Abstract
There is the spatial auto-correlation method (SPACM) for estimation of the phase velocity of surface wave using the array observation of microtremors. It needs to install the seismometers on the circle and at the center. In this paper, a relation between the number of the seismometers and an error in the estimation is made clear. Furthermore, the theoretical applicability of SPACM using two or three seismometers is investigated. As a result, it is found that we can estimate the phase velocity of surface waves having wavelength of more than 3.4 times of the array's radius when three seismometers are arranged at vertices of a right triangle. However, when SPACM are applied to array observation using two seismometers, there is a possibility that the estimation includes a large error. Therefore, a new procedure is proposed to improve the estimation.
