TMD. A record of the macroscopic thermal neutron cross sections of the formation (SF) and the borehole (SB).
An electronic neutron generator produces bursts of pulses of 14-MeV neutrons which spread into the formation and borehole. Following each burst, the neutrons are quickly slowed to thermal energies by successive collisions with nuclei in the surrounding media. The thermalized neutrons are then captured by elements within the formation and borehole, producing gamma radiation. Gamma radiation intensity is sampled in two detectors at six time intervals (time gates) following each burst. These data are used to compute SF and SB. Also, the ratio (R) of counts in the near-spaced to far-spaced detector is recorded and used to estimate formation porosity.
Of the common earth elements, chlorine is by far the strongest neutron absorber and is found mainly in the formation water (as sodium chloride) rather than in the formation matrix. SF primarily a function of the salinity and amounts of water present in the pore volume. Therefore, SF is used with porosity from R to compute formation hydrocarbon saturation if the formation water is saline. The SF curve is relatively unaffected by casing and tubing. SB is used with SF to obtain a very accurate true (intrinsic) formation cross section and an improved porosity estimate. In addition, SB can indicate a variety of well bore conditions such as gas between the tubing and casing, and oil-water contacts. Thermal Multigate Decay Log (TMD) is a Welex trademark.