Very-low-frequency passive matched time difference localization using a deep ocean-bottom seismograph

The penetration of sound into the seabed increases as frequency decreases, aggravating bottom mismatch issues in ray-based matched time difference processing (Ray-MTDP) methods and thereby degrading ranging accuracy and subsequent depth estimation. To address this problem, an auto-correlation-function-envelope (AFE)-based matched time difference processing method is proposed for ranging at very-low-frequencies (VLFs; f≤ 100 Hz). The matching feature is extended from time differences (TDs) of the isolated eigenrays to the global AFE, which enables the consideration of layered bottom influence under the normal-mode framework and facilitates generalized automated matching without prior multipath identification or manual alignment. Analysis of four ocean-bottom seismograph datasets shows that, for the South China Sea, a sediment layer with a positive sound speed gradient over a half-space basement mitigates VLF bottom mismatch and increases the mean probability of credible localization (PCL) by 68.5 percentage points relative to Ray-MTDP. Accordingly, trajectories are then reconstructed based on azimuth estimates. For depth estimation, a notable finding is that the PCL obtained using the TDs associated with once-bottom-reflected paths is, on average, 57.1% points higher than that obtained using direct-path TDs. In addition, both the AFE used for ranging and the selected depth-sensitive TD exhibit strong tolerance to the SSP perturbations.