Cross correlation of the mismatched traces. A zero time-lag between the traces would correspond to a value of 4 s on the vertical axis. It is seen that the traces are phase shifted by as much as 30 ms. Further, there is a substantial phase shift at the shallowest geophone level (depth 1840 ft), indicating substantial contributions from shallow depths, possible in the near-surface. The phase shifts oscillate with depth indicating deep contributions to the data matrix asymmetry.