Detection of fluid flow variations at the Nankai Trough by electric and magnetic measurements in boreholes or at the seafloor

Detection of changes in the flow rate of expelled fluids in accretionary pr isms by monitoring of electric and magnetic fields is discussed. A numerica l model of the electric and magnetic fields associated with fluid flow vari ations at the Nankai Trough is presented which gives a numerical solution o f the coupled system of electric convection currents and conduction current s that directly determines the magnetic anomaly itself. Measurements in a b orehole located between two vents are shown to be well adapted to detection of fluid flow variations using the vertical gradient of the electric poten tial and the horizontal magnetic field. The vertical electric field is abou t 10 mV/km up to 500 m depth where there is a lithologic reflector and abou t 50 mV/km below this reflector. The horizontal gradient of the magnetic fi eld is 2 nT/km at the seafloor. Modelization with a lower fault conductivit y and a larger decollement thickness has also been modeled. The vertical gr adient of the horizontal magnetic field is similar to 5 to 15 nT/km. A vari ation of 3 mV and 1.5 to 3 nT at 600 m depth in a borehole could reveal a f luid flow rate variation of 20%, which is a reasonable fluid flour change b ased on some observations at short-scale time. Since a 1.5 to 3 nT anomaly seems easier to detect than a 3 mV anomaly, it is likely that the variation of the magnetic field would more sensitively reveal fluid flow variations. When monitoring the magnetic field at the seafloor, a change of 0.4 nT/km in the horizontal gradient could reveal a fluid flow rate variation of 20%.