Simple assessment of shallow velocity structures with small-scale microtremor arrays: interval-averaged S-wave velocities

Cho, Ikuo; Urabe, Atsushi; Nakazawa, Tsutomu; Sato, Yoshiki; Sakata, Kentaro

doi:10.1071/eg18020

Cited by 6 publications

(3 citation statements)

References 10 publications

(18 reference statements)

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“…By the use of the sets of the two parameters (radius and NSR) given in Section 4, we can obtain a relation between the array radius and the NSR at the ULF as shown in Figure 10. The figure shows that NSRs generally decrease with the decrease of the array radii, supporting our experiences: we have had very small values of NSR (e.g.,

ε 〈 10^{- 3}

) when we use very small arrays with radii about 1 m (e.g., Cho, 2020; Cho & Iwata, 2019; Cho et al., 2018, 2013). In such cases, we can treat long wavelength ranges reaching several tens of an array radius or more by the SPAC method.…”

Section: Discussionsupporting

confidence: 82%

“…We used microtremor array data observed within the premises of the National Institute of Advanced Industrial Science and Technology (AIST) in Ibaraki Prefecture, Japan to validate the theory developed in Section 3. This site has a simple geological structure with deep PS logging data, with the logging point located at (36.06366°N, 140.13179°E), reaching the seismic basement at a depth around 600 m, so that we have used this site for various microtremor experiments since 2005 (Cho, 2018; Cho & Iwata, 2018; Cho et al., 2013, 2006c). The readers can refer to the quoted papers for the details of the geoenvironment of this site and the experimental studies.…”

Section: Analysis Of Field Datamentioning

confidence: 99%

“…The theoretical effective mode of Rayleigh waves was calculated in the previous studies (Cho, 2018; Cho et al., 2013, 2006c), based on the one‐dimensional velocity structure model beneath the observation site (Figure 3 of Cho et al. [2006c]).…”

Section: Analysis Of Field Datamentioning

confidence: 99%

See 2 more Smart Citations

Limits and Benefits of the Spatial Autocorrelation Microtremor Array Method Due to the Incoherent Noise, With Special Reference to the Analysis of Long Wavelength Ranges

Cho

Iwata

2021

JGR Solid Earth

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Microtremor array surveys determine Rayleigh wave phase velocity to infer the subsurface velocity structures (e.g., Okada, 2003). The spatial autocorrelation (SPAC) method of Aki (1957) is a technique that has been most popularly used to determine phase velocities (Okada, 2003). In earthquake engineering, the demands of microtremor surveys have been increasing year by year, because of their cost-effectiveness, so the demands of the SPAC method have. The analyzable wavelength ranges of the SPAC method, as those of the other methods do, generally depend on array size. The motivation of this study is that, albeit the markedly increasing popularity, the upper limit of the analyzable wavelength ranges (upper limit wavelength, ULW) of the SPAC method has not been clearly identified yet. For example, Foti et al. (2017) state that, "there is no clear maximum wavelength criterion" for the SPAC method and that it is "not recommended to try and extract wavelengths greater than 2-3 times the maximum aperture of the passive array" (i.e., their recommendation is the ULWs from 4r to 6r, where r means array radius). In this study, we first describe what is the signal and noise in microtremor records in Section 2. This is a premise for detailed examinations, as well as a help to interpret the analysis results in the later sections. In Section 3, we present a theory for determining and evaluating a crucial factor for the ULW of the SPAC method. In Section 4, we validate our theory based on field data. Finally, inspired by the results of these sections, we discuss and examine the relation between noise-to-signal ratio (NSR) and the distance between seismic sensors for novel use of the SPAC method to evaluate the attenuation of soils (Section 5). Each of these three sections details the effects of the incoherent noise on the SPAC analysis in long wavelength ranges and collectively constitutes the main subject.

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ε 〈 10^{- 3}

Section: Discussionsupporting

confidence: 82%

Section: Analysis Of Field Datamentioning

confidence: 99%

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Limits and Benefits of the Spatial Autocorrelation Microtremor Array Method Due to the Incoherent Noise, With Special Reference to the Analysis of Long Wavelength Ranges

Cho

Iwata

2021

JGR Solid Earth

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High-resolution spatial elastic wave velocity variation detected using multiple-linear-array microtremor measurements

Chimoto,

Yamanaka

2024

Exploration Geophysics

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Basic performance of a spatial autocorrelation method for determining phase velocities of Rayleigh waves from microtremors, with special reference to the zero-crossing method for quick surveys with mobile seismic arrays

Cho

Senna

Wakai

et al. 2021

Geophysical Journal International

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Summary We theoretically and empirically demonstrate the usability of the zero-crossing method for quick microtremor surveys in earthquake engineering (i.e. microtremor array surveys), namely shallow (< a few kilometres) surveys with small-scale (< 1 kilometre in radius) mobile seismic arrays with a short observation time (< a few hours). The zero-crossing method is a type of spatial autocorrelation (SPAC) method that determines phase velocities based on multiple frequencies at which the SPAC coefficient curve crosses zero. It is theoretically shown that the zero-crossing method is robust against incoherent noise and that the use of the first zero crossings (i.e. those at the lowest frequencies) is more robust against inadequate conditions of the microtremor wavefield than the use of later zero crossings (i.e. those at higher frequencies). We used microtremor array data with maximum array radii and observation durations of 400 m and 120 min on average, respectively, at 445 observation sites in the Kanto Plain, Japan, for validating the practicality of using the first zero crossings. As an illustration of the robustness against low signal-to-noise ratios (SNRs), we show that with the zero-crossing method, low-sensitivity (i.e. low-SNR) seismometers provide the same analysis results as those obtained with high-sensitivity seismometers, even when the power spectral densities for the low-sensitivity seismometers are close to the self-noise level. We then show that a reference phase velocity dispersion curve (RPVDC), created mainly based on the first zero crossings at each site, has a spatial distribution that well corresponds to the geology and topography and is consistent with that obtained in a previous study. We inverted five RPVDCs to model one-dimensional S-wave profiles and validated them using S-wave profiles obtained from velocity logs at nearby deep (e.g. hundreds of metres) boring wells. The accuracy of phase velocities at the later zero crossings for three-sensor/four-sensor arrays and all zero crossings for two-sensor arrays are statistically examined (maximum of 9805 data) based on a comparison with the RPVDCs. The disadvantage of the zero-crossing method is that it can only provide information on phase velocities at discrete wavelengths up to a maximum wavelength of 2.6r (i.e. corresponding to the first zero-crossing point), where r is the radius of a seismic array. Therefore, the RPVDCs were then used to examine the upper limit of the analysable wavelength ranges for the conventional SPAC method for microtremor array surveys. Based on a few hundred three-sensor/four-sensor arrays, it was found that for arrays with radii larger than several tens of metres, three-quarters of the upper limit wavelengths (ULWs) stayed within 5r. For arrays with radii smaller than this value, the ULWs strongly depended on the array radius; the ULWs dramatically increased with decreasing array radius. For example, for arrays with an r value of 0.6 m, half of 336 data ranged between 26r and 54r, and the maximum ULW reached 186r. This strong size dependence can be explained by differences in SNR.

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Simple assessment of shallow velocity structures with small-scale microtremor arrays: interval-averaged S-wave velocities

Cited by 6 publications

References 10 publications

Limits and Benefits of the Spatial Autocorrelation Microtremor Array Method Due to the Incoherent Noise, With Special Reference to the Analysis of Long Wavelength Ranges

Limits and Benefits of the Spatial Autocorrelation Microtremor Array Method Due to the Incoherent Noise, With Special Reference to the Analysis of Long Wavelength Ranges

High-resolution spatial elastic wave velocity variation detected using multiple-linear-array microtremor measurements

Basic performance of a spatial autocorrelation method for determining phase velocities of Rayleigh waves from microtremors, with special reference to the zero-crossing method for quick surveys with mobile seismic arrays

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