U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2013–1028

Version 1.1, March 2013

Prepared in cooperation with the Town of Woodbury, Connecticut 

Mapping Bedrock Surface Contours Using the Horizontalto-Vertical Spectral Ratio (HVSR) Method Near the Middle 

Quarter Area, Woodbury, Connecticut

The bedrock surface contours in Woodbury, Connecticut, 

were determined downgradient of a commercial zone known 

as the Middle Quarter area (MQA; fig. 1) using the novel, 

noninvasive horizontal-to-vertical (H/V) spectral ratio (HVSR) 

passive seismic geophysical method. Boreholes and monitoring 

wells had been drilled in this area to characterize the shallow 

subsurface to within 20 feet (ft) of the land surface, but little 

was known about the deep subsurface, including sediment 

thicknesses and depths to bedrock (Starn and Brown, 2007; 

Brown and others, 2009). Improved information on the altitude 

of the bedrock surface and its spatial variation was needed for 

assessment and remediation of chlorinated solvents that have 

contaminated the overlying glacial aquifer that supplies water to 

wells in the area.

Depth to bedrock near the MQA was first determined using 

the HVSR method, which uses three-component seismometers 

to record the ambient seismic noise, such as that induced by 

distant weather or ocean waves impacting the shore. In areas 

that have a strong acoustic impedance contrast between the 

bedrock and overlying sediments, such as near the MQA, the 

seismic noise induces resonance at frequencies in the range 

of about 0.1 to 100 hertz. The ratio of the average horizontaland vertical-component amplitude spectrums produces a 

spectral ratio curve with peaks at fundamental and higher order 

resonance frequencies. Resonance frequencies are a function 

of both sediment layer thickness and average layer shearwave velocity.

Measurements were made along several transects 

downgradient of the MQA (fig. 1) using the field methods 

described in Lane and others (2008). Additional measurements 

were collected at several surrounding bedrock wells where 

the altitude of the bedrock surface is known. These were used 

to calculate a local regression line based on the relationship 

between the sediment thickness (z) and the resonance frequency 

(f

r

; in hertz) of the main peak in H/V spectra:

z = a f

r

-b. (1)

where

a and b are empirically derived coefficients (Ibs-von 

Seht and Wohlenberg, 1999).

Once calculated, the local regression line was used to 

determine sediment thickness from observed frequencies 

within the same geologic framework. The values for a (209.17) 

and b (0.798) were determined from the observed resonance 

frequencies and corresponding depths to bedrock (table 1; fig. 2) 

at six well locations around the MQA. Using these values for a 

and b, the depth of the bedrock (y; in feet) is calculated to be:

y = 209.17f

r

-0.798 . (2)

Once the depths to bedrock were calculated using the local 

regression, bedrock altitude was determined by subtracting 

the depths determined from a 2-ft contour light detection and 

ranging (LiDAR) digital elevation model (State of Connecticut, 

2011) or from land-surface altitudes previously surveyed at 

monitoring wells; these bedrock altitudes and regional-scale 

studies of the bedrock surface (Starn and Brown, 2007; Burton, 

2011) constrained by bedrock outcrops and drilling logs from 

domestic wells were used to draw bedrock altitude contours 

(fig. 3).

The bedrock-surface contour map shows a high area to 

the southwest of the MQA within a larger bedrock valley. 

The bedrock surface could affect the movement of dense 

nonaqueous phase liquids (DNAPLs) in the glacial aquifer. The 

modeled bedrock surface provides a tool to help target drilling 

locations for characterization of DNAPL contaminants in the 

source area.

By Craig J. Brown, Emily B. Voytek, John W. Lane, Jr., and Janet R. Stone
Figure 1. Aerial photograph of study area showing locations of seismometer readings, the study transects, and the 

monitoring wells in Woodbury, Connecticut.

Figure 2. Regression plot and line equation for resonance 

frequency and depth to bedrock in Woodbury, Connecticut. 

(f

r

, resonance frequency; r

2

, coefficient of determination; 

y, depth to bedrock).

Calibration wells

Regression line

2 10 20

Depth to bedrock, in feet

Resonance frequency, in hertz

y = 209.17f

r

-0.798

r 2 = 0.9876

1,000

100

10

EXPLANATION

WY79

WY83

WY91

WY95

WY77

WY69

WY106 WY72

WY89

WY84

73°12'25" 73°12'20" 73°12'15" 73°12'10"

41°31'40"

41°31'35"

41°31'30"

41°31'25"

Base map from Google Earth, 2012

Projection: Connecticut State Plane Feet

0 0.05 0.1 MILE

0 0.05 0.1 KILOMETER

WY75

7–8

7–3 7–1

5–9

5–8

5–6

5–5

5–4

5–3

5–2

5–1 3–1

2–8

2–7

7–10

5–21

5–20

7–2

5–7

2–5

2–3

6–2

6–3

6–1

6–0

3–4

3–3

3–2

2–6

2–4

2–2

2–1

7–0b

MQT–1

7–0a

4–4

4–5

4–2 4–1

3–99

3–9

3–8

3–7

3–6

3–5

EXPLANATION

Data point and identifier 3–9

WY83 Monitoring well and identifier

Downgradient flow in glacial

aquifer

T2 Transect and identifier

MIDDLE QUARTER AREA

COMMERCIAL ZONE

T5

T7

T4 T3

T2

T6
Seismometer 

reading sites

Resonance 

frequency, 

in hertz

Estimated 

bedrock depth, 

in feet

Estimated 

bedrock 

altitude, 

in feet 

(NAVD 88)

7–10 4.41 64.0 151.5

WY83 8.16 39.2 206.2

WY84 4.92 58.7 184.1

WY931 7.28 42.9 208.7

WY75 4.09 68.0 150.5

Calibration 

well

Resonance 

frequency, 

in hertz

Bedrock depth, 

in feet

Bedrock 

altitude, 

in feet 

(NAVD 88)

WY69 3.73 66.9 166.3

WY77 4.48 63.0 175.1

WY89 9.83 37.1 226.0

WY981 16.39 21.7 230.6

WY106 3.75 70.9 148.9

WY111 2.06 125.0 140.0

1

Outside of the area shown in figure 1.

Seismometer 

reading sites

Resonance 

frequency, 

in hertz

Estimated 

bedrock depth, 

in feet

Estimated 

bedrock 

altitude, 

in feet 

(NAVD 88)

2–1 4.16 67.1 161.4

2–2 4.36 64.6 164.3

2–3 5.28 55.4 174.0

2–4 5.63 52.7 176.6

2–5 6.04 49.8 179.4

2–6 5.45 54.1 175.4

2–7 4.42 63.9 160.1

2–8 4.06 68.4 147.3

3–1 3.45 77.9 152.0

3–2 3.48 77.3 153.2

3–3 3.94 70.0 160.2

3–4 3.98 69.5 158.3

3–5 4.22 66.3 159.5

3–6 4.27 65.7 160.8

3–7 4.31 65.2 161.3

3–8 3.81 71.9 155.1

3–9 4.31 65.2 162.9

3–99 4.73 60.5 168.2

4–1 3.39 79.0 147.0

4–2 3.25 81.7 145.1

4–4 3.22 82.3 146.2

4-5 2.73 93.8 144.4

5–1 3.47 77.5 152.8

5–2 3.63 74.8 158.0

5–3 3.81 71.9 161.3

5–4 3.80 72.1 160.8

5–5 3.86 71.2 160.4

5–6 3.80 72.1 159.4

5–7 3.75 72.8 158.6

5–8 3.52 76.6 153.0

5–9 3.59 75.4 149.7

5–20 3.44 78.0 140.5

5–21 3.47 77.5 137.9

6–0 3.06 85.7 143.5

6–1 3.17 83.3 145.4

6–2 3.75 72.8 159.5

6–3 3.70 73.6 157.1

MQT-1 3.17 83.3 146.6

7–0a 3.72 73.3 159.4

7–0b 3.77 72.5 159.3

7–1 3.93 70.2 160.4

7–2 4.08 68.1 161.9

7–3 4.19 66.7 161.1

7–8 4.19 66.7 155.1

Table 1. Resonance frequencies and estimated bedrock depths and altitudes for seismometer reading sites and calibration wells in 

Woodbury, Connecticut.

[NAVD 88, North American Vertical Datum of 1988, in feet]

References Cited

Brown, C.J., Starn, J.J., Stollenwerk, K.G., Mondazzi, R.A., 

and Trombley, T.J., 2009, Aquifer chemistry and transport 

processes in the zone of contribution to a public-supply well 

in Woodbury, Connecticut, 2002–06: U.S. Geological Survey 

Scientific Investigations Report 2009–5051, 158 p., 

http://pubs.usgs.gov/sir/2009/5051/.

Burton, W.C., 2006, Bedrock geologic map of the early 

Mesozoic Pomperaug basin surrounding basement rocks, 

Litchfield and New Haven counties, Connecticut: U.S. 

Geological Survey Open File Report 2006–1011, 1 sheet, 

http://pubs.usgs.gov/of/2006/1011/.

Ibs-von Seht, Malte, and Wohlenberg, Jürgen, 1999, 

Microtremor measurements used to map thickness of soft soil 

sediments: Bulletin of the Seismological Society of America, 

v. 89, no. 1, p. 250–259.

Lane, J.W., Jr., White, E.A., Steele, G.V., and Cannia, J.C., 2008, 

Estimation of bedrock depth using the horizontal-to-vertical 

(H/V) ambient-noise seismic method, in Symposium on the 

Application of Geophysics to Engineering and Environmental 

Problems, April 6–10, 2008, Philadelphia, Pennsylvania, 

Proceedings: Denver, Colorado, Environmental and 

Engineering Geophysical Society, 13 p., http://water.usgs.gov/

ogw/bgas/publications/SAGEEP2008-Lane_HV/.

Starn, J.J., and Brown, C.J., 2007, Simulations of groundwater flow and residence time near Woodbury, Connecticut: 

U.S. Geological Survey Scientific Investigations Report 

2007–5210, 45 p. 

State of Connecticut, 2011, Connecticut 2 ft contours (revised): 

Hartford, Connecticut, State of Connecticut, Department of 

Environmental Protection, http://www.cteco.uconn.edu/metadata/

dep/document/CONTOUR_2000_2FT_FGDC_Plus.htm.
Figure 3. Aerial photograph of the study area showing locations of seismometer readings, monitoring wells, and the 

bedrock altitudes and contours in Woodbury, Connecticut. NAVD 88, North American Vertical Datum of 1988.

For more information on the USGS—the Federal 

source for science about the Earth, its natural 

and living resources, natural hazards, and the 

environment, visit http://www.usgs.gov/ or call 

1–888–ASK–USGS.

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and publications, 

visit http://www.usgs.gov/pubprod/.

To order this and other USGS 

information products, visit 

http://store.usgs.gov/.

For more information, contact:

Craig Brown

New England Water Science Center

Connecticut Office

101 Pitkin St.

East Hartford, CT 06108

Telephone: 860–291– 6766

Email: cjbrown@usgs.gov

73°12'25" 73°12'20" 73°12'15" 73°12'10"

41°31'40"

41°31'35"

41°31'30"

41°31'25"

Base map from Google Earth, 2012

Projection: Connecticut State Plane Feet

160

0 0.05 0.1 MILE

0 0.05 0.1 KILOMETER

150

140

170

160

160

160

170

170

180

180

190

190

200

200

150

150

EXPLANATION

Data point and bedrock altitude, in feet 

above the North American Vertical Datum

 of 1988

168

175 Monitoring well and bedrock altitude, in

feet above the North American Vertical

Datum of 1988

160

Altitude, top of bedrock

10-foot contour interval, in feet above the

the North American Vertical Datum of 1988

"

166

150

226

147

206

184

175

152

155

161 162

160

159

159

157

160

145

144

138

149

140

150

153

159

159

160

161

161

158

153

146

144

145 147

168

163

155

161 161

160

158

160

153

152

147

160

175

179

177

174

164

161