1. n. [Formation Evaluation]

A feature on a sonic log caused by low signal amplitude that results in erroneously long traveltimes. Sonic logs that do not record waveforms measure the acoustic traveltime between transmitter and receiver by detecting the first signal at the receiver above a certain threshold (first motion detection). The threshold is small so that the signal is detected just after it crosses the zero signal baseline. However, if the threshold is set too high, or the signal is too small, the system will not trigger at the sharp zero crossing but at some later point on the waveform. This increases the apparent transmitter-receiver time. Delta-t stretch is more likely at the far receiver, where signals are weaker, so that the apparent traveltime calculated between receivers is too large. In the extreme case, the system triggers on the next cycle of the waveform, known as cycle skipping.

See related terms: acoustic traveltime,  cycle skip,  delta t,  sonic measurement,  zero crossing