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 invasion and conductive invasion are plotted, the chart is called a butterfly chart. When only one is plotted, it is known as a tornado chart.