IMPROVED BOREHOLE IMAGING BY ULTRASONICS

A new ultrasonic tool for borehole and casing imaging has been develop ed based on recent cementation-imaging technology. A rotating ultrason ic transducer scans the borehole at a high sampling rate to provide de tailed images of echo amplitude and radius. A 250- or 500-kHz focused transducer gives high resolution, penetration in heavy mud, and low se nsitivity to tool eccentering. The echoes are analyzed by a downhole d igital signal processor to optimize the accuracy and reliability of th e radius measurement. The measurements are corrected for eccentering, and the image color scales are dynamically adjusted for optimum sensit ivity in real time by the surface computer. Comparisons with electrica l imaging tools show that the ultrasonic amplitude measurement tends t o respond to lithology indirectly by changes in borehole radius or rug osity. Ultrasonic imaging is unique in making quantitative high-resolu tion measurements of borehole geometry that are useful for borehole-st ability analysis. Examples of automatic hole-shape analysis are shown. The tool can also evaluate internal casing corrosion and detect holes .