Delineation of Selected Lithologic Units Using Airborne Electromagnetic Data near Cedar Rapids, Iowa

Scientific Investigations Map 3423
Prepared in cooperation with the City of Cedar Rapids
By: , and 

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Abstract

The U.S. Geological Survey, in cooperation with the City of Cedar Rapids, began a study in 2013 to better understand the effects of drought stress on the Cedar River alluvial aquifer. After an evaluation of the existing groundwater-flow models for the alluvial aquifer, a plan was begun to construct an updated groundwater-flow model capable of evaluating the effect of prolonged drought and increased demand. As part of the effort to update the existing groundwater-flow model, data were collected during an airborne electromagnetic (AEM) survey in May 2017. The study area for the AEM survey encompasses about 53 square kilometers of the Cedar River Basin, west of Cedar Rapids, Iowa, and includes a 19-kilometer reach of the Cedar River. The AEM survey of the Cedar River alluvial aquifer and adjacent areas was completed to characterize the subsurface geology of the area to refine a lithologic framework. The collected AEM data were postprocessed by numerical inversion using the program EM1DFM to produce subsurface apparent resistivity cross sections. Changes observed in resistivity profile values with depth were used to infer lithologic changes and delineate three of the four lithologic units designated in the lithologic framework for this area: alluvial deposits, glacial till, and bedrock; hereafter referred to as the “lithologic framework.” The fourth unit, composed of surficial eolian sediments, was not delineated in these profiles because these units are thin and discontinuous and are not reliably distinguishable from flood plain alluvial deposits. For the purposes of delineating lithologic units using the AEM data, bedrock was assumed to be the lowest unit in a profile, glacial till was deposited on a bedrock surface, and alluvium was deposited on erosional till or bedrock surfaces.

A three-dimensional fence diagram was created as part of the lithologic framework to further define the extent and thickness of the lithologic units near the Cedar River alluvial aquifer. The fence diagram shows a three-dimensional perspective of unit thickness, extent, and orientation of the delineated lithologic framework. A lithologic framework, by design, is intended to represent a simplification of a more complex natural system through data interpolation between known points, which usually are lithologic logs. The resistivity profiles produced from the AEM survey allow for continuous mapping and accurate interpolation of lithology between lithologic logs; however, the apparent resistivity value may reflect several characteristics of subsurface materials including variations in lithology, porosity, water quality, grain sorting, and degree of saturation. In this study, the only variables considered were those related to changes in the subsurface material.

Suggested Citation

Valder, J.F., Haj, A.E., Bristow, E.L., and Valseth, K.J., 2018, Delineation of selected lithologic units using airborne electromagnetic data near Cedar Rapids, Iowa: U.S. Geological Survey Scientific Investigations Map 3423, 2 sheets, 9-p. pamphlet, https://doi.org/10.3133/sim3423.

ISSN: 2329-132X (online)

Study Area

Table of Contents

  • Acknowledgments
  • Abstract
  • Introduction
  • Lithologic Setting
  • Airborne Electromagnetic Investigation Methods
  • Delineation of Selected Lithologic Units Using Airborne Electromagnetic Data
  • Data and Interpretive Limitations
  • References Cited

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Delineation of selected lithologic units using airborne electromagnetic data near Cedar Rapids, Iowa
Series title Scientific Investigations Map
Series number 3423
DOI 10.3133/sim3423
Year Published 2018
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Dakota Water Science Center
Description Pamphlet: vi, 9 p.; 2 Sheets: 42.0 x 24.0 inches and 44.0 x 28.0 inches; Data Release
Country United States
State Iowa
City Cedar Rapids
Online Only (Y/N) Y
Additional Online Files (Y/N) Y