Time series analysis of hydrologic data obtained from a man-made undersea LPG cavern

Characteristics of hydrologic time series data obtained from an undersea LPG (butane and propane) storage cavern were analyzed in time and frequency domains. The analyzed data included cavern seepage rates, cavern operating pressures (gas pressures), water levels of monitoring boreholes in and around the cavern, surrounding tidal levels and site precipitations of long and short periods. The seepage rates showed annual periodic behaviors, which were related to seasonal LPG use pattern. The gas operating pressures were the most dominant controlling factor for the seepage rates. The tidal level revealed an apparent negative cross-correlation with the seepage rate. However, the long-term close correlation between the tidal level and the seepage rate might be attributed to coincident concurrence of the seasonal LPG use and the tidal level variation. The water levels of the monitoring boreholes showed significantly different auto-correlations and spectral densities with locations. The water levels near the butane cavern exhibited high memory effects and low cross-correlations with the tidal level while those of the propane cavern exhibited short memory effects and relatively higher cross-correlations with the tidal level. The difference in all the hydrologic responses (seepage and water level) of the two caverns to the outer stresses such as precipitation, gas operating pressure and tidal level, can be mostly related to the hydrogeologic stratification surrounding the caverns. The low permeable silty clay layer is very thick above the butane cavern while it is very thin about the propane, such that greater hydraulic connection to the overlying sea could be expected.