T85 Advances in Characterization of Groundwater Flow Processes

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room M101AB

INTEGRATED HELICOPTER ELECTROMAGNETIC AND BOREHOLE INTERPRETATION OF AQUIFERS IN ALLUVIAL VALLEYS AND UNDER DISSECTED TILL PLAINS, NEBRASKA, USA

Paper number: T85-14
Program: Technical Sessions
Time: 11:20 AM – 11:35 AM

Authors

Abstract

The Eastern Nebraska Water Resources Assessment (ENWRA) integrates Helicopter Electromagnetic (HEM) resistivity models with borehole data to produce new hydrostratigraphic models of aquifers in eastern Nebraska.

At the Ashland, Nebraska site, in the Platte River Valley, HEM resistivity profiles clearly identify hydrostratigraphic boundaries in Quaternary sediments and underlying bedrock to a depth of 60 meters. Six hydrostratigraphic units are recognized: loess, fine-grained alluvium, fluvial sand and gravel, undifferentiated glacial deposits, sandstone, mudrock, and limestone/shale. Digital 3-dimensional interpretations show that the alluvial aquifer is hydrologically connected to the Dakota Formation sandstone aquifer, except in an area where a discontinuous mudrock unit separates them. The high contrast in resistivity values between aquifer materials and Paleozoic limestones and shales enables major refinement of the base-of-aquifer map.

At the Firth, Nebraska site, on loess-mantled dissected till plains, we delineate hydrostratigraphic units on the basis of HEM resistivity values greater than 20 ohm/m, but the limited depth of HEM investigation restricts the characterization of deep materials to borehole data alone. We identify aquifers at three stratigraphic levels: (1) a lower aquifer, as much as 80 m thick, in sands and gravels filling a W-E trending sub-till paleovalley, and in probable outwash sands and gravels; (2) a localized middle aquifer, as much as 40 m thick, within glacial deposits; and (3) an upper aquifer, as much as 67 m thick, consisting of a broad, W-E-trending, lenticular sand body connected with multiple, smaller “ribbon-like” sand bodies (potential glacial tunnel channels), which are interpreted almost exclusively from HEM data. These putative sand bodies are 60-1500 m in width and15-30 m in thickness. HEM models further lead us to interpret subsurface loess-till contacts, buried fine-grained channel fills, and modern valley constrictions related to the geometry of a till sheet.

At both sites, 3-dimensional renderings of aquifer units reveal complex geometries and relationships undeterminable from borehole data alone. Furthermore, HEM identifies buried high-resistivity units—potential aquifers—that were previously unknown.