Voldi Maki, Stan Gianzero, Robert Strickland, H. Neil Kepple and Marc V. Gianzero
Halliburton Logging Services, Austin Research Center, Austin, Texas
ABSTRACT
A theoretical and experimental study was conducted to demonstrate the superiority of dynamically focused transducers over standard unfocused transducers as well as transducers which are focused via curvature. The multi-stage focused transducer covers the entire range of borehole sizes for 6.5 inches to 20 inches and works satisfactorily in 15 pound per gallon muds.
The original borehole televiewer (BHTV) utilizing a relatively high frequency transducer was not focused. The imaging resolution was determined by size of the element and relied upon the location of the near field to far field transition region to provide some focusing. Many environmental effects limited the usefulness of the tool. The introduction of concave elements permitted lower operating frequencies and larger diameter transducers thereby reducing the attenuation in heavy drilling muds. Furthermore, the imaging resolution improved because the focusing concentrated more energy into a smaller area. This increased aperture due to focusing reduced the stand-off effects. However, such transducers have a reduced depth of field. Consequently, their imaging resolution is poor wherever the hole diameter departs from the ideal size prescribed by the fixed focal length.
An electronically focused transducer has been developed which provides all of the advantages of a curved transducer with the additional capability of dynamic focusing. The electronics in the system allows for the focusing to be continuously optimized to the borehole diameter. In its most basic form, the transducer consists of annular rings which act as individual transducers. Each ring is attached to a transmitter and receiver circuit. Time delays are used in both the transmitted and received modes to produce the focusing effect. The focusing may be changed for each measurement based upon the last measured distance to the borehole wall.
Both modeled and measured responses of the electronically focused transducer are shown and compared with the modeled spatial responses of unfocused and curvature focused transducers. The comparison clearly demonstrates the improved capability of the dynamically focused transducer to log in difficult borehole conditions.