# Applied Engineering Using Schumann Resonance for Earthquakes Monitoring
## Materials and Methods
### Worldwide Map of ELF Observatories and Seismic Events  
The number of stations or observatories ELF today is not too large, due to many factors, among which is the difficulty of the measurement process or the study of such signals. This study is an issue that is being widely developed in recent years. ELF/VLF stations or observatories exist today can be classified into several types:  
Figure 4: Data from the Calar Alto Sensor (Spain): (**a**) The 30 min periodogram. (**b**) The 24 h Spectrogram. 1. Observatories that measure the disturbances caused by seismic activity on the ELF radiation, considering subELF observatories. These subELF observatories usually record the possible variations in the Earth's magnetic field, where some have very low frequencies (less than 0.03 Hz). An example of this is the station of Uchimura (Japan), which allows for studying the data collected in the range comprised between 0.01 to 0.033 Hz frequencies [52]. In other publications, the signals of lower frequencies (0.01-0.02 Hz) are analyzed [54] to establish anomalies or unaccounted perturbations of the earth's magnetic field before the occurrence of several earthquakes of interest [53]. Considering the development of such stations of great interest for possible applications in the field of early detection is considered as an emerging theme in the study. 2. Other observatories had recorded signals in the 1 Hz to 50/60 Hz spectrum. The latter are important frequencies due to the presence of strong interference signals in this band [63]. It might be interesting to consider the different sub-bands within this range [64]. 3. There are also observatories that monitor the exact value of frequencies. Mainly, these values happen to be multiples of the principal frequency depending on the country. These frequencies are usually 223 Hz (in countries having 60 Hz as principal frequency) and 233 Hz (in countries having 50 Hz as principal frequency) [65]. Other observatories use a frequency of 17 Hz [66]. All of the observatories mentioned earlier measure signals of original nature, but there are other stations that monitor artificial signals that are generated by human activity, such as signals in the VLF band that are issued for the navigation aid [67]. Furthermore, not all of the stations have the same measuring devices. The most common are seismometers, acoustic meters, or telluric current meters. These instruments are of great importance to correlate the different parameters of interest with the detection of natural phenomena. The data above can be related to other meteorological data, such as wind speed, humidity, temperature, and atmospheric pressure [48]. Table 1 shows the World-Wide ELF/VLF observatories. This list is in descending order, showing the estimated station range according to the references listed in the table itself. The observatories with range above 1000 km are usually used for communications and shorter ranges are aimed for the detection of natural disasters. The station's sensitivity range is the maximum distance at which it can detect an atmospheric phenomenon. Therefore, this range depends on the magnitude of the different phenomena it can detect.