Active acoustic logging: comparison of short offset (1 m) and large offset (3 m) acoustic sections recorded in borehole B1 (a, poorly cemented borehole) and in borehole B2 (b, open hole part). The different wave trains are identified by letters: C casing resonances, P refracted P-wave, S refracted S-wave, ST Stoneley wave, F fluid waves, Criss-cross. The acoustic sections are normalized and displayed with a color scale ranging from 0 to 1.