scholarly journals The effective antenna noise figureFafor a vertical loop antenna and its application to extremely low frequency/very low frequency atmospheric noise

Radio Science ◽  
2007 ◽  
Vol 42 (4) ◽  
pp. n/a-n/a ◽  
Author(s):  
Antony C. Fraser-Smith
Keyword(s):  
Low Frequency ◽  
Loop Antenna ◽  
Very Low Frequency ◽  
Extremely Low Frequency ◽  
Atmospheric Noise

scholarly journals Researches on very low frequency acoustic signal propagation characteristics in different shallow elastic wedge bottoms

2019 ◽  
Vol 283 ◽  
pp. 02003
Author(s):  
Jun Zhu ◽  
Hanhao Zhu ◽  
Jun Tang ◽  
Guangxue Zheng
Keyword(s):  
Acoustic Signal ◽  
Low Frequency ◽  
Signal Propagation ◽  
Acoustic Energy ◽  
Propagation Characteristics ◽  
The Finite Element Method ◽  
Very Low Frequency ◽  
Extremely Low Frequency ◽  
Elastic Bottom ◽  
Sloping Bottom

Targeted at the issue of extremely low-frequency (<100Hz) acoustic propagation in complex shallow elastic bottom environments. The influence law of different complex elastic bottoms on the acoustic signal propagation at very low frequency by acoustic energy flux has been analyzed with the simulation, which is based on the finite element method. The elastic bottoms which have been studied are the shallow horizontal elastic bottom, and the up-sloping and the down-sloping elastic bottom. The results show that the acoustic signal propagating in the up-sloping and down-sloping elastic bottom environments is more complex than that propagating in the horizontal elastic bottom, and the acoustic energy leaking into those elastic bottoms has very different influence on the acoustic signal propagation, especially in the up-sloping bottom.

Extremely low frequency detection of electrical discharges at Minamidake crater (Sakurajima volcano, Japan)

2020 ◽  
Author(s):  
Caron E.J. Vossen ◽  
Corrado Cimarelli ◽  
Alec J. Bennett ◽  
André Geisler ◽  
Damien Gaudin ◽  
...  
Keyword(s):  
Low Frequency ◽  
Japan Meteorological Agency ◽  
Frequency Range ◽  
Electrical Discharges ◽  
Very Low Frequency ◽  
Extremely Low Frequency ◽  
Sakurajima Volcano ◽  
Frequency Detection ◽  
The World ◽  
Active Volcanoes

&lt;p&gt;Volcanoes are increasingly better monitored around the world. Nonetheless, the detection and monitoring of volcanic ash plumes remains difficult, especially in remote areas. Intense electrical activity and lightning in volcanic plumes suggests that electrical monitoring of active volcanoes can aid the detection of ash emissions in near real-time. Current very low frequency and wide-band thunderstorm networks have proven to be able to detect plumes of large magnitude. However, the time delay and the relatively high number of non-detected explosive episodes show that the applicability of these systems to the detection of smaller (and often more frequent) ash-rich explosive events is limited. Here we use a different type of thunderstorm detector to observe electrical discharges generated by the persistent Vulcanian activity of Minamidake crater at Sakurajima volcano in Japan. The sensors consist of two antennas that measure the induced current due to the change in electric field with time. In contrast to the current thunderstorm networks, these sensors measure within the extremely low frequency range (1-45 Hz) and can detect lightning up to 35 kilometres distance.&lt;/p&gt;&lt;p&gt;Two detectors were installed at a distance of 3 and 4 kilometres from Minamidake crater and recorded almost continuously since July 2018. Within this period, the ash plumes reached a maximum height of 5.5 kilometres above the crater rim. Using a volcanic lightning detection algorithm and the catalogue of volcanic explosions compiled by the Japan Meteorological Agency (JMA), the number of electrical discharges was determined for each individual explosive event. In addition, the start of electrical discharges was compared to the eruption onset estimated by the JMA.&lt;/p&gt;&lt;p&gt;Preliminary results show that the detector closest to the crater had the highest detection efficiency. It detected electrical discharges during 60% of the eruptions listed by the JMA. This is significantly higher than for the World Wide Lightning Location Network, which detected electrical discharges (in the very low frequency range) within 20 kilometres of Sakurajima for less than 0.005% of the eruptions. Furthermore, the results show that for 40% of the detected eruptions, electrical discharges were detected before the estimated JMA timing. Hence, electrical discharges can mark the inception of the explosion with a higher precision and are an indication of ash emission. This demonstrates the value of the cost-effective sensors used here as a monitoring tool at active volcanoes.&lt;/p&gt;

SOURCE DISTANCE DEPENDENCE OF THE SURFACE‐IMPEDANCE CONDUCTIVITY MEASUREMENT TECHNIQUE

Geophysics ◽  
1969 ◽  
Vol 34 (5) ◽  
pp. 785-788 ◽  
Author(s):  
Peter R. Bannister
Keyword(s):  
Measurement Technique ◽  
Surface Impedance ◽  
Low Frequency ◽  
Conductivity Measurement ◽  
Very Low Frequency ◽  
Extremely Low Frequency ◽  
Source Distance ◽  
Distance Dependence ◽  
Magnetotelluric Method

The surface impedance conductivity measurement technique, commonly called the magnetotelluric method (Wait, 1962a), has been employed for many years at frequencies below 1 Hz. Recently, frequencies in the extremely low frequency (ELF) and very low frequency (VLF) bands have been employed (Watt et al, 1963).

A Method on Underwater Extremely Low Frequency Atmospheric Noise

2013 ◽  
Vol 333-335 ◽  
pp. 2457-2460
Author(s):  
Shi Min Feng ◽  
Sui Hua Zhou ◽  
Hong Xin Zhang
Keyword(s):  
Approximation Method ◽  
Frequency Characteristic ◽  
Low Frequency ◽  
Sampling Frequency ◽  
Frequency Compensation ◽  
Signal To Noise ◽  
Extremely Low Frequency ◽  
Iir Filter ◽  
Atmospheric Noise ◽  
Low Pass

On the basis of low-pass characteristic of the ocean in extremely low frequency (ELF) band , which makes the atmospheric noise distort and signal to noise (SNR) of the receiver worse, magnitude-frequency and phase frequency characteristic of the ocean in ELF band were researched. Influence at different depth of the ocean on the atmospheric noise was simulated and frequency compensation filter was designed with the best uniform approximation method to compensate the distorted atmospheric noise. With the sampling frequency fs=1KHz, at 30m~200m depth, no more than 9th-order IIR filter was used to design frequency compensation filter. The maximal error of the magnitude-frequency characteristic of the designed frequency compensation filter is less than 0.5dB compared with the theoretical. With the filter, the sharpness of the ELF atmospheric noise spikes are restored and the SNR of the receiver is improved.

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