Gravimetry is a well-established tool to probe the deep Earth's processes. Geophysical signals coming from the deep Earth, like the inner core free oscillations, have however never been detected. Challenging quests raise the question of the limits of detection of elusive signals at the Earth's surface. Knowledge of the instrumental limits and of the environmental noise level at a site is fundamental to judge the true sensitivity of an instrument. We perform a noise level comparison of various gravimeters and a long-period seismometer at the J9 gravimetric observatory of Strasbourg (France) to provide a reference of instrumental performances. We then apply a three-channel correlation analysis of time-varying surface gravity from superconducting gravimeter records to isolate the instrumental self-noise from the environmental noise. The self-noise coherence analysis shows that the instrumental noise level remains flat towards lower frequencies till 10^-4 Hz. At seismic frequencies, the self-noise is well explained by a Brownian thermal noise model. At daily and sub-daily time-scales, self-noise is increasing with the period but to a much lesser extent than observed noise level. Observed Earth's ambient noise level at sub-seismic frequencies is hence mostly due to unmodeled geophysical processes. At hourly time-scales, our ability to detect elusive signals coming from the deep Earth's interior is not limited by the instrument capability but is mostly due to the environmental effects.

中文翻译：

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地球上重力信号的检测极限。

重力测量是探测地球深层过程的成熟工具。然而，来自地球深处的地球物理信号，如内核自由振荡，却从未被探测到。具有挑战性的任务提出了地球表面难以捉摸的信号的检测极限的问题。了解仪器极限和现场环境噪声水平对于判断仪器的真实灵敏度至关重要。我们对法国斯特拉斯堡J9重力观测站的各种重力仪和长周期地震仪的噪声水平进行了比较，为仪器性能提供参考。然后，我们对超导重力计记录中的时变表面重力进行三通道相关分析，以将仪器自噪声与环境噪声隔离。自噪声相干性分析表明，仪器噪声水平在 10 ^-4  Hz 之前向较低频率保持平坦。在地震频率下，布朗热噪声模型可以很好地解释自噪声。在每日和次日时间尺度上，自噪声随着时间段的推移而增加，但程度远小于观察到的噪声水平。因此，观测到的亚地震频率下的地球环境噪声水平主要是由于未建模的地球物理过程造成的。在每小时的时间尺度上，我们探测来自地球深处的难以捉摸的信号的能力不受仪器能力的限制，而主要是由于环境影响。