Abstract
Seven two-dimensional (2-D) seismic refraction lines were used to determine the thickness and geometry of a valley train outwash deposit of the Quaternary Henry Formation near Heyworth in southern McLean County, Illinois. These refraction data were collected and processed in 2-D, then imported into a Petrel, a three-dimensional (3-D) geological modeling software package. The 3-D geologic model was built using the velocity attribute of the seismic refraction data. The 3-D velocity model was then verified manually by moving a cross-section through the velocity model at 20 m increments. These selected data points were used to create 3-D horizons, surfaces, and contacts constraining the target Henry Formation from the overlying alluvium of the Cahokia Formation and the underlying Delavan Till. Results of the 3-D model show the Henry Formation outwash trends about S10°E, which is oblique to S55°W-trending modern Kickapoo Creek valley. The Henry Formation outwash is confined to the Kickapoo valley, and consists of well-stratified sand and gravel at that is as much as 25 m in thickness in the channel. The thickness of the Henry Formation in the terrace is 8–10 m. The Cahokia Formation is everywhere about 2 m in thickness. The Henry Formation here is interpreted to be deposited in a subglacial tunnel valley that was deposited about 20,000 years ago as the Laurentide ice sheet retreated from its maximum southerly extent.
Highlights
Henry Formation valley-train outwash near Heyworth, Illinois? We answered this question by shooting multiple refraction lines across and around the perimeter of the study site that allowed us too: (1) identify the location and trend of the valley train throughout the study site in 2-D; (2) construct a 2.5-D model allowing for the correlation and calibration of each line to one another and a borehole drilled at the center of the study site (BH01); (3) create a 3-D geologic model using Petrel to determine the volume of the deposit; and (4) propose an interpretation of the genesis of the Henry Formation in this area
Seismic refraction tomography utilizes travel-time inversion of seismic waves transmitted through geologic materials
The Wisconsin Episode glacier retreated from central Illinois beginning about 20,000 years ago and left a series of recessional moraines
Summary
The principal question addressed with this research is: What is the 3-D architecture of the. Geosciences 2016, 6, 9 two targeted refractors confining the unit of interest, refraction seismology is ideal for this ofshallow subsurface investigation because an increasing velocity gradient is expected. In that the refraction velocity data in itself is imported into Petrel for the have only two targeted refractors confining the unit of interest, refraction seismology is ideal for this construction of the 3-Dinvestigation geologic model. This a distinctly different approach than determining the shallow subsurface because an is increasing velocity gradient is expected.
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