The "hills" and the "tubes" that follows the smooth "grooves" in it are made with 2 isosurfaces. The basis function for the "terrain" in this image is a "sombrero" function that has been displaced vertically by a noise function. The grooves are made by subtracting the function for the "tubes" from the function for the "hills" with the help of a "blobbing" function; the hyperbolic tangent function. The tanh() function has a S-shape that is similar to the shape of the Sigmoid function. Here are two relevant links: [1] [2].