1. n. [Formation Evaluation]

A plot representing the effect of invasion on resistivity measurements that have different depths of investigation. The plot assumes a step profile model of invasion and determines true resistivity, flushed zone resistivity and diameter of invasion from ratios of deep-, medium- and shallow-resistivity measurements. Strictly speaking, when both resistive and conductive invasion are plotted, the chart is called a butterfly chart. When only one is plotted it is known as a tornado chart.

See: conductive invasion,  depth of investigation,  resistive invasion