Analysis of Tidal DC Resistivity Time Series for Periodic Saltwater Mixing Patterns and Determination of Hydraulic Ground Properties

Saline intrusion research using geoelectrical time-lapse monitoring, is often directed towards imaging the saltwater-freshwater boundary and the amount of seawater mixing within a coastal aquifer. However, these time series can contain additional information about subsurface hydrologic properties like hydraulic conductivity and permeability which are crucial elements in coastal groundwater management. In this study, DC resistivity time series from tidal time-lapse monitoring surveys of a shallow coastal sand and gravel aquifer in New Zealand have been analysed for recurring patterns of percentage seawater mixing in different portions of the aquifer. The results show a distinctly different behaviour of percentage seawater change with time for several horizontal locations along two profile lines with varying depth. In addition, the geoelectric time series have been cross-correlated with tidal stage data approximated near the survey location in order to find portions of the aquifer that exhibit different time lags with respect to a diurnal tidal cycle. First results yield a remarkably similar picture to resistivity ratios obtained between high and low tide inversion models of the DC resistivity time series both at different locations and for different seasons. The two methods indicate a correlation between rising and falling tides and the resistivity changes observed from geoelectrical monitoring studies .This may be used to distinguish between more or less hydraulically conductive portions of a coastal aquifer.