Electromagnetic imaging of hydrates and accretionary structure at Hydrate Ridge, Oregon using 2.5 D model studies

We have carried out model studies of controlled source electromagnetic (CSEM) data collected at Hydrate Ridge, Oregon, over the Cascadia accretionary margin. This project was designed to image near-surface gas hydrate using CSEM, but we also collected broad-band marine MT data. A 2D inversion of the MT data provides an image of folding associated with the accretionary complex and is sensitive to conductive fluids within the accretionary prism. In order to improve our understanding of the distribution of hydrate in the shallow section we need similar inversion tools for CSEM data. We compared the use of finite element (Li and Key, 2007) and finite difference (Abubakar et al.,2006) CSEM 2.5 D modeling codes (3D source, 2D model). In particular, we modeled bathymetric changes of up to 800 m along a 20 km, E-W profile across the ridge. The finite element and finite difference codes accommodate bathymetry in different ways: the finite difference approach uses a rectangular grid that extends throughout the entire model space, whereas the finite element approach uses triangular elements that can be refined around any complex geometry without propagating through the model. We compare the efficiency and accuracy of these two codes. Fixed resistivity models incorporating bathymetry were then run to create look-up tables and thus transform our electric field data into apparent resistivities. We compare pseudosections generated this way with those derived using flat-earth 1D calculations. We also attempted to invert the field data using the finite difference 2.5 D pixel inversion code. Inversion results give better depth constraints on resistivity structure, which can then be compared with 2D MT inversion results and seismic data.