Near-Field Seismic Propagation and Coupling Through Mars’ Regolith: Implications for the InSight Mission

Abstract

NASA's InSight Mission will deploy two three-component seismometers on Mars in 2018. These short period and very broadband seismometers will be mounted on a threelegged levelling system, which will sit directly on the sandy regolith some 2-3 meters from the lander. Although the deployment will be covered by a wind and thermal shield, atmospheric noise is still expected to couple to the seismometers through the regolith. Seismic activity on Mars is expected to be significantly lower than on Earth, so a characterisation of the extent of coupling to noise and seismic signals is an important step towards maximising scientific return.In this study, we conduct field testing on a simplified model of the seismometer assembly. We constrain the transfer function between the wind and thermal shield and tripod-mounted seismometers over a range of frequencies (1-40 Hz) relevant to the deployment on Mars. At 1-20 Hz the displacement amplitude ratio is approximately constant, with a value that depends on the site (0.03-0.06). The value of the ratio in this range is 25-50% of the value expected from the deformation of a homogeneous isotropic elastic halfspace. At 20-40 Hz, the ratio increases as a result of resonance between the tripod mass and regolith. We predict that mounting the InSight instruments on a tripod will not adversely affect the recorded amplitudes of vertical seismic energy, although particle motions will be more complex than observed in recordings generated by more conventional buried deployments. Higher frequency signals will be amplified by tripod-regolith resonance, probably reaching peak-amplification at ∼ 50 Hz. The tripod deployment will lose sensitivity at frequencies > 50 Hz as a result of the tripod mass and compliant regolith.We also investigate the attenuation of seismic energy within the shallow regolith covering the range of seismometer deployment distances. The amplitude of surface displacement decays as r −n , where 1.5 < n < 2. This exceeds the value expected for a homogeneous isotropic elastic halfspace (n ∼ 1), and reflects an increase in Young's modulus with depth. We present an updated model of lander noise which takes this enhanced attenuation into account.

Influence of Body Waves, Instrumentation Resonances, and Prior Assumptions on Rayleigh Wave Ellipticity Inversion for Shallow Structure at the InSight Landing Site

“…Less clear peaks occur around 10 Hz for the MPS measurements and 20 Hz for the Vulcano measurements. No indication of resonances between the LVL and the regolith at high frequencies that affect the vertical component only, as observed by Myhill et al (2018) during a field campaign in Iceland, are evident for the two cases in which measurements were conducted on sand. Spectral peaks observed on the vertical component are matched closely on the horizontal ones, as described above, and are also visible on the reference sensor.…”

“…for all sensors around 2 Hz in the CNES case and, less clear, around 1.6 Hz for the measurement on Vulcano. These frequencies are smaller by an order of magnitude than predicted by Myhill et al (2018), and the exactly same effect is also observed on the reference sensors not mounted on the LVL, and, in case of the CNES measurements, not placed on sand, so it is hard to link the Fig. 14 Power density spectra for measurements to characterize the LVL effect.…”

“…i) Same as c) for measurement on Vulcano island amplitude increase to the LVL. Compared to the study by Myhill et al (2018), the reference sensor was not buried here, so it might see some of the same free-surface effects as the sensor on the LVL. An exception is the case of the sensor placed in the PVC tube during the CNES measurements that is not in direct contact with the sand at all.…”

Flexible Mode Modelling of the InSight Lander and Consequences for the SEIS Instrument

“…However, at higher frequencies (>10 Hz), the lander-SEIS distance becomes comparable to the typical wavelengths of seismic propagations and the static deformation hypothesis may no longer be valid. Additionally, in real regolith, the behaviour is unlikely to be fully elastic and there will be some frequency dependant attenuation (Teanby et al 2016;Myhill et al 2018). As we have no frequency dependent attenuation in the elastic ground deformation model used, the amplitudes of the resonances in the observed seismic signal may be overestimated.…”

“…According to Myhill et al (2018), the damping of the ground under the lander feet, D (in kg/s) for a given mass (m) can be expressed as:…”