The development of a numerical solution technique is described to obtain the potential distribution in three-dimensional space due to a point source of charge injection in or on the surface of a half space containing any arbitrary two-dimensional conductivity distribution. Finite difference approximations are made to discretize the governing Poisson's equation with appropriate boundary conditions. The discretization of Poisson's equation by elemental area brought about a numerical formulation for a more effective matrix technique to be utilized to solve for the potential distribution at each node of a discretized half-space. A FORTRAN algorithm named RESIS2D was written to implement the generalized solution method. A brief description of the FORTRAN program in terms of its construction is given. The formal input and output parameters for the relevant subroutines are discussed. The program is designed to be implemented on a CDC 7600 machine. The language of the algorithm is FORTRAN IV; certain programming norms for the CDC 7600 machine and the RUN76 compiler are routinely used. Some variables are stored in the LCM (Large Core Memory) of this machine, and their calling sequence and usage apply to the CDC7600 alone. The resulting solution of the potential distribution can be obtained for amore » current source or sink located on the surface or at any arbitrary surface location. Any arbitrary configuration of transmitter or receiver electrode arrays, therefore, could be simulated to obtain the resistivity response over arbitrarily shaped two-dimensional geologic bodies. For brevity, in the source deck provided in this report, only two electrode arrays commonly used in geothermal reservoir delineation are illustrated. (JGB) « less