ALBERT

Description: For observations of vertical-component acceleration in the normal-mode band (0.3–10 mHz), the detection sensitivity for signals from the Earth’s body can be improved to levels below the Peterson low-noise model (PLNM). This is achieved by deterministic procedures that (at least partly) remove the accelerations originating from atmospheric mass fluctuations. The physical models used in such corrections are still too simple and fail at frequencies above 3 mHz. Anticipating improved atmospheric correction procedures, we explore the prospects of lowering the detection level. From recordings of excellent vertical-component sensors operated under exceptional site conditions at the Black Forest Observatory, we select time windows of very low background signal, for which all of the contributing broadband seismometers showed their best performance. Streckeisen seismometers of type STS-1, STS-2, and STS-6A, a Nanometrics Trillium T360, and the superconducting gravimeter (SG) SG056 manufactured by GWR Instruments take part in this comparison. Because of their low level of self-noise, the STS-1 and the SG056-G1 benefit the most from a correction with the best currently available improved Bouguer plate model for atmospherically induced signals at frequencies below 1 mHz. As far as we know, this is the first case in which the background level of a broadband seismometer could be lowered below the PLNM. At signal periods beyond the normal-mode band (investigated up to 12 hr), the gravimeters show the lowest level of self-noise, directly followed by the STS-6A. In the band from 0.3 to 10 mHz, the STS-1 has the lowest level of self-noise, which is at least 4 dB below the PLNM, directly followed by the T360 and the STS-6A. Sensors of lower self-noise than the currently manufactured STS-6A or T360 are needed before improved atmospheric correction procedures lead to a significantly lower vertical-component detection threshold.

Description: The Seismic Experiment for Interior Structure (SEIS) of the InSight mission to Mars has been providing direct information on Martian interior structure and dynamics of that planet since it landed. Compared with seismic recordings on the Earth, ground-motion measurements acquired by SEIS on Mars are not only made under dramatically different ambient noise conditions, but also include idiosyncratic signals that arise from coupling between different InSight sensors and spacecraft components. This work is to synthesize what is known about these signal types, illustrate how they can manifest in waveforms and noise correlations, and present pitfalls in structural interpretations based on standard seismic analysis methods. We show that glitches (a type of prominent transient signal) can produce artifacts in ambient noise correlations. Sustained signals that vary in frequency, such as lander modes that are affected by variations in temperature and wind conditions over the course of the Martian sol, can also contaminate ambient noise results. Therefore, both types of signals have the potential to bias interpretation in terms of subsurface layering. We illustrate that signal processing in the presence of identified nonseismic signals must be informed by an understanding of the underlying physical processes in order for high-fidelity waveforms of ground motion to be extracted. Whereas the origins of the most idiosyncratic signals are well understood, the 2.4 Hz resonance remains debated, and the literature does not contain an explanation of its fine spectral structure. Even though the selection of idiosyncratic signal types discussed in this article may not be exhaustive, we provide guidance on the best practices for enhancing the robustness of structural interpretations.