Echelle diagrams for the optimal models M1 (upper panel) and M2 (lower panel). Left panel shows the case before applying near-surface corrections. Middle panel shows the case after applying near-surface corrections, according to Eq. (5). Right panel shows the case after applying near-surface corrections, according to Eq. (7). Open symbols refer to the theoretical frequencies, and filled symbols refer to the observable frequencies. Squares are used for l = 0 modes, diamonds for l = 1 modes, triangles for l = 2 modes, and circles for l = 3. The observable frequencies correspond to the average large separation about 170 μHz (see text for details).