EM MWD Gap Sub Telemetry Theory Explanation in Drilling Applications
Mud pulse telemetry is the standard means of transmitting telemetery signals from the drill head to the surface - it modulates the pressure of the mud coolant down the drill pipe and this can be received at the surface but naturally data rates are fairly slow.

Here's what Halliburton say about EM communication and paraphrasing this I expect it means the following: -

Low frequency EM wave - I expect this means the magnetic portion of the EM wave as used by pot-holers comms systems. The E part of the wave wouldn't survive many feet in mud and earth. See this as evidence of using magnetic loop antennas and lowish frequencies. The guy writing the article suggests that: -


  My thoughts on this subject were that the near magnetic field radiated
  by a magnetic antenna is not greatly attenuated by calcareous rock,
  and that the radiated field should penetrate into the earth at least
  the distance of a quarter of a wavelength at the transmitting
  frequency.


If a low modulation frequency were used (say 100kHz) then this will have a wavelength of 3km as an example. But I don't see why even lower frequencies shouldn't be employed. The data rate is a few tens of bits per second and if you said 100 max, a modulation frequency of maybe 1kHz would do the job BUT making a magnetic resonant antenna at 1kHz is a little more difficult but possibly no more difficult that making a search coil to a metal detector. See also this wiki article on the subject.

There is also this article on rescuing trapped miners and being able to contact them: -



And if you need more information, contact Halliburton on sperry@halliburton.com