Ceresco, Nebraska is located approximately 15 miles north of Lincoln, Nebraska. The former USDA CCC site is located on the north edge of the town. Carbon tetrachloride is found in the soil and water samples from wells and test borings in the vicinity of the site and in Ceresco. Concentrations of carbon tetrachloride above MCL have been detected in former public water supply wells. Indications are that the plume is moving south and westward along the eastern edge of the village. To aid in the understanding of the subsurface geology and determine the contaminant flow pathways, a high-resolution shear wave seismic reflection survey was proposed.

Approximately 35-70 feet of Pleistocene age sediments overlie Cretaceous age bedrock. The Pleistocene overburden is composed of glacial outwash deposits, fluvial silts, and interbedded sands and clays capped with a surface layer of loess. The bedrock is the Upper Cretaceous Dakota Formation. At the top of bedrock unconformity the Dakota is either shale or sandstone. The latter probably representing channel deposits. Basal Dakota sandstone found in some borings is interpreted to be laterally continuous. Pre-Pleistocene bedrock valleys have incised into the Dakota Formation east and south of Ceresco. The public water supply wells are completed in the Dakota Formation.

Objective:

The objectives of the seismic survey were to:

• Determine if shear wave seismic reflection data can image the sand features located in the Dakota Formation at or near the contact with the overlying Pleistocene deposits. To use seismic data to assist in mapping the areal extent of these features.

• To use seismic reflection in an attempt to image areas along the contact between the Dakota Formation and the Pleistocene that may suggest hydraulic commutation between the two stratigraphic intervals.

• To test the feasibility of conducting seismic reflection processing in the field in conjunction with other geological / geophysical data collection efforts.

Shear Wave Seismic Data

Four high-resolution shear wave seismic reflection profiles, 2207 CDPs (72 fold), were collected in addition to 2 VSPs. Most of the seismic data were collected using a high frequency shear wave vibrator sweeping 25-225 Hz. A part of Line 1 was collected with a sledge hammer because of access problems. Seismic data acquisition began July 28, 1998 by Bay Geophysical Associates and was completed August 6. The data were processed in the field as acquisition was progressing. After preliminary interpretations were made in the field, ESRI and Bay Geophysical Associates did extensive processing and the final interpretation. 

The seismic data were combined with existing well data to construct a more comprehensive depth to bedrock map. The seismic data revealed areas along the contact between the Pleistocene and the Dakota (bedrock) where the Pleistocene has incised into the bedrock. These areas may act as a conduit for groundwater and contaminants to migrate into the Dakota aquifer. Furthermore, relative stratigraphic sequences within both the shallow portions of the Dakota and the overlying Pleistocene deposits were identified. Lastly, the in-the-field data processing resulted in stack sections that were interpretable and of sufficient quality that major reflecting boundaries were recognized.

(Final stack seismic sections of Line 2 (top) and Line 3 (bottom). Major reflecting interfaces are identified. Note the irregularities (channels, scours) in the Pleistocene section. Horizontal distance approx. 510 ft.)