The use of time domain reflectometry (TDR) techniques for measuring the moisture content of composite materials is a mature art but usually assumes homogeneity of the material in the transverse plane. As the basis of a forward solution to TDR imaging, we describe an integral equation approach to model the response of the TDR system to a lossless heterogeneous dielectric body. Then, in conjunction with a suitable dielectric model of the composite material, the TDR response to moisture content distribution may be quantified. Several methods for integrating the transverse electromagnetic field between the transmission line rods were compared and a method combining a priori information with linear interpolation provided the most consistent integration for three different permittivity distributions. A self-consistency approach was used to compare the modelled propagation velocity with that expected from transmission line theory.