Features in Kansas cyclothems seen by high-resolution seismology

Accurate quantitative modeling of stratal sequences requires verification of model results by means of comparisons with known stratigraphic intervals, and in many cases seismic data provide the best or only means toward that end. In eastern Kansas the scale of variability within Middle and Upper Pennsylvanian units is of the order of meters. Unraveling the stratigraphy and thereby verifying models at such a scale requires high-resolution seismic data. Middle and Upper Pennsylvanian cyclic sequences (cyclothems) are seen as just a few wiggles on standard petroleum exploration reflection seismogram sections with frequency responses of <80 Hz. For instance, the Kansas City and Lansing Groups cannot be distinguished. However, the use of high-resolution reflection seismology, with frequencies up to 500 Hz, allows the detection of thin-bed members of the individual groups and formations to approximately 2-m (7-ft) thickness. Geologic cyclicity and reflection seismology harmonize, and carbonate units are seen as peaks and siliciclastic units (mostly shales) as troughs. Seismic response becomes sensitive to phenomena such as the presence of intrabed sandstone lenses in the shales and the surface roughness (diffusivity) of lithologic interfaces. Complex trace analysis aids the interpretation of bed thickness and the nature and continuity of geologic boundaries. Reflection strength helps determine which geologic boundaries form strong reflectors and therefore have clean, flat interfaces. Instantaneous phase complements reflection strength by showing strong trace-to-trace coherence and by emphasizing stratal truncations, indicating intrabed structure and sandstone channel presence. Instantaneous frequency indicates (1) intrabed structure and sandstone channel presence when it displays a chaotic pattern and (2) the dominant frequency of the reflector response when coherent. Dominant frequency of the reflector response is frequently useful in determining bed thickness

Features in Kansas cyclothems seen by high-resolution seismology

Accurate quantitative modeling of stratal sequences requires verification of model results by means of comparisons with known stratigraphic intervals, and in many cases seismic data provide the best or only means toward that end. In eastern Kansas the scale of variability within Middle and Upper Pennsylvanian units is of the order of meters. Unraveling the stratigraphy and thereby verifying models at such a scale requires high-resolution seismic data. Middle and Upper Pennsylvanian cyclic sequences (cyclothems) are seen as just a few wiggles on standard petroleum exploration reflection seismogram sections with frequency responses of <80 Hz. For instance, the Kansas City and Lansing Groups cannot be distinguished. However, the use of high-resolution reflection seismology, with frequencies up to 500 Hz, allows the detection of thin-bed members of the individual groups and formations to approximately 2-m (7-ft) thickness. Geologic cyclicity and reflection seismology harmonize, and carbonate units are seen as peaks and siliciclastic units (mostly shales) as troughs. Seismic response becomes sensitive to phenomena such as the presence of intrabed sandstone lenses in the shales and the surface roughness (diffusivity) of lithologic interfaces. Complex trace analysis aids the interpretation of bed thickness and the nature and continuity of geologic boundaries. Reflection strength helps determine which geologic boundaries form strong reflectors and therefore have clean, flat interfaces. Instantaneous phase complements reflection strength by showing strong trace-to-trace coherence and by emphasizing stratal truncations, indicating intrabed structure and sandstone channel presence. Instantaneous frequency indicates (1) intrabed structure and sandstone channel presence when it displays a chaotic pattern and (2) the dominant frequency of the reflector response when coherent. Dominant frequency of the reflector response is frequently useful in determining bed thickness

High-resolution seismic study in Miami County, Kansas

Two high-resolution seismic lines were shot by the Kansas Geological Survey near Wellsville, Kansas, during 1981 and 1982. The study site is located near the southeast margin of the Forest City basin. The data were collected using the MiniSOSIE surface seismic-energy source. The purpose of the study was to determine if this high-resolution seismic technique was capable of resolving the presence of thin sandstone bodies, called squirrel sands, located at the top of the Cherokee Group of the Upper Pennsylvanian. The sandstone bodies located within strata were deposited under cyclic conditions associated with the rise and fall of sea level. Seismic data were recorded using both source and geophone arrays to attenuate the contribution of nonvertical seismic waves. Severe low-cut (110 Hz, 24 dB/octave) filters were used to enhance the high-frequency information. After editing and muting, processing sequences were identical for each line. Final conventional stacks produced a coherent record of the subsurface. Because of a lack of sonic well-log data, an algorithm relating neutron porosity to seismic velocity was established for northeastern Kansas. The algorithm was used to produce a pseudo velocity log for a well near the seismic lines. Synthetic seismograms were created from the pseudo velocity log to aid in the interpretation of the seismic data. Analysis of the synthetic seismic data reveals that primary energy is severely attenuated by the cyclothems of the Late Pennsylvanian. There is a 12-dB decrease in transmitted energy at the top of the Cherokee Group resulting from the effects of transmission loss. As a result, it appears that the interbed multiple reflected signal becomes the dominant reflected energy. The seismic data reveal the presence of an anticline as well as several faults in the pre-Mississippian strata. It appears that it is possible to seismically locate squirrel sands that have been deposited in channels using MiniSOSIE. The presence of these channels in the subsurface results in a dimming of the overlying Fort Scott composite reflection due to interference

High-resolution seismic study in Miami County, Kansas

Two high-resolution seismic lines were shot by the Kansas Geological Survey near Wellsville, Kansas, during 1981 and 1982. The study site is located near the southeast margin of the Forest City basin. The data were collected using the MiniSOSIE surface seismic-energy source. The purpose of the study was to determine if this high-resolution seismic technique was capable of resolving the presence of thin sandstone bodies, called squirrel sands, located at the top of the Cherokee Group of the Upper Pennsylvanian. The sandstone bodies located within strata were deposited under cyclic conditions associated with the rise and fall of sea level. Seismic data were recorded using both source and geophone arrays to attenuate the contribution of nonvertical seismic waves. Severe low-cut (110 Hz, 24 dB/octave) filters were used to enhance the high-frequency information. After editing and muting, processing sequences were identical for each line. Final conventional stacks produced a coherent record of the subsurface. Because of a lack of sonic well-log data, an algorithm relating neutron porosity to seismic velocity was established for northeastern Kansas. The algorithm was used to produce a pseudo velocity log for a well near the seismic lines. Synthetic seismograms were created from the pseudo velocity log to aid in the interpretation of the seismic data. Analysis of the synthetic seismic data reveals that primary energy is severely attenuated by the cyclothems of the Late Pennsylvanian. There is a 12-dB decrease in transmitted energy at the top of the Cherokee Group resulting from the effects of transmission loss. As a result, it appears that the interbed multiple reflected signal becomes the dominant reflected energy. The seismic data reveal the presence of an anticline as well as several faults in the pre-Mississippian strata. It appears that it is possible to seismically locate squirrel sands that have been deposited in channels using MiniSOSIE. The presence of these channels in the subsurface results in a dimming of the overlying Fort Scott composite reflection due to interference

Seismic-reflection study in Rice County, Kansas

During the summer of 1983, a MiniSOSIE seismic-reflection study was conducted in Rice County in which an 11.2-km (7-mi) 12-fold common depth point (CDP) profile was shot to investigate several local structural and stratigraphic features. The seismic line was oriented east-west, perpendicular to the local structural grain. Several units, ranging from the Arbuckle through the Mississippian limestones, subcrop beneath the basal Pennsylvanian angular unconformity in this area. The subcrop pattern is dominantly north-south and is related to the eastward dip of these units off the Central Kansas uplift. Reflectors in excess of 1,070-m (3,500-ft) depth are detectable on the seismic profile. The deepest reflectors (0.850 secs) correspond to the Precambrian Rice Formation. Good reflectors occur in the lower Paleozoic section corresponding to a local limestone in the Chattanooga Shale and the subjacent Maquoketa-Viola formations. Several limestones in the Upper Pennsylvanian and Permian section also are good reflectors of seismic energy. Stratigraphic features such as local thinning or thickening and channel cuts can be detected in this part of the stratigraphic section. The Lyons anticline, a local north-south-trending structure currently used for gas storage, also is expressed on the seismic line. The seismic profile shows the structural history of this anticline to be long and complex. Initially, the anticline was a broad, symmetric feature possibly related to the formation of the Precambrian Keweenawan rift. Minor growth may have occurred prior to the deposition of the Chattanooga Shale. A third major period of movement occurred during Late Mississippian to Early Pennsylvanian time when a reverse fault developed on the west flank of the structure, thereby making the structure an asymmetric anticline. Minor structural movement occurred again subsequent to the development of the basal Pennsylvanian angular unconformity

Seismic-reflection study in Rice County, Kansas

During the summer of 1983, a MiniSOSIE seismic-reflection study was conducted in Rice County in which an 11.2-km (7-mi) 12-fold common depth point (CDP) profile was shot to investigate several local structural and stratigraphic features. The seismic line was oriented east-west, perpendicular to the local structural grain. Several units, ranging from the Arbuckle through the Mississippian limestones, subcrop beneath the basal Pennsylvanian angular unconformity in this area. The subcrop pattern is dominantly north-south and is related to the eastward dip of these units off the Central Kansas uplift. Reflectors in excess of 1,070-m (3,500-ft) depth are detectable on the seismic profile. The deepest reflectors (0.850 secs) correspond to the Precambrian Rice Formation. Good reflectors occur in the lower Paleozoic section corresponding to a local limestone in the Chattanooga Shale and the subjacent Maquoketa-Viola formations. Several limestones in the Upper Pennsylvanian and Permian section also are good reflectors of seismic energy. Stratigraphic features such as local thinning or thickening and channel cuts can be detected in this part of the stratigraphic section. The Lyons anticline, a local north-south-trending structure currently used for gas storage, also is expressed on the seismic line. The seismic profile shows the structural history of this anticline to be long and complex. Initially, the anticline was a broad, symmetric feature possibly related to the formation of the Precambrian Keweenawan rift. Minor growth may have occurred prior to the deposition of the Chattanooga Shale. A third major period of movement occurred during Late Mississippian to Early Pennsylvanian time when a reverse fault developed on the west flank of the structure, thereby making the structure an asymmetric anticline. Minor structural movement occurred again subsequent to the development of the basal Pennsylvanian angular unconformity

Geophysical model of the Midcontinent Geophysical Anomaly in northeastern Kansas

A geophysical model of the Midcontinent Geophysical Anomaly (MGA) in northeastern Kansas was derived to fit gravity and magnetic data using an initial model suggested from COCORP seismic sections, along with available drill data. An asymmetric basin filled with interbedded basaltic and clastic rocks in the shallow crust, interpreted by Serpa et al. (1984), appears to be mainly responsible for the primary positive magnetic anomaly of the MGA. The favorable magnetization of 60° inclination and 320° declination indicates that the remanent magnetization is an important factor. Reprocessing part of COCORP seismic data reveals a possible mafic intrusion in the shallow crust beneath a secondary magnetic high. Its favorable net magnetization of 30° inclination and 80° declination suggests it occurred in a different ambient earth's magnetic field than that in which the basaltic rift basin was formed. In addition, the nonmagnetic source of high density at midcrustal levels extending to deep crust and the nonmagnetic Rice Formation basins of low density at shallow crust on both sides of the MGA were proposed to fit the gravity anomaly. The final derived model infers the mafic intrusion in the shallow crust beneath the secondary magnetic high to be of younger age than the Rice Formation

Geophysical model of the Midcontinent Geophysical Anomaly in northeastern Kansas

A geophysical model of the Midcontinent Geophysical Anomaly (MGA) in northeastern Kansas was derived to fit gravity and magnetic data using an initial model suggested from COCORP seismic sections, along with available drill data. An asymmetric basin filled with interbedded basaltic and clastic rocks in the shallow crust, interpreted by Serpa et al. (1984), appears to be mainly responsible for the primary positive magnetic anomaly of the MGA. The favorable magnetization of 60° inclination and 320° declination indicates that the remanent magnetization is an important factor. Reprocessing part of COCORP seismic data reveals a possible mafic intrusion in the shallow crust beneath a secondary magnetic high. Its favorable net magnetization of 30° inclination and 80° declination suggests it occurred in a different ambient earth's magnetic field than that in which the basaltic rift basin was formed. In addition, the nonmagnetic source of high density at midcrustal levels extending to deep crust and the nonmagnetic Rice Formation basins of low density at shallow crust on both sides of the MGA were proposed to fit the gravity anomaly. The final derived model infers the mafic intrusion in the shallow crust beneath the secondary magnetic high to be of younger age than the Rice Formation

Seismic-reflection surveys at sinkholes in central Kansas

Salt-dissolution sinkholes have developed at many localities in Kansas during the past 25 years. Most of the sinkholes subside gradually over a period of years, although catastrophic collapse has occurred in some cases. We have performed high-resolution seismic-reflection surveys across more than a half dozen of these sinkholes. It is possible to discern considerable geologic detail at depths of 50 to 1,000 m (160-3,300 ft) within the sinkholes by seismic-reflection methods. At one site astride I-70, we obtained acoustic images of grabens within the sinkhole that showed approximately 40 to 50 m (130-160 ft) of vertical downdrop at a depth of 400 m (1,300 ft) in an area where surface displacement was less than 5 m (16 ft). At another site we detected two paleosinkholes adjacent to a presently active sink. The paleosinks are filled with alluvial material probably of Pleistocene age; one of them shows indications of two different geologic ages of active sinking. While many of the new sinkholes that have formed appear to be related to oil-field brine disposal or salt-solution mining activities, the detection of the paleosinks by seismic-reflection methods reconfirms the natural occurrence of some salt-dissolution sinkholes in Kansas prior to the encroachment of civilization

Seismic-reflection surveys at sinkholes in central Kansas

Salt-dissolution sinkholes have developed at many localities in Kansas during the past 25 years. Most of the sinkholes subside gradually over a period of years, although catastrophic collapse has occurred in some cases. We have performed high-resolution seismic-reflection surveys across more than a half dozen of these sinkholes. It is possible to discern considerable geologic detail at depths of 50 to 1,000 m (160-3,300 ft) within the sinkholes by seismic-reflection methods. At one site astride I-70, we obtained acoustic images of grabens within the sinkhole that showed approximately 40 to 50 m (130-160 ft) of vertical downdrop at a depth of 400 m (1,300 ft) in an area where surface displacement was less than 5 m (16 ft). At another site we detected two paleosinkholes adjacent to a presently active sink. The paleosinks are filled with alluvial material probably of Pleistocene age; one of them shows indications of two different geologic ages of active sinking. While many of the new sinkholes that have formed appear to be related to oil-field brine disposal or salt-solution mining activities, the detection of the paleosinks by seismic-reflection methods reconfirms the natural occurrence of some salt-dissolution sinkholes in Kansas prior to the encroachment of civilization