Electrical methods can be broadly classified into two groups: those using a controlled (human-generated) energy source and those using naturally occurring electrical or electromagnetic energy as a source. The controlled source methods are most commonly used for shallow investigations, from characterizing surficial materials to investigating resistivities down to depths as great as 1 to [[length::2 km]], although greater depths of investigation are possible with some techniques and under some conditions. The natural source methods are applicable from depths of tens of meters to great depths well beyond those of interest to hydrocarbon development. | Electrical methods can be broadly classified into two groups: those using a controlled (human-generated) energy source and those using naturally occurring electrical or electromagnetic energy as a source. The controlled source methods are most commonly used for shallow investigations, from characterizing surficial materials to investigating resistivities down to depths as great as 1 to [[length::2 km]], although greater depths of investigation are possible with some techniques and under some conditions. The natural source methods are applicable from depths of tens of meters to great depths well beyond those of interest to hydrocarbon development. |