Method for directionally drilling a borehole

A method for directionally drilling a borehole into a horizontal or substantially horizontal earth formation with a tool string that includes a drilling motor that drives a bit on the lower end of a drill string, comprising the steps of:

• curving said borehole until its lower end extends along a path that is between the top and the bottom of said formation;
• drilling ahead while measuring at least two characteristic properties of said formation, wherein one of said characteristic properties is electrical resistivity determined by operating a first electromagnetic means that induces axial flow of current in the walls of said tool string which passes out into said formation and then returns to the walls of said tool string above said first electromagnetic means, and using a second electromagnetic means spaced below said first electromagnetic means to sense the magnitude of the current flow that returns to said walls, said first and second electromagnetic means being insulated with respect to said walls of said tool string such that a portion of said current is prevented from leaving said walls except at locations below said second electromagnetic means, and wherein the measurement of one of said characteristic properties trends upward if the borehole is approaching either the top or the bottom of said formation, and the measurements of another of said characteristic properties trends downward if the borehole is approaching the bottom of said formation;
• and in response to a comparison of said trends, deviating the borehole when necessary so that its path remains between the top and the bottom of said formation.