1. A method of drilling a borehole, wherein the drilling tool including a gamma ray tool providing gamma ray data within a natural gamma ray sphere of influence, the method comprising the steps of: a) selecting a target formation layer in a formation, wherein the formation includes formation layers having different lithologies; b) proposing a borehole trajectory to the target formation layer; c) drilling the borehole and obtaining real time directional and natural gamma ray data at multiple borehole positions along the borehole trajectory; d) processing the real time directional and natural gamma ray data for each of the borehole positions; e) locating a generally horizontally extending boundary of the target formation layer, wherein the boundary is located between the target formation layer and an adjacent formation layer having a different lithology; f 0 determining whether at least two data points of the boundary of the target formation layer are located within the natural gamma ray sphere of influence; g) using the data points to extrapolate the location of the boundary of the target formation layer ahead of the drilling tool; and h) adjusting the borehole trajectory based on the the extrapolated location of the boundary of the target formation layer to maintain the borehole within the target formation layer.