Method for electromagnetic geophysical surveying of subsea rock formations

A method for electromagnetic geophysical surveying of rock formations under a sea-floor, the method comprising:

• towing a first alternating field emitting source at a first depth while emitting a first field having a first phase and first amplitude, said first field propagating partly down through the sea-floor and being reflected and/or refracted through said rock formations and partly propagating back through the sea-floor;
• measuring said first field with electromagnetic receivers and recording corresponding first field registrations;
• towing a second alternating field emitting source at a second depth while emitting a second alternating field having a second phase and second amplitude not necessarily different from said first phase and first amplitude, said second field propagating partly down through the sea-floor and being reflected and/or refracted through said rock formations and partly propagating back through the sea-floor;
• measuring the second field with electromagnetic receivers and recording corresponding second field registrations;
• and
• combining the first and second field registrations to a combined field with a phase or amplitude imposed to at least one of the first or second field registrations for emulating measurements resulting from emission from a phased array emitting antenna set of alternating field emitting sources having directivity in order to transmit a major proportion of combined energy from emission of the alternating fields by the sources downwards through the sea-floor and a significantly smaller proportion of the combined energy upwards.