Présentations Colloques

Oral Presentation
Session 8.02: Groundwater Development and Protection in Coastal and Volcanic Environments with Complex Geological Structures
Shirahata Katsushi
Digital filters for application to tidal groundwater time series
Observation time-series data including significant tidal signals are commonly encountered in studies of groundwater in small islands or coastal areas. Groundwater data collected in such areas often include effects of other non-periodic agents, for example, the effect of a few day-long drops in atmospheric pressure when an intense low pressure system passes. When groundwater time-series data are subject to quantitative analysis of tidal or non-tidal component, separation of these components is an important preliminary step. **In the field of oceanography, digital low-pass filters have long been used to smooth tidal time-series data. The filters suppress semidiurnal and diurnal tidal components in the observation data and disclose longer-period signals. However, uses of digital filters in groundwater hydrology are limited. **A comparative study was made on digital filters presented in the literature, focusing on nonrecursive types that can be immediately used with a prevalent spreadsheet application with its built-in functions. In addition, new excellent tide-killer low-pass filters were produced as per a design method proposed in an oceanographic literature and compared. Following low-pass filters were evaluated- (1) running-mean filters, (2) selected-mean filters, (3) cosine-type filters using windows, and (4) optimized tide-killer filters newly produced. The desired low-pass filter in this study was what suppresses components of semidiurnal to diurnal tidal band, especially known eight major tidal components, with longer-period components (longer than two days) being completely preserved. **The last two types of the studied filters above include desirable digital filters. The low-pass filters studied can easily be transformed into high-pass filters through an arithmetic procedure. High-pass filters transformed from the newly produced tide-killer low-pass filters are excellent in terms of preserving eight major tidal components. The new high-pass filters are optimal for use prior to quantitative analysis of specific major tidal components. **