Source driven problems for weakly non-separable waveguiding environments have been addressed in a number of ways. Data generated via various numerical codes occasionally lead to confusion due to discrepancies between them while apparently dealing with the same problem. Recently a group of benchmark problems has been introduced [J. Acoust. Sec. Am., Vol. 87, No. 4, 1497-1545, April 1990] as a frame of reference wherein different approaches could be set to test and their respective merits could be assessed with some confidence. In this paper we undertake one of these benchmark problems (the non-penetrable wedge) and construct two and three dimensional Green's function representations in terms of intrinsic modes (IM). Our numerical results indicate that IM formulations are capable of providing approximate Green's function representations for weakly non-separable waveguiding environments that are accurate everywhere except in the immediate vicinity of sources.