Recent studies illustrate the sensitivities of the amplitude and waveform of a GPR reflection to the contrast in electromagnetic properties across the reflecting contact. Interpretation of amplitudes is generally complicated by uncertainties in the form and amplitude of the downgoing pulse and lack of constraints on media properties on either side of contacts. Nevertheless, useful information about heterogeneities in electromagnetic properties in sedimentary structures can be derived where local ground-truthing is available or where layering is repetitive. Examples will be shown of forward modeling of radar traces with finite-difference time-domain (FDTD) simulations of radar wave propagation. The modeling serves to constrain the electromagnetic properties of structures compatible with the observed GPR records. Results will be illustrated from both fluvial and barrier island settings. These studies demonstrate that surveying with multiple antenna frequencies is critical to assessing the relative amplitudes of dielectric permittivity, conductivity, and/or magnetic permeability variations over a range of length scales. Further constraints on the absolute values of permittivity variations can be derived from amplitude versus offset (AVO) analysis of common mid-point surveys.