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Description
The Einstein Telescope (ET) is a proposed third-generation underground gravitational-wave observatory designed to observe signals at frequencies as low as a few hertz. Newtonian noise is expected to limit its low-frequency sensitivity, particularly in the 3--15~Hz band. Seismic displacement changes the distribution of mass in the surrounding ground, producing time-dependent gravitational forces on the suspended test masses known as seismic Newtonian noise.
We develop a numerical workflow that calculates seismic Newtonian noise from simulated seismic displacement. Spectral-element simulations resolve how seismic waves propagate through the ground and interact with geological structures and underground caverns. We validate the workflow against analytical solutions in simplified models.
We study a near-surface sedimentary basin and find that it traps seismic energy, producing resonance-related peaks in the Newtonian-noise spectrum. Our cavity experiments show that scattering depends on cavity size relative to seismic wavelength and on burial depth: for cavities of the same size, shallower burial produces stronger scattering through interactions among Rayleigh waves, the cavern, and the free surface.
We have also constructed a numerical model based on the ET-L tunnel design and used it to simulate Newtonian noise, revealing seismic-wave scattering around the caverns.
These results show how local geology and underground geometry affect Newtonian noise, providing a basis for site assessment and future mitigation strategies.