Controlled source electro-magnetic
The Controlled Source Electromagnetic (CSEM) method is an offshore geophysical technique, employing electromagnetic remote-sensing technology to indicate the presence and extent of hydrocarbon accumulations below the seabed.
The CSEM survey uses a dipole source that is towed just above the seafloor to transmit a time-varying electromagnetic field into the earth. This field is modified by the presence of subsurface resistive layers and these changes are detected and logged by an array of receivers placed on the seabed. Because hydrocarbon-bearing formations are highly resistive compared with surrounding formations, a CSEM survey can indicate the presence of oil and gas in offshore situations.