In fact, here you can see the inverse Prony map. Each surface corresponds to a sphere in the four-dimensional moment space with given radius (\(r=\{1.5, 3.5, 5.5, 7.5\}\) for red, green, blue, and purple surfaces respectively) and center (\((1, 1, \mu_U)\)). This means that if \(S_U\) and \(S_V\) belong to inner region of a surface of radius \(r\), the corresponded distance in the moment space will less that \(2r\).