Two systems of electromagnetic units are in common usage in the geophysical literature: The venerable cgs system, also known as electromagnetic units (emu), and the more modern and internationally accepted Système Internationale (International System, abbreviated SI). Geophysical journals now require the use of SI units, and for the most part SI units are adhered to in this text. Geophysical journals published prior to 1980, however, employed emu, and even some recently published textbooks (e.g., Butler) continue its usage. Consequently, reading the geophysical literature requires conversion between the two systems, a common source of frustration because the two systems are significantly different.

The following table summarizes these two important systems of units and the conversion factors between them. Additional discussions can be found in textbooks by Panofsky and Phillips, Butler, and the Society of Exploration Geophysicists, and in papers by Shive, Payne, and Moskowitz.