Test of the validity of the assumptions that enter the derivation of the calibration curve, shown as a noise-reduced signal in travel times (left panel) and flow amplitude (center and right panels) as a function of scale. If all assumptions (A2)–(A5) were satisfied, the different curves in each panel would not cross each other, but be ordered from higher to lower values in the same sense as they are ordered from top to bottom in the legends (e.g., Eq. (12) ≥ Eq. (13) ≥ Eq. (15) ≥ Eq. (17) for the left panel).