A new technique for obtaining high-resolution pore pressure records in thick claystone aquitards and its use to determine in situ compressibility

Laboratory tests are commonly used to determine properties (vertical compressibility, alpha; specific storage, SS; and vertical hydraulic conductivity, K-v) of claystone aquitards; however, whether data representative of in situ conditions can be obtained from disturbed samples is questionable. Here, we present a method to determine the in situ alpha and S-S of a thick sequence of Cretaceous aged claystone by estimating the loading efficiency (gamma) of a formation from pore pressure responses to barometric pressure fluctuations. We installed 10 vibrating wire pressure transducers at different depths (25-325 m below ground) in a thick claystone aquitard by placing them directly within the cement-bentonite grout. Two years of continuous transducer records using this method appeared to provide pore pressure data with a resolution of better than one part in 10(5), equivalent to millimeter of hydraulic head change. Pore pressure responses to barometric pressure changes, earth tides, and precipitation events can be clearly identified, and the barometric responses can be easily analyzed. The resulting values of gamma (0.6-0.93), alpha (2.5 x 10(-7) to 2.2 x 10(-6) kPa(-1)), and S-S (2.6 x 10(-5) to 4.5 x 10(-6) m(-1)) all decrease with depth. The results are comparable with the limited existing data for in situ estimates of S-S and are as much as an order of magnitude smaller than laboratory estimates of SS for similar aquitard deposits. Our findings suggest that the fully grouted transducer method can provide an accurate and reliable means to monitor pore pressure changes and to determine in situ parameters for bedrock aquitard systems. Citation: Smith, L. A., G. V. Kamp, and M. J. Hendry (2013), A new technique for obtaining high-resolution pore pressure records in thick claystone aquitards and its use to determine in situ compressibility, Water Resour. Res., 49, doi: 10.1002/wrcr.20084.