Results from borehole geophysical logs, full waveform sonic logs, VSP and laboratory core sample measurements indicate that lithologic variations within the Sudbury Igneous Complex (SIC) and footwall rocks are the primary cause of reflections observed regionally on the multi-channel seismic reflection profiles. The effects of macroscopic fracturing and low-grade alteration on the seismic response are only important to a depth of ~320 m. The major lithologic units of the SIC and footwall rocks in the North Range of the Sudbury structure have contrasting physical properties: Felsic norite and quartz-gabbro have higher Vp (~6300 m/s) than granophyre (~6000 m/s) due to their higher pyroxene content and the presence of quartz rather than calcic plagioclase in the granophyre. Velocities are higher (Vp~6500 m/s) within the brecciated footwall rocks due to an overall increase in mafic mineral content. The contrasting velocities and densities of these units imply that the granophyre/quartz-gabbro contact and the SIC/footwall transition can be mapped regionally using seismic reflection methods. Subunits within these units are also highly reflective, consistent with the nature of the seismic data, but are likely discontinuous laterally. ¿ American Geophysical Union 1994