OCS Study MMS 2001-0047

BBN Technologies 

BBN Technical Memorandum No. 1270 

ANIMIDA Phase I: Ambient and Industrial Noise 

Measurements Near the Northstar and Liberty 

Sites During April 2000 

Report 

Contract No.143501-99-CT-30998

August 2001 

G. W. Shepard, P.A. Krumhansl, M.L. Knack, C. I. Malme 

Arctic Nearshore Impact Monitoring in the Development Area 

(ANIMIDA) 

Prepared for:

 

 

 U.S. Department of the Interior 

 Minerals Management Service

 Alaska Outer Continental Shelf Region 
OCS Study MMS 2001-0047

This study was funded by the U.S. Department of the Interior, 

Minerals Management Service (MMS), Alaska Outer Continental Shelf Region, 

Anchorage, Alaska, under Contract No. 143501-99-CT-30998, 

 as part of the MMS Alaska Environmental Studies Program 

The opinions, findings, conclusions, or recommendations expressed in this report or product 

are those of the authors and do not necessarily reflect the views of the U.S. Department 

of the Interior, nor does mention of trade names or commercial products constitute endorsement 

or recommendation for use by the Federal Government.

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OCS Study MMS 2001-0047

Table of Contents 

LIST OF TABLES............................................................................................... IV

LIST OF FIGURES ............................................................................................. IV

LIST OF ACRONYMS AND ABBREVIATIONS................................................. VI

EXECUTIVE SUMMARY......................................................................................1

1.0 OVERVIEW..................................................................................................1

1.1 Introduction ............................................................................................................................................ 1

1.2 Objectives................................................................................................................................................ 2

1.3 Field Measurement Summary ............................................................................................................... 2

2.0 STUDY AREA DESCRIPTION AND METHODS............................................4

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

2.2 Acoustic and Vibration Measurements and Analysis Methods.......................................................... 7

2.2.1 Measurement System ...................................................................................................................... 7

2.2.2 Analysis Methods........................................................................................................................... 10

3.0 RESULTS .....................................................................................................11

3.1 Northstar Site........................................................................................................................................ 12

3.1.1 Site NS-Trench1............................................................................................................................. 12

3.1.2 Site NA3.......................................................................................................................................... 15

3.1.3 Site NA2.......................................................................................................................................... 23

3.1.4 Site NA5.......................................................................................................................................... 25

3.1.5 Site NA6.......................................................................................................................................... 36

3.1.6 BSMP Site 5(5)............................................................................................................................... 42

3.2 Liberty Site............................................................................................................................................ 46

3.2.1 Site LA2.......................................................................................................................................... 46

3.2.2 Site LA3.......................................................................................................................................... 48

4.0 DISCUSSION................................................................................................53

4.1 Underwater Noise Environment.......................................................................................................... 53

4.2 In-air Noise Environment .................................................................................................................... 57

4.3 Ice Vibration Environment.................................................................................................................. 60

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OCS Study MMS 2001-0047

5.0 SUMMARY AND CONCLUSIONS ...............................................................63

5.1 Noise and Vibration During Northstar Vibrahammer Operation ................................................... 63

5.2 Noise and Vibration without Northstar Vibrahammer Operation .................................................. 64

6.0 ACKNOWLEDGEMENTS.............................................................................65

7.0 LITERATURE CITED ...................................................................................65

APPENDIX A: COMPLETE LISTING OF THE COLLECTED DATA SET........66

APPENDIX B: THIRD OCTAVE BAND CENTER FREQUENCIES, HZ ............66

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OCS Study MMS 2001-0047

List of Tables 

Table 1-1. Noise and vibration Measurement sites and environmental conditions. ........... 2 

Table 2- 1. Measurement site locations and relative position re Northstar. ....................... 4 

Table 2- 2. Amplifier gains for each data channel and measurement site.....................8 

List of Figures 

Figure 2- 1. Map of ANIMIDA study area........................................................................ 5 

Figure 2- 2 Photograph of Northstar Island showing sheet piling and the vibrahammer 

suspended from a crane................................................................................................ 6 

Figure 2- 3. Photograph of site NA3 preparations.............................................................. 7 

Figure 2- 4. Acoustic and vibration measurement system................................................. 8 

Figure 2- 5. Hydrophone system characteristics................................................................ 9 

Figure 2- 6. Measured geophone and microphone electronic noise floors........................ 9 

Figure 3.1.1- 1. Near-bottom underwater Noise near Northstar ice road (2-2700). ........ 13 

Figure 3.1.1- 2. Mid depth underwater Noise near Northstar ice road (2-2700). ............ 13 

Figure 3.1.1- 3. Airborne noise near Northstar ice road (2-2700). .................................. 14 

Figure 3.1.1- 4. Measured ice vibration levels near Northstar ice road (2-2700)............ 15 

Figure 3.1.2- 1. Near-bottom underwater noise at Site NA3 (3-4610)............................ 16 

Figure 3.1.2- 2. Mid channel depth underwater noise at Site NA3 (3-4610). ................. 16 

Figure 3.1.2- 3. Near bottom underwater noise at Site NA3 (3-1315). ........................... 17 

Figure 3.1.2- 4. Mid channel depth underwater noise at Site NA3 (3-1315). ................. 18 

Figure 3.1.2- 5. Airborne noise at Site NA3 (3-4610)..................................................... 18 

Figure 3.1.2- 6. Second example of airborne noise at Site NA3 (3-1315). ..................... 19 

Figure 3.1.2- 7. Ice vibration levels from vibrahammer at Site NA3 (3-1315). .............. 19 

Figure 3.1.2- 8. Near bottom underwater noise at Site NA3, plowing noise (3-8100).... 20 

Figure 3.1.2- 9. Mid channel depth underwater noise at Site NA3, plowing (8-8100). . 21 

Figure 3.1.2- 10. Near bottom underwater noise at Site NA3; quiet (3-2500). ............... 21 

Figure 3.1.2- 11. Mid channel depth underwater noise at Site NA3; quiet (3-2500). ..... 22 

Figure 3.1.2- 12. Airborne noise at Site NA3; plowing (3-8100).................................... 22 

Figure 3.1.2- 13. Second example of airborne noise at Site NA3 (3-2500). ................... 23 

Figure 3.1.2- 14. Ice vibration noise spectra at Site NA3; plowing (3-8100).................. 24 

Figure 3.1.2- 15. Second example of ice vibration noise at Site NA3 (3-2500).............. 24 

Figure 3.1.3- 1. Near bottom underwater noise at site NA2. (4-4000)............................. 25 

Figure 3.1.3- 2. Mid channel underwater noise at site NA2. (4-4000)............................. 26 

Figure 3.1.3- 3. Measured in-air noise levels at site NA2. (4-4000) ............................... 26 

Figure 3.1.3- 4. Measured ice vibration levels at site NA2. (4-4000) .............................. 27 

Figure 3.1.4- 1. Near bottom underwater noise levels at site NA5; lunch break. (8-4400)

.................................................................................................................................... 28 

Figure 3.1.4- 2. Mid-channel underwater noise levels at site NA5; lunch break. (8-4400)

.................................................................................................................................... 29 

Figure 3.1.4- 3. Measured in-air noise levels at site NA5; lunch break. (8-4400) ........... 29 

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OCS Study MMS 2001-0047

Figure 3.1.4- 4. Measured ice vibration levels at site NA5; lunch break. (8-4400) ........ 30 

Figure 3.1.4- 5. Near bottom underwater noise levels at site NA5; vibrahammer. (8-1200)

.................................................................................................................................... 31 

Figure 3.1.4- 6. Mid channel underwater noise levels at site NA5; vibrahammer. (8-1200)

.................................................................................................................................... 31 

Figure 3.1.4- 7. Near bottom underwater noise levels at site NA5; strong vibrahammer. 

(10-0000).................................................................................................................... 32 

Figure 3.1.4- 8. Mid channel underwater noise levels at site NA5; strong vibrahammer. 

(10-0000).................................................................................................................... 33 

Figure 3.1.4- 9. In-air noise levels at site NA5; vibrahammer. (10-0000) ....................... 33 

Figure 3.1.4- 10. Second sample of in-air noise levels at site NA5; vibrahammer. (9-

6000)........................................................................................................................... 34 

Figure 3.1.4- 11. Measured in-air noise levels at site NA5; no vibrahammer. (8-1200).. 35 

Figure 3.1.4- 12. Second example of in-air noise levels at site NA5; no vibrahammer. 

(10-1300).................................................................................................................... 35 

Figure 3.1.4- 13. Ice vibration noise levels at site NA5; vibrahammer. (10-0000).......... 36 

Figure 3.1.4- 14. Measured ice vibration levels at site NA5; vibrahammer. (10-1300)... 37 

Figure 3.1.5- 1. Near bottom underwater noise levels, site NA6; vibrahammer. (11-1300)

.................................................................................................................................... 38 

Figure 3.1.5- 2. Mid channel underwater noise levels, site NA6; vibrahammer. (11-1300)

.................................................................................................................................... 38 

Figure 3.1.5- 3. Near bottom underwater noise levels at site NA6; no vibrahammer. (11-

5330)........................................................................................................................... 39 

Figure 3.1.5- 4. Mid channel underwater noise levels at site NA6; no vibrahammer. (11-

5330)........................................................................................................................... 39 

Figure 3.1.5- 5. Measured in-air noise levels at site NA6; vibrahammer. (11-1300)....... 40 

Figure 3.1.5- 6. Measured in-air noise levels at site NA6; no vibrahammer. (11-5330).. 41 

Figure 3.1.5- 7. Measured ice vibration levels at site NA6; vibrahammer. (11-1300)..... 41 

Figure 3.1.5- 8. Measured ice vibration levels at site NA6; no vibrahammer. (11-5330) 42 

Figure 3.1.6- 1. Near bottom underwater noise levels at site 5(5); lower winds. (5-0400)

.................................................................................................................................... 43 

Figure 3.1.6- 2. Near bottom underwater noise levels at site 5(5); higher winds. (5-6050)

.................................................................................................................................... 43 

Figure 3.1.6- 3. Mid channel underwater noise levels at site 5(5); lower winds. (5-0400)

.................................................................................................................................... 44 

Figure 3.1.6- 4. Mid channel underwater noise levels at site 5(5); higher winds. (5-6050)

.................................................................................................................................... 44 

Figure 3.1.6- 5. Measured in-air noise levels at site 5(5). (5-0400) ................................. 45 

Figure 3.1.6- 6. Measured ice vibration levels at site 5(5). (5-0400) ............................... 46 

Figure 3.2.1- 1. Near bottom underwater noise level measured at site LA2 (6-1200). ... 47 

Figure 3.2.1- 2. Mid channel underwater noise level measured at site LA2 (6-1200). ... 47 

Figure 3.2.1- 3. Airborne noise level measured at site LA2 (6-1200)............................. 48 

Figure 3.2.1- 4. Ice vibration noise level measured at site LA2 (6-1200). ...................... 49 

Figure 3.2.2- 1. Near bottom underwater noise level measured at site LA3 (7-5000). ... 49 

Figure 3.2.2- 2. Mid channel underwater noise level measured at site LA3 (7-5000). ... 50 

Figure 3.2.2- 3. Airborne noise level with flare measured at site LA3 (7-5000)............. 51 

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OCS Study MMS 2001-0047

Figure 3.2.2- 4. Airborne noise level with no audible flare measured at site LA3 (7-1200).

.................................................................................................................................... 51 

Figure 3.2.2- 5. Ice vibration noise spectrum measured at site LA3 (7-5000). ............... 52 

Figure 4.1- 1. Vibrahammer 23 Hz tonal spectrum level as a function of range as 

received on the near-bottom hydrophone (H1) and mid-channel depth hydrophone 

(H2). ........................................................................................................................... 54 

Figure 4.1- 2. Comparison of 1/3 OB spectra at multiple sites collected during 

vibrahammer operation using the hydrophone positioned near the bottom (H1). ..... 55 

Figure 4.1- 3. Comparison of 1/3 OB spectra at multiple sites collected during 

vibrahammer operation using the hydrophone positioned in the middle of the water 

column (H2). .............................................................................................................. 56 

Figure 4.1- 4. Comparison of 1/3 OB noise spectra at multiple sites without vibrahammer 

noise collected using the Hydrophone positioned near the bottom (H1). .................. 57 

Figure 4.1- 5. Comparison of 1/3 OB noise spectra at multiple sites without vibrahammer 

noise collected using the Hydrophone positioned in the middle of the water column 

(H2) ............................................................................................................................ 58 

Figure 4.2- 1. Comparison of 1/3 OB airborne noise spectra at multiple sites with 

vibrahammer in operation at least at sites NA3, NA5, and NA6. .............................. 59 

Figure 4.2- 2. Comparison of 1/3 OB airborne noise spectra at multiple sites without 

vibrahammer excitation.............................................................................................. 59 

Figure 4.3- 1. Comparison of 1/3 OB ice vibration noise spectra at multiple sites with 

vibrahammer in operation at least at sites NA3, NA5, and NA6. .............................. 60 

Figure 4.3- 2. Vibrahammer 23 Hz tonal received ice vibration spectrum level as a 

function of range. ....................................................................................................... 61 

Figure 4.3- 3. Comparison of 1/3 OB ice vibration noise spectra at multiple sites without 

vibrahammer excitation.............................................................................................. 62 

List of Acronyms and Abbreviations 

ANIMIDA Arctic Nearshore Impact Monitoring in the Development Area 

BBN Bolt Beranek and Newman 

BSMP Beaufort Sea Monitoring Posts 

c speed of sound in water (typically near 1500 m/s) 

Cm centimeter (1 cm = 0.39 inches) 

CTD Conductivity, Temperature, Depth profile measurement tool 

d water depth 

DAT Digital Audio Tape 

dB re 1 μPa decibels in relation to a reference pressure of 1 microPascal 

dBV decibel in relation to a reference of 1 volt 

fc center frequency of a 1/3-octave band 

Fc acoustic cutoff frequency from normal mode theory 

FFT Fast Fourier Transform 

ft foot or feet (1 ft = 0.305 m) 

Hz Hertz (cycle per second) 

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OCS Study MMS 2001-0047

ITC International Transducer Corporation 

kHz kilo-Hertz (1000 cycle per second) 

km kilometer (1 km = 3281 feet or 0.54 nautical mile) 

kt knot (1 nautical mile per hour) 

m meter (1 m = 1.09 yards or 3.28 feet) 

MATLABTM Mathematical Software by The Mathworks Inc. 

MLLW mean lower low water 

MMS Minerals Management Service, U. S. Dept. of the Interior 

nm nautical mile (1 nm = 1.852 km) 

OB octave band 

OCS Outer Continental Shelf 

SNR Signal-to-Noise Ratio 

SPL Sound Pressure Level

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OCS Study MMS 2001-0047

Executive Summary 

As part of the Minerals Management Service (MMS) program entitled “Arctic Nearshore 

Impact Monitoring in the Development Area,” a winter field survey was conducted from 

25 April to 30 April, 2000. Measurements of site-specific underwater noise, in-air noise, 

and ice vibrations were made at eight locations near and between the Northstar and 

Liberty prospects. Both prospects are located in the nearshore portion of the outer 

continental shelf in the Beaufort Sea near Prudhoe, Alaska. 

The goal of this effort was to characterize noise and vibration conditions near the 

Northstar site which was under construction and near the Liberty site which is planned 

for development in the near future. 

Analysis results are presented that document the underwater noise at two water depths, 

the airborne noise conditions, and the ice vibration levels at ranges varying from about 

0.08 nm (0.15 km) to >2.2 nm (4 km) from Northstar. The results present measured 

levels caused by construction activities at Northstar including sheet pile driving using a 

vibrahammer, plowing operations, general truck movement on and near the island, island 

mounted machinery generated noise, and trench backfilling operations. 

Similar measurements were made at and near the Liberty site which document the 

conditions with minimal, if any, man-made noise present. 

1.0 Overview 

1.1 Introduction 

As onshore and offshore oil and gas exploration, development, and production activities 

increase across Alaska’s North Slope, concern is also on the rise regarding the long-term 

effects of these activities. 

This study is part of a multidisciplinary impact monitoring program of the first Federal 

oil development offshore of Alaska, in the nearshore Beaufort Sea. The overall goal is to 

verify the projected impacts from development and production and to provide 

information to support post-leasing decisions to minimize further impacts. Specifically, 

this study is the initial step of the ANIMIDA program to monitor impacts associated with 

the development activities and initial production of oil from Northstar and Liberty Units 

in the nearshore portion of the Outer Continental Shelf (OCS) in the Beaufort Sea near 

Prudhoe Bay. 

During April of 2000, a team of scientists made several daily trips via snowmobile to 

various sites near and around both the Northstar and Liberty sites. The following 

physical environmental parameters were measured: 

Sediment samples to determine contaminant concentrations in local sediments and 

any residual mud and cuttings from earlier exploration activities, 

•

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OCS Study MMS 2001-0047

Suspended sediments to determine whether resuspension from construction activities 

will impact turbidity levels in the water or ice cover, and 

•

• Acoustic and vibration noise levels to characterize the ambient and construction noise 

conditions. 

 

This report documents the results of the third bullet, acoustic and vibration 

characterization. 

1.2 Objectives 

The goal of this study was to conduct a preliminary characterization of the site-specific 

noise and vibration environment at Northstar and Liberty sites. Specifically, the 

objectives were to measure and document both ambient and industrial noise in the air and 

underwater both in open water and ice-covered conditions, and, during the winter/spring, 

vibrations in the ice. The measurement of ambient noise levels was intended to address 

the basic question: Do the construction and operational activities at Northstar and Liberty 

result in noise levels and spectral shapes that differ from naturally occurring background 

conditions? These results will ultimately be used to examine the validity of the working 

hypothesis: Noise levels and related acoustic features in the study area differ from 

background levels as a result of Northstar and Liberty activities. 

1.3 Field Measurement Summary 

Two field measurement periods were chosen: an open water period from 16-26 August 

1999, and an ice-covered period from 26-29 April 2000. Unfortunately, the data 

collected during August were severely contaminated with wave slap sounds (both in air 

and underwater) against the aluminum hull of MMS Launch 1273. Attempts were made 

in the field to separate the sensors from the vessel but these efforts proved unsuccessful. 

We also tried to identify “uncontaminated” snippets and statistically separate the wave 

slap characteristics from the underlying ambient noise also met with failure. 

Consequently, this data set was abandoned. 

The data collection efforts in April were much more successful. Over the course of five 

days, acoustic and vibration measurements were collected at eight sites. The locations 

and their associated environmental conditions are provided in Table 1-1. Note: all 

latitudes and longitudes in NAD83 datum. 

Station 

ID Type Date Time, Local Latitude Longitude 

Wind Dir/ 

Speed, kts 

Air Temp, 

deg C 

Water 

Depth (m) 

Ice Depth 

(m) 

5(5) BSMP 04/27/2000 04/27/2000 70o 26.108 148o 18.090 N/9 -9 6.5 1.6 

LA2 Liberty 04/28/2000 04/28/2000 70o 18.970 147o 37.928 Calm -9 6 1.6 

LA3 Liberty 04/28/2000 04/28/2000 70o 16.778 147o 33.529 SW/2 -9 6.1 1.6 

NS-Trench1 Northstar 04/25/2000 04/25/2000 70o 28.806 148o 41.607 N/7 -13 9.7 1.6 

NA2 Northstar 04/26/2000 04/26/2000 70o 30.817 148o 36.351 N/3 -11 10.8 1.7 

NA3 Northstar 04/26/2000 04/26/2000 70o 29.569 148o 41.460 N/10 -14 11.5 2.4 

NA5 Northstar 04/29/2000 04/29/2000 70o 29.889 148o 40.734 N/5 -11 11.8 1.8 

NA6 Northstar 04/29/2000 04/29/2000 70o 30.372 148o 39.911 W/9 -6 12.4 1.8 

Table 1-1. Noise and vibration Measurement sites and environmental conditions. 

At each site, a suite of four sensors was deployed. 

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OCS Study MMS 2001-0047

Two ITC 6050C hydrophones to measure underwater noise both near the bottom and 

at mid-water depth. 

•

•

•

A 0.5 inch Electret-Condenser microphone to collect in-air acoustic noise. 

A Geo Space GS-30CT geophone which was frozen into the ice surface to measure 

ice vibrations. 

Data from each sensor were amplified and recorded to a TEAC DAT recorder operating 

at double speed resulting in a measurement band from 2 Hz to 20 kHz. In addition, local 

environmental parameters were measured including air temperature, wind speed and 

direction, and CTD (conductivity, temperature, and depth) profiles of the water column. 

3 
OCS Study MMS 2001-0047

4 

2.0 Study Area Description and Methods 

2.1 Study area 

The study area is shown in Figure 2-1. The sampling protocol consisted of collecting 

data at two sites near the Liberty Prospect and a set of six sites at varying ranges from the 

Northstar Prospect. The Liberty sites are intended to establish a measurement basis prior 

to construction activities. The Northstar sites were selected at incremental ranges from 

the prospect to sample the range dependent aspects of noise and vibration excitation. 

Table 2-1 shows the location of the sites sampled near Northstar and the range and 

direction from Northstar to each site. 

Site ID Latitude Longitude 

Range to 

Northstar, km 

Direction from 

Northstar to Site 

NS-Trench 70o 28.806 148o 41.607 1.8 S 

NA3 70o 29.569 148o 41.460 0.15 E 

NA5 70o 29.889 148o 40.734 1 NNE 

NA6 70o 30.372 148o 39.911 2 NNE 

NA2 70o 30.817 148o 36.351 4 NNE 

5(5) 70o 26.108 148o 18.090 15 SE 

Table 2- 1. Measurement site locations and relative position re Northstar. 

The Liberty site has yet to see any development activities so the measurements collected 

are primarily ambient conditions with the possible exception of noise/vibration radiating 

from a natural gas flare located at the end of the Endicott dock. 

Two measurement sites were visited in April of 2000: LA2 and LA3, which are both 

marked in Figure 2-1. LA2 is located approximately 2.7 nm (5 km) NNW of the Liberty 

site over the Boulder Patch in 19.7 ft (6.0 m) water. The measured ice thickness was 5.2 

ft (1.6 m). Site LA3 is at the center of the future Liberty artificial island where the water 

is 20.0 ft (6.1 m) deep and the ice thickness was also 5.2 ft (1.6m). 

At Northstar, human activities were in nearly constant motion during our measurement 

periods. By the time of our measurements, the artificial island at Northstar was mostly 

complete. The main activities consisted of driving sheet piles into the island perimeter, 

backfilling the pipeline trench to shore, and truck movements plowing and moving earth 

on and about the island. 

Figure 2-2 is a photograph taken on 26 April of Northstar. Suspended from the larger 

crane in the picture is a vibrahammer which is used to drive the sheet pilings around the 

perimeter of the island. Vibrahammer noise and vibration was a significant source during 

the measurement period and is described in Section 3. 
OCS Stud

5 

Northstar Prospect

Liberty P

Prudhoe Bay

West Dock Endicott Development Island

Kuparuk River

5(10)

5(5)

5(1)

5(0)

8D

8C

8E

8F

8A 8B

5F

5A

5B

5E

5D

4C5H

4B

4A

0.5km

Alaska

6 FATHOMS

3 FATHOMS

wydyr Bay

Jones Islands Reindeer Island

Cross Island

Boulder Patch

G

4km2km1km

4km2km1km0.5km

Sagavanirktok River

Spoils Disposal

P

ip

e

lin

eAlternate Pipeline

Alternate (Optional) Transect

NA2

NA1

NA4

LA1

LA4

LA2

LA3

Figure 2- 1. Map of ANIMIDA study area. 

NA3

NA5 

NA6 

10 nm Scale10 km 

72o 30’N72o 20’N

149o W 148o W 
OCS Study MMS 2001-0047

Figure 2- 2 Photograph of Northstar Island showing sheet piling and the 

vibrahammer suspended from a crane. 

Six measurement sites near Northstar were visited. The first site, labeled as NS-Trench1 

in Table 1-1, was near the ice road connecting the shore to the artificial island at a 

location roughly 1 nautical mile (nm) from the island. The sea ice was somewhat soft 

and slushy. The pipeline trench had already been opened and the pipe inserted. 

Backfilling of the trench was in progress. Truck movement and plowing noises were also 

present throughout the data set. Road graders were occasionally present working the ice 

road. During this period, sheet pile driving was not apparent to the scientific team. 

The five other sites near Northstar include sites NA2, NA3, NA5, NA6, and BSMP site 

5(5). The site positions varied in range from Northstar from 0.08 nm (150 m) (site NA3) 

to approximately 8.1 nm (15 km) (site 5(5)). Measurements were collected during 

daylight hours while activities continued on/near the island. 

In general, the ice conditions at all sites consisted of shore-fast ice varying in depth from 

roughly 4.9 to 8.2 ft (1.5 to 2.5 m). The skies were overcast most of the time and light 

snow was in the air almost continuously. The ice was covered with light fluffy snow to a 

depth ranging from 2-6 inches (5-15 cm) which undoubtedly produced higher in-air 

propagation loss than would have been evident with bare pack ice. The photograph in 

Figure 2-3 was taken at site NA3 near the Northstar site and clearly shows the snow 

conditions. 

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OCS Study MMS 2001-0047

Figure 2- 3. Photograph of site NA3 preparations. 

2.2 Acoustic and Vibration Measurements and Analysis Methods 

2.2.1 Measurement System 

The measurement system consisted two hydrophones, a microphone, and a geophone that 

was frozen into the ice surface. Figure 2-4 shows a block diagram of the system. 

The hydrophones were both ITC Type 6050C calibrated sensors each with a nominal 

sensitivity of –161 dBV re 1 μPa. In the passband from 2 Hz to 20 kHz, the sensitivity 

ripple was +0.8 dB. System electronic noise was measured in the laboratory by shorting 

out the channel input and recording the resulting noise through the system and onto the 

DAT recorder which was subsequently analyzed to define the noise floor. Figure 2-5 

compares the measured noise floor with historical Arctic and open water noise spectra. 

The two hydrophones were deployed through a hole in the ice. Hydrophone #1 was 

installed at a depth within 6.5 ft (2 m) of the bottom. Hydrophone #2 was lowered to a 

mid water column depth. The signals from each hydrophone were passed through a 

signal conditioning amplifier with a gain of 0 dB for impedance matching and then 

amplified using a BBN manufactured (Model 392) low noise amplifier whose gain varied 

from site to site as the noise conditions warranted. Amplifier gains for all channels and 

measurement sites are documented in Table 2-2. 

The microphone was a calibrated 1⁄2-inch Genrad Condenser microphone with a 

sensitivity of -134 dBV re 20μPa over the frequency band from 15 Hz to 19 kHz. The 

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OCS Study MMS 2001-0047

ITC 6050C (mid-depth)

ITC 6050C (~1 m off bottom)

A

B

Signal

Conditioner Low Noise

Amplifiers

TEAC

DAT

Recorder

Real Time Analysis

(Spectrum Analyzer)

GS Geophone

1/2” Condenser 

Microphone w/ Windscreen

Figure 2- 4. Acoustic and vibration measurement system. 

geophone was a Geo Space model GS-32CT close tolerance geophone. Its intrinsic 

voltage sensitivity was 0.698 V/in/sec and its sensitivity at 70% damping was 0.500 

V/in/sec. The microphone signal was passed through a 0 dB gain signal conditioner and 

both sensors were amplified using BBN’s 392 low noise amplifiers. As with the 

hydrophones, noise floors were measured in the laboratory and are presented in Figure 2-

6. 

All sensors were recorded on a 16-channel TEAC RD-145T DAT recorder set up to 

operate in a 4-channel mode. Recordings were made at “double speed” which digitally 

samples the data at 44.1 kHz producing an accurate measurement band out to 20 kHz. 

All equipment was battery powered during the data collection periods. At each site, data 

was recorded for a minimum of 30 minutes. 

Site 

H1 Gain, 

dB 

H2 Gain, 

dB 

Geophone 

Gain, dB 

Microphone 

Gain, dB 

NS Trench 30 30 30 30 

NA3 30 30 30 30 

NA2 40 40 40 40 

5(5) 30 30 30 20 

LA2 40 40 40 40 

LA3 40 40 40 40 

NA5 30 30 40 30 

NA6 30 30 40 30 

Table 2- 2. Amplifier gains for each data channel and measurement site. 

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Hydrophone System Noise & Reference Ambient 

Noise Levels

0

10

20

30

40

50

60

70

1 10 100 1000 10000 100000

Frequency, Hz

Level, dB//uPa^2 per Hz

System Noise

[1] Noise under shore-fast 

 ice, Fig. 13, 10 Feb.

[1] Quiet Ambient Noise

 Spectra, Fig. 6

Sea State 0

(Knudsen)

Figure 2- 5. Hydrophone system characteristics. 

Geophone and Microphone Noise Floors

-160

-140

-120

-100

-80

-60

-40

-20

0

1 10 100 1000 10000 100000

Frequency, Hz

Geophone Level, dB re in*sec

-25

-20

-15

-10

-5

0

5

10

Microphone Level, dB re 20 uPa/Hz 

Microphone

Geophone

Figure 2- 6. Measured geophone and microphone electronic noise floors. 

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OCS Study MMS 2001-0047

2.2.2 Analysis Methods 

The first step in the analysis procedure was to qualitatively assess the recorded data to 

identify data segments for evaluation. For each site, a hydrophone and the microphone 

were played back through a headset and a log of the aurally observable activities was 

generated. Two minute segments were selected to highlight trucking noises, plowing 

activities, vibrahammer operation, the Endicott gas flare, and ambient noise without 

manmade contributions. As many as four data cuts were examined for each site. 

Two minute data segments were selected because the man-made noises, such as the 

vibrahammer, were only present for short durations. For example, a typical vibrahammer 

operation lasted for 1 minute or less. Also, noise from plowing and truck movement was 

erratic and short in nature (typically 10 seconds or less). Longer analysis windows 

tended to obscure characteristics of the dominant noise source by averaging in more noise 

from other sources. For consistency, all analysis durations were set at 2 minutes. 

Each identified data segment was then processed to estimate the 5th, 50th, and 95th

percentile levels of the site-specific noise statistics. Results are reported in both 

narrowband spectra with a 1 Hz resolution and 1/3-octave bands from 2 to 20 kHz. The 

analysis parameters were as follows: 

• Data sampling rate: 48 kHz, 

• Fast Fourier Transform (FFT) size: 217 (131,017), 

• FFT overlap: 50%, and 

• FFT time domain window: Hanning. 

The process included downloading the DAT files to a personal computer in a format 

compatible with Matlab TM software system. Matlab scripts were then used to generate 

the narrowband and 1/3-octave band spectra. The 1/3-octave band spectra were then 

further processed to estimate the percentile levels. 

10 
OCS Study MMS 2001-0047

3.0 Results 

The recorded data were analyzed to develop a short-term statistical description of the 

variation of the underwater, in-air, and in ice noise conditions at each site for each 

identified noise source. In the following sections, results from six sites near Northstar 

and two sites near the future Liberty site are presented. No observers were stationed at 

Northstar to log activities so the conclusions and observations relating to the source of the 

measured noise provided in this section are inferred based on the measured spectral 

characteristics. 

All results are presented as spectra in a consistent format. Each figure consists of five 

traces. The narrowband 1 Hz resolution power spectrum is an average over the selected 

two minute data segment. It provides insight into the tonal structure of the noise 

environment which is one part of overall acoustic environment; the other being the 

broadband component. Third octave band (OB) data are presented at the 5th, 50th, and 

95th percentiles over the same two minute window with the 50th percentile trace marked 

with open circles. Finally, each figure includes a trace showing the 1/3 OB electronic 

noise floor of the measurement system for comparison to the 1/3 OB noise data. The 

noise floor is an average and as such includes some variability. This means that the 5th

percentile noise estimate can dip below the average 1/3 OB curve. As an example, in 

Figure 3.1-4 below, the 50th percentile noise estimate above 50 Hz is only slightly above 

the noise floor; the difference primarily due to the tonals that appear in the narrowband 

spectra above the electronic noise floor. In this case, the 1/3 OB data represent an upper 

limit. 

Third octave band data are reported three times per octave where an octave is a doubling 

of frequency. Each 1/3-octave sample is a sum of the energy over a range of frequencies. 

Thus in comparing 1/3-octave data and narrowband data, the 1/3-octave samples are the 

sum of the 1 Hz samples over a specified frequency band. And therefore, average or 50th

percentile 1/3-octave data are equal to or higher than the averaged narrowband samples. 

This is also why the narrowband spectra can fall below the 1/3-octave band 

instrumentation noise floor. The bandwidth of each sample is proportional to the center 

frequency, in this case 23%. For example, starting at 2 Hz, 1/3 octave band data are 

reported at 2, 2.5, 3.15, 4, 5, and 6.3 Hz, and continue in this fashion to any desired upper 

frequency limit. Each sample covers the band from 0.891fc to 1.122fc where fc is the 

band center frequency. Sound levels are often reported in 1/3-octave bands because in 

humans and some animals, hearing is roughly 1/3-octave. 

At each site visited, data are typically is set of four: two hydrophones, the microphone, 

and the geophone. These sets were generated over the same 2 minute time interval. The 

reader will see that each figure has a label in its lower right-hand side which identifies the 

DAT tape number and the foot count at which the data analysis was begun. With this 

reference, we can easily re-examine any data segment accurately. 

11 
OCS Study MMS 2001-0047

3.1 Northstar Site 

The data collected near the Northstar site varied in range from the island from 0.08 to 8.1 

nm (150 m to 15 km). The goal was to collect sample data versus range to qualitatively 

assess the extent of Northstar radiated noise with range. No attempt was made to 

estimate noise source levels or to derive transmission loss estimates since these were not 

project goals. 

3.1.1 Site NS-Trench1 

The first site visited was near the ice road about 1 nm from Northstar. Data were 

recorded between 7 and 8 PM local time. Backfilling operations were in progress to 

cover the trench carrying the pipeline to shore. The dominant audible sounds were 

generated by trucks and plows. 

Underwater noise spectra from the near bottom hydrophone (labeled #1) is presented in 

Figure 3.1.1-1 and the mid water hydrophone (labeled #2) in Figure 3.1.1-2. The 

amplifier gains for both sensors were set at 30 dB. Hydrophone #1 (H1) was deployed at 

a depth of 26 ft (8 m) and hydrophone #2 (H2) at 16.5 ft (5 m). The water depth was 32 

ft (9.7 m). Both sensors show a rich tonal structure above 25 Hz and below 600 Hz with 

H1, the deeper sensor, having higher level tonals. The source of the broad spectral peak 

near 700 Hz is unknown and was not observed at the other locations. However, it is also 

present in the microphone data discussed below. Field personnel noted “gurgling” water 

sounds at this site but it is not clear if this was the source of the spectral peak. 

The noise at this site is dominated by the trucking and plowing operations which were 

nearly continuous during this data segment. Note that the 1/3 OB 50th percentile is 

approximately evenly spaced between the 5th and 95th percentiles indicating a reasonably 

stable noise environment. This is not always the case. For reference, the level difference 

between the 5th and 95th percentile is quite uniform across the analysis band at 

approximately 20 dB. As will be shown in the next section, the vibrahammer was not in 

operation during this period because its characteristic narrowband component at 23 Hz is 

not present. 

The measured in-air noise level is shown in Figure 3.1.1-3. The amplifier gain was set at 

30 dB. Here, the tonal content is significant from about 45 Hz up to roughly 2 kHz. 

Above 5 kHz, the noise level is approaching the noise floor of our measurement system. 

Some of the same tonals seen underwater are present in the air such as near 44 Hz and at 

350 and 400 Hz. But the dominant feature is the broadband low frequency noise. 

Compared to measurement made near Liberty in a very quiet environment (for example, 

see Section 3.2.1), the airborne noise here is 10 to 25 dB higher at frequencies below 200 

Hz. 

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OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NT1: Trench, backfill site

Hydrophone 1: Trucks/plowing

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.1- 1. Near-bottom underwater Noise near Northstar ice road (2-2700). 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NT1: Trench, backfill site

Hydrophone 2: Trucks/plowing

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.1- 2. Mid depth underwater Noise near Northstar ice road (2-2700). 

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OCS Study MMS 2001-0047

In ice vibrations were measured using a geophone frozen into the ice surface and an 

amplifier gain of 30 dB. Vibrations can couple into the ice from four sources: 

• vibrations that radiate from the bottom, through the water column and into the ice, 

•

•

•

excitation also from the bottom transmit into the ice through shore-fast ice pathways, 

vibrations that flow from ice floe to ice floe through the plate boundaries, and 

in-air noise that couples into the ice through the snow covered ice surface. 

Figure 3.1.1-4 shows the measured ice vibrations at the NS-Trench1 site. In general, the 

levels are quite low with most of the energy in the band from 2 to 40 Hz. Some 

narrowband tonals can be seen above the noise up to a few kHz. The most significant 

feature is the high level of the 95th percentile noise. This indicates multiple short 

duration or bursts of high level vibrations in the data segment. Its broadband nature is 

consistent with impulsive noise most likely due to the bangs associated with plowing and 

dumping operations. 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NT1: Trench, backfill site

Microphone: Trucks/plowing

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.1- 3. Airborne noise near Northstar ice road (2-2700). 

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OCS Study MMS 2001-0047

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

dB re in*sec per Hz and per 1/3 Octave Band

Site NT1: Trench, backfill site

Geophone: Trucks/plowing

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.1- 4. Measured ice vibration levels near Northstar ice road (2-2700). 

3.1.2 Site NA3 

Site NA3 was located approximately 0.08 nm (150 m) from Northstar in a water depth of 

38 ft (11.5 m). A little after 1 PM local time when the data were collected, the site was a 

hot bed of activity with periodic vibrahammer sheet pile driving, trucks moving about, 

and plowing operations. Four data segments were selected for analysis, two with the 

vibrahammer in operation, one with normal trucking and plowing, and a fourth collected 

during the quietest period observed. 

Figures 3.1.2-1 and 3.1.2-2 present the strongest underwater noise levels collected during 

vibrahammer operation. Amplifier gains were set at 30 dB. With H1 suspended at a 

depth of 33 ft (10 m) and H2 at a mid channel depth of 20 ft (6 m), the near-bottom 

sensor exhibits higher level noise than the mid-channel hydrophone because of its 

proximity to the bottom from which we believe the noise is primarily radiating. The 

spectral characteristics of vibrahammer noise include a strong tonal at 23 Hz, a broad 

level peak in the 2 to 15 Hz band. A second broad peak centered at 30 Hz, and a steep 

roll-off of energy above about 80 Hz. Above 1 kHz, except for the few peaks, the noise 

is generally consistent with the quiet conditions measured near the Liberty site. 

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OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Hydrophone 1: Vibrahammer

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 1. Near-bottom underwater noise at Site NA3 (3-4610). 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Hydrophone 2: Vibrahammer

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 2. Mid channel depth underwater noise at Site NA3 (3-4610). 

16 
OCS Study MMS 2001-0047

A second set of vibrahammer underwater noise measurements is shown in Figures 3.1.2-3 

(near bottom sensor) and 3.1.2-4 (mid depth sensor). The spectral characteristics are 

consistent with the previous set except the levels are roughly 2-5 dB lower. This 

variability should be expected as the coupling of energy into the ground (and thereafter 

into the water) depends on soil conditions at each sheet piling and how the vibrahammer 

is attached to the sheet piling. The higher frequency energy peaks at 700 Hz, 1 kHz, and 

1.3 kHz in this example have generally higher levels and are due to the vibrahammer 

operation as they do not appear in any other data set. 

Airborne radiated noise from the vibrahammer collected at the same two measurement 

times are presented in Figures 3.1.2-5 and 3.1.2-6. In the first example, strong peaks 

dominate the noise environment at 12 and 23 Hz, potentially an harmonically related pair. 

In the second sample, the 12 Hz contribution has disappeared and the 23 Hz energy is 10 

dB lower. As noted above, the differences are thought to be due to the variability in sheet 

piling coupling to variable soil conditions on the island. In both cases, the vibrahammer 

dominates nearly the whole the spectrum from 2 Hz up to about 15 kHz. 

Vibrahammer energy is also significant in the ice producing vibrations that can span the 2 

Hz to 2 kHz band. Figure 3.1.2-7 provides an example. The characteristic vibrahammer 

spectral shape seen in the underwater environment is readily apparent. However, only 

energy below 200 Hz seems to couple well into the ice. 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Hydrophone 1: Vibrahammer and truck noises

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 3. Near bottom underwater noise at Site NA3 (3-1315). 

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OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Hydrophone 2: Vibrahammer and truck noises

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 4. Mid channel depth underwater noise at Site NA3 (3-1315). 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Microphone: Vibrahammer

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 5. Airborne noise at Site NA3 (3-4610). 

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OCS Study MMS 2001-0047

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Microphone: Vibrahammer and truck noises

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 6. Second example of airborne noise at Site NA3 (3-1315). 

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

dB re in*sec per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Geophone: Vibrahammer and truck noises

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.1.2- 7. Ice vibration levels from vibrahammer at Site NA3 (3-1315). 

19 
OCS Study MMS 2001-0047

As was observed along the ice road in Section 3.1.1, plows and trucks operated generally 

continuously. Figures 3.1.2-8 and 3.1.2-9 show the measured underwater noise levels 

collected at site NA3 while these activities were underway around Northstar. The 

vibrahammer was not in operation at this time but undoubtedly generators and related 

equipment were in operation. 

As has been noted before, the lower hydrophone (H1) exhibits a slightly higher noise 

environment. Both sensors show strong manmade noises in the sub-10 Hz region, strong 

tonal presence between 30 and 120 Hz, and significant energy all the way out to 20 kHz 

when compared to the noise measurements collected near the Liberty site. A second set 

of underwater measurements is presented in figures 3.1.2-10 (H1) and 3.1.2-11 (H2). 

These examples were collected during the quietest time period during our data recording 

effort at this site. In general, the broadband noise level is lower but the narrowband 

contribution to the noise field is much more evident. 

Two examples of the in-air noise environment at NA3 when the vibrahammer is not 

operating are shown in figures 3.1.2-12 and 3.1.2-13. Manmade noise dominates the 

airborne noise environment at all frequencies below about 6 kHz. As in the water below, 

numerous narrowband tonals produced by nearby machinery are a significant feature. 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Hydrophone 1: Plowing noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 8. Near bottom underwater noise at Site NA3, plowing noise (3-8100). 

20 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Hydrophone 2: Plowing noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 9. Mid channel depth underwater noise at Site NA3, plowing (8-8100). 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Hydrophone 1: Plows & Trucks

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 10. Near bottom underwater noise at Site NA3; quiet (3-2500). 

21 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Hydrophone 2: Plows & Trucks

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 11. Mid channel depth underwater noise at Site NA3; quiet (3-2500). 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Microphone: Plowing noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 12. Airborne noise at Site NA3; plowing (3-8100). 

22 
OCS Study MMS 2001-0047

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Microphone: Plows & Trucks

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.2- 13. Second example of airborne noise at Site NA3 (3-2500). 

Finally, figures 3.1.2-14 and 3.1.2-15 provide samples of ice vibration levels near 

Northstar when the vibrahammer is off. In these examples, only energy between about 3 

and 140 Hz, both broadband and narrowband, couples well enough into the ice to be 

observable above our system electronic noise floor. 

3.1.3 Site NA2 

Site NA2 was located approximately 2.2 nm (4 km) NNE of the Northstar site. This site 

was specifically chosen to assess the Northstar noise radiation with range. The data were 

collected on 26 April starting at 6:30 PM local time. In a water of depth 35 ft (10.8 m), 

the lower hydrophone was deployed at a depth of 30 ft (9 m) while the mid water sensor 

was at a depth of 18 ft (5.5 m). 

In general, the noise environment at site NA2 showed hardly any apparent manmade 

noise contamination. Two samples of underwater noise spectra, shown in Figures 3.1.3-1 

and 3.1.3-2, are quite close to the conditions measured in [1] under quiet ambient 

conditions (see Figure 2-5) and is approximately 12 dB lower than Knudsen’s sea state 

zero [2]. No narrowband tonals are present and the 1/3 OB levels at both water depths 

are nearly identical. There is a small energy peak in the mid channel hydrophone near 20 

Hz whose source is unknown but may be produced by the ice; see the geophone data 

below. 

23 
OCS Study MMS 2001-0047

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

Site NA3: ~150 m from island

Geophone: Plowing noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

dB re in*sec per Hz and per 1/3 Octave Band Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.1.2- 14. Ice vibration noise spectra at Site NA3; plowing (3-8100). 

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

dB re in*sec per Hz and per 1/3 Octave Band

Site NA3: ~150 m from island

Geophone: Plows & Trucks

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.1.2- 15. Second example of ice vibration noise at Site NA3 (3-2500). 

24 
OCS Study MMS 2001-0047

The in-air noise at site NA2 was very quiet. Winds were light and ice cracking noise was 

not detected aurally. Figure 3.1.3-3 shows a sample spectrum. While a few narrowband 

lines are present, the levels represent very quiet conditions. 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA2, ~4 km NE of NorthStar

Hydrophone 1: Ambient noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.3- 1. Near bottom underwater noise at site NA2. (4-4000) 

Ice vibration noise is shown in Figure 3.1.3-4. The only spectral region with valid data 

above system noise is in the band from 12 to 18 Hz. This appears to be the same peak 

observed with the mid water hydrophone and is probably generated in the ice. 

3.1.4 Site NA5 

Based on the measurements made close to the Northstar artificial island, we attempted to 

collect noise data at increasing ranges to assess the noise level versus range of the 

vibrahammer and other machinery sources at Northstar. Site NA5 was located roughly 

0.54 nm (1 km) to the northeast of the artificial island. The water depth was 39 ft (11.8 

m) and the ice thickness was 6 ft (1.8 m). On April 29th when our measurements were 

made, the winds were light at 5 kts (2.6 m/s) out of the north. Light snow had been 

falling over the last several days resulting in a fresh snow cover of from 2-6 inches (5-15 

cm). The ice was quite rough with ridges rising to heights up to five feet (1.5 m). 

25 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA2, ~4 km NE of NorthStar

Hydrophone 2: Ambient noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.3- 2. Mid channel underwater noise at site NA2. (4-4000) 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA2, ~4 km NE of NorthStar

Microphone: Ambient noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.3- 3. Measured in-air noise levels at site NA2. (4-4000) 

26 
OCS Study MMS 2001-0047

Data collection commenced at 11:45 local time and continued through the construction 

workers’ lunch time and beyond. This provided a unique opportunity to capture the 

radiated noise without vibrahammer operation or the steady stream of trucks and plows 

that were typically operating in the area. 

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

dB re in*sec per Hz and per 1/3 Octave Band

Site NA2, ~4 km NE of NorthStar

Geophone: Ambient noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.1.3- 4. Measured ice vibration levels at site NA2. (4-4000) 

3.1.4.1 Site NA5 During Lunch Break 

The measured underwater noise environment during the lunch time lull is represented in 

Figures 3.1.4-1 and 3.1.4-2. The first figure is from the near bottom hydrophone at a 

depth of 33 ft (10 m) and the second from the mid water hydrophone at 21 ft (6.4 m). 

The dominant noise source is most likely due to electrical generators and/or machinery 

left running. Significant features include the peak near 6-7 Hz and the tonal set from 30 

to 60 Hz. Compared to site LA2 near Liberty reported below in Section 3.2.1 which was 

very quiet, the only spectral regions with manmade sounds are below 10 Hz and from 20 

to 60 Hz. The hydrophone positioned near the bottom exhibits slightly higher levels 

indicating that the energy is likely propagating primarily through the bottom. 

The airborne noise spectra recorded during the lunch break is shown in Figure 3.1.4-3. 

Again comparing these conditions to the quiet period at site LA2, the only manmade, i.e., 

machinery, sounds above ambient at this site are at frequencies below 80 Hz. At 80 Hz 

27 
OCS Study MMS 2001-0047

the two sites had equal levels. The difference increases as frequency decreases so that at 

2 Hz the site NA5 levels are nearly a 15 dB higher than measured at LA2. Also note that 

the microphone picked up a few tonals, notably at about 180 and 290 Hz. When work 

resumes, these tonals are masked by the site crew at work as discussed below. 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Hydrophone 1: Distant Vehicles & Machinery

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 1. Near bottom underwater noise levels at site NA5; lunch break. (8-4400) 

Ice vibration levels 0.54 nm (1 km) away from Northstar at lunch time are shown in 

Figure 3.1.4-4. The only energy detectable above the measurement system noise floor is 

below about 30 Hz and includes three of the tonals observed in the underwater data set 

discussed above. 

3.1.4.1 Site NA5 During Working Hours 

With lunch time over, vibrahammer operations resumed and trucks and plows again were 

active. Multiple time cuts were examined both before and after lunch. They all fell into 

one of two categories: moderate or high level vibrahammer operation. The difference 

between the two is presumed to be due to the different soil conditions encountered as 

each sheet piling was driven into the island base. 

28 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Hydrophone 2: Distant Vehicles & Machinery

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 2. Mid-channel underwater noise levels at site NA5; lunch break. (8-4400) 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Microphone: Distant Vehicles & Machinery

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 3. Measured in-air noise levels at site NA5; lunch break. (8-4400) 

29 
OCS Study MMS 2001-0047

Underwater noise from the two sensors during moderate vibrahammer excitation are 

shown in Figures 3.1.4-5 and 3.1.4-6 for the deep and mid channel hydrophones, 

respectively. The dominant features are the tonal near 23 Hz, the high energy below 10 

Hz, and a few tonals above the strong 23 Hz peak. As before, the near bottom sensor has 

slightly more noise than the mid-channel sensor. Under moderate vibrahammer noise, 

the 1/3 OB containing the 23 Hz tonal was seen to vary in level from 96 to 106 dB re μPa 

on H1 and 92 to 102 dB re μPa on H2. Over the rest of the spectral band, the noise 

spectra levels remained quite consistent (+2 dB). 

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

dB re in*sec per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Geophone: Distant Vehicles & Machinery

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.1.4- 4. Measured ice vibration levels at site NA5; lunch break. (8-4400) 

High level vibrahammer induced underwater noise spectra are presented in Figures 3.1.4-

7 and 3.1.4-8. The differences between this and the moderate operation levels are: 

• much stronger, about 10 dB, noise levels around 7 Hz, 

•

•

higher level 23 Hz tonal sound levels, and 

richer narrowband structure from about 40 to 150 Hz. 

Under these noisier conditions, the 23 Hz tonal was seen to vary from 102 to 112 dB μPa. 

There is also an energy peak centered near 500 Hz. This feature was sometimes observed 

under moderate vibrahammer operation although at a lower level. 

30 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Hydrophone 1: Distant Vehicles & Vibrahammer

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 5. Near bottom underwater noise levels at site NA5; vibrahammer. (8-1200) 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Hydrophone 2: Distant Vehicles & Vibrahammer

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 6. Mid channel underwater noise levels at site NA5; vibrahammer. (8-1200) 

31 
OCS Study MMS 2001-0047

At site NA5, the microphone data included times with the vibrahammer clearly in 

operation and times when the dominant noise was due to distant plowing and truck 

sounds. Two samples of airborne noise with vibrahammer excitation are shown in 

Figures 3.1.4-9 and 3.1.4-10. In the first case, the 23 Hz tonal is clearly evident with 

about a 12 dB signal-to-noise ratio (SNR). Additional tonals in the 350 to 1800 Hz band 

are also present. In the second case, the soil resistance around the sheet piling must have 

be less than the previous case resulting in a much lower level 23 Hz tonal and a 

considerably sparser narrowband tonal set. In general, the quieter example had lower 1/3 

OB levels by from 6 to 10 dB out to 2 kHz. Above 2 kHz, the noise level is only a lower 

bound due to the measurement system noise floor. 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Hydrophone 1: Vibrahammer and truck noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 7. Near bottom underwater noise levels at site NA5; strong vibrahammer. (10-0000) 

When the vibrahammer 23 Hz tonal was not observable with the microphone at site NA5, 

the average 1/3 OB in-air noise levels showed considerable variability. The audible 

sounds were those of the plowing and trucking vehicles which were also present when the 

23 Hz tonal was detectable. However, as will be seen in the ice vibration discussion 

immediately below, even though the 23 Hz tonal noise was not detectable, it was still 

present during these times. Other noise sources, most likely machinery and vehicle 

sounds, overrode the tonal level. 

32 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Hydrophone 2: Vibrahammer and truck noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 8. Mid channel underwater noise levels at site NA5; strong vibrahammer. (10-0000) 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Microphone: Vibrahammer and truck noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 9. In-air noise levels at site NA5; vibrahammer. (10-0000) 

33 
OCS Study MMS 2001-0047

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Microphone: Plowing and truck noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 10. Second sample of in-air noise levels at site NA5; vibrahammer. (9-6000) 

Figures 3.1.4-11 and 3.1.4-12 are two examples when the 23 Hz is not detectable in the 

air. Comparing the two, the former shows 2 to 8 dB higher noise levels in the 8 to 150 

Hz band but 2-3 dB lower noise in the 300 to 1200 Hz region. The 1/3 OB levels are 

nearly comparable to the levels measured with the 23 Hz tonal present. This implies that 

the primary airborne noise sources are the machinery and vehicle noises at Northstar and 

individual samples are dependent on the amount of this activity at the time of data 

collection. 

Ice vibrations at site NA5 were only impacted by Northstar activities at frequencies 

below about 100 Hz. Two representative examples are provided in Figures 3.1.4-13 and 

3.1.4-14. The most energetic measured vibration levels are shown in Figure 3.1.4-13. 

The dominant features are the narrowband tonals at 23, 39, 50, 61, 75 and 100 Hz plus 

the broad low frequency noise hump below 20 Hz. In-air noise levels at this time were 

shown in Figure 3.1.4-9 while the underwater noise levels were presented in Figures 

3.1.4-7 and 3.1.4-8. All four plots show a strong 23 Hz tonal. 

34 
OCS Study MMS 2001-0047

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Microphone: Distant Vehicles & Vibrahammer

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 11. Measured in-air noise levels at site NA5; no vibrahammer. (8-1200) 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Microphone: Distant Vehicle & Machinery Noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.4- 12. Second example of in-air noise levels at site NA5; no vibrahammer. (10-1300) 

35 
OCS Study MMS 2001-0047

In the second, quieter ice vibration example, the low frequency hump is still prominent 

but fewer significant tonals are present (23, 38, 50, and 73 Hz). Measured in-air noise 

shown in Figure 3.1.4-12 did not exhibit a strong 23 Hz tonal. However, both 

hydrophones (spectra not provided) had strong 23 Hz and other tonal components 

present. These observations lead to the conclusion that the vibrahammer noise 

propagates to the ice primarily through the ground and then briefly into the water. It is 

unclear whether the in-air vibrahammer noise is due to direct through-air propagation or 

is radiated from the ice into the air. 

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

dB re in*sec per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Geophone: Vibrahammer and truck noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.1.4- 13. Ice vibration noise levels at site NA5; vibrahammer. (10-0000) 

3.1.5 Site NA6 

Site NA 6 was occupied on 29 April 2000 with data collection starting at 4:23 PM local 

time. The location was about 1.1 nm (2 km) from the Northstar Prospect. Weather 

conditions were light snow with gusty winds around 10 kts (5.2 m/s) from the west. The 

water depth was 41 ft (12.4 m) with an ice thickness of 6 ft (1.8 m). The near bottom 

hydrophone was lowered to a depth of 33 ft (10 m) while the mid-channel sensor was at 

21 ft (6.5 m). In general, the ambient noise conditions were very quiet with occasional 

ice clicks and pops. 

36 
OCS Study MMS 2001-0047

Two examples of the conditions at NA6 are presented: one with strong vibrahammer 

noise and the other without. Figures 3.1.5-1 and 3.1.5-2 are spectra from the lower and 

mid-channel hydrophones respectively. Both exhibit the strong 23 Hz tonal, high low 

frequency noise levels below 10 Hz, and a tonal near 50 Hz. We hypothesize that the 50 

Hz tonal is a machinery line and the low frequency energy is due to vehicle and 

machinery noise at Northstar. 

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

dB re in*sec per Hz and per 1/3 Octave Band

Site NA5, ~1 km NE of NorthStar

Geophone: Vibrahammer & Distant Vehicles

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.1.4- 14. Measured ice vibration levels at site NA5; vibrahammer. (10-1300) 

Figures 3.1.5-3 and 3.1.5-4 show spectra for the same two sensors when the 23 Hz 

vibrahammer sound is not present. The other features, namely the 50 Hz tonal and the 

high low frequency energy below 10 Hz are still present, albeit somewhat reduced in 

level. Over the rest of the band, the levels are nearly identical to those observed with the 

vibrahammer in operation. In both pairs of spectra, the deeper sensor exhibits slightly 

higher noise levels indicating noise radiation from the bottom. 

37 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA6, ~2 km from NorthStar

Hydrophone 1: Vibrahammer and Ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.5- 1. Near bottom underwater noise levels, site NA6; vibrahammer. (11-1300) 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA6, ~2 km from NorthStar

Hydrophone 2: Vibrahammer and Ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.5- 2. Mid channel underwater noise levels, site NA6; vibrahammer. (11-1300) 

38 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA6, ~2 km from NorthStar

Hydrophone 1: Distant Vehicles & Ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.5- 3. Near bottom underwater noise levels at site NA6; no vibrahammer. (11-5330) 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site NA6, ~2 km from NorthStar

Hydrophone 2: Distant Vehicles & Ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.5- 4. Mid channel underwater noise levels at site NA6; no vibrahammer. (11-5330) 

39 
OCS Study MMS 2001-0047

Measured in-air noise levels for the same two time periods are shown in Figures 3.1.5-5 

and 3.1.5-6. Both exhibit no significant narrowband spectral features. In the first case, 

the vibrahammer is in operation and the 1/3 OB noise levels are 2-10 dB higher than the 

second case without the vibrahammer excitation. In the band from 4 to 800 Hz, the 

narrowband noise spectral noise falls at a rate of about 10 dB per octave and the 1/3 OB 

noise decreases at 7 dB per octave. 

As seen at site NA5, even when the microphone can’t detect the vibrahammer’s 23 Hz 

tonal, the geophone can detect the tonal. Figure 3.1.5-7 presents measured ice vibration 

levels with the vibrahammer in operation. The 23 Hz tonal is clearly evident. Only 

energy at 23 Hz and below 10 Hz is above the measurement system noise. The very low 

frequency noise is most likely due to vehicle and machinery noise at Northstar. In Figure 

3.1.5-8, the 23 Hz tonal has disappeared leaving only measurable low frequency energy 

between about 2 and 6 Hz. 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA6, ~2 km from NorthStar

Microphone: Vibrahammer and Ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.5- 5. Measured in-air noise levels at site NA6; vibrahammer. (11-1300) 

40 
OCS Study MMS 2001-0047

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site NA6, ~2 km from NorthStar

Microphone: Distant Vehicles & Ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.5- 6. Measured in-air noise levels at site NA6; no vibrahammer. (11-5330) 

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

dB re in*sec per Hz and per 1/3 Octave Band

Site NA6, ~2 km from NorthStar

Geophone: Vibrahammer and Ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.1.5- 7. Measured ice vibration levels at site NA6; vibrahammer. (11-1300) 

41 
OCS Study MMS 2001-0047

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

dB re in*sec per Hz and per 1/3 Octave Band

Site NA6, ~2 km from NorthStar

Geophone: Distant Vehicles & Ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.1.5- 8. Measured ice vibration levels at site NA6; no vibrahammer. (11-5330) 

3.1.6 BSMP Site 5(5) 

Site 5(5) is one of the Beaufort Sea Monitoring Posts located roughly 8.1 nm (15 km) 

from the Northstar prospect in 21 ft (6.5 m) of water. The site was occupied on 27 April 

with data collection beginning at 7:54 PM local time. Winds were from the north at 9 kts 

(4.6 m/s). In the distance to the northwest, a caravan of vehicles was observed which 

were thought to be a seismic crew. However, no evidence of seismic noise was detected. 

The underwater noise field was sampled with H1 at a depth of 16 ft (5 m) and H2 at 10 ft 

(3 m). Two data segments are presented, one with low winds and the second with higher 

gusting winds. 

Figures 3.1.6-1 and 3.1.6-2 show measured noise spectra for the deeper hydrophone 

under both wind conditions. The average 1/3 OB levels are nearly identical. However, 

the 95th percentile level is considerably higher over the band from 40 Hz to 2 kHz. The 

same is true of the noise field at mid depth as shown in Figures 3.1.6-3 and 3.1.6-4. The 

noise is quite similar to those measured at site NA2 2.2 nm (4 km) from Northstar except 

for a much more significant energy peak centered at 22 Hz and a broad increase in the 

noise level from 1 to 5 kHz. 

42 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site 5(5)

Hydrophone 1: Ambient noise, low winds

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.6- 1. Near bottom underwater noise levels at site 5(5); lower winds. (5-0400) 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site 5(5)

Hydrophone 1: Ambient noise, higher winds

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.6- 2. Near bottom underwater noise levels at site 5(5); higher winds. (5-6050) 

43 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site 5(5)

Hydrophone 2: Ambient noise, low winds

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.6- 3. Mid channel underwater noise levels at site 5(5); lower winds. (5-0400) 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site 5(5)

Hydrophone 2: Ambient noise, higher winds

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.6- 4. Mid channel underwater noise levels at site 5(5); higher winds. (5-6050) 

44 
OCS Study MMS 2001-0047

Airborne noise at site 5(5) was higher than that observed at site NA2 under lower wind 

conditions. Figure 3.1.6-5 is a typical example. Below about 2 kHz, the noise level here 

is approximately 10 dB higher that at NA2. The other significant feature is a peak near 

200 Hz, possibly caused by the distant caravan noted above. Data above 2 kHz is 

approaching the system noise floor limit. 

The ice vibration levels at 5(5) were, as at site NA2, generally at or below our system 

noise floor (figure 3.1.6-6). As at NA2, only the small band from 12 to 30 Hz shows any 

sign of excitation above the noise floor. Here, the energy centered at about 22 Hz has a 

higher level by roughly 10 dB than at NA2 based on the narrowband spectra. It is not 

clear whether this excitation is from the same source as observed at NA2. The 5(5) peak 

is slightly higher in frequency. It is unlikely that the source of this vibration is 

vibrahammer operation at Northstar. Vibrahammer data collected at sites NA3, NA5, 

and NA6 all show a stable narrowband tonal at a slightly higher frequency and with a 

much smaller bandwidth. 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site 5(5)

Microphone: Ambient noise, low winds

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.1.6- 5. Measured in-air noise levels at site 5(5). (5-0400) 

45 
OCS Study MMS 2001-0047

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

Site 5(5)

Geophone: Ambient noise, low winds

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

dB re in*sec per Hz and per 1/3 Octave Band Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.1.6- 6. Measured ice vibration levels at site 5(5). (5-0400) 

3.2 Liberty Site 

Data were collected at two sites at/near the future Liberty site. Aural examination of the 

data indicates that the only manmade noise was most likely the faint sounds from the 

Endicott flare. Otherwise, only natural ambient noises were present. The ice at both sites 

was quite smooth compared to the west and the snow was deeper, approaching 10 inches 

(25 cm) in places. 

3.2.1 Site LA2 

Site LA2 is located approximately 2.7 nm (5 km) NW of the future Liberty site. It is just 

about in the center of the Boulder Patch. The water depth was 20 ft (6 m) and the ice was 

5 ft (1.6 m) thick. Winds were calm and the noise underwater and in the air were very 

quiet. Data collection started at 1 PM local time on 28 April. 

Underwater noise spectra from the near bottom hydrophone at a 16 ft (5 m) depth and the 

mid depth sensor at 10 ft (3 m) are shown in Figures 3.2.1-1 and 3.2.1-2. Both plots have 

spectra nearly identical to those collected at site NA2. The only significant differences 

are a rise in noise level at high frequencies above about 7 kHz and higher levels of noise 

in the 20-30 Hz band. As before, the mid depth sensor measured a higher level of this 

20-30 Hz energy than the near bottom hydrophone. As at NA2, this low frequency 

energy is also present in the ice vibration. 

46 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site LA2, near Liberty

Hydrophone 1: Quiet ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.2.1- 1. Near bottom underwater noise level measured at site LA2 (6-1200). 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site LA2, near Liberty

Hydrophone 2: Quiet ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.2.1- 2. Mid channel underwater noise level measured at site LA2 (6-1200). 

47 
OCS Study MMS 2001-0047

In-air noise at LA2 is presented in Figure 3.2.1-3 while 3.2.1-4 displays the ice vibration 

spectrum collected using the geophone. The airborne noise level is also similar to those 

measured at NA2 except that the level approximately 5 dB quieter below 20 Hz. Energy 

in narrowband tonals is low with fewer lines present compared to NA2. As at NA2, the 

geophone detected very little energy except between 20 and 30 Hz. 

3.2.2 Site LA3 

Liberty site LA3 is located where Liberty Prospect development will take place. The 

scientific team collected data starting around 4:30 PM local time. Winds were light and 

the measured data are almost identical to the conditions encountered at LA2. 

With a water depth of 20 ft (6.1 m), the two hydrophones were deployed to the same 

depths used at LA2, namely, 10 and 16 ft (3 and 5 m). The spectra in Figures 3.2.2-1 and 

3.2.2-2 display the data from the near bottom and mid depth sensors, respectively. The 

noise levels are comparable to the data collected to LA2 except that the increase above 7 

kHz is not present. As encountered before, the energy peak in the 20-30 Hz band is much 

more energetic at mid water column than near the bottom. These data represent quiet 

conditions with potentially some contribution from the Endicott flare to the west. 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site LA2, near Liberty

Microphone: Quiet ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.2.1- 3. Airborne noise level measured at site LA2 (6-1200). 

48 
OCS Study MMS 2001-0047

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

Site LA2, near Liberty

Geophone: Quiet ambient

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Level, dB re in/sec per Hz and per 1/3 OB dB re in*sec per Hz and per 1/3 Octave Band

Figure 3.2.1- 4. Ice vibration noise level measured at site LA2 (6-1200). 

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site LA3, near Liberty

Hydrophone 1: Ambient noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.2.2- 1. Near bottom underwater noise level measured at site LA3 (7-5000). 

49 
OCS Study MMS 2001-0047

100 101 102 103 104 10

20

30

40

50

60

70

80

90

100

110

120

Frequency, Hz

dB re uPa per Hz and per 1/3 Octave Band

Site LA3, near Liberty

Hydrophone 2: Ambient noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.2.2- 2. Mid channel underwater noise level measured at site LA3 (7-5000). 

The only sensor with which we could detect the Endicott flare was the microphone. It 

sounded like a low rumble which faded in and out. The data from LA3 contained a time 

period when the flare was faintly but consistently evident and a time when the flare was 

undetectable aurally. Spectra from these two periods are presented in Figures 3.2.2-3 and 

3.2.3-4. The noise levels are practically identical except in the 70 to 500 Hz band where 

as much as a 10 dB increase is seen in Figure 3.2.2-3. With no other evidence, we 

conclude that this rise in energy is due to the flare located approximately 10.3 nm (19 

km) to the west-north-west. 

Measured ice vibration levels at LA3 are shown in Figure 3.2.3-5. As noted at LA2 and 

NA2, the only observable energy above the noise floor is in the 20-30 Hz band, the same 

spectral region that peaks in the mid depth hydrophone data. The source of this energy is 

unknown at this time. 

50 
OCS Study MMS 2001-0047

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site LA3, near Liberty

Microphone: Endicott Flare Noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.2.2- 3. Airborne noise level with flare measured at site LA3 (7-5000). 

100 101 102 103 104 −20

−10

0

10

20

30

40

50

60

70

80

90

Frequency, Hz

dB re 20 uPa per Hz and per 1/3 Octave Band

Site LA3, near Liberty

Microphone: Ambient noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Figure 3.2.2- 4. Airborne noise level with no audible flare measured at site LA3 (7-1200). 

51 
OCS Study MMS 2001-0047

100 101 102 103 104 −160

−150

−140

−130

−120

−110

−100

−90

−80

−70

−60

−50

Frequency, Hz

dB re in*sec per Hz and per 1/3 Octave Band

Site LA3, near Liberty

Geophone: Ambient noise

1/3 Octave 

5% & 95% Limits 

1/3 Oct. Noise Floor

Narrow Band 

Level, dB re in/sec per Hz and per 1/3 OB

Figure 3.2.2- 5. Ice vibration noise spectrum measured at site LA3 (7-5000). 

52 
OCS Study MMS 2001-0047

4.0 Discussion 

4.1 Underwater Noise Environment 

Underwater man-made noise emanating from Northstar island is primarily caused by 

three sources: vibrahammer operation, machinery noise such as diesel electric generators, 

and vehicle operations including plowing and truck movement. When driving sheet piles 

on the island, the vibrahammer produces the highest noise levels observed during our 

measurement period. 

A vibrahammer is a heavy device that is attached to a standing sheet pile and caused to 

vibrate vertically driving the piling into the ground. The open-cell pilings are driven into 

the ground to a top elevation of about 7 ft (2 m) above mean lower low water (MLLW) 

along the south side of the island for a docking area [3]. Sheet piling are also installed to 

reduce potential contaminant releases into the marine environment by preventing damage 

to island facilities [3]. Energy is thought to be supplied by hydraulic motors which in 

turn are powered by diesel generators. Thus this suite of systems creates a rich noise 

environment. Depending on the soil conditions encountered by the sheet piling, the load 

on this suite can vary considerably and consequently the radiated noise levels, in the air 

and through the ground, fluctuate accordingly. 

Vibrahammer noise is easily recognizable due to its strong, quite narrowband, tonal at 23 

Hz. This tonal radiates out to at least a range of 1.1 nm (2 km) and probably further. 

Measured levels as a function of range are shown in figure 4.1-1. These data are 2 

minute averages and were taken over several days. As noted above, the levels vary 

significantly from piling to piling; more than 15 dB at site NA5 0.54 nm (1 km) from 

Northstar. Because of this variability, the transmission loss as a function of range cannot 

be determined from this data set. However, spectra calculated over the same time period 

invariably resulted in higher levels measured near the sea bottom compared to the sensor 

located half way down the water column. This implies that the primary propagation path 

is through the earth and into the water column rather than directly into the water column 

at Northstar. 

At ranges beyond 1.1 nm (2 km), no clear evidence of the vibrahammer’s 23 Hz tonal 

was observed. This may have been due to data collection occurring at times when the 

vibrahammer was not operational. However, an interesting spectral peak spanning 

several Hertz in the 20-25 Hz region was observed at sites 5(5), LA2, and LA3; see, for 

example figure 3.1.6-1. It is possible that this energy is caused by the vibrahammer but 

without corroborating evidence, the association is not clear. 

Similar measurements of tonal levels were made in the Prudhoe Bay area in March of 

1979 [4]. Low frequency tonal levels (<50 Hz) versus range were recorded near the 

Reindeer Island Cost Well (on a natural barrier-beach island) and the Niakuk 3 Well (on 

a man-made gravel island). The trend in that data set showed a 30-40 dB per decade 

spreading loss plus an absorption loss of about 1 dB per mile. The data presented in 

figure 4.1-1 show a 25 dB/decade spreading loss which is less than observed in [4]. 

53 
OCS Study MMS 2001-0047

However, the earlier data were obtained in water depths of only 10-23 ft (3-7m) and 

transmission loss is expected to be higher in shallow water. 

Vibrahammer 23 Hz T onal Level vs Range

8 0

8 5

9 0

9 5

100

105

110

115

120

0 0.5 1 1.5 2

Range, km

Level, dB re 1 uPa

H1

H2

Figure 4.1- 1. Vibrahammer 23 Hz tonal spectrum level as a function of range as received 

on the near-bottom hydrophone (H1) and mid-channel depth hydrophone (H2). 

From normal mode theory, the acoustic cutoff frequency (Fc) for a 33 ft (10 m) water 

channel over a hard bottom is 

 Fc = c/4d = 37.5 Hz, 

where c is the speed of sound in water (approximately 4920 ft/s or 1500 m/s) and d is the 

water depth. Thus the vibrahammer energy and low frequency sound measured must 

involve propagation through the bottom water-saturated sediment. This means that the 

bottom sediments are acoustically similar to the water column since there is no indication 

of a low frequency cutoff in propagation as also observed in [4] and therefore the 

effective propagation channel depth near Northstar is larger than 33 ft (10 m) and the 

cutoff frequency lower than predicted using this shallow water depth.

The other significant feature observed underwater during vibrahammer operation is an 

increase in energy below 10 Hz. A comparison of figures 3.1.4-5 (with vibrahammer 

noise) and 3.1.4-1 (without vibrahammer excitation), both taken from hydrophone 1 at 

site NA5 0.54 nm (1 km) from Northstar shows that noise below 10 Hz is 5-15 dB higher 

in the former case. Above 60 Hz, the measured noise levels with and without 

vibrahammer operation are nearly identical indicating that noise in this band is not 

coming from the vibrahammer itself. The comparison also shows that the narrowband 

54 
OCS Study MMS 2001-0047

lines seen in both are not specifically related to the vibrahammer and are more likely 

associated with machinery sources on Northstar. 

When the vibrahammer is not driving sheet piles, the radiated noise is dominated by 

vehicle noise and machinery. These noise sources are primarily confined to frequencies 

below about 500 Hz. They consist of broadband noise and narrowband tonals which are 

most likely caused by machinery. 

At a range of 0.08 nm (150 m) from Northstar (site NA3), the man-made noise consists of 

a broad band peak centered around 7 Hz and a large set of narrowband tonals spanning 

the frequency band from 25 to 500 Hz; for example, see figure 3.1.2-10. Moving out to a 

range of 0.54 nm (1 km) (site NA5), the low frequency peak has fallen by 10-15 dB and 

the only man-made sounds are relegated to the spectral region below 70 Hz (see figure 

3.1.4-1). 

Figure 4.1-2 presents the measured 1/3 OB noise spectra collected at four sites near 

Northstar plus site 5(5) and site LA2. All spectra from sites NA3, NA5, and NA6 were 

calculated while the vibrahammer was in operation. It is not known if the vibrahammer 

was operational during the other analyzed segments. 

Close to Northstar at NA3 (range = 0.08 nm or 0.15 km), underwater noise is dominated 

by radiation from the island below 500 Hz, rising as much as 45 dB above quiet 

background noise as represented by the spectra taken at site NA2. Half a nautical mile (1 

 

100 101 102 103 104 40

50

60

70

80

90

100

110

120

Frequency, Hz

Level, dB re 1 μPa per 1/3 OB

H1 1/3 OB Noise Spectra Comparison

NA3 

NA5 

NA6 

NA2 

5(5)

LA2 

Range to Northstar 

NA3 0.15 km 

NA5 1 km 

NA6 2 km 

NA2 4 km 

5(5) 15 km 

LA2 44 km

Figure 4.1- 2. Comparison of 1/3 OB spectra at multiple sites collected during vibrahammer 

operation using the hydrophone positioned near the bottom (H1). 

55 
OCS Study MMS 2001-0047

km) from Northstar (site NA5), man-made noise dominates only in the spectral region 

below about 100 Hz. And at a range of 1.1 nm (2 km) (NA6), only energy in the 23 Hz 

tonal and at very low frequencies below about 7 Hz is detectable above ambient levels. 

Figure 4.1-3 shows the measured 1/3 OB noise spectra for the mid-water column depth 

hydrophone (H2) collected at the same sites and times as shown in the previous figure 

with the vibrahammer in operation. As with H1, close to Northstar the underwater noise 

field is dominated by radiation from the island below 500 Hz, again rising as much as 45 

dB above quiet background noise levels. One kilometer (.54 nm) from Northstar, manmade noise dominates only in the spectral region below about 100 Hz. And at a range of 

1.1 nm (2 km) (NA6), again only energy in the 23 Hz tonal and at very low frequencies 

below about 7 Hz is detectable above normal ambient conditions. Unlike the 1/3 OB data 

from H1, both sites 5(5) and LA2 have a spectral peak near 23 Hz. While it is possible 

this energy is due to vibrahammer operation, it is unlikely because of the long range and 

the fact that H1 did not have the same characteristic. The cause, therefore, is unknown. 

100 101 102 103 104 40

50

60

70

80

90

100

110

120

Frequency, Hz

Level, dB re 1 μPa per 1/3 OB

H2 1/3 OB Noise Spectra Comparison

NA3 

NA5 

NA6 

NA2 

5(5)

LA2 

Range to Northstar 

NA3 0.15 km 

NA5 1 km 

NA6 2 km 

NA2 4 km 

5(5) 15 km 

LA2 44 km

Figure 4.1- 3. Comparison of 1/3 OB spectra at multiple sites collected during vibrahammer 

operation using the hydrophone positioned in the middle of the water column (H2). 

Figures 4.1-4 and 4.1-5 present comparisons of measured 1/3 OB spectral levels at the 

same sites when the vibrahammer was not in operation for H1 and H2, respectively. In 

both instances, the underwater sound energy at NA3 is dominated by island noise 

particularly in the band centered around about 8 Hz and in the band from 20 to 200 Hz. 

In general, the levels are roughly comparable with the near-bottom sensor receiving 

56 
OCS Study MMS 2001-0047

slightly higher levels. At ranges beyond 1.1 nm (2 km), the man-made noise contribution 

to the ambient noise conditions is low to negligible across the measurement band. 

The underwater data described above are consistent with the results obtained from other 

measurements of radiated noise radiated from activities on icebound islands. These are 

documented in [5], section 6.4. 

100 101 102 103 104 40

50

60

70

80

90

100

110

120

Frequency, Hz

Level, dB re 1 μPa per 1/3 OB

H1 1/3 OB Noise Spectra Comparison

NA3 

NA5 

NA6 

NA2 

5(5)

LA2 

Range to Northstar 

NA3 0.15 km 

NA5 1 km 

NA6 2 km 

NA2 4 km 

5(5) 15 km 

LA2 44 km

Figure 4.1- 4. Comparison of 1/3 OB noise spectra at multiple sites without vibrahammer 

noise collected using the Hydrophone positioned near the bottom (H1). 

4.2 In-air Noise Environment 

The impact of airborne noise radiating from Northstar has a smaller footprint (i.e., manmade sounds rising above naturally occurring ambient noise levels) than island generated 

noise impacts the underwater environment. Except for the 23 Hz tonal due to the 

vibrahammer and some moderately significant noise in the 500 Hz to 1.5 kHz band, at 

ranges 0.54 nm (1 km) and greater, no man-made sounds were detectable using the 1/3 

OB data. 

Figure 4.2-1 is a comparison of 1/3 OB measured in-air noise levels at 6 measurement 

sites. The vibrahammer was in operation during the data segments displayed for sites 

NA3, NA5, and NA6. As can be seen, at short ranges from Northstar, island noise 

dominates the airborne noise over almost the whole measurement band (10 Hz to about 

12 kHz). But the measurements at site NA5, only 0.54 nm (1 km) from Northstar, the 

only apparent man-made energy above the background noise is due to the 23 Hz tonal 

and the .5-1.5 kHz energy. The variability in the broadband characteristics is thought to 

57 
OCS Study MMS 2001-0047

be due to varying wind conditions. The lower noise levels measured at sites LA2 and 

NA2 encountered the lowest wind speeds (see Table 1-1). 

100 101 102 103 104 40

50

60

70

80

90

100

110

120

Frequency, Hz

Level, dB re 1 μPa per 1/3 OB

H2 1/3 OB Noise Spectra Comparison

NA3 

NA5 

NA6 

NA2 

5(5)

LA2 

Range to Northstar 

NA3 0.15 km 

NA5 1 km 

NA6 2 km 

NA2 4 km 

5(5) 15 km 

LA2 44 km

Figure 4.1- 5. Comparison of 1/3 OB noise spectra at multiple sites without vibrahammer 

noise collected using the Hydrophone positioned in the middle of the water column (H2) 

The airborne man-made noise (radiated while driving sheet pilings) in the spectral region 

above a few hundred Hertz measured close to Northstar is driven by a rich set of 

reasonably narrow energy peaks as was shown in figures 3.1.2-5 and 3.1.2-6. At a range 

of 0.54 nm (1 km), these peaks have fallen by about 40 dB as can be seen by comparing 

the levels in figure 3.1.4-5 with those read from figure 3.1.2-5. The energy levels and 

spectral width of these peaks is significantly less when the vibrahammer is off as 

discussed next. 

When the vibrahammer is not in operation, the airborne noise levels are considerably 

lower near Northstar. Figure 4.2-2 presents a comparison of measured 1/3 OB noise 

levels at 3 sites near Northstar. Vehicle and machinery noise drive the in-air noise levels 

at site NA3 but appear to have minimal impact at site NA5 located 0.54 nm (1km) from 

Northstar. The increased noise level at site NA6 from 100-500 Hz is unexplained. 

58 
OCS Study MMS 2001-0047

100 101 102 103 104 0

10

20

30

40

50

60

70

80

90

Frequency, Hz

Mic 1/3 OB Noise Spectra Comparison

NA3 

NA5 

NA6 

NA2 

5(5)

LA2 

Level, dB re 1 μPa per 1/3 OB Level, dB re 20 

μPa per 1/3 OB

Range to Northstar 

NA3 0.15 km 

NA5 1 km 

NA6 2 km 

NA2 4 km 

5(5) 15 km 

LA2 44 km

Figure 4.2- 1. Comparison of 1/3 OB airborne noise spectra at multiple sites with 

vibrahammer in operation at least at sites NA3, NA5, and NA6. 

100 101 102 103 104 0

10

20

30

40

50

60

70

80

90

Frequency, Hz

Level, dB re 1 μPa per 1/3 OB

Mic 1/3 OB Noise Spectra Comparison

NA3

NA5

NA6

Level, dB re 20 

μPa per 1/3 OB

Range to Northstar 

NA3 0.15 km 

NA5 1 km 

NA6 2 km 

Figure 4.2- 2. Comparison of 1/3 OB airborne noise spectra at multiple sites without 

vibrahammer excitation. 

59 
OCS Study MMS 2001-0047

4.3 Ice Vibration Environment 

Ice vibrations at the sites located away from Northstar were in general at or below the 

noise floor of the measurement system. Thus the measured levels above 200 Hz 

represent an upper limit. Two factors contribute to the low vibration levels. First, the ice 

is shore-fast and not moving, and second, the stable and reasonably warm temperatures 

coupled with the snow cover that acts as an insulating layer between the air and the ice 

effectively stopped thermal ice cracking noise. 

Near Northstar, man-made noise dominates the ambient noise in the frequency band 

below 200 Hz. Figure 4.3-1 displays measured 1/3 OB ice vibration spectra from 6 sites. 

The vibrahammer was in operation at sites NA3, NA5, and NA6 in this data sample. It is 

not known if the vibrahammer was working during the other data samples but the spectral 

peaks suggest that it was operating. However, the narrowband spectra at sites 5(5) and 

LA2 (see figures 3.1.6-6 and 3.2.1-4) show a broad peak in the 20-30 Hz band that is up 

to 10 Hz wide. This broadened peak is not consistent with the quite narrowband 23 Hz 

tonal observed at sites closer to Northstar (for example, figures 3.1.4-13 and 3.1.5-7). A 

mechanism that could expand a narrowband tonal to such an extent is not known. 

Therefore, the source of the 1/3 OB energy in the 24 Hz band cannot be defined and may 

possibly be due to some noise or vibration contamination caused by the measurement 

equipment. 

100 101 102 103 104 −140

−130

−120

−110

−100

−90

−80

Frequency, Hz

Level, dB re 1 μPa per 1/3 OB

Geo 1/3 OB Noise Spectra Comparison

NA3 

NA5 

NA6 

NA2 

5(5)

LA2 

Range to Northstar 

NA3 0.15 km 

NA5 1 km 

NA6 2 km 

NA2 4 km 

5(5) 15 km 

LA2 44 km

Level, dB re in/sec per 1/3 OB

Figure 4.3- 1. Comparison of 1/3 OB ice vibration noise spectra at multiple sites with 

vibrahammer in operation at least at sites NA3, NA5, and NA6. 

60 
OCS Study MMS 2001-0047

Measured ice vibration spectral shape varies considerably from one site to another. Close 

in at NA3, the vibrahammer and other island noises exhibit a broad peak from 8 to 20 Hz, 

the 23 Hz tonal, and decaying energy from 30 Hz up to 200Hz. At a range of 0.54 nm (1 

km; site NA5), island induced ice vibrations are most significant in the 2-15 Hz band 

probably due to machinery excitation plus two narrow spectral regions centered on 23 

and about 50 Hz. The higher frequency peak is most likely due to generator vibration. 

At a 1.1 nm (2 km) range (site NA6), only Northstar energy below 8 Hz and in the 23 Hz 

tonal was detectable above the measurement equipment noise floor. And as noted above, 

the source of the energy in the 1/3 OB centered at 24 Hz for sites NA6, 5(5), and LA2 is 

not known. When the vibrahammer was known to be in operation, the level of the 23 Hz 

narrowband tonal versus range is shown in figure 4.3-2. A comparison of the measured 

vibrahammer 23 Hz tonal level versus range collected underwater in figure 4.1-1 with the 

range dependence of the ice vibration levels depicted in figure 4.3-2 shows that the falloff is comparable. This indicates that the ice is being driven by water-borne energy 

rather than direct transmission from the island into the ice. 

Vibrahammer 23 Hz Tonal Level vs Range

-125

-120

-115

-110

-105

-100

-95

-90

-85

-80

0 0.5 1 1.5 2 2.5

Range, km

Vibration Level, dB re in/sec

Figure 4.3- 2. Vibrahammer 23 Hz tonal received ice vibration spectrum level as a 

function of range. 

Figure 4.3-3 presents 1/3 OB ice vibration levels for three sites while the vibrahammer 

was not in operation. Trucking and machinery are seen to be the dominant source of ice 

vibrations below about 150 Hz. The ice vibration levels fall off significantly at a 0.54 nm 

61 
OCS Study MMS 2001-0047

(1 km) range (NA5) and except for the man-induced vibrations below 10 Hz, have 

disappeared at a range of 1.1 nm (2 km) (site NA6). 

100 101 102 103 104 −140

−130

−120

−110

−100

−90

−80

Frequency, Hz

Level, dB re 1 μPa per 1/3 OB

Geo 1/3 OB Noise Spectra Comparison

NA3

NA5

Range to Northstar NA6

NA3 0.15 km 

NA5 1 km 

NA6 2 km 

Level, dB re in/sec per 1/3 OB

Equipment Noise 

Floor Limit

Figure 4.3- 3. Comparison of 1/3 OB ice vibration noise spectra at multiple sites 

without vibrahammer excitation. 

62 
OCS Study MMS 2001-0047

5.0 Summary and Conclusions 

In April of 1999, environmental noise and vibration levels were measured at eight sites 

near and between the Northstar and Liberty prospects in the nearshore portion of the 

outer continental shelf in the Beaufort Sea near Prudhoe Bay. The goal was to 

characterize the site-specific underwater and in-air noise and ice vibration environment. 

Data were collected using two hydrophones (one near the sea bottom and one in the 

middle of the water column), a microphone positioned roughly 5 ft (1.5 m) above the ice 

about 100 ft (30 m) from the hydrophone hole, and a geophone which was frozen into the 

ice surface near the hole. 

The Northstar prospect was under construction including sheet pile driving, trench 

backfilling operations, plowing operations, and trucking activity. In addition, machinery 

on the artificial island was in operation to supply power and operate the vibrahammer 

used to drive the sheet piles. All of these noise sources contributed to a rich noise 

environment close to Northstar. 

The Northstar noise environment was measured under two conditions: 1) high level noise 

conditions during vibrahammer operation with vehicle and machinery sound sources, and 

2) moderate noise conditions with truck and plow activity with machinery in operation, 

but without vibrahammer noise. 

5.1 Noise and Vibration During Northstar Vibrahammer Operation 

The noisiest conditions were encountered near Northstar at site NA3 (located 

approximately 0.08 nm (150 m) from the island, see Table 1-1) while sheet piles were 

driven into the island soil using a vibrahammer. Vibrahammer and related machinery 

produced 1/3 OB noise levels as much as 50 dB above ambient noise conditions as 

derived from measurements at far removed sites such as 5(5) and LA2. The dominant 

radiated noise feature from the vibrahammer was a 23 Hz tonal which was easily detected 

in the underwater environment and within the ice at least out to a range of 1.1 nm (2 km). 

Our in-air microphone only detected the 23 Hz tonal out to a range of 0.54 nm (1 km). 

During vibrahammer operation, the noise and vibration levels varied as a function of 

range from Northstar. At a distance of 0.08 nm (150 m), the 1/3 OB underwater noise 

environment was dominated by island noise in the spectral region below about 700 Hz. 

Third octave band levels varied from 10 to 50 dB above ambient levels. Often, the third 

OB noise was driven by one or a few narrowband tonals with the band. See for example, 

figure 3.1.2-3. At a range of 0.54 nm (1 km) from Northstar (site NA5), vibrahammer 

noise only impacted the underwater environment in the spectral region below about 120 

Hz with 1/3 OB levels ranging from 5-45 dB above ambient. Finally, at a range of 1.1 

nm (2 km) (site NA6), the underwater noise field is only impacted by vibrahammer 

induced noise in the 23 Hz tonal and at low frequencies below 8 Hz. At longer ranges, 

there was evidence of man-made noise near 23 Hz, but its narrowband spectral 

characteristics were not consistent with those observed at closer range and therefore it is 

not clear if this energy was due to vibrahammer operation. 

63 
OCS Study MMS 2001-0047

In all cases, the underwater noise due to man-made sounds was higher near the bottom 

compared to measurements taken at mid water column depth. This implies that the 

dominant propagation mode for vibrahammer noise is through the earth and then locally 

into the water rather than into the water near the island and thence forth through the water 

channel. 

The contribution of man-made sound to the airborne noise environment during 

vibrahammer operation is primarily a local effect. At a range of 0.08 nm (150 m) from 

Northstar, vibrahammer, vehicle and machinery can noise dominate the 1/3 OB spectral 

region from 8 Hz up to about 12 kHz. Measured levels varied 0-50 dB above ambient 

noise conditions with the highest levels above ambient in the 200 Hz to 8 kHz bands. 

And often the 1/3 OB levels were driven by narrowband tonals (e.g., figure 3.1.2-5). 

Further out in range at 0.54 nm (1 km), the only man-made airborne energy detectable 

above the ambient noise was in the 1/3 OB containing the 23 Hz tonal and a band from 

500 to 2,000 Hz. The former was only about 5 dB above ambient while the latter peaked 

at a level 12 dB over ambient levels. At longer ranges, only naturally occurring in-air 

ambient noise consistent with the weather/snow conditions was observed. 

Vibrahammer operation also impacted local ice vibration levels. Close to Northstar, 

island related sounds drove the ice vibration levels at frequencies from 2 to 200 Hz with 

levels rising as much as 35 dB above ambient conditions. Most of the energy is 

concentrated in the 1/3 OB from 5 to 100 Hz. Northstar construction noise with the 

vibrahammer in operation is still significant at a range of 0.54 nm (1 km) but only in the 

spectral region below about 80 Hz. And at a 1.1 nm (2 km) range, man-made sounds 

were only detectable in the spectral region at and below 8 Hz and in the 23 Hz 

vibrahammer tonal. At longer ranges (2.2 nm or 4 km and higher), only energy in the 10-

30 Hz band rose above our equipment noise floor and as with the hydrophone data, it is 

not clear if this was due to vibrahammer related excitation or some other source. 

5.2 Noise and Vibration without Northstar Vibrahammer Operation 

When the vibrahammer was not in operation, the underwater environment was not as 

severely affected by man-made sounds. Vehicle and machinery noise was seen to 

dominate the noise field at close range (0.08 nm or 150 m) over the 1/3 OB spectral 

region from 2 Hz to about 1 kHz with levels rising as high as 40 dB above ambient 

conditions. The highest levels were seen in the 5-12 Hz and 20-100 Hz bands. The high 

levels in the latter band are due primarily to several narrowband tonals. See, for example, 

figures 3.1.2-10 and 3.1.2-11. 

At a range of 0.54 nm (1 km) (site NA5), man-made noise under the ice was significantly 

less, rising above ambient only in the 2-8 Hz and 25-60 Hz bands with a 1/3 OB 

maximum peak only about 15 dB above ambient. And at 1.1 nm (2 km), only energy 

below about 7 Hz appears to be due to Northstar activity. Finally, the man-made noise 

levels were consistently higher near the sea bottom compared to levels measured at mid 

water column depth. 

64 
OCS Study MMS 2001-0047

Airborne noise from Northstar when the vibrahammer was inactive generally produced 

man-made sounds only moderately above ambient levels at short range (0.08 nm or 150 

m) but little appears to have been present at longer distances. See figure 4.2.2. At site 

NA3, measured 1/3 OB noise above ambient occurred generally across the band from 2 

Hz to about 8 kHz but the majority was only 0-5 dB higher. Significant man-made noise 

was observed only in the 30-70 Hz (due to a limited set of narrowband tonals; figure 

3.1.2-12) and 200 Hz to 4 kHz regions. 

Ice vibration induced by construction activities when sheet piles were not being driven 

were quite localized. At a range of 0.08 nm (150 m), vibration levels above our 

measurement system noise floor only occurred in 1/3-octave bands below about 120 Hz 

(figure 4.3-2) with a maximum observed level of 30 dB above the lower limit. 

Narrowband tonals contributed the majority of the energy in the third octave bands 25-

120 Hz; for example, see figure 3.1.2-5. At longer ranges, no significant man-made ice 

vibration levels were detected. 

6.0 Acknowledgements 

The authors wish to thank the contractor team headed by Paul Boehm and John Brown 

from Arthur D. Little Corporation. In addition, the work would not have been possible 

without the excellent logistics support provided by Kinnetic Labs headed by Mark 

Savoie. We also appreciated the in-field assistance provided by John Trefry from the 

Florida Institute of technology. 

7.0 Literature Cited 

[1] Milne, A.R., and J.H. Ganton: Ambient Noise under Arctic Sea Ice, J. Acoust. Soc. 

 Am., 36:855 (1964). 

[2] Knudsen, V.O., Alford, R.S. and Emling, J.W.: Underwater Ambient Noise, J. Mar. 

Res. 7(3):410-429 (1948). 

[3] U.S. Army Corps of Engineers, Alaska: Final Environmental Impact Statement, 

Beaufort Sea Oil and Gas Development/Northstar Project, February 1999. 

[4] Malme, C.I., and Mlawski, R.: Measurements of Underwater Acoustic Noise in 

Prudhoe Bay, Alaska, BBN Technical Memorandum No. 513, April, 1979. 

[5] Richardson, W. J., et. al., Marine Mammals and Noise, Academic Press, San Diego, 

CA, 1995. 

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Appendix A: Complete Listing of the Collected Data Set 

Site Date Latitude Longitude 

Start Time, 

Local 

End Time, 

Local Observations 

NS-Trench 26 April 70o 28.806 148o 41.607 19:12 20:10 Plowing 

Trucks Moving 

Trench Backfilling 

NA3 26 April 70o 29.569 148o 41.460 13:17 14:27 Vibrahammer 

Plowing 

Trucks Moving 

NA2 26 April 70o 30.817 148o 36.351 18:30 19:07 Very Quiet 

5(5) 27 April 70o 26.108 148o 18.090 19:47 20:42 Quite Quiet 

Wind Noise 

LA2 28 April 70o 18.970 147o 37.928 12:55 13:42 Very Quiet 

Low Freq. Rumble 

LA3 28 April 70o 16.778 147o 33.529 16:30 17:18 Very Quiet 

Low Freq. Rumble 

NA5 29 April 70o 29.889 148o 40.734 11:45 

12:48 

13:49 

12:46 

13:40 

14:05 

Trucks Moving 

Plowing 

Lunch Break 

Plowing 

Trucks Moving 

Vibrahammer 

NA6 29 April 70o 30.372 148o 39.911 16:23 17:12 Vibrahammer 

Appendix B: Third Octave Band Center Frequencies, Hz 

2.00 12.59 79.43 501.19 3981.07 

2.51 15.85 100.00 630.96 5011.87 

3.16 19.95 125.89 794.33 6309.57 

3.98 25.12 158.49 1000.00 7943.28 

5.01 31.62 199.53 1258.93 10000.00 

6.31 39.81 251.19 1584.89 12589.25 

7.94 50.12 316.23 2511.89 15848.93 

10.00 63.10 398.11 3162.29 19952.62 

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The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility for most of our 

nationally owned public lands and natural resources. This includes fostering sound use of our land and water 

resources; protecting our fish, wildlife, and biological diversity; preserving the environmental and cultural values of 

our national parks and historical places; 

and providing for the enjoyment of life through outdoor recreation. The Department assesses our energy and 

mineral resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major responsibility for 

American Indian reservation communities and for people who live in island territories under U.S. administration. 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary responsibilities 

are to manage the mineral resources located on the Nation's Outer Continental Shelf (OCS), collect revenue from 

the Federal OCS and onshore Federal and Indian lands, and distribute those revenues. 

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program administers the 

OCS competitive leasing program and oversees the safe and environmentally sound exploration and production of 

our Nation's offshore natural gas, oil and other mineral resources. The MMS Royalty Management Program

meets its responsibilities by ensuring the efficient, timely and accurate collection and disbursement of revenue from 

mineral leasing and production due to Indian tribes and allottees, States and the U.S. Treasury. 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being responsive to the 

public's concerns and interests by maintaining a dialogue with all potentially affected parties and (2) carrying out its 

programs with an emphasis on working to enhance the quality of life for all Americans by lending MMS assistance 

and expertise to economic development and environmental protection. 

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