1. . 
With reference to invasion, an abrupt change from the flushed zone to the undisturbed zone, with no transition zone or annulus. This simple model is used most commonly in connection with older resistivity logs since it allows the invasion to be represented by three parameters: flushed-zone resistivity, undisturbed-zone resistivity and diameter of invasion. The model assumes equal invasion at all azimuths. Newer array logs allow more complex invasion models to be interpreted.