Brownian Noise and Temperature Sensitivity of Long-Period Lunar Seismometers

As long-period ground motion holds the key to understanding the interior of the Earth's Moon, reducing long-period noise sources will be an essential area of focus in the design of future lunar seismometers. For the proposed Lunar Geophysical Network (LGN), the International Lunar Network (ILN) Science Definition Team specifies that an LGN enabling seismometer will need to be more sensitive than any previous seismometer at frequencies below 1 Hz. In an effort toward lowering the seismometer noise floor for lunar geophysical missions, we evaluate the 1= f Brownian noise and the temperature sensitivity of a seismometer. Temperature sensitivity of a seismometer is related to an important component of the seismometer output noise that is proportional to the temperature noise in the environment. The implications of the ILN requirement are presented in the context of the state-of-the-art InSight Seismic Experiment for Interior Structure (SEIS) Very Broad Band (VBB) planetary seismometer. Brownian noise due to internal friction was estimated for future lunar operation after accounting for the rebalance of the product of mass and distance to the center of gravity of the pendulum for the SEIS-VBB sensor. We find that Brownian noise could be a limiting factor in meeting the ILN requirement for lunar seismometers. Further, we have developed a formalism to understand the temperature sensitivity of a seismometer, relating it quantitatively to the local gravity, the thermoelastic coefficient of the spring, change in center of gravity, and the coefficient of thermal expansion of the mechanical structures. We found that in general the temperature sensitivity of a seismometer is proportional to the local gravity, and so the temperature sensitivity can be reduced when operating on a planetary body with lower gravity. Our Brownian noise and temperature sensitivity models will be useful in the design of the next generation of planetary seismometers.