Denoising InSight: Determination of Mars’ lateral crustal variations through surface-wave identification

For over three Earth years, NASA’s InSight mission has returned data from the Seismic Experiment for Interior Structure (SEIS) on Mars with over 1300 recorded marsquakes (Horleston et al., this issue), leading to numerous breakthroughs in determining the planet's structure (Khan et al., 2021; Stähler et al., 2021; Knapmeyer-Endrun et al., 2021). However, a large number of detected marsquakes are contaminated by SEIS’s complex interaction with the Martian atmosphere which injects noise and couples into the seismic signal. Identification of surface waves, polarisation analysis and clear body-wave arrivals is therefore often inhibited by this strong atmospheric noise. Despite numerous identifications, only 12 events have so far been fully located. Detection and analysis of the seismic component thus requires separation from such aseismic signal sources.

Here we present a novel denoising model which exploits the strong correlation between the ground motion and the atmospheric conditions recorded at InSight. The approach exploits linearities in the noise injected by the atmosphere by using weather data measured by the lander, or in the absence of such data due to power constraints, the excitation of wind-sensitive lander modes that have been proven an effective atmospheric proxy in estimating this injection on Mars (Charalambous et al., 2021; Clinton et al., 2021; Dahmen et al., 2021). The output results in “demodulation”, i.e., the cancellation of any environmental noise and the effective isolation of the seismic signal power in time-frequency space. Our denoising approach is thus able to remove and decouple the environmental noise from the seismic signal to reveal features in the seismic event waveform that were previously hidden in the noise.

Using the proposed denoising technique, we investigate all marsquake events above magnitude 3 to search for surface waves not identifiable with pre-existing methods. To correctly identify and associate these dispersive wave packets as Rayleigh and Love waves, we examine the characteristics of each through frequency-dependent polarisation analysis. The residual power from the denoised signal allows an accurate examination of the partitioning of energy between the horizontal and vertical components (H/V ratio) of the event. This energy isolation in turn informs an H/V-weighted polarisation analysis which enhances the characteristics of body and surface waves. This approach effectively cancels out the contribution of other polarised signals from the environment, enhancing the characteristics in the signal of interest. The residuals thus not only allow us to correctly identify surface waves, but also locate events which could not previously be located by refining back-azimuths and distances through clear P- and S-wave arrivals.

Using these results, we investigate the lateral variation in the crustal properties with events that are located close to an equatorial transect of Mars, including both major- and minor-arc detections. Events not on the transect further allow us to populate a map of lateral variations in thickness of the crust revealing geographical differences in the crustal structure, but also constraining the average thickness of the crust and the range of allowable crustal densities. This places additional constraints on the seismic velocities of the crust and lithosphere. Finally, with demodulation we further explore the identification and validation of multi-orbit Rayleigh waves and second-arriving Love wave packets from InSight’s largest marsquake recorded so far at a moment magnitude of 4.7 (S1222a), providing further constraints to the model.

 

References

Charalambous, C., Stott, A. E., Pike, W. T., ... & Banerdt, W. B. (2021). A Comodulation Analysis of Atmospheric Energy Injection Into the Ground Motion at InSight, Mars. Journal of Geophysical Research: Planets, 126(4).

Clinton, J. F., Ceylan, S., van Driel, ... & Stott, A. E. (2021). The Marsquake catalogue from InSight, sols 0–478. Physics of the Earth and Planetary Interiors, 310, 106595.

Dahmen, N. L., Zenhäusern, G., Clinton, ... & Banerdt, W. B. (2021). Resonances and lander modes observed by insight on Mars (1–9 Hz). Bulletin of the Seismological Society of America, 111(6), 2924-2950.

Horleston, A., Clinton, J., Ceylan, S., … Banerdt, W. B. The seismicity of Mars as recorded by InSight’s Marsquake Service, this issue

Khan, A., Ceylan, S., van Driel, M., ... & Banerdt, W. B. (2021). Upper mantle structure of Mars from InSight seismic data. Science, 373(6553), 434-438.

Stähler, S., Khan, A., Banerdt, W. B., ... & Smrekar, S. (2021). Seismic detection of the martian core. Science, 373(6553), 443-448.

Knapmeyer-Endrun, B., Panning, M. P., Bissig, F., ... & Banerdt, W. B. (2021). Thickness and structure of the martian crust from InSight seismic data. Science, 373(6553), 438-443.