This monograph looks primarily at deep electromagnetic exploration. However, a few topics on shallow electromagnetic methods are included. Geop hysicists use the widest range of frequencies of electromagnetic waves. Frequency ranging from cycles 100/hour to Giga Hertz, with a penetration depth of a few centimetres used in bore hole geophysics are included. The chapters cover different areas of electromagnetics, including: magnetotellurics; tellurics; MHD sounding; and deep dc resistivity traverses. A separate section on ocean bottom magnetotellurics and ocean bottom magnetometric measurements is presented.