scholarly journals Estimating the seismotelluric current required for observable electromagnetic ground signals

2010 ◽  
Vol 28 (8) ◽  
pp. 1615-1624 ◽  
Author(s):  
J. Bortnik ◽  
T. E. Bleier ◽  
C. Dunson ◽  
F. Freund
Keyword(s):  
Simple Model ◽  
Current Source ◽  
Signal Power ◽  
Earthquake Hypocenter ◽  
Detectable Range ◽  
Source Current ◽  
Cardinal Directions ◽  
Electromagnetic Signals ◽  
Ground Conductivity ◽  
Detectability Threshold

Abstract. We use a relatively simple model of an underground current source co-located with the earthquake hypocenter to estimate the magnitude of the seismotelluric current required to produce observable ground signatures. The Alum Rock earthquake of 31 October 2007, is used as an archetype of a typical California earthquake, and the effects of varying the ground conductivity and length of the current element are examined. Results show that for an observed 30 nT pulse at 1 Hz, the expected seismotelluric current magnitudes fall in the range ~10–100 kA. By setting the detectability threshold to 1 pT, we show that even when large values of ground conductivity are assumed, magnetic signals are readily detectable within a range of 30 km from the epicenter. When typical values of ground conductivity are assumed, the minimum current required to produce an observable signal within a 30 km range was found to be ~1 kA, which is a surprisingly low value. Furthermore, we show that deep nulls in the signal power develop in the non-cardinal directions relative to the orientation of the source current, indicating that a magnetometer station located in those regions may not observe a signal even though it is well within the detectable range. This result underscores the importance of using a network of magnetometers when searching for preseismic electromagnetic signals.

scholarly journals Ionospheric influence on the seismo-telluric current related to electromagnetic signals observed before the Wenchuan <i>M</i><sub>S</sub> = 8.0 earthquake

2016 ◽  
Author(s):  
Mei Li ◽  
Handong Tan ◽  
Meng Cao
Keyword(s):  
Current Source ◽  
Ionospheric Effect ◽  
Electrical Fields ◽  
Model Free ◽  
Ionosphere Model ◽  
The Earth ◽  
Main Fault ◽  
Electromagnetic Signals ◽  
The Waves ◽  
Detectability Threshold

Abstract. A three-layer (Earth-air-ionosphere) physical model, as well as a two-layer (Earth-air) model, is employed in this paper to investigate the ionospheric effect on the wave fields for a finite length dipole current source co-located with the main fault of an earthquake when the transmitter-receiver distance is up to one thousand kilometers or even more. The results show that all electrical fields are free of the ionospheric effect for different frequencies in a relative short range, e.g., ~ 300 km for f = 1 Hz, implying the ionospheric influence on electromagnetic fields can be neglected within this range that becomes smaller as the frequency increases. However, the ionosphere can give a constructive interference to the waves passed through and make them decay slowly when an observation is out of this range and the ionosperic effect can be up to 1–2 magnitudes of the electrical fields. For an observed 1.3 mV/m signal at 1,440 km away for the Wenchuan MS = 8.0 earthquake, the expected seismo-telluric current magnitude for the Earth-air-ionosphere model is of 5.0 × 104 kA , which is of one magnitude smaller than the current value of 3.7 × 105 kA obtained by the Earth-air model free of ionospheric effect. This indicates that the ionosphere facilitates the electromagnetic wave propagation, as if the detectability of the system is improved effectively and it is easier to record a signal even for stations located at distances beyond their detectability threshold.

scholarly journals An analytical expression for early electromagnetic signals generated by impulsive line-currents in conductive Earth crust, with numerical examples

2016 ◽  
Vol 59 (2) ◽  
Author(s):  
Ken'ichi Yamazaki
Keyword(s):  
Seismic Waves ◽  
Ground Surface ◽  
Space Model ◽  
Whole Space ◽  
Source Current ◽  
Em Field ◽  
Electromagnetic Signals ◽  
Analytical Expressions ◽  
The Magnetic Field ◽  
Ground Conductivity

<p>Changes in the electromagnetic (EM) field after an earthquake rupture but before the arrival of seismic waves (“early EM signals”) have sometimes been reported. Quantitative evaluations are necessary to clarify whether the observed phenomena are accounted for by known theories and to assess whether the phenomenon can be applied to earthquake early warning. Therefore, analytical expressions for the magnetic field generated by an impulsive line-current are derived for a conductive half-space model, and for a two-layer model; the somewhat simpler situation of a conductive whole-space is also considered. By analyzing the expressions obtained for the generated EM field, some expected features of the early EM signals are discussed. First, I verify that an early EM signal arrives before the seismic waves unless conductivity is relatively high. Second, I show that early EM signals are well approximated by the whole-space model when the source is near the ground surface, but not when it is at depth. Third, I show that the expected amplitudes of early EM signals are within the detection limits of commonly used EM sensors, provided that ground conductivity is not very high and that the source current is sufficiently intense. However, this does not mean that the EM signals are easily distinguishable, because detector sensitivity does not account for additive noise or false positive detections.</p>

On: “An inverse slope method of determining absolute resistivity” by P. V. Sanker Narayan and K. R. Ramanujachary (GEOPHYSICS, vol. 32, no. 6, p. 1036–1040, December 1967)

Geophysics ◽  
1968 ◽  
Vol 33 (5) ◽  
pp. 843-845
Author(s):  
George V. Keller
Keyword(s):  
Current Density ◽  
Horizontal Plane ◽  
Current Source ◽  
Potential Drop ◽  
Specific Model ◽  
Inverse Slope ◽  
Slope Method ◽  
Source Current ◽  
The Absolute ◽  
A Current

It is important to point out that the method of cumulative conductances described by the authors of this note does not provide a means for determining the “absolute resistivity” of a sequence of layers. The basic technical fallacy in this note is the use of Hummel’s “principle,” stated in equation (1) without consideration of the limits for its applicability. The development of Hummel’s principle is based on a very specific model, that of a sequence of conducting layers resting on an insulating substratum. In such a case, at very large distances from a current source, current must flow entirely horizontally because none can leave either the upper or lower boundaries of the conducting layers. In order that the current can flow in horizontal planes, the rate of potential drop along any horizontal plane must be the same. This condition can be satisfied only if the current density is inversely proportional to the resistivity in each of the conducting layers. Then, the layers may be treated as resistances in parallel, and Hummel’s principle applies.

scholarly journals Potential of on-scalp MEG: Robust detection of human visual gamma-band responses

2019 ◽  
Author(s):  
Joonas Iivanainen ◽  
Rasmus Zetter ◽  
Lauri Parkkonen
Keyword(s):  
Spatial Resolution ◽  
Gamma Band ◽  
Gamma Activity ◽  
Noninvasive Detection ◽  
Signal Power ◽  
Robust Detection ◽  
Gamma Power ◽  
Electromagnetic Signals ◽  
Gamma Band Activity ◽  
Band Activity

AbstractElectrophysiological signals recorded intracranially show rich frequency content spanning from near-DC to hundreds of hertz. Noninvasive electromagnetic signals measured with electroencephalography (EEG) or magnetoencephalography (MEG) typically contain less signal power in high frequencies than invasive recordings. Particularly, noninvasive detection of gamma-band activity (> 30 Hz) is challenging since coherently active source areas are small at such frequencies and the available imaging methods have limited spatial resolution. Compared to EEG and conventional SQUID-based MEG, on-scalp MEG should provide substantially improved spatial resolution, making it an attractive method for detecting gamma-band activity.Using an on-scalp array comprised of eight optically-pumped magnetometers (OPMs) and a conventional whole-head SQUID array, we measured responses to a dynamic visual stimulus known to elicit strong gamma-band responses. OPMs had substantially higher signal power than SQUIDs, and had a slightly larger relative gamma-power increase over the baseline. With only eight OPMs, we could obtain gamma-activity source estimates comparable to those of SQUIDs at the group level.Our results show the feasibility of OPMs to measure gamma-band activity. To further facilitate the noninvasive detection of gamma-band activity, the on-scalp OPM arrays should be optimized with respect to sensor noise, the number of sensors and inter-sensor spacing.

scholarly journals Ionospheric influence on the seismo-telluric current related to electromagnetic signals observed before the Wenchuan <i>M</i><sub><i>S</i></sub> 8.0 earthquake

Solid Earth ◽  
2016 ◽  
Vol 7 (5) ◽  
pp. 1405-1415 ◽  
Author(s):  
Mei Li ◽  
Handong Tan ◽  
Meng Cao
Keyword(s):  
Power Distribution ◽  
Current Source ◽  
Electric Signal ◽  
Ionospheric Effect ◽  
Electrical Fields ◽  
Ionospheric Effects ◽  
Model Free ◽  
Ionosphere Model ◽  
The Earth ◽  
Electromagnetic Signals

Abstract. A three-layer (Earth–air–ionosphere) physical model, as well as a two-layer (Earth–air) model, is employed in this paper to investigate the ionospheric effect on the wave fields for a finite length dipole current source co-located at a hypocenter depth and along the main fault of an earthquake when the distance between the epicenter and an observing station is up to 1000 km or even more. The results show that all electrical fields are free of ionospheric effects for different frequencies in a relative short range, e.g.,  ∼  300 km for f =  1 Hz, implying the ionospheric influence on electromagnetic fields can be neglected within this range, which becomes smaller as the frequency increases. However, the ionosphere can give a constructive interference to the waves passing through and make them decay slowly when an observation is out of this range; moreover, the ionospheric effect can be up to 1–2 orders of magnitude of the electrical fields. For a ground-based observable 1.3 mV m−1 electric signal at f =  1 Hz 1440 km away from the Wenchuan MS 8.0 earthquake, the expected seismo-telluric current magnitude for the Earth–air–ionosphere model is of 5.0  ×  107A, 1 magnitude smaller than the current value of 3.7  ×  108A obtained by the Earth–air model free of ionospheric effects. This indicates that the ionosphere facilitates the electromagnetic wave propagation, as if the detectability of the system were improved effectively and it is easier to record a signal even for stations located at distances beyond their detectability thresholds. Furthermore, the radiating patterns of the electrical field components |Ex| and |Ey| are complementary to each other, although any two-dimensional (2-D) power distribution of these components shows strong power areas as well as weak ones, which is advantageous to register a signal if the observing system is designed to measure both of them instead of only one.

scholarly journals An Impedance Network-Based Three Level Quasi Neutral Point Clamped Inverter with High Voltage Gain

Energies ◽  
2020 ◽  
Vol 13 (5) ◽  
pp. 1261 ◽  
Author(s):  
Muhammad Aqeel Anwar ◽  
Ghulam Abbas ◽  
Irfan Khan ◽  
Ahmed Bilal Awan ◽  
Umar Farooq ◽  
...  
Keyword(s):  
High Voltage ◽  
Low Voltage ◽  
Research Work ◽  
Current Source ◽  
Neutral Point ◽  
Voltage Source ◽  
Duty Ratio ◽  
Voltage Gain ◽  
High Voltage Gain ◽  
Source Current

Due to the impediments of voltage source inverter and current source inverter, Z-Source Inverter (ZSI) has become notorious for better power quality in low and medium power applications. Several modifications are proposed for impedance source in the form of Quasi Z-Source Inverter (QZSI) and Neutral Point Clamped Z-Source Inverter (NPCZSI). However, due to the discontinuity of the source current, NPCZSI is not suitable for some applications, i.e., fuel cell, UPS, and hybrid electric vehicles. Although in later advancements, source current becomes continuous in multilevel QZSI, low voltage gain, higher shoot-through duty ratio, lesser availability of modulation index, and higher voltage stress across switches are still an obstacle in NPCZSI. In this research work, a three-level high voltage gain Neutral Point Clamped Inverter (NPCI) that gives three-level AC output in a single stage, is proposed to boost up the DC voltage at the desired level. At the same time, it detains all the merits of previous topologies of three-level NPCZSI/QZSI. Simulations have been done in the MATLAB/Simulink environment to show the effectiveness of the proposed inverter topology.

scholarly journals Small Signal Modeling of Inverter-based Grid-Connected Microgrid to Determine the Zero-Pole Drift Control with Dynamic Power Sharing Controller

2019 ◽  
Vol 9 (1) ◽  
pp. 3790-3795 ◽  
Author(s):  
E. Pathan ◽  
S. A. Zulkifli ◽  
U. B. Tayab ◽  
R. Jackson
Keyword(s):  
Current Source ◽  
Low Frequency ◽  
Dynamic Effect ◽  
Power Sharing ◽  
System Stability ◽  
Small Signal ◽  
Control Parameters ◽  
Grid Connection ◽  
Space Modeling ◽  
Source Current

This paper presents a small signal state space modeling of three-phase inverter-based microgrid (MG) system with consideration of improved droop control. The complete system matrices for one distribution source-grid connects to the local load have been elaborated by applying high, medium and low-frequency clusters to the system without considering the switching action on the inverter during power-sharing. Moreover, the final matrices will be used to determine the location of the eigenvalues for the control parameters gains due to dynamic effect of the MG, by observing the root locus graph on cluster identification. Sensitivity analysis of all types of frequency cluster showed that power-sharing control parameters such as load current, source current, and inverter voltage are influencing system stability and must be considered when designing the proportional-integral (PI) control when different load scenarios have been applied from the zero-pole drifting. Those eigenvalues of the system model are indicating the frequency and damping oscillatory components when there is sudden changed at the inverter-grid connection. The matrices’ eigenvalues are being plotted using MATLAB/Simulink to identify system stability region and find the PI controller parameters.

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