Ultrasonic beam focus adjustment for single-transducer ultrasonic assembly tools

1. A method of ultrasonic borehole logging using an ultrasonic borehole imaging tool in a borehole intersecting the earth formation, the method comprising:

adjusting a focus for an ultrasonic beam generated from a single-transducer ultrasonic assembly of the ultrasonic borehole imaging tool, comprising selecting a lens in dependence upon a size of the borehole and coupling the lens with a transducer of the single-transducer ultrasonic assembly;

using a receiver to generate measurement information responsive to an ultrasonic signal caused by the ultrasonic beam; and

generating a borehole image from the measurement information.

2. The method of claim 1 , wherein adjusting the focus comprises:

selecting the lens having at least one focal parameter corresponding to the size of the borehole from a set of interchangeable ultrasonic lenses, wherein each lens of the set has focal parameters unique within the set corresponding to the size of the borehole;

coupling the lens with the transducer of the ultrasonic borehole imaging tool; and

generating the ultrasonic beam using the lens.

3. The method of claim 2 , further comprising, upon a change in the size of the borehole to a second borehole size, performing:

removing the lens from the transducer;

selecting a second lens having at least one second focal parameter corresponding to the second borehole size from the set of interchangeable ultrasonic lenses;

coupling the second lens with the transducer of the ultrasonic borehole imaging tool;

generating a second ultrasonic beam using the second lens; and

generating the measurement information with the second lens.

4. The method of claim 3 , wherein the transducer of the ultrasonic borehole imaging tool comprises a transducer cup and wherein coupling the lens with the transducer of the ultrasonic borehole imaging tool comprises inserting the lens within the transducer cup; and removing the lens from the transducer comprises removing the transducer cup from a tool body of the ultrasonic borehole imaging tool and removing the lens from the transducer cup.

5. The method of claim 2 , further comprising, upon a change in a state of operation of the lens:

removing the lens;

selecting a replacement lens;

coupling the replacement lens with the transducer; and

generating the measurement information with the replacement lens.

6. The method of claim 1 , wherein the ultrasonic beam is focused with a focal zone at a borehole wall configured to produce a beam spot of a selected diameter.

7. The method of claim 1 , further comprising adjusting the focus in dependence upon environmental conditions, the environmental conditions comprising at least one of: i) standoff between the ultrasonic borehole imaging tool and a wall of the borehole; and ii) borehole annulus conditions.

8. The method of claim 1 , wherein the selected lens is configured to work with the transducer on the ultrasonic borehole imaging tool to locate a 6 dB focal zone of the single-transducer ultrasonic assembly proximate a wall of the borehole at a standoff distance associated with the size of the borehole and the ultrasonic borehole imaging tool.

9. An apparatus for ultrasonic borehole logging in a borehole intersecting the earth formation, the apparatus comprising:

an ultrasonic borehole imaging tool comprising:

a single-transducer ultrasonic assembly configured for adjusting a focus for an ultrasonic beam generated from the single-transducer ultrasonic assembly by coupling of a lens corresponding with a size of the borehole with a transducer of the single-transducer ultrasonic assembly;

a receiver configured to generate measurement information responsive to an ultrasonic signal caused by the ultrasonic beam; and

at least one processor configured to generate a borehole image from the measurement information.