ALBERT

Description: Gas hydrates are a potential energy resource, a possible factor in climate change and an exploration geohazard. The University of Toronto has deployed a permanent seafloor time-domain controlled source electromagnetic (CSEM) system offshore Vancouver Island, within the framework of the NEPTUNE Canada underwater cabled observatory. Hydrates are known to be present in the area and due to their electrically resistive nature can be monitored by 5 permanent electric field receivers. However, two cased boreholes may be drilled near the CSEM site in the near future. To understand any potential distortions of the electric fields due to the metal, we model the marine electromagnetic response of a conductive steel borehole casing. First, we consider the commonly used canonical model consisting of a 100 m, 100 m thick resistive hydrocarbon layer embedded at a depth of 1000 m in a 1 m conductive host medium, with the addition of a typical steel production casing extending from the seafloor to the resistive zone. Results show that in both the frequency and time domains the distortion produced by the casing occurs at smaller transmitter-receiver offsets than the offsets required to detect the resistive layer. Second, we consider the experimentally determined model of the offshore Vancouver Island hydrate zone, consisting of a 5.5 m, 36 m thick hydrate layer overlying a 0.7 m sedimentary half-space, with the addition of two borehole casings extending 300 m into the seafloor. In this case, results show that the distortion produced by casings located within a 100 m safety zone of the CSEM system will be measured at 4 of the 5 receivers. We conclude that the boreholes must be positioned at least 200 m away from the CSEM array so as to minimize the effects of the casings.

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.