Seismic Response of Pennsylvanian Cyclothems in Douglas County, Kansas

Abstract

Middle and Upper Pennsylvanian cyclic sequences (cyclothems) in eastern Kansas are ineffectively imaged on standard petroleum-exploration reflection seismogram sections with frequency responses less than 80 Hz. The Kansas City and Lansing Groups, despite the number of interlayered high-contrast beds they contain, often cannot be distinguished and are grouped as a single characteristic response. However, the use of high-resolution reflection seismology, with dominant frequencies of 250-300 Hz and maximum frequencies to greater than 500 Hz, allows the detection of thin-bed units within the individual groups and formations to approximately 2-m thickness. Alternating thin limestones and shales that comprise cyclothems lead to resonance of the seismic wave. Geological cyclicity and reflection seismology harmonize, and carbonate units are recorded as peaks and siliciclastic units (mostly shales) are recorded as troughs. Further, seismic response is sensitive to such phenomena as the presence of intrabed sandstone lenses in shales and the surface roughness (diffusivity) of lithologic interfaces. The use of an instantaneous phase response display accentuates low-amplitude interbed features such as channels within sandstones and shales.

The ability of high-frequency seismic data to resolve features within the stratigraphically heterogeneous Pennsylvanian of eastern Kansas is demonstrated. To effectively image at the scale of variability of these strata (meters to tens of meters), resolution must be maximized and spurious noise minimized. Acquisition, processing, and display must all aim to this purpose. Small-scale features that are observed on a seismic section acquired across a geologically well-defined stratigraphic interval are documented, and observations of reflection character are correlated to observed geology.