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Abstract Vigorous convection in Earth's outer core led to the suggestion that it is chemically homogeneous. However, there is increasing seismic evidence for structural complexities close to the outer core's upper and lower boundaries. Both body waves and normal mode data have been used to estimate awave velocity,, at the top of the outer core (thelayer), which is lower than that in the Preliminary Reference Earth Model. However, these lowmodels do not agree on the form of this velocity anomaly. One reason for this is the difficulty in retrieving and measuringarrival times. To address this issue, we propose a novel approach using data from seismic arrays to iteratively measure‐differential travel times. This approach extracts individualsignal from mixed waveforms of theseries, allowing us to reliably measure differential travel times. We successfully use this method to measuretime delays from earthquakes in the Fiji‐Tonga and Vanuatu subduction zones.time delays are measured by waveform cross correlation betweenand, and the cross‐correlation coefficient allows us to access measurement quality. We also apply this iterative scheme to syntheticseismograms to investigate the 3‐D mantle structure's effects. The mantle structure corrections are not negligible for our data, and neglecting them could bias theestimation of uppermost outer core. After mantle structure corrections, we can still see substantial time delays of,, and, supporting a lowat the top of Earth's outer core.