Assessing the Reliability of the Single Circular-Array Method for Love-Wave Ambient-Noise Surveying

Abstract

The single circular array (SCA) method is a spatial autocorrelation (SPAC)-like technique for ambient noise exploration. Its main feature is the possibility of calculation of Love-wave dispersion curves by using centerless circular arrays of 3-component seismometers, allowing independent processing of each circle. Situations in which Rayleigh-wave and Love-wave arrivals or waves coming from different azimuths are mutually correlated are also correctly dealt with in this method. An algorithm for practical calculation of the SCA coefficient B is described. The algorithm includes averaging over a set of time windows and minimizes the number of spectral ratios to be computed for the purposes of stability. Numerical tests show that SCA coefficients estimated in this way have quite a robust behavior. Bias due to use of a finite number of sensors, as well as to effects of nonpropagating incoherent noise, has been theoretically studied in both the deterministic and the stationary random-field formulations. Using a finite number of stations is a cause of bias even under isotropic illumination conditions. Nevertheless, its effect can be neglected for wavelengths-to-radius ratios above a threshold that depends on the number of evenly distributed sensors. By contrast, uncorrelated noise may affect the whole frequency band and is behind the limitations of the method at low frequencies.Finally, we present the first real data test of this method, consisting of a comparison between theoretical and experimental Love-wave dispersion curves for a site where the structure is known. In practice, the minimum wavelength for direct velocity retrieval for a pentagonal array with radius r was approximately λ min L ∼ 3r, although this value depends on the signal-to-noise ratio. Experiments demonstrate that the usable range can be extended, mainly toward shorter wavelengths, if the effects of noise and of the finite number of sensors are included in the analysis.

The inversion of spectral ratio H/V in a layered system using the diffuse field assumption (DFA)

“…The depth of bedrock is about 0.65 km and agrees well with geophysical prospecting developed in the early '80s by the Spanish Geological Survey (IGME 1983). Solid line represents a ( ) model obtained for a nearby site with a seismic method (García-Jerez et al 2010b). Tokimatsu et al (1992) and Lunedei and Albarello (2016) have stressed the need to distinguish between effective and modal dispersion curves (DC) because in practice the energy transport can be accomplished by a multi modal trade-off.…”

Shear‐wave velocity structure from MASW and SPAC methods: The case of Adra town, SE Spain

“…Like MASW, the standard SPAC method involves Rayleigh‐wave dispersion curve retrieval from vertical‐component records. Recent extensions to 3‐component analysis have been developed by García‐Jerez et al (, ) and García‐Jerez, Luzón and Navarro (). The HVSR was proposed by Nakamura () as a rough estimation approach of the seismic response of the subsoil from three‐component records of ambient noise at a single seismic station.…”

The SPAC Method for a Finite M-Station Circular Array Using Horizontally Polarized Ambient Noise

“…derived the same result in a different mathematical framework. Recent papers by García-Jerez, Luzón, Navarro, and Pérez-Ruiz (2008) and García-Jerez et al (2010) describe several alternative methods for dealing with the horizontal components of microtremors. Tada et al (2009) have presented new formulas for the SPAC method that allow one to infer phase velocities of Love waves in a simple manner using two-component, horizontal-motion, circular-array records of microtremors.…”