Estimating Aquifer Parameters from Analysis of Forced Fluctuations in Well Level - an Example from the Nubian Formation near Aswan, Egypt .2. Poroelastic Properties

We discuss the in situ estimation of aquifer poroelastic properties from observed water level fluctuations in five wells sampling the 0.30 porosity Nubian sandstone near Aswan Reservoir, Egypt. The study area is close to ideal for the application of poroelastic theory, because of the relatively simple stratigraphy and the absence of cultural and climatological noise. In this paper we use approximately 2 years data sampled at 0.2-hour intervals to calculate frequency-dependent admittances between the well levels and the recognized forcing functions of the earth tides and barometric fluctuations; the effect of variations in lake level is treated in a companion paper. The time series analysis techniques that we employ are described in detail. Three of the wells show equilibrium-confined response at tidal frequencies, which allows various in situ poroelastic parameters of the aquifer to be derived from the tidal and barometric admittance values. We make extensions to earlier treatments of poroelastic theory in order to include the effect of horizontal strain resulting from barometric loading; the modifications proved of minor importance for our data set because of the high compliance of the aquifer material and the shallow depth to basement. In situ shear modulus-mu is determined as 5.4-6.4 x 10(9) Pa, and uniaxial strain loading efficiency as 0.33. Each of these parameters depends on the field measurements of well level and air pressure, and mu also depends strongly on a theoretical estimate of the solid earth areal strain tides; the principal source of error in mu-lies in these areal strain estimates. We measured Poisson's ratio in the laboratory and found that it lies in the range 0.13-0.18. This, together with the admittance values, allows estimates of uniaxial strain modulus (13-19 x 10(9) Pa, with the smaller values at shallower depth), specific storage (1.5-1.9 x 10(-6) m-1, with the larger values at shallower depth), porosity (0.2-0.3), Skempton's constant (0.54-0.58), and Biot's constant (0.78-0.85). Laboratory measurements of uniaxial strain modulus are about 50% higher than the in situ estimates, probably reflecting scale effects. Despite the high porosity, use of a theory of aquifer response in which the solid constituent is considered incompressible produces significant errors in these estimates.