Matrix identification Matrix identification (MID) refers to a lithology solution in petrophysics where the bulk density and photoelectric factor are used to estimate the lithologies of the rocks being logged. When a standard triple combo log is run in a well, the tool is utterly naive to the true identity of the rocks it encounters. The bulk density for example really measures electron density, and the neutron log (grossly simplified) measures hydrogen, and neither has any clue about what lithology it is looking at.
Introduction A caliper log records the borehole size. When a logging tool is pulled up the well, a caplier arm opens or closes as it encounters zones of washout, mudcake, or zones of stable hole condition. If there is abundant washout or mudcake, many logging tools (bulk density, neutron, etc.) do not collect valid petrophysical data. Often a petrophysicist will use some cutoff (2.5 inches of washout, maybe) to determine whether they can use the data over that interval in their analysis or if they should instead throw it out.