1. As a rotary well is drilled, samples of the circulating mud and its suspended drill cuttings are examined for lithology and hydrocarbons by visual observation (including ultra-violet fluoroscopy) and for gas-phase constituents by a Wheatstone bridge ‘‘hot-wire’’ partition gas chromatograph or hydrogen-flame ionization analyzer. A mud log is a plot of such measurements, together with a lithologic log and a drilling-time log (showing rate of penetration). Used to detect fluids that have entered the mud from the formations. Hydrocarbons may be evidenced by fluorescence, by chromatographic analysis, gas, and other ways. The mud is also monitored for salinity and viscosity to indicate water loss. Plots of such data are usually included with a sample log and drill-time log. 2. A log made with a microlog sonde with the arms collapsed so that the measuring pad is not pressed against the borehole wall. Measures resistivity of the mud at in-hole conditions.