# Applied Engineering Using Schumann Resonance for Earthquakes Monitoring
## Materials and Methods
### ELF Sensor  
ELF sensors are built by combining vertical electric field sensors and horizontal magnetic induction coils [54]. This process is very complex due to the large number of factors involved, such as its low frequency, the weakness of the signal to be detected, the non-uniformity of the ionosphere, the manifestation of natural electromagnetic noise, and the interference caused by industrial zones,  
Figure 1: Phenomenon of Schumann Resonance. appearing as background noise in the process [55]. These are some of the reasons behind the small number of ELF sensors compared to other observatories; although that number has increased fourfold since the five observatories accounted for in 1999. These observatories are located throughout the world. However, the largest concentration of such observatories is in Japan, where the occurrence of seismic events is greater [56]. Many of these observatories are attempting to establish a correlation between the ELF signals measured and seismic activity, and to compare with data from other stations. For this, a correlation is established within the different components of the magnetic field [57], and the development of theoretical models for describing the intensity of the SR by disturbances in the ionosphere over the top of the epicenter of an earthquake. One of these theoretical models is based on ionosphere's vertical conductivity profile model, describing the regular Earth-ionosphere cavity. The possible disturbance is introduced with the modified model. The localized ionosphere modification shows a Gaussian radial dependence; it has a 1-Mm radius, with the decrease reaching 20 km in the lower ionosphere height over the epicenter of the earthquake (Taiwan). Also, the diffraction problem in the Earth-ionosphere cavity with a disturbance can be resolved by using the Stratton-Chu integral equation [58]. One of these ELF stations has been developed and steadily improved by the research group TIC019 of Almeria University and it is located in Calar Alto (Almeria, Spain), Figure 2. This observatory began operation in 2011, being the first observatory of its kind in Spain. The technology used in this type of observatories has evolved significantly in recent years. The general aspects of it are described in a few papers, but only a handful of articles explicitly set the methodology used to capture and measure ELF signals. Figure 3 shows the block diagram of Calar Alto ELF observatory (Almeria, Spain). As the diagram shows, the system consists of a set of stages from signal capture and local real-time processing to transmission to the University of Almeria and the insertion of post-processed data into an interactive database. The stages of the measuring system are as follows:  
1. Signal Capture: formed by the ELF magnetic sensors. 2. Stage of conditioning: of signal, constituted by a high gain differential amplifier and a level scaled adapter. Through this process a suitable signal is obtained at the input of the next stage, with a maximum dynamic range input and the lowest output clipping. A passband filter ranging from 3  
Figure 2: Calar Alto Extremely Low Frequency (ELF) Station (Spain). to 100 Hz is also included in this stage. This filter attenuates signals outside the desired band, preventing undesired intermodulation or cross-modulation phenomena. 3. The stage of the Analog/Digital converter (A/D converter): This stage has two channels, one for each sensor. It converts the analog signal into 24-bit samples per channel with a sample rate of 196 samples/s. 4. The last stage is the data transmission and storage. It is constituted by a local information storage (data logger), which serves as failsafe from temporary interruptions in the transmission system. Additionally it allows data transmission from the A/D converter to the installations of the University of Almeria by means of a digital radio link. In such facilities, the data is inserted into an interactive database for post-processing and study.