# EM MWD Gap Sub Telemetry Theory Explanation in Drilling Applications

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

• Let me just comment on the needed antenna length. You don't need the antenna to match the wavelength. It can be much shorter. Think about AM radio, at 1 MHz the wavelength is about 300m, but a ~1/2 meter length of wire can be used as an antenna. (but longer is better.. to a point.) Nov 12 '14 at 14:35
• I suspect that the term EM Field is referring to the near-field which does have a frequency dependance and the term EM wave is a far-field or radiated component. en.wikipedia.org/wiki/Near_and_far_field Nov 12 '14 at 20:50

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: -

First, EM telemetry begins with the gap sub. A modulated voltage/current is applied across this gap to send data. The frequency is in the range of 2 to 12Hz. The power level is in the range between 10 to 30 watts; sometimes more.

Now, steel pipe has a resistivity on the order of 10^-8 ohm-meters (less than a micro ohm-meters). The resistivity of the earth is in the range of 1 ohm-meter to 1000 ohm-meters more or less. In other words, the steel pipe is about a million times more conductive than the earth. Given a choice, the current will to travel the low resistance path up/down the pipe. Except that the current wants to return to the other side of the gap to complete the circuit.

So, think about the pipe just above and below the gap. Current will leak from the pipe just above the gap to the bit of pipe just below the gap. Since the pipe is so much more conductive, pipe above and below this area will also leak current and conduct through the formation. You can imagine an expanding spherical shell of current centered at the gap until you get to the surface.

At the surface, the pipe has a low resistance direct path to the top of the gap. The voltage drop though the pipe from the surface to the top of the gap could be measured in millivolts or microvolts.

The problem is getting the connection to the bottom of the gap. That connection is made indirectly. Because there is this spherical shell of voltage/current, there is a voltage/current gradient near the surface So, electrodes are placed into the ground some distance away from the BOP. And if the gods smile, there is a voltage difference between the BOP and the ground rod driven into the ground. Once that difference is measured, data can be received.

So, your friend is correct. A current travels up the drill pipe to the receiver, and returns to the bottom of the gap through the formation.

You don't need the theory of electromagnetic waves to explain EM telemetry. You don't need a resonate antenna the length of the diameter of the earth to receive the signal. EM telemetry can be understood in terms of voltage gradients and current conduction. That's all.

There are various schemes for the transmission of ultra-low frequency electromagnetic waves while drilling information. The first is that the earth is used as a loop conductor, and the drill pipe is used as a loop conductor, transmitted to the ground, and then filtered and amplified by the ground receiving equipment to decode. This method has disadvantages. It is greatly affected by formation resistivity. The second method is a direct electromagnetic wave transmission method. This method requires high-power ultra-low frequency transmitters and high-sensitivity receivers to transmit in true electromagnetic waves. This method is not affected by formation resistivity. Impact. The working frequency of these two methods is generally selected between 2Hz-20Hz, the antenna structure is different.