Effect of temporal and spatial variations of the primary signal on VLF total‐field surveys

Most of the airborne and ground VLF instruments presently used measure the total‐field response in addition to field ratios. Results of surveys using these instruments are adversely affected by spatial and temporal variations in the VLF primary field. Until now, the nature of such variations has not been studied from the point of view of geophysical surveying practice. Spatial variations are analyzed using radio propagation models. The most important result is the identification of primary field minima where surveys would be unreliable. Their dependence on the transmitter location is rather complex, and modeling should be carried out before specifying VLF stations for a survey area. Spatial and temporal variations have been studied using field monitoring of the transmitted signal. The results of field experiments indicate that the nature of the received VLF fields changes significantly even over moderate distances (20–30 km) and that data cannot be reliably corrected over larger distances. This observation has a significant implication for VLF total‐field surveys, particularly airborne, in which base stations have been routinely used to monitor the primary field strength and to correct the survey data. The results of primary signal monitoring are also used to demonstrate the effect of solar flares on VLF surveys. Because of the large intensity and complex electromagnetic character of solar flares, survey data recorded during such events cannot be used for map compilation and interpretation.