Dimensionless Time‐Drawdown Plots of Late Aquifer Test Data

Abstract

ABSTRACT The Jacob straight‐line method is concerned with the late time Theis type curve. The Jacob method gives a straight line on semilogarithmic paper. In practice, the Jacob straight line is taken for granted by ground‐water hydrologists, hydrogeologists, and engineers, and applied almost universally. Not every straight line on semilogarithmic paper, however, warrants the application of the Jacob method. The Jacob straight‐line method has physical restrictions (in addition to the mathematical requirements of having the dimensionless time factor, u, less than 0.01) which are ignored. Therefore, determination of the storage coefficient and transmissivity from the semilogarithmic plot of data yields either over or underestimations. The objective of this paper is to show that the slope of the straight line on semilogarithmic paper can provide useful quantitative information for the proper application of the Jacob method. Differences in the slopes of the theoretical and field lines implies various physical circumstances. In practice, the Jacob method must be used with great care. The physical construction of the aquifer and flow conditions must be considered.