Seismic Autocorrelation Analysis of Deep Mars

The InSight mission deployed one seismic station on Mars, providing a chance to apply single-station-based autocorrelation analysis to investigate Martian subsurface structures. However, recent analysis indicated the low-frequency autocorrelation signals may originate from quasi-periodic high-amplitude instrumental "glitches" rather than the reflection response of deep Mars. In this study, we detected and removed these high-amplitude glitches in raw seismic data and employed autocorrelation on the clean vertical component waveforms filtered between 0.05 and 0.1 Hz. We observed signals at the expected times for the olivine-wadsleyite transition and core-mantle boundary (CMB) as estimated by other methods. This result suggests that the low-frequency autocorrelation signals are the reflection response from the olivine-wadsleyite transition in the mantle and the Martian CMB region, rather than a noise phenomena. A grid search method to fit the observed PcP waveform was used to identify a layer intermediate in velocity between the Martian mantle and core at the Martian CMB. The InSight mission deployed one seismic station on Mars at the end of 2018 to measure the interior structure of Mars. A recent study autocorrelated the InSight ambient noise data to retrieve reflection signals from subsurface discontinuities. Two seismic signals that reflected from deep within Mars, the olivine-wadsleyite transition, a pressure-temperature dependent mineral phase change in the mantle, and the core-mantle boundary (CMB), are observed. However, some analyses suggested that the arrival times of these signals coincide with the recurrence time of high-amplitude glitches within the raw seismic data, leading to an incorrect interpretation of the autocorrelation function (ACF). To resolve this issue, we detected and removed the high-amplitude glitches before further processing the ambient noise data. The autocorrelation analysis of clean continuous vertical-component waveforms recovers signals at times corresponding to the olivine-wadsleyite transition and CMB, whereas ACF of glitch-only waveforms does not. This suggests that signals on the low-frequency ACF are from the seismic discontinuities at olivine-wadsleyite transition and CMB rather than being noise artifacts. We further identified velocity layering near Martian CMB by matching the complexity of the PcP signal observed in the ACF of clean data set with synthetic seismograms. Autocorrelation of clean Seismic Experiment for Interior Structure waveforms recovers the prominent signals reflected from deep MarsThe autocorrelation tests on raw, clean and glitch-only data suggest the signals identified are from deep Mars rather than instrumental noiseA velocity transition model of the Martian core-mantle boundary better fits the observed PcP phase than a sharp velocity drop