THE RHOmaa - Umaa CROSSPLOT
The interpretation of simple minerals and lithologies is generally straightforward from a visual inspection of the photoelectric index curve with the neutron/density overlay. In cases of mixed lithologies or mysterious minerals, a crossplot of a zone log responses can be helpful both for purposes of identification and for semi-quantitative estimations of volumetric proportions.
The RHOmaa - Umaa crossplot was introduced to utilize measurements of the photoelectric index, neutron porosity, and bulk density for matrix mineral evaluation. The two dimensions of the crossplot require the three log variables to be condensed in some manner. This is done with a similar approach to the M-N plot methodology, through the suppression of porosity as a variable and calculation of the apparent properties of the rock matrix. The log variables that are needed for the calculation are shown diagramatically in Figure 50.