Identification of heterogeneous aquifer transmissivity using an AE-based method

Determination of hydraulic head, Η, as a function of spatial coordinates and time, in ground water flow is the basis for aquifer management and for prediction of contaminant transport. Several computer codes are available for this purpose. Spatial distribution of the transmissivity, Τ(x,y), is a required input to these codes. In most aquifers, Τ varies in an erratic manner, and it can be characterized statistically in terms of a few moments: the expected value, the variance, and the variogram. Knowledge of these moments, combined with a few measurements, permits one to estimate Τ at any point using geostatistical methods. In a review of transmissivity data from 19 unconsolidated aquifers, Hoeksema and Kitanidis (1985) identified two types of the logtransmissivity Υ = ln(Τ) variations: correlated variations with variance σ_Υc² and correlation scale, Ι_Υ, on the order of kilometers, and uncorrelated variations with variance σ_Υn². Direct identification of the logtransmissivity variogram, Γ_Υ, from measurements is difficult because T data are generally scarce. However, many head measurements are commonly available. The aim of the paper is to introduce a methodology to identify the transmissivity variogram parameters (σ_Υc², Ι_Υ, and σ_Υn²) using head data in formations characterized by large logtransmissivity variance. The identification methodology uses a combination of precise numerical simulations (carried out using analytic element method) and a theoretical model. The main objective is to demonstrate the application of the methodology to a regional ground water flow in Eagle Valley basin in west-central Nevada for which abundant transmissivity and head measurements are available.