Modeling of ground-penetrating radar (GPR) data in 2.5 dimensions is implemented by superposition of 2-D finite-difference, time-domain solutions of Maxwell's equations for different horizontal wavenumbers. Dielectric, magnetic, and conductive losses are included in a single formulation. Attenuations associated with dielectric and magnetic relaxations are introduced by superposition of Debye functions at a set of relaxation frequencies and using memory variables to replace convolutions between the field variables and the decay functions. Better fits to data may always be obtained using the superposition method than by the Cole-Cole model. Good fits to both loss-tangent versus frequency data from lab measurements, and to 500 and 900 MHz field GPR profiles of a buried pipe and the surrounding layers, demonstrate the flexibility and viability of the modeling algorithm. Discrepancies between lab and in-situ measurements may be attributed to scale differences and local variations that make lab samples less representative of the site than the GPR profile.