CSAMT exploration at Sellafield: Characterization of a potential radioactive waste disposal site

The long term disposal of radioactive waste in an underground repository re quires the detailed geological evaluation of a potential site. Owing to the ir inherent sensitivity to the presence of fluids in rocks, electromagnetic (EM) methods have an important role in this assessment. Controlled-source EM techniques are especially useful in strong anthropogenic noise environme nts such as industrial locations. However the complexity of modeling and in version can limit the quantitative interpretation of controlled-source EM d ata. A potential radioactive waste disposal site at Sellafield in Great Britain has been investigated using a variety of EM exploration techniques. Control led-source audio-frequency magnetotelluric (CSAMT) data have given the best subsurface information in an environment that has a high level of cultural noise. One-dimensional inversions of the Sellafield CSAMT data were found to be inadequate; 2.5-D forward modeling and inversion were used to interpr et the data. The resulting resistivity models show good agreement with well log data collected at the site. These resistivity models show the presence of a large zone of hypersaline groundwater extending 1 km inland towards t he potential repository and indicate the effect of faults on the hydrogeolo gy.