The MFAM Magnetometer samples at 1000 Hz, which in turns captures a lot of unique waveforms. When viewing the data raw, it can therefore appear to be a bit noisy. But a closer examination of the data will reveal a real variation of the magnetic field which is caused caused by the power distribution network. Proper filtering is required to reduce the power line caused variations and reveal the strong signal of interest.
It is not obvious that 60 or 50 hertz electromagnetic radiation is real, since in ordinary experience any power line “noise” is electrostatically coupled into a system (think 60 hertz hum on a stereo system) and is a fault that needs to be fixed. In this case however the variation in the magnetic field is induced by the power grid and is real. The magnetometer is simply and dutifully reporting the variation.
These power line variations are to some extent present everywhere – even miles from the nearest power line. But obviously being close to power lines will increase the amplitude of the variations a lot. Often on a MagArrow survey the power line variations will be larger at one end of the survey area than the other. Poking in the GPS coordinates at the survey area nearest the larger variations into Google Earth will usually reveal the power lines from an aerial view – even if they are not visible on the ground.
After applying a Fourier Frequency Transform on the MFAM data to identify the noise sources, 50 and 60 Hz noise amplitudes are easily observed. Also observable is the likely to be 20.8 Hz Schumann resonance of the third node and some other ultra-low frequency electro magnetic radiation produced naturally by the Earth. Harmonics of 60 Hz are also present.
Another common question is “Why is the power line variations not a sine wave like the power line voltage?” Remember that voltages do not make magnetic fields. Only current generates magnetic fields, and the current being drawn is not a sine wave at all. Many loads, for example, only draw current at the voltage peaks. This makes for a non-sinusoidal magnetic field that is rich in harmonics. Also note that most power distribution system use a 3 phase topology. The ripple current in such a system will be 150 or 180 Hz. Thus you will often see large peaks in the power spectrum at these frequencies and their harmonics.