Well logging apparatus having toroidal induction antenna for measuring, while drilling, resistivity of earth formations

A measurement-while-drilling system for determining the resistivity of formations surrounding a borehole being drilled in the earth by a drill bit at the end of a drill string, comprising:

• a drill collar in said drill string, said drill collar comprising an elongated tubular body formed of an electrically conductive material;
• a toroidal antenna mounted on said body in an insulating medium, said antenna comprising a coil mounted on a toroidal core having an axis substantially coincident with the drill collar axis;
• means for energizing said toroidal antenna to induce a current which travels in a path that includes said body and said formations;
• an electrode disposed on said body;
• and
• means for measuring at said electrode an electrical signal resulting from said current, said electrical signal being an indication of the resistivity of said formations.