I have been doing an extensive amount of reading on EM (Electromagnetic) Telemetry in MWD (Measurement While Drilling) applications. From my reading, there are a few considerable gaps in my understanding which I am trying to fill.
Now this is how I understand the theory works. Just behind a drill bit, a gap sub is placed which provides an electrical isolation from the two ends of the drill pipe. This isolation creates an altered voltage between the two sides of the gap sub. An electric field is emitted (and orthogonally a magnetic field) from the gap sub. On the surface, an antenna is created using a ground rod and the top of the drill string. This antenna is used to retrieve the signal created by the EM field. That signal is then decoded to retrieve the data.
First, many of the sources stated that the signal generated is on the order of 5-10 Hz. If this is the case, would not the antenna have to be several hundred kilometers wide (since wavelength = speed of light / frequency) in order to retrieve the signal? This leads to my second question. All of sources stated that an EM FIELD is generated. An EM field is not the same as an EM wave. How does a field have a frequency? I feel like I am overlooking something fundamental here in regards to EM theory.
Lastly, a co-worker and I had a very long discussion about how this works. He stated that an EM wave actually isn't emitted at all, but a voltage gradient is created across the gap sub. He also stated that the drill string is essentially used as one leg of a transmission wire and the earth is used as the other. In other words, the drill string is used to transmit data up to the surface and the earth is used to receive data from the surface, or vice versa. To me, this is just seems plain wrong, but perhaps I am the one mistaken. If this was the case, would not the two sides of the transmission wires (the earth and drill string) essentially short themselves out since they are in constant contact with each other? His reasoning was that current follows the path of least resistance and thus will not dissipate to the earth but travel up the drill string to the surface.
Any help, insight, or other reference is greatly appreciated. Here is about the best image I could find to help describe the general theory http://www.netl.doe.gov/Image%20Library/technologies/oil-gas/natural-gas/projects-n/ep/DCS_31103.jpg