A detection system of geological formations, particularly but not exclusively used for oil exploration, and for characterization and monitoring of hydrocarbon reservoirs. The detection system includes an electromagnetic transmission device positioned on a surface of a geological formation close to a first well having a vertical development direction, the first well being covered by a metal casing, and an electromagnetic reception device positioned deep inside the first well, the electromagnetic reception device including an electromagnetic detector. The electromagnetic transmission device includes a primary electromagnetic field generator configured to generate a primary electromagnetic field focalized in a direction of the metal casing, the metal casing irradiating the primary electromagnetic field into the geological formation.