SURFACE-GEOPHYSICAL TECHNIQUES USED TO 

DETECT EXISTING AND INFILLED SCOUR HOLES

NEAR BRIDGE PIERS

By Gary Placzek and P.P. Haeni

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 95-4009

Prepared In cooperation with the

FEDERAL HIGHWAY ADMINISTRATION

Hartford, Connecticut 

1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For additional information Copies of this report can be

write to: purchased from:

Chief, Branch of Geophysical Applications U.S. Geological Survey

and Support, Office of Ground Water Earth Science Information Center

U.S. Geological Survey Open-File Reports Section

450 Main Street Box 25286, MS 517

Room 525 Denver Federal Center

Hartford, CT 06103 Denver, CO 80225
CONTENTS

Abstract.........................................................................................................................................!

Introduction....................................................................._^

Purpose and scope.............................................................................................................2

Previous investigations.....................................................................................................3

Principles of surface-geophysical techniques ...............................................................................3

Fathometer profiling .........................................................................................................5

Continuous seismic-reflection profiling............................................................................5

Ground-penetrating radar..................................................................................................7

Equipment and modification.........................................................................................................8

Fathometer systems...........................................................................................................8

Continuous seismic-reflection profiling systems............................................................ 10

Ground-penetrating radar system.................................................................................... 12

Position-recording system...............................................................................................13

Data collection near bridge piers ................................................................................................14

Collection of geophysical data........................................................................................ 14

Collection of verification data......................................................................................... 15

Processing techniques applied to surface-geophysical and position data................................... 16

Fathometer and position data.......................................................................................... 16

Color Fathometer and continuous seismic-reflection profiling data............................... 19

Ground-penetrating radar data........................................................................................22

Results of surface-geophysical techniques at selected bridge sites........................................... 24

Old Baldwin Bridge, Old Saybrook................................................................................24

New Baldwin Bridge, Old Saybrook..............................................................................26

Route 14 Bridge, Canterbury..........................................................................................31

Bulkeley Bridge, Hartford..............................................................................................31

Route 4 Bridge, Farmington ...........................................................................................36

Route 2 Bridge, Norwich................................................................................................37

Future equipment modifications and research needs ..................................................................38

Summary and conclusions..........................................................................................................39

References cited..........................................................................................................................42

FIGURES

1. Map showing the bridge sites in Connecticut used for this study ............................................4

2. Positioned 200-kilohertz Fathometer cross-section 5 feet upstream of the piers

at the Bulkeley Bridge, Hartford, Conn......................................................................... 17

3. Three-dimensional bathymetric contour plot upstream from the Bulkeley Bridge,

Hartford, Conn................................................................................................................ 18

4. Unprocessed positioned 14-kilohertz continuous seismic-reflection profiling data collected 

10 feet upstream from pier 4 of the Bulkeley Bridge, Hartford, Conn...........................21

5. Digitally filtered, horizontally corrected, deconvolved 14-kilohertz data from figure 4........21

6. Gray-scale amplitude plot of 2- to 16-kilohertz swept-frequency record collected

10 feet upstream from pier 7 at the new Baldwin Bridge, Old Saybrook, Conn.

(after construction)..........................................................................................................22

in
FIGURES-continued

7. Unprocessed 300-megahertz ground-penetrating radar data collected 2 feet

upstream from the pier at the Route 14 bridge in Canterbury, Conn.............................23

8. Wiggle-trace plot of digitally filtered and migrated 300-megahertz ground-penetrating

radar data from figure 7 ..................................................................................................23

9. Unpositioned 200-kiloheitz Fathometer profile collected 10 feet upstream from the piers

at the old Baldwin Bridge, Old Saybrook, Conn............................................................25

10. Unpositioned 3.5-kiloheitz continuous seismic-reflection profiling data collected 10 feet

upstream from pier 8 at the old Baldwin Bridge, Old Saybrook, Conn.........................25

11. The old and new Baldwin Bridge pier locations, Old Saybrook, Conn................................27

12. Unpositioned 200-kiloheitz Fathometer profile collected along the proposed bridge

alignment of the new Baldwin Bridge, Old Saybrook, Conn, (before construction) .....28

13. Unpositioned 200-kiloheitz Fathometer data collected 15 feet upstream of the new

Baldwin Bridge, Old Saybrook, Conn, (during construction) ........................................29

14. Unpositioned 200-kiloheitz Fathometer data collected 15 feet upstream of the new

Baldwin Bridge, Old Saybrook, Conn, (after construction) ...........................................29

15. Unpositioned 3.5-kiloheitz continuous seismic-reflection profiling data collected 15 feet

upstream from the new Baldwin Bridge, Old Saybrook, Conn, (after construction) .....29

16. Unpositioned 14-kilohertz continuous seismic-reflection profiling data collected 15 feet

upstream from the new Baldwin Bridge, Old Saybrook, Conn, (after construction) .....30

17. Black-and-white photograph of a 20-kilohertz color Fathometer cross section collected

12 feet upstream from pier 4 of the Bulkeley Bridge, Hartford, Conn........................... 3 3

18. Gray-scale amplitude plot of 1.5- to 10-kilohertz swept-frequency record collected

5 feet upstream from piers 4 and 5 at the Bulkeley Bridge, Hartford, Conn..................34

19. Unpositioned 7-kilohertz continuous seismic-reflection profiling data collected 20 feet

upstream from the Bulkeley Bridge, Hartford, Conn.....................................................34

20. Positioned 100-megahertz ground-penetrating radar data collected 15 feet upstream

from piers 4 and 5 of the Bulkeley Bridge, Hartford, Conn...........................................35

21. Unpositioned 300-megahertz ground-penetrating radar data collected 4 feet east

of the center pier at the Route 4 Bridge, Farmington, Conn...........................................36

22. Unpositioned 200-kilohertz Fathometer data collected 3 feet upstream from the piers at

the Route 2 Bridge, Norwich, Conn...............................................................................37

TABLES

1. Typical reflection coefficients...................................................................................................6

2. Approximate values for electromagnetic properties of selected materials............................... 8

3. Summary of performance criteria for selected surface-geophysical techniques ....................39

4. Summary of geophysical data collected at six bridge sites in Connecticut............................40

IV
CONVERSION FACTORS AND ABBREVIATIONS

Multiply_________________By_________To obtain___________

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Other abbreviations used:

dB, decibel

kB/s, kilobyte per second

kHz, kilohertz

kW, kilowatt

MB, megabyte

MHz, megahertz

ms, millisecond

ns, nanosecond

jiiS/cm, microsiemens per centimeter

W, watt
SURFACE-GEOPHYSICAL TECHNIQUES USED TO 

DETECT EXISTING AND INFILLED SCOUR HOLES

NEAR BRIDGE PIERS

By Gary Placzek and P.P. Haeni

ABSTRACT

Surface-geophysical techniques were 

used with a position-recording system to study 

riverbed scour near bridge piers. From May 

1989 to May 1993, Fathometers, fixed- and 

swept-frequency continuous seismic-reflection profiling (CSP) systems, and a groundpenetrating radar (GPR) system were used 

with a laser-positioning system to measure the 

depth and extent of existing and infilled scour 

holes near bridge piers. Equipment was purchased commercially and modified when necessary to interface the components and (or) to 

improve their performance.

Three 200-kHz black-and-white chartrecording Fathometers produced profiles of the 

riverbed that included existing scour holes and 

exposed pier footings. The Fathometers were 

used in conjunction with other geophysical 

techniques to help interpret the geophysical 

data. A 20-kHz color Fathometer delineated 

scour-hole geometry and, in some cases, the 

thickness of fill material in the hole. The signal 

provided subbottom information as deep as 10 

ft in fine-grained materials and resolved layers 

of fill material as thin as 1 ft thick.

Fixed-frequency and swept-frequency 

CSP systems were evaluated. The fixedfrequency system used a 3.5-, 7.0-, or 14-kHz 

signal. The 3.5-kHz signal penetrated up to 50 

ft of fine-grained material and resolved layers 

as thin as 2.5-ft thick. The 14-kHz signal 

penetrated up to 20 ft of fine-grained material 

and resolved layers as thin as 1-ft thick. The

swept-frequency systems used a signal that 

swept from 2- to 16-kHz. With this system, up 

to 50 ft of penetration was achieved, and fill 

material as thin as 1 ft was resolved. Scourhole geometry, exposed pier footings, and fill 

thickness in scour holes were detected with 

both CSP systems.

The GPR system used an 80-, 100-, or 

300-megahertz signal. The technique produced 

records in water up to 15 ft deep that had a specific conductance less than 200 |LiS/cm. The 

100-MHz signal penetrated up to 40 ft of resistive granular material and resolved layers as 

thin as 2-ft thick. Scour-hole geometry, the 

thickness of fill material in scour holes, and 

riverbed deposition were detected using this 

technique.

Processing techniques were applied 

after data collection to assist with the interpretation of the data. Data were transferred from 

the color Fathometer, CSP, and GPR systems 

to a personal computer, and a commercially 

available software package designed to process GPR data was used to process the GPR 

and CSP data. Digital filtering, predictivedeconvolution, and migration algorithms were 

applied to some of the data. The processed 

data were displayed and printed as color 

amplitude or wiggle-trace plots. These processing methods eased and improved the interpretation of some of the data, but some 

interference from side echoes from bridge 

piers and multiple reflections remained in the 

data.
The surface-geophysical techniques 

were applied at six bridge sites in Connecticut. 

Each site had different water depths, specific 

conductance, and riverbed materials. Existing 

and infilled scour holes, exposed pier footings, 

and riverbed deposition were detected by the 

surveys. The interpretations of the geophysical 

data were confirmed by comparing the data 

with lithologic and (or) probing data.

INTRODUCTION

Riverbed scour near bridge piers is a 

widespread problem throughout the United 

States. Scour near bridge piers has caused 

scour holes to develop, piers to fail, and 

ultimately, bridges to collapse. Many bridge 

failures throughout the United States have 

been attributed to this problem. From 1961 to 

1976,46 of 86 major bridge failures were a 

result of scour near piers (Murillo, 1987). 

More bridge failures during that period were 

caused by scour than by earthquakes, wind, 

structural, corrosive, accidental, and 

construction-related causes. Streambed 

aggradation and degradation, contraction 

scour, and local scour were found to be the 

main contributing factors to scour near bridge 

piers (Richardson and others, 1991).

Measurements of riverbed scour and 

deposition near bridge piers are essential to 

studying the scour process. By understanding 

the scour process, better initial construction 

techniques and scour countermeasures can be 

developed to minimize scour effects. 

Maximum scour depths near a bridge pier 

usually occur during the peak of a flood when 

measurements are difficult, dangerous, or even 

impossible to collect. In addition, bed load on 

alluvial channels is redeposited shortly after 

the peak flow but long before the flood has 

receded (Trent and Landers, 1991), so scour 

holes can be infilled. If the scour hole has been 

infilled, data collected after the flood's peak 

using a sounding weight or a high-frequency

Fathometer will not measure the maximum 

scour depth. After the flood recedes, surfacegeophysical techniques may be able to detect 

and measure the maximum scour depth that 

occurred during the peak of the flood. Surfacegeophysical records collected near bridge piers 

contain a continuous profile of the water 

bottom and, at some sites, can delineate 

reflectors in the subbottom. These subbottom 

reflectors can be correlated to old scour 

surfaces, pier foundations, old channel beds, or 

geologic layers.

In 1989, the U.S. Geological Survey 

(USGS) and the Federal Highway 

Administration (FHWA) began a cooperative 

study to evaluate and improve the 

effectiveness of several surface-geophysical 

techniques used to detect riverbed scour near 

bridge piers. The study included evaluating 

surface-geophysical equipment, developing 

data-collection and interpretation methods, and 

applying these results to bridge sites in 

different physical settings. Six bridge sites in 

Connecticut were chosen for this study (fig. 1), 

and geophysical techniques were used to 

examine existing and infilled scour holes near 

piers at these sites. The sites differ in river 

width and depth, specific conductance of water 

(freshwater or saltwater), and in the types of 

river bottom and subbottom materials. Two of 

the larger sites, where geologic information 

was available, were designated as primary test 

sites. New or modified instrumentation and 

new data collection and interpretation methods 

were tested at the primary test sites.

Purpose and Scope

The purpose of this report is to describe 

(1) the surface-geophysical techniques used to 

measure existing and infilled scour holes near 

bridge piers; (2) the new or modified equipment, data-collection methods, and interpretation methods developed for this application; 

and (3) the interpretation of data collected
from selected bridge sites in different physical 

settings. This report includes descriptions of 

the basic principles of the surface-geophysical 

techniques and equipment used during the 

study. Additional details are provided on datacollection methods and data processing and 

interpretation. Each of the six bridge sites are 

described with interpretations of the geophysical records. Finally, advantages and limitations 

of each geophysical technique and future 

development needs in this field are discussed.

Previous Investigations

Scour processes have been extensively 

studied, and mathematical models have been 

developed from physical models and field 

data. Numerous scour-prediction equations 

have been published (Anderson, 1974; Hopkins and others, 1980; Richardson and Richardson, 1989), but predicted scour depths can 

vary substantially when equations are applied 

outside the range of conditions for which they 

were developed (Jarrett and Boyle, 1986). 

Field measurements of scour depths are 

needed to put practical constraints and limits 

on these prediction equations (Trent and Friedland, 1992). Some of these field measurements 

can be made using geophysical techniques.

Surface-geophysical techniques have 

been used successfully to map the subbottom 

stratigraphy in lakes and rivers (Wright and 

others, 1984; Morrissey and others, 1985; 

Haeni, 1986; Hansen, 1986; Haeni and others, 

1987; Reynolds and Williams, 1988, Cardinell 

and others, 1990; Beres and Haeni, 1991). 

Continuous seismic-reflection profiling (CSP) 

and ground-penetrating radar (GPR) techniques have been used to evaluate scour holes 

near bridge piers (Gorin and Haeni, 1989; 

Crumrine, 1991; Haeni and Placzek, 1991; 

Haeni and others, 1992; Placzek and others, 

1993; Mueller and others, 1994). One study 

has shown that continuous seismic-reflection 

profiles could be used to determine quantitative material classification data (Hart and

Caulfield, 1989). The geophysical data collected for these studies were useful in determining depth of existing scour holes, thickness 

of fill material in infilled scour holes, depth to 

and extent of buried rip-rap, location of old 

river channels, depth to bedrock, and geometry 

of pier foundations.

PRINCIPLES OF SURFACEGEOPHYSICAL TECHNIQUES

All the surface-geophysical techniques 

used in this study operate using the same basic 

principles of wave propagation and reflection. 

From the water surface, a signal is transmitted 

into water and is reflected back to the water 

surface by interfaces between materials with 

different physical properties. By observing the 

reflected signals, information about the depth 

to and the type of interface can be determined. 

The main differences between the individual 

geophysical techniques are the types of signals 

transmitted and the physical property changes 

that cause reflections. Fathometer and CSP 

systems use seismic signals, and reflections are 

caused by interfaces between materials with 

different acoustic properties. The GPR system 

uses electromagnetic signals, and reflections 

are caused by interfaces between materials 

with different electrical properties.

Seismic and electromagnetic signals 

behave differently as they travel through the 

same material. They travel at different velocities, and different characteristics of the materials can attenuate, scatter, or reflect the energy. 

Seismic signals can be scattered by gas or air 

bubbles, therefore, CSP techniques are not 

used in settings with gassing organic materials. 

Electromagnetic signals are sensitive to electrical conductivity and are attenuated quickly 

in saltwater or conductive geologic materials 

such as silts and clays; therefore, GPR techniques are not used in saltwater or to penetrate 

silt and clay formations.
73°

01127000

EXPLANATION

USGS stream-flow gaging station

BRIDGE Number indicates name and location

1 Old Baldwin Bridge, Old Saybrook

2 New Baldwin Bridge, Old Saybrook

3 Bulkley Bridge, Hartford

4 Route 4 Bridge, Farmington

5 Route 2 Bridge, Norwich

6 Route 14 Bridge, Cantebury

30 MILES

10 KILOMETERS

Figure 1 . Bridge sites in Connecticut used for this study.
Fathometer Profiling

Fathometers determine water depth by 

repeatedly transmitting seismic energy through 

the water column and recording the arrival 

time of the reflected energy from the water 

bottom. The time required for the seismic signal to travel from its source to a reflector and 

back is known as the two-way travel time, and 

it is measured in milliseconds (ms) (equal to 

1 x 10~3 seconds). The Fathometer calculates 

the depth to a water bottom by dividing the 

two-way travel time by two and multiplying 

the result by the velocity of sound through 

water:

o - vj

"where'. D = depth to the water bottom (ft), 

V = velocity of sound through

water (ft/s), and 

t = two-way travel time (s).

The depth value is printed as a continuous graphic profile and (or) displayed as a 

numeric value by the Fathometer. Fathometers 

are calibrated by adjusting the value of V (eq. 

1), which may vary slightly depending on 

water type. Most Fathometers use a narrowbandwidth 200-kHz seismic signal. These 

Fathometers provide accurate depth data, but 

little or no information about the subbottom. 

Fathometers that use a lower frequency signal, 

such as 20 kHz, can detect reflected energy 

from subbottom interfaces, such as the bottom 

of an infilled scour hole. A color Fathometer is 

calibrated to measure and display in color the 

amplitude of the reflected signal, which, in 

constant water depths, can be related to characteristics of the bottom material.

Continuous Seismic-Reflection 

Profiling

Continuous seismic-reflection 

profiling (CSP) systems transmit seismic 

energy from a transducer located just beneath 

the water surface. The energy passes through

the water column and into the subbottom. 

Where a change in acoustic impedance occurs, 

such as at the water/bottom interface, part of 

the seismic energy is reflected back to the 

water surface. The remainder of the seismic 

energy is transmitted further into the material. 

The amount of energy that is reflected by an 

interface is determined by the reflection 

coefficient of that interface, which is 

dependent upon the acoustic impedance of the 

material above and below the interface. 

Acoustic impedance is defined as the product 

of the density of a material and the velocity of 

sound through the material:

Z = pV (2)

where: Z = acoustic impedance (g/m2s), 

p = density of the material (g/m3),

and 

V = velocity of sound through the

material (m/s).

The reflection coefficient is defined as 

the difference of the acoustic impedances of 

the materials at an interface divided by their 

sum:

^-"7 7 7 ^M **r 4

R = z^z! = ii (3)

where: R = reflection coefficient for this

interface, 

Zi,2/2 = acoustic impedance of

material above and below the

interface, and 

Ap AI = amplitude of the reflected and

incident waves at the

interface.

Typical reflection coefficients for selected 

geologic interfaces are shown in table 1. These 

reflection coefficients indicate that the 

amplitude of the reflected signal from a 

water/sand interface (R= 0.3) is larger than the 

amplitude of the reflected signal from a 

water/mud interface (R=0.05), in similar water 

depths (At- is constant). In systems that have 

color amplitude printouts or displays, each
color represents a range of amplitudes of the 

reflected signal (Ar). When the water depth is 

constant, the amplitude of the incident wave 

(Az-) at the water/bottom interface is constant, 

and the color displayed representing that 

interface can be related to its reflection 

coefficient. A number of studies have shown 

that reflection coefficients are related linearly 

to bulk density, porosity, and median grain size 

of marine sediments (Faas, 1969; Hamilton, 

1970; Parrott and others, 1980).

The depth scale of a CSP record is calculated using equation 1 where D is the depth 

to any reflector and V is the velocity of sound 

through the material above it. Typically, the 

velocity of sound through water (about 4,888 

ft/s; Sheriff, 1984) and saturated sediments 

(about 5,075 ft/s; Haeni, 1988) is assumed to 

be 5,000 ft/s, and the same vertical scale for 

the water column is used to measure depths in 

the subbottom. When more precision is 

needed, a separate depth scale for the water 

column and for saturated sediments is calculated using appropriate velocities.

The frequency of the seismic signal 

used by the CSP technique determines the 

maximum penetration depth and resolution. A 

high-frequency signal has a short wavelength 

and is attenuated by subbottom materials but 

provides high resolution of subbottom interfaces (Sylwester, 1983). A low-frequency signal has a longer wavelength and is attenuated 

less by subbottom materials but provides 

poorer resolution. The fixed-frequency CSP 

technique uses a narrow-bandwidth fixed-frequency signal usually centered at 3.5, 7.0, or 

14 kHz. The CSP technique using a 14-kHz 

signal can be used in water as shallow as 4 ft, 

can penetrate up to 20 ft into the subbottom in 

certain materials, and can detect fill material as 

thin as 1 ft in a scour hole. The CSP technique 

using a 3.5-kHz signal can be used in water as 

shallow as 7 ft, can penetrate up to 100 ft into 

the subbottom in certain materials, and can 

detect fill material as thin as 2.5 ft in a scour 

hole. Swept-frequency (chirp) CSP techniques 

that sweep from 2 to 16 kHz can be used in

Table 1 . Typical reflection coefficients

[Modified from Sylwester, 1983]

Interface Reflection coefficient

Water/air 

Water/limestone

Water/clay or silt 

Water/mud 

Mud/clay or silt 

Clay/sand 

Sand/limestone

-1.0 

0.5

Sand/granite

.1-.2

-$p 

.1

'$^

.2

.3 

.4
water as shallow as 1 ft, can penetrate 200 ft 

into the subbottom in silts and clays, and can 

detect fill material as thin as 3 in. in a scour 

hole (Schock and LeBlanc, 1990).

The frequency of the seismic signal 

used also determines the size and weight of the 

transducer required to generate the signal. 

Generally, transducers that generate low-frequency signals are larger, heavier, and have 

wider beam angles than transducers that generate high-frequency signals. The beam angle of 

a transducer or antenna is the area beneath the 

device that contains most of the energy of the 

signal. A wide beam angle allows energy to be 

reflected by objects alongside of as well as 

under the transducer. These objects include the 

shoreline and the sides of piers. Echoes from 

the shoreline or from piers (side echoes) can 

interfere with the data.

Another source of interference in geophysical data is caused by multiple reflections. 

When a signal travels from the transducer to 

the water bottom, it is reflected by the bottom. 

When the signal returns to the surface it can be 

reflected at the air/water interface and transmitted back to the bottom. It is reflected by the 

bottom again and is received at the transducer 

a second time. This is called a second arrival 

or a water-bottom multiple. The energy may 

continue to ring in the water column causing 

more than one multiple reflection. Water-bottom multiples are most evident when the 

water-bottom reflection coefficient is large, as 

in rivers with hard bottoms.

Ground-Penetrating Radar

The principles used by the GPR technique are similar to those used by Fathometer 

and CSP techniques. GPR systems, which can 

be used on land or in water, radiate short 

pulses of electromagnetic energy from a 

broad-bandwidth transmitting antenna. When 

used on water, the energy is transmitted from 

the antenna floating on the water surface. This

energy passes through the water column and 

into the subbottom. Where a change in the 

electrical properties occurs, such as at the 

water/bottom interface, part of the energy is 

reflected back to the water surface. The 

remainder of the energy is transmitted further 

into the material. The reflected signal received 

by an antenna is transformed to the audio-frequency range, amplified, recorded, processed, 

and displayed. The ability of the electromagnetic energy to penetrate a material depends 

upon the specific conductance of the material.

The GPR record consists of a continuous graphic display of reflected energy over a 

set time interval. The set time interval is the 

two-way travel time, measured in nanoseconds 

(equal to 1 x 10"9 seconds). The depth of water 

and the thickness of geologic material in the 

subsurface can be determined if the velocity of 

the electromagnetic energy (Vm) through the 

material is known. Vm through a particular 

material is calculated as follows:

(4)

where: Vm = velocity of electromagnetic

energy (ft/s), 

c - speed of light in free space (ft/s)

(approximately 3 x 108), and 

Er = relative dielectric permittivity 

(dimensionless ratio).

Er is a measure of the capacity of a 

material to store a charge when an electric field 

is applied relative to the same capacity in a 

vacuum (Sheriff, 1984). Approximate values 

of Er and Vm of selected materials are given in 

table 2. The depth to a reflector can be 

calculated using equation 1. Note that Vm 

through water differs from Vm through 

saturated sediments, so the vertical depth scale 

calculated for the water column cannot also be 

used for the subbottom. Separate depth scales 

are generated for materials with different Vm 

values.
The penetration depths achieved using 

GPR techniques depend on the frequency of 

the signal used and on the specific conductance 

of water and bottom materials. In general, the 

GPR system will penetrate resistive materials 

and will not penetrate conductive materials. 

Dense, moist clays impede performance of 

GPR, and saltwater makes the technique 

ineffective. GPR techniques work best in 

shallow (less than 20 ft) freshwater with 

granular bottom and subbottom sediments. 

Electromagnetic energy seldom penetrates 

more that 100 ft into the subbottom (Wright 

and others, 1984) and, in highly conductive 

material, may only penetrate a few feet 

(Olhoeft, 1984). Using the 80-MHz antennas, 

penetration depths of at least 20 ft into the 

subbottom have been achieved through water 

less than 20 ft deep with a specific 

conductance of 155 jiS/cm (Gorin and Haeni, 

1989). The frequency used determines the 

resolution of the technique, which can range 

from 1 ft with a 300-MHz antenna to 2.5 ft 

with an 80-MHz antenna. Multiple reflections 

are less common in GPR data than in CSP 

data, but side echoes from piers can cause 

interference in the GPR data.

EQUIPMENT AND MODIFICATION

The geophysical instruments used in 

this study were commercially available, and 

some modifications were required to assemble 

the systems. Fathometer, CSP, and GPR 

systems were used as purchased, but 

improvements were made to each system to 

better suit them for scour surveys.

Fathometer Systems

Three black-and-white chart-recording 

Fathometers were used during this study. One 

of the Fathometers used was the Lowrance 1 

model X-15B, a 192-kHz black-and-white 

chart-recording Fathometer. The system 

required a 12-volt dc power supply and 

consisted of a lightweight (5 Ib) control unit 

with a built-in belt-driven chart recorder and a 

3-lb transducer. Bathymetric data were 

recorded graphically on the chart, and pier 

locations and comments were written directly

The use of trade names is for identification 

purposes only and does not constitute endorsement 

by the U.S. Geological Survey.

Table 2. Approximate values for electromagnetic properties of selected materials

[Modified from Geophysical Survey Systems, Inc., 1987; JlS/cm, microsiemens per centimeter at 25 degrees 

Celsius; ft/ns, feet per nanosecond]

Material Specific conductance

(|lS/cm)

Relative dielectric

permittivity (Er)

(dimensionless ratio)

Velocity of

electromagnetic

energy, Vm (ft/ns)

IpiSil^^^

Dry sand

^ilp^^i;--:;

Saltwater

;-::^;:; :;';;;:.; :-. /;.. :: ;; o ;.;; ;;

0.001 - 10

' M^-^^^.f

40,000

;.;:.; : --.-':--';: c::.:- : ;

4-6

;:;;-. ::'; v.'! 8'i---v ...-. : . : :-i-. :

81

.;;:;;,; ., i:-:- :;.:,;. ,;.'

0.416-0.498

.111

.111

10-100 

1,000-10,000

10 .321 

.0301-.375
on the chart record during data collection. The 

amplitude of the reflected signal was displayed 

using a gray scale. A hard bottom appeared as 

a wide, dark gray line indicating a strong 

reflection, and a soft muddy bottom appeared 

as a narrow, light gray line indicating a weak 

reflection. The depth data were accurate to 

within 0.6 percent of the full-scale depth. For 

example, with a 50-ft full-scale depth, the 

depth to the water bottom was determined to 

within 4 in. Side echoes rarely occurred due to

o

the narrow (8 ) beam angle of the transducer. 

The tops of exposed pier footings were 

detected when the transducer passed over the 

footings. The 192-kHz Fathometer signal did 

not penetrate most bottom sediments because 

of its high frequency and, consequently, did 

not detect fill in the scour holes. This 

Fathometer was not configured to output 

digital depth data so it could not be interfaced 

with a laser-positioning system.

The Raytheon 2154D model DE-1190, 

a 200-kHz black-and-white chart-recording 

Fathometer, produced records similar to those 

of the Lowrance X-15B. The Raytheon 

required a 12-volt dc power supply and consisted of a 25-lb control unit with a built-in 

belt-driven chart recorder and a 4-lb transducer. An Odom Digitrace module was added 

to the standard Raytheon unit so that digitized 

depth data could be transferred from the Fathometer to a positioning system via an RS-232 

serial communication port.

The 45-16 Innerspace Technology 

model 448 black-and-white chart-recording 

Fathometer produced records similar to those 

from the Raytheon 2154D but used a solidstate thermal printer instead of a mechanical 

belt and stylus. An RS-232 serial interface was 

built into the 30-lb unit so it could be interfaced with a laser-positioning system. The 5-lb 

transducer generated a 208-kHz signal with a 

3° beam angle, and an optional 125-kHz transducer is available for use with this Fathometer. 

This Fathometer was controlled by an internal

microprocessor and was the most advanced 

Fathometer used in this survey.

All three black-and-white chart-recording Fathometers were portable and provided 

clear, accurate information on water depth, 

existing scour holes, the top of submerged 

debris, and the top of exposed pier footings. 

One of the Fathometers was used simultaneously with other geophysical systems to provide bathymetric records, which aided in the 

interpretation of CSP and GPR data.

A variable-frequency color Fathometer, the SeaTex model HE-730, manufactured 

by Si-Tex, Inc. was used to profile and detect 

fill in existing scour holes. The purchased system required a 12-volt dc power supply and 

consisted of a 35-lb control unit and color 

monitor and a 15-lb transducer. One of six possible frequencies (20,40, 60, 75, 88, or 100 

kHz) was transmitted by a single transducer, 

with a frequency-dependent beam angle. A 20- 

kHz signal with a beam angle of 44 was used 

in this study to maximize penetration depths, 

but the wide beam angle caused side echoes to 

appear in the records. A minimum of 5 ft of 

water was required to operate the system, and 

a maximum of 10 ft of penetration into the 

subbottom was observed. Subbottom layers 

were resolved to within 1 ft with this unit. A 

color video display of the data appeared during 

data collection, but a printed bathy metric profile was not produced during data collection.

The HE-730 system assigned different 

colors to different amplitudes of the reflected 

signals. A minimum threshold was set by the 

sensitivity control that controlled the smallest 

amplitude plotted on the screen. The smallest 

amplitude reflection was displayed as light 

blue. Signals with amplitudes that were 6 dB 

higher, or about twice as large, were displayed 

as violet. Subsequent color changes occurred 

in 6-dB steps, and four additional colors were 

used: white, green, yellow, and red. Assuming 

a constant water depth, a bottom represented
by red had a higher reflection coefficient and 

was harder than that represented by light blue.

A recording-interface box, made by SiTex, was added to the system so the analog 

data was recorded by a standard audio tape 

recorder requiring a 110-volt ac power source. 

The data were stored on two channels of a 

standard audio cassette. The left channel 

recorded a modulated trigger pulse, which 

indicated the time of the beginning of each 

individual sounding. The right channel stored 

a modulated data signal from each sounding. 

During data collection, the data were positioned by noting the tape count at landmarks 

such as piers.

The HE-730 system initially had several limitations that eventually were overcome. 

The color step size was fixed at 6 dB, so 

smaller changes in the signal could not be displayed as a color change on the color monitor. 

An event mark button was not available on the 

system so position data relied solely on tape 

counts noted in the field. A hard copy of the 

data was not produced by the system, and the 

only way to review the data was to replay the 

data through the system from tape and observe 

the color monitor. A hard copy could be 

obtained by photographing the video monitor.

Modifications and improvements were 

made to the HE-730 color Fathometer system 

during this study. A hardware modification 

allowed the color step size to be changed from 

6 to 4 dB in order to show smaller changes in 

the return signal and to improve the visual display. An event mark button was added that 

allowed vertical-line marks to be recorded in 

the data during data collection. The cassette 

tape recorder was replaced by a digital-audio 

tape (DAT) recorder. This improved the quality of the recorded signal by using a digital format rather than an analog format. A method to 

transfer color Fathometer data to a personal 

computer (PC) was developed that allowed 

digital processing of the color Fathometer data 

and printing capability.

Continuous Seismic-Reflection 

Profiling Systems

Two types of CSP systems were used 

for this study a fixed-frequency system and a 

swept-frequency system. The fixed-frequency 

CSP system was tuned to a single frequency 

between 3.5 and 14.0 kHz and was used to profile existing and infilled scour holes. The system from ORE International, Inc. consisted of 

a model 5430A transmitter, a model 5210A 

receiver, one of two transducer configurations, 

an EPC model 8700 thermal graphic recorder 

and annotator, and a DAT recorder. The signal 

frequency and signal power were set on the 

transmitter, gain and filter settings were set on 

the receiver, and a real-time graphic plot was 

generated by the thermal graphic recorder. The 

analog data were recorded in digital format by 

the DAT recorder onto standard DAT cartridges. The transmitter, annotator, receiver, 

and DAT recorder were rack-mounted in two 

cases to increase the portability of the system. 

The total weight of all the equipment, not 

including the transducers, was about 220 Ibs. 

All units required a 110-volt ac power supply 

and were powered by a portable 1,000-W generator.

Two different types of transducer configurations were used with the fixed-frequency 

CSP system. A 150-lb rectangular array of 

four model 134 transducers transmitted a signal tuned between 3.5 and 7.0 kHz in two 

modes of operation. When two transducers 

transmitted and the other two received energy 

(separate mode), the beam angle was wider 

and penetration depths were less than when all 

four transducers transmitted and then received 

the reflected energy (combined mode). Side 

echoes were detected in both modes of operation, and ringing within the transducer produced parallel lines across the top of the CSP 

record that interfered with data from shallow 

water. The presence of multiple reflections in 

the records increased in the combined mode as 

did the transducer ringing, which complicated

10
data interpretation. In the combined mode, the 

3.5-kHz beam angle was 55 and the 7.0-kHz 

beam angle was 35 . In the separate mode, the 

3.5-kHz beam angle was 120 0^0by 55 and the

7.0-kHz beam angle was 80 by 35 . The transducer array was usually operated at 7.0 kHz, 

and the separate mode was used in shallow 

water. A minimum water depth of 5 ft was 

required to avoid the ringing interference. 

When the transducer was tuned to 3.5 kHz, 

interfaces 2.5 ft thick could be resolved. In 

fine-grained material, up to 100 ft of subbottom penetration was obtained, whereas in 

coarser materials, subbottom penetration was 

limited to 20 to 30 ft.

The second transducer configuration 

was a single element, 50-lb, model T14, cylindrical 14-kHz transducer. Internal ringing was 

a problem with this transducer, which often 

produced poor records at scour sites especially 

in very shallow water. This transducer was 

able to resolve interfaces to within 1 ft.

CSP data saved on DAT cartridges 

were reviewed and processed two ways. In the 

first method, the tape was replayed through the 

receiver and the gains, filters, and verticaldepth scales were re-adjusted as the data were 

printed on the graphic recorder. Regions of 

interest on the record were enhanced if the 

acquisition parameters were not set optimally 

during data collection or if a new region of 

interest developed. The second technique 

involved transferring the data to a PC for processing. A discussion of this method is presented in the processing section of this report.

The two major shortcomings of the 

standard CSP system were the ringing in the 

14-kHz transducer and the ringing in the outgoing transmitter pulse. In a new 50-lb 14-kHz 

transducer (also model T-14), the transmitter 

and receiver elements were separated, which 

reduced some of the internal transducer ringing. Implementing an active ring-damping circuit into the transmitter sharpened the 

outgoing electronic pulse to the transducer and

reduced ringing in the 3.5, 7.0, and new 14- 

kHz records. The new 14-kHz transducer, 

using the ring-damping circuit, was designed 

to work in water as shallow as 3 ft.

A modification to the EPC graphic 

recorder allowed the operator to print both the 

positive and negative components of the return 

signal or just the positive portion of the signal. 

This modification aided in the interpretation of 

some of the data.

Two swept-frequency CSP systems 

were used during this study the Datasonics 

CAP-6000 and the Precision Signal X-Star 

system offered by EG&G Marine Instruments. 

The two systems have differences, but their 

basic designs are similar. The systems consisted of a computer-based control unit and 

signal processor in one 120-lb case and a color 

monitor in another 80-lb case. The control unit 

recorded the data onto 8-mm Exabyte tape cartridges. An external printer and a transducer 

mounted in a tow fish were also part of the system. The Datasonics 1.5- to 10-kHz tow fish 

weighed 360 Ib, and the Precision Signal 2- to 

16-kHz tow fish weighed 280 Ib. The fins of 

the Datasonics tow fish were cut for use near 

piers in scour projects, but its weight was not 

reduced substantially. A 1,000-W, 110-volt ac 

generator was used to power each system. 

Minimal setup parameters were needed to 

begin data collection, and the data-collection 

routine was controlled by a menu-driven program. The color monitor provided a real-time 

color amplitude display of bathymetric and 

subbottom information. A hardness or sediment classification plot of the river bottom also 

was displayed during data collection.

Both swept-frequency CSP systems 

produced high-resolution records of existing 

scour holes, fill in the holes, and geologic layers in the subbottom. Both swept-frequency 

systems were easier to operate but more difficult to troubleshoot than the ORE system 

because many controls were automatically set 

by the equipment.

11
Since the conclusion of the study, new 

swept-frequency CSP systems have become 

commercially available. Precision Signal has 

introduced the Mini Star, which sweeps from 4 

to 20 kHz using a 55-lb transducer and 20-lb 

Spare II workstation. DataSonics has introduced the Chirp II system, which uses the 

same transducer as the earlier DataSonics system but with a 80486 processor. ORE International has introduced the GeoChirp, which 

sweeps from 2 to 8 kHz or 1.5 to 11.5 kHz 

using the same transducer from its fixed-frequency CSP system and upgraded electronic 

controls.

Ground-Penetrating Radar System

A GPR system using 80-, 100-, or 300- 

MHz antennas was used to profile existing 

scour holes and to detect infilled scour holes. 

The SIR-8 GPR system, manufactured by 

Geophysical Survey Systems, Inc. (GSSI), 

consisted of a control unit, a digital tape 

recorder, a graphic recorder, and an antenna. 

The system was powered either by a 12-volt dc 

or 110-volt ac power supply. The control unit 

controlled the gain, filter, maximum time 

(depth) recorded, and other setup parameters; 

the digital tape recorder recorded the data; the 

graphic recorder printed the profile during data 

collection; and the antenna determined the frequency of the signal used. The total weight of 

the instrumentation, not including the antennas, was about 100 Ib. The antenna weights 

were 80 Ib for the single 80 MHz, 25 Ib for the 

single 100 MHz, and 30 Ib for the single 300 

MHz antenna. The antennas were used in a 

mono-static mode, requiring one antenna, and 

in a bi-static mode, requiring two antennas 

separated by a fixed distance. In the monostatic mode, one antenna transmitted and 

received a signal; in the bi-static mode, one 

antenna transmitted a signal and the other 

received it. The 80 and 100-MHz antennas 

were used in both the mono-static and bi-static

modes. The 300-MHz antenna had separate 

transmitter and receiver antennas built into a 

single housing, so although it was contained a 

single housing, it operated in the bi-static 

mode. A removable electronic module located 

in the antenna housings determined the amount 

of power transmitted by the antennas. A lowpower (20 or 40 W) transmitter was used in the 

300 MHz antenna and the mono-static 80- and 

100-MHz antenna configurations. A highpower (2.5 or 5 kW) transmitter was used in 

the bi-static 80- and 100-MHz antenna configurations. The high-power, bi-static 80-MHz 

antennas provided up to 20 ft of penetration 

into the subbottom, in 10 ft of freshwater, and 

resolved layers as thin as 2.5 ft. On land, the 

80-MHz antennas penetrated more than 50 ft 

of resistive material. The low-power 300-MHz 

antenna provided up to 10 ft of penetration into 

the subbottom in 2 ft of freshwater, and it 

resolved layers as thin as 1 ft.

Data recorded on 3M DC600HC tape 

cartridges were reviewed and processed two 

ways. In the first method, the tape was 

replayed through the control unit, and the 

gains and various print controls were readjusted as the data were printed on the 

graphic recorder. The vertical-depth scale was 

fixed during data collection. The second 

method involved transferring the data to a PC 

for processing, which allowed the verticaldepth scale to be shortened and processing 

algorithms to be applied. A discussion of this 

method is presented in the processing section 

of this report.

A major limitation of the standard 

radar antennas was that they were not designed 

to be used in water. The wheels were removed 

from the 80- and 300-MHz antennas, and the 

cases were sealed so water did not leak into 

them. Because the removable electronic modules and the cable were connected on the top 

side of the standard antennas, care was used 

during data collection not to splash water near 

those parts. New fiberglass cases for the 1 GO12
MHz antennas were designed and built with 

water-resistant connectors mounted on top of 

the cases, and a 5-kW transmitter and a singlestage receiver were permanently mounted 

inside the waterproof cases. These could be 

used only in the bi-static mode of operation. 

The tops of both antennas were shielded to 

reduce interference from reflections from 

bridge decks. The use of a fiber-optic link 

between the antennas, rather than a standard 

coaxial connection, improved the quality of 

the GPR data. The modified 100-MHz antennas provided the same quality data as the standard antennas, but were better suited to collect 

data in scour investigations. The standard and 

modified 100-MHz antennas were much 

lighter and easier to maneuver near bridge 

piers than the 80-MHz antennas and provided 

comparable penetration depths and better resolution.

Position-Recording System

Position data were used to correlate the 

geophysical records with the pier locations, to 

correct the horizontal scale of the geophysical 

records and to review the tracklines of the 

boat's paths during data collection to ensure 

complete coverage of the survey area. The 

Hydro I, a laser-positioning system manufactured by IMC, was used with a Fathometer and 

a data logger or Hypack acquisition software 

to collect position data before 1993. After 

1993, an upgraded version of the system, the 

Hydro II, manufactured by Laser Technology, 

Inc., was used with a Fathometer and acquisition software on a PC to collect position data.

The Hydro position-recording systems 

recorded the boat's position (x, y) relative to a 

fixed point and the corresponding water depth 

(z). Although the Hydro II's laser is accurate to 

within 0.3 ft, the practical accuracy, which 

includes tracking errors, is closer to 2 ft. A 

shore station and a ship station were required 

by these systems. The shore station consisted

of a laser distance-measuring device, a theodolite (an angle-measuring device), and a radio 

link that transmitted the coordinates of the 

boat's location to the boat. A person was 

required on shore to manually track the boat 

with the laser. The ship station consisted of a 

data logger/radio link (or a PC/radio link), 

which received the position (x, y) data from 

the shore station and the depth data (z) from a 

Fathometer. The data logger (or PC) on the 

ship recorded the boat's position and the water 

depth at a programmed distance or time interval. Usually, the x-y-z values were recorded 

every time the boat travelled a distance of 5 ft. 

A mark signal was transmitted to the geophysical equipment at 10-ft distance intervals. During data collection, the data logger (or PC) also 

aided in navigation by indicating how far the 

boat was to the left or right of the programmed 

trackline. When using the data logger, the data 

were transferred from the data logger to a PC. 

The Hydro I hydrographic software was used 

to review the boat's tracklines and bathymetric 

profiles and to edit and print two-dimensional 

plots of the tracklines and profiles.

An additional data-collection and processing software package, Hypack, written by 

Coastal Oceanographies, was used on an IBMcompatible PC in the boat during data collection. At the Bulkeley Bridge, the geometry and 

locations of the bridge piers were programmed 

into Hypack from bridge-construction plans. 

At the Baldwin Bridge, the locations of the 

piers were programmed into Hypack on site by 

positioning the boat at the front and back of 

each pier and recording the position. Hypack 

provided a real-time graphic display of the 

bathymetry and the boat's position in relation 

to the piers, desired trackline, and previous 

boat paths. By monitoring these data during 

data collection, complete coverage of the scour 

site, as well as the quality of the position and 

depth data, were assured. On the boat, Hypack 

replaced the data logger and recorded the data 

directly to a file on the PC in ASCII format. 

The PC used two serial ports. One port read

13
depth data from a Fathometer and the other 

read position data from the radio-link unit. 

After data collection, Hypack or the Hydro I 

software was used to edit and print data. Additional programs, Isomate and Surfer, were used 

to create two-dimensional and three-dimensional contour plots from the data. Further 

details are included in the processing section 

of this report.

DATA COLLECTION NEAR BRIDGE 

PIERS

Successfully collecting surface-geophysical data near bridge piers begins with the 

choice of an appropriate technique. The appropriate choice of the geophysical technique(s) 

to use for detecting and measuring existing and 

infilled scour surfaces depends on two factors. 

First, the chosen technique must be able to 

provide the penetration and resolution 

required. For example, if the purpose of the 

survey is to detect infilled scour holes, a 200- 

kHz Fathometer will not provide the subbottom information required. If the purpose is to 

map the bathymetry of existing scour holes, 

the 200-kHz Fathometer probably will provide 

adequate results. Second, the chosen technique 

must work in the setting at the scour site. For 

example, CSP techniques will not work in 

water depths less than 3 ft, and GPR techniques will not work well in water with a specific conductance greater than 1,000 (iS/cm.

Once a technique is chosen, the logistics at the site need to be considered. The 

equipment may be operated from a small or 

large boat or from the shore or a bridge deck. 

When operating from a boat, the transducer or 

antenna is supported from the side of a boat. A 

skilled boat operator is required to maneuver 

near piers especially in rivers with high flow 

velocities. The best records are acquired when 

the boat's velocity is kept to a minimum. The 

slow speed reduces the amount of noise in the 

record, and expands the horizontal scale,

which makes the geometry of the scour hole 

and subbottom reflectors easier to interpret 

from the data.

When operating from the shore, the 

equipment is set up on the bank of a river, and 

the transducer is slowly maneuvered near the 

piers by a person or cable. This set-up is used 

at sites with narrow rivers or where boat access 

is limited. When operating from a bridge deck, 

the equipment is set up in a vehicle or cart, 

which is wheeled across the bridge deck. A 

cable extends over the bridge deck to the transducer or antenna, which is guided by a person 

or cable. This set-up is used on low bridges 

where high-flow velocities or debris near piers 

and obstructions on the bridge do not pose a 

problem. When traversing upstream of the 

piers, better position control of the transducer 

can be obtained when operating from a boat or 

river bank than from a bridge deck.

Complete coverage of the areas that 

have the potential to scour is essential. Multiple cross-sections of data are usually collected 

at selected intervals along the upstream side of 

the piers. Data also are collected along the 

downstream side of the piers. Additional 

cross-sections are collected farther upstream 

and downstream to determine the extent of 

contraction scour, and lateral lines alongside 

the piers are collected to determine the lateral 

extent of the scour holes.

Collection of Geophysical Data

The Fathometer systems were the simplest to operate, most portable, and least 

expensive. The control unit usually was operated from a boat, but occasionally was operated from a bridge deck. Two people were 

required to conduct a Fathometer survey, and 

the Fathometer was usually used in conjunction with a more complex geophysical instrument. When operating from a boat, the 

transducer was either mounted on the hull of 

the boat or suspended rigidly from the side of

14
the boat. When operating from a bridge deck, 

the transducer was attached to a sounding 

weight, lowered over the side of the bridge, 

and moved across the bridge to produce a 

bathymetric profile. Multiple lines of data 

along the upstream side of the piers were collected only when the Fathometer was operated 

from a boat. The location of the transducer was 

noted on the Fathometer record during data 

collection.

The fixed-frequency CSP system was 

more complex to operate than the Fathometer 

systems, and required a field crew of two or 

three people, including at least one trained 

individual. The equipment was operated from 

different-sized boats and from the banks of rivers. When operating from a large boat, the 

150-lb transducer was suspended from the side 

of the boat by a fixed support. At small river 

sites and at sites where a boat launch was not 

available, a 12-ft inflatable boat was used. At 

these sites, the 150-lb transducer was suspended by a truck-tire inner tube and guided, 

but not supported, by the small boat. When 

operating the equipment from a river bank on 

narrow rivers, the transducer suspended by the 

inner tube was manually pulled across river.

The swept-frequency CSP systems 

were much heavier than the fixed-frequency 

systems, so swept-frequency data were not 

collected from the small boat. A minimum 

water depth of 2 ft was required when using 

the swept-frequency system; this was because 

the boat required this depth, not the geophysical equipment. The swept-frequency CSP data 

were collected from a 22-ft Boston Whaler 

equipped with an over-the-side crane. The tow 

fishes were large and heavy and were suspended in the water alongside the boat by the 

crane. Eddy currents near piers made maneuvering near piers with this tow fish difficult. To 

change the frequency range, the transducers 

and tow fish had to be changed.

The GPR system was a little more 

complex to operate than the fixed-frequency

CSP system, but a survey could be conducted 

by a crew of two or three people, with at least 

one trained person. The GPR equipment was 

operated from large and small boats and on 

land across shallow or dry river channels. The 

antennas were floated alongside the boat as the 

boat maneuvered slowly upstream, downstream, and alongside the piers. An aluminum 

boat can interfere with the data when antennas 

are next to the boat, therefore, an inflatable or 

fiberglass boat was used to reduce this interference. In shallow streams or rivers, the antennas 

were pulled across scour holes by hand.

Two position-recording methods were 

used in this study visual estimations and precise measurements collected with a laser-positioning system. At sites on narrow rivers with 

low-velocity water, where the transducer was 

manually pulled across the river, visual estimations of the transducer's location were noted 

on the field records. Locations of the beginnings of the lines, sides and center of piers, and 

the ends of the lines were noted on the records. 

At sites on large rivers or where future studies 

may require data to be re-collected along the 

same line, a position-recording system was 

used. Changes in water velocities caused by 

eddy currents near bridge piers made maneuvering at a constant velocity difficult. Because 

the boat travelled at different speeds, the horizontal scale of the geophysical record was distorted. Data from the laser-positioning system 

was used to correct the horizontal scale. That 

procedure is detailed in the processing section 

of this report.

Collection of Verification Data

Geophysical data were verified, if possible, by comparing them with physical measurements made at the site. Hard bottoms, soft 

bottoms, exposed footings, and less than 5 ft of 

fill material can be detected in rivers less than 

15 ft deep by probing the bottom of the river 

with a 1-in. diameter hollow metal rod. The

15
information obtained by probing was used to 

verify the interpretations of the geophysical 

data. Geophysical data interpretations also 

were compared with cores obtained at a site to 

confirm or identify subbottom interfaces. A 

vibracore system consisting of a concrete 

vibrator attached to a 3-in. diameter 20-ft long 

aluminum irrigation pipe with a core-catcher 

attached at its end was operated from a 22-ft 

boat. The pipe was lowered to the river bottom 

through 7 ft of water and was vibrated through 

about 9 ft of bottom material. A 4-ft core was 

retrieved that confirmed the presence of a 

sand/clay interface at the bottom of an old 

scour hole that was detected by the geophysical techniques. The vibracore system is 

described in Thompson and others (1991), and 

its application in a scour hole is described in 

Haeni and Gorin (1989).

Subsurface lithologic logs at bridge 

sites and bridge plans also were used to help 

verify subbottom reflectors detected by the 

geophysical techniques. Although the lithologic logs did not provide scour data, they did 

provide information about the depth to geologic interfaces. This information confirmed 

the presence of geologic interfaces detected by 

geophysical techniques. It also confirmed that 

these interfaces were not caused by scour, but 

existed naturally before the bridge was built. 

The depths to footings obtained from the 

bridge plans were used to confirm the interpretation of footings in geophysical records.

PROCESSING TECHNIQUES APPLIED

TO SURFACE-GEOPHYSICAL AND

POSITION DATA

The application of digital-signal processing (DSP) techniques to surface-geophysical and position data improved the 

interpretation of unprocessed geophysical 

records (Haeni and Placzek, 1991; Haeni and 

others, 1992). The application of DSP routines 

to geophysical data sets increased the accuracy

and ease of the data interpretations. These processing routines were not applied to every data 

set, however, because much of the unprocessed 

field data contained sufficient information to 

satisfy the goal of the survey. A geophysical 

record was processed if more information was 

required from the unprocessed field record or 

if a processing routine was being tested. Processing the data generally improved the 

records but did not provide information from a 

record if good data had not been collected initially.

Processed geophysical data were easier 

to interpret because horizontal scales and some 

of the geometry of geologic layers were corrected, noise and interference patterns were 

reduced in the data, and the resolution of the 

data display was improved. Position information was used to correct for differences in the 

speeds and paths of the boats and to produce a 

geophysical profile with a horizontally corrected scale. Digital band-pass filtering routines eliminated some system and 

environmental noise from the field data sets, 

and a predictive-deconvolution algorithm 

improved the display resolution of some 

records. A Kirchoff migration algorithm 

applied to horizontally corrected data sets corrected the geometry of the bottom of the scour 

holes and reduced interference patterns from 

point reflectors, piers, or rip-rap bottoms.

Fathometer and Position Data

Fathometer and position data were processed using a PC after data collection. Position and depth data acquired using the Hydro I 

data logger were transferred from the data logger, through a standard RS-232 serial link, to a 

PC. Position and depth data acquired using the 

Hypack software were automatically saved to 

an ASCII file on the PC. The Hydro I and 

Hypack software were used to edit the data, 

and Isomate and Surfer were used to create 

contour plots from the edited data.

16
The Hydro I hydrographic software 

was used after data collection to graphically 

edit, display, and print two-dimensional 

bathymetric and trackline plots. First, the 

software was used to review bathymetric plots 

and tracklines and edit or remove any 

obviously erroneous data points. Erroneous 

data points were identified because they were 

extremely large or small compared with 

nearby data points. Erroneous position data 

were caused by the laser missing the target on 

the boat and by radio interference during data 

transmission from shore to ship. Incorrect 

depth data were obtained when the Fathometer 

digitized a depth that was not the water 

bottom, in which case the analog chart record 

was reviewed to determine the correct depth. 

After the individual tracklines and depths were

edited, two-dimensional plots of the individual 

tracklines and bathymetric profile data were 

printed. An example of an edited twodimensional bathymetric profile printed using 

the Hydro I software is shown in figure 2.

The Hypack software package was 

used to monitor bathymetric and trackline 

plots during data collection and to graphically 

edit and print two-dimensional plots after data 

collection. Isomate, an optional software 

module to Hypack, was used to create a twodimensional contour plot of the data. First, 

multiple lines of the data were combined into a 

single grid file used to create the contour map. 

For example, three lines of data upstream from 

a pier, a line downstream and a line alongside 

the pier were combined into a single grid file.

Pier 6 Pier 7 Pier 8

100 200 300 400 500 600 

DISTANCE, IN FEET

700 800 900

Figure 2. Positioned 200-kilohertz Fathometer cross-section 5 feet upstream from the 

piers at the Bulkeley Bridge, Hartford, Conn. (Modified from Haeni and others, 1992.)

17
The grid size selected during the gridding 

process determined the spatial resolution of the 

plot. A grid size of 5 ft indicates that the 

survey area was divided into 5-ft square blocks 

and a single depth was determined for each 

block. If a depth measurement was not 

available for a block of the grid, the program 

interpolated a depth value for that block from 

surrounding depth measurements. From the 

completed grid file, a plot file was created 

from which a printout of the contour data was 

made. From the grid file, Isomate also can 

create a DXF file, which can be used in many 

computer-aided design (CAD) packages to 

display, enhance, and print data.

Surfer (version 4.15), by Golden Software, Inc., was used to edit, display, and print

two- and three-dimensional plots of the position and depth data. Surfer uses a numerical 

rather than graphical editor, so typically, data 

were edited using the Hydro I software or 

Hypack and then transferred to Surfer. Grid 

and plot files were created by Surfer in a similar way as with Isomate. In addition to twodimensional contour plots, Surfer also produced three-dimensional surface and contour 

plots (fig. 3). Surfer 4.15 can use only 14,000 

x-y-z data points per plot, but the version of 

Surfer for Windows can use more than 

250,000 data points per plot when used on a 

PC with 8 MB of RAM. More data points can 

be used if the amount of RAM is increased.

200 FEET

0 50 METERS 

Approximate horizontal distance 

Vertical exaggeration x10

Figure 3. Three-dimensional bathymetric contour plot upstream from the Bulkeley 

Bridge, Hartford, Conn. (Modified from Haeni, 1992.)

18
Color Fathometer and Continuous 

Seismic-Reflection Profiling Data

Recorded color Fathometer and CSP 

data were transferred to a PC and processed 

using the same hardware and software. A commercially available 100-kHz analog to digital 

(A/D) converter card (DT2823 from Data 

Translations) was used in an IBM-compatible 

80386-based PC to transfer the data. DATIN, a 

program written by the USGS, controlled the 

operation of the A/D card during the datatransfer procedure. DISPLAY, a program also 

written by the USGS, was used to display and 

print the data on a Hewlett Packard PaintJet 

printer. RADAN3, a commercially available 

program written by GSSI, was used to process, 

display, and print the data. RADPRINT, also 

written by GSSI, was used to display and print 

the data on a printer supported under Windows.

The color Fathometer and CSP systems 

stored a data signal and a trigger signal on a 

DAT cartridge. The data signal from the DAT 

cartridge was demodulated, amplified, and filtered before it was input into the A/D card. 

The trigger signal was demodulated, and its 

pulse width was shortened before it was input 

into the A/D card. DATIN controlled the transfer of these signals through the A/D card to a 

binary file on a hard drive. Each time a transducer sounded, an individual data trace and its 

corresponding trigger pulse were recorded on 

the tape. Each individual data trace transferred 

to the PC by DATIN was represented by 1,024 

16-bit samples. The sampling rate, record 

length, and the number and size of buffers used 

during the transfer process were selected by 

the user. These variables determined the datatransfer rate, which was kept below 512 kB/s, 

the limit of the optimized hard drive. The sampling rate determined the highest frequency 

component that could be preserved during the 

transfer process. The sampling rate always was 

set to at least twice the highest frequency component contained in the filtered data. The

record length, equivalent to the total two-way 

travel time, determined the maximum depth 

represented by the data.

To transfer data from the Color Fathometer to a PC, the data were replayed through 

the color Fathometer system from the DAT 

cartridge, and data and trigger signals were 

simultaneously input into the A/D card. In one 

instance, 2 minutes of color Fathometer data 

contained 720 traces. After applying a 20-kHz 

low-pass filter to each trace, each trace was 

sampled at 42 kHz for 24 ms and was represented by 1,024 16-bit samples. The transfer 

process required a data transfer rate of 13 kB/s, 

the size of the data file was 1.48 MB, and the 

file included data to a depth of 60 ft.

Once transferred to a PC, the color 

Fathometer data were displayed as a color 

amplitude plot and printed using DISPLAY. 

DISPLAY allowed display parameters, such as 

the color palette, color threshold levels, and 

vertical scale to be adjusted. Periodic wiggle 

traces were plotted over the color amplitude 

plot to provide more detailed amplitude and 

phase information about individual traces. The 

tops and bottoms of records were eliminated to 

focus on particular features of a scour hole.

To transfer recorded 3.5, 7.0, and 14.0- 

kHz CSP data to a PC, data were replayed 

through the CSP system from a DAT cartridge 

and data and trigger signals were input into the 

A/D card. In one instance, 2 minutes of 14- 

kHz CSP data contained 1,044 traces. After 

applying a 28-kHz low-pass filter to each 

trace, each trace was sampled at 68.3 kHz for 

15 ms and was represented by 1,024 16-bit 

samples. The transfer process required a data 

transfer rate of 17.9 kB/s, the size of the data 

file was 2.14 MB, and the file included data to 

a depth of 37 ft.

The CSP data file created by DATIN 

was then converted to a file that was read by 

RADAN3 and processed by RADAN3. Digital 

filtering, rubber sheeting, predictive deconvolution, and migration processes were applied

19
to the CSP data. Horizontal digital filtering 

eliminated some noise in the record caused by 

transducer ringing and also eliminated flat 

reflectors. Vertical digital filtering narrowed 

the band-pass region of the signal, therefore, it 

reduced some ambient noise caused by the system or the boat's engine. A rubber-sheeting 

technique used position data to correct the horizontal scale of the geophysical profiles. 

Unevenly spaced marks made on the geophysical record by the positioning system representing 10 ft of boat travel indicated that the boat's 

velocity was not constant. Rubber sheeting 

corrected for the inconsistent velocity by interpolating or deleting traces to evenly space the 

marks. A predictive-deconvolution algorithm 

improved the resolution of the display by narrowing the displayed width of reflected signals 

caused by single reflectors. A Kirchoff migration algorithm geometrically corrected the 

record by collapsing hyperbolas in the record 

caused by side echoes or boulders on the river 

bottom to the location of their sources. In addition, the slope of the sides of a scour hole were 

migrated to their actual shape. An unprocessed 

14-kHz CSP record from the Bulkeley Bridge 

is shown in figure 4 and the same record, after 

having been digitally filtered, rubber sheeted, 

and deconvolved, is shown in figure 5. The filtering reduced some noise, the rubber sheeting 

improved the geometric representation of the 

shape of the hole, and the deconvolution algorithm resolved thin layers of fill material representing possible past scour surfaces.

The swept-frequency CSP systems 

incorporated signal processing techniques into 

their control units that automatically 

deconvolved the signal pulse during data 

collection. Additional processing of the data 

was done in real time and the processed data 

were displayed and printed as they were 

collected. The Precision Signal system used a 

Spare II workstation to handle the intensive 

DSP routines requiring real-time digital 

filtering and decon volution. The DataSonics 

system used an 80386 microprocessor and had 

similar data-processing capabilities. 

Horizontal-scale corrections and migration 

routines were not applied to the sweptfrequency data because positioning 

information was not collected while testing 

these systems. Although possible, sweptfrequency data were not transferred to a PC to 

be processed for this study. A gray-scale 

swept-frequency record collected at the 

Baldwin Bridge with a Precision Signal system 

is shown in figure 6.

20
*&v Previous scour surface

Figure 4. Unprocessed positioned 14-kilohertz continuous seismic-reflection profiling 

data collected 10 feet upstream from pier 4 of the Bulkeley Bridge, Hartford, Conn. 

(Modified from Haeni, 1992.)

20 40 

HORIZONTAL DISTANCE, IN FEET

Figure 5. Digitally filtered, horizontally corrected, deconvolved 14-kilohertz data from 

figure 4.

21
w

Pier 7 center-line !

60-

50 FEET

0 

Approximate horizontal distance

15 METERS

Figure 6. Gray-scale amplitude plot of 2- to 16-kilohertz swept-frequency record 

collected 10 feet upstream from pier 7 at the new Baldwin Bridge, Old Saybrook, Conn, 

(after construction).

Ground-Penetrating Radar Data

GPR data recorded on 3M data 

cartridges were transferred to a PC and 

processed. A commercially available tape 

drive read the 3M data cartridges, and 

READTAPE, a USGS program, was used to 

transfer the radar data to a file that could be 

read by RADAN3. Digital filtering, rubber 

sheeting, deconvolution, and migration 

processes were applied to the GPR data using 

RADAN3. An example of an unprocessed 

GPR record from the Route 14 bridge in 

Canterbury, Conn., is shown in figure 7. The 

wiggle-trace plot of the same record, after 

applying a band-pass filter and a migration

routine, is shown in figure 8. The hyperbolas 

on the river bottom east of the pier have been 

collapsed to their point of origin.

Care was used when the radar profiles 

were migrated and interpreted, because at least 

two signal velocities were present. Table 2 

shows that the electromagnetic energy velocity 

is approximately 0.11 ft/ns in freshwater and is 

approximately 0.18 ft/ns in saturated sand. A 

fully migrated section must use both of these 

velocities. The migration routine in RADAN3 

uses only one velocity. The velocity in water 

was used, therefore, only the water/sediment 

interface was correctly migrated, and reflectors 

in the subbottom were not interpreted as 

correctly migrated.

22
15 METERS

Approximate horizontal distance

Figure 7. Unprocessed 300-megahertz ground-penetrating radar data collected 2 feet 

upstream from the pier at the Route 14 bridge in Canterbury, Conn.

LLJ 

LLJ

rr 4-

x

D.

15 METERS 

Approximate horizontal distance

Figure 8. Wiggle-trace plot of digitally filtered and migrated 300-megahertz groundpenetrating radar data from figure 7.

23
RESULTS OF SURFACEGEOPHYSICAL TECHNIQUES 

AT SELECTED BRIDGE SITES

Geophysical data were collected at six 

bridge sites in Connecticut (fig. 1). The sites 

were chosen because of their different physical 

settings and potential for scour. The river at 

each site ranged from 200 to 1,800 ft wide and 

from less than 1 to 40 ft deep. The specific 

conductance of the water ranged from 64 to 

40,000 (iS/cm. The bottom and subbottom 

materials also were different from site to site. 

The number of piers in the water ranged from 

1 to 8. The sites were chosen so that the utility 

of each geophysical technique applied in 

different physical settings could be assessed.

The Bulkeley Bridge and the Baldwin 

Bridge sites were chosen as primary test sites, 

where new or modified instrumentation and 

new data collection and interpretation methods 

were tested during this study. These two sites 

were chosen because historic surface-water 

flow records, complete copies of bridgeconstruction plans, and a complete set of 

standard geophysical records were available. A 

USGS streamflow-gaging station at the 

Bulkeley Bridge provided historic discharge 

measurements of the Connecticut River since 

1905. A USGS streamflow-gaging station near 

Middletown (20 mi north of the Baldwin 

Bridge) provided historic discharge 

measurements of the Connecticut River 

Estuary since 1965. Bridge-construction plans, 

including test lithologic logs, were available at 

both sites, and cores were collected in a scour 

hole at the Bulkeley Bridge site (Haeni and 

Gorin, 1989). Data collected with new or 

modified equipment and techniques were 

evaluated by comparing the data with the 

standard data sets.

Old Baldwin Bridge, Old Saybrook

The Old Baldwin Bridge carried 

Interstate 95 over the 1,800-ft wide 

Connecticut River Estuary near Old Saybrook, 

Conn, and had eight round-nose piers spaced 

about 230 ft apart. The piers in the saltwater

estuary were numbered from 2 to 10 (west to 

east) to correlate with Connecticut Department 

of Transportation bridge plans. The average 

water depth across the section was 20 ft, and a 

40-ft deep channel was between piers 8 and 9. 

Because of tidal influence at this site, flow 

reversal occurs. The river level fluctuates 

within a 4-ft range during a full tidal cycle, and 

depth scales were corrected for varying tidal 

stages. A maximum discharge of 186,000 ft3/s 

was recorded on June 2, 1984 near Middletown (Cervione and others, 1992). The river at 

this site has a mud bottom and a sand and silt 

subbottom (Bingham and others, 1975, p. 48; 

Home and Patton, 1989). Unpositioned 3.5- 

kHz CSP, 20-kHz color Fathometer, and 200- 

kHz Fathometer data were collected at the site 

in May 1989. GPR data were not collected at 

this site because saltwater prevented the 

effective use of this technique.

The 200-kHz Fathometer data 

collected 10 ft upstream from the piers 

indicated the presence of scour holes near piers 

4 through 9 and a 40-ft deep channel between 

piers 8 and 9 (fig. 9). The largest scour hole 

detected was located at pier 8 and was 

estimated to be 10 ft deep. Interference from 

echoes from the footing prevented an 

interpretation of the actual bottom of this hole 

from the Fathometer record.

The 3.5-kHz CSP data collected 

simultaneously indicate the same bottom 

features as the Fathometer but contained some 

subbottom information. At pier 8 (fig. 10), 

many hyperbolic reflectors near the bottom of 

the hole indicate that coarse material, such as 

gravel, is present. A 2-ft thick layer of fill 

material is interpreted along the east side of the 

hole. A 2-ft thick layer of fill material also was 

detected at pier 7. At this site, the CSP 

technique was able to detect the river bottom 

and the thickness of the fill material on the east 

side of the holes at piers 7 and 8, whereas the 

Fathometer could not detect the fill material. 

The setting of this site was favorable for the 

CSP technique although interference from side 

echoes was present in the data.

24
w Pier 4 Pier 5 Pier 6 Pier? Pier 8 Pier 9 Pier 10

150 METERS 

Approximate horizontal distance

Figure 9. Unpositioned 200-kilohertz Fathometer profile collected 10 feet upstream 

from the piers at the old Baldwin Bridge, Old Saybrook, Conn.

Pier 8 

Center-line

' - / ' ^'*fSJJ^/J&\^*&*+3S.I2****>*L 

r

0 30 METERS

Approximate horizontal distance

Figure 10. Unpositioned 3.5-kilohertz continuous seismic-reflection profiling data 

collected 10 feet upstream from pier 8 at the old Baldwin Bridge, Old Saybrook, Conn.

25
New Baldwin Bridge, Old Saybrook

The new Baldwin Bridge, constructed 

60 ft south of the old Baldwin Bridge, now 

carries Interstate 95 over the Connecticut 

River Estuary (fig. 11). Geophysical data were 

collected at the new bridge location before, 

during, and after construction (Placzek and 

others, 1993). The new bridge has eight 

rounded-nose piers spaced 240 ft apart in 

water ranging from 10 to 40 ft deep. Each pier 

consists of two columns, each 8 ft wide and 

36 ft long. The piers in the water were 

numbered 3 to 9 (west to east) to correlate with 

Connecticut Department of Transportation 

bridge plans. A navigation channel is between 

piers 8 and 9, and a wood fender system is in 

place alongside those piers. During 

construction, each pier was protected by a 

rectangular cofferdam that was 36 ft wide and 

144 ft long; the flow of the river was 

perpendicular to the 36-ft side. After 

construction, each cofferdam was cut off at the 

riverbed level or the top of the footing, 

whichever was higher.

Since construction began on this 

bridge, the maximum discharge of the 

Connecticut River near Middletown (the 

closest USGS streamflow-gaging station) was 

94,000 ft3/s on March 22, 1990 (Cervione and 

others, 1991). This discharge represents a peak 

flow equivalent to a 3-year recurrence-interval 

flood; therefore, no significant floods have 

occurred since the construction of this bridge.

Fathometer and CSP data were 

collected before, during, and after the 

construction of the new Baldwin Bridge. The 

specific conductance of saltwater is 40,000 

|iS/cm (table 2), so GPR techniques were not 

used at this site. This bridge was under 

construction from 1990-91, and unpositioned 

Fathometer and 3.5- and 7-kHz CSP data were 

collected on May 25, 1989; October 31, 1990; 

July 22, 1992; and December 15,1992. 

Unpositioned swept-frequency CSP data,

positioned Fathometer, and positioned 7-kHz 

CSP data were collected on May 18, 1993. 

Approximate horizontal scales were 

determined for the unpositioned records by 

correlating visually estimated positions with 

bridge plans. Before construction of the new 

bridge, the data were collected along the 

proposed bridge alignment, and the centerlines of the old bridge piers were marked on 

the record. The new piers are located 50 ft 

west of the corresponding old piers. During 

construction, the sides of the cofferdams were 

marked on the record; after construction, the 

center-lines of the new piers were marked on 

the records.

Fathometer data collected along the 

proposed bridge alignment before construction 

(fig. 12) show an undulating river bottom with 

about 6 ft of relief, a flat river bottom at the 

proposed pier 7 location, and a 40-ft deep 

navigation channel east of the proposed pier 8 

location. The 7-kHz CSP data collected before 

construction show subbottom reflectors from 

geologic layers up to 25 ft deep. This 

information was collected before bridge 

construction so the geologic layers could be 

identified in future records and would not be 

interpreted as scour surfaces.

Fathometer data collected 15 ft 

upstream from the piers during construction 

(fig. 13) show scour holes 5 ft deep at pier 6, 

11 ft deep at pier 7, and 7 ft deep at pier 8. The 

7-kHz CSP data collected simultaneously did 

not detect any fill material in these scour holes. 

Fathometer data collected 15 ft upstream from 

the piers after construction (fig. 14) show no 

scour hole at pier 6, and scour holes 21 ft deep 

at pier 7 and 15 ft deep at pier 8. The top of the 

footings also were detected on the records at 

piers 7 and 8. The deepest scour hole 

downstream from the piers was 4 ft deep at 

pier 7. The 3.5-kHz CSP data collected 15 ft 

upstream from the piers after construction (fig. 

15) show that 5 ft of material was deposited in 

the scour hole at pier 6 and the scour holes 

near piers 7 and 8 increased in depth, but had

26
"^1 Connecticut River

f' ''-I

Fieri ^2345

D D ODD

I I | I I I

Fieri 2 *A 3 4 5

1

6

D

I

6

7

D

I

7

X

8

D

I

8

j

Main 

hanne /

9

D

I

9

L

10

D

1

10

D

N

QOId bridge pier locations 

| New bridge pier locations

V

Long Island Sound

600 FEET

I

I 

200 METERS

Figure 11. The old and new Baldwin Bridge pier locations, Old Saybrook, Conn.

27
1LLJ 

Q

0 150 METERS 

Approximate horizontal distance

Figure 12. Unpositioned 200-kilohertz Fathometer profile collected along the proposed 

bridge alignment of the new Baldwin Bridge, Old Saybrook, Conn, (before construction).

no fill material in them. Side echoes from the 

piers were present in the record but did not 

interfere with the interpretation of the data. 

The tops of the footings at piers 7 and 8 also 

were detected. CSP data on the downstream 

side of the piers show geologic layers in the 

subbottom but no filled scour holes.

Additional Fathometer data were 

collected laterally alongside old and new pier 7 

on December 15, 1992. This line extended 

from 350 ft north of the old bridge to 250 ft 

south of the new bridge 10 ft east of both 

piers. Scour depths of 5 ft upstream from old 

pier 7 and 20 ft upstream from new pier 7 were 

detected. Footings on the sides of both piers 

were detected on the record. Little or no scour 

was observed on the south side of either pier.

The single-element 14-kHz CSP 

transducer produced poor-quality records at 

the new Baldwin Bridge, but the new dualelement 14-kHz CSP transducer using the

active ring-damping circuit produced 

improved records. Data collected in December 

1992 using the new transducer and circuit 

detected a thin, 1-ft layer of fill material along 

the west side of the scour hole at pier 7 (fig. 

16). A deeper previous scour surface also was 

present in this record on the west side of the 

hole. The east side of the hole contained many 

hyperbolic reflections indicating that it may be 

armored with gravel or rip-rap. Multiple 

reflections, side echoes from the piers, and an 

echo from the top of the sheet piling or footing 

also appear in the record.

Positioned 7-kHz CSP and 2- to 16- 

kHz swept-frequency CSP data, collected in 

May 1993, detected the same deeper layer of 

fill material on the west side of the hole at pier 

7. The high-resolution swept-frequency CSP 

data (fig. 6) also show the 1-ft thick layer of fill 

material on the west side of the hole. No 

additional fill material was detected in the 

other scour holes.

28
0 30 METERS 

Approximate horizontal distance

Figure 13. Unpositioned 200-kilohertz Fathometer data collected 15 feet upstream from 

the new Baldwin Bridge, Old Saybrook, Conn, (during construction). (From Placzek and 

others, 1993.)

Pier 8 Pier? Pier 6

50 METERS

Approximate horizontal distance

Figure 14. Unpositioned 200-kilohertz Fathometer data collected 15 feet upstream from 

the new Baldwin Bridge, Old Saybrook, Conn, (after construction). (From Placzek and 

others, 1993.)

16-1 Pier? Pier 6 W

0 100 FEET

| ' ' ' ' -1 Approximate horizontal distance

0 30 METERS

Figure 15. Unpositioned 3.5-kilohertz continuous seismic-reflection profiling data 

collected 15 feet upstream from the new Baldwin Bridge, Old Saybrook, Conn, (after 

construction). (From Placzek and others, 1993.)

29
At the new Baldwin Bridge site, the 

CSP data showed the same bathymetric data as 

the Fathometer records, but in addition, 

provided information about the subbottom. Fill 

material up to 5 ft thick in a scour hole was 

interpreted using the CSP techniques at this

site, which was saltwater with a mud bottom 

and a subbottom consisting of sand and silt. 

The fixed-frequency and swept-frequency CSP 

systems produced similar records, although the 

resolution of the swept-frequency data was 

higher than the 3.5- or 7.0-kHz data.

Pier? 

Center-line

Approximate 

^ qriginal 

Hverbottbm

0 15 METERS 

Approximate horizontal distance

Figure 16. Unpositioned 14-kilohertz continuous seismic-reflection profiling data 

collected 15 feet upstream from the new Baldwin Bridge, Old Saybrook, Conn, (after 

construction).

30
Route 14 Bridge, Canterbury

The Route 14 bridge spans the 210-ft 

wide Quinebaug River in Canterbury, Conn. A 

single pier was located in water that had a 

maximum depth of 5 ft at the time of the study. 

The maximum discharge prior to August 1989 

at the nearest USGS streamflow-gaging 

station, located 8 mi downstream near Jewett 

City, was 42,000 ft3/s on August 20, 1955 

(Cervione and others, 1990). The pier was 

skewed to the flow creating a potential for 

scour, but no debris was present around the 

pier. A lithologic log from the bridgeconstruction plans indicated that the riverbed 

material consisted of a sand and gravel bottom 

and a sand and gravel subbottom with some 

clay; depth to bedrock was more than 45 ft. 

The specific conductance of the water near 

Jewett City during the 1989 water year ranged 

from 87 to 150 |iS/cm (Cervione and others, 

1990).

GPR techniques were used at this site 

because the water was shallow and the specific 

conductance was low. CSP techniques were 

not used at this site because of the shallow 

water depths. In August 1989, 80- and 300- 

MHz GPR data were collected at this site. The 

equipment was operated from a 12-ft fiberglass 

boat. Multiple cross-sections and lateral 

sections were collected near the bridge pier. 

The position of the antenna relative to the pier 

was visually estimated and noted on the 

record. The single 80-MHz antenna was 

difficult to maneuver near the bridge pier and 

did not provide sufficient resolution, so the 

300-MHz antenna was used. Riverbed 

deposition was detected west of the pier in the 

300-MHz GPR data collected 2 ft upstream 

from the pier (fig. 7). The record shows 2 ft of 

fill material west of the pier. Hyperbolic 

images in the record east of the pier indicate a 

gravel or rip-rap bottom and prevent any 

shallow subbottom reflectors from being 

identified. Multiple, smooth, flat-lying, parallel

reflectors west of the pier were interpreted as a 

sand bottom and subbottom; this interpretation 

was verified by the lithologic log from the 

bridge-construction plans.

Figure 7 was processed using a 

migration routine, and the results are shown in 

figure 8. The migration routine collapsed the 

hyperbolas east of the pier to their point 

sources. The gravel surface is interpreted to be 

1 ft below the river bottom, and it may be an 

old scour or riverbed surface. No subbottom 

information was obtained below the gravel 

surface in this record. Processing the data also 

improved the subbottom interface west of the 

pier that is interpreted as 4 ft of sand overlying 

an old channel surface.

The interpreted GPR records at the 

Route 14 bridge indicate 4 ft of riverbed 

deposition west of the pier and a gravel surface 

1 ft below the river bottom east of the pier. 

Processing the data allowed the bottom gravel 

layer to be interpreted.

Bulkeley Bridge, Hartford

The Bulkeley Bridge carries Interstate 

84 over the Connecticut River between East 

Hartford and Hartford, Conn. A total of eight 

bridge piers span the 1,060-ft wide river. The 

piers were numbered from 1 to 8, starting with 

the first pier in the water on the west side. Piers 

1 and 6 are 170-ft long and 40-ft wide, and the 

other six piers are 140-ft long and 20-ft wide. 

The riverbed materials at this site consist of 

sand overlying glacial lacustrine silt and clay 

(Haeni and Gorin, 1989). Water depths near 

the piers ranged from 2 to 30 ft during the 

study, and the specific conductance of the 

water from October 1988 to September 1992 

ranged from 69 to 155 |iS/cm (Cervione and 

others, 1993). The average water depth 100 ft 

upstream from the piers was 8 ft.

A streamflow-gaging station is located 

at the Bulkeley Bridge, and the annual mean

31
discharge of the Connecticut River between 

May 1990 and May 1993 was about 17,000 

ft3/s (Cervione and others, 1993). However, 

the Connecticut River often floods, and the 

flow increases to greater than 10 times the 

average (Ryder and others, 1981). Since 1935, 

there have been four spring floods during 

which the river rose more than 30 ft above 

base level in Hartford (Bell, 1985). The 

maximum flow was 282,000 ft3/s on March 20, 

1936 (Cervione and others, 1993).

In June and July 1987, prior to this 

study, unpositioned 200-kHz black-and-white 

Fathometer and 20-kHz color Fathometer, 

14-kHz CSP, and 80-MHz GPR data were 

collected at the Bulkeley Bridge (Gorin and 

Haeni, 1989). Infilled scour holes were 

detected at piers 3,4, 5, and 6. In 1988, three 

cores were collected from a scour hole 

upstream from pier 4 to verify the geophysicaldata interpretations.

For this study, positioned 200-kHz 

black-and-white Fathometer, 20-kHz color 

Fathometer, 14-kHz CSP, and 100-MHz GPR 

data were collected and processed in May and 

October of 1990 (Haeni and Placzek, 1991). In 

September 1991, unpositioned 1.5- to 10-kHz 

swept-frequency CSP data were collected, and 

the following month, a new 14-kHz transducer 

was used to collect positioned CSP data. In 

May 1993, positioned 7-kHz CSP data were 

collected, and new 100-MHz antennas were 

used to collect positioned GPR data.

A 200-kHz Fathometer was used with 

the Hydro I positioning system and software to 

produce horizontally corrected bathymetric 

profiles of the river bottom in May 1990. A 

profile of the river bottom, 5 ft upstream from 

the piers, shows depressions in the river 

bottom indicating existing scour holes (fig. 2). 

Multiple lines of Fathometer data were 

collected upstream, downstream, and 

alongside pier 4 to determine the size of the 

scour hole. The hole upstream from pier 4 was 

about 80 ft wide and 150 ft long. Seven lines

of the positioned Fathometer data collected at 

different distances upstream from the piers 

were used to create a three-dimensional 

contour plot of the topography of the river 

bottom upstream from the piers (fig. 3). The 

navigation channel and five scour holes at 

piers 2 through 7 are present. The plot 

produced a clear three-dimensional image of 

the size and shape of the scour holes, but the 

depth of the holes was interpreted more 

accurately from the 200-kHz Fathometer 

profiles of the individual cross sections.

Positioned 20-kHz color Fathometer 

data were collected in May 1990 upstream 

from the piers, and the data indicated the same 

bathymetric profile as the 200-kHz Fathometer 

data. The 20-kHz color Fathometer data also 

showed fill material in some of the scour holes. 

At pier 4, 5 ft of fill was detected in the scour 

hole 12 ft upstream from the pier (fig. 17). This 

record contains a multiple reflection that does 

not interfere with the area of interest. The 

positioned 14-kHz CSP data and 100-MHz 

GPR data, collected in May, also indicate 5 ft 

of fill material in the scour hole at pier 4. The 

CSP and GPR data were subsequently 

processed, which clarified the data 

interpretation (Haeni and Placzek, 1991).

In September 1991, the Datasonics 

1.5- to 10-kHz and the 8- to 23-kHz sweptfrequency CSP systems were used to collect 

CSP data. The low-frequency system detected 

scour holes at piers 2 through 7. More than 4 ft 

of fill was detected in the holes at piers 3 

through 6, and less than 2 ft of fill was detected 

in the holes at piers 2 and 7. As much as 6.5 ft 

of fill was detected at pier 4, and as much as 4 

ft of fill was detected at pier 5 (fig. 18). The 

high-frequency swept-frequency system did 

not produce as good a record as the lowfrequency system. The high-frequency system 

had difficulty locking on the river bottom and 

had a large amount of system and (or) 

environmental noise, which degraded the 

quality of the record.

32
w

50 FEET

15 METERS

Approximate horizontal distance 

Vertical exaggeration x3

Figure 17. Black-and-white photograph of a 20-kilohertz color Fathometer cross 

section collected 12 feet upstream from pier 4 of the Bulkeley Bridge, Hartford, Conn. 

(Modified from Gorin and Haeni, 1989.)

Positioned 14-kHz CSP data collected 

in October 1991 also indicated six scour holes 

at six piers of the Bulkeley Bridge. Fill material was detected in five of these holes. Five ft 

of fill material was detected in the scour hole 

10 ft upstream from pier 4. The field record 

(fig. 4) shows an existing 5-ft hole with 5 ft of 

fill material, and the distance between position 

marks changed, indicating an inconsistent boat 

speed. The horizontally corrected, filtered, and 

deconvolved record shows the same scourhole depth and fill material thickness, but also 

shows steeper sides of the hole and multiple 

old scour surfaces (fig. 5). The multiple scour 

surfaces can be explained if the hole had 

scoured to its maximum depth during the highest recorded flow and then filled gradually over 

time, or if the hole rescoured and filled during 

subsequent flood events.

In May 1993, positioned 7-kHz CSP 

data were collected 20 ft upstream from the 

piers. The data provided a clearer image of the 

deep subbottom stratigraphy than the 14-kHz 

CSP data or the 80- and 100-MHz GPR data. 

Existing scour holes were detected 20 ft 

upstream from piers 3, 5, 6, and 7. Although 

no existing scour hole was evident, as much as 

6 ft of fill in an old scour hole was detected at 

pier 4 (fig. 19), and as much as 5 ft of fill was 

detected at piers 5 and 6. Because of the datacollection mode in which the positioning 

system was operated, the position marks on the 

record appear evenly spaced but do not 

indicate a constant boat speed. The marks 

signify where position data points were 

collected and are not necessarily 10-ft 

increments.

33
Pier 5 

Center-line

Pier 4 

Center-line

n

30 METERS

Approximate horizontal distance

Figure 18. Gray-scale amplitude plot of 1.5- to 10-kilohertz swept-frequency record 

collected 5 feet upstream from piers 4 and 5 at the Bulkeley Bridge, Hartford, Conn.

0 30 METERS 

Approximate horizontal distance

Figure 19. Unpositioned 7-kilohertz continuous seismic-reflection profiling data 

collected 20 feet upstream from the Bulkeley Bridge, Hartford, Conn.

34
Positioned 100-MHz GPR data also 

were collected in May 1993 using the modified 

antennas built for this study. Infilled scour 

holes were detected near piers 2 to 7. The data 

showed 5 ft of fill material at the bottom of a 

10-ft deep hole 15 ft upstream from pier 5 and 

3 ft of fill material on the east side of a 13-ft 

deep hole 15 ft upstream from pier 4 (fig. 20). 

A 9-ft deep scour hole detected at pier 2 in 

1987 (pier 7 in Gorin and Haeni, 1989, p. 11) 

had filled in and was only 4 ft deep in 1993. 

The GPR data did detect the old scour surface 

from 1987, 5 ft below the 1993 river bottom.

The 20-kHz color Fathometer, 7- and 

14-kHz CSP system, 1.5-10 kHz sweptfrequency CSP system, and the 100-MHz GPR 

system provided subbottom information used 

to interpret the thickness of fill material in 

scour holes at this site. The geophysical data 

collected with all of the techniques indicated

that about 5 ft of material filled the hole at pier 

4. The seismic data provided subbottom 

information to 25 ft (to bedrock) and the radar 

data provided subbottom information to 10 ft 

(to the glacial lacustrine clay). The water 

depth, specific conductance, and physical 

setting allowed the color Fathometer, CSP, and 

GPR techniques to detect scour surfaces in the 

subbottom of the river at this site.

The three core samples previously 

collected at pier 4 correlated with all the 

geophysical data collected at this site. The 

grain-size distribution of the fill material in the 

scour hole ranged from fine sand near the 

water bottom to very coarse sand, with a few 

large pebbles at the bottom of the fill material. 

Glacial lacustrine silt and clay underlie the fill 

material. Detailed results of the coring are 

described in Haeni and Gorin (1989).

cc

LLJ

QL. 

LLJ 

Q

I_____I_____I_____I

100 FEET

j____i

0 30 METERS 

Approximate horizontal distance

Figure 20. Positioned 100-megahertz ground-penetrating radar data collected 15 feet 

upstream from piers 4 and 5 of the Bulkeley Bridge, Hartford, Conn.

35
Route 4 Bridge, Farmington

The Route 4 bridge spans the 310-ft 

wide Farmington River in Farmington, Conn. 

Three piers were located in water that was less 

than 10 ft deep during this study. The nearest 

USGS streamflow-gaging station is 3.5 mi 

upstream near Unionville, and the maximum 

discharge recorded prior to October 1991 was 

140,000 ft3/s on August 19, 1955 (Cervione 

and others, 1993). Rip-rap was present on both 

river banks, and no debris was present around 

the piers at the time of the study. A lithologic 

log from the bridge plans indicated that the 

riverbed material consists of a sand bottom and 

subbottom. The specific conductance of the 

water near Unionville from August 1990 to 

October 1991 ranged from 64 to 100 jiS/cm 

(Cervione and others, 1993).

In August 1990, 100-MHz GPR data 

and 200-kHz Fathometer data were collected

simultaneously by a two-person crew in a 13-ft 

inflatable boat powered by a 6-horsepower 

engine. The 100-MHz GPR data showed no fill 

material in a 3-ft scour hole on the upstream 

side of the center pier. In October 1991, 300- 

MHz GPR data were collected. A 3-ft deep 

scour hole was detected 4 ft to the left of the 

upstream side of the center pier, and a mound 

of material was detected at the downstream 

side (fig. 21). No fill material in the scour hole 

was detected.

GPR records were collected 

successfully at this site, but no deep scour 

holes were detected. One interpretation of this 

is that little scour has occurred at this site. The 

detection of a coarse sand or gravel layer 

beneath a finer sand layer elsewhere in the 

records indicates that the GPR technique did 

work in this physical setting.

Downstream 

edge of pier

Upstream 

edge of pier

15 METERS

Approximate horizontal distance

Figure 21 . Unpositioned 300-megahertz ground-penetrating radar data collected 4 feet 

east of the center pier at the Route 4 Bridge, Farmlngton, Conn.

36
Route 2 Bridge, Norwich

The Route 2 bridge spans the 200-ft 

wide Shetucket River in Norwich, Conn., and 

two round-nose piers are located in water that 

has a maximum depth of 15 ft. A maximum 

discharge at this site of 68,000 ft3/s on August 

20, 1955 was calculated using data collected 

7 mi upstream on the Quinebaug River near 

Jewett City and 16 mi upstream on the 

Shetucket River near Willimantic (Cervione 

and others, 1990). The riverbed materials 

consist of a sand and gravel bottom and a sand 

subbottom (Cervione and others, 1968, p. 44). 

The river was wide and deep enough to use a 

22-ft boat for data collection. The location of 

the transducers was estimated and noted on the 

records. GPR techniques were not used at this 

site because saltwater was present.

In June 1991, 14-kHz CSP data were 

collected from a 13-ft boat and in October 

1991,20- and 200-kHz Fathometer and 7-kHz 

CSP data were collected from a 22-ft boat. The

Fathometer data (fig. 22) detected that the top 

of the footing 3 ft upstream from pier 1 was 

exposed at a depth of 13 ft and the river bottom 

was at 17 ft, indicating that a 4-ft scour hole 

was present. No evidence of scour was 

detected near pier 2. No side echoes from piers 

were present on the Fathometer record, but the 

7- and 14-kHz CSP data did contain side 

echoes. The CSP data contained the same 

bathymetric data as the Fathometer data and 

did not indicate the presence of any infilled 

scour holes. The 20-kHz color Fathometer data 

confirmed the presence and depth of the 

footing and the existing scour hole, but 

provided no subbottom information.

The CSP and 20-kHz Fathometer 

records did not detect an old scour surface at 

pier 1 or 2, indicating either that no infilled 

scour holes were present, or that the fill 

material was less than 1 ft thick, the maximum 

resolution of the techniques. Probing near the 

hole confirmed that the footing was exposed 

and that there was no fill material in the hole.

Fieri 

Center-line

Pier 2

0 50 FEET

I____I____i i i i i i i i 

0 15 METERS

Approximate horizontal distance

Figure 22. Unpositioned 200-kilohertz Fathometer data collected 3 feet upstream from 

the piers at the Route 2 Bridge, Norwich, Conn.

37
FUTURE EQUIPMENT

MODIFICATIONS AND RESEARCH

NEEDS

During this study, modifications were 

made to equipment, and field procedures and 

interpretation methods were developed for 

studying scour near bridge piers. Future 

equipment modifications and other areas of 

research include:

(1) The 14-kHz dual-element 

transducer has not been fully tested with the 

active ring-damping circuit at bridge-scour 

sites. This system has the potential to be used 

in very shallow (3 ft) water. Further 

applications of this transducer and an 

evaluation of its performance are needed.

(2) The GPR system can record field 

data onto DAT cartridges using the GSSI 

model 38 interface. By implementing this 

interface, GPR data can be stored on a material 

common to all systems, making future datatransfer procedures simpler than present ones.

(3) Geophysical data recorded on a 

DAT cartridge can be transferred directly to a 

PC by a strictly digital link, rather than 

through a digital-to-analog and analog-todigital card. The digital link would provide an 

easier transfer procedure with less noise and 

less hardware dependence on unique systems. 

The digital link required is commercially 

available, although some software may need to 

be developed.

(4) A processing procedure could be 

developed to correct for vertical-scale changes 

in GPR data when the signal passes through 

materials with different velocities. A single, 

corrected vertical scale would provide a 

geophysical record that more closely 

resembles the actual geometry of the site.

(5) Development and documentation of 

multiple suppression, side-echo reduction, and 

adaptive filtering techniques may help reduce

the noise and interference in geophysical data 

collected near bridge piers.

(6) A variable time-varying gain may 

account for the varying amplitude of the 

incident wave in different water depths and 

can relate the amplitude of the reflected signal 

to the reflection coefficient, which can be 

related to material types.

(7) New geophysical instruments that 

could be used to detect and measure riverbed 

scour are constantly being developed. Some 

commercially available systems that may 

apply to scour studies include:

(a) Swept-frequency systems that use a 

smaller transducer and a broader bandwidth 

signal (4 to 20 kHz), and are less expensive 

than the systems used in this study.

(b) Dual-frequency color Fathometers 

that allow a dual-display of high- and lowfrequency data.

(c) Low-frequency Fathometer 

systems, such as the 128-kHz InnerSpace 45- 

16, may provide limited penetration depths 

and high resolution required in a scour study.

(d) Stationary scanning multi-beam 

bathymetric sonar devices, such as the Scour 

Fish developed by American Inland Divers, 

Inc. or Sea Bat developed by Reson, 

Inc.,which provide bathymetric data that can 

be used to evaluate scour conditions.

(e) High-resolution multiplefrequency subbottom profilers that are used by 

Rijkswaterstaat in Holland to obtain 

subbottom information for dredging. The 

System 80, manufactured by Ulvertech 

Limited in England, has a small transducer 

with a beam width less than 5 over the 5- to 

20-kHz frequency range. The system generates 

this narrow beam, low-frequency signal from 

two offset high-frequency transmissions.

38
SUMMARY AND CONCLUSIONS

Fathometer, CSP, and GPR techniques 

were used to detect and measure the depth and 

extent of existing and infilled scour holes near 

bridge piers. Methods were developed to apply 

these surface-geophysical techniques near 

bridge piers and to process and interpret the 

data. The geophysical techniques used at a 

given site were chosen on the basis of penetration depth and resolution requirements, as well 

as physical site characteristics, such as water 

depth and specific conductance. Performance 

criteria of the surface-geophysical techniques 

used in this study are summarized in table 3. 

This includes idealized and achieved values of 

resolution capability and penetration depth. 

Achieved values may differ from idealized 

values because the techniques were not 

applied under ideal conditions or the geologic 

conditions were not present to adequately test 

the technique.

Most of the geophysical equipment 

was standard and commercially available, but 

some modifications were required. The 80- 

and 100-MHz GPR antennas were waterproofed, and a new case was designed and 

constructed for the 100-MHz antenna. An 

active ring-damping circuit and a dual-element

14-kHz transducer improved the quality of the 

CSP data. All geophysical instruments were 

mounted in shipping cases that increased portability and field durability of the equipment.

Geophysical data were collected at six 

bridge sites in Connecticut. Scour holes both 

with and without fill material in them were 

detected. In addition, riverbed deposition from 

a migrated river channel, depth to geologic 

layers, buried rip-rap, and exposed pier footings were detected. A summary of the sites, 

their characteristics, and the results of the techniques used at the sites is shown in table 4.

After data collection, the geophysical 

data were interpreted, and all available data 

from the site were correlated. A process developed to transfer data from tape cartridges to 

the PC allowed Fathometer, CSP, and GPR 

data to be processed using commercially available software. The processing of the data 

resulted in more accurate interpretations and 

improved data displays. Bathymetric profiles, 

position data, and verification data supplemented the geophysical data to confirm the 

depth and extent of scour holes, thickness of 

fill material in the holes, location of exposed 

footings, location of previous channels, and 

depth to other geologic layers.

Table 3. Summary of performance criteria for selected surface-geophysical techniques

[kHz, kilohertz; MHz, megahertz; idealized value is the manufacturer claim or theoretical maximum; achieved 

value is that obtained during this study; , no manufacturer claim]

Resolution capability

(feet)

Penetration depth

(feet)

Geophysical 

technique

Fathometers

Continuous

seismic-reflection

profilers

Ground-penetrating

radar system

Material type

Fine-grained

(silts and clays)

Fine-grained

(silts and clays)

Electrically

resistive and

granular

Signal 

frequency

200kHz

20kHz

14kHz

7kHz

3.5 kHz

2 to 16 kHz

300MHz

100 MHz

80MHz

Idealized

0.3

0.5

1

2

2.5

0.3

1

2

2.5

Achieved

0.5

1

1

2

2.5

1

1

2

2.5

Idealized

-

20

40

100

200

40

40

100

Achieved

0

10

20

25

50

50

10

40

50

39
Table 4. Summary of geophysical data collected at six bridge sites in Connecticut

o [ft, feet; ^,S/cm, microsiemens per centimeter; ft /s, cubic feet per day; kHz, kilohertz, MHz, megahertz; GPR, ground-penetrating radar; NA, not applicable]

Site, location. 

Date(s) data 

collected.

Old Baldwin

Bridge, Old

Saybrook, Conn.

May 1989.

New Baldwin

Bridge, Old

Saybrook, Conn.

May 1989

October 1990

July 1992

May 1993Route 14 Bridge,Canterbury, Conn.

August 1989

Water depth (ft) 

and bottom/ 

subbottom 

materials

8-40.

Mud/sand and silt.

10-40.

Mud/sand and silt.0-5.

Sand and

Typical specific 

conductance 

()j,S/cm) and 

discharge (ft3/s)

Saltwater (40,000).

Tidal.

Saltwater (40,000).

Tidal.

87-150.

1,000 average;

Fathometer

200 kHz:

bathymetry of river

bottom.

200 kHz: used with

positioning system;

scour holes and top

of exposedfootings detected.20 kHz: bathymetryof river bottom,

some fill detected.

Fathometer data notcollected.

Results

Continuous 

seismicreflection 

profiling

3.5 kHz at pier 8:

2.5 ft of fill

detected in scour

hole.3.5, 7.0, 14, and 2-

16 kHz: 5 ft of fill

in a scour holedetected. Sweptfrequency and 14kHz detected 1-ftlayers.

NA, water depthtoo shallow.

Groundpenetrating 

radar

NA, due tosaltwater.NA, due tosaltwater.

80-MHz antennas

provided lowresolution.

Notes

Scour holes at 5 of

7 piers. Some fillmaterial detected

in 2 of 5 scour

holes.

Data collectedbefore, during, and

after construction.

A 20-ft scour holedeveloped within

3 years without asignificant flood.

Fill detected.

Processing

improved GPR

records.

gravel/sand and gravel.

42,000 maximum. 300-MHz antenna 

shows 4 ft of 

riverbed deposition 

and 1ft of fill 

material.
Table 4. Summary of geophysical data collected at six bridge sites in Connecticut --Continued

«

[ft, feet; jiS/cm, microsiemens per centimeter; ft /s, cubic feet per day; kHz, kilohertz, MHz, megahertz; GPR, ground-penetrating radar; NA, not applicable]

Site, location. 

Date(s) data 

collected.

Bulkeley Bridge,

Hartford, Conn,May 1990October 1990

September 1991October 1991

May 1993Route 4 Bridge,

Farmington, Conn.

August 1990October 1991

Water depth (ft) 

and bottom/ 

subbottom 

materials

2-30.

Sand/silt and clay.

0-10.

Sand/sand with

boulders.

Typical specific 

conductance 

(fxS/cm) and 

discharge (ft3/s)

69-155.

17,000 average;

282,000 maximum.

64-100.745 average;

140,000 maximum.

Fathometer

20kHz:bathymetric plot, 5

ft of fill detected inone hole,

200kHz:bathymetric plot200 kHz: good

bathymetric profile.

Results

Continuous 

seismicreflection 

profiling

Positioned 7- and

14-kHz, 1. 5- to 10-

kHz Chirp: 5 ft of

fill detected by allsystems. Chirp

data detected thin

layers of fill.

NA, water depth

too shallow;

average water

depths less than 5ft.

Groundpenetrating 

radar

Positioned 100

MHz: 5 ft of fill

detected.Modified 100-MHz

antennas workedwell.

100 MHz: profiled

scour hole,detected about 2 ft

of fill.

300 MHz: no fill in

hole; fill deposited

downstream from

pier.

Notes

Three-dimensionalcontour plot ofbathymetry.Processingimproved 14-kHz

records. 6 of 8 piershad scour holes

and fill. Confirmed

by previous coring.

Scour holes

developed upstreamfrom pier andmaterial depositeddownstream.

Route 2 Bridge,

Norwich, Conn.

June 1991

October 1991

8-15.

Sand and

gravel/sand.

Saltwater.

1,300 average;

68,000 maximum.

200 kHz: goodbathymetric profile.

14 kHz: exposed NA, saltwater.

footing detected; nofill detected.

7 kHz: similar to 14

kHz but less

resolution.

Exposed footing

4 ft above river

bottom detected.

No fill. Confirmed

by probing. I
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