|Budget Amount *help
¥3,400,000 (Direct Cost : ¥3,400,000)
Fiscal Year 2002 : ¥1,300,000 (Direct Cost : ¥1,300,000)
Fiscal Year 2001 : ¥2,100,000 (Direct Cost : ¥2,100,000)
We investigated the practical applicability, to large arrays of seismometers with radii up to 600 meters, of the tentatively named Enlarged Henstridge method, a recently proposed method of microtremor exploration to obtain, in a fairly simple way, the S-wave velocity structure which in one of the most important parameters to represent the underground structure. The Enlarged Henstridge method, which allows one to obtain the dispersion curve of the phase velocity of surface waves by measuring microtremors with an array of seismometers placed at least 3 locations along a circumference, is characterized by the lack of necessity to place a seismometer at the center of the circle and by the tolerance for uneven spacing of seismometers along the circumference. The applicability of the method had been demonstrated for arrays with radii of 5 and 15 meters, but it had never once been applied to larger arrays. In order to investigate the practical applicability of the Enlarged Henstridge Method t
o large arrays consisting of 3 stations, we conducted an exploration of microtremors, at a location where the underground structure is known, by deploying circular arrays of 3 seismometers spaced at even or uneven intervals along circumferences 25 to 600 meters in radius.
We first measured microtremors with an array of 4 stations, or 3 stations spaced at nearly equal intervals along a circumference plus a fourth station at the center of the circle, and estimated the dispersion curve a beneath the observation site by applying the SPAC method to the data obtained. We then estimated the dispersion curve with the Enlarged Henstridge method by using arrays of 3 evenly or unevenly spaced stations, and examined the adequacy of the analysis results by comparing them with the dispersion curve as estimated with the SPAC method. We found that the Enlarged Henstridge method was comparable to the SPAC method in terms of the accuracy of the analysis results, and that the wavelength range, over which the dispersion curve could be evaluated adequately, tended to shift toward longer wavelengths as compared with the case of the SPAC method.
Uneven spacing of seismometers has influences, by way of observation errors and other factors, on the estimation accuracy for the dispersion curve. We empirically investigated the tolerable extent of unevenness in the configurations of the stations for which appropriate dispersion curves could be obtained with a high likelihood, as well as the wavelength ranges over which those appropriate dispersion curves were obtained in that case. Less