OCS Study 

•

MMS 2006-050 

Simulation of a Subsurface Oil 

Spill by a Marine Hydrocarbon 

Seep

Final Technical Summary 

Final Study Report 

U.S. Department of the Interior 

Minerals Management Service

Pacific OCS Region 

OCS Study 

MMS 2006-050 

Simulation of a Subsurface Oil 

Spill by a Marine Hydrocarbon 

Seep 

Final Technical Summary 

Final Study Report 

Authors 

Ira Leifer 

Jordan Clark 

Bruce Luyendyk 

Principal Investigators 

Prepared under MMS Cooperative 

Agreement No. 14-35-01-00-CA-31063 

by 

Coastal Marine Institute 

Marine Science Institute 

University of California 

Santa Barbara, CA 93106-6150 

U.S. Department of the Interior 

Minerals Management Service Camarillo

Pacific OCS Region September 2006
OCS Study 

•

MMS 2005-xxx 

Disclaimer 

This report has been reviewed by the Pacific Outer Continental Shelf Region, Minerals Management 

Service, U.S. Department of the Interior and approved for publication. The opinions, findings, 

conclusions, or recommendations in this report are those of the author, and do not necessarily reflect the 

views and policies of the Minerals Management Service. Mention of trade names or commercial products 

does not constitute an endorsement or recommendation for use. This report has not been edited for 

conformity with Minerals Management Service editorial standards. 

Availability of Report

Extra copies of the report may be obtained from: 

U.S. Dept. of the Interior 

Minerals Management Service 

Pacific OCS Region 

770 Paseo Camarillo 

Camarillo, CA 93010 

Phone: 805-389-7621 

A PDF file of this report is available at: 

http://www.coastalresearchcenter.ucsb.edu/CMI/ 

Suggested Citation 

The suggested citation for this report is: 

Leifer, I., J. Clark, and B. Luyendyk. Simulation of a Subsurface Oil Spill by a Marine Hydrocarbon 

Seep. MMS OCS Study 2006-050. Coastal Research Center, Marine Science Institute, University of 

California, Santa Barbara, California. MMS Cooperative Agreement Number 14-35-01-00-CA-31063. 81 

pages. 
Table of Contents 

FINAL TECHNICAL SUMMARY........................................................................ 1 

FINAL STUDY REPORT....................................................................................... 5 

 INTRODUCTION ............................................................................................... 5 

 I-1 Project Goals ............................................................................................ 5 

 I-2 Methodological Approach........................................................................ 5 

 I-3 Summary of Accomplishments ................................................................ 6 

 I-4 Study Area................................................................................................ 7 

 I-4-1 Coal Oil Point Seep Field ............................................................ 7 

 I-4-2 Shane Seep Area .......................................................................... 8 

 

 METHODOLOGY .............................................................................................. 9 

 II-1 Seabed Morphology................................................................................ 9 

 II-2 Seep Bubble Measurements.................................................................... 10 

 II-2-1 Overview of Bubble Measurement Systems .............................. 10 

 II-2-2 Seep Bubble Measurement System ............................................ 13 

 II-2-3 Bubble Measurement System Calibration - Blurring ................. 15 

 II-2-4 Bubble Measurement System Calibration - Off-axis Rays ........ 19 

 II-3 Methodology - Gas Geochemistry.......................................................... 24 

 II-4 Methodology - Fluid Motions................................................................. 24 

 

 RESULTS ............................................................................................................ 25 

 III-1 Results - Seabed Morphology................................................................ 25 

 III-2 Seabed Morphology Significance.......................................................... 28 

 III-3 Results - Bubble Observations .............................................................. 30 

 III-3-1 Seabed Vent Bubble Observations............................................ 30 

 III-3-2 Sea Surface Bubble Observations ............................................. 36 

 III-4 Results - Bubble and Aqueous Gas Measurements............................... 37 

 III-4-1 Dissolved Methane and Higher Alkanes................................... 38 

 III-4-2 Dissolved Oil............................................................................. 40 

 III-4-3 Fluid Motions and Plume/Ocean Interactions........................... 41 

 III-4-4 Atmospheric Methane Emission Plumes................................... 41 

 III-4-5 Extreme Emission Events - BLOWOUT .................................. 47 

 

 NUMERICAL BUBBLE STUDIES.................................................................... 53 

 IV-1 Theoretical Description of a Bubble ..................................................... 53 

 IV-2 Numerical Bubble Model Overview ..................................................... 55 

 IV-2-1 Single Bubble Studies ............................................................... 56 

 IV-2-2 Contamination........................................................................... 58 

 IV-2-3 Upwelling Flow......................................................................... 60 

 IV-3 Sensitivity Studies ................................................................................. 60 

 IV-4 Plume Simulations................................................................................. 61 

 
 DISCUSSION...................................................................................................... 65 

 V-1 Seabed Morphology Discussion ............................................................. 65 

 V-1-1 Seep Seabed Plasticity................................................................ 65 

 BIBLIOGRAPHY................................................................................................ 68 

 
Figures 

Figure I-1. Coal Oil Point Seep Field ....................................................................... 8 

Figure II-1. Schematic of video-based Bubble Measurement System (BMS)........... 13 

Figure II-2. Pool test of the seep Bubble Measurement System................................ 13 

Figure II-3. Schematic of closed seep Bubble Measurement System........................ 15 

Figure II-4. Images of bubbles of 670 μm at varying shutter speeds......................... 17 

Figure II-5. Schematic of bubble blur calibration experiment................................... 18 

Figure II-6. Bubble radius versus shutter speed......................................................... 18 

Figure II-7. Schematic of experimental set-up .......................................................... 19 

Figure II-8. Enlarged bubble image and surface intensity plot.................................. 20 

Figure II-9. Size histogram for bubbles ..................................................................... 21 

Figure II-10. Schematic of bubble edge halo-ring..................................................... 22 

Figure II-11. Calculated screen distance of origin versus bubble angle.................... 23 

Figure III-1. Shane Seep seabed surveys from 2000-2003........................................ 26 

Figure III-2. Seep bubble images showing different vent types ................................ 30 

Figure III-3. Vertical velocities for vents .................................................................. 32 

Figure III-4. Bubble emission flux distributions........................................................ 34 

Figure III-5. Integrated cumulative emission flux distributions ................................ 35 

Figure III-6. Vertical velocity as a function of radius ............................................... 36 

Figure III-7. Image of bubbles from Shane Seep....................................................... 37 

Figure III-8. Image of bubbles from Shane Seep using the closed BMS .................. 37 

Figure III-9. Alkane ratio from sea floor to sea surface ............................................ 39 

Figure III-10. Shane Seep surface expression showing features ............................... 40 

Figure III-11. Plot of excitation-emission fluorescence ............................................ 41 

Figure III-12. The Coal Oil Point seep field.............................................................. 43 

Figure III-13. Hours per year wind direction in 2° bins. ........................................... 44 

Figure III-14. Transect through the Shane Seep plume ............................................. 46 

Figure III-15. Contour map of Shane Seep atmospheric THC concentration............ 46 

Figure III-16. Contour plot of Gaussian plume model for sea surface CH4.............. 47 

Figure III-17. Contour plot of vertical integration of Gaussian plume...................... 47 

Figure III-18. Seabed map of central ......................................................................... 49 

Figure III-19. Video captures of large gas ejection at Shane Seep............................ 50 

Figure III-20. West campus air pollution station (WCS) measurements................... 52 

Figure IV-1. Flowchart of the numerical bubble model ............................................ 55 

Figure IV-2. Numerical simulation of a clean, r = 5000 μm bubble.......................... 57 

Figure IV-3. Numerical simulation of a clean, r = 500 μm bubble............................. 58 

Figure IV-4. Rise velocity parameterizations for clean and dirty bubbles ................ 59 

Figure IV-5. Numerical simulation of a dirty, r = 2000 μm bubble........................... 60 

Figure IV-6. Sensitivity results showing predicted ratio ........................................... 61 

Figure IV-7. Bubble emission size-distributions versus bubble radius ..................... 62 

Figure IV-8. Predicted vertical CH4 transport for three simulations......................... 64 
Tables 

Table III-1. Summary of bubble plume characteristics ............................................. 32 

Table III-2. Natural hydrocarbon seeps visited.......................................................... 38 

Table III-3. Summary of major gases bubble partial pressures ................................. 38 

Table III-4. Summary of trace gas bubble observations............................................ 39 

Table III-5. Atmospheric gas composition above Shane Seep .................................. 45 

 

Table IV-1. Model-predicted bubble gas mole-fraction at the sea surface................ 63 

 
Simulation of a Subsurface Oil Spill 

1 

FINAL TECHNICAL SUMMARY 

STUDY TITLE: Simulation of a Subsurface Oil Spill by a Marine Hydrocarbon Seep 

REPORT TITLE: Simulation of a Subsurface Oil Spill by a Marine Hydrocarbon Seep 

CONTRACT NUMBER(S): 14-35-01-00-CA-31063; Task # 17611 

SPONSORING REGION: Pacific 

APPLICABLE PLANNING AREAS: Southern California 

FISCAL YEAR(S) OF PROJECT FUNDING: FY 00/01 

COMPLETION DATE OF REPORT: September 2006 

COST(S): FY 00/01 - $110,935; FY 01/02 - no cost 

CUMULATIVE PROJECT COST: $110,935 

PROJECT MANAGER: Russell J. Schmitt 

AFFILIATION: University of California, Santa Barbara 

ADDRESS: Coastal Research Center, Marine Science Institute, University of California, 

Santa Barbara, CA 93106-6150 

PRINCIPAL INVESTIGATORS: 1

Ira Leifer, 2Jordan Clark, and 3Bruce Luyendyk 

ADDRESSES: 1

Marine Science Institute, University of California, Santa Barbara, CA 93106-

6150; 2

Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 

93106-9610, 3

Institute for Crustal Studies, University of California, Santa Barbara, CA 93106 

KEY WORDS: hydrocarbon seeps, oil spills, buoyant oil and gas flow, bubble plume, Coal 

Oil Point Seep Field, surfacing location, dissolved methane 

OBJECTIVES: To understand the roll of gas bubbles and gas bubble plumes in transporting 

oil from the seabed to the seasurface, thereby improving predictions of the fate (i.e., surfacing 

location) of oil (and gas) from an underwater pipeline leak 

SIGNIFICANT CONCLUSIONS: Several discoveries resulted from this project. The bubble 

plumes from seeps were found to modify their environment by raising the dissolved methane 

levels in the water and creating upwelling flows. These enhance bubble survival and thus 

decreases the vertical distance they can transport oil, minimizing its surfacing footprint. Bubble 

distributions from natural seeps were measured (first ever) and were either broad distributions, 
Final Technical Summary - Leifer 

2 

or narrow and about 2000-3000 μm radius. Oil was found to form small gassy droplets in both 

cases, although the mechanism was different. It was also discovered that the trajectories of the 

gassy oil droplets were different from the oily bubbles. 

STUDY PRODUCTS: 

Publications: 

Boles, J.R., J.F. Clark, I. Leifer, and L. Washburn. 2001. Temporal variation in natural 

methane seep rate due to tides, Coal Oil Point area, California. J. Geophys. Res. 

106C11, 27077-27086. 

Leifer, I., and J. Clark. 2002. Modeling trace gases in hydrocarbon seep bubbles. Application 

to marine hydrocarbon seeps in the Santa Barbara Channel, Russian Geology and 

Geophysics {Original - Geologiya I Geofizika} 43(7): 613-621.

Leifer, I., J. Clark, and R. Chen. 2000. Modifications of the local environment by a natural 

marine hydrocarbon seep, Geophys. Res. Lett. 27(22): 3711-3714. 

Leifer, I., G. De Leeuw, and L.H. Cohen. 2003. Optical measurement of bubbles: System‚ 

design and application. J. Atm. Ocean. Tech. 20(9): 1317-1332.

Leifer, I., and A.G. Judd. 2002. Oceanic methane layers: A bubble deposition mechanism from 

marine hydrocarbon seepage. Terra Nova 16: 471-425. 

Leifer, I., and I. MacDonald. 2003. Dynamics of the gas flux from shallow gas hydrate 

deposits: Interaction between oily hydrate bubbles and the oceanic environment. Earth 

Plan. Sci. Lett. 210(3/4): 411-424. 

Leifer I., and R. Patro. 2002. The bubble mechanism for transport of methane from the 

shallow sea bed to the surface: A review and sensitivity study. Cont. Shelf Res. 22: 

2409-2428.

MacDonald, I.R., I. Leifer, R. Sassen, P. Stine, R. Mitchell, and N. Guinasso Jr., 2002. 

Transfer of hydrocarbons from natural seeps to the water column and atmosphere, 

Geofluids 2: 95-107.

Roy, L.A., S. Steinert, S.M. Bay, D. Greensteain, Y. Sapozhnikova, O. Bawardi, I. Leifer, and 

D. Schlenk, 2003. Biochemical effects of petroleum exposure in hornyhead turbot 

(Pleuronichthys verticalis) exposed to a gradient of sediments collected from a natural 

petroleum seep in CA, USA. Aquatic Toxicology 65(2): 159-169.
Simulation of a Subsurface Oil Spill 

3 

Presentations: 

Conferences and Workshops

Bergmann, S., M. LaMontagne, I. Leifer, and P. Holden. Assessment of the effect of marine 

oil seeps on sediment bacterial diversity and community structure by PCR-TRFLP. 

American Society of Microbiologists General Meeting, Salt Lake City, Utah, May 19-

23, 2002.

Leifer, I., Bubble transport to the sea surface of seep methane. USGS Workshop on Naturally 

Occurring Fossil Methane Seepage as a Factor in Global Climate Change, Portland, 

Oregon, May 2-4, 2001. 

Leifer, I., J. Boles, J. Clark, P. Holden, M. LaMontagne, B. Luyendyk, C. Olman, and L. 

Washburn. Predicting the fate of oil in the marine environment from the seabed to 

surface oil slicks and beyond. California and the World Oceans ‘02, Santa Barbara, 

CA, Oct. 27-30, 2002. 

Leifer, I., J. Clark, B. Chen, L. Washburn, B. Luyendyk, and T. Egland. Observations of gas 

bubbles from the Coal Oil Point marine hydrocarbon seeps in the Santa Barbara 

Channel. Shallow Gas Group 6th International Conference, Gas in Marine Sediments, 

St. Petersburg, Russia, Sept. 5-9, 2000. 

Leifer, I., J. Clark, and B. Luyendyk. From the Seabed to the Sea Surface: Measurments and 

implications of hydrocarbon seepage in the Santa Barbara Channel (abstract), AAPG 

HEDBERG CONFERENCE “Near-Surface Hydrocarbon Migration: Mechanisms and 

Seepage Rates” April 7-10, 2002, Vancouver, BC, Canada, Search and Discovery, 

Article #90006, 2002. 

Leifer I., and A. Judd. Seep bubble layer deposition due to sharp bubble dissolution. XI 

European Union of Geosciences, Strasbourg, France, 8-12, April, 2001. 

Lorenson, T., W. Ussler III, J. Dougherty, C. Paull, S. Mitra, R. Keaten, K. Kvenvolden, and I. 

Leifer. Dissolved methane concentrations adjacent to natural oil and gas seeps off Pt. 

Conception, California. AGU San Francisco, 8-11 Dec. 2002. 

Seminars

1. MMS Camarillo, CA, May 16, 2002. Marine hydrocarbon seeps: A natural laboratory for 

studying processes related to petroleum exploration, spill mitigation, and ecological 

adaptation 

2. Woods Hole Oceanographic Institute, Woods Hole, MA. Sep. 2002. The contribution to 

marine hydrocarbon seepage from catastrophic seepage events. 

3. Venoco Inc., Carpenteria, CA. Aug. 2002. The Shane Seep blowout and our understanding 

of the role of marine hydrocarbon seeps to the global environment 

4. California State Lands Commission, Long Beach, CA. Oct. 2001. The unique environment 

of marine hydrocarbon seeps. 
Final Technical Summary - Leifer 

4 

Educational Products: 

1) Narrated Video - Surveying Shane Seep, 16 minutes 

2) www.bubbleology.com website most recently receiving 14500 hits per month (see 

www.bubbleology.com/stats) presents some of the research including allowing manuscripts to 

be downloaded. 
Simulation of a Subsurface Oil Spill 

5 

FINAL STUDY REPORT 

I. INTRODUCTION 

I-1. Project Goals 

The project “Simulation of a sub-surface oil spill by a marine hydrocarbon seep” proposed to 

understand by in-situ measurements the mechanism by which oil travels from a source at the 

sea floor to the sea surface. Currently, numerical models of the behavior of the buoyant oil and 

gas flow arising from a leak must make numerous untested assumptions. To address these 

shortcomings, field studies were undertaken to identify areas of uncertainty and improve our 

understanding of this process. 

Motivation for this study arises from the fact that oil and gas typically are emitted together 

both at seeps or from an underwater spill, since the two phases are actually part of a spectrum 

stretching from the lightest hydrocarbon, methane, to much larger n-alkanes, and other oil 

components (too numerous to list, many unidentified). And when emitted together, the 

(surface active) oil travels on bubble surfaces, thereby rising much faster than if traveling as oil 

droplets. As a result, the bubble’s buoyancy changes the oil’s fate and its surfacing footprint is 

greatly reduced. 

Thus the primary justification for this research is that improved prediction of surfacing location 

of an underwater oil spill means better preparedness with regards to positioning oil spill 

cleanup equipment. And as oil exploration moves to deeper and deeper waters, the importance 

of better understanding bubble, bubble plume, and oily bubble processes becomes of increasing 

importance. 

To address these issues, we undertook to measure the fluid dynamics, gas chemistry, and 

bubble distributions of an active area of seepage in the Coal Oil Point Seep Field. The Seep 

Field is located just a few kilometers from the UCSB campus, and is one of the better 

characterized seep fields in the world, and among the largest in magnitude. Within the seep 

field, a very active and diver accessible seep was chosen to provide greater flexibility in the 

measurements and more rapid turn-around in the development of techniques. Since diver 

accessible seeps are by their nature shallow, measurements were also performed in the Gulf of 

Mexico (in Collaboration with Texas A&M) at seepage from a hydrate mound in water 550 m 

deep. These observations showed numerous similarities with processes at the shallower seeps. 

I-2. Methodological Approach 

The project goal was to understand the fate of oil and gas escaping from the seabed from 

natural seeps to understand better the underlying processes, which are shared with an undersea 

oil spill. The starting point was numerical sensitivity studies (Leifer and Patro, 2002) which 

indicated a sensitivity to many parameters that were largely, or completely unknown in the 

literature, and for which measurement techniques had not been developed or described in the 

literature. Thus our basic approach was to develop techniques for measuring the parameters 
Final Study Report - Leifer 

6 

needed to initialize and run a numerical model, and also to improve the numerical model to 

consider new data on new processes necessary for successful simulations. 

I-3. Summary of Accomplishments 

An advantage of embarking upon a project to do what has not been done before is that success 

often engenders numerous discoveries of great interest to the community, including many that 

unanticipated. A highlight of some of the most interesting discoveries and accomplishments: 

1. Discovery that strong upwelling flows do exist in marine hydrocarbon seep bubble streams 

both in the Santa Barbara Channel (Leifer et al., 2000a) and the Gulf of Mexico (Leifer and 

MacDonald, 2003). 

2. Discovery that the fluid in marine hydrocarbon seep bubble streams becomes enhanced or 

even saturated with gas (Leifer et al., 2000). 

3. Confirmation of numerical study predictions (Leifer and Patro, 2002) that dissolved gas 

concentrations and upwelling flows are important to the fate of the bubbles, increasing 

bubble survivability and thus the flux of hydrocarbons to the sea-surface (Leifer et al., 

2000a). 

4. Confirmation that the bubble transport mechanism is critical to predicting the surfacing 

footprint of a hydrocarbon seep, which is enormously smaller and closer to the location of 

the seepage than if the oil rises as a pure oil droplet (Leifer and Boles, 2005a). 

5. Discovery that oil coating on bubbles enhances bubble survivability and thus the transport of 

hydrocarbons to the sea surface (MacDonald and Leifer, 2002). 

6. Discovery that the seep system is extremely dynamic with ejections and “blowouts” 

occurring frequently. These events make seeps an even better natural laboratory for 

studying undersea oil spill processes (Leifer et al., 2004). 

7. Discovery that the bubble distribution of hydrocarbon seeps (in the SB Channel and the Gulf 

of Mexico) are primarily of bubbles larger than 2-mm diameter, and that smaller bubbles 

(to 0.5 mm diameter) are produced by vents with bubble tearing and breakup, but do not 

contribute appreciably towards the total mass transfer. Also, the first published bubble 

distributions for a marine hydrocarbon seep (Leifer and MacDonald, 2003). 

8. Discovery that bubble gas composition fractionates (Leifer and Clark, 2003; Clark et al., 

2003) as the bubbles rise, as predicted by Leifer and Patro (2002). 

9. Discovery that optical bubble measurements (and measurements calibrated with optical 

means) during the last century have a bias due to off-axis rays (Leifer et al., 2003a). 
Simulation of a Subsurface Oil Spill 

7 

10. Discovery that bubble dissolution can lead to the formation of methane layers (Leifer and 

Judd, 2002). These layers have the potential to trap (temporarily) oil transported on bubble 

surface from either a seep, or an undersea oil spill. 

11. First quantitative observations of “catastrophic seepage” including plume processes 

during which emissions increase enormously. Numerical studies showed that during 

catastrophic seepage virtually all of the methane reaches the atmosphere (Leifer et 

al., 2005b) 

12. Development (and first use) of an air pollution plume monitoring approach to 

studying seep emissions (Leifer et al., 2005b). 

13. First field-validated bubble plume model of seepage (Leifer and Clark, 2002). 

14. Production of educational video (“Surveying Shane Seep”) on the importance of 

these processes to the fate of seeps, and the impact on natural fauna and flora that 

interact and with the seeps. Conservatively, the video has been seen by thousands of 

viewers. 

15. Maintenance of the www.bubbleology.com website that disseminates seep and bubble 

process information, publications, and various movie clips. The website currently receives 

circa 17,000 hits per month from circa 1000 unique users in circa 50 countries (see 

www.bubbleology.com/stats). Due to www.bubbleology.com, roughly 3 specific requests 

are received per month for advice/help on bubble and seep research in diverse countries 

(many developing). Several have led to long-term collaborations. 

I-4. Study Area 

Studies were conducted in the Coal Oil Point (COP) Seep Field, located a few kilometers from 

the University of California, Santa Barbara in the Santa Barbara Channel, offshore California. 

The COP Seep Field is one of the largest known areas of active marine seepage in the world, 

and arguably the best studied. Seepage spans a wide range of magnitudes and oil to gas ratios. 

Furthermore, much of the seepage is diver accessible. All these features make it an ideal 

natural laboratory for the study of hydrocarbon processes in the ocean. 

I-4-1. Coal Oil Point Seep Field 

The COP Seep Field is shown in Figure I-1. Several studies have quantified seep area (e.g., 

Allen et al. 1970; Fischer and Stevenson 1973; Fischer 1978) and emission fluxes (e.g., 

Hornafius et al. 1999; Quigley et al. 1999; Clark et al. 2000) using sonar techniques, ocean 

geochemistry, and direct gas capture. Fischer and Stevenson (1973) noted changes in 

hydrocarbon seeps on decadal time scales in the field with significant decrease in seepage area 

between 1946 and 1973. On the basis of on a comparison of sonar data and oil company seep 

maps, they attributed this drop to offshore production. During the last seven years, the UCSB 

seep group has mapped the seeps in the area using sonar images and quantified seepage flux 

from sonar and direct gas capture using a flux buoy (Washburn et al. 2001). Results indicate 
Final Study Report - Leifer 

8 

that ~1.5 x 105 m3 dy-1 (4.5 x 1010 g yr-1) of seep gas is emitted to the atmosphere from ~3 

km2 of sea floor (Hornafius et al. 1999) with roughly an equal amount dissolved into the 

coastal ocean (Clark et al. 2000). The seeps also release about 80 barrels dy-1 - 5x106 liters 

yr-1 (Clester et al. 1996) with oil slicks a common channel feature (Leifer et al. 2005b) and it 

has been noted that oil emissions vary with tides (Mikolaj and Ampaya 1973). 

Seep locations are controlled by anticlines and lie along three depth trends. The inner trend is 

~20-m deep and includes Shane Seep and IV Super Seep. A second trend at ~ 40 m deep 

includes the Horseshoe seeps and COP Seeps. The deepest trend (~70 m depth) includes the 

La Goleta Seeps and Seep Tent Seeps as well as Platform Holly. 

Figure I-1. The Coal Oil Point Seep Field, Santa Barbara Channel off the coast of Santa Barbara, California. 

Upper panel shows the overall channel including the Santa Barbara and Los Angeles airports. Gray areas in lower 

panel indicate regions of high bubble density as indicated by sonar returns (Hornafius et al. 1999). Inshore seeps 

i.e., (Shane Seep, Ira Seep, IV Super Seep, and Farrar Seep) were too shallow for the sonar survey. Seep names 

are informal and GPS coordinates locations are provided on figure. Table coordinates referenced to 34° N, 119° 

W. 

I-4-2. Shane Seep Area 

One of the most intense seep areas in the COP Seep Field is in Shane Seep (34° 24.37’ N, 119° 

53.41’ W), located in 22 m deep water – i.e., diver accessible. For example, Washburn et al. 

(2004) recorded the highest single flux measurement for the COP Seep Field at Shane Seep. 

Seabed surveys, described in detail below showed that seepage at Shane Seep consisted of 

numerous (order 103

-104) small vents that emit streams of similarly sized bubbles rising in 

lines over an area approximately 30 m by 20 m. (600 m2). These minor vents surround 
Simulation of a Subsurface Oil Spill 

9 

hydrocarbon volcanoes (the number has changed over the years) with one or more major vents. 

The volcanoes are termed hydrocarbon because they consist primarily of tar and sand. Minor 

vents produce bubbles that are nearly mono-disperse in size, while major vents produce a broad 

size spectrum of bubbles. Major vents also have significantly greater gas flux minor vents. 

Bubbles rise in several intense plumes to the sea surface, each plume corresponding to a 

hydrocarbon volcano. These bubble plumes drive strong upwards water flows, called the 

upwelling flow. The strong upwelling flow generates strong outward flows at the surface. 

Bubbles burst at the surface, leaving very fine slicks that the outwelling flow rapidly advects. 

Furthermore, the flow is unsteady, both pulsing and shifting spatially. The central plumes are 

surrounded by a much wider and more diffuse bubble plume. Surveys over the years have also 

shown significant changes in seabed morphology. 

For all these aspects, our approach has been to intensively study Shane Seep and its interaction 

with the environment, although other seeps in the field have been investigated. 

II. METHODOLOGY 

II-1. Seabed Morphology 

Most of the research studies were conducted at Shane Seep, one of the most active, and 

concentrated seep areas in the seep field. In the process of studying seepage phenomena at 

Shane Seep, we documented numerous changes in magnitude, extent, and seabed features 

within the seep. The most active vents were located within hydrocarbon volcanoes several 

meters across, which were documented to vary spatially over the years of the study. 

Furthermore, these changes appeared associated with large, transient gas emissions, and are 

proposed related to tar flux (Leifer and Boles, 2005a). 

To understand the transient seep behavior, and its effects on seabed morphology (or 

alternatively, how to infer the dynamic nature of seep emissions based on seabed morphology) 

a project of seabed mapping was undertaken, described in Leifer et al. (2004). 

For the last three years, (2000-2003), SCUBA divers have periodically visited Shane Seep (34° 

24.37’ N, 119° 53.41’ W) in the Coal Oil Point (COP) Seep Field and conducted both video 

surveys and measurement surveys to document the changes in seabed morphology with time. 

Measurement surveys were done with a fiberglass meter tape. Since Shane Seep is in an area 

of otherwise unremarkable sandy seabed, North-South and East-West steel-link transect cables 

(2.5 cm links, 60 feet long) were laid down late October, 2001. Previously, sand anchors and 

yellow transect lines were used; however, they often disappeared between surveys, possibly 

due to dragging boat anchors and/or fishermen’s nets. The transect was centered on a position 

3.5 meters WNW of two prominent hydrocarbon volcanoes. The volcanoes are termed 

hydrocarbon (HC) volcanoes to denote that the volcano walls are a combination of tar and sand 

rather than mud (La Montagne et al. 2004). Every meter along the chain, 30-centimeter 

lengths of yellow, polypropylene rope were tied at one end to the chain. Polypropylene rope is 

buoyant and the free ends remained visible even in places where the chain became buried. A 
Final Study Report - Leifer 

10 

250-kg mooring anchor was placed at the intersection of the transect chains and connected by 

line to a spar buoy ~2 m above the seabed. The line continued to the sea surface and a small 

buoy. Each HC volcano had an associated major vent, and one meter lengths of rebar were 

hammered halfway into the sediment nearby. Closed cell foam numbers, 15-cm high, were 

pushed onto the rebar and secured above and below by plastic wire ties. At the four cardinal 

points and 4 intermediate directions (i.e., NE) at a distance of 10 m from the transect center, 

rebars also were hammered into the sediment. Wire ties were attached, with ends sticking out 

at the rebar top and held in place by expanded polypropylene rope. A code was used, where 

counterclockwise from North, 1, 2, 3, and 4 plastic cable ties were used, with and additional 

half length plastic cable ties to indicate intermediate directions (i.e., 1 and 1/2 for NE). Video 

surveys were conducted along circular lines at a given radius from the center. This was 

accomplished by having the videographer hold one end of a tape measure, while a second diver 

held the other end at the mooring point. The videographer maintained the video camera at a 

constant angle and zoom (wide-angle) setting. Divers also used tape measures to measure 

precisely the distance to the main morphological features, including their height above the sea 

floor, wall height and caldera width. 

II-2. Seep Bubble Measurements 

For the purposes of predicting the fate of gas bubbles, and hence oil transported by gas 

bubbles, the bubble size distribution is critical, small bubbles tend to dissolve, while large 

bubbles may be able to reach the surface with most of their original gas content. Bubble rise 

speed, and the strength of the upwelling flow also are size dependent, as is the amount of 

carried on the bubbles. As a result, the fate of the oil arrives depends upon the bubble size. 

Thus one of the main tasks of this research was to measure the bubble distribution, of particular 

importance since seep bubble distributions were entirely absent in the literature. To do so, we 

developed a bubble measurement system optimized for marine seeps. 

II-2-1. Overview of Bubble Measurement Systems 

The following summary is from Leifer et al. (2003b), written during this project. This section 

summarizes the various methods used to measure bubbles for bubbles generated from breaking 

waves at the water surface. Since this research project produced the first quantitative 

measurements of the bubble distribution from hydrocarbon seeps, design of the bubble 

measurement system was based on system considerations from measurements of bubbles from 

breaking waves at the sea-surface. Design considerations specific for seep bubbles from Leifer 

and MacDonald (2003) are also presented. 

Breaking-wave bubble measurement systems – Design Considerations

Different approaches have been used to measure bubbles, from acoustics to optical (laser and 

video/photo). Each technique has advantages and disadvantages, and is effective over different 

size ranges and bubble density regimes. Laser techniques are non-invasive and use a laser 

beam that has its illumination blocked when a bubble crosses the narrow measurement volume 

(Hwang et al. 1990), or multiple beams whose intersection defines the measurement volume 

and the bubble size is determined by its interaction with the laser interference pattern (Baldy 
Simulation of a Subsurface Oil Spill 

11 

and Bourguel 1987; Asher and Farley 1995). Laser systems have problems when multiple 

bubbles enter the beam(s), ie., high bubble density results in coincidence, and with large (r > 

1000 μm, where r is radius) bubbles whose mean shape is ellipsoidal but variable due to shape 

oscillations. The shape of large bubbles may cause aliasing wherein large bubbles are sized as 

smaller bubbles (Asher and Farley 1995). Since bubble concentration typically decreases with 

r-2 to r-4, (e.g., Haines and Johnson 1995), aliasing only significantly (under) biases large 

bubble concentrations. Acoustic approaches include resonance, pulse propagation, or 

multifrequency backscatter. Acoustic methods have high rejection of non-bubble particles, 

e.g., 10-9 for acoustic resonators (Vagle and Farmer 1998), and can non-invasively map large 

regions of the water column, allowing investigations into bubble plume distributions (Thorpe 

1982; Medwin and Breitz 1989). However, acoustical methods have difficulties, in common 

with laser methods, with non-spherical (i.e., r > 500 μm in a turbulence flow, where r is radius) 

bubbles and at high bubble concentrations. Optical methods (photographic and video) are able 

to measure at high bubble densities and over a wide r range including very large bubbles (the 

optics determines the size range). Optical approaches use either a constrained measurement 

volume (e.g., Monahan et al. 1994; Wang and Monahan 1994), or an unconfined measurement 

volume (Walsh and Mulhearn 1987; Haines and Johnson 1995; Johnson and Cook, 1979; Jähne 

and Geiβler 1994; Deane and Stokes 1999). Optical systems are generally invasive since for 

clarity in turbid water, or for high bubble concentrations, the measurement volume must be 

near the system's viewing port. However, invasive systems may significantly disturb the 

bubbly flow. 

Bubbles are typically backlit, although side and front lighting (discussed below) have been 

used. The collected images are analyzed and the number (or probability) of bubbles of each 

size class determined. The bubble concentration distribution, based on the measurement 

volume, can be calculated from the probability distribution. The measurement volume for 

constrained systems varies only with the effect of bubbles touching the image edge (i.e., a large 

bubble must be further from the edge than a small one to avoid its images being partially cut). 

In contrast, one of the major disadvantages of an unconstrained optical BMS is the difficulty in 

determining their measurement volume, since the depth of field varies with bubble size, optics, 

and the analysis method. Furthermore, the measurement volume may vary with illumination 

and water clarity. 

In unconstrained systems, apparent bubble size varies with analysis technique (e.g., the 

threshold level, image preprocessing, etc.) and distance to the bubble unless telecentric lenses 

are used (only feasible for longer focal lengths and thus higher magnification). Johnson and 

Cooke (1979) used three forward pointing strobes. Bubbles were "in focus" when the three 

spots did not overlap, and the measurement volume was a simple function of r up to the large r

limit of ~300 μm. Walsh and Mulhearn (1987) used a similar approach and found that the spot 

location on the bubble is complicated by optical effects due to refraction and reflection on the 

curved bubble surface, and estimated a sizing error of ~10%. Haines and Johnson (1995) side 

illuminated bubbles in a 5-cm thick slab. For bubbles in this slab, the size error was 

determined by moving a bubble attached to a wire along the camera axis, and was found to be 

~10% across the 5 cm slab for a r = 250-μm bubble. 
Final Study Report - Leifer 

12 

Constrained systems also may have the problem of evolution of the size distribution between 

entry into the device and reaching the measurement volume since bubble rise velocity increases 

with r. If this distance is short, segregation is negligible. An extreme example is a bubble tube 

where bubbles enter and rise through a clear tube above the water surface for imaging. The 

entry distribution can be calculated using the bubble rise velocity (Bowyer 1992). 

Seep Bubble Measurement Systems - Design considerations

The intent of bubble visualization was to record a video sequence showing the bubbles next to 

a calibrated scale of known size and to provide sufficient illumination to distinctly render each 

bubble (i.e., no motion blur) in the individual video frames. Bubble visualization requires the 

depth of field to be sufficient to maintain good focus for bubbles in the plume. However, 

bubble-sizing errors can become significant if the bubble plume occupies too large a crosssection of the camera’s depth of field because the accuracy of size estimation depends upon 

distance between the size reference and the measured bubble. For broad plumes, baffles can be 

used to block off all the bubbles outside of a slab. Imaging a bubble with distinct edges 

requires a shutter speed sufficiently fast to freeze bubble motion. For auto-iris video cameras, 

this requires sufficient illumination to force the shutter speed. Three illumination schemes 

were attempted in the present observations. With front illumination, the principal light sources 

were quartz flood lamps mounted in the same plane as the camera. For side illumination, a 

collimated light source was placed perpendicular to the camera’s point of view and 

immediately adjacent to the bubble plume. For back illumination, lamps placed behind the 

camera plane were reflected back into the bubbles by a white panel placed immediately behind 

the bubble plume. 

Of the three illumination techniques, the auto-iris shutter speed of the video camera was too 

slow (due to insufficient illumination) and bubbles appear as streaks. Back illumination 

provided the best results, with in-focus bubbles showing as dark outlines with bright centers. 

With back illumination, the white panel had markings to indicate size scale. Side illumination 

with the collimated light source provided images that were the most difficult to analyze. 

Bubbles appeared as half moons with the dark side edge extremely difficult to distinguish, and 

required hand analysis. Although for spherical bubbles it was easy to estimate the far edge, for 

the irregular shapes of most of the larger seep bubbles, this was difficult, thereby increasing the 

sizing uncertainty. Bolts on the collimated light source were used for size scale. Front 

illumination provided two difficulties, it was almost impossible to determine if a bubble was in 

focus, and secondly, bubble edges were less distinct. Size comparison for bubbles that initially 

were front-illuminated, and then back-illuminated (by rising above the panel top) showed that 

front illumination underestimated bubble size. Front illumination worked best with a black 

background. 
Simulation of a Subsurface Oil Spill 

13 

II-2-2. Seep Bubble Measurement System 

Figure II-1. Schematic of video-based, Bubble Measurement System (BMS), open configuration. 

Bubble measurement systems and analysis approaches are reviewed in Leifer et al. (2003b). 

The video-BMS has several key components, shown schematically in Figure II-1 and in 

underwater images in Figure II-2. Bubbles are backlit by two 300-W, wide dispersion, 

underwater lights (ML3010, DeepSea Power and Light, CA), shining on a translucent screen. 

The back of the measurement volume is defined by a clear screen mounted in front of the 

translucent screen which has a series of size scale markings at the bottom of the video camera’s 

field of view. Backlighting causes each bubble to appear, ideally, as a dark ring surrounded by 

a central bright spot, allowing, at least in principle, computer analysis, whereas side-lighting 

produces half moons that require manual outlining (Leifer and MacDonald, 2003). When a 

bubble is too close to the backlighting screen, off-axis rays obscure the bubble’s edges, 

decreasing contrast and biasing bubble size towards a potentially significant underestimate 

(Leifer et al., 2003a). 

Figure II-2. Pool test of the seep Bubble Measurement System in open configuration. 
Final Study Report - Leifer 

14 

To ensure the bubbles are at a known distance, where the resolution can be related to the screen 

scale markings, bubble blockers are positioned underneath the spaces between the camera and 

the measurement volume and between the lights and the clear screen. Parallax errors are 

minimized (and can be calibrated for) with long focal length - i.e., high zoom - settings. The 

underwater video camera (SuperCam 6500, DeepSea Power and Light, CA) allows complete 

remote control, including most importantly shutter speed, which is set high enough to prevent 

bubble blurring. All components are mounted on aluminum optical railing and/or tubing for 

easy repositioning. Teflon-lined railing sleds are locked into place with bolts tapped into the 

sleds. All bolts are graphite particle lubricated to prevent seizing after recovery. Light and 

video cables are secured to the frame (and boat) with strain reliefs. The whole system is 

slightly negatively buoyant, with the buoys maintaining a vertical orientation. Cables and a 

buoy line are taped together into a neutrally buoyant cable bundle. Video is recorded by an 

onboard Mini-digital video (DV) video recorder (Sony Video Walkman, Sony, Japan) and 

displayed on a flat screen monitor. A diver communication system maintains contact with the 

SCUBA equipped diver positioning the BMS, although it can also be mounted on a remotely 

operated vehicle (ROV). Either a custom-built camera controller (Speed Vision, CA) or a 

laptop controls the camera settings. 

Video clips are acquired directly by Firewire (IEEE-1594) at 60 fields per second at full digital 

video DV resolution (720 x 240 pixels) and analyzed using routines written in NIH Image 

(NIH Image 2000). The routines (see Leifer et al. 2003b; Leifer and MacDonald 2003) extract 

the fields to 60 frames per second, remove background intensity variations, and remove 

pixelation noise. Images are thresholded and all bubbles analyzed in each frame. For each 

bubble, the position, major and minor axes, angle, and area are recorded as well as the frame 

number. From the major (ra) and minor (rb) axes, the equivalent spherical radius is calculated 

(Sam et al., 1996) 

 r = (rararb)1/3 (1) 

Statistically significant fractions of the bubbles are tracked between frames to determine the 

velocity function, V(r). The measurement series are analyzed further with Matlab scripts 

(Mathworks, MA). Sequences associated with tracked bubble are identified and the mean 

trajectory angle calculated with a least-squares, linear-regression analysis. A polynomial, 

least-squares, linear-regression analysis of all tracked bubbles is used to calculate V(r), which 

includes the effects of buoyant rise, turbulence, and bubble-induced upwelling flows. All 

bubbles are r and time, t, segregated and then histogrammed. The histogram segregation bin 

widths are chosen so a statistically significant number of bubbles are counted in bins near the 

distribution peaks. Size bins are spaced logarithmically. Normalization of the histogram by 

the radius increment and depth interval yields the bubble size layer population distribution, Ψ, 

which is the total number of bubbles in each size class per meter of depth. Ψ is what sonar 

observes and can be combined with V(r) to calculate the bubble size emission population 

distribution, Φ. In this way, multiple counting of bubbles as they rise across the field of view 

is accounted for. If the bubble stream is larger than the blockers, the population is scaled to the 

entire stream based on overview video, or the blockers can be repositioned. Finally, 

distributions are fit with a least-squares, linear regression analysis over selected r by a function 

of the form: 

Φ(r,t) = k(t)r-S(t) (2) 
Simulation of a Subsurface Oil Spill 

15 

where F is Φ or Ψ, k is a constant, and S is a power law exponent. The fit is performed on 

 log(F) = -S log(r) + log(k) (3) 

For surface measurements, the BMS is reconfigured in a closed mode, shown in Figure II-3. 

Measuring bubbles at the sea surface is more challenging than at the seabed for several 

reasons. For computer identification, the bubble need to be back illuminated, thus at the sea 

surface, the lights are competing with sunlight. Thus the primary goal of the closed 

configuration is to block out the sunlight. Currents at the sea surface can be large (to 1 m/s are 

common), and these currents have a tendency to move bubbles rapidly towards the camera, or 

cause highly turbulent fluid flows around the screen, preventing accurate measurement of the 

vertical velocity (for flux calculations). As bubbles get closer to the camera, their apparent size 

increases, while as they move towards the illumination screen their contrast with the 

background decreases, sometimes to the point of invisibility. Thus, a second function of the 

closed BMS is to isolate the fluid in the measurement volume from motions associated with 

currents or motions of the boat. 

Figure II-3. Schematic of closed seep Bubble Measurement System (BMS). 

The BMS design allows quick conversion between open and closed orientation. The major 

components of the bubble measurement system (camera, lights, and screen) are mounted on 

sleds that slide on optical rails. These sleds are held in place by bolts tapped through the sleds 

that are tightened to lock the sled in place. The components can be slid out from the open 

BMS framework and slid onto a railing in the closed BMS framework, which is a heavy (7-mm 

walls) 30-cm diameter PVC tube with a gas for bubbles to enter. Buoys are attached to the 

BMS system to maintain the orientation and keep the BMS approximately 30 cm below the sea 

surface (if too shallow, the BMS will form bubbles at the sea surface). Two PVC pipes are 

attached to the BMS to maintain orientation, and the cables are connected to strain reliefs. The 

closed system also has a clear screen at the camera side of the measurement volume that fits 

very tightly to the walls, preventing any water motions along the BMS axis.

II-2-3. Bubble Measurement System Calibration – Blurring 

Given sufficient illumination, current video cameras allow selection of shutter speeds 

sufficiently fast to prevent bubble blurring. However, if illumination is limited, a choice must 

be made between aperture size (i.e., depth of field) and shutter speed (blurring). The decision 
Final Study Report - Leifer 

16 

depends upon the bubble distribution and bubble dynamics of interest. For example, in a 

constrained system, the DOF need only be the distance between windows, while in an 

unconstrained system, where the goal is visualization of bubble plumes; a large DOF may be 

required. The effect of bubble blurring depends upon the bubble’s velocity (pixels per second) 

and size (pixels). While in the laboratory, it is easy to design a BMS with sufficient 

illumination; at sea (shipboard or submersible/ROV, etc.) power may be a limiting factor. 

Furthermore, the illumination required depends upon the size of the FOV. For example, a 

small FOV is required to study small bubbles, and thus light easily can be focused to evenly 

and intensely illuminate the small FOV. In contrast, a large FOV requires significantly more 

illumination power. 

Blurring smears the bubble image but the result may be complex. For blurring of ~1 pixel or 

less, blurring can increase sharpness. Consider a bubble whose edge is half in two pixels. 

Smearing the bubble in one direction can place more of the edge in one pixel, creating a 

sharper edge. Alternatively, if the edge is almost entirely in one bubble, blurring spreads the 

intensity across two pixels, decreasing sharpness. 

Also, since blurring occurs along only one direction, blurring affects only one axis. Examples 

are shown in Figures II-4a and II-4d for a 670-μm bubble. These two images were taken for 

shutter speeds of 1/20,000 s and 1/500 s, respectively. Furthermore, since bubble motion is on 

average - due to buoyancy - vertical, blurring occurs mostly along the minor rather than the 

major axis (Equation 1). Additional complexities arise in the relationship between blurring and 

apparent size since non-blurred bubbles exhibit intensity gradients at their edges (e.g., Figure 

II-4b) and bubble size is determined by thresholding. Finally, to determine an acceptable level 

of blurring, blurring induced errors should be compared with other errors and natural 

variability. 
Simulation of a Subsurface Oil Spill 

17 

Figure II-4. Images of bubbles of 670 μm bubbles for 1/20000 s (a) and 1/500 s (b) shutter speeds. Intensity 

profiles (c,d) for (a,b), respectively for dashed lines in (a,b). Also shown are 670 μm bubbles at lower 

magnification for 1/10000 (c) and 1/500 s (f) shutter speeds. Size scales located under images. 

To understand the effect of bubble blurring upon apparent bubble size, a series of bubble-blur 

calibration experiments were conducted in a glass tank at the University of California, Santa 

Barbara. A schematic is shown in Figure II-5. A regulated airflow was fed through one of two 

drawn capillary tubes to generate single bubble streams of bubbles (Leifer et al., 2000b). 

Bubbles were imaged with a high speed video camera (Kodak, Ektapro, 1000 FPS) for shutter 

speeds ranging from 1/60 to 1/20000 s. Backlighting was provided by a 1000 W spotlight with 

2° dispersion illuminating a translucent plastic screen located behind the tank. Illumination 

was controlled by a variac and varied to allow consistent background intensity for an aperture 

of F8, except at the fastest shutter speeds (≤ 1/10000 s) where the aperture was F5.6. A 55-mm 

telecentric lens (Computar, Japan) was used, thereby minimizing errors due to uncertainty in 

the distance to the bubble. From the highest shutter speeds, capillaries one and two produced r

= 670 and 2240-μm bubbles, respectively. Bubble sizes were consistent during each run. 

Image sequences were digitized at a resolution of 720 X 480 pixels, cubic spline interpolated to 

double resolution and thresholded 10 intensity units above the local background (determined 

by a 10 pixel wide ring around each bubble). Major and minor axes (and angle) were 

determined from a least squares fit of an ellipse to the bubble outline. 
Final Study Report - Leifer 

18 

Figure II-5. Schematic of bubble blur calibration experiment. 

For the three runs, the measured bubble radii versus shutter speeds are shown in Figure II-6. 

Errors bars are one standard deviation. For 670-μm radius bubbles and the narrow field of view 

(Run #1), blurring has negligible effect for shutter speeds greater than 1/1000 s (6.2 pixels blur, 

~ 50% of r). At slower shutter speeds, the error grows rapidly. For the same bubbles but with 

a wide field of view (Run #2) bubbles were ~5 pixels in radius and thus sizing errors are larger. 

In fact, the size overestimate in Run #2 is ~1 pixel. The lower resolution decreases the effect 

of blurring with increasing shutter speed, only becoming significant for speeds of 1/250 s 

where the blur is ~7 pixels (140%). For the larger, 2240 mm oscillating bubble (Run #3), 

several effects combined to make bubble blurring less significant. The greater size (in pixels), 

slower velocity, and greater eccentricity decrease the error due to blurring, particularly with 

respect to the greater uncertainty in bubble size due to oscillations. 

Figure II-6. Bubble radius versus shutter speed for experimental runs showing increase in apparent bubble size 

due to blurring. Run 2 is for the same bubble as Run #1, but with a wider field of view. 
Simulation of a Subsurface Oil Spill 

19 

In summary, bubble blurring of less than a pixel is always negligible; however, greater bubble 

blurring is typically tolerable, particularly for bubbles large enough to show a white central 

spot (~10 pixels). For bubbles smaller than 10 pixels, bubble blurring is generally less 

important compared to errors associated with pixilation. 

II-2-4. Bubble Measurement System Calibration – Off-axis Rays 

While performing some highly accurate measurements of bubble hydrodynamics (Leifer et al. 

2000b), the authors were forced to address potential error sources, including: Where is the 

actual as opposed to apparent bubble edge? This question partially arises from the fact that 

the edge of a bubble image is not a step function in intensity and as a result, different intensity 

threshold levels yield different bubble sizes. Thus, determination of the appropriate threshold 

level is critical to correctly measure bubble size. Calibration experiments were conducted with 

respect to bubble size was measured both optically and non-optically using the displaced mass 

method, reported in (Leifer et al., 2003b). A stream of bubbles was optically imaged with a 

video system and counted by their disruption of a laser beam. The bubbles also were collected 

and the bubble-displaced water “weighed.” In the displaced mass method, a balance outside 

the tank is connected by a lever arm balanced on a knife-edge to a sealed, inverted, funnel in 

the tank. As water is displaced from the funnel, buoyancy decreases the downward pull from 

the funnel, causing an increase in mass on the balance. Using a balance allows measurement 

of much smaller displaced volumes without the uncertainty of the precise location of the 

meniscus and thus the displaced volume. From the gas volume and the number of bubbles, the 

size can be calculated, thereby allowing a non-optical calibration of bubble size. The 

experimental set-up is shown in Figure II-7 below. 

Figure II-7. Schematic of experimental set-up. 
Final Study Report - Leifer 

20 

The air supply was regulated to provide a constant stream of bubbles. Water vapor can cause 

bubble size to rapidly grow after release. To prevent this, the air was presaturated. Bubble 

size can also change due to gas (air) exchange with the water. Thus, the tank was vigorously 

bubbled with large bubbles prior to each experiment to ensure that the tank water was close to 

equilibrium with air. For the bubbles studied, since the surface tension force (La Place 

pressure) was negligible, (Leifer and Patro, 2002) the dissolved air in the tank water should be 

in equilibrium with atmospheric plus 1/2 the hydrostatic pressure for the tank water height. 

The equivalent spherical bubble radius, i.e., the radius the bubble would have if it was a sphere, 

was calculated using two different approaches. The displaced-mass effective bubble radius, 

rM, was calculated by using the collected gas volume, calculated from the measured mass, M, 

and the water density, ρ, and the volume of a sphere, and is: 

 

r M =(

3M / ρ

4πN )

1/3

 (4) 

where N is the number of bubbles counted, ρ was calculated from a polynomial fit to tabulated 

values of ρ from the CRC (1995; p. F-5) as described in Leifer et al. (2000). The collected 

volume was corrected for the difference in hydrostatic pressure between the depth of the 

collection funnel and the imaging depth. Since r depends upon the threshold intensity, the 

background intensity local to the bubble was determined, and images were thresholded 10, 15, 

20, and 25 units above (darker) than the local background intensity. Image intensity varies 

between 0 and 255 and was adjusted so that the images spanned most of this range. The local 

background rather than the overall intensity background was used because of intensity 

variations across the images which although small (typically < 10) were a potential error 

source. The local background was determined from the median intensity in a 10 pixel wide 

ring surrounding each bubble. 

An example bubble image is shown in Figure II-8. For this experiment, rM = 748 μm from a 

bubble mass of 7.87 x10-5 g water displaced per bubble. The halo ring surrounds the bubble in 

Figure II-8a, was ~3 pixels wide, and appears as a ring-like depression surrounding the bubble 

in the surface plot shown in Figure II-8b. 

Figure II-8. Enlarged bubble image (200%) (a) and surface intensity plot (b) of a 748-μm radius bubble with 

intensity scale.
Simulation of a Subsurface Oil Spill 

21 

A histogram of ro for images thresholded 25 units above the background is shown in Figure II9a. For this figure, a series of 1100 frames comprising ~800 bubbles was analyzed. The 

dominant peak represented an elliptical mode in the bubble shape oscillation, while the smaller 

peak represented a spherical mode. Both peaks were very narrow (half width ~5 μm) and 

separated by just 10 μm. 

Figure II-9. Size histogram for bubbles in Figure II-8, thresholded 25 units above the local background intensity 

(a). Optically determined radius, ro, versus threshold intensity (b). Also shown is a quadratic fit, ro with 

extrapolation to background, and mass-derived radius, rM. Data key on figure.

The median values for ro at different intensities are shown in Figure II-9b and clearly show a 

decreasing trend in ro with increasing threshold intensity. A quadratic curve of ro with 

intensity was determined by a least-squares, linear-regression analysis, and is also shown in 

Figure II-9b. The extrapolation of this curve to the background intensity is shown by a circle. 

The horizontal dashed line shows the value of rM. Thus, the actual bubble edge is at a 

threshold value of 147, i.e., below the background intensity. In fact, the bubble edge is hidden 

in the “halo” ring, which renders the method of bubble size determination by simple 

thresholding unrealistic or at best problematical. The size bias also varies depending upon the 
Final Study Report - Leifer 

22 

chosen threshold. For example, if thresholded just 5 intensity units above the background 

(implying good image quality) yields r too small by 5%. 

Based on a series of simple experiments, it was concluded that the major uncertainty in bubble 

sizing was due to reflection. The “halo”, while important for explaining internal reflections 

does not produce an external bright ring. A similar experiment was conducted by Vigneault, 

Panneton & Raghavan (1992). In their experiment, bubbles were produced from a drawn 

capillary tube also supplied by a carefully regulated air supply. The value of ro for a threshold 

slightly above the background was compared with the flow rate derived r. They showed very 

good agreement between the two methods. However, since they did not pre-saturate the air, 

they most likely underestimated the flow rate derived r due to bubble growth from water vapor 

(i.e., bubbles were larger by the time they rose to the level of the video camera, ~2%). They 

also did not correct for hydrostatic pressure. Had corrections for water vapor and hydrostatic 

pressure been made, they also would have found that the correct threshold was slightly below 

the local background intensity. 

It was found that the shape of the backlighting screen determined the “halo” shape. 

Specifically, rotating a rectangular screen changed the orientation of the halo, suggesting that 

off-axis light rays were responsible. A schematic of the geometry is shown in Figure II-10, 

where radial symmetry about the camera axis was assumed for simplicity. For backlit bubbles, 

the screen required to provide even illumination is typically significantly larger than the 

bubble. 

As a result, light emitted from above (or below) the bubble can be reflected off the front bubble 

face towards the camera, thereby appearing to originate at the bubble’s edge, obscuring the 

actual edge. The angles of interest are shallow enough that the bubble can be considered a 

perfectly reflecting sphere. 

Figure II-10. Schematic of bubble edge halo-ring caused by off axis light reflections. Light source from back 

illuminated screen to the right. See text for details. 

If the distance from the camera backplane to the bubble axis, C, and the distance from the 

bubble to the screen, S, are known, then for different angles within the camera’s field of view, 

the lighting screen location for origin of the reflected light, L, can be calculated and is: 
Simulation of a Subsurface Oil Spill 

23 

L = rsin(β) +

S + r cos(β)

tan(α − 2β − π 2)

 (5) 

with β given by 

β = π − sin−1{

C

r

sin(α)}. (6) 

The derivation of (5) is found in Leifer et al. (2003b) Alternatively, for a specified screen 

radius, LS, the fraction of the bubble image that is front illuminated (i.e., not dark) can be 

calculated. 

A calculation of L using (5) is shown in Figure II-11 for r = 10-3 m, C = 1 m over a range of S. 

Values of L are shown versus fraction of the maximum bubble angle, αM, which for small 

angle is simply: 

 αM = tan(r/C) (7) 

and for this case was 10-3 radians. For LS=20 cm (dashed line) and S varying from 1.0 m to 0.1 

m, the forward reflection calculation predicts a ring that appears from ~0.5% to ~15% from the 

bubble edge, respectively. Since α was small, L was largely insensitive to C or r, unless C

was very small (on the order of a few centimeters). 

For the experiment shown in Figure II-11, C, S, and LS were ~75 cm, ~25 cm, and ~15 cm, 

respectively. Equation (5) predicts a radius fraction of ~4%, in general agreement with the 

observed ~2.5% difference between rM and ro for thresholding at the background. The 

discrepancy probably arises because the reflected intensity is added to the bubble intensity. 

This simple model (Figure II-11) shows how off-axis rays can obscure the bubble edge, but the 

calculated bias assumes thresholding at the background. If a simple threshold is chosen to size 

the bubbles, then the bias will be greater, perhaps significantly, unless the illumination screen 

angle (φ) is small. When bubble distributions are strongly r dependent (e.g. oceanic bubble 

distributions vary as r-3 - (De Leeuw and Cohen, 1994; Dietz 1979), even small biases in r will 

produce large biases in parameters such as total bubble surface area and volume. 

Figure II-11. Calculated screen distance of origin, L, versus fractional apparent bubble angle (α/αM) for several 

screen distances, S. See text for details.
Final Study Report - Leifer 

24 

II-3. Methodology - Gas Geochemistry 

Bubble dissolution depends upon the equivalent gas concentration difference between the 

bubble and the surrounding water. Thus for predicting the dissolution rate (gas outflow rate) of 

a bubble, the dissolved gas concentration in the bubble plume must be measured, as well as the 

partial pressure of the different gas components in the bubbles. Although methane, nitrogen, 

and oxygen are the most significant gases in the system, trace gas measurement are useful for 

model validation. 

Dissolved gases were collected in glass syringes at the seabed and sea surface. To get the 

dissolved gas composition in the bubble stream, divers blocked the rising bubbles with their 

bodies by leaning backwards, and collected the samples from in front of their chests. Several 

approaches were used to collect bubble gas samples. In one method, Zip Lock bags were filled 

with gas and 50-ml glass syringes used to remove ~15 ml of gas (so that syringes would not 

overflow at the surface). Another method used evacuated Teflon bags connected to a funnel 

that were filled with seep gas, and finally, small BOD bottles were used, inverted over a small 

funnel placed over a bubble stream. In this method, the bottle was flushed with several 

volumes of gas before sealing with a septa cork, and crimping. 

The Teflon bag method allows gas to be collected with minimal contact with water; however, it 

was only able to function where the flow of gas was large enough to “inflate” the bag, thus the 

preferred method for small seeps is the BOD bottles. However, this method was only 

implemented starting in 2003, and thus all data reported here is for either Teflon bag collection, 

or syringe extraction from Ziploc bag. 

Water collection was using glass syringes. The syringes were held onto a flat piece of plastic 

which allowed water collection without removing the syringe from the plastic support plate. 

The plate was marked with the sample location (e.g., 10 m, 20 m, surface, etc.) and thus 

prevented confusion in where samples were collected. Syringes were drawn slowly so that 

there was no cavitation. 

In the laboratory, gas samples were transferred directly to vacutainers in a container filled with 

argon. Gas in the water samples was extracted by the headspace method (Dietz 1979). 

Basically, the headspace gas, argon, was injected into the syringe and the syringe shaken to 

allow dissolved gas to reach equilibrium with gas in the headspace. Headspace gas is then 

transferred into vacutainers. Argon rather than nitrogen was used to allow GC determination 

of the nitrogen. Gas samples were analyzed using standard gas chromatography techniques at 

Zymax Forensics (San Luis Obispo, Ca). 

II-4. Methodology – Fluid Motions 

Numerical studies also show a strong sensitivity to the upwelling flow, the principle exploited 

by the turbine-seep tent to measure gas flux. This flow is important because bubbles are 

assumed entrained within the flow (Woolf and Thorpe 1991), rising at their buoyancy velocity, 

VB, relative to the flow. Thus the upwelling flow reduces the transit time of the water column. 

Another effect is the upwelling of methane rich water (McDougal 1978; Leifer and Judd 2002). 
Simulation of a Subsurface Oil Spill 

25 

Both effects increase the methane flux to the atmosphere. The upwelling flow was inferred 

from the BMS-derived V(r), but required assumptions about the bubble cleanliness and 

oiliness. Alternatively, dye tracer studies allow direct measurements of the upwelling flow. 

Fluorescein dye (commonly used in oceanographic studies and available from marine suppliers 

as rescue marker dye) was introduced into the bubble stream and the time to reach the surface 

measured. First, the dye was dissolved in seawater in a clear, 10-cm diameter plastic tube. 

The tube bottom was capped by a large rubber stopper held in place with an elastic cord, while 

the tube top has a sheet of aluminum foil held loosely in place by electrical tape and secured by 

another large rubber stopper. At the seabed and adjacent to a major vent, the diver removed 

the top and bottom stoppers while maintaining the tube in a vertical orientation. The tube was 

then moved laterally over the vent and the rising bubbles immediately popped off the loosely 

held aluminum foil, dragging the dye out of the tube. Before moving the tube over the bubble 

stream, the diver gave a countdown over the acoustic diver communication system to scientists 

onboard the boat who timed the dye arrival at the sea surface. Previous attempts to inject the 

dye into the stream from syringes, etc., often missed or significantly disturbed the bubble 

stream, resulting in a significant delay of several seconds before the dye was entrained in the 

upwelling flow. 

III. RESULTS 

III-1. Results - Seabed Morphology 

When first visited in November 2000, Shane Seep was characterized by two East-West HC 

volcanoes sharing a center ridge, each ~3-m diameter and nearly circular. The location of the 

central ridge is shown as a dashed line in Figure III-1a, although precise measurements were 

not taken at the time. At the sea surface, there were two distinct plumes that formed two 

outwelling flows with a strong convergence zone between. Over the following three years 

numerous changes occurred (see Figure III-1) including the appearance of four new major 

volcanoes along an East-West line, while the original vents dwindled in importance. Although 

video surveys were conducted before November 2001, Figure III-1a shows results from the 

first survey with measuring tapes. 

During 2000, the seabed morphology changed very little, until the appearance of volcano #3 

during a three-week period (between two video surveys) in October 2001. Volcano #3 was 

discovered during a dive to place a chain link transect chain on the seabed (Figure III-1a). 

During this time period there was a (potentially coincidental) magnitude 2.4 earthquake at 10-

km depth and within 500 m of Shane Seep, on October 17, 2000 at 2300, PST. However, 

given the absence of direct observations, a causal tectonic mechanism remains hypothetical. 

At this time, the walls of volcano #3 rose ~25 cm above the surrounding seabed and were 

extremely circular, centered on the main vent. Many large tar blocks (up to 1-m diameter and 

30-cm high) were observed in its interior. Significant changes also were observed for 

volcanoes #1 and #2, whose walls were generally lower and their central dividing ridge 

completely gone. Volcano #2 was also deeper and larger. Two weeks later (Figure III-1B), all 

vents had increased in extents and wall heights, and volcano #3's caldera was rocky, although 
Final Study Report - Leifer 

26 

tar blocks were still observed. The location of vent #2 had shifted almost 1-meter North within 

its caldera. 

Figure III-1. Shane Seep seabed surveys from 2000-2003. Dates shown on panels, small grid is 1 meter, large 

grid is 5 meters. Profile vertical heights are to scale. and indicated by vertical lines. For each hydrocarbon (HC) 

volcano, vent numbers (#) show locations of main vents. Lines indicate direction of increasing slope towards HC 

volcanoes. Snake-like shapes are tar ridges. See text for details. 

The next measurement survey was March 7, 2002 (Figure III-1C), and showed additional 

changes in seabed morphology. The extents of volcanoes #1 & #2 had decreased and the walls 

were shorter and less steep, possibly due to sand scouring by winter storms during the previous 

months. The walls of volcano #3 also were less steep, but its extent had increased 

significantly. Most significantly, the transect chain now disappeared into the walls of volcano 

#3, although not the other volcanoes. Beyond volcano #3 a circular mini-caldera, ~1-m wide 

and 2-cm high was videoed from which a few sporadic bubbles would appear from time to 

time (shown at approximate location by dotted circle in Figure III-1C). Its significance was 

not appreciated at the time, and thus measurements were not taken. 

Almost a year passed before the next mapping survey in March 11, 2003, during which feature 

locations were mapped, but profiles were not measured. A new area of very active seepage 
Simulation of a Subsurface Oil Spill 

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had appeared during the previous two months near the mooring point (volcano #0), and the 

major vent locations had moved. Rebar stakes marking vents #1 and #2 were found toppled 

over and relocated to the new vent locations. The transect chain still penetrated the walls of 

volcano #3, which now was on a slightly raised plateau. Also, vent #4 had developed into a 

major feature. Vent #4 now contained many minor vents producing bubble streams (although 

no clear major vent) and was located on a roughly circular, raised plateau. Its walls were also 

higher and its caldera descended below the plateau level by a ~10 cm. Also noted in this 

survey were North-South aligned tar ridges about 1-m high and 5-m long further east than 

volcano #4. Minor vents were more common on top of the ridges than in the surrounding area. 

It is unclear when the ridges appeared as previous video and measurement surveys generally 

did not extend further than Vent #3. Note, on this dive visibility was a relatively clear 2.6 m, 

thus features a few meters beyond the survey extent easily could remain unobserved. Also 

shown on Figure III-1D are the locations where the turbine-seep tents were deployed. 

Significant changes were observed by the next survey, April 9, 2003 (Figure III-1E). In 

particular, sand had been deposited throughout Shane Seep and obscured (although they may 

also have been eroded) many of the features. The rebar stake for Vent #2 was found sticking 

15 cm out of the sandy sediment, instead of the 50 cm when it was relocated during the 

previous survey mission. Volcano walls were largely gone, their previous location shown by 

dashed lines. In their place, ridges were observed, with the transect chain passed through the 

walls of both ridges in its path. These ridges may have been portions of the now buried 

volcano walls. However, where the South walls for volcanoes #1 and #2 wall had been, a 41-

cm high ridge that extended to the N-S transect chain was observed. The south wall of #3 had 

disappeared, but the area North of the ridge was elevated above the seafloor to the East and 

West. In all previous surveys (video and mapping) this area was nearly flat. Furthermore, the 

major vent for #3 had moved ~3 meters east, with the observed ridge centered about this new 

main vent location. Vent #4 had grown significantly, was still circular, and now was centered 

almost two meters further west. Finally, an area of very active seepage had appeared at the 

mooring point, including a small depression (~30 cm from the video surveys). At the time it 

was unclear that this would develop into a major feature and precise measurements were not 

made. 

In the most recent survey, June 30, 2003 (Figure III-1F), the dominant seepage feature was 

vent #0, which had become oblong. Its southern wall was two step, with a raised plateau ~20 

cm wide, halfway up the wall. The location of vents #1 and #2 had shifted southward and 

emitted a fraction of the gas they had several years earlier. Volcano walls had reappeared, 

although partially this may be explained by the observation that the area no longer appeared 

buried in sand. Vent #3 had relocated north of the transect chain at the same distance from the 

mooring point, while vent #4 had moved further south and east. Volcano #4 still was raised 

relative to the surrounding seabed, but less so than before. However, the seabed now sloped 

upwards nearly a meter between volcanoes #3 and #4. Furthermore, the profile of volcano #4 

had changed, it now resembled more a mound than an elevated volcano on a plateau. A final 

change observed but not measured was the appearance of volcano #5, not shown, ~3 m west of 

the mooring point. Divers estimated it was 0.75 m across and with walls than 5 cm tall, but 

very active in terms of gas emissions. Given the absence of walls, it is unlikely to have been in 

existence during previous surveys. It was discovered after it was noted that Shane Seep's seasurface expression suggested five upwelling plumes, a feature not noted during previous visits 
Final Study Report - Leifer 

28 

to Shane Seep. Also, the total seepage at Shane Seep had increased significantly from previous 

surveys, the area of active seepage (defined as more than 3 or 4 vents m-2), which previously 

had extended about 7-m from the East-West transect chain, now extended to almost 13 meters 

to the South. The area immediately surrounding volcano #3 also was far more active. 

III-2. Seabed Morphology Significance 

The seabed surveys demonstrate that seepage features at Shane Seep are both semi-permanent 

and “plastic.” The sediment overburden is Late Quaternary, and for the outer COP seep trends 

is thin, < 1 m. At Shane Seep, the overburden is thicker, 2-3 meters (Fischer 1978), consisting 

primarily of very fine sand, and has modern total organic carbon of 1 - 2% (Fischer 1978). The 

sand overlies fractured Monterey formation basement. The upper 30 cm of sand is cemented 

by tar (La Montagne et al. 2004) and highly cohesive. The seabed near Shane Seep is also 

heavily coated with bacterial mats, and large tar balls can be found within the hydrocarbon 

(HC) or tar volcanoes. These volcanoes are termed hydrocarbon volcanoes rather than mud 

volcanoes because of their high tar content, which provides the necessary cohesion to form the 

volcano walls, and fixed vent locations. 

That the walls represent a depositional process was dramatically demonstrated by the burying 

of the transect chain after it was draped across the newly formed volcano #3. Since elsewhere 

the chain, even two years later, lay on top of the sandy bottom, only a depositional process 

could explain the manner that the chain disappeared into the caldera's walls. Furthermore, the 

appearance of stones in the bottom of volcano #3 suggests that not only tar, but also sand was 

lofted during these events, leaving behind the rocks. 

There are several processes evidenced in the changes in seabed morphology over the years. 

These include deposition of tar from ejections along with the removal of seabed material 

during large ejection events, feature erosion and burial by storms, and the plastic deformation 

of the seabed. Vent locations remain fixed for a period of years but then re-express themselves 

nearby. While ejection events build up caldera walls they also destroy them. Together these 

changes provide strong evidence of the dynamic nature of hydrocarbon seeps. 

During the formation of volcano #3, 100,000 kg of sediment was displaced. This most likely 

was due to either a single or a series of "explosive" events as indicated by highly circular 

caldera shape. And in general, when volcano walls grew higher, they also grew circularly 

(except for Vent #0). Meanwhile, much smaller ejections (but larger than that observed by the 

turbine tents) caught on video did not evidence any significant displacement of seabed 

sediment. Clearly the formation of volcano #3 was a large event (or series of events). The 

source of the event that precipitated the formation of volcano #3 could have occurred deep or 

shallow. If we assume shallow blockage responsible, it still must have been deep enough to 

stop the flow through both existing volcanoes #1 and #2. Otherwise pressure could not have 

built up behind them to then cause destruction of the ridge. Pressure behind the blockage 

would have increased until the blow through destroying the ridge separating the two vents. But 

this raises the question: How does an event that could destroy the ridge between volcanoes #1 

and #2, despite their being highly active (i.e., open) vents, overcome the significantly greater 

resistance represented by the sediment displaced in the formation of volcano #3? Thus it is far 
Simulation of a Subsurface Oil Spill 

29 

more plausible that a deep event was responsible. In this case, a massive pulse of gas suddenly 

was released and traveled rapidly upwards through the fractures. At the seabed, the two 

existing vents were inadequate to flux the great volume of gas, and thus represented a bottleneck behind which the pressure increased until it created a new pathway, volcano #3. The 

giant tar blocks found on volcano #3’s caldera floor must have been squeezed out through the 

fractures immediately under volcano #3, since similar tar blocks were not found in volcanoes 

#1 and #2. It is unclear from the observations whether the deep event was the clearance of a 

blockage in the fracture system or sudden access to the fracture system by a previously 

unconnected gas reservoir. 

Tar does not necessarily migrate in punctuated transient events, for example there are beach tar 

oozes and forms mounds at the south edge of the Carpinteria State Park, CA. This tar seep is a 

long-term feature, having been used in boat making by indigenous peoples for centuries or 

longer (Galloway 1998). Given sufficient pressure, tar migrates. And such pressure may 

explain the appearance of elevated seabed features such as the elevated plateau under volcano 

#4, or the ridges further to the east. Here, some of the fractures may be completely clogged 

with tar, implying an enormous pressure buildup to cause a blow through, which is unlikely 

(i.e., rare) to happen while gas can flow (and pressure be relieved) through other vents. In this 

case, the elevated pressure forces the tar to slowly ooze forwards, gradually lifting the 

sediment layer. Water flow may also be involved. And certainly the very gradual (over a year) 

formation of volcano #4 suggests a gradual process in sharp contrast to the sudden appearance 

of volcano #3. 

The difference between volcanoes #3 and #4 was that volcano #4 formed gradually and 

included a raised mound. Not only did the plateau height gradually increase with time, but also 

the gas flux through the volcano #4’s vents and the size and height of its caldera walls. When 

first identified, volcano #4 was only a few centimeters tall with a few bubble streams. By 

2003, the caldera was meters across, the walls were half a meter high, and there were many 

active vents. This is consistent with the tar slowly being forced from the fractures during the 

initial stages until the gas flux was sufficiently great (i.e., fractures were sufficiently opened) to 

allow the blow through mechanism to begin depositing the caldera walls. 

In fact, the decrease in relative flux, and then impermanence of volcano #3 seen after March 

2003, may relate to a large tar and or fluid flow clogging the vent. This is supported by the 

lifted seabed North of volcano #3 observed in April 2003, prior to the volcano's relocation to 

this area. Similarly, the expansion of volcano #4 between April and June, 2003, was preceded 

by a large (50 cm) increase in plateau height, noted in March 2003. 

The survey mapping showed that Shane Seep has undergone significant changes in seepage on 

a monthly time scale. Seabed changes were linked to large transient seepage events that are 

most probably related to tar blockage. These transient events must occur with some frequency, 

and thus demonstrate that not only do hydrocarbon seeps provide a model of a steady state 

leakage from an undersea pipeline or well, but that they also (transiently) provide an 

opportunity to study more violent leakage processes, such as blow outs. 
Final Study Report - Leifer 

30 

III-3. Results - Bubble Observations 

III-3-1. Seabed vent bubble observations 

Seep Vent Classification Scheme

Within the active seepage area defined as Shane Seep, there are literally thousands of vents 

spanning a wide range of fluxes and distributions. Survey video suggests that vent bubble 

plumes can be classified into four categories, major, minor, elastic, and obstructed, examples 

of which are shown in Figure III-2. 

Figure III-2. Seep bubble images showing different vent types. A) is for volcano #1 (2000) B) shows North-East 

quadrant, ~10 m from mooring point. C) - F) are bubble measurement system images. Arrow shows vertical 

direction, ticks are 1 cm apart. Bubble streams α and β are from major vents, while γ, δ, and ε are from minor 

vents. C) and D) show a major vent, E) shows a large bubble minor vent - e.g., γ, while F) shows a small bubble 

minor vent - e.g., δ and ε. 

Figure III-2A shows volcano #1’s caldera and walls from which both minor and major vents 

are emitting bubble streams. Outlying areas contained only minor vents, an example of which 

is shown in Figure III-2B for ~10 m northwest of volcano #1. Minor vents are characterized 

by a stream of bubbles rising in trains, and have narrow, sharply peaked size distributions, 

generally large bubbles with r>2000 μm. Here large bubbles are defined as r>700 μm, since 
Simulation of a Subsurface Oil Spill 

31 

smaller bubbles do not oscillate and behave dirty, while larger bubbles oscillate and behave 

hydrodynamically clean, even in seawater (Patro et al. 2002). 

Major vents have much higher emission rates and produce bubbles spanning a broad size range 

from small to giant (up to several centimeters in diameter). Both minor and major vents can be 

simulated in the laboratory by blowing air through a capillary tube. At low flow rates, the 

bubble size is determined by the capillary mouth size and is insensitive to flow rate (Blanchard 

and Syzdek 1977). However, at high flow rates, the bubbles begin to coalesce before they can 

separate from the capillary mouth, forming larger bubbles. With increasing flow, turbulent 

intensities increase, and smaller bubbles begin to appear. Finally, at very high flow rates, a 

broad spectrum of bubbles is produced (Slauenwhite and Johnson 1999; Tsuge et al. 1981). 

Elastic vents occur where the seabed behaves like an elastic material, such as tar. In this case 

the bubble flux depends upon tensile failure of the seabed, which occurs when the buoyancy 

force becomes greater than the seabed material’s tensile strength. Thus the vertical flux is 

described by punctuated vertical motions at points and times of structural failure followed by 

static periods of gas accumulation until the next structural failure event. As a result, the time 

and location of the emission of bubbles is random, or quasi random (Johnson et al. 2002). 

Furthermore, the bubbles are typically emitted in “packets” that can include very large bubbles. 

An extreme example is shown in Figure III-2I for Ira Seep, located a few hundred meters east 

of Shane Seep (see Figure III-1) also at 20-m depth. Here, every few minutes the seabed 

distorts and expands until it ruptures and produces enormous (to 10s of centimeters) bubbles. 

The tar mound was ~75-cm diameter and ~30-cm high at the time of this image. 

The final vent type, obstructed, is a catch-all class that covers vents not easily classified in the 

other categories. Obstructed vents occur when a physical feature of or above the vent mouth, 

for example, rocks or kelp fronds, either causes bubble breakup or coalescence into streams of 

very large bubbles. Two obstructed vents are shown in Figures III-2G & III-2H. In Figure III2G, rocks in the vent mouth break the bubbles. For this vent, the gas flow was large enough to 

lift one of the rocks (~5-cm long), in Figure III-2H a log caused coalescence of bubbles from 

vents under its surface, but upon rising around the log they were broken up while passing 

through a sea anemonae (Metridium giganteum). A sarcastic fringehead (Neoclinus blanchrdi) 

is visible in the image. 
Final Study Report - Leifer 

32 

Figure III-3. A) - C) Vertical velocities for vents β, γ, and δ shown in Figure III-2, respectively. Also shown are 

the clean and dirty bubble rise velocity parameterizations in stagnant water, and polynomial, least-squares fit to 

the data. Polynomial fits are provided in Table III-1. Diamonds are outliers. Data key on upper panel. D)-F) 

shows the total layer volume as a function of time for vents β, γ, and δ, respectively. 

Table III-1. Summary of bubble plume characteristics. 

Plume β γ δ Anemone 

Type Major Minor Minor Obstructed 

Mass Flux (mMol s-1) 4.76 0.48 0.22 4.39 

Vol Flux (cm3 s-1) 16.8 3.5 1.63 32.2 

Vol Layer (cm3 m-1) 35.0 9.7 5.27 50.25 

Peak r (μm) 700 3500 2200 30000 

∫Φ 5(μm) 6000 3800 2400 15000 

Vup (cm s-1) 40 18 10 30 

Vz1 (cm s-1) 36.7 17.9 21.1 49.5 

Vz2 (μm s-1) 69.0 53 59.5 10.1 

Vz3 (μm s-1) -0.003 - - - 

 
Simulation of a Subsurface Oil Spill 

33 

Vup is upwelling flow, Vz is a polynomial of form Vz = Vz1 + rVz2 with r, bubble radius, in 

cm. ∫Φ 5 is the radius at which the integrated mass flux accounts for 50% of the plume mass. 

Volume flux and layer volume were corrected to STP. 

Seabed observations of Major and Minor Vents

As discussed in the methods section, calculation of Φ requires both the vertical velocity and Ψ. 

The vertical velocities for the vents shown in Figure III-2, labeled β, γ, and δ are shown in 

Figure III-3A-C. The flux from the major vent (b) consisted of pulses of very large bubbles 

emitted every ~0.1 s, that rose very fast, entraining other bubbles and generating significant 

turbulence. As a result, the standard deviation in the velocities is very large. Also shown in 

the figure are the clean and dirty parameterizations (Clift et al. 1978) for stagnant water. The 

difference in velocity between these parameterizations and the data is due to the upwelling 

flow. A third-order polynomial fit to the data also is shown and largely follows the dirty curve 

for r<5000 μm, strongly suggesting that oil contamination has caused bubbles to behave 

hydrodynamically dirty for r<5000 μm. The sharp increase for r>5000-μm bubbles is due to 

the much higher upwelling flow associated with the pulses of the largest bubbles, although 

these largest bubbles were not in the field of view sufficiently long to measure their velocity. 

The mean upwelling flow can be estimated from the difference between the dirty 

parameterization curve and the fit, and was ~30 cm s-1. During the bubble pulses, though, it 

increased to more than 80 cm s-1. Given the smaller flux, it is not surprising that smaller 

upwelling flows were observed for the minor vents. Assuming bubbles from these smaller 

vents are as oily as the main vent, the estimated upwelling flows for γ and δ, were 15 and 20 

cm s-1, respectively. 

Ψ was calculated for each distribution for each frame, and integrated over r to calculate the 

total layer bubble volume (cm3 per vertical meter) as a function of time, and is shown in 

Figures III-3D - F. The volume flux for the major vent is dominated by the largest bubbles that 

escape each ~0.15 s, and causes peaks in Figure III-3D. The minor vent bubble streams also 

show evidence of temporal variability, or pulsing. Note that volume fluxes for the major vent 

are ~20 to 30 times greater than for the minor vents. 

The Φ distributions for the three vents shown in Figure III-2 are shown in Figure III-4. The 

major vent had a very broad distribution including bubbles as small as 275-μm radius (near the 

lower resolution limit). Very large bubbles also were observed, with the largest ~1.5-cm 

radius, one of which is visible in Figure III-2C. In contrast, the two minor vents produced 

narrow bubble distributions, with peaks at 2200 μm and 3500 μm for vents δ and γ, 

respectively. Also, note that the peak in Φ for the minor vents was greater than for the major 

vent at the same radius; thus the much larger flux from the major vent is accounted for by the 

broadness and presence of very large bubbles. 
Final Study Report - Leifer 

34 

Figure III-4. Bubble emission flux distributions for A) Major and B) Minor vents. Distributions are for BMS 

images in Figures III-2C- F. Data key on upper panel. Also shown is fit to Major vent data. Error bars are 1 

standard deviation. 

To identify which bubbles are most important to the mass flux, the normalized cumulative (i.e., 

integrated) mass flux for the three vents in Figure III-4 was calculated and is shown in Figure 

III-5. The dashed lines show the e-1, 0.5, and 1-e-1 levels. The width between e-1 and 1-e-1

indicates the narrowness of the mass dominating bubbles and was comparable for both minor 

vents (~700 μm), while the major vent spanned a much broader range (~2000 μm). For both 

minor peaks, the 50% level was reached at r only slightly larger than the peak Φr, indicating 

the peaks were narrow and symmetric. The major vent; however, had 50% of its mass 

contained in bubbles larger than 6000 μm, while its peak in Φ was at 700 μm. Thus the mass 

distribution was highly skewed, with bubbles at the peak in Φ (700 μm) contributing negligibly 

to the total mass flux. 
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35 

Figure III-5. Integrated cumulative emission flux distributions, ∫Φ, for vents in Figure III-4. Vents labeled on 

figure. 

Seabed Observations of Obstructed and Elastic Vents

It is difficult to generalize to obstructed vents, as they are highly dependent upon the 

obstruction's geometry. An example is shown in Figure III-3H where a log caused coalescence 

into very large bubbles, that upon rising hit a sea anemonae that causes bubble break-up in 

some bubbles. This vent was located ~3-m North of the mooring point and was videoed in 

2001. The function V(r) is shown in Figure III-6A, along with the clean and dirty 

parameterizations and a linear fit to V(r). For bubbles with r>1 cm, surface contamination has 

negligible effect on bubble rise and the clean and dirty parameterizations converge. The values 

of V(r) show a consistent offset of ~30 cm s-1. Since the bubbles were 30 - 50 cm apart, the 

upwelling velocity probably represents that of the general area from all the rising bubbles at a 

height of ~1.5 m above the seabed - other V(r) were measured 30 cm above the seabed). The 

upwelling velocity agrees well with dye release measurements of the upwelling flow, where it 

took approximately 50 seconds for the dye to travel the 22 m to the sea surface during two 

separate dye release experiments (Clark et al. 2003). This upwelling flow implies a significant 

vertical water flux, as evidenced by the observation that it only took a few minutes for the 

water at the seabed to become clear of excess green dye, despite very weak currents at the 

seabed. Also, on days where the swell has stirred up the seabed, bubble plumes at the sea 

surface are observed as brown due to turbidity (apparent in BMS video) compared with the 

surrounding water. 
Final Study Report - Leifer 

36 

Figure III-6. A) Vertical velocity as a function of radius for the obstructed vent shown in Figure III-2H. B) 

Shows emission flux distribution, and integrated cumulative emission flux distributions, ∫Φ.

The emitted bubbles were ~3 cm radius, but smaller bubbles were formed when these bubbles 

fragmented. Bubbles with r<1 cm probably were formed, but were below the minimum size 

resolution. These very large bubbles are unstable during rise and break-up during ascent (Clift 

et al., 1978). Divers followed some of these large bubbles during their rise and observed trails 

of smaller bubbles breaking off the giant bubbles' trailing edges. These smaller bubbles rise 

slower than the giant bubbles, forming trailing bubble chains, similar in appearance to jellyfish 

tentacles. By 10 meters above the seabed giant bubbles were largely absent. Distributions 

from elastic vents were not available; however, surface observations at Ira Seep showed 

bubbles 10 - 30 cm reaching the surface followed by a cloud of smaller bubbles. The key to 

the survival of the enormous elastic vent bubbles at Ira Seep through 20 m of water may be that 

they were not rising in a turbulent bubble plume, and their hydrostatic growth was comparable 

to loss due to fragmentation and dissolution. 

III-3-2. Sea Surface Bubble Observations 

As bubbles rise, they grow due to the decreasing hydrostatic pressure, shrink due to dissolution 

(the outflow of methane and other gas), and grow due to the inflow of dissolved atmospheric 

gases. Thus, the change in bubble size depends upon the vertical velocity (buoyant rise plus 

upwelling flow) and the gas exchange coefficient for the bubble, kbub. Both kBub and VB 

depend upon parameters such as bubble size, temperature, bubble cleanliness, and oil, among 

other parameters. Thus, measurements of the bubble distribution at the sea surface play an 

important role in model validation. The model is initialized with the seabed emission size 

distribution and the predicted bubble size distribution is compared with the measured bubble 

size distribution at the sea surface. 

As with the bubbles at the seabed, bubbles at the sea surface exhibit considerable spatial 

variability, due in part to the seabed variability, and to the trajectory of the bubbles through the 

water column and the effect of currents, upwelling flow, and other fluid motions. Thus bubble 

video was obtained in transects across the seep surface expression. Latitude and longitude are 

recorded every few seconds, as well as the visual appearance of the bubbles plumes. 

Near surface images from Shane Seep (see Figure III-7a) were digitized and analyzed to 

determine the size distribution, Φ (Figure III-7b), defined as the number of bubbles per size 

increment (1 μm) in the entire measurement field. Φ shows a relatively broad peak centered at 
Simulation of a Subsurface Oil Spill 

37 

2500 μm. Minimum resolution for this optical setting was circa 1500 μm. A power law was 

fit to Φ for r > 3000 μm, where r is bubble radius, and showed a steep decrease with Φ ~ r -3.1. 

Total seep gas in this image is ~7 cm3. Φ can be converted to a mass flux by determining the 

vertical velocity, Vz(r), which is the sum of the upwelling flow and bubble buoyancy and 

scaling to the plume cross section. This method was used by Leifer and MacDonald (2003) to 

estimate seep bubble flux in the Gulf of Mexico. 

Figure III-7. Image of bubbles from Shane Seep (a) and bubble size distribution, Φ, as a function of radius, r.(b). 

Figure III-8. Image of bubbles from Shane Seep using the closed BMS. 

III-4. Results - Bubble and Aqueous Gas Measurements 

The fate of the bubble also depends upon the gas composition at the seabed, and also the 

dissolved aqueous composition through the water column. As an extreme example, if the 

bubbles were pure carbon dioxide, a much more soluble gas than methane, they would dissolve 

much faster than if they were pure methane. And if the water was saturated with methane due 

to the gas outflow from the seep bubbles, the bubble dissolution rate would be significantly 
Final Study Report - Leifer 

38 

decreased, and some bubbles might even effervesce, or grow as methane from the water flows 

back into the bubble. Thus saturation of the water column can have a significant impact on 

bubble survivability and the transport of methane to the atmosphere. 

III-4-1. Dissolved Methane and Higher Alkanes 

In addition to measuring the major atmospheric and seep gases, we also measured trace gases, 

such as ethane, butane, etc., at the seabed and sea surface. Since each of these gases has 

different diffusivity and solubility, their concentration will evolve differently with time. As a 

result, they provide a good constraint on numerical model predictions. 

To characterize the seep environment, dissolved gases, bubble gas composition, and fluid 

motions were measured at three seeps of different sizes and depths, La Goleta Seep (LGS), 

Thor CP Seep (TCS), and Seep Tent Seep (STS). Attributes and locations, major gas 

observations, and trace gas observations are presented in Tables III-1, III-2, and III-3. At each 

seep, water and bubble gas samples were collected in the fluid and in the rising bubble plume 

at the surface. These samples were analyzed for carbon dioxide (CO2), oxygen (O2), nitrogen, 

(N2), CH4, and higher n-alkanes to n = 5 (Table III-2). The sea floor gas composition was 

estimated from measurements collected between 1982 and 1996 at the Seep Tents. For a 

discussion of the Seep Tents see Rintoul (1982); Boles et al. (2001). 

Table III-2. Natural hydrocarbon seeps visited. Locations are at 119°W, 34°N. TCS - Thor CP, LGS - La Goleta 

Seep, STS - Seep Tent Seep. 

Name Location Depth Area Vup Relative 

 (Lat. , Lon.) (m) (m2) m s-1 Activity 

TCS 52.442', 23.650' 20 2 - Low 

LGS 51.183', 22.500' 70 25 0.3 Active 

STS 53.350', 23.050' 70 700 >1 Extreme 

Vup is the upwelling velocity. 

Table III-3. Summary of major gases bubble partial pressures and dissolved concentrations. STS2 is STS 

corrected to 2 m for comparison with other seeps. STSB is STS at the seabed.

Seep z P(O2) P(N2) P(CH4) CMET HCMET

 (m) mbar mbar mbar μMol/l mbar 

STS 1 85 251 691 1000 860 

STS2 2 93 274 754 - - 

LGS 2 105 339 700 1200 810 

TCS 2 75 203 758 260 190

 % % % 

STS 1 7.69 22.8 62.8 1000 860 

STSB 70 0.14 0.79 87.5 - - 

z is sample depth, P is partial pressure, C is concentration, MET is CH4, and H is the Henry's Law Constant for 

12°C.
Simulation of a Subsurface Oil Spill 

39 

Figure III-9. Alkane ratio from sea floor to sea surface for Seep Tent seep (34° 53.35' N, 119° 23.05' W) bubble 

gas. 

Aqueous methane, CH4, concentrations near the surface in the bubble plumes have been found 

to be substantially greater (>108 times) than atmospheric equilibrium values. At three of the 

four seeps sampled, CH4 near the surface was slightly supersaturated with respect to the 

bubble's partial pressure (Leifer et al., 2000a). Thus the rate limiting step for CH4 dissolution 

into the water column was not bubble gas transfer, but rather mixing (by advection and 

diffusion) between the saturated bubble plume water and the bulk ocean. 

Partial pressures for the alkane series to pentane (Table III-4) were also determined for the 

bubbles (dissolved n-alkane concentrations were not determined). In all cases n-alkane partial 

pressure decreased with n-alkane number, n. At STS a comparison between seafloor and 

surface bubble composition showed that the lighter n-alkanes (ethane and butane) decreased by 

about 50% and that the higher alkanes decreased significantly less. Overall, during their rise, 

bubbles became enriched in the heavier n-alkanes relative to the lighter n-alkanes. This 

observation can be understood with respect to the process of bubble-mediated gas exchange. 

As a seep gas bubble rises, it exchanges gas with the surrounding water (seep gases outflow 

and atmospheric gases inflow). Since this is a diffusive process, the heavier alkanes with lower 

diffusivity, D, exchange slower, and as a result, bubbles become enriched with these gases.

Table III-4. Summary of trace gas bubble observations at sample depths listed in Table III-3. For STS units are % 

mole fraction.

Seep P(C2H6) P(C3H8) P(C4H10)P(C5H12) 

 mbar mbar mbar mbar 

STS 25.6 19.6 4.54 2.93 

STS2 27.9 21.4 4.95 3.19 

LGS 20.6 16.3 5.14 2.98 

TCS 14.6 17.7 5.95 4.54

 % % % 

STS 2.33 1.78 0.41 0.27 

STSB 5.09 3.07 0.43 0.24 

P is partial pressure. 
Final Study Report - Leifer 

40 

At the sea floor, bubbles are about 90% CH4 and 10% higher hydrocarbons while at the 

surface of the deeper seeps (65 m) they are about 60% CH4, 30% air, and 10% higher 

hydrocarbons. The decrease in CH4 at a shallow seep (20 m) was much less; the surface 

composition was about 70% CH4, 15% air, and 15% higher hydrocarbons. For alkanes heavier 

than CH4 (i.e., crosses on Figure 1), the ratio of the surface to sea floor mole fraction showed a 

linear enhancement (see Figure 1) with increasing alkane number. CH4 behaved differently 

because the water column was saturated with CH4. 

III-4-2. Dissolved Oil 

Preliminary investigations have shown that spectrofluorometry can be used to investigate oil 

dissolution from the bubble plume. Water samples were obtained in a horizontal transect under 

the seep surface expression at a depth of 1 m for fluorometric and gas chromatographic (GC) 

analysis. The features of the surface expression of the seep are shown in Figure III-10a. The 

current was towards the north, while the transect line was cross current (east to west). Within 

this surface expression, there were three different regions, the central upwelling area where 

most bubbles surfaced, an outwelling region where flow from the central region dominated, 

and an outer region where oil slicks were visible. 

Figure III-10. Shane Seep (34°24.37’ N, 119° 53.41’ W) surface expression showing features and sampling 

transect (a), and sea floor map at 20 m depth (b). Squares in (b) are is 1 m by 1 m and dots represent seafloor 

vents. 

Transitions between these regions are marked on Figure III-10a. There were two strong bubble 

plumes in the central region, presumably one for each of the large sea floor vents - located in 

mud volcanoes. A sea floor map of the major Shane Seep features is shown in Figure III-10B, 

corresponding to Figure III-10A, before formal survey measurements were conducted. 

Spectrofluorometry results are shown in Figure III-11A. Excitation was at 337 nm, emission 

was integrated over 350 - 600 nm, and values are normalized to quinine sulfate. Spectra 

showed clear oil signatures. The transect (shown in Figure III-10A) was cross current, and the 

surface slick features corresponding to Figure III-10A are indicated on Figure III-11A. The 0-

m coordinate is centered in one of the bubble plumes. Between the two bubble plumes was a 

surface convergence zone (+2 m) that showed local fluorescence maxima. The highest 

fluorescence was located under the visible surface slick (-10 m), while the lowest was in the 
Simulation of a Subsurface Oil Spill 

41 

center (+3 m). This trend was also observed at 15 m (Figure III-11B), and is consistent with 

advection by the strong outward surface flow of dissolved from the bubble plume. Moreover, 

within the plume, it is likely that the high number of bubbles effectively collects, or sparges, oil 

from the water column. 

Figure III-11. Plot of excitation-emission fluorescence, normalized to quinine sulfate with background 

subtracted, for 1 m depth (a) and 15 m depth (b) for Shane Seep. 

III-4-3. Fluid Motions and Plume/Ocean Interactions 

Strong upwelling velocities were observed at larger seeps and all showed surface divergence. 

Dye released at La Goleta Seep rose at circa 30 cm/s, while at both Shane Seep and the Seep 

Tent Seep, vertical rise was in excess of 1 m/s, and very unsteady. Our present research is 

addressing not only bubble plume dynamics and geochemistry but also ocean-bubble plume 

interactions. For example, saturation of the bubble plume water is very important to the bubble 

dissolution rate (i.e., survivability). Thus, the exchange rate between the bubble plume and the 

bulk ocean (that is significantly undersaturated with respect to the bubbles) is critical for 

determining the fraction that dissolves during transit. Dye was released at the bottom of Shane 

Seep and rose through the 22-m water column in 54 s, or 41 cm/s. During the dye release, the 

dye initially spread out into a cloud ten meters or so in diameter; however, over the subsequent 

minute, all of the dye was entrained and arrived at the surface. 

III-4-4. Atmospheric Methane Emission Plumes 

Overview

Subsurface measurements present a time sequence of the gas emissions from a single vent, or 

for a flux buoy (Washburn et al., 2005) a small fraction of a plume. Sonar has greater potential 

as the time necessary to map a plume is less; however, the inversion problem presents 

numerous challenges (Hornafius et al., 1999). From the time-spatial sequences, a map of the 

emission flux can be derived, based on the assumption that emission is constant with time, or 

that the sampling at each location is statistically-significant and normally distributed with time. 

Unfortunately, variations in seep emission occur on all time scales, from the sub-second to 

swell time-scales (Leifer and Boles, 2005a), to tidal (Boles et al., 2001) and longer. These 
Final Study Report - Leifer 

42 

variations occur due to external forcing, such as swell and tides, and internal processes, such as 

oil-gas interactions and changes in the resistance due to tar accumulation and advection. 

Clearly, the ability to obtain an integrated emission flux from a seep area would be invaluable. 

Efforts were made to use in-situ atmospheric methane measurements to measure the total 

integrated emission from a seep area. These efforts were aimed at interpreting fortuitous 

observations of a blowout. 

Methane and non-methane hydrocarbons in the Santa Barbara County

The 1999 reactive organic gas, ROG, emissions inventory for Santa Barbara County was 80.37 

tons dy-1, excluding vegetation sources. Vehicles (cars, trucks, airplanes, etc.) were the 

dominant human emission source (25.9 tons dy-1). Off-shore activities (petroleum production, 

distribution, and shipping) contributed a small fraction (2.84 tons dy-1). Stationary sources 

include power generation, and industrial activities like surface coating (SBCAPCD 2001). 

Marine hydrocarbon seeps in the Santa Barbara Channel are among the largest in the world, 

significantly affecting air quality in Santa Barbara County. From sonar survey estimates, the 

seeps release 105 m3 day-1 of gas and 100 bbl day-1 oil (Quigley et al. 1999). Assuming a 30% 

volatile component in the oil, marine seeps contribute 35±7 tons ROG day-1, or 43% of the 

county ROG, and thus are a very significant ROG pollution source. This ROG reacts with 

NOx to form ozone thereby lowering air quality standards. 

On-shore observations of seep emissions

The map of the seep field shown in Figure III-12 shows the location of the West Campus Air 

Pollution Station (WCS), which is owned and operated by Venoco, Inc. for the Santa Barbara 

County Air Pollution Control District (SBCAPCD). With respect to Shane Seep, WCS is in 

the onshore direction (47°) and 1.49 km distant and approximately 700 m inshore. Windveering can be considered negligible given the proximity of WCS to the shoreline and lack of 

significant topographic features in the vicinity – WCS is 6 m above sea level. 
Simulation of a Subsurface Oil Spill 

43 

Figure III-12. The Coal Oil Point seep field, Santa Barbara Channel, California. Upper left panel shows location 

in California, lower left panel shows Santa Barbara Channel. Gray areas in main panel indicate regions of high 

bubble density determined from sonar returns (Hornafius et al. 1999). Inshore seeps were too shallow for the 

sonar survey. WCS is West Campus air pollution station (34° 24.915' N, 119° 52.716' W), UCSB is University of 

California, Santa Barbara. 

The significance of the seeps is shown in the THC data from WCS. Figure III-13 shows that 

winds at WCS in 2001 primarily were dominated by the onshore breeze and less so by the 

offshore (night) breeze, although there was also a strong prevailing wind component 

(westerly). Figure III-13A shows the total number of hours the wind blew in each direction 

bin. Average total hydrocarbon, THC, concentrations as a function of wind direction were 

calculated by first segregating all 2001 wind direction measurements into 2° direction bins. 

Then, all THC measurements in each (2° direction) bin were averaged (Figure III-13B). The 

averaged THC data shows the primary THC pollution source for WCS is the Coal Oil Point 

Seeps. High THC values clearly correlate with winds from azimuths to the seeps, with the 

greatest values due south, i.e., from the Coal Oil Point Seeps and the Seep Tent Seep, but also 

from Shane Seep. In contrast, land sources produce lower THC concentrations at WCS. This 

is partially because the sources are farther away, but primarily since the main land based THC 

source (rush hour) only lasts a few hours its average is lower than for the continuous seep 

emissions. 
Final Study Report - Leifer 

44 

 

Figure III-13. A) Hours per year wind direction in 2° bins for WCS in 2001 from hourly readings. B) Total 

hydrocarbon concentration (THC) at WCS for 2001 from hourly measurements as a function of wind direction in 

2° bins. Direction to various seeps labeled on figure. 

Source strength (i.e., total emissions along the wind path can be estimated from back trajectory 

calculations for an atmospheric plume. 

For a Gaussian plume, the ground concentration downwind, C, is (Hanna et al. 1982): 

C(x, y) = k(x, y)Q (1) 

where x and y are the downwind and transverse distances from the source, respectively, Q is 

the source strength, and k describes how Q decreases with distance. If k is expanded (1) can be 

rewritten as: 

C(x, y) = 2πσz(x)σ y(x)Qu e

−1

2

Y

σ y (x )

⎛ 

⎝ 

⎜ ⎞ 

⎠ 

⎟ 

2

e

− 1

2

Z+h

σz( x)

⎛ 

⎝ 

⎜ ⎞ 

⎠ 

⎟ 

2

 (2)

where z is height, σy and σz are the horizontal and vertical diffusion coefficients, respectively u

is the wind speed, and h is the emission height. The value of h can be assumed zero, since 

even though methane is lighter than air it is still a small fraction of the total air gases. Both σy

and σz are described by functions of x that depends upon atmospheric stability, which depends 

upon solar insulation, surface roughness, and u, or can be determined directly from wind 

measurements. For example, for slightly unstable conditions that occur for light sun and 3 < u

< 4 m s-1, or for moderate sun and 2 < u < 3 m s-1, and surface roughness typical of the ocean 

at 3 m s-1, Briggs turbulence yields: 

σ y = 0.11x 1 +10−4x ; σz = 0.08x 1 + 2x10−4x (3) 

Thus for C measured at a known distance from the source, k can be calculated from (1) and Q

estimated. There is some uncertainty in the validity of equations for σy and σz for coastal 

conditions since the parameterizations for σy and σz as in (3) were from land studies. 
Simulation of a Subsurface Oil Spill 

45 

On-water observations of seep emissions

Greater detail about the seep emissions and fluxes to the atmosphere can be measured from insitu THC measurements. A Flame Ion Detector (OVA-88, Foxboro) was used to measure 

methane in-situ and was recorded with a multichannel data logger (OMP-MODL, Omega 

Corp, CT). Although seep gas at the seabed contains significant CO2 (12%), and ethane (3%), 

with trace air and higher n-alkanes, by the time the bubbles reach the sea surface, the more 

soluble components have been stripped out. Air samples collected above Shane Seep in 1-L 

Teflon bags (SKC, PA) were analyzed by the Southern California Gas Company, Engineering 

Analysis Center (Table III-5). Atmospheric composition was 1.91% methane, with combined 

n-alkanes comprising <0.02%. Thus, although non-methane seep gas components do enter the 

atmosphere, hydrocarbons were more than 95% methane. 

Table III-5. Atmospheric gas composition above Shane Seep. 

Gas Seabed (%) 1-m Depth (%) Atmosphere (%)

CH4 84.0 79.4 1.91 

C2H6 1.2 0.80 N/A 

C3H8 1.1 0.79 0.0033 

C4H10 .55 0.46 0.0026 

C5H12 .18 0.18 0.0015 

C6+ N/A N/A 0.0156

Methane is emitted in highly discrete locations due to the rising bubbles. These atmospheric 

plumes rapidly mix, forming a large plume that is advected and diffuse laterally and vertically 

downwind. To create a spatial plume map, the plume was repeatedly transected. A transect 

lasted two to three minutes and was easily resolved the CH4 plume. A sample transect is 

shown in Figure III-14 for August 13, 2003. This transect was very near to Shane Seep and 

methane levels were as high as 12 ppm. The background level represents elevated methane 

levels in this portion of the seep field. In August 2003, Shane Seep was characterized by three 

main plumes, within a larger area of a dispersed bubble emissions, plus a second area of 

seepage ~50 m to the west (W. Shane Seep). Plume structure is clearly visible and arises from 

variability in temporal and spatial emissions and turbulence. Thus, some of the heterogeneity 

is due to resolving the multiple sources of methane. A second source of variability is likely 

due to swell. Leifer and Boles (2005b) showed that seepage at Shane Seep responds at swell 

frequencies. Their study was for the peripheral vent area of the Shane Seep area; however, 

visual observations reveal strong pulsing of the main plume at circa the swell frequency. Thus, 

some of the variability likely is due to swell induced modulation. The dashed line represents a 

column height of methane that is one half the atmospheric column height of methane. 
Final Study Report - Leifer 

46 

Figure III-14 Transect through the Shane Seep plume ~50 m downwind for Aug 15, 2003. Dashed line roughly 

corresponds to a 0.5 g column height of methane. times background CH4. Inset shows location and distance scale 

of transect and is referenced to 119°W, 34°N; circle shows location of Shane Seep. 

A contour map from the same survey is shown in Figure III-15. Two sources are clearly 

identifiable, one due to bubbles entering the atmosphere (the roughly East-West trend) and one 

due to air-sea exchange from the dissolved CH4 plume (34° 24.40'N, 119° 53.40'W). The 

wind-driven plume was heading offshore and trails off towards the South. 

Figure III-15. Contour map of Shane Seep atmospheric THC concentration (%). Lines show boat tracks, with 

ticks indicating measurement points and circles show starting points. Location of Shane Seep and West Shane 

Seep indicated by targets. Distance scale shown on figure. Measurements were gridded to 0.01° latitude-longitude 

bins and measurements in each bin averaged. Winds were light (2 m s-1 to 2.8 m s-1) from WNW. Currents were 

to the north at ~0.5 m s-1. 

The contour map is only a two dimensional representation of methane concentrations near the 

sea surface of a three-dimensional plume; however, all the methane in the plume must be 
Simulation of a Subsurface Oil Spill 

47 

considered for flux estimates. A three-dimensional Gaussian plume was matched to the 

surface data (Figure III-16) to estimate the methane column-height and the emission flux or 

source strength, Q. 

Figure III-16. Contour plot of Gaussian plume model for sea surface CH4 (%). Q is source strength. 

A rough fit to the sea surface data by adjusting the Gaussian plume parameters is shown in 

Figure III-16. Briggs turbulence for moderate sun and moderately stable conditions and 2.9 

m/s wind speed was used. Using the plume formulation (2), the vertical column of CH4 per 

square meter was calculated and is shown in Figure III-17. For a Gaussian plume and for the 

first few hundred meters from the source, over 99% of the methane is found in the lowest 100 

m of the atmosphere. 

Figure III-17. Contour plot of vertical integration of Gaussian plume. Figure III-16 shows the sea level 

expression. 

III-4-5. Extreme Emission Events – BLOWOUT! 

The techniques described and ground truthed in section IV-4-4 were used to study a fortuitous 

observation of a large transient emission from Shane Seep, a blowout. Much of the following 

text is adapted from a manuscript for submission. 
Final Study Report - Leifer 

48 

Introduction

Atmospheric methane, CH4, is an important greenhouse gas at least 20 times more potent than 

carbon dioxide (Khalil and Rasmussen 1995) whose mixing ratio has more than doubled during 

the last century (Rowland 1985). CH4 has both anthropogenic (375 Tg) and natural sources 

(160 Tg) of either biologic or geologic origin (Prather et al. 1995). Natural geologic sources 

include marine seepage associated with hydrate dissociation and leakage from deeper 

hydrocarbon reservoirs. The contribution of these seafloor sources to atmospheric CH4 is 

uncertain due to the likelihood that some or all of the emitted CH4 dissolves into the ocean 

during transit from the seabed to the sea surface (Clark et al. 2003; Heeschen et al. 2003; 

Leifer and Judd 2002). Global estimates from marine seeps (neglecting hydrates) are ~20 Tg 

yr-1 (Kvenvolden et al. 2001), or ~13% of natural emissions. Although seeps are found on all 

continental shelves (Hovland et al. 2003), few quantitative emission rates exist. Furthermore, 

while marine sediments preserve widespread evidence of large transient CH4 releases, the 

magnitude and frequency of these transient events remains unknown. Thus, the seep 

contribution based on gentle bubble emanations typical of steady-state seepage likely is an 

underestimate since it neglects the contribution from large transient events. 

CH4 hydrate dissociation has been proposed to play an important role in atmospheric CH4 

budgets and climate change – e.g., The Clathrate Gun Hypothesis - (Kennett et al. 2003; Katz

et al. 1999; Severinghaus et al. 1998; Dickens et al. 1995). For tropical to mid-latitude oceans, 

CH4 hydrate deposits are at depths where water-column transit may represent a formidable 

barrier. 

Herein, we present the first quantitative observations of a large transient seepage emission, and 

numerical model results based on the observations indicating that these events allow significant 

CH4 to reach the atmosphere even from the deep sea. Given the vast estimated CH4 hydrate 

reserves – 5x106 Tg, any process that enhances the marine geologic CH4 contribution to the 

atmosphere is important. This is particularly relevant since global warming and increased 

ocean temperature likely will increase hydrate dissociation (Hatzikiriakos and Englezos 1993). 

Observations

Observations were at a highly active, shallow (22 m) marine seep area (unofficially named 

Shane Seep; 34° 24.370’N, 119° 53.428’W), in the Coal Oil Point (COP) seep field, near the 

University of California, Santa Barbara (Figure III-12). The Shane Seep area is located at the 

intersection of crossing E-W and NE-SW fault trends (Mark Kammerling, Venoco Inc. pers. 

com., 2004). 

Intense seepage at Shane Seep escapes from vents centered in several pockmark-like 

hydrocarbon (HC) volcanoes (Leifer et al. 2004). On November 7, 2002, two heavy iron 

chains were lain down along N-S and E-W lines, which transected HC volcano #3 that had 

formed a few weeks earlier. Several months later, video surveys showed the chain on the 

seabed except where it penetrated the volcano walls, demonstrating a depositional process for 

the formation of HC volcano walls. Observations described below suggest chain burial likely 

occurred from multiple blowout events. 
Simulation of a Subsurface Oil Spill 

49 

Figure III-18. Seabed map of central zone of Shane Seep area from survey Mar. 7, 2002. Symbols key below A. 

B. Hydrocarbon volcano profiles. Grid on A. and B. is one meter square. C. and D. Video captures from Mar. 8, 

2002 survey showing penetration of transect chain into hydrocarbon (HC) volcano walls. Positions and view 

angles of C. and D. shown by labeled arrows on A. A is from Leifer et al. (2004). 
Final Study Report - Leifer 

50 

Figure III-19. Video captures of large gas ejection at Shane Seep during a dye injection experiment. Before the 

ejection (A) seepage was quiescent. Blowout (B) bubble streams rose and grew rapidly (C). Seconds later the 

diver (outlined in white) injected dye (D) which first reached the sea surface 7 s later. Most of the dye arrived 

slightly later (E). Overflight images show initial arrival (G) and after tens of seconds. Times relative to blowout 

and size scale on figure.

On March 8, 2002, SCUBA divers and videographers were at Shane Seep to measure the 

bubble plume’s upwelling flow by introducing fluorescein dye into the bubble stream at the 

seabed. A diver communication system allows onboard scientists to record the dye arrival time 

at the sea surface and thus the fluid upwelling velocity, Vup. Video cameras were at the 

seabed, 5 m above the seabed, shipboard, and in an airplane. A test dye release at 0845 Local 

Time (LT), yielded a 50-s transit time – Vup~45 cm s-1, comparable to previous values Clark 

et al., (2003). Ten minutes before the airplane’s arrival, divers reported that seabed seepage at 

the main HC volcano had virtually ceased (Figure III-19A). At 0936 LT a large gas ejection 

occurred at the seabed (Figures III-19B & III-19C). Suddenly, three separate gas streams arose 

from the seabed, described by the divers as sounding like a freight train (Shane Anderson, 

UCSB, pers. comm., 2002). The leading bubbles expanded very rapidly so that by 5 m above 

the seabed the bubbles were several meters in diameter. Dye introduced into the bubble flow at 

the seabed a few seconds after the blowout (Figure III-19D) first was observed at the sea 

surface 7 s later (Figure III-19E) – peak Vup ~ 300 cm s-1, although the main mass of dye 

arrived 10 s after dye injection – Vup ~ 200 cm s-1. Bubble plumes lift deeper, cooler water, 
Simulation of a Subsurface Oil Spill 

51 

which at the seasurface formed a divergent surface outwelling flow. This outwelling flow 

which is cooler and thus denser then sinks. The outwelling flow is a perennial feature of Shane 

Seep; however, after the blow-out, the area of outwelling flow rapidly expanded during the 

ejection (Figure III-19G & III-19H). Overflight images clearly show the dyed bubble stream 

throughout the water column, tilted by the currents (Figures III-19I & III-19J). Meanwhile at 

the seabed, tar pieces “rained” down between the videographer and vents, i.e., in the area of the 

volcano walls. Meanwhile at the seabed, tar pieces settled between the divers and vents in the 

area of the volcano walls. After several minutes, the flux slowly decreased until seabed video 

showed a return to approximately normal, both for the main vents and the surrounding smaller 

vents. 

The wind direction was on-shore and towards WCS during the period of the ejection and the 

CH4 plume from the ejection was detected at WCS at precisely the time predicted based on the 

wind speed. Total hydrocarbon (THC) levels were elevated an average 1.05 ppm above 

background for 6 minutes before returning to normal. 

The mean wind speed, u10, from 0930 to 0950 LT was 1.57±0.15 m s-1, yielding an advection 

time of 15.8±1.6 min, for predicted arrival at 0952 LT. The recorded total hydrocarbon (THC) 

(Figure III-20) shows a THC pulse arriving at 0952 LT. THC levels were elevated an average 

1.05 ppm above background for 6 minutes. Background was defined as the mean THC from 

0935 to 0945 LT and was 2.28 ppm. At 1000 LT, the wind began shifting southwards. From 

1010 to 1020 LT, a plume from the Seep Tent Seep area (195° - 3.6 km) was advected over 

WCS. Although more distant, normal emissions from the Seep Tent seep area (at 195°, 3.6 km 

from WCS), and THC levels at WCS in the plume, are an order of magnitude greater than from 

Shane Seep. 
Final Study Report - Leifer 

52 

Figure III-20. West campus air pollution station (WCS) measurements of total hydrocarbon (THC) for Mar. 8, 

2002 and B), WCS wind direction data for same day. Grayscale bar on A) shows wind direction. 

Source strength, Q (m3 s-1), was back-calculated from the WCS THC data using a Gaussian 

plume model (Hanna et al., 1982). We assumed the 10-m wind speed, u10, wind direction, θ, 

and atmospheric stability were constant between Shane Seep and WCS and that wind-veering 

was negligible given the proximity of WCS to the shoreline and lack of significant topographic 

features in the vicinity – WCS is 6 m above sea level. For this clear morning, two atmospheric 

stability cases were simulated, “Briggs Turbulence” for slightly unstable conditions with 2 < 

u10 < 3 m s-1 and surface roughness typical of the ocean at these wind speeds for light sun 

(Case 1) or for moderate sun (Case 2). 

For Case 1 and elevation of THC levels by 1.05 ppm above background at WCS, Q = 0.52 m3

s-1 (Figure III-20). Sensitivity to θ was tested by calculating Q for θ = 226, 232° with 

variability based on the ±3° accuracy of anemometer (Model 020C, Met One Instr., OR). For θ

= 226° and 232°, Q was 0.56 and 0.52 m3 s-1, respectively. Sensitivity is small because the 

wind was almost directly towards WCS. For Case 2, Q = 0.235 m3 s-1, with a similar small 

sensitivity to θ. Sensitivity to u for a Gaussian plume is linear (for constant stability class), 

thus uncertainty in Q from u was ~10%. The main uncertainty was associated with stability 

class – i.e., Case 1 versus Case 2. For the entire blowout event, the total emission, Qtot, was 

~160 m3 or ~70 m3, for Cases 1 and 2, respectively. In this study, the average Q from the two 

cases (Q = 0.4 m3 s-1 ±25%, or ~120 m3

 for the entire event) is used. 
Simulation of a Subsurface Oil Spill 

53 

This Q is comparable to the output of the entire seep field, Q = 1.15 m3 s-1 (Hornafius et al. 

1999) and was 10 to 15 times the normal Shane Seep Q of 0.038 m3 s-1 (Washburn et al.

2005). Thus, for the entire event, ~120 m3

 of methane escaped at the sea surface. Due to 

bubble dissolution, Q (in moles) at the seabed must have been larger than at the sea surface. 

While large, compared to the field, the emission rate was not significant. However, a 

numerical study described in Section V showed greatly reduced gas loss to the water column 

for the blowout due to plume processes, specifically. Thus, the model study showed that the 

true significance of the event was that plume processes enhanced enormously CH4 transport to 

the sea surface. 

For hydrocarbon seeps, blowouts likely are caused by tar blockage of seep vents (Leifer and 

Boles, 2005a); Hovland, 2002). A tar block at the main vent would explain the decrease and 

then absence of seepage prior to the ejection. We propose that for this event, the ejection 

resulted from an increase in pressure behind the blockage until the passage was blown free. 

For this mechanism, the ejection size (for the same vent and tar blockage) would be similar at 

greater depths since it is a differential pressure across the blockage that creates the ejection 

(Leifer et al., 2005a). During this event, a small amount of tar pieces were observed raining 

down onto the HC volcano walls, suggesting that burial of the chain here is explained by 

multiple blowout events. Seafloor pockmarks, or a concave crater-like depression that 

commonly occurs on muddy seabeds are common seepage-related features on all continental 

shelves. Pockmarks are proposed to result from an explosive formation process (Hovland, 

2002), although likely due to other factors than tar. If the proposed pockmark-formation 

mechanism is similar to our observations, – blowout emissions and deposition – then blowout 

seepage may be common and a potentially important seepage process. Blowout seepage has 

greater potential to contribute to atmospheric budgets than low flux seepage due to bubbleplume processes, discussed below. These processes can significantly enhance the efficiency of 

bubble-mediated transport across the water column. 

IV. NUMERICAL BUBBLE STUDIES 

An existing bubble propagation model described in Leifer and Patro (2002) and Leifer and 

Judd (2002) was applied to the hydrocarbon seeps in the Santa Barbara Channel. This model 

has been used for sensitivity studies on data collected in the Santa Barbara Channel, as well as 

seep bubbles in the Gulf of Mexico (Leifer and MacDonald, 2003; MacDonald et al., 2002) 

and North Channel (Leifer and Judd, 2002). 

IV-1. Theoretical Description of a Bubble 

The model solves the differential equations describing bubble behavior. The bubble 

dissolution rate is equivalent to a mass flux (FM) and is 

FM = ∂n

∂t = AkBΔC = 4πr

2

kB(r,D(T))(C − PB / H(T)) (4)
Final Study Report - Leifer 

54 

where n is the number of moles in the bubble, t is time, A is the bubble surface area, r is the 

bubble equivalent spherical radius, kB is the bubble gas transfer rate, C is the aqueous 

concentration, ∆C is the concentration difference between the gas and liquid phases, D is the 

gas diffusivity, PB is the bubble pressure which is primarily the hydrostatic pressure, PH, with 

a small contribution from surface tension (2σ/r). kB is a function of r and D. H is the Henry’s 

Law constant (mol cm-3 Atm-1) that describes the equilibrium between the dissolved and 

gaseous phases (Sanders, 2005). Both H and D are temperature, T, dependent. Note, H is now 

defined as the inverse of Leifer and Patro (2002) and Leifer and Judd (2002). 

From (4), ∆C is the driving force behind bubble gas outflow and depends upon the gas 

solubility as expressed by H. Thus, changes in solubility from temperature have a direct effect 

on the bubble dissolution rate. 

As the bubble dissolves, it shrinks, although as it rises, it grows from the decreasing 

hydrostatic pressure. The size change of the bubble, is described by, 

∂r

∂t = RT

∂N

∂t − 4πr33 ρ W g

∂z

∂t

⎧ ⎨ ⎩ ⎫ ⎬ ⎭ 

 4πr 2 (PA + ρ W gz +

2σ

r ) − 4πr 33

2σ

r 2

⎧ ⎨ ⎩ ⎫ ⎬ ⎭ 

−1

 (5) 

where R is the universal gas constant, ρW is the density of water, g is the gravitational constant, 

and z is depth. If a factor q is defined from the ideal gas law that converts moles into 

atmospheres, where q = RT/V, where V is volume, (5) can be rearranged to yield: 

∂r

∂t = r q

∂N

∂t − ρ W g

∂z

∂t

⎧ ⎨ ⎩ ⎫ ⎬ ⎭ { } 3(PA + ρ W gz) + 4σ / r −1

 (6) 

Whether a bubble grows or shrinks depends upon the numerator of (6). Also needed is the 

differential equation describing the change in pressure within the bubble, 



∂PB

∂t = − 3PBr

∂r

∂t

+ q

∂n

∂t (7)

As the bubble rises, PH decreases, ∆C decreases and the mass flux from the bubble decreases. 

Bubbles rise at their stagnant rise velocity, VB, relative to the surrounding fluid, which in a 

bubble plume is typically moving upwards due to drag from the rising bubbles with an 

upwelling flow, VUP,. Thus, the bubble depth, z, is described by: 

∂z

∂t = −VB +VF (8)

The upwelling flow decreases the time for bubbles to reach the surface, as well as increasing 

∂PH /∂t. For most seep bubbles (r~1000 to 3000 μm), VB is 20 - 30 cm s-1. Upwelling flows 

have been documented in various seepage systems from large plumes spanning up to tens of 
Simulation of a Subsurface Oil Spill 

55 

meters (Leifer et al., 2000; Clark et al., 2003) to bubble streams arising from a single vent 

(Leifer and MacDonald, 2003; Leifer and Boles, 2005a). 

Although both kB and VB are parameterized in terms of r, in reality they are determined by the 

detailed hydrodynamic flow around the bubble. This flow is strongly affected by bubble 

buoyancy, i.e., r. However, anything that affects the bubble surface state, such as surface 

active substances, surfactants, affects the hydrodynamics and thus kB and VB. Contaminated 

bubbles have immobile surfaces and which cause them to exchange gas slower and rise slower 

than clean bubbles, which have mobile interfaces. Outside the laboratory, bubbles always have 

some contamination, which is pushed towards the downstream hemisphere by the flow, thus 

bubbles typically have partially immobilized surfaces. However, unless the immobile portion 

of the bubble’s interface extends far enough from the bubble’s downstream pole – 45° (Sadhal 

and Johnson, 1983) – it has negligible effect on the bubble’s behavior and the bubble behaves 

clean. The result is that for a given contamination, bubbles larger than a transition radius 

behave clean, while smaller bubbles behave dirty with a transition between the two cases. 

Patro et al. (2002) found this transition occurred at approximately r ~ 500 - 700 μm. 

Figure IV-1. Flowchart of the numerical bubble model. From (Leifer and Judd, 2002) 

IV-2. Numerical Bubble Model Overview 

A model flowchart is shown in Figure IV-1. Initially, physical, fluid dynamical, plume, and 

chemical parameters are loaded. Next, the seabed bubble emission flux distribution, φ(r), is 

calculated from the observed VB(r) and the bubble emission size distribution - i.e., Figure III-5. 

Where feasible, look-up tables are used for computational speed, including parameterizations 
Final Study Report - Leifer 

56 

for kBub and VB, and upwelling profiles. Bubbles in each size class are simulated including an 

imposed upwelling flow, Vup. Each bubble size class is initialized at the sea floor with 

observed partial pressures and allowed to rise due to buoyancy and upwelling flows. The 

model solves the coupled differential equations describing the change in moles gas, n, r, bubble 

pressure, PB, and depth, z, as a bubble rises, exchanges gases, and changes in size due to gas 

exchange and decreasing PH. Numerical integration is by a third-fourth order Runge-Kutta 

routine with variable step size and is terminated when the bubble reaches the surface or 

dissolves to 5-μm radius. Then the output for each size class is interpolated to a smooth depth 

grid. The gas flux (methane, alkanes) for the plume into the water column and vertically in the 

bubbles at each depth is calculated from the mass flux integrated over the bubble emission flux 

at each depth. Each bubbles size class is individually simulated and is combined with the size 

flux distribution (i.e., the number of bubbles of each size emitted per second) to calculate total 

flux of each gas. Gas in the bubbles at the sea-surface is the flux to the atmosphere. 

Bubble model predictions are strongly influenced by the rise speed, VB, and gas transfer, kBub, 

parameterizations, which are strongly dependent upon bubble size, oiliness, and water 

temperature. Parameterizations are based upon a combination of field observations, laboratory 

experiments, and best estimates from the literature. Gas kinetics (solubilities and diffusivities) 

are based on the literature (e.g., Waninkhof, 1992; Sander, 2000), although several are 

unreported and thus estimated values are used (based on theoretical calculations – Sander, 

2000). 

Regarding fluid motions, the model allows specification of a 2D flow field which can include 

complex motion generated by the rising bubbles (upwelling flow), waves orbital motions, 

turbulence, and Langmuir cells. Currently, the model is not fully 2D; bubbles are assumed to 

originate from a single source, and can move in 2D due to the flow field; however, the 2D 

position and gas fluxes are not stored; model output consists of only the overall gas fluxes at 

each depth. 

A bubble tends to grow due to air inflow and changes in hydrostatic pressure while it tends to 

dissolve due to outflow of CH4 and other hydrocarbons. Whether the bubble dissolves or not 

depends upon the balance between these two processes. Because oil collects at the bubbles' 

gas-water interface and is entrained in the upwelling flow, the model can calculate the oil flux 

and whether or not the oil is “deposited” in subsurface layers. 

IV-2-1. Single Bubble Studies 

Simulations were run for surfactant-contaminated bubbles rising from 22 m (e.g., Shane Seep), 

and 13°C using the gas composition for the Seep Tent at the seabed (Table III-3) where all 

trace n-alkanes are simulated at ethane (11.6 %), thus the air gases are trace. For the first 

simulations, bubbles were clean, dissolved methane was trace (~1/250 saturation) and there 

was no upwelling flow. 

Simulation output for a clean r = 5000 μm bubble is shown in Figure IV-2. As the bubble 

rises, methane escapes while atmospheric gases enter, so that by the surface, approximately 

86% of the methane has been lost, although this bubble is still ~40% methane (Figure IV-2A). 
Simulation of a Subsurface Oil Spill 

57 

Thus the change in bubble composition at the sea-surface as compared with at the seabed does 

not indicate how much methane has been lost, because the bubble size also changes. This can 

be understood by a simple conceptual model in which we assume that in our system there is 

only methane. In this case, the bubble would have lost most of its methane to the surrounding 

ocean, but we would still be pure methane (since there is no air to enter the bubble). Also note 

that ethane, representative of the trace n-alkane gases, becomes enhanced in the bubble relative 

to methane due to its lower diffusivity. 

For the first 60 seconds of its rise (~10 m), the bubble shrinks (Figure IV-2D) due to the rapid 

outflow of methane faster than its growth due to hydrostatic pressure (Figure IV-2B). For the 

rest of its rise, it grows as the air inflow and hydrostatic pressure become more significant than 

the (slowing) outflow of methane. At ~100 seconds (Figure IV-2D) the bubble arrives at the 

sea surface, only slightly smaller than when it left the seabed (Figure IV-2C). 

Figure IV-2. Numerical simulation of a clean, r = 5000 μm bubble in 13°C water with trace dissolved methane 

and no upwelling flow. A) shows molar evolution, B) shows partial pressure evolution, C) shows radius evolution, 

and D) shows depth evolution. 

Unlike the 5000 μm, which makes it to the surface with most of its methane intact, a 500-μm 

radius bubble, shown in Figure IV-3, dissolves after rising only a few meters (Figure IV-3D). 
Final Study Report - Leifer 

58 

While the methane and ethane outflow the bubble, air gases inflow, becoming the dominant 

gases at ~7 s (Figure IV-3B). As a result, the dissolution rate (i.e., decrease in radius with 

time) slows down (Figure IV-3C). Methane continues to outflow until it becomes a trace gas at 

~12 s; thereafter, it no linger affects bubble size. Meanwhile the inflow of air gases has also 

nearly ceased (12 s), since they are roughly in equilibrium with dissolved gases in the 

surrounding water. However, since the bubble is now less than 100 μm, surface tension begins 

to play a role, causing all gases to outflow. The effect of surface tension becomes apparent 

towards the end of the bubble’s life, when the bubble pressure begins to rise (Figure IV-3B). 

The simulation was terminated at 28 s since the bubble does not significantly rise any further. 

Figure IV-3. Numerical simulation of a clean, r = 500 μm bubble in 13°C water with trace dissolved methane 

and no upwelling flow. A) shows molar evolution, B) shows partial pressure evolution, C) shows radius 

evolution, and D) shows depth evolution. 

IV-2-2. Contamination 

Smaller bubbles dissolve even faster; however, given that these bubbles are oil coated and in 

the ocean, it is likely that bubbles are contaminated to some extent by surface-active substances 

- surfactants. Patro et al. (2002) showed that for seawater, large bubbles behaved clean, while 

small bubbles behaved dirty. In seawater, the transition was at 500 μm and was sharp, in 

lagoon water, the transition was broad, with bubbles 2200 μm and larger behaving clean. 
Simulation of a Subsurface Oil Spill 

59 

Figure IV-4. Rise velocity, VB, parameterizations for clean and dirty bubbles as a function or radius, r. Also 

shown are some experimental laboratory data, and contours of Reynolds number. From Leifer and Patro (2002). 

Whether bubbles are clean or dirty can be significant, as shown in the parameterization for 

clean and dirty bubbles in Figure IV-4. For example, if the same 500-μm radius bubble 

simulated in Figure IV-3 was dirty, it would reach the surface. Although surfactants decrease 

bubble rise speed, and thus increase the time to transit the water column, they have an even 

larger effect on gas exchange. The bubble still reaches the surface with trace methane, but it 

does reach the surface. For comparison, the clean simulation results (Figure IV-3) are shown 

by dashed lines in Figures IV-5b and IV-5c. By 250 s, the bubble begins growing again due to 

decreasing hydrostatic pressure, since the air gases are close to equilibrium. 
Final Study Report - Leifer 

60 

Figure IV-5. Numerical simulation of a dirty, r = 2000-μm bubble in 13°C water with trace dissolved methane 

and no upwelling flow. A) shows molar evolution, B) shows partial pressure evolution, C) shows radius 

evolutions for dirty and clean, and D) shows dirty and clean depth evolutions. Data key on figure. 

Clearly, correctly describing surface state is important. Unfortunately, published descriptions 

of the effect of oil on bubble behavior are unavailable. Bubbles at Shane Seep have trace oil 

coatings and appear to in general, follow the dirty parameterization, with occasional very oily 

droplets that rise much slower and may have significantly lower gas transfer rates. Their fate 

likely will be very different from similar-sized bubbles. While it is clear that small bubbles 

behave contaminated, and it is likely that the largest bubbles behave clean, the transition is 

uncertain. For the remaining simulations, we assume (conservatively) that trace oil 

contamination has no effect beyond that of seawater surfactants (largely algal exudates) with a 

transition at r ~ 700 μm as suggested in Patro et al. (2002). 

IV-2-3. Upwelling Flow 

When bubbles are in a plume, they modify their environment including creating an upwelling 

flow (Leifer et al., 2000a). This flow enhances bubble-mediated gas transport by decreasing 

the transit time to the sea surface, but also by increasing the rate that the hydrostatic pressure 

decreases. It is the concentration difference that drives gas transfer, thus a more rapid process 

in the hydrostatic pressure means the bubble reaches shallower depths where mass loss is 

slower with more of its original gases. 

IV-3. Sensitivity studies 

Using this model, sensitivity of bubble-mediated CH4 transport to ambient conditions was 

studied. Strong sensitivities to aqueous CH4 saturation, upwelling flows (Leifer and Patro 
Simulation of a Subsurface Oil Spill 

61 

2002), and oil contamination were identified. Field data collected as part of the current MMS 

project discovered and quantified these parameters (Leifer et al. 2000a; Leifer and Clark 2002; 

Clark et al. 2003). 

Figure IV-6. Sensitivity results showing predicted ratio of bubble alkane molar content at surface to sea floor 

versus initial bubble radius, r for (a) upwelling flow, Vup, of 10 cm s-1, and (b) 30 cm s-1. Also shown is the 

surface r on the right vertical axis. Data key on plot. 

The model investigated alkane transport to the surface for La Goleta Seep (34° 52.44' N, 119° 

23.65' W), for two upwelling flows, Vup = 10 and 30 cm/s. Dissolved CH4 was 0.9 Atm, small 

bubbles were assumed contaminated, large bubbles clean. The predicted surface r is shown in 

Figure IV-6. For Vup = 10 cm s-1 (Figure IV-6a), a 2000-μm bubble reaches the surface with r

~ 2000 μm, while a 900-μm bubble reaches the surface with r ~ 300 μm. Stronger upwelling 

(Figure IV-6b) causes less bubble dissolution (bubbles are subsurface for a shorter time). 

These two sensitivity studies show a strong sensitivity of bubble survivability to Vup. In one 

scenario a 500-μm bubble dissolves, depositing its oil subsurface. In the other, it doesn’t. 

Also shown in Figure IV-6 is the surface to bottom ratios for alkanes to pentane. For all 

bubbles, the ratio increases with alkane number, except for CH4, which behaves differently due 

to its saturation of the water column. The relative enhancement of heavier versus lighter 

alkanes is greatest for bubbles that dissolved slightly, and least for the largest bubbles (which 

grow). For the largest bubbles this results from the large volume, i.e., during their ascent to the 

surface, gas exchange changes the bubble's total mass little. In contrast, when a bubble 

dissolves, as indicated by a large decrease in CH4 (e.g., r<800 μm), all gases are equally forced 

from the bubble, and thus there is little enhancement of heavier alkanes. As a result, only a 

narrow size range can explain observed alkane enhancements (i.e., minor vent bubble plumes). 

In fact, circa 3000-μm bubbles produce a relative enhancement of 2 for pentane versus ethane. 

For these conditions, bubbles with initial r ~ 3000 μm must be responsible for most of the gas 

transport in the plume. Since these bubbles grow only slightly, this agrees with observed 

surface bubble distribution for La Goleta Seep. 

IV-4. Plume Simulations 

Given the emission size distribution of bubbles in a plume, the numerical model can simulate 

the mass flux for all the bubble size classes in the plume and calculate for each gas, the mass 
Final Study Report - Leifer 

62 

outflow as a function of depth and the vertical mass transport (i.e., bubble content) as a 

function of depth. Simulations were also used to address the impact of blow-out emissions. 

Simulations were conducted for three plumes, a minor, a major, and a blow-out plume, shown 

in Figure IV-7. 

Figure IV-7. Bubble emission size-distributions, Φ, versus bubble radius, r, used in numerical simulations, for 

different seepage types. Case 1 is a minor plume (open circle), Case 2 is a major plume (filled circle), and Case 3 

is for the blowout (diamond) with calculated values based on fit to Case 2 (diamond). Also shown is least squares 

fit to major plume over range of fit. Cases 1 and 2 from Leifer and Boles (2005). 

Bubble plumes were a minor vent, a major vent, and a blowout vent (Figure IV-7; Table IV-1). 

Minor produce single streams of bubbles and have a narrow, peaked emission size-distribution, 

Φ. Φ is the number of bubbles in each size class (μm-1) emitted per second in the entire plume. 

Laboratory (Blanchard and Syzdek 1977; Tsuge et al. 1981) and field (Leifer and MacDonald 

2003; Leifer and Boles 2005a) observations indicate that low gas flux vents produced narrow, 

sharply peaked Φ, while high gas flux vents produce a broad and weakly size-dependent 

Φ, that extends to both very small and very large bubbles. The difference arises because at low 

flow, bubble size depends solely upon vent mouth geometry (Blanchard & Syzdek 1977) 

whereas at high flux, bubble fragmentation at the vent mouth creates the broad size distribution 

(Slauenwhite and Johnson 1999). Thus, for the blowout (Case 3), the functional dependency of 

Φ for the major vent was used for the blowout, Φ = A r-0.59, but the range of bubbles described 

by this power law was assumed to extend to a wider range, from 100 to 15000 μm. The value 

of A was chosen so that the total seabed volume flux was 0.5 m3 s-1 (at STP). The upper radius 

cut-off was based on qualitative observations of bubbles from the seabed video and assumes 

that bubbles larger than r ~ 1.5 cm, which were not seen at the sea surface, rapidly broke up 

and formed bubbles with Φ ~ r-0.59. 

Bubbles were simulated as having a size-varying contamination (Patro et al., 2002). For minor 

vent plumes, Vup was ~10 cm s-1 (Leifer and Boles, 2005a) while the main vent at Shane Seep 

was simulated for normal conditions with Vup ~ 45 cm s-1. For the blowout simulation, the 

measured Vup of 200 cm s-1 was used. 
Simulation of a Subsurface Oil Spill 

63 

At the seabed, the model was initialized with seep gas with a composition that was mostly CH4

(84%) and CO2 (12%), with trace air (2.21%), and n-alkanes decreasing from 3% ethane. Nalkanes larger than CH4 were simulated as ethane with a mole fraction of 3%. Aqueous CH4

was elevated several orders of magnitude above seep-field background and was only weakly 

depth dependent - 51.5 and 49.8 mMol L-1 at seabed and sea surface, respectively (Clark et al., 

2003). Simulations used an aqueous CH4 value of 0.05 mMol L-1. The water temperature was 

14°C. To address the lack of water samples during the unanticipated event, a fourth case 

simulated a blowout with elevated plume CH4 based on CH4 dissolution from Case 3. 

Table IV-1. Model-predicted bubble gas mole-fraction at the sea surface (1-m depth) and gas observations for 

Shane Seep main plume. Remaining bubble composition was higher n-alkanes. 

Case Bubble CH4 Flux O2 N2 CH4 CO2 CH4 loss 

 (mmol s-1) (%) (%) (%) (%) (%) 

1 Minor 0.19a

 5.9 14.9 76.0 0.04 51.6 

2 Major 9.3a

 1.3 3.9 89.6 1.6 10.4 

3 Blowout 2.1x104 b 0.44 2.1 84.8 9.4 1.108 

4 Blowout-II 2.1x104 b 0.44 2.1 84.8 9.4 1.096 

Sample gas at seabed 0.34 1.87 83 12 

Sampled gas at surface 5.3 14 78 0.6 

 

a

 measurements from (Leifer and Boles, 2005a); b derived from calculated Q for blowout. 

As bubbles rise due to buoyancy and the upwelling flow, seep gases exchange with dissolved 

air in the water. For normal conditions, bubbles at the sea surface were observed to have CH4 

(78%), air (19.3%), and trace CO2 (0.6%) and n-alkanes. Although CO2 loss increases the CH4

mole fraction, air inflow decreases the CH4 mole fraction. 

For the minor (Case 1) and major (Case 2) vent bubble plumes, the bubbles were 5% and 22% 

air at the sea surface, respectively (Table IV-1). CO2 outflow was very fast, decreasing to just 

0.04% and 1.6% at the sea surface, for the minor and major plumes, respectively. Mole 

fractions for the major and minor plumes “bracket” the observed sea-surface bubble-mole 

fractions. This is in agreement with the sea-surface gas collection method, which included 

bubbles from both the major vents and the numerous minor vents in the immediate vicinity of 

the main caldera. The major plume simulation better approximated the sea-surface CO2 -mole 

fractions than the minor plume simulation. This is to be expected, since bubble-mediated 

transport of soluble gases increases with size (Leifer and Patro, 2002) and the largest bubbles 

were exclusively found in the major plume. The minor plumes, though appear to be most 

important to overall gas transfer, in agreement with the results from Leifer and Clark (2003). 

Leifer and Clark (2003) ran simulations absent seabed gas composition and seabed bubble sizedistributions. For the simulations, seep-tent gas composition was used. It was found that the 

ratio of n-alkanes in sea surface gas could only be explained if most of the gas was being 

transported by bubbles 2000 μm to 3000 μm radius, i.e., the minor bubble plumes. 
Final Study Report - Leifer 

64 

Figure IV-8. Predicted vertical CH4 transport (in bubbles) for the three simulations. See Table 1 and text for 

details. Note, Cases 1 and 3 are multiplied by 102 and 10-4, respectively 

For the blowout simulation (Case 3), the predicted mole fraction of methane at the sea surface 

was less than for the major plume (Case 2). This is explained by the slower CO2 outflow, i.e., 

the bubble retained more of its original CO2 and thus the methane mole fraction was reduced. 

For the minor and major vents, 51.6% and 10.4%, respectively, of the original CH4 dissolved 

(Figure IV-8), while for the blowout, CH4 loss was just 1.108% or 0.23 mol s-1. Air mole 

fractions were reduced due to the greater CO2 and methane in the bubble and reduced air 

inflow into the bubbles. 

The importance of elevated plume concentrations to bubble-mediated CH4 transport was tested 

by using the dissolution flux for Case 3 to estimate dissolved plume CH4. For the ~10 second 

transit of the water-column during the blowout, ~2.3 moles of CH4 were predicted to have 

dissolved into the plume water. Based on the aerial overflight images, a conical blowout 

plume was assumed, 1-m diameter at the sea-surface (Figure III-19E) and 22-m tall - i.e., 23 

m3. If this plume remained unmixed with the surrounding bulk ocean, i.e., a conservative 

assumption, then plume CH4 concentrations would have been elevated 0.1 mmol L-1 above 

normal plume concentrations. This estimate is conservative as it neglects plume loss to the 

bulk ocean. Using this elevated plume concentration, a simulation was run for a blowout and a 

plume CH4 of 0.15 mmol L-1 (Case 4). All other conditions were as in Case 3. This 

simulation showed a very slight decrease in CH4 loss to the water-column, 1.096% versus 

1.108% for Case 3. The effect was small because 0.15 mmol L-1 CH4 is ~5% of CH4

saturation at 10 m 
Simulation of a Subsurface Oil Spill 

65 

For a blowout from this very shallow seep, the primary factor enhancing CH4 transport to the 

atmosphere was the rapid upwelling flow. There simply was insufficient time for the plume 

water to become significantly saturated. For the blowout vent, a 10 cm s-1 Vup yielded CH4 

loss of 6.8%, ~seven times the loss for Vup = 200 cm s-1. The presence of very large bubbles 

with their greater volume contributed significantly to methane transport, with bubbles larger 

than 5000 μm carrying 75% of the methane. For deeper seeps, the CH4 dissolution increases. 

Simulating the same blowout gas volume from 250-m depth - including compressibility- (Case 

3) showed greater, but still small CH4 loss, ~8.6% of the initial CH4. Thus, if a seep blowout 

of comparable magnitude occurred at greater depths and generated similarly strong upwelling 

flows, potentially a significant fraction of its CH4 can reach the atmosphere. For comparison, 

the major vent plume at 250 m (Case 2) lost ~69% of its original CH4. 

The model was highly illustrative that for blowout seepage, vertical methane transport 

efficiency is enhanced enormously. As a result, almost all of the methane from a shallow seep 

reaches the atmosphere. Furthermore, the model suggested that given sufficient ejection size 

and resultant upwelling flow and large bubble size, efficient transport of methane from the 

seabed to the sea surface is feasible from depths of many hundreds of meters. 

V. DISCUSSION 

V-1. Seabed Morphology Discussion 

The seabed surveys demonstrate that seepage features at Shane Seep are both semi-permanent 

and “plastic.” The sediment overburden is Late Quaternary, and for the outer COP seep trends 

is thin, < 1 m. At Shane Seep the overburden is thicker, 2-3 meters (Fischer 1978), consisting 

primarily of very fine sand, and has modern total organic carbon of 1 - 2% (Fisher 1978). The 

sand overlies fractured Monterey formation basement. The upper 30 cm of sand is cemented 

by tar (La Montagne et al. 2003) and highly cohesive. The seabed near Shane Seep is also 

heavily coated with bacterial mats, and large tar balls can be found within the hydrocarbon 

(HC) or tar volcanoes. These volcanoes are termed hydrocarbon volcanoes rather than mud 

volcanoes because of their high tar content, which provides the necessary cohesion to form the 

volcano walls, and fixed vent locations. 

That the walls represent a depositional process was dramatically demonstrated by the burying 

of the transect chain after it was draped across the newly formed volcano #3. Since elsewhere 

the chain, even two years later, lay on top of the sandy bottom, only a depositional process 

could explain the manner that the chain disappeared into the caldera's walls. Furthermore, the 

appearance of stones in the bottom of volcano #3 suggests that not only tar, but also sand was 

lofted during these events, leaving behind the rocks. 

V-1-1. Seep Seabed Plasticity 

There are several processes evidenced in the changes in seabed morphology over the years. 

These include deposition of tar from ejections along with the removal of seabed material 

during large ejection events, feature erosion and burial by storms, and the plastic deformation 
Final Study Report - Leifer 

66 

of the seabed. Vent locations remain fixed for a period of years but then re-express themselves 

nearby. While ejection events build up caldera walls they also destroy them. Together these 

changes provide strong evidence of the dynamic nature of hydrocarbon seeps. 

During the formation of volcano #3, 100,000 kg of sediment was displaced. This most likely 

was due to either a single or a series of "explosive" events as indicated by highly circular 

caldera shape. And in general, when volcano walls grew higher, they also grew circularly 

(except for Vent #0). Meanwhile, much smaller ejections (but larger than that observed by the 

turbine tents) caught on video did not evidence any significant displacement of seabed 

sediment. Clearly the formation of volcano #3 was a large event (or series of events). The 

source of the event that precipitated the formation of volcano #3 could have occurred deep or 

shallow. If we assume shallow blockage responsible, it still must have been deep enough to 

stop the flow through both existing volcanoes #1 and #2. Otherwise pressure could not have 

built up behind them to then cause destruction of the ridge. Pressure behind the blockage 

would have increased until the blow through destroying the ridge separating the two vents. But 

this raises the question: How does an event that could destroy the ridge between volcanoes #1 

and #2, despite their being highly active (i.e., open) vents, overcome the significantly greater 

resistance represented by the sediment displaced in the formation of volcano #3? Thus it is far 

more plausible that a deep event was responsible. In this case, a massive pulse of gas suddenly 

was released and traveled rapidly upwards through the fractures. At the seabed, the two 

existing vents were inadequate to flux the great volume of gas, and thus represented a bottleneck behind which the pressure increased until it created a new pathway, volcano #3. The 

giant tar blocks found on volcano #3’s caldera floor must have been squeezed out through the 

fractures immediately under volcano #3, since similar tar blocks were not found in volcanoes 

#1 and #2. It is unclear from the observations whether the deep event was the clearance of a 

blockage in the fracture system or sudden access to the fracture system by a previously 

unconnected gas reservoir. 

Tar does not necessarily migrate in punctuated transient events, for example there are beach tar 

oozes and forms mounds at the south edge of the Carpinteria State Park, CA. This tar seep is a 

long-term feature, having been used in boat making by indigenous peoples for centuries or 

longer (Earth Island Institute, 2003). Given sufficient pressure, tar migrates. And such 

pressure may explain the appearance of elevated seabed features such as the elevated plateau 

under volcano #4, or the ridges further to the east. Here, some of the fractures may be 

completely clogged with tar, implying an enormous pressure buildup to cause a blow through, 

which is unlikely (i.e., rare) to happen while gas can flow (and pressure be relieved) through 

other vents. In this case, the elevated pressure forces the tar to slowly ooze forwards, gradually 

lifting the sediment layer. Water flow may also be involved. And certainly the very gradual 

(over a year) formation of volcano #4 suggests a gradual process in sharp contrast to the 

sudden appearance of volcano #3. 

The difference between volcanoes #3 and #4 was that volcano #4 formed gradually and 

included a raised mound. Not only did the plateau height gradually increase with time, but also 

the gas flux through the volcano #4’s vents and the size and height of its caldera walls. When 

first identified, volcano #4 was only a few centimeters tall with a few bubble streams. By 

2003, the caldera was meters across, the walls were half a meter high, and there were many 
Simulation of a Subsurface Oil Spill 

67 

active vents. This is consistent with the tar slowly being forced from the fractures during the 

initial stages until the gas flux was sufficiently great (i.e., fractures were sufficiently opened) to 

allow the blow through mechanism to begin depositing the caldera walls. 

In fact, the decrease in relative flux, and then impermanence of volcano #3 seen after March 

2003, may relate to a large tar and or fluid flow clogging the vent. This is supported by the 

lifted seabed North of volcano #3 observed in April 2003, prior to the volcano's relocation to 

this area. Similarly, the expansion of volcano #4 between April and June, 2003, was preceded 

by a large (50 cm) increase in plateau height, noted in March 2003. 
68 

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73 

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 

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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. 

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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 

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Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

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