Measurement of a Weak Variable Magnetic Field in an Unshielded Space

The article describes the issues involved in comparing the results of measurements of a weak magnetic field in an unshielded space with the use of sensors of various types, which is relevant for geophysics, magnetobiology, and medicine. The issues of measuring a weak variable magnetic field by various magnetic induction sensors in an unshielded space in the absence of obvious geomagnetic variations are considered. The amplitude of natural geomagnetic noise in a quiet geomagnetic field in the absence of geomagnetic variations is shown to have a random nature; therefore, gradient methods for measuring a weak variable magnetic field are limited from below by the level of natural geomagnetic noise. The influence of the size of the sensor of a weak variable magnetic field on the results of measurements of broadband random geomagnetic noise is identified.