Miniature array analysis of microtremors

Abstract

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.

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

“…Konno and Kataoka, 2000;Cho et al, 2008). of 0.6 m, consisting of three sensors spaced evenly around a circle and another at the centre (Cho et al, 2013), and an irregular array of three unevenly spaced sensors (Cho et al, 2004) with side lengths of 5-7 m, were deployed at each site for 15-min recordings at a sampling frequency of 100 Hz.…”

“…The centreless circular array method of Cho et al (2004Cho et al ( , 2008Cho et al ( , 2013 was applied to records of the regular four-sensor arrays and of the irregular three-sensor arrays, to obtain dispersion curves of Rayleigh-wave phase velocities. We also considered a set of three consecutive sensors in a linear array to be a 'centred circular' array of three sensors with a radius of 4 m and applied the spatial autocorrelation method of Aki (1957) to their records.…”

“…We have been working to develop simpler and more robust microtremor methods that use small-scale seismic arrays, such as miniature arrays measuring only 1 m or less in radius and compact arrays of three unevenly spaced sensors measuring only several metres in radius (Cho et al, 2004(Cho et al, , 2008(Cho et al, , 2013. The simpler and more robust microtremor method has enabled large-scale surveys to be conducted in recent years by using large numbers of compact seismic arrays of similar types (Senna et al, 2017).…”

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

“…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.…”

“…They can be a tool to acquire data before attempting other exploration methods (e.g. Cho et al, 2013;Cho and Senna, 2016). However, despite its flexibility in the field, the data processing procedure (i.e.…”