Potential gradient is measured using an EFM-100 Electric Field Mill from Boltek. It is mounted in inverse position about 0.9 m above ground using Boltek 3 feet tripod inverting mounting kit. The EFM-100 is located on the grass away from elevated objects.
Calibration was done using a reference EFM-100 located on the ground surface at several places around the installed EFM-100 to find a correction factor, which is applied on raw data.
Analog output of EFM-100 is sampled at 25 Hz by dedicated datalogger with GPS time stamping. Data are stored in specific binary format.