Drill stem logging with electromagnetic waves and electrostatically-shielded and inductively-coupled transmitter and receiver elements

An apparatus suitable for measuring while drilling formation parameters about a borehole traversing earth formations and containing drilling fluids, comprising:

• a tubular, electrically conductive drill collar threaded at each end for threaded engagement in a string of drill pipe above the drill bit and including a longitudinal passage suitable for conveying drilling fluids therethrough;
• means for transmitting electromagnetic energy into the space surrounding said drill collar, said transmitting means insulated from but disposed on the exterior of said drill collar;
• first means for receiving electromagnetic energy from the space surrounding said drill collar, said first receiving means longitudinally spaced a first distance from said transmitting means and insulated from but disposed on the exterior of said drill collar;
• a first receiver electrostatic shield substantially surrounding said first receiving means to reduce the electric field component of the received electromagnetic energy, including electrical noise, said first shield comprising conductive elements substantially surrounding said receiving means, insulated from said drill collar, having at least one gap to prevent eddy currents therein and maintained at the ground potential of said receiving means;
• and
• first inductive coupling means disposed within said electrostatic shield and inductively coupled to said receiving means for coupling the signal from said receiving means to an electrical conductor for communicating to a data handling means.

An apparatus suitable for measuring a formation parameter about a borehole traversing an earth formation and containing drilling fluid, comprising:

• a tubular, electrically conductive housing means threaded at each end for threaded engagement in a drill string above the drill bit and including a longitudinal passage suitable for conveying drilling fluids therethrough;
• means for transmitting electromagnetic energy into the formation surrounding said housing means, said transmitting means insulated from but disposed about the exterior of said housing means;
• first receiving means for receiving electromagnetic energy from said formation, said first receiving means longitudinally spaced a first distance from said transmitting means and insulated from but disposed about the exterior of said housing means;
• first shield means substantially surrounding said first receiving means for electrically, but not magnetically, shielding said first receiving means by reducing the coupling of the electric field component of the received signal, including noise, to said first receiving means, said first shield means comprising:
• a pair of first conductive shield tubes substantially surrounding said first receiving means and within which said first receiving means is disposed, said first tubes each having a first end, said first ends spaced from one another with a first gap to prevent eddy currents therein, each having a second end, said second ends joined to a first conductive case at a pair of opposed openings in the side walls of said case through which said first receiving means extends, said first shield tubes and said first conductive case insulated from said housing means,
• means mounted within said first conductive case for inductively coupling the signal from said first receiving means to an electrical conductor leading to signal handling means, and
• means for electrically connecting said first shield tubes and said first conductive case to ground potential while insulating them from said conductive housing to reduce electrical noise from ground loop currents therein.