Figure 4

Measurements of the system clocks’ drift using correlation of noise correlations. (a) Correlation between the positive- and negative-time-delay parts of the measured cross-correlation function C₁₂. The function C₁₂(t) is correlated with C₁₂(−t) for the time delays 3.2 s < t < 3.6 s, which correspond to the deterministic acoustic arrivals. Line numbers indicate the quarter (first to fourth) of the available data set used in the C₁₂ calculation. (b) Same as in (a) but for the measured cross-correlation function C₂₃ for the 2–3 pair and 6.2 s < t < 7.0 s. (c) Least-square fits to the time shifts between 1–2 (solid line) and 2–3 (dashed line) instruments measured at different geotimes. The geotime is shown in hours following the pre-deployment clock synchronization.