Ultrasonic transducer with reduced backing reflection

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

an ultrasonic transducer for generating an ultrasonic wave in an acoustic medium of the wellbore, the ultrasonic transducer comprising:

a front layer;

a piezoelectric layer comprising a first side and a second side, the front layer coupled to the first side of the piezoelectric layer;

a backing layer comprising a first side and a second side, the second side of the piezoelectric layer coupled to the first side of the backing layer; and

a rear layer comprising:

a first side coupled to the second side of the backing layer;

a second side of the rear layer positionable to abut the acoustic medium; and

a thickness between the first side and the second side that is one-fourth of a wavelength of a velocity of sound in a material of the rear layer.

2. The well tool of claim 1 , wherein the front layer is a front quarter-wavelength plate.

3. The well tool of claim 1 , wherein the ultrasonic transducer is an ultrasound ultrasonic transducer for outputting the ultrasonic wave at a frequency of 20 kHz or higher.

4. The well tool of claim 1 , wherein the well tool comprises a logging-while-drilling tool or a wireline tool.

5. The well tool of claim 1 , further comprising:

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

a processing device in communication with the ultrasonic transducer and the ultrasonic sensor; and

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

receive the associated signal from the ultrasonic sensor; and

determine a characteristic of the object based on the associated signal.

6. The well tool of claim 5 , wherein the characteristic is an image of the object.

7. The well tool of claim 5 , wherein the characteristic is a measured diameter of part of the wellbore determined by a caliper application, an impedance of borehole material, or an existence of a deformity of a wall of the wellbore.

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

an ultrasonic transducer for generating an ultrasonic wave in an acoustic medium of the wellbore, the ultrasonic transducer comprising:

a front layer;

a piezoelectric layer comprising a first side and a second side, the front layer coupled to the first side of the piezoelectric layer;

a backing layer comprising a first side and a second side, the second side of the piezoelectric layer coupled to the first side of the backing layer; and

a rear layer comprising:

a first side coupled to the second side of the backing layer;

a second side of the rear layer positionable to abut the acoustic medium; and

a thickness between the first side and the second side that is one-fourth of a wavelength of a velocity of sound in a material of the rear layer; and

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

9. The sensor system of claim 8 , wherein the ultrasonic transducer is an ultrasound ultrasonic transducer for outputting the ultrasonic wave at a frequency of 20 kHz or higher.

10. The sensor system of claim 8 , wherein the sensor system is in a logging-while-drilling tool or a wireline tool.

11. The sensor system of claim 8 , further comprising:

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

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

receive the associated signal from the hydrophone; and

determine a characteristic of the object based on the associated signal.

12. The sensor system of claim 11 , wherein the characteristic is an image of the object.

13. The sensor system of claim 11 , wherein the characteristic is a measured diameter of part of the wellbore determined by a caliper application, an impedance of borehole material, or an existence of a deformity of a wall of the wellbore.

14. A method, comprising;

disposing a well tool in a wellbore, the well tool including a hydrophone and an ultrasonic transducer, the ultrasonic transducer comprising:

a front layer;

a piezoelectric layer comprising a first side and a second side, the front layer coupled to the first side of the piezoelectric layer;

a backing layer comprising a first side and a second side, the second side of the piezoelectric layer coupled to the first side of the backing layer; and

a rear layer comprising:

a first side coupled to the second side of the backing layer;

a second side of the rear layer positioned to abut an ultrasonic medium of the wellbore; and

a thickness between the first side and the second side that is one-fourth of a wavelength of a velocity of sound in a material of the rear layer;

outputting, by the ultrasonic transducer, an ultrasonic wave in the ultrasonic medium of the wellbore;

detecting, by the hydrophone, a reflection or a refraction of the ultrasonic wave off an object in the wellbore;

generating, by the hydrophone, a representative signal of the reflection or the refraction of the ultrasonic wave off the object in the wellbore; and

using the representative signal to determine a characteristic of the object.

15. The method of claim 14 , wherein the front layer is a front quarter-wavelength plate,

wherein the ultrasonic transducer is an ultrasound ultrasonic transducer for outputting the ultrasonic wave at a frequency of 20 kHz or higher.

16. The method of claim 14 , wherein the well tool is a logging-while-drilling tool or a wireline tool.

17. The method of claim 14 , wherein the characteristic is an image of the object.

18. The method of claim 14 , wherein the characteristic is a measured diameter of part of the wellbore determined by a caliper application, an impedance of borehole material, or an existence of a deformity of a wall of the wellbore.