1. A method for generating a model function h(x,y,z) implicitly representing geologic horizons, the method comprising: receiving geological data representing a fault network and horizons automatically extracted from seismic data in a studied domain; generating a three-dimensional (3D) mesh covering the studied domain, wherein the 3D mesh is divided into a plurality of fault blocks by the fault network; defining a discrete function h(x,y,z) to have values of the geological data representing horizons at discrete nodes of the 3D mesh; installing normal-direction constraints on the discrete function h(x,y,z) defining surfaces representing horizons, wherein the normal-direction constraints constrain the gradient of the discrete function h(x,y,z) at given locations to be parallel to and oriented in the same direction as vectors normal to planes tangent to horizons at the given locations, without defining the relative positions of the surfaces representing horizons; interpolating the discrete function h(x,y,z) at the nodes of the 3D mesh to create a piecewise continuous function h(x,y,z) that honors the constraints; simultaneously generating a plurality of geological horizons implicitly represented as a plurality of level set surfaces of the discrete function h(x,y,z); and displaying a model of the piecewise continuous horizon function h(x,y,z).