Mississippi Valley Division Engineer Research and Development Center

Application of Chirp 

Acoustic Sub-Bottom Data 

in Riverine Environments: 

Identification of Underlying 

Rocky Hazards at Cape 

Girardeau, Missouri, and 

Thebes, Illinois

MRG&P Report No. 31 • February 2020

Mississippi River Geomorphology and 

Potamology Program 

MRG&P Report No. 31 

February 2020 

Application of Chirp Acoustic Sub-Bottom Data 

in Riverine Environments: Identification of 

Underlying Rocky Hazards at Cape Girardeau, 

Missouri, and Thebes, Illinois

A Report Preparation Tool for ERDC Authors and Contractors, Version 1.02 Heidi M. Wadman and Jesse E. McNinch

Coastal and Hydraulics Laboratory

U.S. Army Engineer Research and Development Center

Field Research Facility

1261 Duck Rd.

Kitty Hawk, NC 27948

Final report 

Approved for public release; distribution is unlimited. 

Prepared for US Army Corps of Engineers

Washington, DC 20314-1000

Under Project 127672; “Mississippi River Geomorphology and Potamology Program”
MRG&P Report No. 31 ii

Abstract

Shallow acoustic reflection (chirp) data have been utilized to map the 

elevation of underlying stratigraphy in a wide range of aqueous 

environments. Of particular concern in riverine regions is the elevation of 

near-surface underlying rock that, if exposed during normal migration of 

sedimentary bedforms, can cause grounding and damage to vessels 

transiting the region during periods of low water. Given the ephemeral 

nature of the rock’s exposure, traditional surveying methods are 

insufficient to map rock when it is covered by a thin veneer of sediment, 

increasing the potential hazard. Accordingly, the US Army Corps of 

Engineers, St. Louis District, (MVS) explored the use of chirp sub-bottom 

surveys to identify buried rock within the Mississippi River in the vicinity 

of Cape Girardeau, MO, and Thebes, IL. Hazard maps showing the 

distribution of buried rock were generated, and the base of the mobile 

sediment layer was identified where possible. These data will allow MVS to 

accurately identify potentially hazardous regions during periods of low 

water. Although the study did not result in the complete mapping of all 

near-surface geologic hazards, regions that warrant further study are 

identified, and modifications to the original survey plan are provided to 

improve the accuracy of future data collection efforts.

DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. 

Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. 

All product names and trademarks cited are the property of their respective owners. The findings of this report are not to 

be construed as an official Department of the Army position unless so designated by other authorized documents.

DESTROY THIS REPORT WHEN NO LONGER NEEDED. DO NOT RETURN IT TO THE ORIGINATOR.
MRG&P Report No. 31 iii

Contents

Abstract................................................................................................................................................... ii

List of Figures.........................................................................................................................................iv

Preface....................................................................................................................................................vi

Acknowledgements..............................................................................................................................vii

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

Background .............................................................................................................................. 1

Objective ................................................................................................................................... 1

Approach................................................................................................................................... 2

Study area................................................................................................................................. 4

2 Methods...........................................................................................................................................5

Data collection.......................................................................................................................... 5

Data processing........................................................................................................................ 5

3 Results.............................................................................................................................................9

Layer resolution........................................................................................................................ 9

Identification of sedimentary layers......................................................................................10

Identification of bedrock........................................................................................................12

Identification of specific reflection surfaces ........................................................................13

R01 .............................................................................................................................................13

R02 .............................................................................................................................................15

Horizontal and vertical error..................................................................................................18

4 Discussion.....................................................................................................................................21

Applicability in riverine environments ...................................................................................21

Interpretation of R01..............................................................................................................21

Interpretation of R02 .............................................................................................................25

Data holidays vs. lack of signal penetration.........................................................................26

5 Conclusions...................................................................................................................................29

References............................................................................................................................................31

Appendix: Sub-Bottom File Metadata...............................................................................................33

Unit Conversion Factors......................................................................................................................36

Acronyms and Abbreviations..............................................................................................................37

Report Documentation Page
MRG&P Report No. 31 iv

List of Figures 

Figure 1. Location of the Cape Girardeau, MO, study area. Panel (A) shows the overall 

region expanded in panel (B). The actual survey lines (red) in the white inset box in panel 

(B) are presented in detail in panel (C), including the general location of each river mile. 

Sub-bottom cross-sections presented in subsequent figures are too small to be imaged 

on the regional map. Instead, the specific seismic profile line from which cross sections in 

subsequent figures were extracted (as annotated on each figure) is noted on panel (D).................. 2

Figure 2. Common geophysical acoustic survey techniques. The type of data collected 

depends primarily on the frequency of the transmitter.......................................................................... 3

Figure 3. The EdgeTech 3100p (216s) utilized in the study. (A) Shows the towfish attached 

to the fixed pole mount along the side of the vessel; (B) shows the towfish being lowered 

into place below the water surface........................................................................................................... 6

Figure 4. Example sub-bottom profile from the Cape Girardeau region. The upper panel (A) 

is in this and all subsequent seismic profile figures is an un-interpreted image of the 

seismic profile, complete with the line file name for identification. The lower panel (B) 

shows interpretation of the riverbed, pertinent reflection surfaces, and the horizontal and 

vertical scales. Note that the image’s scale will differ in subsequent seismic profile 

figures........................................................................................................................................................10

Figure 5. Example of bowtie artifacts in an (A) un-interpreted seismic profile and (B) 

interpreted image of the seismic profile. ...............................................................................................11

Figure 6. Example of buried rock in: (A) un-interpreted seismic profile; and (B) interpreted 

image of the seismic profile (orange R01).............................................................................................12

Figure 7. Example of buried rock and overlying stratigraphic layers in an: (A) un-interpreted 

seismic profile; and (B) interpreted image of the seismic profile (orange line, R01).........................14

Figure 8. Depth of the rock unit R01 under the riverbed in feet. Regions where the rock 

was outcropping on the riverbed are delineated by black lines. .........................................................15

Figure 9. Example showing the base of mobile sand in an (A) un-interpreted seismic 

profile and (B) interpreted image of the seismic profile (green line, R02)..........................................16

Figure 10. Example of the base of the mobile sediment layer overlying the subsurface 

rock in (A) un-interpreted seismic profile and (B) interpreted image of the seismic profile 

(rock: orange R01; mobile sediment base: orange R02)..................................................................... 17

Figure 11. Upper two panels show an example of multiple intersecting layers with the 

same acoustic signature as R02 in (A) un-interpreted seismic profile and (B) interpreted 

image of the seismic profile. Lower two panels show an example of the R02 reflection 

surface being lost due to absorption of the acoustic signal in (A) un-interpreted seismic 

profile and (B) interpreted image of the seismic profile. ...................................................................... 17

Figure 12. Depth of the interpreted base of the mobile sand wave layer, R02, under the 

riverbed in feet. Regions where the rock was outcropping on the riverbed are delineated 

by black lines. ...........................................................................................................................................18

Figure 13. Example of highly variable surface elevation in exposed and buried rock at 

Cape Girardeau: (A) un-interpreted seismic profile and (B) interpreted image of the 

seismic profile (orange line, R01)...........................................................................................................22

Figure 14. Hazard map for sub-surface R01 in the study area, shown as depth below the 

riverbed (thickness) in feet. Red lines indicate outcrops of R01 at the riverbed surface. 

White areas indicate data holidays. ....................................................................................................... 24
MRG&P Report No. 31 v 

Figure 15. Example of an acoustically transparent profile underneath dense (hard) 

sediment. The morphology of the riverbed (sand waves) and the bowties suggest the 

surface was composed of very hard sediment, which reflected most of the acoustic 

energy. However, faint reflection surfaces are mapped below the surface, indicating that 

the riverbed here is not composed of rock. Accordingly, this is interpreted to represent a 

surficial deposit of sand overlying finer-grained (less dense) material...............................................25
MRG&P Report No. 31 vi

Preface

This study was conducted for the US Army Corps of Engineers (USACE) as 

part of the Mississippi River Geomorphology and Potamology (MRG&P) 

Program, under Project 127672; “Mississippi River Geomorphology and 

Potamology Program.” The MRG&P is part of the Mississippi River and 

Tributaries Program and is managed by the USACE, Mississippi Valley 

Division (MVD), and districts. At the time of publication of this report, the 

MRG&P Program Director was Dr. James W. Lewis. The MVD Commander 

was MG R. Mark Toy. The MVD Director of Programs was Mr. James A. 

Bodron.

Mississippi River engineering direction and policy advice were provided 

by the Mississippi River Commission. The Commission members were 

MG Toy, USACE, President; the Honorable James Reeder; the Honorable 

Norma Jean Mattei, PhD; RDML Shepard Smith, National Oceanic and 

Atmospheric Administration; BG D. Peter Helmlinger, USACE; and MG 

Robert F. Whittle, USACE.

The work was performed by the Coastal Observations and Analysis Branch 

of the Flood and Storm Protection Division, US Army Engineer Research 

and Development Center, Coastal and Hydraulics Laboratory (ERDC-CHL). 

At the time of publication of this report, Dr. Jeff Waters was Chief of the 

Coastal Observations and Analysis Branch; Dr. Cary Talbot was Chief of the 

Flood and Storm Protection Division. The Deputy Director of ERDC-CHL 

was Mr. Jeffrey R. Eckstein, and the Director was Dr. Ty V. Wamsley.

The Commander of ERDC was COL Teresa A. Schlosser, and the Director 

was Dr. David W. Pittman.
MRG&P Report No. 31 vii

Acknowledgements

The authors acknowledge the extensive efforts of Mr. Shawn Kempshall, 

Mr. Adam Rockwell, Mr. Mathew Staley, PSM, Mr. Randy Trout, and Mr. 

Rob Davinroy, PE, for the data collection and initial processing attempts. 

In addition, Mr. Eddie Brauer, Mr. Andy Gaines, Mr. Cory Tabbert, 

Mr. John Vest, and Dr. Ty V. Wamsley provided regular discussion 

during the data re-processing effort and further explained, via multiple 

discussions, the complicated regional geology of the area. This study was 

funded by the U.S. Army Corps of Engineers, St. Louis District, Applied 

River Engineering Center, and the U.S. Army Corps of Engineers, 

Mississippi Valley Division.
MRG&P Report No. 31 1 

1 Introduction

Background 

Where outcrops of erosion-resistent geologic units, such as indurated or 

weakly lithified sedimentary layers or rock, are observed along riverbeds, 

the same geologic unit is also present in the shallow, sub-surface 

stratigraphy. However, the spatial extent of these geologic layers below the 

surface of a riverbed is often much more extensive than indicated by just 

the portion of the geology that is exposed at the riverbed surface. When 

the usually buried portion of these geologic layers is temporarily exposed 

above the riverbed via sediment transport of overlying mobile bedforms, 

especially during periods of low water, these erosion-resistant units can 

adversely impact the navigability of the river. In the region of Cape 

Girardeau, MO, and Thebes, IL, an extensive and irregular geologic 

formation of pinnacle-shaped rock underlies portions of the main 

navigation channel of the Mississippi River. During periods of high river 

flow and associated bedform migration, outcrops of this rock are often 

temporarily exposed in the navigation channel. Vessels transiting this 

region during subsequent periods of low water levels risk grounding along 

the pinnacle-shaped rock outcrops. However, due to the highly mobile 

nature of the bedforms in this region (e.g., sand waves), these navigation 

hazards are frequently buried as quickly as they are exposed, making them 

challenging at best to map via traditional geophysical surveys and 

subsequently incorporate into navigation charts. 

Objective 

Although the surficial expression of these units can be mapped via 

sidescan sonar (either via a dedicated sidescan survey or via the 

backscatter from a multibeam bathymetry survey), a veneer of sediment 

on the order of 1-2 inches (in.) in thickness will obscure indurated 

sedimentary layers or rock outcrops from these mapping methodologies. 

In addition, extensive debris on the surface of the riverbed can further 

complicate interpreting the presence and spatial extent of rock in the study 

area. Rather than rely on these traditional mapping technologies, The 

US Army Corps of Engineers (USACE), St. Louis District (MVS), Applied 

River Engineering Center, explored the use of chirp sub-bottom 

geophysical surveys to identify buried rock within the Mississippi River in 
MRG&P Report No. 31 2 

the vicinity of Cape Girardeau, MO, and Thebes, IL (river mile [RM] 55-47 

and RM 47-43, respectively; Figure 1). Hazard maps showing the 

distribution of rock buried within 15 ft of the surface of the riverbed were 

generated. In addition, the base of the mobile sediment layer was 

identified where possible. These maps will allow MVS to accurately 

identify potential hazards in navigable regions during periods of low water 

and to further identify the volume and depth of hazardous rock. Although 

the effort presented here is not a complete delineation of the potential 

buried stratigraphic hazards in this region, the study provides proof-ofconcept of the methodology, and suggestions are provided for future 

efforts in this, and similar, environment.

Figure 1. Location of the Cape Girardeau, MO, study area. Panel (A) shows the overall region 

expanded in panel (B). The actual survey lines (red) in the white inset box in panel (B) are 

presented in detail in panel (C), including the general location of each river mile. Sub-bottom 

cross-sections presented in subsequent figures are too small to be imaged on the regional 

map. Instead, the specific seismic profile line from which cross sections in subsequent figures 

were extracted (as annotated on each figure) is noted on panel (D). 

Approach 

All geophysical mapping methodologies share the same basic relationship: 

a towfish is towed from a survey vessel, and a sound source is generated 

from a transducer in the towfish towards the sediment-water interface 

(referred hereafter as the riverbed). The density change at the riverbed 

reflects the sound back to the surface where it is detected by receivers 

either embedded in the towfish or towed behind the towfish or vessel 

(Figure 2). Since the frequency of the acoustic pulse is known, the two-way 

travel time provides a measure of the distance (or water depth) between 
MRG&P Report No. 31 3 

the towfish and the riverbed, and this relationship provides the basis for 

all bathymetric depth sounders. The acoustic frequency generated by these 

towfish is generally of a high-enough frequency (e.g. ~200 kilohertz [kHz]) 

such that all of the generated sound is reflected off of the riverbed, with 

little or no penetration below the surface.

Figure 2. Common geophysical acoustic survey techniques. The type of data collected 

depends primarily on the frequency of the transmitter. 

The strength of the returned signal (referred to as the signal’s amplitude) 

is a measure of the acoustic impedance of the geologic unit, which in turn 

is directly related to the density of the sediment, and thus the amplitude of 

the acoustic return can be used to delineate the geologic nature of the 

riverbed. Softer, less dense sediment (e.g., silty-clay) reflects less energy 

and thus has a lower amplitude return than more dense sediment (e.g., 

sand). The varying amplitudes require groundtruthing via sediment 

samples to determine what sediment type they are reflective of in a specific 

region, but significantly fewer samples are required to groundtruth 

amplitude data to generate a surface type map than without a survey. The 

frequency of the acoustic signal determines the size of features that can be 

resolved — the resolution — as well as the depth under the riverbed — the 

penetration — to which the signal can penetrate, if at all. Sidescan sonar 

systems measure a swath of high-frequency amplitude returns from the 

riverbed, allowing detailed maps of surficial sediment type and features 

(e.g., ripples) to be generated. However, sidescan sonars cannot penetrate 

more than the uppermost inch or two of the riverbed sediment, precluding 

their use for mapping subsurface stratigraphy (e.g., Mitchell 1993).
MRG&P Report No. 31 4 

To map the stratigraphy below the riverbed, lower acoustic frequencies 

must be utilized. Although multiple strategies exist for generating these 

pulses, frequency-modulated (FM), acoustic chirp reflection systems are 

considered the industry standard for acquiring stratigraphic data in shallow, 

aqueous environments (e.g., Schock and LeBlanc 1990; LeBlanc et al. 1992; 

Roberts and Supan 2000; Schock 2004; Lee et al. 2009). Chirp systems 

produce an FM range of acoustic frequencies with each acoustic pulse 

(similar in audible sound to a bird chirp), providing a range of frequencies 

lower than those used for bathymetry or sidescan sonar applications. These 

lower frequencies are able to penetrate the sediment-water interface and 

thus be transmitted into the riverbed. Rather than getting a single reflection 

from the single frequency acoustic pulse, the different frequencies in a chirp 

pulse are reflected off of changes in density in the subsurface sediment, and 

different frequencies on the same chirp pulse are reflected at different 

amplitudes and time from different density changes and depths, allowing a 

detailed map of subsurface stratigraphy to be created. Similar to sidescan 

sonar data, the amplitude of each return is a function of the different 

acoustic impedance (or density) of each layer, and sediment samples via 

cores can be used to interpret the composition of the mapped layers. 

Acoustic chirp reflection systems have been utilized to map the shallow 

stratigraphy of a wide range of aqueous environments (e.g., Schock and 

LeBlanc 1990; Schock 2004; Roberts and Supan 2000), including rivers 

and estuaries (e.g. Carbotte et al. 2004; Nitsche et al. 2004; Nitsche et al. 

2007; Plets et al. 2009), lacustrine (e.g. Schwamborn et al. 2002; Cukur et 

al. 2013; Cukur et al. 2015), and shallow coastal (e.g. LeBlanc et al. 1991; 

Schock 2004; Lee et al. 2009) environments. Note that the frequency of a 

chirp pulse cannot penetrate the crystalline structure of rock or discern 

differences in rock composition. Lithified and/or rocky layers are thus 

identified by the complete reflection of any acoustic energy off of the surface 

of the rock layer.

Study area 

The framework geology in the region surrounding Cape Girardeau and 

Thebes is comprised primarily of relict deltaic, alluvial, and riverine 

deposits, many of which have been heavily faulted and folded (e.g., 

Pryor and Ross 1962; Johnson 1985; Saucier 1994). Although an 

extensive examination of the local geology is beyond the scope of this 

report, terrestrial mapping efforts have identified several limestone and 

shale formations that outcrop in the vicinity of the Mississippi River in 

this region.
MRG&P Report No. 31 5 

2 Methods

Data collection 

A chirp geophysical survey was conducted from 1-3 March 2016 by MVS 

and the USACE, Jacksonville District (SAJ). Approximately 77 linear miles 

of chirp data were collected between RM 55 and 43, and the track lines of 

all survey lines are shown in Figure 1. Data were collected using an 

EdgeTech 3100p (216s) sub-bottom unit, with an available FM acoustic 

range of 2-16 kHz. An Applanix POS MV 320/DGPS was used for vessel 

positioning via the Missouri Department of Transportation RTK-network, 

and survey line planning and spacing were executed using Hypack. Singlebeam bathymetric data were collected simultaneously during the chirp 

survey to allow for future rectification of sub-surface reflection surfaces to 

a vertical datum. EdgeTech Discovery software was used for collection of 

the chirp geophysical data, and the specific pulse settings, preserved in the 

metadata header of the raw data files, used can be obtained by request 

from the USACE Jacksonville District. Vessel speed was held as constant 

as feasible during the survey to maximize steering but minimize cavitation 

and data loss. 

The EdgeTech towfish was pole-mounted to the survey vessel (Figure 3) 

and deployed at a consistent depth to minimize layback and positioning 

errors, precluding the need for an integrated pressure sensor to measure 

the depth of the towfish below the water surface. A fixed mount often 

reduces the impact of cavitation bubbles from the vessel engine on the 

acoustic signal and further provides an element of safety when towed in 

regions with subsurface hazards. It also allows for a more precise 

calculation of the layback between the GPS antenna and the geophysical 

survey equipment.

Data processing 

Initial processing of the geophysical data was completed by SAJ in Fiscal 

Year 16 using Chesapeake Technology SonarWiz 7. A swell filter was 

applied during this data processing effort to minimize the impact of vessel 

motion (swell) on the data. During data collection, the reflection surfaces 

mapped by the chirp are measured as two-way travel time, and the speed 

of sound of the water column and sediment are used to convert these times 

to depths below the towfish in the EdgeTech Discovery collection software. 
MRG&P Report No. 31 6 

This is a fairly inaccurate method that fails to account for vertical changes 

in the vessel and the towfish mount. Specifically, the freeboard of a vessel 

may change throughout a survey day, as personnel and equipment move 

around the vessel, and fuel is consumed in both the engine and the 

generator running the equipment often resulting in a gradual lightening of 

vessel payload, and thus a measurable change in the relative position of 

the water line on the vessel (and/or towfish) to local water level. 

Accordingly, a vertical datum rectified to the towfish alone is not 

recommended in professional publications referencing chirp sub-bottom 

data unless the vertical position of the towfish is directly measured with a 

RTK-GPS antenna rigidly mounted onto the towfish. Instead, acceptable 

datums include depth below the seafloor, or rectification to a known 

seabed elevation (such as rectification to a bathymetric survey). In order to 

account for the likely variability in the vertical position of the towfish 

during data collection, the original processing effort by SAJ rectified the 

raw elevations below the towfish to the NAVD88 single beam bathymetry 

collected simultaneously during the survey. In addition to the riverbed, 

multiple reflection surfaces were noted and interpreted by SAJ based on a 

qualitative, visual appearance as representing buried bedrock, layers of 

gravel or sand, or anthropogenic features such as buried weirs or cables. 

Unfortunately, project funding precluded the collection of either sediment 

cores or sediment surface grabs to groundtruth the survey data, making 

interpretations of the chirp record challenging at best. 

Figure 3. The EdgeTech 3100p (216s) utilized in the study. (A) Shows the towfish 

attached to the fixed pole mount along the side of the vessel; (B) shows the towfish 

being lowered into place below the water surface. 
MRG&P Report No. 31 7 

Due to uncertainties in the initial data interpretation, largely due to the 

complex geology of the area and the lack of sediment cores, the raw chirp 

files were provided to the U.S. Army Engineer Research and Development 

Center, Coastal and Hydraulics Laboratory (CHL), for a secondary 

processing effort. The raw data were imported into Chesapeake 

Technology SonarWiz 7.01.006, EdgeTech Discovery software collects 

horizontal datum data as geographic x, y, and these values were used to reproject the data to Missouri State Plane, East Zone, feet using Chesapeake 

SonarWiz 7. Only the upper 70% of the data record was imported as the 

lower 30% of the record contained no data (see Appendix for line-specific 

settings). Gain was not applied during importation. Data were smoothed 

upon import using a moving box car filter with an ambient constant of 500 

pings and an observed constant of 20 pings to remove spikes out of the 

acoustic record. A swell filter was initially applied, but the application of 

the filter resulted in numerous artifacts within the shallow record, so it 

was ultimately not used. A speed of sound of 1485 meters per second (m/s) 

(assumed freshwater and an average water temperature of 20 °C) was used 

to convert two-way travel time from milliseconds to depth below the 

towfish in feet.

The riverbed was identified using SonarWiz’s seafloor bottom tracking 

tool, and the specific filters used to optimize this automated detection for 

each line are noted in the Appendix. Once identified, the riverbed on each 

seismic line was individually examined for erroneous data points and then 

converted into a reflection surface (identified in SonarWiz as a feature). 

Pertinent subsurface reflection surfaces, detailed in Chapter 3, below, were 

hand-digitized. Digitization was extended through any apparent heave or 

swell and was completed by a single operator to reduce errors introduced 

by having multiple individuals digitizing surfaces. A total of 855 features, 

including 83 riverbeds and 772 reflection surfaces, were identified and 

hand digitized in the project. Note that SonarWiz refers to the 

water/sediment interface as a seafloor by default; in this case, the term 

seafloor in the appendix or in any files associated with this report refers to 

the riverbed.

Once digitized, reflection surfaces were exported as .csv files containing 

location (latitude, longitude, x, y) and depth below the towfish (a depth of 

zero was used for the riverbed itself). To rectify the subsurface data to an 

acceptable vertical datum during the processing effort for this report, the 

depth below the towfish was converted to depth below the riverbed by 
MRG&P Report No. 31 8 

calculating the difference in the depth below the towfish between the 

riverbed and a pertinent reflection surface (e.g., the thickness between the 

two surfaces was calculated). This is a first-order accepted vertical datum 

for stratigraphy data as the relative distance between the riverbed and 

underlying geologic units is not subject to the same water level-driven 

fluctuations during the time of mapping as the riverbed is to the towfish. 

These data can be further rectified in the future to a specific vertical datum 

(such as a bathymetric survey), but doing so was beyond the scope of this 

processing effort. The thickness files were imported into Microsoft Excel 

and edited for importation into Matlab version 2017a. The data were 

subsequently gridded using nearest neighbor at a variety of grid spacings 

(see Chapter 3 for details) and exported as .asc files for importation into 

ArcGIS. Horizontal and vertical errors associated with the data collection 

and processing methods are detailed in Chapter 3 below.
MRG&P Report No. 31 9 

3 Results

The overarching goals of the survey were (1) to provide proof-of-concept of 

the applicability of using acoustic chirp reflection data to map sub-surface 

stratigraphy in the Mississippi River and (2) to map the distribution and 

depth below the riverbed of potentially hazardous rock in and near the 

navigation channel in the vicinity of Cape Girardeau, MO, and Thebes, IL. 

An additional goal was to better constrain the volume of bedload transport 

by identifying where possible the base of the mobile sediment mapped at 

the surface of the riverbed.

Layer resolution 

The resolution of stratigraphic layers, as well as the depth below the 

riverbed to which the acoustic signal could penetrate, is a function of the 

acoustic frequency used as well as the amplification power contained 

within the towfish. With respect to an EdgeTech 3100p (216s), the overall 

operating frequency ranges from 2-16 kHz, but the entire frequency range 

is not transmitted by the towfish at any one time. Instead, the operator 

selects a portion of the acoustic range to transmit from the towfish at a 

certain power. This allows the operator to focus efforts on obtaining 

(1) maximum layer resolution at the expense of vertical sound penetration, 

(2) maximum vertical penetration with a loss of data resolution, or 

(3) some balance between the two. The higher the frequency used, the 

smaller the layer that can be resolved but the lower the overall penetration 

of the sound into the bottom. Published minimal layer resolution for the 

EdgeTech 3100p (216s) is 2.5 in. to 4 in. (dependent on frequency), and 

published maximum sound penetration is reported to be 18 ft in sand, 

260 ft in mud; however, these values represent maximum resolution 

and/or penetration under optimal conditions, and these standards are 

rarely achieved in the field. The typical minimum layer thickness resolved 

in field use is generally 6-7 in., and maximum penetration depths seen in 

sand vs. mud are on the order of 15 ft vs. 80 ft in sand vs. mud, 

respectively. Note that penetration greatly decreases when maximum data 

resolution is being achieved and vice versa, so the actual layer resolution 

and depth of penetration achieved is dependent on the user-directed 

settings (e.g., a setting that allows resolution of layers of less than 10 in. 

will yield much shallower penetration depths). In addition, the pulse rate 

of each chirp and the receive time between each pulse also directly impacts 

the minimal thickness of the resolved layers as well as the vertical 
MRG&P Report No. 31 10

penetration. Unfortunately, these settings were not available for the CHL 

data processing and interpretation effort described in this report, 

complicating the identification of the reflection surfaces described below.

Identification of sedimentary layers 

The density of sediment below the riverbed changes with changes in 

sediment stratigraphy. At each density change, there is either an increase 

or decrease in speed of sound, depending on the different density of the 

two layers, and a portion of the acoustic energy that penetrates the 

sediment is reflected back to the surface to the towfish receivers. The 

amplitude of the returning signal is converted from an analog pulse to a 

digital signal and presented visually as a dark line drawn below the towfish 

(Figure 4). Darker-grey shading below a reflection surface indicates that 

the acoustic signal is being reflected back to the towfish (i.e., denser 

sediment, such as sand), while bright, more white color below a reflection 

surface usually indicates absorption of the acoustic energy into the 

sediment (i.e., less-dense sediment, such as silty clay), with less overall 

reflection. Harder, denser units may also refract the acoustic energy back 

to the towfish at a ~45° angle from the original contact, resulting in 

artifacts referred to as bowties (Figure 5).

Figure 4. Example sub-bottom profile from the Cape Girardeau region. The upper panel (A) is 

in this and all subsequent seismic profile figures is an un-interpreted image of the seismic 

profile, complete with the line file name for identification. The lower panel (B) shows 

interpretation of the riverbed, pertinent reflection surfaces, and the horizontal and vertical 

scales. Note that the image’s scale will differ in subsequent seismic profile figures. 
MRG&P Report No. 31 11

Figure 5. Example of bowtie artifacts in an (A) un-interpreted seismic profile and (B) 

interpreted image of the seismic profile. 

During data collection, an acoustic pulse is transmitted by the towfish. For 

a brief period of time (determined by the collector and usually noted as the 

record length), the towfish then operates in receiving mode, where it

listens for the reflection of the acoustic data off of changes in underlying 

sediment density. The deeper the penetration desired, the longer the 

necessary listening time. During this time frame, it is possible for the 

sound source to travel through the water column and be reflected off of the 

riverbed to the towfish, where it then is reflected back down through the 

water column and is reflected once again from the riverbed to the towfish. 

The result is the towfish seeing the riverbed more than once. These 

artifacts are referred to as bottom multiples and are always found in exact 

multiples of the depth below the towfish and the riverbed (e.g., Figure 4b). 

The longer the listening time, the more likely that one or more bottom 

multiples will be recorded by the towfish. Because these multiples can 

obscure real reflection surfaces that are coincidently located at similar 

two-way travel time depths below the towfish, they cannot simply be 
MRG&P Report No. 31 12

filtered out from the data record. Accordingly, they are noted wherever 

they occur in the acoustic record.

Identification of bedrock 

Chirp acoustic data, regardless of signal frequency, penetrates 

unconsolidated and weakly consolidated sediment with a range of 

efficiencies. The frequencies used are unable to penetrate rock due to a 

complete lack of pore space, resulting in the reflection of 100% of the 

acoustic energy that reaches the rock back to the towfish receivers at an 

angle from the surface of the rock. Qualitatively, at the surface and with 

depth, the rock appears in chirp data as a distinct reflection surface with 

no data (gray shaded image) below it, illustrating the reflection of the 

acoustic energy off of rock with no penetration through it (Figure 6). If the 

rock layer is sufficiently close to the riverbed and/or is overlain by soft 

sediment such as layers of silt and clay, the reflected acoustic energy is 

often visible as bowties extending into the acoustically transparent region 

below the rock (Figure 5). At greater depths and/or under harder sediment 

(e.g., well-sorted sand), the bowtie artifacts are less visible, and the 

scattered energy appears more as smudges” just below the reflection 

surface (Figure 6).

Figure 6. Example of buried rock in: (A) un-interpreted seismic profile; and (B) interpreted 

image of the seismic profile (orange R01). 
MRG&P Report No. 31 13

Identification of specific reflection surfaces 

Two reflection surfaces with distinctive reflection amplitudes were 

identified in the study region. A brief description of their qualitative 

appearance, amplitude, and spatial distribution is presented below.

R01 

The single-most dominant reflection surface observed in the study site was 

digitized as R01. This surface is characterized by both high amplitudes as 

well as acoustic transparency below it – indicating complete reflection of 

the acoustic signal (Figure 6). The depth of this surface below the riverbed 

was highly variable – ranging from exposed at the riverbed to up to 20 ft 

below it. Where exposed at the riverbed, it has been previously identified 

as outcropping, irregular rock. Both near the surface and with depth, the 

unit was highly irregular in elevation, and was rarely noted at the same 

elevation on parallel survey lines, suggesting a great deal of spatial 

variability in the elevation of the unit. Near the surface, bowtie features 

were occasionally observed below the layer, and with depth the layer often 

had a smudged appearance below it. No reflection surfaces were noted 

below this unit, suggesting complete reflection of the acoustic energy that 

reached it.

The layer was not continuous in elevation; rather, it was characterized by 

rapid changes in elevation from being exposed at the riverbed at one 

location, and then plunging to depths of ~20’ below the riverbed over a 

horizontal distance of 100’ or less (Figures 6, 7). In several regions, R01 

appeared below the riverbed as a smudged surface that cut through 

overlying stratigraphy to come within a few feet of the riverbed (Figures 

6,7), and was often exposed on adjacent, shore-parallel survey lines.

To see these variations in elevation more clearly, the depth below the 

riverbed of R01 was gridded in Matlab using a nearest neighbor algorithm 

with a search radius of 250 ft, and a smoothing interval of 8 cells (Figure 

8). Although smoothing the data risks losing some of the more dramatic 

changes in R01 elevation change below the riverbed, it also reduces 

vertical errors generated by the digitization process (see Chapter 3, below). 

Overall, R01 was either exposed at the riverbed, or within ~6 ft of the 

riverbed: (1) near RM54, north of the town of Cape Girardeau, MO; (2) 

between RM 51 & 52, at Cape Girardeau, MO; and (3) along the right bank 

near RM49; and (4) between RM 46-43.5, near Thebes, IL. R01 was not 
MRG&P Report No. 31 14

observed in the sub-bottom data along the left bank between RM 49 and 

46.5, or in regions near RM 50 (Figures 1, 8).

Figure 7. Example of buried rock and overlying stratigraphic layers in an: (A) un-interpreted 

seismic profile; and (B) interpreted image of the seismic profile (orange line, R01). 
MRG&P Report No. 31 15

Figure 8. Depth of the rock unit R01 under the riverbed in feet. Regions where the rock was 

outcropping on the riverbed are delineated by black lines. 

R02 

A secondary, discontinuous reflection surface was noted throughout the 

study region and, where confidently identified, digitized as R02 (Figure 9). 

This surface was relatively flat-laying and found, on average, 

approximately 6 ft below the riverbed. Near the surface, it occasionally 
MRG&P Report No. 31 16

exhibited bowties, indicating strong reflectance of the acoustic energy back 

to the towfish. However, unlike R01, reflection surfaces were mapped 

below R02, indicating that R02 is composed of less-dense material than 

R01 (e.g., sediment vs. rock; Figure 10). Where R02 met R01, the R02 unit 

stopped at the vertical rise of R01, suggesting latter formation/deposition 

than R01 (i.e., that R02 is stratigraphically younger material). In many 

stretches of the study area, however, the distinct acoustic contrast of R02 

with other sedimentary units became less clear in one of two ways: (1) 

observation of multiple reflection surfaces having similar acoustic 

signatures (Figure 11a) or (2) R02 showing a change in acoustic signature 

along the sub-bottom profile (Figure 11b), suggesting either change in the 

geologic material comprising the sedimentary unit or a lack of acoustic 

penetration into the riverbed. R02 was not digitized in these regions. The 

depth below the riverbed of R02 was gridded in Matlab using nearest 

neighbor with a search radius of 250 ft and a smoothing interval of 8 cells 

(Figure 12). Accordingly, the gridded surface of R02 shows significant data 

holidays (white areas) in between regions of mapped sediment (Figure 12).

Figure 9. Example showing the base of mobile sand in an (A) un-interpreted seismic profile 

and (B) interpreted image of the seismic profile (green line, R02). 
MRG&P Report No. 31 17

Figure 10. Example of the base of the mobile sediment layer overlying the subsurface rock in 

(A) un-interpreted seismic profile and (B) interpreted image of the seismic profile (rock: 

orange R01; mobile sediment base: orange R02). 

Figure 11. Upper two panels show an example of multiple intersecting layers with the same 

acoustic signature as R02 in (A) un-interpreted seismic profile and (B) interpreted image of 

the seismic profile. Lower two panels show an example of the R02 reflection surface being 

lost due to absorption of the acoustic signal in (A) un-interpreted seismic profile and (B) 

interpreted image of the seismic profile. 
MRG&P Report No. 31 18

Figure 12. Depth of the interpreted base of the mobile sand wave layer, R02, under the 

riverbed in feet. Regions where the rock was outcropping on the riverbed are delineated by 

black lines. 

Horizontal and vertical error

Data were collected using an Applanix POS system recording real-time 

Real-Time Kinematic (RTK) corrections. The horizontal error associated 

with RTK corrections is generally less than 1 ft. In addition, the location of 

the towfish was not centered directly below the Global Positioning System 
MRG&P Report No. 31 19

(GPS) antennae, but that offset was not provided to CHL. On a typical, 

small survey vessel, that offset is usually approximately 2-4 ft, though 

applying those numbers here is pure speculation. Based on vessel size and 

the described survey arrangements, a horizontal error of 4 ft was applied. 

Given the line spacing of the survey lines (300 ft apart) and the goals of 

this study as a proof-of-concept, as well as a mapping effort of larger 

indurated or rock material, a horizontal error of 4 ft will not likely impact 

the results of this work. 

Vertical error in the seismic profile is dependent on (1) the assumption of a 

standard speed of sound of 1485 m/s (assuming freshwater and an average 

water temperature of 20 °C) and (2) errors in digitization by the processor. 

Speed of sound, the rate at which sound travels through a sedimentary 

unit, is a function of the sediment’s density, porosity, grainsize, 

temperature, and mineralogy and is higher in coarse-grained, sandy 

sediments vs. finer-grained, muddy sediment. Despite these variations, 

overall the speed of sound in sediment of all types has been determined to 

be very similar to the speed of sound in water (usually 1%-3% less in 

sediment than in water (e.g., Shumway 1960; Hamilton 1963, 1965; 

Gorgas et al. 2002; Yang and Tang 2017). This translates to a potential 

maximum vertical error of <0.5 ft, but many researchers have suggested 

that the effect is so minimal for stratigraphic purposes that it can be 

neglected altogether (e.g., Hamilton 1972; Kibblewhite 1989; Bowles 1997; 

Buckingham 1997; Gorgas et al. 2002). A greater source of vertical error is 

inherent in the digitization process. Chesapeake Technology SonarWiz 

uses sophisticated algorithms to identify the sediment-water interface and 

auto-digitize the riverbed. The process is not perfect and requires visual 

inspection to ensure that the software does not erroneously identify water 

column noise (such as a school of fish) as the riverbed. Subsequent 

digitization of underlying reflection surfaces are completed by hand, and 

slight variations in the position of the mouse will also introduce vertical 

errors. The error in vertical elevation is directly related to subtle variations 

in each digitized point as entered by the processor, and thus is different for 

every given project. For the data processing effort performed by CHL and 

presented in this report, the difference in vertical elevation between 

subsequent horizontal digitized points was determined on multiple 

regions for 12 seismic lines and averaged 0.78 ft, with a standard deviation 

of 0.57 ft. Note that these errors can compound. Accordingly a vertical 

error of ~2 ft should be considered when evaluating these data. This error 
MRG&P Report No. 31 20

would persist even if the data were further rectified to known datum (i.e.,

to a rectified bathymetric survey).
MRG&P Report No. 31 21

4 Discussion

Applicability in riverine environments 

One of the overarching goals of this effort was to determine the suitability 

of acoustic chirp reflection technology to map subsurface indurated and 

rock units in the Mississippi River. Although chirp data have been 

collected successfully in riverine environments, the specific goals of this 

application differ from what has been conducted before in similar 

environments. In addition, the cost to acquire chirp technology and the 

extensive training/educational requirements of data collection personnel 

and data interpretation personnel are significant and support a proof-ofconcept survey.

All 83 survey lines collected were suitable for importation and processing, 

and all contained usable sub-bottom data in the upper portion of the 

record. Although there were limitations with the chirp frequency used on 

any given sonar line, and the overall transmitting power of the towfish 

(discussed in more detail below), the data yield a map of the underlying 

shallow stratigraphy of the Mississippi River. Data collection was 

completed in 3 days, not including mobilization/demobilization time, 

indicating this was a quick and thus economically viable mapping 

technique, especially given that bathymetric data could be collected 

simultaneously. The river conditions did not preclude the collection of 

quality data, and the final survey line plan shows minimal deviations from 

the planned survey, suggesting navigability of the survey lines around 

shallow regions and/or vessel traffic was excellent. Overall, this dataset 

indicates that, with some future modification of equipment and survey 

execution, shallow acoustic chirp geophysical surveys show great potential 

for the mapping of subsurface stratigraphy in the Mississippi River.

Interpretation of R01 

R01 is interpreted to represent the irregular, pinnacle-like rock 

frequently exposed in the Mississippi River. The acoustic signature of 

this surface, including the acoustic bowties seen where it closely 

underlies the riverbed, the acoustic transparency below it, and the 

smudged appearance it has at depth, strongly supports interpretation of 

this unit as rock. In addition, the irregular, pinnacle-like nature of the 

outcropping rock continues below the surface, strongly suggesting this 
MRG&P Report No. 31 22

unit is not sedimentary but rather is lithified rock that has been 

subsequently folded and faulted (Figures 6, 7, 10). Rock is very 

distinctive in shallow reflection imagery, and the lack of sediment cores 

does not preclude this interpretation.

The discontinuous nature of R01 is likely related to several factors. The 

primary factor is the irregularity of the rock surface. At the riverbed, R01 

exhibits significant vertical relief, often in the form of steep pinnacles, and 

similar relief is observed in the subsurface formation (Figure 13). On many 

of the seismic lines, R01 plunges below the penetration limit of the towfish 

used (Figures 7, 10; see below for further discussion of that issue). The 

rock formations in the vicinity of Cape Girardeau are also faulted in many 

regions, meaning that the elevation of the R01 formation at any one 

location is not always spatially consistent. Finally, the cross-shore width of 

any one elevation of R01 was only ~100 ft. Given that the survey lines were 

spaced ~300 ft apart, this complicates extrapolation of this highly 

irregular surface between each survey line.

Figure 13. Example of highly variable surface elevation in exposed and buried rock at Cape 

Girardeau: (A) un-interpreted seismic profile and (B) interpreted image of the seismic profile 

(orange line, R01). 

Despite these limitations, the general nature of R01 is clearly discernible 

from the data. Overall, R01 is highly irregular and found throughout the 

study area. In three major regions, R01 is within 10 ft of the riverbed (near 

RM 54, 51.5, and 43.5-46; Figure 8), representing a potential hazard for 

transiting vessels depending on sand mobility and river stage. To better 

illustrate the regions where R01 might present a hazard to navigation, a 

hazard map was generated for the region based on the gridded data 

(shown in Figure 8) and is presented in Figure 14. On this map, 

everywhere that R01 was mapped within 10 ft of the riverbed is shaded in 
MRG&P Report No. 31 23

gray, and the red lines that overlay the gray regions indicate exposed rock. 

Regions where R01 was mapped between 10 to 15 ft from the riverbed are 

shaded in yellow, and regions where R01 was only mapped at depths of 

15 ft or more below the riverbed are shaded in green. Note that the spacing 

of the survey lines was insufficient to fully characterize the small-scale 

variability in R01, and thus additional, small-scale patches of shallow R01 

might exist outside of the gray and yellow regions, but were not mapped. 

In addition, very small near-surface pinnacles of R01 were likely smoothed 

out in the gridding process. Despite these limitations, the hazard map is 

useful for identifying regions in the study area with greater potential 

navigability hazards than other regions.

Note the significant data holidays (white regions) along the northern 

shoreline between RM 47-48 on Figure 14, as well as along the western 

shoreline between RM 49.5-51. Figure 1 indicates that chirp data were 

collected in these white regions, so the lack of subsurface R01 is not due to 

a lack of data. Instead, these regions where characterized by very dense 

acoustic returns (hard, surficial geology), as evident in the nearly acoustictransparency of the profile below the surface (Figure 15). Faint reflection 

surfaces are visible below the riverbed but are not coherent enough to be 

identified. The shape of the riverbed and the faint reflection surfaces below 

it suggest the sediment here is very hard and dense, likely sand in the form 

of sand waves. These data suggest that the towfish used lacked the ability 

to transmit an acoustic pulse through the entire sand layer, and this 

challenge is explored more in Chapter 4, below.
MRG&P Report No. 31 24

Figure 14. Hazard map for sub-surface R01 in the study area, shown as depth below the 

riverbed (thickness) in feet. Red lines indicate outcrops of R01 at the riverbed surface. White 

areas indicate data holidays. 
MRG&P Report No. 31 25

Figure 15. Example of an acoustically transparent profile underneath dense (hard) sediment. 

The morphology of the riverbed (sand waves) and the bowties suggest the surface was 

composed of very hard sediment, which reflected most of the acoustic energy. However, faint 

reflection surfaces are mapped below the surface, indicating that the riverbed here is not 

composed of rock. Accordingly, this is interpreted to represent a surficial deposit of sand 

overlying finer-grained (less dense) material. 

Interpretation of R02 

In addition to mapping the basement rock of a region (here, R01), acoustic 

chirp reflection technology can be used to produce stratigraphic maps of 

underlying sediment layers. A secondary goal of this effort was to identify 

the base of the mobile sediment layer of the Mississippi River in this 

location. Mobile sediment can take many forms, including mobile sand 

ripples or waves, as well as fluid mud layers. Without sediment cores, 

interpreting various reflection surfaces as the contact between sand and 

mud, or mud and gravel, etc., is tenuous at best.

The Cape Girardeau sub-bottom data clearly show not only multiple layers 

of sediment below the riverbed in many places but also the presence of 

sand waves over much of the study area (e.g., between RM 47-48.5; 

Figures 11, 15). The interpretation that the surface is sand is based on (1) 

the bathymetry of the riverbed showing undulations in depth consistent 

with sand waves, (2) the strong acoustic reflection and occasional bowties 

associated with the surface of the riverbed indicating dense (hard) 

material, and (3) the mostly acoustically transparent profile beneath the 

sand waves, indicating much of the acoustic energy was reflected back to 

the towfish (Figures 10, 11, 15). A discontinuous but coherent reflection 

surface (R02) was discernible in much of this region (Figure 12) and likely 

represents an indurated, erosion-resistant surface on which the sand 

waves are migrating. This reflection surface was mapped, on average, 

within ~6 ft of the riverbed (Figure 12), suggesting the sand deposit 

comprising the waves averages 6 ft in thickness. Note that in some regions, 

multiple reflection surfaces were observed that could represent the base of 

the migrating sand waves (e.g., Figure 11b), complicating the 

interpretation of R02 over the entire study area. This likely reflects 
MRG&P Report No. 31 26

multiple indurated sedimentary units below the riverbed and would be 

consistent with the complex geology of the region. Arbitrarily selecting one 

of the reflection surfaces as R02 is presumptive, however, without 

sediment cores to groundtruth the different layers. In addition, in some 

regions multiple reflection surfaces were noted without the presence of 

sand waves (Figure 4), likely reflecting multiple fine-grained sedimentary 

deposits, as the greater penetration of the acoustic energy is indicative of 

less-dense sediment. Identifying which of these reflection surfaces 

represents the base of a mobile fine-grained layer is not possible without 

sediment cores; however, these data do provide guidance for core 

collection if absolute identification of the base of the various mobile 

sediment layers in this region is needed.

Data holidays vs. lack of signal penetration

Large swaths of the study area provided limited geophysical data — either in 

the identification of the subsurface elevation of R01 or in the identification 

of the base of the mobile sediment layer(s) known as R02. These data 

holidays are most clearly seen as white regions in Figures 8, 12, and 14. 

Multiple factors can result in partial or complete data holidays during a 

geophysical survey, including (1) lack of coverage (e.g., a planned survey line 

was altered due to unexpected bathymetric hazards and/or to avoid 

transiting vessels), (2) too high of an acoustic pulse used (e.g., a higher 

pulse was used to improve minimal layer resolution with a trade-off in 

signal penetration), (3) insufficient record length during collection (e.g., the 

acoustic pulse used was low frequency enough to penetrate to the needed 

depths, but either the vessel moved too quickly or the towfish did not listen 

long enough between pulses to hear the slower, deeper returns), or (4) lack 

of signal penetration due to insufficient towfish transmittal power.

In the case of this study, the transited survey lines as shown in Figure 1 

show very few large deviations from shore-parallel tracks, suggesting that 

the first factor, lack of coverage, is likely not a factor here. Although it has 

been previously noted that the spatial extent of the features being mapped is 

smaller than the survey line spacing (e.g., ≤100 ft vs. ~300 ft, respectively), 

the gridding algorithms used to generate Figures 8, 12, and 14 eliminate any 

data holidays that would result from that gap, at a potential trade-off of 

accuracy. Accordingly, survey line spacing is not considered to be a 

significant factor in the existence of the observed data holidays.
MRG&P Report No. 31 27

Factors 2-4 noted above are a function of the user-determined settings at 

the time of collection, as well as vessel survey speed, and are explained in 

more detail, below. If the user selected a higher frequency pulse to better 

delineate the near-surface stratigraphy, the higher frequency would also not 

penetrate very far into the riverbed, and the result would be the acoustically 

transparent seismic profiles seen, for example, in Figures 9 and 11. 

Alternatively, the user might have selected a lower frequency pulse to 

enhance signal penetration (e.g., Figures 4, 10), but if the pulse rate or 

record length were set high, there would be insufficient listening time by the 

towfish in between each acoustic pulse to record the longer return times 

associated with reflections from greater depths, resulting in low resolution 

near the surface and little or no return from depth. In this case, the 

elevation of the rock near the riverbed surface was a bigger concern than the 

elevation of the rock at depth. Accordingly, the pulse selected by the survey 

team was determined in the field to maximize the trade-off between 

resolution and penetration to achieve the goals of mapping the base of the 

mobile sediment layer (requiring high resolution) and mapping the depth to 

the pinnacle rock layer (requiring high penetration), without requiring 

multiple efforts to collect the same line using different pulses (costprohibitive). In addition, the vessel was held at a constant, slow speed, 

maximizing listening time. Accordingly, although pulse selection might have 

resulted in some data loss at greater depths in regions, neither the pulse 

selection nor the vessel speed during collection are considered to be the 

primary factors in the existence of the observed data holidays.

The final factor that can result in data holidays is a lack of sufficient 

transmitting power by the towfish to penetrate deep enough into the 

riverbed’s stratigraphy to reflect off of deeper layers. As detailed in the 

Methods section above, the EdgeTech 3100p (216s) has a typical 

maximum working penetration of ~15 ft in sand (~80 ft in mud), and this 

reflects, on average, the typical penetration seen below the sand waves in 

this region (~6-8 ft for underlying sediment; ~12-15 ft for underlying 

rock). R01 was mapped at depths of up to ~20 ft under the riverbed in 

some locations, but (1) most of the deeper rock was observed underlying 

finer-grained, less-dense sediment (e.g., acoustically absorbent material 

such as that seen in Figures 6 and 7), and (2) rock provides a powerful 

acoustic return (100% of the transmitted signal) that is often reflected at 

the maximum penetration depth of a pulse and towfish (e.g., ~15-22 ft, 

Figure 10). The transmit power of an acoustic reflection chirp towfish is 

created by piezoelectric ceramic plates, which generate an acoustic pulse. 
MRG&P Report No. 31 28

Larger (and heavier) ceramics generate higher transmit powers (louder 

sounds) and lower frequencies (enhancing penetration at the cost of 

resolution) but also greatly increase the size and weight of the towfish. As 

an example, the EdgeTech 3100p (216s) utilized in this study weighs ~170 

pounds (lb) in air and, as such, is portable and easily towed by a small 

vessel or mounted on a towing plate. In contrast, the EdgeTech 3200 

(512i), which has the potential to penetrate up to 60 ft of sand in ideal 

conditions, weighs ~550 lb in air and requires either a large, deep-draft 

vessel to safely tow, or a sophisticated mount on a smaller towing vessel to 

manage safely. The data collected here suggest that greater penetration is 

needed to map the deeper stratigraphy of R01 and the full spatial extent of 

R02. It is possible that improved signal penetration could be achieved with 

the EdgeTech 3100p (216s), using the lowest available frequency and a 

correspondingly low pulse rate as well as a slower survey speed to 

maximize deeper returns, but this would come at the expense of greater 

resolution in the shallower stratigraphy. Most likely, a different towfish 

will be needed to reliably collect data at depths greater than ~8-10 ft under 

sand.
MRG&P Report No. 31 29

5 Conclusions

Overall, the geophysical surveying conducted in 2016 in the vicinity of 

Cape Girardeau, MO, successfully demonstrated the applicability of 

shallow acoustic chirp sub-bottom profiling for mapping the stratigraphy 

underlying the Mississippi River. Over 77 miles of chirp sub-bottom data 

were collected between RM 55 and 43, and the data collected were 

sufficient to delineate regions of significant subsurface hazards in the form 

of shallow, buried rock that can potentially and ephemerally be exposed in 

the riverbed, representing a hazard to navigation. Although the survey line 

spacing precluded a highly detailed and accurate map of rock elevation at 

the resolution of the feature being mapped, it clearly identified the spatial 

extent of the more hazardous regions of the study area, allowing for more 

focused and cost-effective future surveying. In addition, the data were able 

to partially identify the spatially varying base of the mobile sand layer 

along approximately one-half of the study area, potentially providing more 

accurate bedload volume calculations for the region.

Significant data holidays were noted in the underlying rock maps and in 

the delineation of the base of the mobile sediment layers. These holidays 

are most likely due to limitations in user settings during the initial data 

collection, as well as power limitations inherent in the towfish utilized for 

the survey. In addition, the spacing between the survey lines was greater 

than the spatial extent of the features being mapped, with the result that 

although the gridded data provide an excellent overall view of the 

subsurface elevation of the underlying rock in this region, the exact 

elevation below the riverbed at any one location is not accurate, and the 

gridded elevation can vary by several feet from the actual elevation. 

Despite this, the data clearly identify the more hazardous regions of the 

river, allowing for prioritization of future work.

The region would benefit from additional surveying. A larger, lowerfrequency towfish would provide greater penetration along any one survey 

line. In addition, subsequent surveys to accurately map the specific 

elevation of the underlying rock for navigation and/or removal purposes 

would need a survey line spacing that is closer than the spatial extent of 

the features being mapped. A trained geologist and geophysicist would be 

critical during subsequent data collection efforts, as that individual is 

trained to recognize geologic variability as presented in geophysical data in 

real time and to adjust a survey line plan’s spacing and/or orientation to 
MRG&P Report No. 31 30

accurately map the stratigraphy in question. These accurate data would 

then benefit by being rectified to a standard vertical datum such as North 

American Vertical Datum of 1988 by rectifying them with bathymetric 

data collected with RTK-GPS, such as was originally done during the 

initial data processing effort. From such data, a detailed elevation map 

(with error) could be generated for the region in question. 
MRG&P Report No. 31 31

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Reconstruction of the Grace Dieu, (1439), River Hamble, UK.” Journal of 

Archaeological Science 36: 408-418. 

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and Thebes Quadrangles. Illinois State Geological Survey Circular 332. 

Champaign, IL: Illinois State Geological Survey. 

Roberts, H. H., and J. Supan. 2000. “Acoustic Surveying of Ultra-Shallow Water Bottoms 

(<2.0 m) for Both Engineering and Environmental Applications.” Offshore 

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Schock, S. G., and L. R. LeBlanc. 1990. “Some Applications of the Chirp Sonar.” Oceans

90: 24-26. 

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Studies 13: 259-269. 

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1109. 
MRG&P Report No. 31 33

Appendix: Sub-Bottom File Metadata

The table below lists file-specific importation and interpretation 

information for each sub-bottom profile collected in support of this 

project.

Note the following:

HardB: Indicates outcropping rock on the seismic profile.

R01: Interpreted buried rock.

R02: Interpreted base of mobile sand layer.

Line Name 

% 

Import Bttm Track Settings Shore Orientation Reflectors 

3-01-16 001.002 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-01-16 001.003 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-01-16 001.004 70% Blnk 2.0; Dur 10, Thres 5 parallel R01, R02

3-01-16 001.005 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-01-16 001.006 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-01-16 001.007 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB

3-01-16 001.008 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-01-16 001.010 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-01-16 001.011 70% Blnk 2.0; Dur 10, Thres 5 parallel R01

3-01-16 001.012 70% Blnk 2.0; Dur 10, Thres 5 parallel R02

3-01-16 001.013 70% Blnk 2.0; Dur 10, Thres 7 parallel HardB, R01, R02

3-01-16 001.014 70% Blnk 2.0; Dur 10, Thres 7 parallel HardB, R01, R02

3-01-16 001.015 70% Blnk 2.0; Dur 10, Thres 7 perpendicular HardB, R01

3-01-16 001.016 70% Blnk 2.0; Dur 10, Thres 5 perpendicular HardB, R01, R02

3-01-16 001.017 70% Blnk 2.0; Dur 10, Thres 5 perpendicular HardB, R01

3-01-16 001.018 70% Blnk 2.0; Dur 10, Thres 5 parallel R01

3-01-16 001.019 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-01-16 001.020 70% Blnk 2.0; Dur 10, Thres 7 parallel HardB, R01

3-01-16 001.021 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R02

3-01-16 001.022 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-01-16 001.023 70% Blnk 2.0; Dur 10, Thres 5 parallel R01

3-01-16 001.024 70% Blnk 2.0; Dur 10, Thres 7 parallel R01, R02
MRG&P Report No. 31 34

Line Name 

% 

Import Bttm Track Settings Shore Orientation Reflectors 

3-01-16 001.025 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-01-16 001.026 70% Blnk 2.0; Dur 10, Thres 5 parallel R01, R02

3-01-16 001.027 70% Blnk 2.0; Dur 10, Thres 5 parallel R01, R02

3-01-16 001.028 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-01-16 001.029 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-01-16 001.030 70% Blnk 2.0; Dur 10, Thres 7 perpendicular HardB, R01

3-01-16 001.031 70% Blnk 2.0; Dur 10, Thres 5 perpendicular HardB, R01, R02

3-01-16 001.032 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-01-16 001.033 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-01-16 001.034 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-01-16 001.035 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-01-16 001.036 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-01-16 001.037 70% Blnk 2.0; Dur 10, Thres 5 perpendicular R02

3-01-16 001.038 70% Blnk 2.0; Dur 10, Thres 5 perpendicular HardB, R01

3-02-16 002-CH1 70% Blnk 2.0; Dur 10, Thres 5 parallel R01, R02

3-02-16 002.001 70% Blnk 2.0; Dur 10, Thres 7 parallel R01, R02

3-02-16 002.002 70% Blnk 2.0; Dur 10, Thres 7 parallel HardB, R01

3-02-16 002.003 70% Blnk 2.0; Dur 10, Thres 7 parallel R01, R02

3-02-16 002.004 70% Blnk 2.0; Dur 10, Thres 7 parallel ---

3-02-16 002.005 70% Blnk 2.0; Dur 10, Thres 5 parallel R02

3-02-16 002.006 70% Blnk 2.0; Dur 10, Thres 5 perpendicular R01, R02

3-02-16 002.007 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-02-16 002.008 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-02-16 002.009 70% Blnk 2.0; Dur 10, Thres 7 parallel HardB, R01

3-02-16 002.010 70% Blnk 2.0; Dur 10, Thres 7 parallel ---

3-02-16 002.011 70% Blnk 2.0; Dur 10, Thres 7 parallel ---

3-02-16 002.012 70% Blnk 2.0; Dur 10, Thres 7 parallel R01

3-02-16 002.013 70% Blnk 2.0; Dur 10, Thres 7 perpendicular HardB, R01

3-02-16 002.014 70% Blnk 2.0; Dur 10, Thres 7 parallel R01, R02

3-02-16 002.015 70% Blnk 2.0; Dur 10, Thres 7 parallel ---

3-02-16 002.016 70% Blnk 2.0; Dur 10, Thres 7 parallel ---

3-02-16 002.017 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-02-16 002.018 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02
MRG&P Report No. 31 35

Line Name 

% 

Import Bttm Track Settings Shore Orientation Reflectors 

3-02-16 002.019 70% Blnk 2.0; Dur 10, Thres 5 parallel R01

3-02-16 002.020 70% Blnk 2.0; Dur 10, Thres 5 perpendicular R01

3-02-16 002.021 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-02-16 002.022 70% Blnk 2.0; Dur 10, Thres 5 parallel R01, R02

3-02-16 002.023 70% Blnk 2.0; Dur 10, Thres 5 parallel R01

3-02-16 002.024 70% Blnk 2.0; Dur 10, Thres 5 parallel R01

3-02-16 002.025 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-02-16 002.026 70% Blnk 2.0; Dur 10, Thres 5 parallel R01

3-02-16 002.027 70% Blnk 2.0; Dur 10, Thres 5 perpendicular R01, R02

3-02-16 002.028 70% Blnk 2.0; Dur 10, Thres 5 parallel R01, R02

3-02-16 002.029 70% Blnk 2.0; Dur 10, Thres 5 parallel R01, R02

3-02-16 002.030 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-02-16 002.031 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-02-16 002.032 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB

3-02-16 002.033 70% Blnk 2.0; Dur 10, Thres 7 perpendicular HardB, R00

3-02-16 002.034 70% Blnk 2.0; Dur 10, Thres 7 perpendicular HardB, R01

3-03-16 003-CH1 70% Blnk 2.0; Dur 10, Thres 7 parallel HardB, R01, R02

3-03-16 003.001 70% Blnk 2.0; Dur 10, Thres 7 parallel HardB, R01

3-03-16 003.002 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-03-16 003.003 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-03-16 003.004 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB

3-03-16 003.005 70% Blnk 2.0; Dur 10, Thres 5 perpendicular HardB, R01

3-03-16 003.006 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-03-16 003.007 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01, R02

3-03-16 003.009 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-03-16 003.010 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R01

3-03-16 003.011 70% Blnk 2.0; Dur 10, Thres 5 parallel HardB, R02

3-03-16 003.012 70% Blnk 2.0; Dur 10, Thres 5 perpendicular HardB, R01, R02
MRG&P Report No. 31 36

Unit Conversion Factors

Multiply By To Obtain 

feet 0.3048 meters

inches 0.0254 meters

miles (nautical) 1,852 meters

miles (U.S. statute) 1,609.347 meters

pounds (mass) 0.45359237 kilograms
MRG&P Report No. 31 37

Acronyms and Abbreviations

ft foot(feet)

FM frequency-modulated

FY fiscal year

GPS Global Positioning System

in. inch(es)

kHz kilohertz

lb pound(s)

m/s meters per second

MVS St. Louis District

RM river mile

RTK Real-Time Kinematic 

SAJ Jacksonville District

USACE US Army Corps of Engineers
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data 

sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect

of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations

and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of

law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE 

February 2020

2. REPORT TYPE

Final Report

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Application of Chirp Acoustic Sub-Bottom Data in Riverine Environments: Identification of 

Underlying Rocky Hazards at Cape Girardeau, Missouri, and Thebes, Illinois

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Heidi M. Wadman and Jesse E. McNinch

5d. PROJECT NUMBER

127672

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) (see reverse)

Coastal and Hydraulics Laboratory

U.S. Army Engineer Research and Development Center

Field Research Facility

1261 Duck Rd.

Kitty Hawk, NC 27948

8. PERFORMING

ORGANIZATION REPORT

NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

US Army Corps of Engineers

Washington, DC 20314-1000

10. SPONSOR/MONITOR'S ACRONYM(S)

USACE

11. SPONSOR/MONITOR'S

REPORT NUMBER(S)

MRG&P Report No. 31

12. DISTRIBUTION/AVAILABILITY STATEMENT

Approved for public release; distribution is unlimited.

13. SUPPLEMENTARY NOTES

14. ABSTRACT

Shallow acoustic reflection (chirp) data have been utilized to map the elevation of underlying stratigraphy in a wide range of aqueous 

environments. Of particular concern in riverine regions is the elevation of near-surface underlying rock that, if exposed during normal 

migration of sedimentary bedforms, can cause grounding and damage to vessels transiting the region during periods of low water. Given 

the ephemeral nature of the rock’s exposure, traditional surveying methods are insufficient to map rock when it is covered by a thin 

veneer of sediment, increasing the potential hazard. Accordingly, the US Army Corps of Engineers, St. Louis District, (MVS) explored 

the use of chirp sub-bottom surveys to identify buried rock within the Mississippi River in the vicinity of Cape Girardeau, MO, and 

Thebes, IL. Hazard maps showing the distribution of buried rock were generated, and the base of the mobile sediment layer was

identified where possible. These data will allow MVS to accurately identify potentially hazardous regions during periods of low water. 

Although the study did not result in the complete mapping of all near-surface geologic hazards, regions that warrant further study are 

identified, and modifications to the original survey plan are provided to improve the accuracy of future data collection efforts.

15. SUBJECT TERMS 

Fluvial geomorphology, Geophysical surveys, Mississippi River, River channels, Rocks, Sedimentation and deposition

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 

ABSTRACT

SAR

18. NUMBER 

OF 

PAGES

45

19a. NAME OF RESPONSIBLE PERSON

James W. Lewis a. REPORT

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified 19b. TELEPHONE NUMBER (Include area code)

601-634-5062

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