Theory and Observations - Instrumentation for Global and Regional Seismology
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Keywords:
Seismometer
Instrumentation
Seismic Noise
Broad band
Digital data
A network of 23 analog seismic stations was established in Pakistan to monitor seismic activity. The instrumentation of seismic stations includes short period seismometers with natural period at 1 Hz. The seismic noise conditions of five seismic stations are studied to determine the detection capability in the frequency range from 1.0 Hz to 10.0 Hz regarding monitoring of local and regional seismicity. The Power spectral density of noise level of the records was computed by taking day and night samples during winter and summer seasons. The seismic noise levels at these stations range from –120 db to –157 db between 1.0 to 10 Hz, which is well within the limits defined by the Peterson’s new low and high noise model. The observed noise variations recorded during day and night range from 5 db to 17 db, depending on the site of the station. The detect-ability estimates of the selected five seismic stations installed at Fort Munroe, Cherat, Thammi Wali, Dhulian and Sargodha assume a conservative signal-to-noise ratio of 3. These stations showed detection capability 3.1, 3.3, 3.4, 3.5 and 3.8 magnitudes at 1000 km distance respectively.
Seismometer
Seismic Noise
Ambient noise level
Passive seismic
Seismic array
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Seismometer
Seismic Noise
Ambient noise level
Intensity
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Ambient seismic noise techniques are an excellent choice for imaging the subsurface in areas that are seismically quiet or otherwise unsuitable for active source experiments due to geographic isolation or environmental sensitivity. Recently, decades-long time series were made available for download from the Incorporated Research Institutions for Seismology (IRIS) from permanent network installations, allowing access to long, uninterrupted recordings from seismometers around the world. This has spurred the development of an entire field of applications for passive seismic noise analysis. Over the continental United States, the USArray project has advanced to provide station coverage in relatively dense and regularly spaced arrays, but along the Aleutian Island arc in Alaska and other geographically isolated but seismically active locations, the hazards associated with volcanic eruptions and the difficulty of accessing stations for repair or replacement throughout most of the year has allowed only for sparse coverage. The analysis of ambient seismic recordings generally suits one of two purposes. The first involves the parameterization of the source of each component of the ambient seismic noise spectrum and focuses on both the spatial locations and mechanisms of generation. The second purpose of looking at ambient seismic noise is to create velocity models of the subsurface below the array. The assumptions required for the traditional approach to analysis of ambient seismic noise, namely beamforming and the spatial autocorrelation
Seismometer
Ambient noise level
Seismic Noise
Passive seismic
Seismic array
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Until recently superconducting gravimeters (SGs) have been used principally in tidal studies (periods 6–24 hr) due to their high sensitivity and low drift rates. This paper considers the performance of these instruments as long-period seismometers, particularly in the normal mode band (periods 1–54 min). To judge their suitability in providing useful information to seismology, it is important to determine their noise characteristics compared to other established instruments such as spring gravimeters. We compare several continuously recording instruments: the SGs in Esashi (Japan), Wuhan (China), Strasbourg (France) and Cantley (Canada) and the spring gravimeter ET-19 and seismometer STS-1 at the Black Forest Observatory (BFO, Germany). We also include non-permanent instruments, the SG102 at BFO as well as the ET-18 in Metsähovi (Finland). The five quietest days out of the available records are stacked to obtain the power spectral density of the noise in the frequency band 0.05–20 mHz (50 s to 6 hr). Our reference is the New Low Noise Model designed for seismometers. Only at the BFO site were there several instruments that could be compared; even so, in order to obtain the best individual data for each instrument the records selected were not simultaneous. The noise characteristics of the different instrument–site combinations are compared, leading to conclusions about site selection, instrument modifications and the recent potential of SGs to contribute to seismic normal mode studies. We refer to our previous work on the seismic noise magnitude, a summary statistic derived from the power spectral density which has been used to rank the performance of instrument–site combinations.
Gravimeter
Seismometer
Seismic Noise
Noise power
Frequency band
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Seismometer
Seismic Noise
Seismic array
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SUMMARY Variations in atmospheric pressure have long been known to introduce noise in long-period (>10 s) seismic records. This noise can overwhelm signals of interest such as normal modes and surface waves. Generally, this noise is most pronounced on the horizontal components where it arises due to tilting of the seismometer in response to changes in atmospheric pressure. Several studies have suggested methodologies for correcting unwanted pressure-induced noise using collocated microbarograph records. However, how applicable these corrections are to varying geologic settings and installation types (e.g. vault versus post-hole) is unclear. Using coefficients obtained by solving for the residuals of these corrections, we can empirically determine the sensitivity of instruments in a specific location to the influences of pressure. To better understand how long-period, pressure-induced noise changes with time and emplacement, we examine horizontal seismic records along with barometric pressure at five different Global Seismographic Network stations, all with multiple broadband seismometers. We also analyse three Streckeisen STS-2 broadband seismometers, which are collocated with a microbarograph, at the Albuquerque Seismological Laboratory. We observe periods of high magnitude-squared-coherence (γ2-coherence; γ2 > 0.8) between the seismic and pressure signals which fluctuate through time, frequency, and even between seismic instruments in the same vault. These observations suggest that these tilt-generated signals are highly sensitive to very local (<10 m) site effects. However, we find that in cases where instruments are not located at a large depth (<100 m), the pressure-induced noise is polarized in a nearly constant direction that is consistent with local topographic features or the geometry of the vault. We also find that borehole instruments at a large depth (>100 m) appear to be unaffected by pressure-loading mechanisms outlined by Sorrells (1971) but possibly by Newtonian attraction. Correlating the induced-noise polarization direction with times of high coherence, we work to identify sensors that are ultimately limited by pressure-induced horizontal noise as well as period bands that can benefit from pressure corrections. We find that while the situation is complex, each sensor appears to have its own unique response to pressure. Our findings suggest that we can determine empirical relationships between pressure and induced tilt on a case by case basis.
Seismometer
Seismic Noise
Microseism
Ambient noise level
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The self-noise level of a seismometer can determine the performance of the seismic instrument and limit the ability to use seismic data to solve geoscience problems. Accurately measuring and simultaneously comparing the self-noise models from different types of seismometers has long been a challenging task due to the constraints of observation conditions. In this paper, the self-noise power spectral density (PSD) values of nine types of seismometers are calculated using four months of continuous seismic waveforms from Malingshan seismic station, China, and nine self-noise models are obtained based on the probability density function (PDF) representation. For the seismometer STS-2.5, the self-noise levels on the horizontal channels (E−W and N−S) are significantly higher than that on the vertical channel (U−D) in the microseism band (0.1 Hz to 1 Hz), which is a computing bias caused by the misalignment between the sensors in the horizontal direction, while the remarkably elevated noise on the horizontal channels at the low frequencies (<0.6 Hz) may originate from the local variation of atmospheric pressure. As for the very broadband seismometers Trillium-Horizon-120 and Trillium-120PA, and the ultra-broadband seismometers Trillium-Horizon-360 and CMG-3T-360, there is a trade-off between the microseism band range and low-frequency range in the PSD curves of the vertical channel. When the level of self-noise in the microseism band is high, the self-noise at low frequencies is relatively low. Although compared with the other very broadband seismometers, the self-noise level of the vertical component of the STS-2.5 is 3 dB to 4 dB lower at frequencies less than 1 Hz, the self-noise level of the STS-2.5 at high frequencies (>2 Hz) is slightly higher than others. From our observations, we conclude that the nine seismometers cannot reach the lowest noise level in all frequency bands within the working range.
Seismometer
Microseism
Seismic Noise
Frequency band
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