EM Transmission Link and Detection

Although EM stands for Electro Magnetic, which may suggest radio frequency signals, the very low frequencies used in the drilling environment behave more like DC, or more or less static signals. These signals, as such, can be looked at and analyzed with the help of basic electrical theory. 
The functional EM system consists of several main parts: 
1. EM Transmitter

2. Gap Sub

3. Signal path through drill string 
4. Return signal path through the geological formations 
5. EM Receiver 

There are many different ways to transmit the EM signal. However, the one common function of all of the transmitter types is to control the power of the signal. There is a trade off between a strength of a signal and the power consumption. The best signal can be obtained using maximum available power, but such set up may last only a few hours. Therefore an EM system is designed for a specific, optimal for such system, environmental conditions. There is no EM transmission scheme that will excel in all the possible geological formations, maintaining the optimal electrical performance. For example, some of the EM transmitters will outperform others in very low resistivity formations, but will fail in high resistivity environment. And vice versa. 

 Gap Subs are required by all traditional EM systems. The EM signal is generated by creating a potential difference between the areas above and bellow the gap. The Gap Sub can also the source of major power loss from the transmitter, therefore the layout of the Sub must be carefully designed. Also, the drilling fluid will have a significant contribution to the power loss at the Gap. Low resistivity gels, for example, will cause higher gap losses, but invert mud will extend the battery life. 

The drill string provides an excellent conduction path for the electrical EM signal to reach the surface. This path also includes all connections, cables and grounding rod. However, this path is also the source of main problems of all EM systems, mainly, the source of electrical noise. Since the drill string is directly connected to the rig, it will contain all the noise that is generated at the rig’s various stations. The electrical noise will consist of: 
-Rig Noise. This noise can come from faulty grounding of the generators or pumps, or from any other faulty electrical machinery. 
-Drilling Noise. While drilling, the pipe moves with respect to formations or casing, causing generation of stray frequencies as detected by the surface equipment. 
-Ground Noise. This can come from poor ground conditions (mainly dry sands), lightning storms, sun flares, and the naturally occurring Spontaneous Potential of the geological formations. 

The propagation of the signal through the geological formations is the major challenge for the EM systems in general.
The low resistivity formations, such as salts (including anhydrides) will require a lot of current to support the voltage levels high enough for on surface detection. The high resistivity formations such as coal, will require just a little current, but the voltage levels will have to be very high. In general:

0.1-50 Ohm-m (low resistivity), best for low voltage/high current EM 
50-500 Ohm-m, its the best range of any EM

500-2000 Ohm-m (high resistivity), good for high voltage/low current EM 
0ver 2000 Ohm-m (very high resistivity). Only special EM tools can make it, repeaters or extended reach antennas should be used. 

The Ohm-m values can be obtained from the resistivity (induction) logs. 

EM Tool Specifications:
Downhole Transmitter Transmitter frequency: 2-20 Hz, adjustable 
Transmission Power: 50 W max, adjustable 
Output Voltage: 3-10Vrms 
Operational Temperature: 0 - 150 DegC 
Vibration: 20g RMS (15-1000 Hz)
Shock: 1000g, 1ms, 1/2 Sine

Input voltage: 22-36Vdc 
Available mounting space diameter: 35 mm, length: 1000 mm 
Transmission protocol: PM and FM 
Signal frequency: 2-20 Hz 
Signal amplitude: 100nV-10V 
Environmental noise: Diesel Power Generators, 50-60Hz, 3 phase 
Noise levels: 100V 
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