Identifying Aquifer Type in Fractured Rock Aquifers using Harmonic Analysis

Abstract

Determining aquifer type, unconfined, semi‐confined, or confined, by drilling or performing pumping tests has inherent problems (i.e., cost and complex field issues) while sometimes yielding inconclusive results. An improved method to cost‐effectively determine aquifer type would be beneficial for hydraulic mapping of complex aquifer systems like fractured rock aquifers. Earth tides are known to influence water levels in wells penetrating confined aquifers or unconfined thick, low‐porosity aquifers. Water‐level fluctuations in wells tapping confined and unconfined aquifers are also influenced by changes in barometric pressure. Harmonic analyses of water‐level fluctuations of a thick (∼1000 m) carbonate aquifer located in south‐central Oklahoma (Arbuckle‐Simpson aquifer) were utilized in nine wells to identify aquifer type by evaluating the influence of earth tides and barometric‐pressure variations using signal identification. On the basis of the results, portions of the aquifer responded hydraulically as each type of aquifer even though there was no significant variation in lithostratigraphy. The aquifer type was depth dependent with confined conditions becoming more prevalent with depth. The results demonstrate that harmonic analysis is an accurate and low‐cost method to determine aquifer type.

Characterising Bedrock Aquifer Systems in Korea Using Paired Water-Level Monitoring Data

“…However, if a vadose zone exists on top of the water table, lags can occur as the atmospheric changes pass through the vadose zone. The atmospheric signals of S2 and K1 occur by such lags [29], and the lack of an M2 signal can indicate an unconfined condition of an aquifer [33]. As the thickness of the vadose zone increases, the aquifer becomes contained by greater pressure and exhibits an M2 earth tide signature.…”

“…To apply the method of Rahi and Halihan [33] to the 159 paired monitoring wells, hourly measured time-series data were used to conduct a fast Fourier transform (FFT) analysis, producing a periodogram of the signal power of the frequency (cycles/day) [41][42][43]. Using the MATLAB ® program (R2015a, Mathworks Inc., Natick, MA, USA), spectral analysis was conducted to classify the frequency domains and to analyse the strengths of the signals that were derived from the respective water-level data (Table S1).…”

“…Natural water-level monitoring data include the intrinsic periodicities caused by earth tides and atmospheric pressure fluctuations, which influence the water-level fluctuations in confined, unconfined, and low-porosity aquifers [28][29][30]. Because the degree of influence can differ depending on the characteristics of each aquifer, the periodic features of such signals could be used to identify the type of aquifer more specifically [31][32][33].…”

“…This analysis used the hourly groundwater-level data during the dry season from February to April 2010 to minimise the noise caused by precipitation. Then, using the components, the characteristics of the aquifers were classified based on specific frequency domains, which are shown in the unconfined, semi-confined, and confined conditions [33]. In the study of water-level fluctuation, five major harmonic components of tidal potential are identified: S2, M2, N2, K1, and O1 [28,36] (Table 2).…”

Groundwater Storage Change Estimation Using Combination of Hydrogeophysical and Groundwater Table Fluctuation Methods in Hard Rock Aquifers

“…In this study, the change of atmospheric pressure has been taking into account due to its great influence, compared with earth tides on water-level fluctuations within wells tapping unconfined aquifers [33]. According to Rahi and Halihan [34]. In the case of a well penetrating an unconfined aquifer, atmospheric changes affect the water level within the well instantaneously, whereas the effects on the aquifer, if present, are delayed due to the presence of the air and other gasses within the unsaturated zone.…”