The Marine CSEM Response of a Steel Borehole Casing - Applications for the NEPTUNE Canada Gas Hydrate Observatory

Gas hydrates are a solid, ice-like mixture of water and low molecular weight hydrocarbons. Hydrates are a potential energy resource, a possible factor in climate change, and a geohazard. It is thus important to determine the location, concentration and distribution of the world’s supply of gas hydrates. Because of their ice-like nature, hydrates are electrically insulating. Consequently, a geophysical technique which remotely detects changes in seafloor electrical conductivity, such as the marine controlled source electromagnetic (CSEM) method, is useful for marine gas hydrate exploration. In order to study hydrate deposits, the University of Toronto marine electromagnetics group has deployed a stationary seafloor time domain CSEM system in the framework of the NEPTUNE Canada underwater cabled observatory, offshore Vancouver Island. However, for interdisciplinary studies of the area, the International Ocean Drilling Program is planning to drill two boreholes near the CSEM system and must insert steel casings. In this paper we model the electromagnetic response of conductive steel casings in the seafloor subject to a marine CSEM system. We derive a thin tube modeling algorithm, and then use it to make some initial calculations concerning the effects of these casings on the NEPTUNE system.