I. Introduction
Over the last years, ambient noise tomography has become one of the fastest growing research areas in seismology and exploration geophysics. Compared to earthquake-based seismic tomography methods, ambient noise tomography is particularly useful in imaging shallow earth structures [1], [2]. Moreover, because of the persistent nature of the seismic background noise, temporal variation of the earth structure can be analyzed and monitored by studying the variation in the noise cross-correlation function [3]–[5]. Ambient noise methods have the advantage of being low cost and having resistant repeating sources with a minimal environmental disturbance.