Composite transducer for downhole ultrasonic imaging and caliper measurement

1. An apparatus configured to evaluate an earth formation, the apparatus comprising:

a rotatable transducer assembly surrounded by an acoustically transparent window and immersed in a fluid filling a cavity defined by the acoustically transparent window;

a composite transducer on the rotatable transducer assembly configured to propagate an acoustic signal through the acoustically transparent window into a borehole filled with borehole fluid and receive a reflection from a wall of the borehole; and

at least one processor configured to use the reflection obtained at a plurality of orientations of the transducer during rotation of the transducer assembly to provide an image of the earth formation;

wherein the rotatable transducer assembly comprises an impedance matching material disposed between the composite transducer and the fluid; and

wherein the assembly maintains a distance greater than zero between the composite transducer and the acoustically transparent window during rotation of the assembly, the assembly being configured via the distance selected to: (i) reduce a reverberation time of a reverberation therebetween, and (ii) improve acoustic coupling with the acoustically transparent window.

2. The apparatus of claim 1 wherein the composite transducer further comprises piezoelectric transducer (PZT) rods in a polymer matrix.

3. The apparatus of claim 1 further comprising a backing material on a side of the transducer opposite to the propagation of the acoustic signal through the acoustically transparent window configured to absorb acoustic signals.

4. The apparatus of claim 1 wherein the image produced by the at least one processor is selected from: (i) a traveltime image, and (ii) a reflectance image.

5. The apparatus of claim 1 wherein the rotatable transducer assembly is part of a logging string conveyed into the borehole on a wireline.

6. The apparatus of claim 1 wherein the composite transducer further comprises a ceramic having a high quality factor.

7. A non-transitory computer-readable medium product having instructions thereon that enable at least one processor to perform a method, the method comprising:

producing an image of an earth formation using a reflected signal from a borehole wall resulting from generation an acoustic signal by a composite transducer on a rotatable transducer assembly surrounded by an acoustically transparent window and immersed in a fluid filling a cavity defined by the acoustically transparent window in the borehole and transmission of the generated acoustic signal through the acoustically transparent window on the transducer assembly into the borehole wherein the assembly maintains a distance greater than zero between the composite transducer and the acoustically transparent window during rotation of the assembly, the assembly being configured via the distance selected to: (i) reducing a reverberation time of a reverberation therebetween, and (ii) improving acoustic coupling with the acoustically transparent window;

and wherein the rotatable transducer assembly further comprises an impedance matching material disposed between the composite transducer and the fluid.

8. The non-transitory computer-readable medium product of claim 7 further comprising at least one of: (i) a ROM, (ii) an EPROM, (iii) an EEPROM, (iv) a flash memory, and (v) an optical disk.

9. The apparatus of claim 1 , wherein the distance is about a quarter wavelength of a frequency of the transducer.

10. The apparatus of claim 1 wherein the impedance matching material disposed between the composite transducer and the fluid is configured to acoustically match the acoustic impedance of the composite transducer to the acoustic impedance of the fluid.

11. The apparatus of claim 10 wherein the impedance matching material disposed between the composite transducer and the fluid is configured to maximize transmission of the propagating acoustic signal to the fluid.