oa Seismic Reflection/Refraction Reconnaissance Survey To Measure Pleistocene Sand Aquifer Thickness Along The Lower Wisconsin River Valley

In response to a request for a rapid, inexpensive assessment of the depth to pre-<br>Pleistocene rock along an approximately 160 km (100 mile) length of the Lower Wisconsin<br>River valley, a field program was devised to make six transects at approximately 32 km (20<br>mile) intervals with the hammer seismic technique. The work was done in 3 weeks of field<br>time and with a similar amount of time for analysis and interpretation. Each transect<br>consisted of a number of 24-channel spreads spaced approximately 0.8 to 1.6 km (0.5 to 1<br>mile) from each other, as budgetary and time conditions precluded continuous profiling.<br>Each spread was a composite of two 12-channel spreads using an older signal-enhancement<br>seismograph and was shot from both ends and from the center. A 6 m (19.6 ft) geophone<br>interval was used, which did not always permit refracted first arrivals to be seen at far<br>geophones, but showed clearly the prominent bedrock reflector even at the maximum offset<br>of 141 m (462 ft). The good signal quality and success of the survey were attributed to good<br>coupling due to the presence of a surface loess or compact soil layer; sufficient rainfall to<br>maintain the surface layer moisture; the homogeneous nature of the underlying san section;<br>and the level sites free of static effects. Refraction analysis gave depth to water table and<br>the sonic velocities of the upper two layers. Reflection analysis of the analog records was<br>done simply by the T2-X2 method and resulted in average velocities and depths to the first<br>prominent reflector. Depths were generally 130 - 200 ft (40 - 61 m) across the entire valley,<br>although one transect did confirm a shallow bedrock terrace extending to one-third of the<br>width of the modern valley. These results are being used to guide further hydrogeological<br>studies and to assist in groundwater flow modeling. This study shows that for certain very<br>special conditions, such shallow noncontinuous seismic profiling can be an extremely rapid<br>and cost effective method of orienting further hydrogeologic investigations.