Vol.9 No.2 2016

Research paper : Constructing a system to explore shallow velocity structures using a miniature microtremor array (I. CHO et al.)−89−Synthesiology - English edition Vol.9 No.2 (2016) of the Kanto Plain and the Osaka Plain, and a manual has been created for such measurement.[10][11] Specifically, the arrays [Fig. 1(a)] of large and small scale with radii of several tens of meters to about 1,000 meters are laid out, and the microtremors, whose oscillation sources are natural waves or wind, are measured overnight (in practice, 1 to 2 h or 3 h or more according to the radius of the array) (Table 1). The phase velocity of Rayleigh waves at a frequency of 0.1~10 sec is extracted from the microtremor data obtained, and the information for the seismic basement depth that may reach the depth of several kilometers is obtained.The wavelength range analyzable with one array is dependent on the array size, and it is said to be approximately from twice to several times or dozen times the array radius.[11] Since the maximum depth of exploration is dependent on the maximum wavelength of the analyzable waves, to obtain data at a wide wavelength range that covers the target depth, it is necessary to make measurements using multiple arrays with different radii. For example, Fig. 3 is the dispersion characteristic (called dispersion curve) obtained by measuring microtremors with six types of triangle arrays (arrays of radii of 50 m and 100 m, 250 m and 500 m, and 1,000 m and 2,000 m; these are shown in blue, green, and red, respectively) where Fig. 1(a) are combined multiple times. For example, the 2 Hz phase velocity (blue line) was obtained using an array with 50 m radius, while 0.2 Hz phase velocity (red line) was obtained using an array with 2 km radius. The wavelengths were 125 m and 10 km, respectively, while the depth scale was several tens of meters and several kilometers, respectively. The dispersion curves of the phase velocity obtained for each array were connected to create one dispersion curve, and this is deemed as the data representing the subsurface velocity structure at the measurement points with depths of several tens of meters to several kilometers.However, as shown in the figure, often data obtained with different radii do not connect well. Whether the dispersion curves from arrays of different sizes connect smoothly or not is the index for the reliability of data. If the connections of the analysis results of arrays are extremely poor, re-measurement must be considered. The reason for conducting array measurements of multiple sizes is not only to cover the range of exploration depth, but also to evaluate the reliability of the results.We shall address this issue further. In the microtremor array analysis, it is assumed that the sources of microtremors are generally located sufficiently far from the array. However in practice, rarely is this assumption met. If the microtremor array of several tens of meters is laid out in the urban area, it is unavoidable that the industrial activities such as traffic vibrations will be included within the array. As a result, these add up as unforeseen record components, or noise, add Fig. 3 Microtremor arrays in conventional exploration (left) and an example of the dispersion curve obtained for each array (right) (after Report [12] of the Tokyo Metropolitan Government)In many cases, the data obtained from arrays with different radii do not connect entirely.Table 1. Microtremor exploration methodsNew systemThe conventional microtremor exploration methodExploration depthMeasurement timeItemMicrotremor array layout and sizeAnalyzable wavelength rangeArrays in star-shaped layout that combine four-point arrays [Fig. 1(a)]. The array radii are about several tens of meters to 1,000~ 2,000 m.Several hours. Nighttime measurement when long-period microtremors become dominant is appropriate.The analyzable minimum wavelength is two times the array radius, and the maximum wavelength is about several times to a dozen times.Several tens of meters~a few thousands of metersBasically, four-point arrays with radius of 0.6 m [Fig. 1(b)]. Three-point irregular arrays with 5 m radius [Fig. 1(c)] are used optionally.15 min. Daytime observation when short-period microtremors become dominant is appropriate.The analyzable minimum wavelength is two times the array radius, and the maximum wavelength is about several tens of times to over 100 times if the condition is good.Several meters~several tens of meters50m,100m250m,500m1000m,2000mFrequency (Hz)Phase velocity (km/s) m, 100 m1000 m, 2000 m250 m, 500 m(50 m, 100 m)(250 m, 500 m)(1000 m, 2000 m)(Array radius)


page 33