scholarly journals Temporal variation and frequency dependence of ambient noise on Mars from polarization analysis

2020 ◽  
Author(s):  
Yudai Suemoto ◽  
Takeshi Tsuji ◽  
Tatsunori Ikeda
Keyword(s):  
Temporal Variation ◽  
Frequency Dependence ◽  
Ambient Noise ◽  
Polarization Analysis

Ambient noise field and temporal changes on ambient noise auto/cross-correlogram at the sea bottom inferred from ocean-bottom seismic and pressure arrays

2020 ◽  
Author(s):  
Yoshihiro Ito ◽  
Miyuu Uemura ◽  
Spahr C. Webb ◽  
Kimihiro Mochizuki ◽  
Stuart Henrys
Keyword(s):  
Power Spectrum ◽  
Acoustic Wave ◽  
Temporal Variation ◽  
Wave Field ◽  
Ambient Noise ◽  
Temporal Change ◽  
Temporal Changes ◽  
Seismic Interferometry ◽  
Ocean Bottom ◽  
Bottom Pressure

<p>The interactions of wind with the ocean surface, ocean wave with acoustic wave, acoustic wave with seismic wave below the sea bottom, and the interplay among them drive important energy flows from the atmosphere to the lithosphere. Uncertainty remains regarding the origin of wind-related noise in the ocean and its coupling to seismic noise below the sea floor. Seismic interferometry is a powerful tool that uses microseisms, or ambient noise within solid earth, to monitor temporal seismic velocity change by referring to the auto/cross-correlation as a Green’s function at the sites, and its temporal change. The most important assumption when detecting seismic velocity changes with seismic interferometry is that mutually uncorrelated noise sources are distributed randomly in space and time without any temporal changes in their density and intensity in a fully diffuse wave field. An effect of temporal variation on the ambit noise field to the retrieval of Green’s function is, however, not fully understood, nor is how reliable temporal changes in interferogram noise are, especially when accompanied by large earthquakes and slow slip events. Here, we show relationships among the temporal changes of sea surface wave, acoustic wave, and seismic wave fields, which are observed in ocean bottom pressure gauges and seismometer arrays installed in New Zealand. The temporal variation in the power spectrum obtained from continuous ocean bottom seismometer and pressure records near 200 mHz correlates with the temporal variation in wind speed above the sites, particularly during wind turbulence of more than 5 m/s. The temporal fluctuation in the ocean bottom pressure caused by the ocean surface wave field correlates to that of a microseism near 200 mHz. The temporal variations in the power spectrum from both continuous ocean bottom pressures and microseisms in the 200–800 mHz range show a positive correlation. After calculating the auto/cross-correlation functions (ACF/CCF) from ambient noise in a 200–800 mHz pass band every 6 h, the temporal variation in the correlation between the ACF/CCFs is investigated every 6 h. The temporal variation in the ACF/CCFs correlates with the time derivative of the temporal changes in the power spectrum amplitude of both the bottom pressure and the microseism rather than the temporal changes in the amplitude of the power spectrum. This suggests that the temporal change that occurs in the seismic interferogram owing to ambient noise, is mostly controlled by the temporal change in the ocean wave field undergoing fluctuations by the atmospheric turbulence over the sea surface. The temporal variations in the noise field in space and time may break the assumption on seismic interferometry, and eventually make the apparent temporal change in interferogram noise.</p>

Study on the Structural Characteristics of Martian Regolith by Ambient Noise Data from SEIS Observation

2020 ◽  
Author(s):  
Wanbo Xiao ◽  
Yanbin Wang
Keyword(s):  
Internal Structure ◽  
Ambient Noise ◽  
Landing Site ◽  
Observation Data ◽  
Polarization Analysis ◽  
S Wave ◽  
Long Distance ◽  
Seismic Observation ◽  
Noise Data ◽  
Martian Regolith

<p>For Earth and Moon, the seismic observation data is the most direct and effective means to detect their internal structure. However, due to the long distance between Mars and Earth and the harsh observation conditions on Mars, the exploration of Martian velocity structure model is a very challenging task. The InSight lander deployed the first seismic observation instrument SEIS (Seismic Experiment for Internal Structure) on the Mars’ surface after its successful landing on Mars on November 26, 2018. In this study, we performed horizontal-to-vertical spectral ratio (HVSR) and polarization analysis of three component VBB seismic waveforms recorded by the SEIS station released on the IRIS website. We are trying to constrain the thickness of the Martian regolith at the landing site of InSight from the SEIS data. The VBB ambient noise data we used are in HHV/HHU/HHW channels of ELYSE station in 30 Martian days. These data are predominantly ambient noise data caused by wind effects and do not contain any known marsquake data. We found that the HVSR curves from nearly all released data show two distinct peaks at 11.9 Hz and 24.5 Hz, respectively. Furthermore, we conducted particle motion and polarization analysis on these data in various frequency bands, which indicate that the ground motion at the highest peak show linearly polarized and vertically incident motion with a fixed azimuth. This could be explained by the S-wave resonance of the Martian regolith at the InSight landing site caused by the wave motion source from the wind induced motion of the lander. Using the possible S-wave velocity of the Martian regolith proposed by previous studies and the peak frequencies of the HVSR results in this study, thickness of the Martian regolith at the InSight landing site was obtained that is smaller than the pre-evaluated thickness (3~5 m) for the InSight mission.</p>

Imaging magnetic microstructure with SEMPA

1993 ◽  
Vol 51 ◽  
pp. 1032-1033
Author(s):  
M. H. Kelley ◽  
J. Unguris ◽  
R. J. Celotta ◽  
D. T. Pierce
Keyword(s):  
Electron Microscopy ◽  
Thin Films ◽  
Scanning Electron Microscopy ◽  
Sandwich Structure ◽  
Magnetic Coupling ◽  
Polarization Analysis ◽  
Secondary Electrons ◽  
Magnetic Microstructure ◽  
Escape Depth ◽  
Scanning Electron

By measuring the spin polarization of secondary electrons generated in a scanning electron microscope, scanning electron microscopy with polarization analysis (SEMPA) can directly image the magnitude and direction of a material’s magnetization. Because the escape depth of the secondaries is only on the order of 1 nm, SEMPA is especially well-suited for investigating the magnetization of ultra-thin films and surfaces. We have exploited this feature of SEMPA to study the magnetic microstrcture and magnetic coupling in ferromagnetic multilayers where the layers may only be a few atomic layers thick. For example, we have measured the magnetic coupling in Fe/Cr/Fe(100) and Fe/Ag/Fe(100) trilayers and have found that the coupling oscillates between ferromagnetic and antiferromagnetic as a function of the Cr or Ag spacer thickness.The SEMPA apparatus has been described in detail elsewhere. The sample consisted of a magnetic sandwich structure with a wedge-shaped interlayer as shown in Fig. 1.

On the Frequency-Dependence of the Chirality Pseudoscalar of a Chiral Medium

1995 ◽  
Vol 5 (7) ◽  
pp. 913-918 ◽  
Author(s):  
Frédéric Guérin ◽  
Akhlesh Lakhtakia
Keyword(s):  
Frequency Dependence ◽  
Chiral Medium

Frequency-dependence of the magnetic transverse relaxation in a random anisotropy system: a-DyNi

1989 ◽  
Vol 50 (1) ◽  
pp. 91-98 ◽  
Author(s):  
B. Dieny ◽  
X. Labouze ◽  
B. Barbara ◽  
G. Pillion ◽  
J. Filippi
Keyword(s):  
Frequency Dependence ◽  
Transverse Relaxation ◽  
System A ◽  
Random Anisotropy
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