The field of a circular ultrasonic transducer emitting a single-cycle pulse into water has been observed using a specially constructed small (150 μm) wide-band receiving probe and a compact stroboscopic schlieren system. The theoretically predicted plane-wave and diffracted edge-wave components of the field have been resolved. Good agreement with the theory for a pistonlike source is obtained, except in a region less than 1.5 transducer radii from the transducer. The output of the transducer used in the transmit–receive mode to detect small targets has been measured and the results are in accord with a time-domain principle of reciprocity between transmission and reception. Implications of the results for field plotting and for the location and characterization of small targets are considered.

The transient fields of circular and square wide-band (1-5 MHz) ultrasonic transducers have been studied in water. Transmission and reception of short pulses have been observed using a small (150 pm) wide-band probe as both receiver and transmitter. Transmit-receive mode results from a small target have also been considered. Detailed comparisons with the theoretical waveforms for an ideal transducer (pistonlike in transmission and pressure-sensitive in reception) exhibit generally good agreement. The principle of reciprocity between transmission and reception has been verified. Discrepancies from pistonlike behavior, most marked at short ranges near the axis of each transducer, have been shown by schlieren observations to be caused by an extra ("head") wave which originates from a plate wave propagating laterally across the face. It is concluded that nearfield pulse echo results must be interpreted with some circumspection.

SPE Members Abstract Experiments were conducted to study how sonic and ultrasonic cementation logs are affected by microannuli. By creating both gas- and liquid-filled microannuli of known sizes at the casing-to-cement interface, measurements could be recorded and analyzed. Depending on the nature of the fluid and the size of the microannulus, results showed that all cementation logs were sensitive to microannuli. Gas at the casing-to-cement interface had a very strong effect on the ultrasonic measurement but had minimal effect on traditional sonic Cement Bond Log (CBL) measurement if the microannulus was in the range of one m. CBL measurement was very sensitive to water-filled microannuli. Ultrasonic measurements were capable of evaluating the quality of the cement behind a water-filled microannuli as large as 100 m. Liquid-filled microannuli could be identified through the large coupling attenuation measured with multi-spacing sonic tools. Analysis of these experiments brings new insights to cementation-log interpretation. Introduction Interpretation of cementation logs is without any doubt subject to controversy. For over 30 years, CBL measurements have been used to evaluate cement jobs. More recently, special tools have been developed to overcome some of the limitations of the traditional sonic measurements. Use of ultrasonics, has made it possible to improve cement evaluation, mainly through a spatial resolution which allows a much clearer identification of incomplete mud removal. However, using higher frequency waves renders the signal more sensitive to the local events, such as poor pipe-surface condition. In many cases, field experience and theories have helped people to better understand the response of the tools to specific situations. When a small gap exists between the casing and the cement, the response of acoustic and ultrasonic tools is affected. There is a microannulus effect which renders cement job evaluation even more difficult. Several articles have been published to explain how a microannulus can be created or induced and how this affects cementation logs. Temperature or pressure changes are the most common causes of microannuli (see Appendix 1 for usual formulae). P. 505^

Summary A new ultrasonic tool for borehole and casing imaging has been developed based on recent cementation imaging technology. A rotating ultrasonic transducer scans the borehole at a high sampling rate to provide detailed images of echo amplitude and radius. A 250 or 500 kHz focused transducer gives high resolution, penetration in heavy mud and low sensitivity to tool eccentering. The echoes are analyzed by a downhole digital signal processor to optimize the accuracy and reliability of the radius measurement. The measurements are corrected for eccentering, and the image color scales are dynamically adjusted for optimum sensitivity in real time by the surface computer. Comparisons with electrical imaging tools show the ultrasonic amplitude measurement tends to respond to lithology indirectly via changes in borehole radius or rugosity. Ultrasonic imaging is unique in making quantitative high-resolution measurements of borehole geometry that are useful for borehole stability analysis. Examples of automatic hole shape analysis are shown. The tool can also evaluate internal casing corrosion and detect holes. Introduction Detailed images of the borehole can be produced by three common techniques: video cameras, micro-electrical imagers and ultrasonic scanners. Video cameras operate only in clear liquids or gases. Electrical imagers cannot be used in oil-base mud. The ultrasonic method works in water-base and oil-base muds and is the only technique that provides a high-resolution caliper (borehole geometry) survey. The first ultrasonic instrument, the borehole televiewer, was introduced over 25 years ago. In the 1980s Shell and Amoco updated the design and in the last few years the technique has enjoyed a revival of interest with the introduction of new tools by service companies.

A new ultrasonic tool for borehole and casing imaging has been developed based on recent cementation imaging technology. A rotating ultrasonic transducer scans the borehole at a high sampling rate to provide detailed images of echo amplitude and radius. A 250 or 500 kHz focused transducer gives high resolution, penetration in heavy mud and low sensitivity to tool eccentering. The echoes are analyzed by a downhole digital signal processor to optimize the accuracy and reliability of the radius measurement. The measurements are corrected for eccentering, and the image color scales are dynamically adjusted for optimum sensitivity in real time by the surface computer. Comparisons with electrical imaging tools show the ultrasonic amplitude measurement tends to respond to lithology indirectly via changes in borehole radius or rugosity. Ultrasonic imaging is unique in making quantitative high-resolution measurements of borehole geometry that are useful for borehole stability analysis. Examples of automatic hole shape analysis are shown. The tool can also evaluate internal casing corrosion and detect holes. 1. Introduction Detailed images of the borehole can be produced by three common techniques: video cameras, microelectrical imagers and ultrasonic scanners. Video cameras operate only in clear liquids or gases. Electrical imagers cannot be used in oil-base mud. The ultrasonic method works in water-base and oilbase muds and is the only technique that provides a high-resolution caliper (borehole geometry) survey. The first- ultrasonic instrument, the borehole televiewer, was introduced over 25 years ago [1]. In the 1980s Shell and Amoco updated the design and in the last few years the technique has enjoyed a revival of interest with the introduction of new tools by service companies [2,3].