On locating a ship-towed electromagnetic streamer using a seafloor dipole transmitter

The marine controlled-source electromagnetic method has become a popular technique for mapping the electrical resistivity structure of the seafloor. Electromagnetic fields, produced by an electric dipole transmitter, diffuse through the earth and are recorded on the seabed by nodal or cable based electric dipole receivers. Accurate information on transmitter and receiver geometry is extremely important for proper interpretation; errors in the position and orientation of the transmitter and/or the receivers propagate into errors in the predicted seafloor resistivity. In this paper we consider the special situation where a receiver cable is towed in the vicinity of a transmitter which is located on the seafloor with a known position and orientation. We examine the log-differentiated step response of this system for a selection of one, two, and three dimensional models, and show that while the electromagnetic field recorded at early times contains information about the subsurface resistivity, the late time decay of this pseudo-impulse response is only a function of geometry for all the models we consider. In fact, the shape of this late time decay is almost exactly the same as that of a dipole in a wholespace having the same resistivity as seawater. This shows that late time information can be used to determine the position of the receiver cable, while early time information can be used to determine the resistivity structure of the seafloor.