Evaluating Frequency Band and Stretching Time Window Effects on Noise-Based dv/v for Subsurface Hydrological Monitoring

Environmental seismology increasingly employs relative changes in seismic velocity (dv/v) with time, derived from continuous seismometer recordings, to infer temporal variations in ground properties such as soil moisture and groundwater dynamics. However, despite that widespread use, there is no consensus on optimal frequency bands and stretching time windows used to extract reliable dv/v time series. Previous studies addressing deep, and shallow groundwater dynamics each applied distinct combinations of frequency bands and stretching time windows, raising the key question: how comparable are dv/v results derived from different parameter choices, and how consistently do they represent subsurface hydrological variability?

This work presents an intercomparison of commonly used noise-based dv/v combinations of frequency bands and stretching time windows across two hydrological domains: (1) groundwater at depth, and (2) shallow critical zone hydrology. For each domain, we review and implement published combinations of the previous parameters and assess their influence on the resulting dv/v time series.

To evaluate their methodological impact, we compare the different dv/v estimates amongst themselves and against independent environmental control datasets, including groundwater levels, soil moisture time series, and additional hydroclimatic observations. For both domains, we complement literature case studies with our own two field datasets from Germany, one in the Eifel and one in the Harz region. In the Eifel region, a network of ten seismic stations has been deployed across two sub-catchments in the upper Ahr Valley, composed of intensively folded and fractured Devonian mudstone and carbonate rocks. Continuous seismic records are paired soil moisture sensors at depths down to 40 cm and groundwater well measurements from nearby monitoring sites. In the Harz Mountains, we use additional seismic, top soil moisture, and hydrological observations to extend the comparison to a geologically and hydroclimatically distinct setting, with variable soil cover on weathered granite boulders creating abundant water flow in the shallow subsurface.

We hypothesize that different combinations of frequency bands and stretching time windows will produce systematically distinct dv/v patterns, even when applied to the same dataset, and that their sensitivity will vary across deep groundwater systems and shallow surface hydrology. By identifying parameter combinations that enhance or mask relations with the independently sensed environmental variables, this study aims to better understand methodological controls between these parameters and contribute towards a more consistent and comparable practice for environmental applications.