Beyond the SPAC Method: Exploiting the Wealth of Circular-Array Methods for Microtremor Exploration

Tada, Taku; Cho, Ikuo; Shinozaki, Yuichi

doi:10.1785/0120070058

Cited by 57 publications

(41 citation statements)

References 27 publications

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“…We have developed new methods in recent years for extracting surface wave characteristics from circular array data (Cho et al, 2004(Cho et al, , 2006aTada et al, 2006Tada et al, , 2007Tada et al, , 2009Tada et al, , 2010 on the basis of a rigorous theory derived by generalizing the SPAC method (Cho et al, 2006b). This paper presents an efficient tool that is specialized for the exploration of shallow subsurface structures to the depth of several tens of meters; this tool is based on one of the available derivations of the SPAC method.…”

Section: Introductionmentioning

confidence: 99%

“…This paper presents an efficient tool that is specialized for the exploration of shallow subsurface structures to the depth of several tens of meters; this tool is based on one of the available derivations of the SPAC method. More specifically, this paper proposes a set of analysis methods, the centerless circular array method (CCA; Cho et al, 2004Cho et al, , 2006a, and the noise-compensated CCA method (nc-CCA; Tada et al, 2007), for identifying the phase velocities of Rayleigh waves with wavelengths exceeding several tens or a hundred meters through a 15-min observation of vertical-component microtremors using a circular array with a radius of several tens of centimeters (hereinafter referred to as a "miniature array"). The paper also presents a method for evaluating the analysis limits.…”

Section: Introductionmentioning

confidence: 99%

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Miniature array analysis of microtremors

2013

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We suggest observing microtremors by using a miniature array consisting of vertical-component seismometers that are placed at the center and on the circumference of a circle with a radius of several tens of centimeters, for identifying the phase velocities of Rayleigh waves with wavelengths exceeding several tens or a hundred meters. We present, as tools for the analysis, a set comprised of the analysis methods for the phase velocity and the evaluation method for the analysis limit, which were recently developed by the authors on the basis of a rigorous theory derived by generalizing a spatial autocorrelation method. We conducted miniature-array observations using four or six servo-type seismometers, JU-215 manufactured by Hakusan Corporation, at about 50 observation points throughout a test site and urban areas in Tsukuba City and its surroundings, which have various topographical and geologic environments. The time required from arrival at an observation point to retrieval averaged about 30 min. We could determine the phase velocities of Rayleigh waves with wavelengths of 40 and 100 m at 91% and 51% of all the observation points, respectively. Miniature array analyses can significantly improve the mobility of observation to infer shallow subsurface velocity structures to the depth of several tens of meters.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Miniature array analysis of microtremors

2013

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“…Fitting the curves over multiple maxima and minima improves the resulting estimates of shear-wave velocities, including the resolution of near-surface low-velocity layers, as well as the recognition of higher mode wave propagation in the SPAC curves (Roberts and Asten 2005;Asten , 2009. Cho et al (2006b) and Tada et al (2007) have formulated a general theory for circular array measurements of ambient vibrations based on ratios between different types of spectral densities of Fourier expansions of the set of records with azimuth. Other methods rely on the equivalence of azimuthal averaging by either distributing stations in various azimuths on a circle or using a pair of only two stations, but with recordings of a sufficient duration to assume azimuthally evenly distributed sources (e.g.…”

Section: Methodsmentioning

confidence: 99%

On the repeatability and consistency of three-component ambient vibration array measurements

Endrun

Ohrnberger

Savvaidis

2009

Bull Earthquake Eng

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Ambient vibration measurements with small, temporary arrays that produce estimates of surface wave dispersion have become increasingly popular as a low-cost, noninvasive tool for site characterisation. An important requirement for these measurements to be meaningful, however, is the temporal consistency and repeatability of the resulting dispersion and spatial autocorrelation curve estimates. Data acquired within several European research projects (NERIES task JRA4, SESAME, and other multinational experiments) offer the chance to investigate the variability of the derived data products. The dataset analysed here consists of repeated array measurements, with several years of time elapsed between them. The measurements were conducted by different groups in different seasons, using different instrumentations and array layouts, at six sites in Greece and Italy. Ambient vibration amplitude spectra and locations of dominant sources vary between the two measurements at each location. Still, analysis indicates that this does not influence the derived dispersion information, which is stable in time and neither influenced by the instrumentation nor the analyst. The frequency range over which the dispersion curves and spatial autocorrelation curves can be reliably estimated depends on the array dimensions (minimum and maximum aperture) used in the specific deployment, though, and may accordingly vary between the repeated experiments. The relative contribution of Rayleigh and Love waves to the wavefield can likewise change between repeated measurements. The observed relative contribution of Rayleigh waves is generally at or below 50%, with especially low values for the rural sites. Besides, the visibility of higher modes depends on the noise wavefield conditions. The similarity of the dispersion and autocorrelation curves measured at each site indicates B. Endrun (B) · M. Ohrnberger 123 536 Bull Earthquake Eng (2010) 8:535-570 that the curves are stable, mainly determined by the sub-surface structure, and can thus be used to derive velocity information with depth. Differences between velocity models for the same site derived from independently determined dispersion and autocorrelation curves-as observed in other studies-are consequently not adequately explained by uncertainties in the measurement part.

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“…The proposed method assumes that an S-wave velocity structure is to be estimated from the phase velocities of Rayleigh waves, which are obtained by applying the SPAC method (Aki, 1957) or the centreless circular array (CCA) method (Cho et al, 2004;Tada et al, 2007;Cho et al, 2013) to circular-array data of vertical-component microtremors. A theoretical phase-velocity dispersion curve is calculated with a surface-wave theory assuming a horizontally layered velocity structure.…”

Section: Overviewmentioning

confidence: 99%

Development and numerical tests of a Bayesian approach to inferring shallow velocity structures using microtremor arrays

Cho

Iwata

2018

Exploration Geophysics

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Abstract. We propose an empirical Bayesian approach to inferring shallow (depth ranges from a few to several tens of metres) S-wave velocity structures using microtremor arrays and execute numerical tests to assess the feasibility of this approach. In our approach, the estimate of the S-wave structure (posterior) is derived from an empirical S-wave structure model (prior) and phase velocities of Rayleigh waves obtained with microtremor arrays. In other words, we aim to find a model that is close to the empirical model and is able to explain phase velocities with a 1D surface-wave theory. The inversion is stabilised by the constraints from the prior model so that model parameterisation with many thin layers can be adopted. The velocity structure is individually estimated for each of two cases (assumptions): the case where we assume fundamental-mode dominance and the case where we take into account the higher modes. Optimal values of the model parameters (e.g. a thickness parameter) are found, based on Akaike's Bayesian Information Criterion (ABIC), and the choice of the better assumption of the surface-wave theory is also based on ABIC. Numerical tests, where synthetic data is generated from a horizontally stratified two-layer model, indicate that the relative weight between a prior model and the observed data is appropriately adjusted by ABIC. It is revealed that a value of the thickness parameter required to reproduce the given two-layer model is successfully found by ABIC. We also suggest that we can make a plausible choice of the assumption of the surface-wave theory with ABIC, unless observation error is extremely large.

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Beyond the SPAC Method: Exploiting the Wealth of Circular-Array Methods for Microtremor Exploration

Cited by 57 publications

References 27 publications

Miniature array analysis of microtremors

Miniature array analysis of microtremors

On the repeatability and consistency of three-component ambient vibration array measurements

Development and numerical tests of a Bayesian approach to inferring shallow velocity structures using microtremor arrays

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