The subsurface contrast between Madison’s Isthmus and the West Side is stark. Downtown, near the Capitol, you hit weathered sandstone and dolomite within 20 feet. Move west toward Middleton and the glacial drift thickens to over 100 feet of interbedded silt, sand, and till, often masking irregular bedrock topography. Seismic tomography bridges this uncertainty. Our team runs P-wave refraction and SH-wave reflection surveys to delineate the contact between unconsolidated Quaternary deposits and the Paleozoic bedrock beneath the Yahara River valley. When drill spacing is limited, a MASW survey combined with refraction profiles gives us both a velocity model and VS30 for seismic site class per IBC Chapter 16, which is critical in a city where the drift–bedrock interface can shift 40 feet across a single city block.
We routinely map the 1,200 m/s contour as the top of competent rock across Madison’s glacial terrain, resolving velocity contrasts that boreholes alone miss.
