Electrical Resistivity and Temperature Piezocone Penetration Test in Ice-Rich Permafrost in Nunavik, Quebec, Canada: Signal Processing and Forward Modelling of Electrical Resistivity Signal

摘要

An electrical resistivity and temperature piezocone penetration test (RTCPTu) was carried out in ice-rich permafrost near Umiujaq, Nunavik, Quebec, Canada, to assess the permafrost cryostratigraphy. A penetrometer with temperature and piezometer sensors and electrical resistivity module was used for this RTCPTu. Signal processing of RTCPTu logs and forward modelling to simulate the response of the electrical resistivity module penetrating in ice-rich layered medium were performed to discriminate the ice lenses from the layers of frozen silt within permafrost. The cryostructure of permafrost, made of a complex sequence of ice lenses and layers of frozen silt, induces very noisy RTCPTu logs. Ice-rich permafrost is characterized by extremely variable values of cone resistance, friction ratio, pore pressure, and electrical resistivity. Based on the wavelet analysis of RTCPTu logs, the wavelengths of electrical resistivity are shorter than the ones of cone resistance, friction ratio, and pore pressure while all these sensors penetrated the same ice-rich layered medium. As found from forward modelling of the response of an electrical resistivity module penetrating in a resistive ice lens of different thicknesses embedded in conductive frozen silt, the variations in electrical resistivity as a function of depth are more complex than the ones of cone resistance, friction ratio, and pore pressure. This complex response is due to the four electrodes of the electrical resistivity module which are alternately in contact with the ice lens inducing complex electrical current flow and potential induction in this ice-rich layered medium. Knowing the simulated responses of the electrical resistivity module penetrating in ice-rich layered medium, the ice lenses, and layers of frozen silt in ice-rich permafrost were discriminated using cross-correlation between the electrical resistivity log and these simulated responses. Therefore, the cryostratigraphy of permafrost and ice content per penetrated linear meter of 42% were assessed from this RTCPTu.