Prepared in cooperation with the City of Cedar Rapids

Delineation of Selected Lithologic Units Using Airborne 

Electromagnetic Data near Cedar Rapids, Iowa

Pamphlet to accompany 

Scientific Investigations Map 3423

U.S. Department of the Interior

U.S. Geological Survey
Front cover. Helicopter and the RESOLVE frequency-domain “bird” lifting off from airport to begin 

data collection survey (Photograph taken by J. Valder, U.S. Geological Survey).

Back cover. Helicopter used in airborne electromagnetic study (Photograph taken by Bruce 

Jacobs, City of Cedar Rapids).
Delineation of Selected Lithologic Units 

Using Airborne Electromagnetic Data near 

Cedar Rapids, Iowa

By Joshua F. Valder, Adel E. Haj, Emilia L. Bristow, and Kristen J. Valseth

Prepared in cooperation with the City of Cedar Rapids

Scientific Investigations Map 3423

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

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)
iii

Acknowledgments

The authors thank the City of Cedar Rapids, Iowa, for their support of past and ongoing studies 

that provided valuable information for this study.

Several personnel at the U.S. Geological Survey, including David Smith, Bruce Smith, and Maria 

Deszcz-Pan, assisted with the collection, processing, and analysis of geophysical data. Additionally, Janet Carter, Greg Delzer, Kymm Barnes, Nancy Stamm, and Galen Hoogestraat (U.S. 

Geological Survey) provided insightful edits and comments for this report.

v

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Location and Description of Study Area...........................................................................................2

Lithologic Setting ...........................................................................................................................................2

Airborne Electromagnetic Investigation Methods...................................................................................5

Delineation of Selected Lithologic Units Using Airborne Electromagnetic Data................................6

Delineation of Lithologic Units in Resistivity Profiles .....................................................................6

Map Interpolation from Lithologic Units ...........................................................................................8

Data and Interpretive Limitations................................................................................................................8

References Cited............................................................................................................................................8

Figures

1. Map showing study area for airborne electromagnetic survey, location of 

wells with lithologic information, and location of streamgages...........................................3

2. Photograph showing the RESOLVE frequency-domain airborne 

electromagnetic system (the “bird”) used to transmit and receive 

electromagnetic energy to characterize the Cedar River alluvial aquifer, 

suspended from the helicopter ..................................................................................................4

3. Map showing airborne electromagnetic survey lines............................................................7

4. Examples of delineated lithologic changes interpreted from four selected 

resistivity profiles within the study area........................................................................sheet 1

5. Interpolated maps of the study area ..............................................................................sheet 2

Table

1. Well identification for 25 wells with lithologic logs in the study area .................................5
vi

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2) 0.3861 square mile (mi2)

Datum

Vertical coordinate information is referenced to the Geodetic Reference System 1980 (GRS 80).

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.

Abbreviations

AEM airborne electromagnetic

USGS U.S. Geological Survey
Delineation of Selected Lithologic Units Using Airborne 

Electromagnetic Data near Cedar Rapids, Iowa

By Joshua F. Valder, Adel E. Haj, Emilia L. Bristow, and Kristen J. Valseth

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.

Introduction

The Cedar River alluvial aquifer is the principal source 

of municipal water supply for the city of Cedar Rapids, Iowa. 

The alluvial aquifer, hereafter defined in this report as “alluvial deposits,” is generally composed of sands and gravels 

with spatially discontinuous overlying eolian sediments near 

valley margins where wind has reworked the older surficial 

alluvial deposits (Schulmeyer and Schnoebelen, 1998; Quade 

and others, 1998). Historically, the aquifer has been capable of 

supporting the water supply needs of the city of Cedar Rapids 

and the surrounding area, which has an estimated population 

of about 126,000 as of 2010 (U.S. Census Bureau, 2018). 

Between July 2011 and February 2013, Iowa experienced 

severe drought conditions (generally referred to as the “2012 

drought”) that affected water availability throughout Iowa 

for those communities that rely on alluvial aquifers to supply 

their water needs (Hillaker, 2012). During this time, the city of 

Cedar Rapids observed water-level declines in their production wells (B. Jacobs, City of Cedar Rapids, written commun., 

2018), raising concern about the reliability of the alluvial aquifer. Riverbed leakage accounts for an estimated 74 percent of 

recharge to the alluvial aquifer (Schulmeyer and Schnoebelen, 

1998). As the streamflow in the Cedar Rapids area decreases, 

riverbed leakage to the underlying alluvial aquifer decreases 

and water levels in the alluvial aquifer decline (Schulmeyer 

and Schnoebelen, 1998). As a result, the underlying carbonate 

bedrock, hereafter referred to as “bedrock,” may have become 

the primary source of recharge to the alluvial aquifer. The 

bedrock is generally composed of 22 lithologic formations 

ranging from Silurian to Devonian in age (University of Iowa, 

2013; Tucci and McKay, 2006; Iowa Department of Natural 

Resources, 2004).
2 Delineation of Selected Lithologic Units Using Airborne Electromagnetic Data near Cedar Rapids, Iowa

The U.S. Geological Survey (USGS), 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. Previously published groundwater-flow models provided flow analysis based on the existing understanding of the physical properties of the alluvial aquifer and the 

effects of pumping on source waters (Schulmeyer and Schnoebelen, 1998; Turco and Buchmiller, 2004). After an evaluation of the existing groundwater-flow models for the alluvial 

aquifer (Schulmeyer and Schnoebelen, 1998; Turco and 

Buchmiller, 2004), a plan was begun to construct an updated 

groundwater-flow model capable of evaluating the effects of 

prolonged drought and increased demand. As part of the effort 

to update the existing groundwater-flow models, data were 

collected during an airborne electromagnetic (AEM) survey 

in May 2017 (Deszcz-Pan and others, 2018; fig. 1) to better 

characterize subsurface materials and more accurately define 

the extent of the alluvial aquifer near Cedar Rapids. A total of 

25 lithologic logs available from the Iowa Geological Survey 

(2018) were used for interpretation and validation of the AEM 

data collection (fig. 1).

An AEM survey can overcome some of the time and 

land-access constraints of ground-based geophysics and can 

be used to produce a comprehensive and continuous dataset 

for delineating horizontal extents and vertical depths of the 

aquifers and characterizing landscape-scale geologic structures (Valseth and others, 2018). In the AEM method, electromagnetic signals transmitted from an airborne device induce 

currents in the subsurface that are then measured by receivers in the device. In the RESOLVE frequency-domain AEM 

system used for this survey, the transmitters and receivers are 

housed in an apparatus, referred to as a “bird,” suspended by 

a tow line about 30 meters (m) below a helicopter and about 

30 m above the ground (Valseth and others, 2018; fig. 2). The 

data collected by the AEM system indicate apparent electrical 

resistivity of the subsurface, and these data can be interpreted 

to delineate lithologic boundaries when processed (Valder and 

others, 2016; Valseth and others, 2018).

The City of Cedar Rapids contracted with CGG Canada 

Services Ltd. to collect the AEM data in May 2017. CGG 

Canada Services Ltd. provided the final AEM dataset to the 

City of Cedar Rapids and the USGS, who inverted the data 

using the program EM1DFM (Farquharson, 2000) to produce 

resistivity depth sections for three-dimensional visualization 

of subsurface electrical variations. Data generated during this 

study are available as a USGS data release (Deszcz-Pan and 

others, 2018).

Purpose and Scope

The purpose of this report is to present the delineation 

of selected lithologic units (alluvial deposits, glacial till, and 

bedrock) using AEM data collected in May 2017 near Cedar 

Rapids, Iowa (fig. 1). Data collected during the 2017 AEM 

survey are presented in a separate data release (Deszcz-Pan 

and others, 2018). This report describes (1) the lithologic setting near Cedar Rapids, Iowa; (2) AEM investigation methods; 

(3) delineation of selected lithologic units; and (4) data and 

interpretive limitations of the methods used.

Location and Description of Study Area

The study area for the AEM survey encompasses about 

53 square kilometers (km2) of the Cedar River Basin, west 

of Cedar Rapids, Iowa (fig. 1), and includes a 19-kilometer 

(km) reach of the Cedar River. The area includes a flat alluvial 

valley underlain by fluvial and glaciofluvial sands and gravels 

and uplands generally underlain by glacial tills and eolian 

sediments (Quade and others, 1998). The location and extent 

of the AEM survey area were planned with several considerations: gathering data near the Cedar River alluvial deposits; 

avoiding electromagnetic interference and built structures; 

and targeting representative alluvial deposits, glacial till, and 

eolian sediments that are in the study area (fig. 1). A total of 

two USGS streamgages, Cedar River at Blairs Ferry Road 

at Palo, Iowa (USGS streamgage 05464420), and Cedar 

River at Edgewood Road at Cedar Rapids, Iowa (USGS 

streamgage 05464480), are near the study area (fig. 1); streamflow data for these streamgages are available from U.S. Geological Survey (2018). The locations of 25 wells with lithologic logs (table 1) that were used for interpreting AEM data 

also are shown in figure 1 (Iowa Geological Survey, 2018).

Lithologic Setting

For the interpretation of AEM survey results, the regional 

lithologic system was simplified to four main units. Of these 

units, three consist of surficial deposits: glacial tills, eolian 

sediments (windblown), and alluvial (stream) deposits; the 

fourth is the underlying carbonate bedrock composed of 

22 formations ranging from Silurian to Devonian in age (University of Iowa, 2013; Tucci and McKay, 2006; Iowa Department of Natural Resources, 2004). These lithologic units are 

components of the Southern Iowa Drift Plain and Iowan Surface landform regions in the study area and include sediment 

deposits and weathering zones formed during several glacial 

and interglacial intervals over the past 2.2 million years (fig. 1; 

Hallberg, 1986; Prior, 1991; Bettis and others, 2005).

The Southern Iowa Drift Plain is a region that was glaciated several times from 2.2 million to 500,000 calendar years 

before present, referenced to calendar year 1950 (Hallberg, 

1986). These glaciations deposited predominantly basal tills 

on an irregular bedrock surface. These tills were assigned to 

the Pleistocene-age Alburnett Formation and younger Wolf 

Creek Formation by Hallberg (1980). Both formations consist 

of multiple depositional till units and paleosols (weathering 

surfaces with soil formation) and include deposits of several 

glacial and interglacial intervals. These tills are primarily silty 
Lithologic Setting 3

Figure 1. Study area for airborne electromagnetic survey, location of wells with lithologic information, and location of 

streamgages.

9

8

7

6

5

4

3

2

1

25

24

23

22 21 20

19

16 17

15

13

12

11

10

Hiawatha

Palo

Robins 05464420

05464480

18

14

91°48' 91°46' 91°44' 91°42'

42°04’

42°02'

42°00'

Map image is the intellectual property of Esri and is used herein under license. 

Copyright © 2018 Esri and its licensors. All rights reserved.

Universal Transverse Mercator, Zone 15

North American Datum 1983

0 1 MILE

0 1 2 KILOMETERS

Cedar River

Cedar 

Cedar Rapids

BLAIRS FERRY ROAD

BLAIRS FERRY ROAD

Covington

¬«94

¬«94

#

EXPLANATION

Well location with map identifier (table 1)

Iowan Surface

Southern Iowa Drift Plain

Airborne electromagnetic study area

U.S. Geological Survey streamgage and number

Landform region (Prior, 1991)

05464480

12

Wooded marsh 

Woodland 

Natural area/park

Urban area

River

Study area

IOWA

 I–380
4 Delineation of Selected Lithologic Units Using Airborne Electromagnetic Data near Cedar Rapids, Iowa

Figure 2. The RESOLVE frequency-domain airborne electromagnetic system (the “bird”) 

used to transmit and receive electromagnetic energy to characterize the Cedar River alluvial 

aquifer, suspended from the helicopter (photograph by Joshua Valder, U.S. Geological 

Survey).

Helicopter

Tow line

The “bird”—RESOLVE frequency-domain 

airborne electromagnetic system 

Approximate 

location of 

transmitter Approximate 

location of receiver
Airborne Electromagnetic Investigation Methods 5

Table 1. Well identification for 25 wells with lithologic logs 

in the study area. The well identification number is the unique 

number identifying each well in the GeoSam database (Iowa 

Geological Survey, 2018).

Map identifier (fig. 1) Iowa Geological Survey well 

identification number

1 4975

2 5082

3 5084

4 12660

5 17042

6 22775

7 22783

8 23575

9 80616

10 81091

11 81096

12 81301

13 81309

14 83011

15 83012

16 83013

17 83014

18 83015

19 83016

20 83017

21 83018

22 83019

23 83020

24 83021

25 84479

clay and sandy clay loams with interbedded, discontinuous 

sand and gravel bodies that range in thickness from 1 to 3 m.

After the last pre-Illinoian glaciation, a drainage network 

formed, similar to the one observed on the modern landscape, 

as represented by the Iowan Surface. On the uplands, deep 

weathering profiles and erosional surfaces developed, removing till entirely in places; in the lowlands, stream channels 

incised, in some areas to bedrock, and, over the millennia 

(during the 500,000 calendar years before present), many 

cut-and-fill episodes left behind sand and gravel deposits 

(Hallberg, 1980). During the Wisconsinan glaciation, the last 

advance of continental ice in North America was between 

23,000 and 11,000 calendar years before present. Although 

ice did not reach the study area during this last advance, two 

episodes of eolian sediment (loess) deposition (Prior, 1991) 

were on high alluvial terraces and upland surfaces near the 

Cedar River valley.

In the study area, the Cedar River alluvial aquifer is 

generally composed of 13–22 m of sand and gravel deposits that are Pleistocene to Holocene in age and eolian sand 

deposits in areas of the valley where wind has reworked older 

alluvial deposits (Schulmeyer and Schnoebelen, 1998; Quade 

and others, 1998). Other surficial deposits in the study area are 

primarily glacial till overlain by discontinuous sand and silt 

(eolian sediment) deposits (Quade and others, 1998). Tills in 

the study area contain interbedded sand and gravel bodies, but 

these interbedded bodies are typically too deep or too thin for 

the AEM survey RESOLVE system to distinguish the resistant 

material from surrounding till (David Smith, U.S. Geological 

Survey, oral commun., 2017). This geologic interpretation of 

subsurface materials was used to guide the lithologic interpretations of AEM data in the section “Delineation of Selected 

Lithologic Units Using Airborne Electromagnetic Data.”

Airborne Electromagnetic Investigation 

Methods

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. Using 

the AEM method provided a more detailed characterization 

of the aquifer that could not be otherwise achieved using 

traditional methods; for example, lithologic log characterization or surface observation of geology (Valseth and others, 

2018). Initial ground-based resistivity surveys were used to 

characterize the lithologic targets (depth) and frequencies and 

to demonstrate electrical resistivity contrasts in the subsurface 

that made AEM a suitable method for the area. The shallow 

depth of the Cedar River alluvial aquifer supported the use of 

the RESOLVE frequency-domain AEM system for the survey.

The AEM survey was completed in May 2017 and consisted of 66 flight lines for a total of about 600 survey linekilometers, including a grid of east-west flight lines (“primary 

flight lines”) with a spacing of 200 m and several north-south 

“tie” lines (fig. 3). Several free-flight, curving flight lines 

along the Cedar River channel also were surveyed. These 

“free-flight” lines were flown over other areas of interest that 

were too small or isolated to be captured by the survey grid 

(fig. 3); however, these data are not included in the delineation 

of lithologic units described in this report but are available as 

part of a USGS data release (Deszcz-Pan and others, 2018).

Horizontal spacing of the apparent resistivity measurements averaged around 2.5 m and varied based on the 

10-hertz sampling frequency and the speed of the RESOLVE 

frequency-domain AEM system as it moved over the land surface. The collected AEM data were postprocessed by numerical inversion using the program EM1DFM (Farquharson, 

2000) to produce subsurface apparent resistivity cross sections, hereafter referred to as “resistivity profiles,” below all 

flight lines and are available in a USGS data release (DeszczPan and others, 2018).
6 Delineation of Selected Lithologic Units Using Airborne Electromagnetic Data near Cedar Rapids, Iowa

Delineation of Selected Lithologic 

Units Using Airborne Electromagnetic 

Data

Changes observed in resistivity profile values with 

depth derived from the AEM data 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” (fig. 4, sheet 1, available at 

https://doi.org/10.3133/sim3423). 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. Lithologic logs from the Iowa Geological Survey were 

used to verify and modify the lithologic framework (table 1; 

Iowa Geological Survey, 2018). Finally, delineated resistivity 

profiles were used to interpolate maps of the glacial till surface 

elevation, the bedrock surface elevation, alluvial deposit thickness, and depth to bedrock (figs. 5A–D, respectively, sheet 2, 

available at https://doi.org/10.3133/sim3423) using similar 

techniques described in Valseth and others (2018).

Delineation of Lithologic Units in Resistivity 

Profiles

Resistivity profiles created from the AEM survey flight 

lines provided the foundation for building the lithologic 

framework (fig. 4, sheet 1). Each profile created by DeszczPan and others (2018) was used to delineate three lithologic 

units: alluvial deposits, glacial till, and bedrock. Generally, 

lithologic unit boundaries were placed on profiles based on 

resistivity values and relative changes in values with depth. 

Land-surface elevation, geomorphology, and resistivity values 

of overlying material also were considered in lithologic unit 

boundary placement. Previously published investigations have 

documented that changes in electrical resistivity can be correlated to changes in bulk material properties in the subsurface 

and used for conceptualization in a lithologic framework 

(Valseth and others, 2018; Valder and others, 2016; Ball and 

others, 2011; Reynolds, 2011). Expected ranges in resistivity 

values for geologic materials were developed from published 

literature (Valseth and others, 2018), from measured resistivity 

values in geophysical logs of wells available from the Iowa 

Geological Survey (2018), and from comparison of resistivity values in the postprocessed AEM data with lithologic logs 

(Deszcz-Pan and others, 2018).

Some overlap in the expected resistivity value ranges for 

the three lithologic units made interpretation of contacts based 

upon resistivity values alone difficult. Some alluvial deposits 

in the study area have a range of resistivity values similar to 

the range of the underlying bedrock. In resistivity profiles 

where alluvial deposits overlie bedrock, the boundary between 

these units may not be clearly defined; for example, as shown 

in figure 4B. Interpretation of the resistivity profiles in these 

areas was informed by well lithologic logs that consistently 

indicated the contact between the alluvial deposits and the 

bedrock to be deeper than the area of resistivity contrast in 

the AEM data. Eolian sediments and alluvial deposits in the 

study area also have similar ranges of resistivity values and, 

in some profiles, are indistinguishable from one another; for 

example, eolian sediments were mapped adjacent to alluvial 

deposits by Quade and others (1998), yet a distinct boundary between these units cannot be observed in the resistivity 

values as shown in figure 4B. Eolian sediments were also 

mapped by Quade and others (1998) on the upland surfaces 

near the valley margin. The resistivity values of these eolian 

sediments contrast sharply with the values of the underlying 

glacial till (fig. 4A). Although this boundary is apparent, eolian 

sediments were not delineated as a separate lithologic unit. 

Other examples of potential undelineated eolian sediments are 

shown in figures 4A and B.

Subsurface delineation of lithologic units was informed 

by comparison of lithologic logs (Iowa Geological Survey, 

2018) to resistivity profiles created from the AEM survey data 

(Deszcz-Pan and others, 2018). 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. This is illustrated in figure 4A, where 

a wedge of high-resistivity material was interpreted as a 

paleochannel alluvial sand and gravel deposit, not as bedrock. 

This interpretation is made on the premise that the alluvial 

deposit overlies and crosscuts a conductive unit interpreted as 

glacial till, which, in turn, overlies the lowest resistant unit, 

interpreted as bedrock.

Finally, the thickness of conductive material can alter the 

resolution and values of underlying resistant materials (Reynolds, 2011). This effect is illustrated in figure 4B, where resistivity values used to delineate the bedrock surface beneath till 

differ from values used beneath alluvial deposits. This effect 

was considered when delineating unit boundaries within each 

of the 66 profiles (1 for each flight line). An example from the 

66 profiles illustrating a typical resistivity profile within the 

study is shown in figure 4C.

In some parts of the resistivity profiles, data seemed to be 

affected by interference from conductive or electrified structures such as roadways, power lines, and railroads, hereafter 

referred to as “cultural interference.” Cultural interference can 

cause sharp and abrupt contrasting values in the resistivity 

profile that expand laterally with increasing depth (fig. 4D). 

Cultural interference suspected from such data irregularities 

was verified for this study using aerial imagery from Google 

Earth Pro (© 2016 Google), which shows power lines, 

railroads, and other anthropogenic structures, and location 

data provided by local utilities and the City of Cedar Rapids.
Delineation of Selected Lithologic Units Using Airborne Electromagnetic Data 7

Figure 3. Airborne electromagnetic survey lines.

Hiawatha

Palo

Robins

91°48' 91°46' 91°44' 91°42'

42°04’

42°02'

42°00'

Study area

IOWA

 I–380

Map image is the intellectual property of Esri and is used herein under license. 

Copyright © 2018 Esri and its licensors. All rights reserved.

Universal Transverse Mercator, zone 15

North American Datum 1983 0 1 MILE

0 1 2 KILOMETERS

Cedar River

Cedar River

Cedar Rapids

BLAIRS FERRY ROAD

BLAIRS FERRY ROAD

Covington

¬«94

¬«94

EXPLANATION

Airborne electromagnetic study area

Wooded marsh 

Woodland 

Natural area/park

Urban area Airborne electromagnetic primary flight line

Airborne electromagnetic tie line

Airborne electromagnetic free-flight line
8 Delineation of Selected Lithologic Units Using Airborne Electromagnetic Data near Cedar Rapids, Iowa

Map Interpolation from Lithologic Units

Each of the 66 resistivity profiles (Deszcz-Pan and 

others, 2018) with interpreted lithologic unit boundaries 

were imported into a geographic information system (Esri, 

2018) using python scripts to georeference and analyze each 

resistivity profile and to interpolate each line spatially within 

the study area. The methods described in Valseth and others 

(2018) were used for all the processing of the data, except for 

the binning of existing wells based on perpendicular distance 

from the flight line. For this study, each well was used to 

validate the interpolation between lines. This validation was 

done to support the lithologic interpretations from interpolated 

resistivity values in areas between flight lines. Lithologic units 

were delineated in areas not flown (outside of the flight area) 

using the same interpolation methods but were designated as 

low-confidence areas (figs. 5A–D, sheet 2) because no resistivity profiles were available. The AEM flight data, resistivity 

profiles, interpolated maps, and delineation of the lithologic 

units are illustrated in the fence diagram in figure 5E and 

document the relative thicknesses of the three mapped lithologic units in the study area: alluvial deposits, glacial till, and 

bedrock. This three-dimensional fence diagram was created 

as part of the lithologic framework to further define the extent 

and thickness of the selected lithologic units in the area near 

Cedar Rapids. The fence diagram shows a three-dimensional 

perspective of unit thickness, extent, and orientation of the 

delineated lithologic framework (fig. 5E).

The thickness of the alluvial deposits was determined 

using the difference between the elevation of the glacial till 

surface and the land-surface elevation using a 3-m digital 

elevation model available through the Iowa Geodata website (https://geodata.iowa.gov/dataset/three-meter-digitalelevation-model-iowa-derived-lidar). The approximate 

thickness range of alluvial deposits is from 0 to 54 m, and the 

thickest alluvial deposits are in the northwest part of the study 

area (fig. 5C) where a paleochannel in the bedrock surface 

has been mapped (fig. 5B). The glacial till unit thickness was 

determined using the difference between the glacial till surface 

elevation (fig. 5A) and the bedrock surface elevation (fig. 5B). 

The approximate thickness range of the glacial till unit is 

from 0 to 72 m, and the thickest areas are in the southwestern 

part of the study area coinciding with low elevation of the 

bedrock surface and high elevation of the glacial till surface. 

The bedrock unit elevation does not have a delineated bottom 

because of the limitation of the AEM investigation depth, so 

no thickness was calculated. The depth to bedrock was calculated using the difference of the land-surface elevation (digital 

elevation model) and the bedrock surface elevation (fig. 5B). 

The approximate depth to bedrock range is from 0 to 72 m 

(fig. 5D).

Data and Interpretive Limitations

Interpolations made between data points inherently contain some uncertainty. 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 (Reynolds, 2011). In this study, the only variables 

considered were those related to changes in the subsurface 

material. Those factors—variations in lithology, porosity, and 

sorting—were considered when delineating lithologic units in 

the resistivity profiles by comparing values to characteristics 

described in lithologic logs. Given the few lithologic logs 

available in the Cedar Rapids area, some uncertainty in exact 

placement of lithologic boundaries may exist. Delineating the 

different lithologic units provides an estimate of aquifer geometry, horizontal extents, and aquifer thicknesses, which are 

useful for future groundwater and hydrologic investigations.

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For more information about this publication, contact:

Director, USGS Dakota Water Science Center

1608 Mountain View Road

Rapid City, SD 57702

605–394–3200

For additional information, visit: https://www.usgs.gov/centers/dakotawater

Publishing support provided by the 

Rolla Publishing Service Center

Valder and others—Delineation of Selected Lithologic Units Using Airborne Electromagnetic Data near Cedar Rapids, Iowa—Scientific Investigations Map 3423

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3423