A novel approach to groundwater monitoring in the Maltese Islands : analysis of auto- and cross-correlation functions of ambient seismic noise

Abstract

The Maltese islands, approximately 315 km2 in area with a high population density, face high levels of water stress due to low amounts of rainfall and a dependence on groundwater abstraction. Up till now, in-situ borehole readings are the only utilised method to monitor the quantitative status of groundwater in Malta. This study investigates an innovative, cost-effective approach to groundwater monitoring in an island environment; by computing cross-correlations and autocorrelations of ambient seismic noise recorded by two types of seismic stations in the Maltese islands: six broadband from the Malta Seismic Network (MSN) and six short-period from the FASTMIT network. Six years of seismic noise data was utilised from the broadband stations, and a year-long dataset was available from the short-period stations. Interstation distances in this study ranged from 3-35 km. The MSNoise program was used for the data management and processing. Seismic velocity changes (δv/v), which are possibly induced by changes in groundwater level, were detected using the Moving-Window Cross-Spectral (MWCS) technique. Both types of stations can provide similar sensitivity to the δv/v when taking into consideration appropriate filters (0.1-1 Hz and 0.3-3 Hz for the broadband and short-period stations respectively). The results demonstrate that seasonal and annual changes in the groundwater levels can be detected by changes in seismic velocity. The method works for both cross-correlation (maximum δv/v variation of ∼0.3%) and autocorrelation functions (maximum δv/v variation of ∼2%) of ambient seismic noise, the latter being an order of magnitude more sensitive. Seasonal and annual variations of the δv/v from autocorrelations of some stations were found to be more pronounced than from cross-correlations. Clear seasonal variations in δv/v were observed as a result of cross-correlations between short-period stations as well as short-period and broadband stations. The quality of the δv/v deteriorates at longer interstation distances when seasonal variations in the δv/v are less obvious. Presumably, this is because longer interstation paths tend to traverse more complex geology, different types of aquifers, or even the sea. Generally, the comparison of the δv/v with groundwater level readings from nearby boreholes show highly encouraging similarities in the seasonal variations, indicating that δv/v may be used as a proxy for monitoring groundwater levels in the Maltese islands in regions where borehole measurements are sparse.