EXAMPLES OF FRACTURE CONNECTIVITY IN WISCONSIN’S PRECAMBRIAN CRYSTALLINE BEDROCK

Fracture networks are the primary means of groundwater flow in crystalline bedrock. However, it can be difficult to predict connections between fractures identified in boreholes. Here at the Wisconsin Geological and Natural History Survey, we are using borehole geophysics and flow logging to better understand these fracture networks.

We present two examples of fractures connectivity in crystalline bedrock that were not anticipated. The first example is from a 1000 foot deep well in Price County in northern Wisconsin. This well had several small hydraulically connected fractures at depths of around 100 feet and a larger fracture at 330 feet. Flow logging and subsequent packer testing verified that the shallow fracture network was not well connected to the deeper fracture. The head in the deeper fracture is consistently 5 feet higher than the shallow fracture network. The heads in the shallow fracture network are nearly the same as those seen in a wetland located about 100 feet from the well. The higher head in the deep fracture could only be from higher elevation wetlands located several thousand feet from the well, implying a better connection to those wetlands than the nearby wetlands. The second example is from three test wells for Pittsville, WI. These wells were all drilled in a line. Flows to two of the wells were dominated by a single fracture while flows were observed in four separate fractures in the third well. Pumping from the central well, while measuring flows in the third well showed that two of the deep fractures in the third well were connected to the pumping well while the two shallow fractures in the third well were not, even though the shallow fractures were at a similar elevation as the fracture in the pumping well. Our structural analyses show that each of these three boreholes have unique fracture orientation patterns with little or no similarity to each other. Also, the dominant fracture orientations change with depth in all three boreholes.

These two examples illustrate some of the challenges present in characterizing fracture connectivity in Wisconsin’s crystalline rocks and demonstrate the utility of using borehole geophysics and flow logging to understand fracture networks. As such, we recommend these techniques be applied when considering groundwater flow in crystalline bedrock.