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| # | Post Title | Result Info | Date | User | Forum |
| MagArrow Heading Error Compensation Flight FAQ | 3 Relevance | 3 years ago | Gretchen Schmauder | Application | |
| To perform a heading error compensation flight, fly the UAV with MagArrow attached up to 100-150 meters in a low gradient area. Hover the drone in a single spot and rotate it slowly through 360 degrees while logging magnetic data the with MagArrow. By keeping the drone location stationary the mag field will be also be constant. Thus we are only left with the sensor reading as a function of orientation. The MagArrow has two MFAM sensors, and the way they are arranged ensures that when one sensor is in its dead zone the other is at its optimum orientation, and vice versa. The readings from the two sensors are combined to produce one magnetometer reading only. The two sensor readings are weighted such that as one sensor approaches its dead zone it is weighted much less (down to zero in the dead zone) while the optimum oriented sensor is weighted more fully. Thus you get only one magnetometer reading with no dead zones whatsoever. In addition, the weighted averaging of the sensors still does partial heading error cancelling. | |||||
| Understanding Acquisition Filters in Seismographs - Their Use and how to Filter | 3 Relevance | 3 years ago | Gretchen Schmauder | Software | |
| Low Cut: , 10, 15, 25, 35, 50, 70, 100, 140, 200, 250, 280, 400 Notch: 50, 60, 150, 180 High Cut: 32, 64,125, 250, 500 or 1000 Hz The first recommendation for cases when you are having trouble getting sufficient signal to noise would be to increase your signal via stacking the data with multiple source events or get a more powerful seismic source. This will usually produce better results than the application of filters. Another approach would be acquire data when the noise sources are less present. That may mean collecting data at night when the area is closed or the traffic is less. Early morning can be better for areas where the wind tends to increase during the day. The selection of filters is very site dependent and can depend on a variety of factors as well as the type of survey being performed. 1) Typically the Notch filters are to remove noise due to electrical power lines (50 or 60 Hz and their harmonic frequencies depending on the country you are in). 2) Low cut filters are generally used for noise due to wind and moving vehicles, but care must be taken not to remove too much bandwidth from generated seismic signal. Often the noise sources have the same frequencies as the seismic data you are interested in and can’t be effectively removed using frequency filtering. 3) High cut filters can be used to remove noise from high frequency vibratory signals such as compressors or airplanes. In general it is best to record the data without any frequency filters and filter in post processing or only on the displayed data in our software. It will be a matter of experimentation to determine the best filters at your site. Modern 24-bit seismographs (Geode, Stratavisor, ES-3000, etc) have a much wider range of signal amplitudes that they can record accurately. This means that they can still accurately record smaller seismic signals even in the presence of larger noise signals. Therefore there is a reduced need for analog filters that are applied prior to digitization of the signals. Digital filters are more flexible and can be more specifically applied to the noise that is recorded rather than the “Broader Brush” of analog filters. Digital filters also have the benefit of being able to go back to the original data if the wrong filter is applied, which is not the case with Analog filters. The general approach in the seismic industry is now to record everything – including the noise – and the filter out what you don’t want later. | |||||
| 1000 Hz Sample rate and Powerline Variations | 3 Relevance | 3 years ago | Gretchen Schmauder | Application | |
| 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. | |||||
| 1000 Hz Sample Rate and Powerline Variations | 3 Relevance | 3 years ago | Gretchen Schmauder | MFAM | |
| 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. | |||||
