We provide several analytical solutions for reservoir impedance here: a square, 5-spot TDF well pattern in cartesian coordinates (Sec. 2.1.1), a concentric circle approximation of an ID (Sec. 2.1.2) in radial coordinates, and a more exact solution for an ID using a stream potential function (Sec. 2.1.3). In Sec. 2.1.4, we present a hybrid 5-spot TDF solution. This hybrid solution combines desirable aspects of cartesian and radial coordinate systems used in Secs. 2.1.1 and 2.1.3. Pressure contours for the ID and TDF approaches are illustrated in Fig. (a) and Fig. (b), respectively. These contour lines represent the cross-sectional area over which flow occurs divided by the reservoir thickness.