Variations in intensity of the primary VLF-EM field have a significant effect on the quality of airborne and ground surveys in which the total field is measured. An analysis of the results of a recent VLF-EM survey and base station records in New Brunswick, Canada, revealed major variations in the primary VLF-EM field due to changes in the transmitting power and to solar flares. To study the effect of systematic primary field variations over the survey area, VLF-EM field intensity was calculated using radio propagation model based on uniform conductive earth and anisotropic-layered ionosphere. The model correctly predicted a decrease in the primary field generated by the NAA transmitter by about 40 percent in a direction perpendicular to a survey line. Such a change is sufficiently important to require repeated calibrations of the receiver between the survey lines. The model also indicated that the phase difference between contributions of the ground and sky waves should vary over the survey area. Such variations explain opposite polarities of sudden amplitude variations due to solar flares that were observed during the survey.