Results of Geophysical Surveys at Hocomonco Pond,
Westborough, Massachusetts
U.S. GEOLOGICAL SURVEY
Open-File Report 92-646
Prepared in cooperation with the
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASTE MANAGEMENT DIVISION, REGION I
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950R93O04
RESULTS OF GEOPHYSICAL SURVEYS AT HOCOMONCO POND,
WESTBOROUGH, MASSACHUSETTS
By Bruce P. Hansen
U.S. GEOLOGICAL SURVEY
Open-File Report 92-646
Prepared in cooperation with the
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASTE MANAGEMENT DIVISION, REGION I
Marlborough, Massachusetts
1993
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U.S. DEPARTMENT OF THE INTERIOR
BRUCE BABBIT, Secretary
U.S. Geological Survey
Dallas L. Peck, Director
For additional information
write to:
District Chief
U.S. Geological Survey
Massachusetts-Rhode Island District
28 Lord Road, Suite 280
Marlborough, MA 01752
Copies of this report can be
purchased from:
U.S. Geological Survey
Books and Open-File Reports Section
Box 25425, Federal Center
Denver, CO 80225
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CONTENTS
Abstract
Introduction
Purpose and scope
Previous investigations
Geophysical methods
Seismic methods
Seismic refraction
Continuous seismic reflection ....
Ground-penetrating radar
Results of geophysical surveys
Seismic refraction
Continuous seismic reflection
Ground-penetrating radar
Suggestions for additional data collection
Summary and conclusions
References
Page
1
1
3
3
3
3
3
5
5
7
7
11
11
16
16
17
ILLUSTRATIONS
Figure 1-3.
4.
5.
6.
7.
8.
Page
Maps showing:
1. Location of the Hocomonco Pond study site 2
2. Altitude of the bedrock surface on the southeast side of
Hocomonco Pond 4
3. Location of seismic-refraction lines and 80- and 300-megahertz
ground-penetrating radar surveys 8
Diagram showing geohydrologic sections interpreted from
seismic-refraction data for lines 1-4 9
Map showing location of continuous seismic-reflection profiling surveys .... 12
Diagram showing interpretation of representative
continuous seismic-reflection profiling record 13
Diagram showing interpretation of 300-megahertz radar record
of survey section A-B 14
Diagram showing interpretation of 80-megahertz radar record
of survey section A-B 15
• * •
111
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TABLES
Table 1. Approximate values of conductivity, relative dielectric permittivity,
and radar-wave velocity for selected materials ,
Page
6
CONVERSION FACTORS AND VERTICAL DATUM
Multiply
By
To obtain
foot (ft)
mile (mi)
foot per second (ft/s)
foot per nanosecond (ft/ns)
square foot (ft2)
square mile (mi )
Length
0.3048
1.609
Velocity
0.3048
0.3048
Area
0.0929
2.590
meter
kilometer
meter per second
meter per nanosecond
square meter
square kilometer
Sea Level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD
of 1929)-a geodetic datum derived from a general adjustment of the first-order level nets of the United
States and Canada, formerly called Sea Level Datum of 1929.
IV
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Results of Geophysical Surveys at Hocomonco Pond,
Westborough, Massachusetts
By Bruce P. Hansen
ABSTRACT
INTRODUCTION
Seismic-refraction, continuous seismic-reflection
profiling, and ground-penetrating radar surveys
were done from May through July 1991 at the
Hocomonco Pond Superfund site in Westborough,
Mass,, to delineate the configuration of a bedrock
surface buried below glacial deposits. Analysis of
seismic-refraction data from four locations near
the pond indicate that the bedrock surface ranges
in depth from 10 to 250 feet below land surface.
Continuous seismic-reflection-profiling surveys
on the pond detected the bedrock surface at sev-
eral locations and consistently identified an irreg-
ular contact between shallow coarse-grained
sediments and overlying fine-grained sediments.
Ground-penetrating-radar surveys with 80- and
300-megahertz antennas penetrated to depths of
45 and 22 feet, respectively. The irregular coarse-
grained surface covered by fine-grained and soft
pond-bottom sediments that was identified by the
continuous seismic-reflection surveys was deline-
ated in more detail by the ground-penetrating
radar surveys. The radar surveys also identified
debris resting on the pond bottom. Best resolution
was obtained by the 300-megahertz radar survey.
Drilling or alternative geophysical surveys would
help to delineate the bedrock surface in areas
where the methods used did not give conclusive
results.
The Hocomonco Pond Superfund Site is in
the town of Westborough, in south-central
Massachusett (fig. 1). Soil and ground water at
the site have been contaminated by creosote from
a wood-treating facility that was located on the
south side of the pond. Site investigations by
consulting firms are providing hydrogeologic in-
formation needed to design remediation pro-
grams.
Creosote in the saturated zone has been de-
tected in drill cuttings and ground-water samples
collected from test wells. Creosote and related
high-molecular-weight compounds dissolve
slowly in ground water. They can remain as dense
nonaqueous-phase liquids (DNAPL's), which can
migrate downward to confining geologic units
such as day, dense till, or bedrock. If DNAPL's
reach a confining layer, such as bedrock, they can
migrate along the surface of that layer to areas
of lower altitude. A bedrock-surface-altitude
map (fig. 2) prepared from boring data and the
results of a gravity survey (Keystone Environ-
mental Resources, Inc., 1990) indicates that a
closed depression is present in the bedrock sur-
face on the southeast side of the pond. The bed-
rock depression could act as a sink for the
DNAPL's that are present in the saturated un-
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73°00'
71°00"
42° 17'
42°16'
B«w Iron U.S. Onnaytat Sun*y
Martboiough. Mm.. 1M3.
125.000
1MILE
_\
1 KILOMETER
CONTOUR INTERVAL 3 METERS
DATUM IS SEA LEVEL
Figure l.-Location of the Hocomonco Pond study site.
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consolidated deposits under the site. No data are
available to confirm the existence of the dashed
closed contours on the northwest side of the bed-
rock depression. Supplemental bedrock-configu-
ration data are needed to evaluate possible
migration pathways. The U.S. Geological Survey
(USGS), in cooperation with U.S. Environmental
Protection Agency (USEPA), conducted geophys-
ical surveys on Hocomonco Pond and adjacent
land areas with the purpose of obtaining informa-
tion on depth to bedrock and, if possible, on the
structure and distribution of the unconsolidated
deposits above bedrock. From May through July
1991, seismic-refraction surveys were done adja-
cent to Hocomonco Pond, and continuous seismic-
reflection profiling (CSP) surveys and
ground-penetrating radar (GPR) surveys were
done on the pond. This work was done as part of
a technical-assistance agreement between the
USGS and USEPA.
Purpose and Scope
This report presents the results of the geo-
physical surveys that were done from May
through July 1991 at the Hocomonco Pond Super-
fund site, Westborough, Mass. The report briefly
describes the seismic-refraction, CSP, and GPR
geophysical methods used, presents selected re-
cords of the data collected, and discusses the
results. V
Previous Investigations
Information on bedrock-surface altitude was
available from test borings made during a num-
ber of investigations at the site (Burgess and
others, 1985; Keystone Environmental Re-
sources, Inc., 1990,1991). Test-well logs for sev-
eral locations on the north and west shore of the
pond were available from several unpublished
water-supply studies by the town of
Westborough.
GEOPHYSICAL METHODS
All geophysical methods make use of the fact
that different rock types have different physical
properties. The physical properties of interest
for this study are seismic velocity, acoustic im-
pedance, and relative dielectric permittivity.
Readers can find detailed descriptions of meth-
ods used in several references cited in the text.
Seismic Methods
Seismic methods make use of reflected and
refracted waves of seismic energy, and they are
based on the time that it takes energy generated
at a point source to travel through the ground to
receivers called geophones on land and hydro-
phones in water. By use of the seismic-refraction
method, the velocity of sound through various
layers can be calculated and, as a result, the
structure and depth of various layers in the sub-
surface can be determined. The seismic-reflec-
tion method shows subsurface structure, but the
depth of each subsurface layer can be determined
only if the velocity of each layer is known.
Seismic Refraction
The use of the seismic-refraction method re-
quires the assumption of a layered earth in which
tile velocity of seismic energy in each layer is
greater than the velocity of seismic energy in the
layer above it. When this condition is met, seismic
energy originating from a sound source travels
downward into the ground until it meets a re-
fracting surface. The energy that is refracted
along this surface continually generates seismic
waves that travel upward to land surface, where
they may be detected by a series of geophones. In
some cases, thin, intermediate-velocity layers
cannot be detected. Descriptions of seismic-re-
fraction theory and interpretation methods are
given in Dobrin (1976), Haeni (1988), Mooney
(1981), and Redpath (1973).
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71°38'55"
71°38'40"
42°16>25-
48°16>20"
EXPLANATION »
-250— BEDROCK CONTOUR-Shows altitude of bedrock surface. I
Dashed where approximately located. Contour Interval
variable, in feet above sea level.
TEST WELLS-Number Is well number as*igned by consulting
companys. Number In parenthesis Is bedrock altitude,
in toet above sea level.
400 FEET
too METERS
Figure 2.—Altitude of the bedrock surface on the southeast side of Hocomonco Pond. (Modified
from Keystone Environmental Resources, Inc., 1990, fig. 5).
Seismic-refraction data at Hocomonco Pond
were collected by means of a 12-channel signal-
enhancement seismograph. Distance between
the first and last geophone in each geophone
spread-the arrangement of geophones in rela-
tion to the position of the energy source-was
550 ft and spacing between geophones was 50 ft.
Small two-component explosive charges were
used as a source of seismic energy. These explo-
sive charges were buried 3 to 4 ft below land
surface at a selected shotpoint. Altitudes and
locations of all geophones and shotpoints were
recorded. The seismic-refraction data were in-
terpreted by computer-modeling techniques that
incorporate delay-time and ray-tracing proce-
dures (Scott and others, 1972; Scott, 1977).
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Continuous Seismic Reflection
The seismic-reflection method detects seis-
mic energy reflected from changes in the acoustic
impedance in subsurface layers. Changes in
acoustic impedance are common at geologic
boundaries. Seismic-reflection surveying can be
used on land or water, but the continuous seis-
mic-reflection profiling (CSP) method can only be
used on water. Because data are collected almost
continuously, this method can be used to define
the hydrogeologic units beneath a large water-
covered area in a short time.
Further descriptions of the theory and adap-
tation of the method to shallow water surveying
can be found in Ewing and Tirey (1961), Hersey
(1963), Trabant (1984), Haeni (1971), EG&G En-
vironmental Equipment Division (1977), and
Sylwester (1983). The applications of this
method to engineering, hydrology, and geology
have been discussed by Moody and Van Reenan
(1967), Haeni (1971), Haeni and Sanders (1974),
Missimer and Gardner (1976), Freeman-Lynde
and others (1982), Wolanssky and others (1983),
Haeni and Melvin (1984), Morrissey and others
(1985), Haeni (1986,1989), Hansen (1986,1989),
Reynolds and Williams (1989), and Tucti and
others (1991).
In the CSP method, an acoustic signal trans-
mitted from a sound source near the water sur-
face is reflected from interfaces between
materials of different acoustic impedance. The
acoustic impedance of any material is equal to the
product of the velocity of sound in the material
and the bulk density of the material.
The graphic recordings of CSP data repre-
sent a nearly continuous record of subsurface
acoustic-property changes, which has the ap-
pearance of, and many times is a close represen-
tation of, the geologic section along the survey
line. The lithology, structure, and contacts be-
tween geologic units can often be interpreted
directly from the CSP records (Haeni, 1989). The
depth to any contact shown on the record can be
determined by dividing the two-way traveltime
by 2 and multiplying the results by the velocity
of sound through the subsurface deposits. For
this study, a velocity of 5,000 ft/s for water and
saturated unconsolidated deposits was used to
compute depth to individual reflectors. In New
England, the velocity of sound through saturated
stratified glacial deposits generally ranges from
4,000 to 6,000 ft/s (Haeni, 1988, p. 41). Reflector
depths determined from the records were not
corrected for differences in the velocity of sound
as it traveled through the water column. Correc-
tions for the water column would have little effect
because the velocity of sound in water (4,800 ft/s)
is almost equal to the velocity of sound in strati-
fied glacial deposits.
The CSP system that was used consisted of
a graphic recorder, a high-voltage power supply,
a high-resolution boomer sound source, a hydro-
phone streamer, a filter-amplifier, and two 110-
watt generators. The acoustic signals generated
by the sound source travel through the water
column and into the subsurface. The sound re-
flected back to the surface is received by the
hydrophone streamer and is amplified, filtered,
and graphically recorded. In this study the un-
filtered (raw) signal was also recorded on a digital
audio-tape recorder. Such recorded data can be
used to replay or postprocess the data.
A 17-ft outboard-powered boat was used to
convey the CSP system and a two-man crew. The
electronic equipment was powered by a small
800-watt generator. A second 2.5-kilowatt gener-
ator was used to power the high-voltage power
supply. The sound source, mounted on a small
catamaran, and the hydrophones were towed on
opposite sides of the boat at about 0.5 ft under the
water surface and with about a 10-ft separation.
To minimize noise during surveying, The crew
used a small electric fishing motor to propel the
boat.
Ground-Penetrating Radar
Ground-penetrating radar (GPR) is the elec-
tromagnetic analogue of the CSP method;
graphic displays of data from the two methods
are similar in appearance. GPR systems transmit
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pulses of radio-frequency electromagnetic energy
into the ground. This energy travels through the
ground until it arrives at an interface between
materials having different electrical properties.
At this interface, some energy is reflected back
toward the surface and the remaining energy
continues deeper into the subsurface. Many of
the materials found in geohydrologic settings ex-
hibit different electrical properties, which are
determined by water content, dissolved minerals
in ground water, and expansive-clay and heavy-
mineral content (Wright and others, 1984;
Olhoeft 1984,1986; Haeni and others, 1987). The
reflected energy received by an antenna at the
surface is amplified, converted to audio frequency
range, recorded, processed, and displayed on a
graphic recorder. The record shows the relative
amplitude of the reflected signal and the total
traveltime for the signal to pass down through
the subsurface, reflect from electrical interfaces,
and return to the surface. The two-way travel-
time, which is measured in nanoseconds (1 nano-
second = 10"9 second), and a property called
relative dielectric permittivity are used to com-
pute depth to a reflector by means of equations
presented by Beres and Haeni (1991). The dielec-
tric permittivity is a measure of the capacity of a
material to store a charge when an electric field
is applied to it relative to the same capacity in a
vacuum (Sheriff, 1984). Approximate values of
relative dielectric permittivities and radar-wave
velocities are listed for selected materials in
table 1.
Transmission frequencies used in GPR sur-
veys range from 80 to 1,000 MHz (megahertz).
The frequency used for a given study is selected
to provide an acceptable compromise between
high resolution and deep penetration. High-fre-
quency signals produce high-resolution records
but have a limited depth of penetration. The
principal factor limiting the depth of penetration
of radar waves is the attenuation of the electro-
magnetic waves by earth materials. Radar-signal
penetration depends on the electrical conductiv-
ity of the materials present and the frequency of
the radar signal. Studies in areas of low electri-
cal-conductivity materials, such as clay-free sand
and gravel, show that low-frequency radar waves
Table 1 .-Approximate values of conductivity,
relative dielectric permittivity, and radar-wave
velocity for selected materials
[data from Ulriksen, 1982; Maria, 1988; tt/ns, foot per nanosecond;
mho/m, mho per meter]
Material
Air
Pure water
Sea water
Freshwater
ice
Sand (dry)
Sand
(saturated)
Silt (saturated)
Clay
(saturated)
Rich
agricultural soil
Sandstone
(wet)
Shale (wet)
Limestone
(dry)
Limestone
(wet)
Basalt (wet)
Granite (dry)
Granite (wet)
Bedded salt
Conductivity
0
10'4to3x10'2
4
ID'3
lO^tolO"3
lO^tolO"2
lO^tolO'2
10'1 to 1
10'2
4x10'2
10'2
10'9
2.5x10
ID'2
10-8
10'3
IQ^tolO-4
Relative
dielectric
permittivity
1
81
81
4
4 to 6
30
10
8 to 12
15
6
7
7
8
8
5
7
3 to 6
Radar wave
velocity (ft/ns)
0.98
.11
.11
.49
.49 to 40
•17
.31
.35 to .28
.25
.40
.37
.37
.35
.35
.44
.37
.57 to .40
can penetrate to depths of 90 ft (Wright and
others, 1984; Olhoeft, 1984,1986). In highly con-
ductive materials, such as clay-rich materials,
the penetration depth of radar waves can be less
than 3 ft (Wright and others, 1984; Olhoeft, 1984,
986). Approximate values of conductivity for se-
lected materials are listed in table 1.
6
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Selected radar records were interpreted on
the basis of configuration, amplitude, continuity,
and terminations of reflections. The electromag-
netic velocities generally used for depth interpre-
tation were 0.11 ft/ns in water and 0.20 ft/ns in
saturated unconsolidated sediments. Ldthologies
of units were interpreted from the radar records
by comparing the character of the reflected radar-
wave configurations to a chart of radar-wave
configurations from known unconsolidated de-
posits and depositional sequences in the glaci-
ated Northeast (Beres and Hanei, 1991, p. 379).
The GPR equipment and a 1,000-watt power
generator were mounted in a small inflatable
rubber boat, which was propelled by a small
outboard motor. The antennas, which were
mounted in floating fiberglass enclosures, were
towed next to the boat. Two radar surveys were
done, one with dual 80-MHz center-frequency
transmitting and receiving antennas and the
other with 300-MHz antennas. The near-surface
resolution of these surveys is 1 to 2 ft for the
80-MHz antennas and 0.3 to 0.5 ft using the
300-MHz antennas. The radar traverses were
referenced to known landmarks, which were
noted on the radar records. A constant course
and constant speed were maintained between the
landmarks.
RESULTS OF GEOPHYSICAL
SURVEYS
The results of the seismic-refraction surveys
and selected results of the CSP and GPR surveys
that were conducted at Hocomonco Pond are pre-
sented in this section. All of the original data
from these surveys are in the files of the U.S.
Geological Survey Massachusetts-Rhode Island
District Office in Marlborough, Mass.
Seismic Refraction
The locations of the four seismic-refraction
lines are shown on figure 3, and geohydrologic
sections interpreted from seismic-refraction data
are shown on figure 4.
Lone 1, on the northern side of Smithvalve
Parkway, has a maximum bedrock-surface alti-
tude of 305 ft on the eastern end of the line. This
altitude decreases slightly to about 297 ft at point
IB in the middle of the line. From this point the
bedrock-surface altitude declines to about 230 ft
on the western end of the line. Bedrock altitudes
determined from seismic refraction along the
eastern half of the line are consistent with test-
well data in this area (Burgess and others, 1985).
The presence of a bedrock outcrop about 200 ft
south of the eastern end of the line supports the
interpretation of a shallow bedrock surface in
this area.
Lone 2 is northeast of Hocomonco Pond along
Otis Street. The bedrock-surface altitude, 253 ft
on the north end of the line, decreases to about
160 ft near the south end of the line. This line
consisted of two geophone spreads. The data from
the second spread indicate an intermediate-ve-
locity layer (8,100 ft/s) between saturated strati-
fied deposits (5,100 ft/s) and bedrock (16,000 ft/s).
The intermediate layer was not indicated by the
data from the first spread because the layer was
either absent or too thin to be detected. The two
spreads were interpreted separately, and the in-
terpretations were combined into the geohydro-
logic section shown. The bedrock-surf ace
altitudes on the south end of this line are in
approximate agreement with data from two wells
near the line (Keystone Environmental Re-
sources, Inc., 1991). The center of a buried bed-
rock valley is defined by the bedrock-surface
altitude of 150 ft at station 2B.
Lone 3, along the south shore of the pond, has
an irregular bedrock surface that decreases from
an altitude of 195 ft on the western end to 178 ft
on the eastern end. The time-distance data from
this line did not indicate any material with a
velocity between the typical velocities for satu-
rated stratified deposits and bedrock. Data from
a well near the eastern end of the line indicates
a 34-ft layer of saprolite (Keystone Environmen-
tal Resources, Inc., 1991). The nondetection of an
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42°16'40"
71° 39'
71"38'40"
42°16'20"
1B
EXPLANATION
J^> LOCATION OF 80- AND 300-MHz RADAR SECTONS-
Shown on figures 7 and 8
— 300-MHz RADAR SURVEY-Approximate location
-B 80-MHz RADAR SURVEY-Approximate location
JA SEISMIC-REFRACTION-SURVEY LNE-tmerpretaUon
—I profiles shown on figure 4
1000 FEET
300 METERS
Figure 3.-Location of seismic-refraction lines and 80- and 300-megahertz
ground-penetrating-radar surveys.
intermediate layer may be a result of too large a
geophone spacing, or a "blind zone problem". The
blind zone (Soske, 1959; Sander, 1978) occurs
when a layer has an increase in seismic velocity
with depth, but the velocity contrast or the layer
thickness is too small to cause a return of first-
arrival energy at land surface. This problem
cannot be overcome by any change in the layout
of the geophones or shotpoints. If this interme-
diate layer is present, then the computed depth
to bedrock, for at least the east end of the line,
will be in error, and the actual depth to bedrock
will be greater than that shown. This error will
probably be less than 50 percent (Redpath, 1973).
Line 4 on the north shore of the pond has a
minimum indicated bedrock altitude of 30 ft.
From this low point, the bedrock-surface rises
toward the western and eastern ends of the line
to 155 ft and 270 ft, respectively. The indicated
8
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UNSATURATED UNCONSOUDATED
DEPOSITS
V-1.133 IMI air Moond
SATURATED STRATIFIED
: UNCONSOUOATED DEPOSITS
V. 4.415 (Mtpviwond
220
200 FEET
J I
0 50 METERS
VERTICAL EXAGGERATION X 3.9
DATUM IS SEA LEVEL
LINE 2
i!
a
2B
2C
SOUTH
UNSATURATEO UNCONSOUDATED
DEPOSITS
V.1.400feltp«Meond
SATURATED STRATIFIED:
• UNCONSOUDATED DEPOSITS
V-5,IOO(»«tp«f~cond
Till, WEATHERED ROCK.
SAPROUTE
V. 8,100 (Ml fMTMCond
BEDROCK
V . 14.000 tort px Moond
100
I
200 FEET
0 50 METERS
VERTICAL EXAGGERATION X 2.9
DATUM IS SEA LEVEL
Figure 4.-Geohydrologic sections interpreted from seismic-refraction data for lines 1-4.
9
-------
a
i
3B
WEST
300 1
280-
260
240-
LINE 3
3A
EAST
K 220
2
200
180-
160-
LAND SURFACE
UNSATURATED UNCONSOUDATED
DEPOSITS
V> 1.300 (Ml pvwcond
: SATURATED STRATIFIED
: UNCONSOUDATED DEPOSITS
;V-S.e70lMtpWMcand
V. VELOCITY
\ X \.
200 FEET
LINE 4
4A
7" UNSATURATED UNCONSOUDATED AO
WEST DEPOSITS HD
LAND SURFACE V. WK) h.t p« »«nd |
I SO METERS
VERTICAL EXAGGERATION X 2.6
DATUM IS SEA LEVEL
4C
EAST
SATURATED STRATIFIED
: UNCONSOUDATED DEPOSITS
V . 5.167 iHlpH Meant
100 METERS
VERTICAL EXAGGERATION X 4
DATUM IS SEA LEVEL
Figure 4,—Geohydrologic sections interpreted from seismic-refraction data for lines l-4~Continued.
10
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bedrock altitude in the middle of the line is in
general agreement with a drilling "refusal" alti-
tude for a water-supply test well drilled by the
town of Westborough near that location.
Continuous Seismic Reflection
The location of the CSP survey lines are
shown on figure 5. A section of record from sta-
tions 2 through 10 is shown on figure 6.
In general, analysis of the record shown on
figure 6 and of other records from the pond iden-
tified a very irregular coarse-grained surface
(hummocky and chaotic reflectors), which has
been filled and covered by fine-grained deposits
(flat, parallel reflectors). The thickness of the
fine-grained deposits is variable. At station 4,
fine-grained deposits are about 20 ft thick (water-
bottom to coarse-grained contact). Between sta-
tion 4 and 5, the thickness of the fine-grained
deposits increases to at least 70 ft. At station 5,
the coarse-grained contact is near the pond bot-
tom. None of the continuous reflectors clearly
indicates a till or bedrock surface at depth.
•
Sections of good record where deep structure
is indicated (referred to in this report as "win-
dows") are present at several locations. Because
of the discontinuity of the sections where deep
structure is indicated, the interpretation of these
sections is subjective, and alternative interpreta-
tions are possible. One of these windows starts
halfway between stations 7 and 8 and extends to
station 8. The reflections (high amplitude) at
80 ft are interpreted to be the top of a coarse-
grained unit possibly till. The darker reflections
just below a depth of 105 ft at station 8 may
represent the bedrock surface. In the section
halfway between stations 8 and 9, the strong
chaotic reflections, beginning about 115 ft below
the pond surface and extending to a depth of at
least 160 ft, indicate cross-bedded, coarse-
grained deposits. Based on the seismic-reflection
record, bedrock at this location may be at a depth
of about 160 ft. Several deep, possibly continuous,
reflections indicate a till or bedrock surface. One
of these reflections starts at station 2 at a depth
of 125 ft and trends across the record to a depth
of about 100 ft at station 4. There is some evi-
dence on the original record that this reflection
is laterally more extensive, but the record is so
weak that a conclusive determination is not pos-
sible.
The lack of a clearly defined bedrock or till
surface at depth is probably due to attenuation
and refraction of the acoustic energy by the thick
sequence of coarse deposits or by methane gas in
the shallow sediments. In addition, a small
acoustic impedance contrast across the stratified
coarse-grained sediments, till, weathered bed-
rock, and bedrock boundaries (contacts), may re-
sult in only weak reflected energy or no reflected
energy, being returned to the surface from these
contacts.
Ground-Penetrating Radar
Two GPR surveys were done on the
Hocomonco Pond to test the utility of this method
for detecting and delineating a bedrock surface
at depth and to compare this method to the CSP
method. The locations of the GPR surveys are
shown on figures 3 and 5. The 80- and 300-MHz
records were compared by running surveys lines
at the same approximate location at both fre-
quencies. The 300-MHz record (fig. 7) has a
higher resolution than the 80-MHz record (fig. 8).
Figures 7 and 8 are distorted because the radar-
wave velocity in water is about one-half the
radar-wave velocity in saturated unconsolidated
sediments (table 1). Two depth scales, one for
the depth of water and one for the depth of
saturated sediment, are shown on the radar sec-
tions.
Interpretation of the 300-MHz record (fig. 7)
shows a pond depth of about 5 ft near the left edge
of the record (bottom of the area where no reflec-
tions follow direct arrivals), which decreases to
about 2.4 ft at B. In the center.there appears to
be about 2 ft of soft bottom sediment (low-ampli-
tude parallel reflections), which thins to about 1
ft at B. Below the soft sediment, the parallel
reflectors indicate a 7-ft-thick sequence of fine-
11
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42°16'40"
71°39'
71° 38'40"
42016>20" —
EXPLANATION
1000 FEET
I I I
2 A B
-e—r-+
LOCATION OF CONTINUOUS SEISMIC-REFLECTION SURVEYS-
Numbers Indicate station numbers shown on figure 6.
Letters Indicate location of 80- and 300-MHz radar
sections shown on figures 7 and 8.
Figure 5.~Location of continuous seismic-reflection profiling surveys.
grained deposits that are probably glacial-lake
deposits but also may be a continuation of the
recent bottom deposits with thin imbedded re-
flecting beds. These fine-grained deposits appear
to thin toward B as they lap onto coarser grained
deposits (hummocky or chaotic reflections). The
contact between the fine and coarse deposits be-
comes shallower toward B. The maximum depth
of observed penetration on the 300-MHz record
is about 22 ft below the pond surface.
On the 80-MHz record (fig. 8), the water
depth is shown to be about 5 ft in the center of
the record and about 4 ft at B. No soft bottom
sediments can be seen. About a 30-ft thickness
of fine-grained sediments (parallel reflectors) can
be seen. The contact between the fine-grained
deposits and coarse-grained deposits (hummocky
or chaotic reflectors) is at a depth of about 35 ft
in the center of the section and rises to about a
depth of 5 ft at B. The maximum depth of radar-
12
-------
CO
Fln»-gr»ined
deposits
-ir'v
Bedrock SUl1ac«?~^V.Caanawirainad< W • '.ZfeK^ '
.09
LINE OF SECTION SHOWN ON FIGURE 6
200
I
400
600
I
800
VERTICAL EXAGGERATION X 5
APPROXIMATE HORIZONTAL DISTANCE. IN FEET
EXPLANATION
J UNEOFSECTION-Wtnd
' 300 Mureconfc shown on
flgurt»7«nd8.
2 STATION LOCATION AND
NUMBER
-275
-250
-200 • CO
-175 §
h-
-150 HI
-125
-100
^50
a
Figure 6.-- Interpretation of representative continuous seismic-reflection profiling record.
-------
B
o-i
Bf
s 10-
1
U) 15-
w 20-
25-
30 J
Hyperbolic reflections from
dabrto on pond bottom
Contact between fine
and coarse-grained
i. deposits
.' ', is •'» • •
Approximate location shown
on figure 3 and 5
20
l
40 FEET
1-0
-50
-100
-150
-200
-250
"-300
APPROXIMATE HORIZONTAL DISTANCE. IN FEET
Figure 7.-Interpretation of 300-megahertz radar record of survey section A-B.
-------
WATER SURFACE
Fine-grained
deposits
Contact between fine
and coarse-grained
deposits
HyperboDc reflection from pond
bottom loos, boulders, and debris
Approximate location shown
on figure 3 and 5
20
40 FEET
APPROXIMATE HORIZONTAL DISTANCE. M FEET
r-0
-140
-280
-420
^seo
8
ul
Figure 8.-Interpretation of 80-megahertz radar record of survey section A-B.
-------
wave penetration shown in this section is about
45 ft below the pond surface.
The hyperbolic reflections that originate at
or near the pond bottom are present on both
radar records. These reflections are probably
from point reflectors such as boulders, logs, and
other debris that are resting on or are buried in
the soft bottom deposits.
SUGGESTIONS FOR ADDITIONAL
DATA COLLECTION
None of the geophysical methods used in this
study or other data available conclusively delin-
eate the bedrock surface along the southeast edge
of Hocomonco Pond. Drilling might be required
to obtain the necessary information; however,
several geophysical methods may provide a cost-
effective alternative to a drilling program along
the poorly accessible southeast shore of the pond.
Rerunning a CSP survey along the edge of
the pond using a sound source power level two or
three times larger than that used in the original
survey might provide a clearer definition of deep
reflectors that were only weakly and inconclu-
sively indicated by the original data. Two land-
seismic techniques, the generalized reciprocal
refraction method (Palmer, 1980; and Lankston,
1989) and multichannel land reflection (Dobrin,
1976; and Hajnal, 1978), also might provide the
required information. Both of these methods re-
quire a close geophone spacing and have better
resolution than the seismic-refraction method
used in this study. A gravity survey on the pond
surface (ice), or multi-channel land reflection and
(or) refraction with hydrophones on the pond
bottom might also provide the required informa-
tion.
SUMMARY AND CONCLUSIONS
Geophysical surveys involving seismic re-
fraction, continuous seismic-reflection profiling,
and ground-penetrating radar methods were
done to determine the depth to bedrock beneath
and adjacent to Hocomonco Pond.
The bedrock-surface altitude under Line 1,
south of the pond and along Smithvalve Parkway,
ranges from 305 ft at the eastern end of the line
to 230 ft under the western end of the line. The
bedrock-surface altitude under Line 2, northeast
of the pond and along Otis Street, is 253 ft on the
northern end and decreases to about 158 ft near
the southern end of the line. The bedrock-surface
altitude under Thrift 3, along the southeast shore
of the pond, is 178 ft on the eastern end and raises
slightly to 195 ft on the western end of the line.
The bedrock-surface altitude at the eastern end
of this line may be lower than indicated because
of the presence of a faster velocity layer indicated
by data from a nearby well but not indicated by
the seismic data. The bedrock-surface altitude
under Line 4, on the north side of the pond, is
270 ft on the eastern end, decreases to 30 ft near
the center, and then rises to 155 ft on the western
end of the line.
Continuous seismic-reflection profiling on
the pond identified a bedrock surface in only
several locations. The bedrock was interpreted
to be at 160 ft below the surface of the pond at
one location. The CSP surveys indicated that the
pond is underlain by an irregular, coarse-grained
surface that is overlain by fine-grained sedi-
ments, which are probably glacial-lake deposits.
Two ground-penetrating radar surveys with
80- and 300-MHz antennas show
depths of penetration of 45 and 22 ft, respectively.
T.ilcp the CSP results, analysis of the GPR data
delineated an irregular coarse-grained surface
covered by fine-grained sediment at most loca-
tions. The resolution of the 300-MHz records
allows delineation of the presence and approxi-
mate thickness of a layer of soft pond-bottom
sediment. The ground-penetrating radar sur-
veys also identified the location of debris resting
on or near the pond bottom.
None of the geophysical methods used, or the
other data available, conclusively delineates the
bedrock surface along the southeast edge of the
16
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pond. A conclusive determination of the bedrock-
surface configuration in this area may require
test drilling. Several geophysical methods, in-
cluding a high power CSP survey, multichannel
land seismic-reflection profiling, generalized re-
ciprocal seismic refraction, and a gravity survey
on the pond (ice) surface, could be a cost-effective
alternatives to a test-drilling program along the
poorly accessible shore of the pond.
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