This is an abbreviation for a well log used in the oil and gas industry. You might have encountered this acronym MDT in your log evaluation software or any (ASCII) log files provided by the logging company. The full name for this log used in the industry is 'Mud Delta-T'. Usage of this log is not high and encountering it is a rare occasion.

This log displays readings of the acoustic compressional slowness (Delta-T) of the drilling fluid. In general this log therefore displays units of the fluid compressional slowness (Acoustic slowness of a fluid; the slowness of the compressional wave).

Concluding, this is a measure of the time it takes for a compressional wave to traverse a fixed distance; reciprocal of Compressional_Velocity. At a higher level, this can be seen as a measure of the interval transit time (The travel time of a wave over a unit distance, hence the reciprocal of wave velocity).

This log usually has the unit 'Acoustic Slowness'. This is the time interval per unit length, Delta-T, necessary to attenuate a traveling acoustic wave. Applies to all types of Delta-T: Compressional, Shear and Stoneley.

Data for this log is produced by a tool that uses a acoustic logging technique and can be used to investigate the acoustic slowness of the mud. A similar log might be: Delta-T Mud (DTMD).