Method for hydrocarbon reservoir mapping and apparatus for use when performing the method

A method for an electromagnetic survey of electrically resistive target objects that potentially contain hydrocarbons, said method comprising:

• determining electrical characteristics of a stratum being surveyed by use of a transverse magnetic mode of at least one electromagnetic field source and registration of a transverse magnetic response;
• generating an intermittent source current pulses with a sharply defined termination in the at least one electromagnetic field source;
• transferring the intermittent source current pulses to a submerged, essentially vertical transmitter antenna and into the strata;
• intercepting a medium response with at least one receiver deployed in an induction zone and provided with at least one submerged, essentially vertical receiver antenna, in time between consecutive current pulses;
• and
• measuring strata response in the induction zone, wherein the horizontal distance between the at least one transmitter antenna and the at least one receiver is equal to R, and R≦(tρα(t)/μ0)1/2, where t is the time lag counted from the instant after the electromagnetic field source has been switched off, μ0=4π10−7 H/m, and ρα(t) is the apparent resistivity of a substratum in the period t.