Two orthogonal wide-aperture seismic refraction lines were acquired in the New Madrid Seismic Zone by the U.S. Geological Survey in 1991. These N-S and E-W oriented lines cross the Blytheville Arch and adjacent portions of the Reelfoot Rift. Ten shots were fired along the E-W line; eight shots were fired along the N-S line. Iterative numerical modeling, including scattering and viscoelasticity, has been performed for all shots on both lines using a new eighth-order staggered-grid (two dimensional) finite difference solution based on superposition of relaxation mechanisms. The main features in the data can be explained by the existence of a strong 15-km-wide low-velocity cap lying beneath the center of the Blytheville Arch between 1.5 and 2.5 km depth; the data do not allow estimation of its extent along the axis of the arch, but it is at least 30 km long and appears to become less prominant toward the southwest. The arch itself has a low-velocity background that extends to at least 5 km depth. An additional low-velocity layer (the Elvins shale) lies generally between 3 and 4 km depth, is upwarped at the edges of the arch, and thins toward the edges of the Reelfoot Rift. There is no evidence for anomalously low intrinsic Q, but the data may be interpreted to indicate strong local scattering that occurs west of the low-velocity cap of the Blytheville Arch at depths that are similar to, and slightly shallower than, the cap. Additional strong scattering occurs at the western edge of the Reelfoot Rift. An alternative interpretation attributes the apparent scattering at the Blytheville Arch to instrument noise. ¿ American Geophysical Union 1996