# Methods for removal of unwanted signals from gravity time-series: comparison using linear techniques complemented with analysis of system dynamics

## TL;DR

This study compares linear and system dynamics methods for removing dominant tidal signals from gravity time-series, demonstrating effective reduction of tides to reveal smaller geophysical signals like hydrological events.

## Contribution

It introduces and compares three conceptual approaches—frequency filtering, physical modeling, and data-based modeling—for tide removal in gravity data, highlighting their limitations and applications.

## Key findings

- Tides were reduced sufficiently to observe geophysical events above 10 nm s^{-2}
- Different methods leave residual tides or distort signals
- The approaches are generalizable to other data affected by periodic signals

## Abstract

The presence of undesirable dominating signals in geophysical experimental data is a challenge in many subfields. One remarkable example is surface gravimetry, where frequencies from Earth tides correspond to time-series fluctuations up to a thousand times larger than the phenomena of major interest, such as hydrological gravity effects or co-seismic gravity changes. This work discusses general methods for removal of unwanted dominating signals by applying them to 8 long-period gravity time-series of the International Geodynamics and Earth Tides Service, equivalent to the acquisition from 8 instruments in 5 locations representative of the network. We compare three different conceptual approaches for tide removal: frequency filtering, physical modelling and data-based modelling. Each approach reveals a different limitation to be considered depending on the intended application. Vestiges of tides remain in the residues for the modelling procedures, whereas the signal was distorted in different ways by the filtering and data-based procedures. The linear techniques employed were power spectral density, spectrogram, cross-correlation and classical harmonics decomposition, while the system dynamics was analysed by state-space reconstruction and estimation of the largest Lyapunov exponent. Although the tides could not be completely eliminated, they were sufficiently reduced to allow observation of geophysical events of interest above the $10 \text{ nm s}^{-2}$ level, exemplified by a hydrology-related event of $60 \text{ nm s}^{-2}$. The implementations adopted for each conceptual approach are general, so that their principles could be applied to other kinds of data affected by undesired signals composed mainly by periodic or quasi-periodic components.