Fig. 1

Settings of problems and parameters. The open circles, p, q, and s, are receivers. The closed circles, 1, 2, \(\ldots\), L, are microtremor sources, where L is the total number of sources. \(r_{pq}\) and \(r_{ps}\) are the distances between two receivers of p–q, and p–s, respectively. The sources are far enough from the receivers as shown in wiggly lines, and the lines from the receivers to a source are parallel with an azimuth, \(\theta _{\ell }\). \(\psi\) is the angle formed by spq