altered zone

1. n. [Formation Evaluation]
A near-wellbore formation zone, a few inches thick, whose acoustic velocity has been affected by impregnation with drilling fluids, stress relief, or both. The acoustic velocity of the rock in the immediate vicinity of the borehole wall can be much slower than that in the virgin formation. To measure the formation velocity, it may be necessary to use a sonic logging tool that has a greater spacing between transmitter and receiver array (about 10 to 15 ft [3 to 4.5 m]) than the standard sonic tool (about 3 to 5 ft [0.9 to 1.5 m]). The altered zone may also give rise to different acoustic modes, for example the hybrid mode or a second Stoneley wave.