Magnetic Field Noise in the Ultra-low Frequency (ULF) Band and Historical Comparisons

The results of low-frequency (< 6 kHz) magnetic field noise measurements at underground coal mines are presented. A comparison of these results to measurements made 35 to 40 years ago suggests that the magnetic field noise has increased substantially (20–30 dB) since this period of time. The ambient noise level is an important factor in the operation of Through- The-Earth (TTE) communications systems, and the data presented herein are an essential consideration in the design of future TTE systems.