Precise well positioning is crucial to the success of drilling advanced horizontal and extended-reach wells. To overcome problems related to incorrect positioning of the well path because of uncertainties attached to directional surveys and seismic data, a new measurement-while-drilling (MWD) concept fdr geological positioning has been developed. By applying borehole acoustic reflection techniques, a direct measure of the well position with respect to critical fluid contacts and stratigraphic interfaces parallel to the well can be obtained. The radial detection range has been extended from a few meters with current geosteering tools to about 20 m. Theoretical analysis and tests with a full-scale research prototype have demonstrated the feasibility of this technology.