The 4th Site Characterization Board Workshop will be held on 14 April 2026 at the Department of Physics, Sapienza University of Rome. This workshop is conceived as a focused technical meeting aimed at consolidating the current status of site characterization activities for the Einstein Telescope and at advancing a more coherent framework for the next phase of analysis and coordination. It follows the earlier workshop series, which served as a forum for updates and discussion across candidate ET sites and related work packages.
The scientific goal of this edition is to move from a general exchange of information toward a more structured comparison of methods, datasets, and modelling requirements relevant to site assessment. For this reason, the agenda has been built around a set of tightly connected objectives: reviewing the present understanding of the environmental noise budget, discussing measurement and data-sharing strategies, and identifying the common physical parameters and modelling assumptions needed to improve the reliability and comparability of site characterization studies.
Particular attention will be devoted to the main noise channels that are critical for underground gravitational-wave infrastructures, namely acoustic, magnetic, and seismic noise. Within this framework, the workshop is intended to promote convergence on measurement practices, data interpretation, and cross-site consistency, with the broader aim of strengthening the scientific basis on which future site-related evaluations will rely.
A central element of the workshop will be the discussion of Newtonian Noise, addressed both from the perspective of ongoing activities in the different groups and from that of model construction. Special emphasis will be placed on the parameterization required for robust NN estimates, including the seismic and geological quantities that enter predictive models. In this sense, the workshop is meant not only to report progress, but also to clarify which inputs, standards, and levels of description are needed to make NN assessment more systematic and actionable.
The afternoon session will focus on the next organizational and scientific steps, with dedicated discussion on the development of the first 3D geological models and on the coordination of the corresponding work within the Site Characterization Board. The overall structure of the meeting is therefore designed to support a concrete outcome: improving alignment between measurements, data products, and modelling tools, and defining a clearer path toward an integrated site characterization strategy for the Einstein Telescope.