Assessment of the Alluvial Sediments in the 

Big Thompson River Valley, Colorado

By Adrienne Barnett and Karl J. Ellefsen

U.S. Geological Survey Digital Data Series DDS-66

U.S. Department of the Interior

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

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

First replication 2000

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80255

This report is also available online at:

http://greenwood.cr.usgs.gov/pub/dds/dds-066/

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Version 1.0

Published in the Central Region, Denver, Colorado

Manuscript approved for publication May 23, 2000

Graphics by the author

Photocomposition by Norma J. Maes
III

Contents

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

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

Geology and Field Conditions............................................................................................................. 1

S-wave Refraction Method ................................................................................................................ 2

General Description.................................................................................................................... 2

Data Collection ............................................................................................................................ 2

Data Processing .......................................................................................................................... 3

Interpretation ........................................................................................................................................ 3

 Interpretation of the Velocity Profiles...................................................................................... 3

 Construction of the Cross Sections.......................................................................................... 5

 Interpretation of Cross Section A-A’ and B-B’ ...................................................................... 6

 Interpretation of Cross Section C-C’........................................................................................ 6

 Additional Observations............................................................................................................. 7

Conclusions........................................................................................................................................... 9

Acknowledgments ............................................................................................................................... 10

References ............................................................................................................................................ 10

Appendix................................................................................................................................................ 11

Figures

 1. Exposure showing the soil and the underlying post-Piney Creek alluvium.................. 2

 2. The seismic source used to generate the Sh-waves and the spread used to 

collect S-wave refraction data ............................................................................................ 3 

 3. Map showing the layout of spread A7 and seismograms from shots 3 and 4 in 

 the spread. .............................................................................................................................. 4

 4. Dot plots and statistics of the velocities and the thicknesses for the top layers of 

 the profiles............................................................................................................................... 6

 5. Dot plots and statistics of the velocities for the middle layers of those profiles with 

 three layers, for the upper middle layers of those profiles with four layers, and 

 for the lower middle layers of those profiles with four layers........................................ 7

 6. Dot plots and statistics of the velocities for the bottom layers along cross sections 

A-A’, B-B’, and C-C’ ................................................................................................................ 8

 7. Dot plots and statistics of the velocities associated with the post-Piney Creek 

 alluvium and the Broadway Alluvium.................................................................................. 8

 8. Dot plots and statistics of the alluvium thicknesses for cross section A-A’, B-B’, 

 and C-C’.................................................................................................................................... 9

 

 Tables 

 1. Velocities and thicknesses estimated from the S-wave refraction data ..................... 5

Plates 

 1. Geology of the Big Thompson River valley, Colorado

 2. Velocity profiles and cross sections A-A’, B-B’, and C-C’ across the

 Big Thompson River valley, Colorado
1

Assessment of the Alluvial Sediments in the Big Thompson 

River Valley, Colorado

By Adrienne Barnett and Karl J. Ellefsen

Abstract

To obtain subsurface geologic information about the alluvium in the Big Thompson River valley, S-wave refraction data 

were collected along three roads that cross the valley. The traveltimes were processed to estimate velocities and thicknesses for a 

layered-earth model; from these models, three cross sections of 

the river valley were constructed. The river valleys are covered 

by a layer of soil, which is 0.2 to 1.5 m thick. Beneath the soil, 

there is one layer of alluvium at some locations and two layers at 

other locations. For the two westernmost cross sections, the total 

thickness of the alluvium ranges from about 6 to 10 m near the 

center of the valley and from about 2 to 6 m near the sides of the 

valley. The easternmost cross section is somewhat more complex 

than the other two, because it is near the confluence of the Big 

Thompson and the Little Thompson Rivers. In this cross section, 

the thickness of the alluvium ranges from about 8 to 10 m in the 

southern half of the valley and from about 3 to 13 m in the northern half. In all three cross sections, the alluvium overlies bedrock, which is the upper transition member of the Pierre Shale. 

Introduction

The USGS Front Range Infrastructure Resources Project is 

designed to provide geologic, hydrologic, and geographic information to cities and towns in the Front Range Urban Corridor of 

Colorado. Because ground water and aggregate are so economically important to these communities, one part of the project 

involves evaluating these resources. Aggregate is defined as 

crushed stone, sand, and gravel that is used to construct homes, 

dams, highways, and so on (Langer and Glanzman, 1993, p 1). 

The Big Thompson River valley is close to three cities in 

the Front Range Urban Corridor: Loveland, Longmont, and 

Greeley. Because of large size of the valley, it could be a significant resource of both ground water and aggregate to these cities. 

Consequently, the geology of the valley was investigated as part 

of the current project. 

Geologic information was obtained with a seismic (S-wave) 

refraction survey because there are not enough significant exposures of the alluvial sediments, from which inferences about the 

entire river valley could be made. Data were collected along 

three roads that cross the Big Thompson River valley (pl. 1): 

(1) County Road 3 1300 S, which is about 7 km northwest of 

Johnstown; (2) County Road 13, which is about 5 km northwest 

of Johnstown; and (3) County Road 21 and State Highway 257, 

which are near Milliken. These three roads are referred to as 

CR 3, CR 13, and CR 21, respectively, in the rest of this report.

This report describes the collection of the seismic data, its 

processing, and its interpretation. The results show the thickness 

and the gross stratigraphy of the alluvial sediments, both of 

which are needed to evaluate the ground water and the aggregate 

resources. On the CD-ROM are the field data, software for viewing the field data, and a file called README.TXT; this file contains information on executing the software and on the format of 

the field data. 

Geology and Field Conditions 

The sediments in the Big Thompson River valley were 

eroded from Front Range bedrock, which crops out about 20 km 

west of the study area; during the Quaternary Period, the sediments were carried downstream and deposited in the valley. 

Throughout the valley, the sediments are overlain by soil. 

The following description of the Quaternary sediments in 

the Big Thompson River valley is from Colton (1978). The postPiney Creek alluvium is crossed by CR 3, CR 13 and CR 21 

(pl. 1). This alluvium is about 2 to 5 m thick and consists mostly 

of sand and gravel with some small lenses of clay and silt 

(fig. 1). In the large river valleys of the Front Range, the postPiney Creek alluvium is underlain by an older gravel alluvium; it 

is not known if this older alluvium is in the Big Thompson River 

valley. The Piney Creek alluvium is crossed by CR 21 (pl. 1). 

This alluvium is about 0 to 6 m thick and consists mostly of sand 

and gravel; it also includes organic matter and a weakly developed soil at its top. Near the edges of the river valley, the Piney 

Creek alluvium grades into colluvium. The Broadway Alluvium 

is crossed by CR 13 and CR 21 (pl. 1). This alluvium is about 3 

to 5 m thick and consists mostly of sand and gravel; it also has a 

weakly developed soil at its top. The Broadway alluvium is 

found on the upper terraces of the river valley.

In the study area (pl. 1), the bedrock is the upper transition 

member of the Pierre Shale (Scott and Cobban, 1965). Its name 

is derived from its stratigraphy: the sedimentary beds are a transition from the underlying Pierre Shale to the overlying Fox 

Hills Sandstone. The beds are composed of friable sandstone 

and soft, shaly sandstone. The formation is about 600 m thick 

and dips between 1° and 4° to the east and southeast.

Roads CR 3, CR 13, and most of CR 21 border fields used 

for crops and pasture, and along CR 13 and CR 21 some fields 

are fenced making access somewhat difficult. Roads CR 3 and 

CR 13 had little traffic, and consequently there was little ambient noise in the seismograms. In contrast, CR 21 north of 

Highway 60 had a great deal of traffic, especially large trucks. In 

addition, trains occasionally traveled on two railroad tracks, 
2 Assessment of the Alluvial Sediments in the Big Thompson River Valley, Colorado

which crossed CR 21. East of CR 21, heavy machinery was 

being used to construct new houses. Consequently, there was a 

great deal of ambient noise.

S-wave Refraction Method 

General Description

The most common refraction method, P-wave refraction, 

was tested in the Big Thompson River valley. Although refracted 

P-waves from the water table were recorded, refracted P-waves 

from the bedrock were not (see the Appendix). Without the 

waves from the bedrock, the depth to the bedrock cannot be estimated; thus, P-wave refraction was unsuitable for this survey. 

Instead, S-wave refraction was used, and the method is briefly 

described here, as it is somewhat uncommon.

The seismic source, which was invented by Hasbrouck 

(1983), weighs about 36 kg and is about 0.3 m wide and 0.6 m 

long (fig. 2A). It has 8 spikes on its bottom that couple it to the 

ground; because of these spikes, the source is nicknamed the 

golf shoe. When the golf shoe is struck by a sledgehammer, it 

generates horizontally polarized S-waves (as well as other types 

of seismic waves). During a field test of various S-wave sources, 

the S-waves generated by the golf shoe had higher amplitudes 

than those generated by most other sources (Miller and others, 

1992). 

For field conditions like those in the Big Thompson River 

valley, the golf shoe generates high-amplitude Sh-waves that 

propagate perpendicular to the direction of the sledgehammer 

strike (Kähler and Meissner, 1983). In this propagation direction, 24 geophones are laid out along a line (fig. 2B). The geophones detect the horizontal component of particle velocity and, 

hence, the Sh-waves. The voltages generated by the geophones 

are recorded by an ES-2401 Exploration Seismograph (EG&G 

Geometrics, 1992).

Each side of the golf shoe is usually struck about 5 times 

with the sledgehammer, and the seismograms from each strike 

are added together by the seismograph. This technique, which is 

called vertical stacking, minimizes the random noise in the seismograms. If there is significant ambient noise when the data are 

collected, each side of the golf shoe is struck about 9 or 10 

times. All strikes on one side of the golf shoe constitute one 

shot. There are usually two shots at each location of the golf 

shoe. For one shot, the golf shoe is struck on one side; for the 

other shot, the golf shoe is struck on the other side (fig. 2A). The 

two shots generate waves with opposite polarities, and this difference helps with the identification of the various waves in the 

seismograms (Hasbrouck, 1987). The two shots are often called 

a shot pair.

The layout of the shot pairs and the geophones is defined 

as the spread. If any refractor has significant dip (that is, greater 

than about 5°), a shot pair is needed at each end of the spread to 

determine the dip. Where a refractor is deep compared to the 

length of the spread, few waves from this refractor, if any, are 

recorded. In this case, additional shot pairs are added at both 

ends of the spread; the chosen locations depend upon the field 

conditions. 

Data Collection

For many seismic refraction surveys, data are collected 

along a straight, continuous profile (see, for example, Telford 

and others, 1976, p. 363–366). However, in this investigation 

continuous profiles were not used for the following reasons: 

(1) All of the county roads border fields with crops, some of 

which have fences. If continuous profiles were used, some 

crops and some fences would have been destroyed. (2) In the 

South Platte River valley and other river valleys of the Front 

Range, the topography of the bedrock changes gradually (Smith 

and others, 1964). Consequently, in the Big Thompson River 

valley, the topography is assumed to change gradually. In this 

case, the numerous estimates of the depth to bedrock that would 

be obtained with a continuous profile would be redundant. 

(3) The buildings, the roads, and the railroad tracks near CR 21 

make it impossible to layout a straight, continuous profile. 

Because a continuous profile would cause so many problems, the seismic data were collected with single spreads that 

were located either in or alongside fields. After permission was 

obtained from the landowners, the spreads were laid out to 

avoid damaging the crops, the fences, and so on. Generally, the 

chosen locations were well suited for data collection for the following reasons: (1) The elevation changes along the spreads 

were almost zero. (2) The areas around the spreads were clear 

of obstacles and large vegetation. 

The spreads were generally 200 to 400 m apart. There 

were 7 spreads along CR 3, 6 along CR 13, and 9 along CR 21 

(pl. 1). The spreads are numbered from south to north; each 

number has the prefix A, B, or C corresponding to CR 3, CR 13, 

and CR 21, respectively. Along CR 21, the spreads were 0.1 to 

0.3 km from the road to minimize the ambient noise from the 

traffic. 

Figure 1. Exposure showing the soil and the 

underlying post-Piney Creek alluvium. This 

exposure is along the bank of the Big Thompson River near cross section B-B’. 
Interpretation 3

Geophones

detecting

Sh-waves 

Golf shoe being 

struck by the 

sledgehammer

Direction, along the 

ground, that

Sh-waves with 

largest amplitude

propagate

A B

To record the seismograms for each spread, the seismograph digitized the analog voltages using a 15 bit converter. The 

sample interval for the digitization was 0.1 ms, which was small 

enough to record the seismograms with high fidelity. The 

Nyquist frequency corresponding to this sample interval was 

5 kHz; because the frequencies of the recorded seismic waves 

were less than about 160 Hz, there was no aliasing. Some high 

frequency noise was removed with a high-cut filter set to 

250 Hz. The recording times of the seismograms were 204.8, 

307.2, or 409.6 ms; for each spread the recording time was chosen to make the refracted S-waves appear in about the middle of 

the seismograms.

For each spread, several test shots were made to determine 

both a suitable spacing between the geophones and a suitable 

spread length. Figure 3 shows a typical spread and the seismograms from a shot pair. Because the geophones in the spread are 

close together, the direct and the refracted S-waves are densely 

sampled. Consequently, there are many traveltimes for estimating layer thicknesses and velocities. Furthermore, the velocity of 

the refracted S-wave is about 1000 m/s, which is typical of bedrock in this area. Thus, the spread is long enough to record the 

refracted S-wave from the deepest layer that is important to the 

investigation.

Data Processing

The data processing consisted of three steps. First, the data 

files were converted from SEG-2 format (Subcommittee of the 

SEG Engineering and Groundwater Geophysics Committee, 

1990) to SU format (Cohen and Stockwell, 1999). Second, the 

initial traveltimes of the direct and the refracted S-waves were 

picked using program “pick_sw” (Ellefsen, 2000). Third, the 

traveltimes were processed with a commercial computer program, the “Seismic Refraction Interpretation Program” (Rimrock Geophysics Inc., 1995). For this program, the ground is 

represented by homogeneous, isotropic layers; the interfaces 

between the layers may be curved. The program estimates the 

velocities and the thicknesses of the layers. For this survey, the 

curvatures of the interfaces are small over the length of each 

spread, and so the curvatures are ignored. 

The estimated velocities and thicknesses for all spreads are 

listed in table 1. In this report, the velocities and the thicknesses 

that are estimated for one spread are called a velocity profile. 

The profiles have either three or four layers. For those profiles 

with three layers, the middle layer is referred to as simply Middle Layer. For those profiles with four layers, the two middle 

layers are referred to as Upper Middle Layer and Lower Middle 

Layer. All velocities are rounded to two significant digits. All 

thicknesses are rounded to two significant digits, except those 

less than 1.0 m, which are rounded to the nearest 0.1 m. The 

velocity profile for each spread is plotted in plate 2. 

Interpretation

Interpretation of the Velocity Profiles 

The distributions of both the velocities and the thicknesses 

from the profiles (table 1, pl. 2) are shown with dot plots (Swan 

and Sandilands, 1995), and a typical example is shown in

figure 4A. This plot indicates that there are 4 velocities between 

50 and 100 m/s, 17 velocities between 100 and 150 m/s, and 1 

velocity between 150 and 200 m/s. Thus, a dot plot is like a histogram, except that the dot plot shows the data points. Each dot 

plot is characterized by a range, a median, a mean, and a standard deviation, all of which are rounded to two significant digits. 

Both the dot plots and the statistical measures show trends that 

Figure 2. A, The seismic source, nicknamed the golf shoe, used to generate the 

Sh-waves. Red arrows indicate where the 

golf shoe is struck by the sledgehammer. 

B, The spread used to collect S-wave refraction data. Note that the direction in 

which the golf shoe is struck is perpendicular to the line of geophones 
4 Assessment of the Alluvial Sediments in the Big Thompson River Valley, Colorado

0 5 10 15 20 25

DISTANCE IN X DIRECTION, IN METERS

-4

0

4

DISTANCE IN Y 

DIRECTION,

IN METERS

5 10 15 20

Geophone Number

Shot 1

Shot 2

Shot 3

Shot 4

A

0.05

0.10

0.15

0.20

TIME, IN SECONDS

5 10 15 20

SEISMOGRAM NUMBER

Refracted Wave

Direct Wave

Low-amplitude

noise

B

help with the interpretation of the velocity profiles and with the 

construction of the cross sections. 

For the top layers in the profiles, all velocities are between 

80 and 150 m/s; both the median and the mean are 110 m/s 

(fig. 4A). This range in velocities is similar to that for soil or 

loam (Kudo and Shima, 1981; Suyama and others, 1986). All 

thicknesses are between 0.2 and 1.5 m; both the median and the 

mean are 0.6 m (fig. 4B). This range is consistent with the 

observed thickness of the soil that is exposed along river banks 

(fig. 1). Furthermore, the S-wave data were collected in fields 

and pastures covered with soil. For these reasons, the top layers 

in all velocity profiles are interpreted as being soil. 

In the velocity profiles with three layers, the velocities of 

the middle layers range from 140 to 250 m/s (fig. 5A). The 

median is 180 m/s, and the mean is 190 m/s. In the profiles with 

four layers, the velocities of the upper middle layers range from 

100 to 260 m/s (fig. 5B). Both the median and the mean are 

160 m/s. In other words, the velocities for the middle layers (in 

the three-layer profiles) and upper middle layers (in the fourlayer profiles) are similar; they cannot be distinguished on the 

basis of S-wave velocity. These velocities are typical of clays, 

sands, and gravels that are unconsolidated and are near the 

ground surface (Kudo and Shima, 1981; Meissner and others, 

1985; Suyama and others, 1986). Moreover, such sediments are 

exposed along the river banks (fig. 1) and in local quarry pits. 

Thus, these layers in the velocity profiles are interpreted as being 

alluvium and are called alluvium with low S-wave velocity. 

In the velocity profiles with four layers, the velocities of the 

lower middle layers range from 240 to 530 m/s (fig. 5C). The 

median is 420 m/s, and the mean is 400 m/s. These velocities are 

typical of alluvial and diluvial clays, sands, and gravels that are 

unconsolidated and are near the ground surface (Kudo and 

Shima, 1981; Meissner and others, 1985; Suyama and others, 

1986); thus, these layers are interpreted as being alluvium. 

Because the velocities for these layers are high compared to 

those for the other alluvium (fig. 5A, B), these layers are called 

alluvium with high S-wave velocity. 

Along cross sections A-A′ and B-B′, the velocities of the 

bottom layers in the profiles range from 910 to 1400 m/s; both 

the median and the mean are 1100 m/s (fig. 6A). Along cross 

section C-C′, the velocities of the bottom layers range from 560 

to 1000 m/s; both the median and the mean are 830 m/s (fig. 6B). 

Both sets of velocities are much higher than typical velocities for 

unconsolidated sediments that are near the ground surface (Kudo 

and Shima, 1981; Meissner and others, 1985; Suyama and others, 1986). Thus, the bottom layers are interpreted as being 

Figure 3. A, Map showing the layout of spread A7. B, Seismograms from shots 3 and 4 in the spread. The

seismogram numbers correspond to the geophone numbers. The amplitudes have been adjusted using automatic gain control with a window of 0.08 s. This adjustment boosts some low-amplitude noise in seismograms 12 through 24, and it diminishes the amplitude of the refracted wave in seismograms 12 through 15. 
Interpretation 5

bedrock. The bedrock in this area is friable sandstone and soft, 

shaly sandstone; measurements of S-wave velocity in such rocks 

were not found in any publications. 

Construction of the Cross Sections

The interpreted velocity profiles were used to construct the 

cross sections. Consider, for example, the top layers in the velocity profiles in cross section A-A′ (pl. 2). In between velocity profiles, the thickness of the top layer is unknown, and therefore it 

was estimated with interpolation. This procedure was used for 

all other layers. In some places, the alluvium with high S-wave 

velocity pinches out (for example, between profiles A1 and A2 

(pl. 2)); such pinch outs are represented by jagged lines because 

their precise location is not known.

At three locations along cross section A-A′ (pl. 2), the 

depths to bedrock, which were determined with drilling (Colton 

and Fitch, 1974), are about 1 to 2 m shallower than the depths 

estimated with the S-wave refraction data. The reason for the difference is interpreted to be weathering of the bedrock surface. If 

the weathered bedrock is thin and its velocity is lower than the 

velocity of the underlying, unweathered bedrock, then the presence of the weathered bedrock cannot be detected with S-wave 

Velocity (m/s) Thickness (m)

Spread 

Name

Top Layer

Middle 

Layer of 

3-Layer 

Velocity 

Profile

Upper 

Middle 

Layer of 

4-Layer 

Velocity 

Profile

Lower 

Middle 

Layer of 

4-Layer 

Velocity 

Profile

Bottom 

Layer

Top Layer

Middle 

Layer of 

3-Layer 

Velocity 

Profile

Upper 

Middle 

Layer of 

4-Layer 

Velocity 

Profile

Lower 

Middle 

Layer of 

4-Layer 

Velocity 

Profile

A1 140 170 --- --- 1100 0.6 7.0 --- ---

A2 90 --- 120 340 1100 0.3 --- 2.3 6.6

A3 100 --- 160 460 1400 0.6 --- 2.6 7.9

A4 90 --- 200 490 1100 0.6 --- 2.8 5.6

A5 100 --- 130 450 1100 0.4 --- 2.4 5.5

A6 80 --- 120 320 1200 0.3 --- 2.2 5.6

A7 110 160 --- --- 1000 0.4 3.7 --- ---

B1 150 250 --- --- 1100 1.0 7.6 --- ---

B2 100 --- 160 460 910 0.4 --- 2.9 7.5

B3 130 --- 190 530 910 0.5 --- 3.3 4.9

B4 120 --- 160 500 1100 0.2 --- 3.8 7.7

B5 80 --- 100 340 1200 0.2 --- 2.0 3.9

B6 110 160 --- --- 910 0.2 5.0 --- ---

C1 130 250 --- --- 780 1.2 9.0 --- ---

C2 120 220 --- --- 820 0.9 8.2 --- ---

C3 100 --- 160 350 850 1.5 --- 2.3 7.1

C4 130 --- 190 390 830 0.7 --- 2.7 6.2

C5 110 180 --- --- 1000 0.8 10. --- ---

C6 130 --- 260 470 800 0.9 --- 2.1 4.8

C7 140 --- 190 320 970 0.4 --- 3.8 9.6

C8 100 --- 160 330 840 0.8 --- 2.7 6.6

C9 110 140 --- --- 560 0.3 2.8 --- ---

Table 1. Velocities and thicknesses estimated from the S-wave refraction data, which were collected in the Big Thompson 

River valley, Colorado. 

[The velocity profiles have either three or four layers. For those with three layers (for example, A1), the table entry with the leaders (---) pertains to a four layer profile. Conversely, for those with four layers (for example, A2), the table entry with the leaders (---) pertains to a three 

layer profile. The velocities and thicknesses are plotted in pl. 2.]
6 Assessment of the Alluvial Sediments in the Big Thompson River Valley, Colorado

0 100 200 300 400 500 600

VELOCITY, IN METERS PER SECOND

Range: 80 to 150 m/s

Median = 110 m/s

Mean = 110 m/s

Standard Deviation = 20 m/s

Number of samples = 22

0 1 2 3 4 5 6 7 8 9 10 11

THICKNESS, IN METERS

0

5

10

15

20

COUNT

0

5

10

15

20

COUNT

Range: 0.2 to 1.5 m

Median = 0.6 m

Mean = 0.6 m

Standard Deviation = 0.3 m

Number of samples = 22 A B

refraction (Redpath, 1973, p. 19–23; Haeni, 1988, p. 13–16). 

Thus, a layer of weathered bedrock, which is 1 to 2 m thick, is 

included in cross section A-A′ on the basis of the drill hole data. 

The velocities of the bedrock in cross section B-B′ are similar to those in cross section A-A′; furthermore, cross section B-B′

is close to cross section A-A′. For these reasons, the lithology of 

the bedrock in both cross sections is assumed to be similar. Thus, 

cross section B-B′ also includes a thin layer representing weathered bedrock. 

Interpretation of Cross Sections A-A’ and B-B’

Cross sections A-A′ and B-B′ (pl. 2) are geologically similar, and so they are interpreted together. The elevation of the bedrock (determined only by the velocity profiles) changes 

gradually along the cross sections (except between profiles B4 

and B5). These gradual changes are consistent with the topography in the other river valleys of the Front Range Urban Corridor 

(Colton and Fitch, 1974). In both cross section, the elevation of 

the bedrock is lower in the middle of the valley (1461 to 1464 m 

in A-A′ and 1454 to 1458 m in B-B′) than at the sides (1465 to 

1467 m in A-A′ and 1459 to 1463 m in B-B′). In both cross sections, the total thickness of the alluvium is about 6 to 10 m in the 

middle of the valley and about 2 to 6 m at the sides. (Precise 

thickness cannot be stated because the thickness of the weathered bedrock is not known.) 

In all velocity profiles, the alluvium with low S-wave velocity is present. In the middle of the valley where it overlies the 

alluvium with high S-wave velocity, its thickness ranges from 

2.0 to 3.8 m. Near the sides of the valley, the alluvium with low 

S-wave velocity constitutes all of the sediments; its thickness is 

roughly 5 m on the southern side and 2 to 3 m on the northern 

side. At all velocity profiles except B1, this alluvium is associated with the post-Piney Creek alluvium. All thicknesses 

estimated with the seismic data are consistent with thicknesses 

reported by Colton (1978). 

The alluvium with high S-wave velocity is present only in 

the middle of the valley and is buried beneath the alluvium with 

low S-wave velocity. The alluvium with high S-wave velocity is 

about 2 to 6 m thick and is interpreted to be the older gravel alluvium beneath the post-Piney Creek alluvium.

Interpretation of Cross Section C-C’

At profile C1 in cross section C-C′ (pl. 2), the elevation of 

the bedrock is 1450 m. The elevation generally decreases from 

C1 to C7 (except at C6) and increases from C7 to C9. Between 

profiles C1 and C5, the total thickness of the alluvium is somewhat uniform, ranging from 8.2 to 10 m. In contrast, between 

profiles C5 and C9, the total thickness is highly variable, ranging from 2.8 to 13.4 m. 

In all velocity profiles, the alluvium with low S-wave velocity is present. In the middle of the valley where it overlies the 

alluvium with high S-wave velocity, its thickness ranges from 

2.1 to 3.8 m. This range is consistent with the thicknesses for 

cross sections A-A′ and B-B′. Near the sides of the valley, the 

alluvium with low S-wave velocity constitutes all of the sediments; it is about 9 m thick on the south side and 3 m on the 

north side. The alluvium with high S-wave velocity was deteced 

only at profiles C3, C4, C6, C7, and C8 and is 4.8 to 9.6 m thick. 

It is buried beneath the alluvium with low S-wave velocity.

Along cross section C-C′ (pl. 1, 2), velocity profiles C1 

through C4 are associated with the Broadway Alluvium, C5 with 

the Piney Creek Alluvium, C6 with the Broadway Alluvium, C7 

with the Piney Creek Alluvium, C8 with the post-Piney Creek 

alluvium, and C9 with the Piney Creek Alluvium. Because of 

this complex order, associating these three alluviums with the 

alluviums with low and high S-wave velocities is difficult. 

Figure 4. Dot plots and statistics of A, the velocities and B, the thicknesses for the top layers of the profiles. These layers 

are interpreted as soil. The velocities and the thicknesses are listed in table 1 and are plotted in plate 2. 
Interpretation 7

Range: 140 to 250 m/s

Median = 180 m/s

Mean = 190 m/s

Standard Deviation = 43 m/s

Number of samples = 8

0 100 200 300 400 500 600

0

5

10

15

20

COUNT

A

UPPER MIDDLE LAYERS

Range: 100 to 260 m/s

Median = 160 m/s

Mean = 160 m/s

Standard Deviation = 41 m/s

Number of samples = 14

LOWER MIDDLE LAYERS

Range: 240 to 530 m/s

Median = 420 m/s

Mean = 400 m/s

Standard Deviation = 88 m/s

Number of samples = 14

0 100 200 300 400 500 600

0

5

10

15

20

COUNT

B

0 100 200 300 400 500

VELOCITY, IN METERS PER SECOND VELOCITY, IN METERS PER SECOND

VELOCITY, IN METERS PER SECOND

600

0

5

10

15

20

COUNT

C

MIDDLE LAYERS

Nonetheless, a few associations are straightforward for the 

alluvium with low S-wave velocity. At profiles C3, C4, and C6, 

it is associated with the Broadway Alluvium, and its thicknesses (2.3, 2.7, and 2.1 m, respectively) are only slightly less 

than the 3 to 5 m range reported by Colton (1978). At profiles 

C7 and C9, it is associated with the Piney Creek Alluvium, and 

its thicknesses (3.8 and 2.8 m, respectively) are within the 0 to 

6 m range reported by Colton (1978). At profile C8, it is 

associated with the post-Piney Creek alluvium, and its 

thickness (3.8 m) is within the 2 to 5 m range reported by Colton (1978). 

Additional Observations 

While interpreting the velocity profiles and the cross sections, three additional observations regarding the sediments 

and the bedrock were made. Although these observations are 

Figure 5. Dot plots and statistics of the velocities A, for the middle layers of those profiles with three layers, B, for the upper 

middle layers of those profiles with four layers, and C, for the lower middle layers of those profiles with four layers. These 

layers are interpreted as alluvium. The velocities are listed in table 1 and are plotted in plate 2. 
8 Assessment of the Alluvial Sediments in the Big Thompson River Valley, Colorado

VELOCITY, IN METERS PER SECOND VELOCITY, IN METERS PER SECOND

COUNT

A B

CROSS SECTIONS A-A' AND B-B' CROSS SECTION C-C'

500 700 900 1100 1300 1500

0

2

4

6

8

COUNT

500 700 900 1100 1300 1500

0

2

4

6

8

Range: 910 to 1400 m/s

Median = 1100 m/s

Mean = 1100 m/s

Standard Deviation = 140 m/s

Number of samples = 13

Range: 560 to 1000 m/s

Median = 830 m/s

Mean = 830 m/s

Standard Deviation = 130 m/s

Number of samples = 9

not especially useful for interpreting the cross sections, they 

may be helpful in future investigations. First, the range, the 

median, and the mean of the bedrock velocities for cross 

sections A-A′ and B-B′ are substantially higher than the corresponding quantities for cross section C-C′ (fig. 6). The 

difference is interpreted as a change in lithology in the upper 

transition member of the Pierre Shale. Cross sections A-A′ and 

B-B′ are near the stratigraphic bottom of this formation, 

whereas cross section C-C′ is near the stratigraphic top (pl. 1). 

Second, the range, the median, and the mean for the post-Piney 

Figure 6. Dot plots and statistics of the velocities for the bottom layers along A, Cross sections A-A’ and B-B’ and B, Cross 

section C-C’. These layers are interpreted as bedrock. The velocities are listed in table 1 and are plotted in plate 2. 

VELOCITY, IN METERS PER SECOND

COUNT

A B

VELOCITY, IN METERS PER SECOND

COUNT

POST-PINEY CREEK ALLUVIUM

Range: 100 to 200 m/s

Median = 160 m/s

Mean = 150 m/s

Standard Deviation = 28 m/s

Number of samples = 13

BROADWAY ALLUVIUM

100 150 200 250 300

0

2

4

6

8

10

100 150 200 250 300

0

2

4

6

8

10

Range: 160 to 260 m/s

Median = 240 m/s

Mean = 220 m/s

Standard Deviation = 39 m/s

Number of samples = 6

Figure 7. Dot plots and statistics of the velocities associated with A, the post-Piney Creek alluvium and B, the Broadway 

Alluvium. The velocities profiles associated with these two alluviums are shown in plate 2. The velocities are from the middle 

layers (for those profiles with three layers) and from the upper middle layers (for those profiles with four layers). 
Conclusions 9

Creek alluvium are substantially lower than the corresponding 

quantities for the Broadway Alluvium (fig. 7). Thus, the type of 

alluvium affects the S-wave velocity. Third, in cross sections 

A-A′, B-B′, and C-C′, the mean thicknesses are 7.7, 8.1, and 8.7 

m; the median thicknesses are 7.9, 7.9, and 9.0 m, respectively 

(fig. 8). Thus, the thickness increases about 1 m between cross 

sections A-A′ and C-C′, which corresponds to the downstream 

direction of the Big Thompson River. The increase, however, is 

small compared to the standard deviations, which range from 

2.1 to 2.8 m. 

Conclusions 

The velocity profiles, which were estimated with S-wave 

refraction data, have either three or four layers. The top layer 

always has velocities between 80 to 150 m/s and thicknesses 

A

COUNT

B

COUNT

C

Range: 3.7 to 10.5 m

Median = 7.9 m

Mean = 7.7 m

Standard Deviation = 2.1 m

Number of samples = 7

0

0

COUNT

CROSS SECTION A-A’

3 6 9 12 15

THICKNESS, IN METERS

1

2

3

4

5

6

0

0

CROSS SECTION B-B’

3 6 9 12 15

THICKNESS, IN METERS

1

2

3

4

5

6

0

0

CROSS SECTION C-C’

3 6 9 12 15

THICKNESS, IN METERS

1

2

3

4

5

6

Range: 5.0 to 11.5 m

Median = 7.9 m

Mean = 8.1 m

Standard Deviation = 2.5 m

Number of samples = 6

Range: 2.8 to 13. m

Median = 9.0 m

Mean = 8.7 m

Standard Deviation = 2.8 m

Number of samples = 9

Figure 8. Dot plots and statistics of the alluvium thicknesses for A, cross section A-A’, B, cross section B-B’, and C, cross 

section C-C’. The thicknesses are listed in table 1 and are plotted in plate 2. 
10 Assessment of the Alluvial Sediments in the Big Thompson River Valley, Colorado

between 0.2 and 1.5 m; this layer is interpreted as soil. The middle layers (in profiles with three layers) and the upper middle 

layers (in profiles with four layers) have velocities between 100 

and 260 m/s; these are interpreted as alluvium and are called 

alluvium with low S-wave velocity. The lower middle layers (in 

profiles with four layers) have velocities between 240 and 

530 m/s; these also are interpreted as alluvium but are called 

alluvium with high S-wave velocity. The bottom layers have 

velocities between 910 and 1400 m/s near cross sections A-A′

and B-B′ and between 560 and 1000 m/s near cross section C-C′. 

The bottom layers are interpreted as bedrock, which is the upper 

transition member of the Pierre Shale. 

In cross section A-A′ and B-B′, the alluvium with low 

S-wave velocity is associated with the post-Piney Creek alluvium (except at one location). The thickness ranges from about 2 

to 5 m, which is consistent with measurements for the postPiney Creek alluvium in other river valleys in the Front Range. 

The alluvium with high S-wave velocity ranges in thicknesses 

from about 2 to 6 m. It may be a gravel alluvium that, in other 

river valleys in the Front Range, is beneath the post-Piney Creek 

alluvium. The combined thickness of both alluviums ranges 

from about 6 to 10 m near the middle of the valley and from 

about 2 to 6 m near the sides of the valley.

In cross section A-A′, the depth to the bedrock determined 

from drilling is consistently about 1 or 2 m less than that determined from the refraction data. The likely reason for this discrepancy is that the top of the bedrock is weathered and 

consequently has a somewhat low velocity. The thickness of this 

thin, weathered zone cannot be determined from the refraction 

data but is estimated to be about 1 to 2 m. 

In the southern half of the valley in cross section C-C′, the 

combined thickness of both alluviums ranges from 8.2 to 10 m ; 

in the northern half of the valley, the combined thickness of both 

alluviums ranges from 2.8 to 13.4 m. The stratigraphy in this 

cross section is somewhat complex because of its proximity to 

the confluence of the Big Thompson and Little Thompson Rivers. 

Acknowledgments

The landowners in the Big Thompson River valley allowed 

us to collect data on their property. Shay Beanland, Richard 

Eden, Jeffrey Lucius, and Kim Oshetski helped collect the data. 

Funding for this investigation was provided by the U.S. Geological Survey, Front Range Infrastructure Resources Project.

References

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Appendix

The P-wave refraction method was tested to determine 

whether it would be suitable for a survey of the Big Thompson 

River valley. The test was conducted along CR 3 (pl. 1), where 

the bedrock is about 8 m below the surface, which was determined from drilling. The source was a metal plate lying on the 

ground, which was excited by striking it with a sledgehammer. 

The spread consisted of 24 geophones that measured the vertical 

component of particle velocity. The geophones were spaced 2 m 

apart; thus the spread length was 46 m, which was long compared to the bedrock depth. The seismograms were recorded by 

an ES-2401 exploration seismograph (EG&G Geometrics, 

1992). When the source was 2, 50, and 98 m from the end of the 

spread, the seismograms included a direct P-wave and a highamplitude, refracted P-wave from the water table. The seismograms, however, did not include a refracted P-wave from the 

bedrock, probably because the velocity contrast between the saturated sediments and the bedrock is too small. 

Without a detectable refracted wave from the bedrock, the 

depth to the bedrock cannot be estimated. Thus, a significant 

goal of the seismic survey would not be attained, and so P-wave 

refraction was unsuitable for the survey of the Big Thompson 

River valley.