refraction of the acoustic wave.

18. The method of claim 16 , further comprising:

determining a sound velocity that is characteristic of the object based on the reflection or refraction of the acoustic wave.