1. n. [Formation Evaluation]
Log recorded by a sonic tool with a longer transmitter-to-receiver spacing (generally 10 to 15 ft) than a standard sonic tool. The rock near the borehole is sometimes altered by drilling fluids, stress relief, or both, causing a thin zone whose velocity is lower than that of the true formation. With standard spacings, the wave traveling through the altered zone may arrive first at the receiver, since this zone is closer to both transmitter and receiver. The increased spacing permits the wave traveling through the true formation to arrive first and be measured. The depth of investigation varies with slowness and transmitter-receiver spacing but is of the order of 2 to 3 ft. An increased transmitter-to-receiver spacing also allows better separation of waveforms relating to different acoustic waves, such as compressional, shear and Stoneley arrivals.
