The dependence of resistivity on pressure is well known in water saturated porous sedimentary rocks. A new electromagnetic fractal model has been employed successfully to study the fractal parameters for double layer polarisation in the low-to-intermediate frequency range. This model addresses the roughness of the pore space and it has been employed to study the over burden effects on the complex resistivity under in situ conditions in the frequency range 10Hz-100kHz and pressure range 1MPa-40MPa. This model includes three relaxation times: bulk relaxation, double layer relaxation and fractal relaxation of pore roughness surfaces; a grain ratio resistivity relating the resistivity of blocking grains to the electrolyte resistivity; the DC resistivity of the rock; the chargeability; and the fractal frequency exponent given by the fractal nature and roughness of the pore surfaces.