ALBERT

Description: A typical marine controlled-source electromagnetic system consists of an electric dipole transmitter and one or more electric dipole receivers. The objective of a survey is to determine the seafloor resistivity by recording the electromagnetic transients, which diffuse through the earth from the transmitter to the receivers. Accurate knowledge of system geometry is crucial for proper interpretation; errors in the position and orientation of the transmitter and/or the receivers propagate into errors in the predicted seafloor resistivity. We show theoretically that for certain multireceiver set-ups and crustal electrical profiles that the geometry and the seafloor resistivity may be determined independently. A specific example is an experiment proposed in association with NEPTUNE Canada. Here, we have already deployed an electric dipole transmitter with a known orientation in a known location. A cabled streamer of receivers may be towed by a survey vessel in the vicinity of the transmitter on a known heading. For this configuration, an eigenparameter analysis of two seafloor models consisting of (1) a halfspace and (2) a resistive layer buried within a halfspace shows that the resistivity structure of the seafloor can be independently resolved from the cable location. Further studies of these two models also indicate that the position of the streamer must be roughly known in advance on the order of a hundred metres to be used as a suitable starting model in a non-linear inversion. The crucial information is contained in the parts of the pulse which travel through the seawater and which act as a calibration path. Such information is absent for a static DC method.