Electrokinetic signature of the Nankai Trough accretionary complex: preliminary modelling for the Kaiko-Tokai program

Abstract

Fluid flow in the Nankai Trough accretionary complex has been observed during the previous phases of the Kaiko project to be of order 100 m year −1 in small-scale high-permeability conduits, and 10 m year −1 where shallow detachment faults intersect the seafloor. The rate of flow was also variable, increasing up to 30% over a month. Monitoring the fluid flow will be one of the principal objectives of the next phase of the Kaiko program, known as Kaiko-Tokai. One method is to measure the electric and magnetic fields generated by the electrokinetic phenomena (or streaming potential), resulting from fluid flow in the porous sediments. This paper describes modelling of fluid flow in the accretionary prism and the resulting electrokinetic signals which may be observed from sensors on the seafloor or in shallow boreholes. A two-dimensional finite-difference model is used to study the hydrogeology of a realistic section of the Nankai Trough, with constraints from seismic reflection data and Ocean Drilling Program boreholes. Fluid sources owing to porosity reduction in the prism and of distant external origin are modelled. We find that porosity reduction alone is insufficient to reproduced the observational estimates of excess pore pressure and fluid flux. Additional fluid source terms are therefore required along a low-angle décollement, or detachment fault, which separates the deformed prism sediments from the undeformed underthrust sediments beneath. The décollement must be significantly more permeable than the adjacent sediments (our models suggest by approximately four orders of magnitude), and is probably anisotropic parallel to the main structural trends. From the hydrogeological model, the electric potential and electric and magnetic fields within the ocean and sediments were calculated from the electrokinetic phenomena. Seafloor electric potentials are found to be of the order of a few millivolts, with seafloor horizontal and vertical electric fields of the order of 0.5 μVm −1 close to the deformation front. As the electric potential rises significantly with depth through the sediments, boreole measurements may be particularly useful. We conclude with some comments about noise sources and experimental logistics for the Kaiko-Tokai program.