Abstract

We use data recorded by four arrays of portable instruments to investigate the propagation of short period (0.2≤T ≤2.0 sec) surface waves within the Ozark Uplift and Illinois Basin. At the regional scale, we construct group velocity dispersion curves for five suites of propagation paths, and invert them for shear velocity structure. The best model in each case consists of a single layer above a halfspace, and we can correlate the model units with geologic formations. The upper layer in the two Ozark Uplift models represents Ordovician and Cambrian carbonate strata, the halfspace corresponds to the Precambrian crystalline basement. Differences between our new models and an earlier one reflect the different parts of uplift that were sampled, and show the thickening of the Paleozoic section away from the uplift core. In the Illinois Basin models, the upper layer represents Pennsylvanian age elastics and the halfspace represents older Paleozoic carbonates. This interpretation is substantiated by a velocity log from a nearby deep well. Again, differences between our new models and earlier efforts result from different passband data that sample different parts of the basin.

We also extract interstation phase velocities from array data recorded at the western edge of the Illinois Basin, over Mississippian age outcrop. By comparing this local dispersion curve with one calculated from the appropriate regional scale model, we conclude that the local structure can also be modeled as a single layer above a halfspace. Local shear velocities are, however, 30% faster than the regional average, which reflects the absence of the slow Pennsylvanian elastic strata around the basin’s periphery.

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