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The High Resolution Acoustic Televiewer (HRAT) provides a continuous high-resolution oriented ultrasound image of the borehole wall. The probe uses a fixed acoustic transducer and a rotating acoustic mirror to scan the borehole walls with a focussed ultrasound beam. The amplitude and travel time of the reflected acoustic signal are recorded as separate image logs. Features such as fractures reduce the reflected amplitude and appear as dark sinusoidal traces on the log. The travel time log is equivalent to a 360-arm caliper and shows diameter changes within open fractures and ‘break-outs’. Directional information is also recorded and used to orient the images in real time.