OCS Study

MMS 2001-050

Coastal Marine Institute

Improved Geohazards and

Benthic Habitat Evaluations:

Digital Acoustic Data with

Ground Truth Calibrations

Final Report

Cooperative Agreement

Coastal Marine Institute

Louisiana State University

U.S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region
U.S. Department of the Interior Cooperative Agreement

Minerals Management Service Coastal Marine Institute

Gulf of Mexico OCS Region Louisiana State University

OCS Study

MMS 2001-050

Coastal Marine Institute

Improved Geohazards and

Benthic Habitat Evaluations:

Digital Acoustic Data with

Ground Truth Calibrations

Final Report

Author

Harry H. Roberts

July 2001

Prepared under MMS Contract

14-35-0001-30660-19910

by

Coastal Studies Institute

Louisiana State University

Baton Rouge, Louisiana 70801

Published by
iii

Disclaimer

This report was prepared under contract between the Minerals Management Service

(MMS) and Coastal Studies Institute, Louisiana State University. This report has been

technically reviewed by the MMS and approved for publication. Approval does not signify that

the contents necessarily reflect the views and policies of the Service, nor does mention of trade

names or commercial products constitute endorsement or recommendation for use. It is,

however , exempt from review and compliance with MMS editorial standards.

Report Availability

Extra copies of the report may be obtained from the Public Information Office (Mail Stop

5034) at the following address:

U.S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region

Public Information Office (MS 5034)

1201 Elmwood Park Boulevard

New Orleans, Louisiana 70123-2394

Telephone Number: (504) 736-2519

or 1-800-200 GULF

Citation

Suggested citation:

Roberts, H.H. 2001. Improved geohazards and benthic habitat evaluations: digital acoustic data

with ground truth calibrations. OCS Study MMS 2001-050. U.S. Dept. of the Interior,

Minerals Mgmt. Service, Gulf of Mexico OCS Region, New Orleans, La. 116 pp +

appendices.
v

Preface

The Digital High Resolution Acoustic Data for Improved Benthic Habitat/Geohazards

Evaluation study is focused on developing a better understanding of seafloor geology of the Gulf

of Mexico continental slope in order to increase our predictability of geohazards and

chemosynthetic communities, particularly in areas of hydrocarbon seepage-venting. Those are

the areas that are commonly accompanied by acoustic wipe-out zones in high resolution acoustic

data sets. This report is a summary of research sponsored by the U.S. Department of Interior

Minerals Management Service (MMS), Gulf of Mexico Region OCS through the Coastal Marine

Institute at Louisiana State University (Contract no. 30660/19910).

This study was conducted by the PI as both a field and a laboratory investigation. Field

data sets consist of high resolution seismic profiles and side-scan sonar data collected by

instrumentation assembled by the Marine Geology subprogram at Coastal Studies Institute at

Louisiana State University with technical support from the Coastal Studies Institute Field

Support Group. Supporting high resolution acoustic data sets and 3D-seismic profiles and

surface amplitude extraction data have been contributed by numerous major companies in the

petroleum industry. In addition, very important seafloor verification data acquired by a manned

submersible have been primarily supported by NOAA’s National Underseas Research Center at

the University of North Carolina (Wilmington), as well as dives supported by the Louisiana

Quality Education Fund (LEQSF). This combined research has led to talks at regional, national,

and international professional meetings and peer-reviewed scientific publications (listed

separately in the Technical Summary accompanying this report). This report describes the

rationale for the study, techniques applied, data sources, results of two successful field data

collection cruises, sites analyzed and used in the evaluation, data gaps, results, interpretation, and

recommendations.

The study was undertaken with the intent of producing a practical guide for interpreting

geohazards, particularly those associated with hydrocarbon seepage and venting, and the

presence or absence of associated chemosynthetic communities. In the course of conducting this

investigation, fundamental knowledge about the linkage between slope geology, seepage rate,

and biologic response has emerged. This new appraisal of the impacts of hydrocarbon seepage

and venting on slope geology and biology is discussed in the summary sections of this report. In

addition, the strong influence of high quality 3D-seismic data on geohazards evaluations is

discussed along with the variability of seafloor types that produce a common response as

acoustic wipe-out zones on high resolution seismic profiles. Interpretation criteria are still

evolving, but the most rigorous interpretation scheme is available in this final report.

In keeping with MMS guidelines, this report has been written for an audience of

knowledgeable lay-people. The report is accompanied by an extensive list of scientific

publications that can function as sources for specific scientific information and a glossary of

terms used in geohazards and hydrocarbon seep/vent research.
vii

Acknowledgments

This research would not have been possible without the cooperative input of ideas and

data from many petroleum companies working on the Gulf of Mexico continental slope. These

companies willingly shared high resolution acoustic data taken in support of geohazard surveys

and engineering projects. In addition, many companies shared 3D-seismic surface attribute data

for specific sites being addressed by this investigation sponsored by the Minerals Management

Service through a Cooperative Agreement (Contract No. 14-35-0001-30660) with the Coastal

Marine Institute at Louisiana State University. Companies that deserve special recognition for

their help in this study are Amerada Hess, BHP, BP, Chevron, Conoco, Mobil, Shell,and Texaco.

Personnel in these companies who have helped the author assemble data for the examples

included in this study, and also for areas not used, are too numerous to list. The author wishes to

thank these people for their helpful cooperation. In addition, it is important to call attention to

other funded research projects that produced data directly used in this study. The Louisiana

Education Quality Support Fund (Contact No. LEQSF 1994-1997- RD-B-12 ) provided six days

of submersible dive time on sites used in this MMS-CMI study. These direct observations have

been valuable “calibration points” for linking acoustic data to real seafloor conditions.

Regarding the seafloor verification part of the study, the author is especially appreciative of the

valuable submersible funding that has been supplied by the NOAA National Underseas Research

Center, University of North Carolina at Wilmington (Grants NA 80AA-8-00081, NA88 AA-DUR004 and NA36 RU0060). The submersible work and the support ship of Harbor Branch

Oceanographic Institution are acknowledged for their excellent cooperation in meeting the

objectives of our dive programs. Finally, the author would like to thank his scientific colleagues

both at Louisiana State University and Texas A&M University for the exchange of ideas and

scientific products relating to hydrocarbon seep research.
ix

Table of Contents

I. Introduction ...........................................................................................................1

A. Geologic Framework....................................................................................4

B. Seafloor Geology..........................................................................................5

1. Dome-Top Mounds.................................................................................6

2. Dome-Top Erosional Features ................................................................7

3. Gas Hydrate Outcrops and Associated Mounds .....................................7

4. Mud Vents and Mud Volcanoes .............................................................8

5. Slope Instability Features........................................................................9

C. Data Base....................................................................................................10

1. Seismic Facies.......................................................................................13

II. Case Studies .........................................................................................................17

A. Case Study: Large Complex Mound, Garden Bank Block 161 ................17

1. Introduction...........................................................................................17

2. Geologic Setting ...................................................................................19

3. Available Data ......................................................................................19

4. Feature Characteristics..........................................................................20

5. Feature Synopsis and Interpretation......................................................23

B. Case Study: Mound, Garden Banks Block 189.........................................26

1. Introduction...........................................................................................26

2. Geologic Setting ...................................................................................26

3. Available Data ......................................................................................27

4. Feature Characteristics..........................................................................28

5. Feature Synopsis and Interpretation......................................................31

C. Case Study: Expulsion Centers, Garden Banks Blocks 338 and 382........34

1. Introduction...........................................................................................34

2. Geologic Setting ...................................................................................35

3. Available Data ......................................................................................37

4. Feature Characteristics..........................................................................38

5. Feature Synopsis and Interpretation......................................................40

D. Case Study: Diapriric Mound, Garden Banks Block 427 .........................45

1. Introduction...........................................................................................45

2. Geologic Setting ...................................................................................46

3. Available Data ......................................................................................47

4. Feature Characteristics..........................................................................47

5. Feature Synopsis and Interpretation......................................................49

E. Case Study: Three Separate Features and Distinct Bottom Types,

Green Canyon Block 53...................................................................................55

1. Introduction...........................................................................................55

2. Geologic Setting ...................................................................................55

3. Available Data ......................................................................................57

4. Feature Characteristics..........................................................................58

5. Feature Synopsis and Interpretation......................................................64

F. Case Study: Mounded Carbonates, Green Canyon Block 140..................70
x

1. Introduction...........................................................................................70

2. Geologic Setting ...................................................................................71

3. Available Data ......................................................................................72

4. Feature Characteristics..........................................................................73

5. Feature Synopsis and Interpretation......................................................76

G. Case Study: Mound with Gas Hydrates and Chemosynthetic

Communities, Green Canyon Block 185 ...................................................79

1. Introduction...........................................................................................79

2. Geologic Setting ...................................................................................80

3. Available Data ......................................................................................81

4. Feature Characteristics..........................................................................82

5. Feature Synopsis and Interpretation......................................................83

III. Summary of Project Results .................................................................................89

A. Geologic and Biologic Responses to Venting and Seepage ......................89

1. Feature Spectrum ..................................................................................90

2. Criteria for Making Better Interpretations of Seep-Vented Related

Seafloor Features from Acoustic Data...............................................100

IV. Project Conclusions ............................................................................................103

V. Recommendations................................................................................................107

VI. References .......................................................................................................109

Appendix A. Industry Data Sets Reviewed for MMS-CMI Project ........................ A-1

Appendix B. Equipment Used on MMS/CMI Project..............................................B-1

Appendix C. A Summary of Sites Surveyed under MMS/CMI Support

and Types of Data Collected.................................................................C-1

Appendix D. Glossary.............................................................................................. D-1
xi

List of Figures

Figure Page

1. Complexity of the Louisiana-Texas continental slope are revealed

in high resolution and computer enhanced Seabeam bathymetry...............................2

2. (a) High resolution sparker profile across a dome tope illustrating

the mounded seafloor, acoustic wipe-out zones beneath these mounts,

and truncated bedding on the dome flanks. (b) Large clasts of authigenic

carbonate are common constituents of the sediments on the dome tope

and around the mounded carbonates...........................................................................6

3. A high resolution seismic profile across a known gas hydrate complex

in Green Canyon Block 185........................................................................................7

4. (a) A small mud volcano from Green Canyon Block 272 shows all

the features typical of larger forms that reach heights of over 30 m

above the surrounding seafloor. (b) This high resolution seismic profile

of a mud volcano with two distinct vents is located in Green Canyon Block 97. ......9

5. This picture shows submarine slump deposits that have originated

from the flank of an adjacent salt diapir in East Breaks Area, Block 211................10

6. Map of the Louisiana-Texas continental slope, deep water leasing areas,

and the locations of case history sites.......................................................................13

7. Map showing the location of Garden Banks Block 161. ..........................................18

8. Bathymetric map illustrating the large mound-like feature that

nearly occupies the entire GB Lease block...............................................................18

9. An E-W profile of the GB161 mound digitally acquired as part

of this investigation using a Seismic Systems S-15 water gun as

an acoustic source. ....................................................................................................20

10. (a) A map of 3D-seismic surface amplitude data from the GB161 mound area.

(b) An amplitude slice made through the subsurface extension of the feature

 at the 1 sec horizon. .................................................................................................21

11. X-ray radiographs of two cores from Boring 1 illustrate (a) a massive

nonstratified structure and (b) suites of unusual coarse-grained components

in a fine-grained matrix.............................................................................................22

12. This 3D-seismic profile of the GB161 mound illustrates the complex

subsurface configuration, the chaotic and acoustically turbid zone

beneath the mound, and the position of Boring 1. ....................................................24

13. Location map for GB189. .........................................................................................26

14. Bathymetric map of the GB189 area showing the symmetrical mound

in the south-central part of the block and the graben-like trough that

trends to the south away from the.............................................................................28

15. A 3.5 kHz profile oriented north-south across the feature........................................29

16. This north-south oriented 15 in3

 water gun profile reveals

important characteristics about the GB189...............................................................29

17. This process multichannel sparker profile oriented north-south

across the GB189 mound clearly illustrates the high amplitude internal

reflection event as well as the very reflective upper mound surface. .......................30
xii

18. A bottom photography from JSL-1, Dive 3306, which shows the

small coralline algal buildups that occur on the upper mound. ................................30

19. This 3D-seismic amplitude map indicates that his feature has an

acoustic impedance similar to the surrounding sea floor with no

evidence of a phase inversion over the feature, which is generally

associated with gas-charged sediments.....................................................................31

20. Location map for Garden Banks Block 338 and 382................................................34

21. A bottom features map of the GB338-427 area illustrating the

expulsion centers in GB338 and GB382, gravity-driven deposits that have

been transported downslope, and the large diapiric mound in GB427.....................34

22. Bathymetric map for the GB338-382 area illustrating the NE-SW ridge-like

topography which is a reflection of underlying shallow salt. ...................................35

23. Side-scan sonar mosaic of flows originating from the GB338 expulsion center......36

24. A 3.5 kHz profile across the expulsion center in GB338. ........................................38

25. A 3.5 kHz profile across the expulsion center in GB382. ........................................25

26. A 15-in3 water gun profile across the expulsion center in GB338...........................39

27. An E-W oriented side-scan sonar swath across the crestal area of

the GB382 expulsion center mound..........................................................................39

28. A NW-SE oriented side-scan sonar swath across the crestal area

of the GB338 expulsion center mound. ....................................................................40

29. A 3D-seismic surface amplitude map of the GB338 and 382 area

illustrating the amplitude deviations from the background. .....................................41

30. A 3D-seismic profile through the GB382 expulsion center illustrating

the acoustically opaque interior of this feature which extends deep within

the subsurface, the strong surface reflector defining the mound, and the

positive to negative phase reversal over the mound crest.........................................41

31. Barite cones found near the apex of the GB338 mound. ..........................................42

32. Small barite chimneys common to a large area of the GB338 mound surface.........42

33. Chemosynthetic mussels (Bathymodiolus sp.) cemented together with barite.........43

34. Location map for Garden Banks Block 427. ............................................................45

35. Bathymetric map for GB427 and surrounding areas. .............................................45

36. Migrated multichannel seismic profile (TriCluster data) across the

GB427 mound illustrating the strong internal reflector which is less than

100 m from seafloor at the mound’s apex. ...............................................................47

37. High resolution seismic profile across the GB427 mound illustrating the

acoustically opaque mound interior, bedded sediments on the mound flank,

remnants of a hemipelagic drape deposit over part of the feature, and its irregular

surface of variable reflectivity. .................................................................................48

38. A 3D-seismic profile from the GB338 expulsion center to the GB427

diapiric mound. .........................................................................................................49

39. A deep-tow side-scan sonar mosaic of the GB427 mound illustrating

important surface characteristics of this feature such as the irregular and highly

reflective areas and the linear patterns that are radial to the mound crest area. .......49

40. A map of 3D-seismic surface amplitude map of the GB427 mound........................50

41. Disarticulated and scattered lucinid-vesycomyid clams at one of the

dormant vent sites near the crest of the GB427 mound............................................51
xiii

42. (a) massive rocks of authigenic carbonate are found on the surface and

(b) nodular masses of host sediment cemented with 13C-depleted Mg-calcite

are found in the subsurface at the mound crest as well as along its flanks,

especially near the flow pathways from upslope expulsion centers. ........................51

43. Lithification on the flank of the GB427 mound........................................................52

44. Site-specific areas of side-scan sonar data as extracted from the mosaic

of the GB427 mound, illustrated in Figure 38. .........................................................53

45. Location map for Green Canyon Block 53...............................................................55

46. Bathymetric map for GC53 and surrounding areas. .................................................56

47. High resolution seismic profile (3.5 kHz) across the mounded area

in the SW sector of GC53. ........................................................................................58

48. A 3D-seismic profile oriented NE-SW across GC53 showing the relationships

between salt, faults, and important surface features. ................................................59

49. Photographs of the GC53 mounded area in the middle of the block........................60

50. Side-scan sonar image of the central mounded area in GC53. .................................60

51. A 3.5 kHz profile across the inactive mud volcano in the NE section of GC53. .....61

52. Bottom photography of a gully on the flank of the GC53 inactive mud

volcano along its SW side.........................................................................................62

53. A 3.5 kHz profile across the collapse depression in the SW quadrant

of the GC53...............................................................................................................63

54. This side-scan sonar record, acquired as part of this project, illustrates the

venting area within the collapse depression, newly deposited mud flows, and

a set of roughly circular low-relief scarps indicating local subsidence ....................63

55. This figure represents the cross section of a combined gas and suspended

sediment plume acquired by a precision depth recorder prior to a manned

submersible dive on this feature in the summer of 1997. .........................................64

56. This 3D-seismic surface amplitude map of GC53 clearly identifies that

three bottom types and features discussed in this report...........................................65

57. Location map showing the position of Block 140 in the Green Canyon

leasing area................................................................................................................70

58. Bathymetric map of the GC140 dome and adjacent areas........................................71

59. A 50-in3 air gun profile run N-S across the GC140 dome .......................................73

60. This figure represents a side-scan sonar mosaic of the GC140 dome top area.........73

61. A 3D-seismic profile oriented in a northwest-southeast direction across the

GC140 dome to the mounds that are known to contain gas hydrates in GC185. .....74

62. A photography of the base of an authigenic carbonate mound in GC140

showing large blocks of well-lithified Ca-Mg carbonate forming this feature.........75

63. A crevasse in a GC140 authigenic carbonate mound ...............................................75

64. A 3D-seismic surface amplitude map of the GC53 area...........................................76

65. Location map for Green Canyon Block 185.............................................................79

66. Bathymetric map of the GC mound complex indicating that two distinct

mound-like features exist along the GC 184-185 border..........................................80

67. Schematic figure representing the present salt and sediment subsurface

configuration of the GC140, 184, and 185 area........................................................81

68. High resolution seismic profile fun E-W across the northernmost GC

185 mound. ...............................................................................................................82
xiv

69. A mosaic of 77 kHz side-scan sonar data collected from the northern GC

185 mound using the manned submersible NR-1.....................................................83

70. This 3D-seismic surface amplitude map of the GC185 mound complex clearly

identifies areas that deviate significantly from the background of seafloor

surrounding the mounds............................................................................................84

71. A 3D-seismic profile across the northern GC185 mound.........................................85

72. These three photographs are representative of seafloor conditions and

communities typical of the high topography of both the northern and southern

GC185 mounds. ........................................................................................................85

73. This bottom photography of the mud vent at the base of the GC185 mound

illustrates the active escaping gas and mud extrusion taking place at the site..........86

74. Authigenic carbonate mounds are aligned along a fault that is clearly visible

in high resolution seismic data..................................................................................89

75. A schematic illustration of the relationship between rate of delivery of fluids

and gases to the seafloor and geologic/biologic response. .......................................91

76. (a) A high resolution seismic profile 15-in3 water gun) across a mud volcano

in Green Canyon Block 143. (b) This submersible-acquired picture shows

gas escaping from the central crater of the GC143 mud volcano. ..........................93

77. A small gas hydrate mound in Green Canyon Block 232 showing

outcropping ledges of gas hydrate at its base and along its flanks and

orange bacterial mats on the surface.........................................................................96

78. (a) This outcrop of authigenic carbonate with associated tube worms and

mussels occurs in Green Canyon Block 232. ...........................................................96

79. Small fluid mud expulsion areas in Green Canyon Block 232 exhibit

sheets of extruded mud on which lucinid clam shells litter the surface....................97
xv

List of Tables

Table Page

1. Sites and features that were the focal point for data collection

in support of this study...................................................................................................1

2. Pore fluid salt concentration for Boring 1, Garden Banks Block 161. .........................22

3. Side-scan sonar image characteristics for GB161 case study.......................................25

4. Seismic facies for GB161 case study............................................................................25

5. Surface reflection characteristics for GB161 case study. .............................................25

6. Side-scan sonar image characteristics for GB189 case study.......................................33

7. Seismic facies for GB189 case study............................................................................33

8. Surface reflection characteristics for GB189 case study. .............................................33

9. Side-scan sonar image characteristics for GB338 case study.......................................44

10. Seismic facies for GB338 case study..........................................................................44

11. Surface reflection characteristics for GB338 case study. ...........................................44

12. Side-scan sonar image characteristics for GB427 case study.....................................54

13. Seismic facies for GB427 case study..........................................................................54

14. Surface reflection characteristics for GB427 case study. ...........................................54

15. Side-scan sonar image characteristics for GC53, mounded carbonates,

case study..................................................................................................................67

16. Seismic facies for GC53, mounded carbonates, case study........................................67

17. Surface reflection characteristics for GC53, mounded carbonates, case study. .........67

18. Side-scan sonar image characteristics for GC53, mud volcano, case study ...............68

19. Seismic facies for GC53, extinct mud volcano, case study. .......................................68

20. Surface reflection characteristics for GC53, extinct mud volcano, case study...........68

21. Side-scan sonar image characteristics for GC53, collapse depression, case study.....69

22. Seismic facies for GC53, collapse depression, case study..........................................69

23. Surface reflection characteristics for GC53, collapse depression, case study. ...........69

24. Side-scan sonar image characteristics for GC140 case study.....................................78

25. Seismic facies for GC140 case study..........................................................................78

26. Surface reflection characteristics for GC140 case study. ...........................................78

27. Side-scan sonar image characteristics for GC185 case study.....................................88

28. Seismic facies for GC185 case study..........................................................................88

29. Surface reflection characteristics for GC185 case study. ...........................................88

30. General relationships between delivery rate of fluids and gases to the

seafloor and response in seafloor geology, biology and

hydrocarbon degradation ..........................................................................................92
1

I. Introduction

Oil and gas exploration and production in deep water (beyond the shelf break) of the

northern and northwestern Gulf of Mexico are currently being carried out on a much greater

scale than in any equivalent setting in today’s oceans. New evaluation techniques based on

improved acquisition and processing of 3D-seismic data suggest that 2.33 billion barrels of liquid

hydrocarbons and about 27.5 trillion cubic feet of gas exist in “proved reserves” in 754 active

fields in the Gulf of Mexico OCS region. Within the U.S. Exclusive Economic Zone these

resources from the Gulf of Mexico are by far the most important from a strategic and economic

point of view. With growing world populations that need hydrocarbons and the potential for

political instability in the Middle East and other important oil-producing areas, it is in the best

interest of the United States to reduce dependency on foreign oil reserves by developing our own

resources. The most promising hydrocarbon frontier in America is off Louisiana and Texas in

water depths ranging from 300-2500 m.

Renewed interest in deep water exploration in the Gulf of Mexico has stimulated

economic growth in oil-related industries and presented scientists and engineers with a new set

of challenges uniquely associated with little known areas of the deep slope environment. In 1996

an exploratory well was drilled on top of a mammoth structural feature at the base of the slope in

over 2500 m of water (Alaminos Canyon). At the time, this drilling project represented a

glimpse of the future for the Gulf’s northern continental slope. The 1995 Minerals Management

Service offshore lease sale (OCS Sale 161) resulted in leasing over 3 million acres to 62 different

companies. Follow-up drilling of prospects acquired during this sale has led to to significant

discoveries in the deep-water province of the Gulf of Mexico. In July of 1999 BP Amoco

announced four major oil discoveries, including the largest made to date by any company. The

largest of these discoveries is the “Crazy House” prospect, located in a water depth of 1830 m in

Mississippi Canyon Blocks 776,777, and 778. This field alone is estimated to contain over a

billion barrels of recoverable oil. The other three discoveries range in depth from 1220 m to

1980 m. For BP Amoco these three discoveries add about 600 million barrels of recoverable oil

to their reserves in the southern Green Canyon area. The total Gulf of Mexico reserves for BP

Amoco alone is about 3 billion barrels. Although there are many technological challenges

associated with working in ultra deep water of the Gulf, most major petroleum companies are

actively exploring these areas.

Hydrocarbon exploration and production activities are taking place in a dynamic and

complex geologic framework for the northern Gulf of Mexico continental slope, which is

inherited from the interplay of massive sediment input since Cretaceous times and the

compensating movement of allochthonous Jurassic salt. The end product is perhaps the most

geologically and ecologically complex continental slope setting in today’s oceans (Figure 1).

Therefore, it becomes important for basic science, applied science, and regulatory reasons to

develop a better understanding of slope processes and seabed characteristics in this province

where “ground truth” data are scattered and have been collected primarily for site-specific

reasons.
2

Figure 1. Complexity of the Louisiana-Texas continental slope are revealed in high resolution and computer

enhanced Seabeam bathymetry. The dome and basin topography is related to the interplay between the

input of massive volumes of sediment and the deformationof underlying salt.

The petroleum industry and the Minerals Management Service (MMS) are both mandated

to understand the physical-biological-geological complexities of the outer continental shelf and

upper continental slope region (OCS) of the northern Gulf of Mexico because of the combined

demands for resource utilization and management. Within simple geologic frameworks the tasks

of both resource extraction-utilization and rational environmental management of these efforts

are reasonably well understood. However, in extremely complex settings like the northern Gulf

of Mexico OCS, MMS must anticipate environmental management problems in a geographic

region that has received minimal financial support to determine physical-biological-geological

characteristics and spatial-temporal variability of these properties. With continuing interest in oil

and gas exploration in deeper and deeper areas beyond the continental shelf edge, industry has

met the challenge with innovative engineering concepts and designs for drilling, production

facilities, and resource transport systems. In order to effectively manage OCS activities, MMS

must incorporate the best possible information from industry and academia to develop new

concepts and technologies for environmental assessment. It is in this spirit that this study was

undertaken in order to provide criteria and concepts for improved interpretation of sea floor

geology and habitats of sensitive benthic communities.
3

The intent of this program is to use both digital and analog high resolution acoustic data

sets (geohazards data) as the primary data sources with digital 3D-seismic surface attribute data

as an added source of interpretive information and direct observation-sampling by research

submersible for sea floor verification. As this program has progressed, it has become absolutely

clear that the sea floor verification step is necessary on multiple examples within each feature

category before reliable interpretation criteria can be developed. In addition, sea floor attribute

mapping (Hill 1996; Roberts et al. 1996) from 3D-seismic data has added a new dimension

which provides a means of mapping gas in surface sediments and the presence of hard

mineralized bottoms. Combined with high resolution acoustic data sets and sea floor verification

data, the surficial information provided by 3D-seismic makes a powerful combination for

interpreting sea floor geology and thereby benthic habitats.

Operational, engineering, and regulatory guidelines require industry to evaluate potential

drilling areas, platform sites, and pipeline routes for the purpose of both identifying potential

geohazards and the presence of federally protected biologic communities (particularly

chemosynthetic communities). On the regulatory side, MMS strives to formulate environmental

management guidelines in the general framework of rapidly expanding resource development.

Both industry and MMS need the best available data sets to meet their objectives. Our

understanding of the sea floor at continental slope depths in the Gulf of Mexico is based

primarily on scattered site-specific data sets acquired with a variety of instrumentation types.

Many sea floor features such as faults and submarine landslides plus other forms of mass

movement are relatively easy to identify even on mediocre acoustic data sets. There are

however, many sea floor features that morphologically range from mounds to depressions that

represent themselves on high resolution seismic profiles as acoustic wipe-out zones or zones of

no organized subsurface reflection horizons. These features present a particularly difficult

problem for those tasked with interpreting sea floor geology from remotely sensed high and

medium resolution acoustic data. Most interpreters consider acoustic wipe-out zones to be the

product of bubble phase gas in surface and shallow subsurface sediments (Bryant 1981; Hovland

and Judd 1988). Previous studies, which have incorporated ground-truth verification of zones of

no acoustic return through cores and direct observations using an ROV or manned submersible,

demonstrate that acoustic wipe-out zones can be associated with many different types of features

and sea floor types. They range from mud volcanoes (Neurauter and Bryant 1990; Neurauter and

Roberts 1994) to mud mounds containing gas hydrates (Brooks et al. 1985) to areas of lithified

sea floor (Roberts and Aharon 1994) and extinct expulsion sites (Roberts et al 1998). Through

manned submersible inspection of these sea floor types and features which have acoustically

turbid interiors, it has been determined that most of them are the products of fluid and gas

expulsion. Liquid and gaseous hydrocarbons are commonly found associated with acoustic

wipe-out zones, and the presence of chemosynthetic organisms is common but not absolute. So,

the challenge becomes one of definitive identification of various types of sea floor features that

have a common general response on high resolution seismic data sets. Therefore, the overall

objective of this study is to apply state-of-the-art digital acoustic data, both high resolution

seismic and lower resolution, but deeper penetrating 3D-seismic as well as side-scan sonar data,

for developing a conceptual framework and reliable criteria for identification of sea floor

features and areas that are the products of hydrocarbon venting-seepage. Within this overall aim

of the study a parallel purpose is to better understand how to interpret the presence or absence of

chemosynthetic communities from standard high resolution acoustic data collected for
4

geohazards evaluation and 3D-seismic surface amplitude data. The establishment of reliable

interpretation criteria coupled to digitally acquired data has the potential to greatly increase the

reliability of geohazards evaluations suitable for compliance with MMS regulations for

chemosynthetic communities without the added expense of bottom video and/or photographic

survey.

This study differs from other attempts to classify sea floor types in the northernnorthwestern Gulf of Mexico (e.g. McDonald et al. 1996) in that four separate data sources are

focused on identification of a spectrum of different feature types that possess the common

acoustic response that has been linked to hydrocarbon seepage venting and presence of

chemosynthetic communities, acoustic wipe-out zones. The data sources are: (1) standard high

resolution geohazards acoustic data sets (surface tow and deep tow), (2) project-acquired

surface-tow high resolution acoustic data, (3) 3D-seismic surface amplitude extraction data, and

(4) sea floor verification of sites using a manned submersible. Concurrent research projects

sponsored by NOAA, the petroleum industry, and Louisiana Quality Education Fund have

provided data sets necessary to make a regional comparison of areas displaying acoustic wipeout characteristics. The variability of acoustic response within and between areas provides the

key information on which a meaningful conceptual framework and interpretation criteria can be

formulated. Systematic analysis of this variability in acoustic response as correlated to real

characteristics of the sea floor makes this investigation different from previous research.

The hypothesis to be tested by this study is: Systematic analysis of digital high resolution

acoustic data and 3D-seismic coupled with sea floor verification provided by manned

submersible observations over a wide variety of sea floor features exhibiting acoustic wipe-outs

will produce a set of interpretation criteria that will significantly improve our ability to predict

the character of the modern OCS sea floor and the occurrence of chemosynthetic communities.

The presence or absence of chemosynthetic communities is directly dependent on delivery of

hydrocarbons to the sea floor. This delivery system is a product of the continental slope’s

geologic framework. Therefore, understanding the geology of hydrocarbon seep-venting sites is

viewed in this study as fundamental to being able to predict general sea floor geologic and

biologic responses.

A. Geologic Framework

In order to understand the geologic framework and stratigraphic architecture of the

northern Gulf continental slope, an appreciation of the evolution of allochthonous salt, associated

faults, and intraslope sedimentary basins is essential. Seismic profiles and well data (various

types of well logs and associated micropaleontological data) provide the main elements for

evaluating the evolving depositional environments, sedimentary facies, and salt characteristics.

Some authors have produced sequential structural restorations from these types of data sets

(Worrall and Snelson 1989; Diegel et al. 1995; Peel et al. 1995; Rowan 1995; and McBride

1995). Such reconstructions help explain the evolution of salt structures coincident with

sedimentation. This new understanding of the dynamic changes that have taken place through

time to give us the present slope configuration is possible because of improved seismic imaging

technology (Ratcliff 1993), better physical modeling of salt-sediment systems (Vendeville and

Jackson 1992), and the application of sequential restorations (McBride 1996). Such innovative
5

new work on the slope has shown us that tabular allochthonous salt sheets and nappes are not

new to the slope, but have occurred previously and have undergone various stages of

deformation and evacuation (Diegel and Cook 1990). The emplacement and eventual evacuation

of allochthonous salt appears to vary spatially and temporally throughout the northern Gulf of

Mexico basin. No single model for salt movement can explain the array of salt geometries

presently imaged in the subsurface. However, it is clear that original salt geometries and the

manner in which they interact dictate the positions of later minibasins, remnant salt diapirs,

extensional growth faults, contractional structures, and strike-slip deformation (McBride 1996).

In addition, it is clear that this framework provided by salt deformation and sediment loading is

the template for understanding the complexities of the modern sea floor.

Salt deformation and ultimately slope configuration is largely linked to the ways

sediments are input to deep water. As is now widely recognized by the geologic community,

sediment input to the outer shelf and continental slope is strongly modulated by sea level

changes (Suter and Berryhill 1985). During periods of sea level lows, fluvial systems entrench

themselves as they prograded across the shelf to eventually deposit their sediments in thick and

discrete shelf-edge deltas (Suter and Berryhill 1985; Roberts et al. 1991; Sydow and Roberts

1994; Anderson et al. 1996). In addition, these rapidly deposited deltas load the shelf margin, an

inherently unstable area, frequently causing shelf-edge failures that contributed large volumes of

sediment to downslope depositional sites (Coleman et al. 1983). The interplay between intense

periods of sedimentation, largely at low sea levels, and compensating salt tectonics has resulted

in a present-day slope configuration that is characterized regionally by numerous domes and

basins (Martin 1980) as can be seen in Figure 1. As a product of the processes outlined above,

the interdome basins are filled with thick sedimentary sequences composed of sand-rich slope

fans and turbidities as well as thick clay-rich units. Because of sea level forcing of fluvialdeltaic sediment input to the continental slope, during periods of high sea level, fine-grained

hemipelagic sediments with a high pelagic foraminiferal content drape the slope topography,

except on topographic highs in outer shelf and upper slope settings, where physical processes

may remove much or all of this deposit. These sediments vary in thickness but are typically 3-5

m thick on the mid-to upper-slope.

B. Seafloor Geology

Faulting is a process that occurs on many scales within the continental slope setting, from

major growth faults that cut thousands of meters of sedimentary section to much smaller

compensating faults related primarily to salt movement in the shallow subsurface. In addition to

off-setting the sea floor and creating local topography with oversteepened slopes that lead to

various forms of mass movement, faults are responsible for numerous constructional sea floor

features related to the vertical flux of fluids and gases and expulsion of these products at the

ocean bottom. At one end of the feature spectrum are large mud volcanoes (Neurauter and

Bryant, 1990; Neurauter and Roberts, 1994) formed by fine-grained sediment forced up faults.

Hedberg (1974) identifies the process of sediment flux by gas-filled formation fluids up faults as

being responsible for creating mud diapirs as well as mud volcanoes. At the other end of the

spectrum, vertical flux of gases and fluids may be very slow. Microbial degradation of both

hydrocarbon gases and crude oil associated with this process can catalyze by-products such as

calcium-magnesium carbonates that create a variety of sea floor features including hard grounds
6

and mound-like structures of various dimensions (Roberts et al 1992 a,b). This process has been

described from other settings, such as accretionary prisms, where salt tectonics is not a factor and

only biogenic methane is the hydrocarbon source (Ritger et al., 1987; Paull et al., 1992). The

following discussion highlights important small-to-mesoscale features on Louisiana’s continental

slope that make the surficial geology of this province very complex.

1. Dome-Top Mounds

Common features on the sea floor over shallow subsurface salt diapirs are carbonate

mounds having various dimensions and frequencies of occurrence. Although almost every upper

slope diapir crest thus far investigated in this study has carbonate mounds of some description,

one of the best examples of a wide-spread mound complex occurs in the Green Canyon Area,

Block 140. Each one of these mounds is the site of slow seepage of both hydrocarbon gases and

crude oil. The mounds have developed from deposition of both calcium- and magnesium-rich

carbonates, a by-product of microbial activity at the seep sites. Carbonates derived from this

process have been described in detail from other localities by Ritger et al. (1987) and Paull et al.

(1992). This process produces carbonate that is 13C depleted and at this site δ13C values of -48 to

-55 are common. Most samples analyzed thus far have been composed of Mg-calcite, although

dolomite comprises up to 40% of some samples (Roberts et al. 1992b). The average relief of

these mounds (Figure 2) is about 10 m with some greater than 20 m. Details of the moundforming process are currently being interpreted from submersible-derived data sets. At this

location, the mounds have developed during several late Pleistocene cycles of sea level change

(Roberts and Aharon 1994) and display coarse sediment lags which seem to be evidence of

considerable dome-top erosion. On surface amplitude data derived from 3D seismic (Roberts et

al. 1992a), this site displays a “bright” high amplitude response related to a hard, reflective

bottom, but no indication of gas in near surface sediments. This response is consistent with a

reflective, irregular, carbonate interface where seepage is a very slow process. On the upper

slope near the shelf edge, seep-related mounds with similar amplitude signatures are veneered

with biogenic carbonates developed primarily during periods of Late Pleistocene lowered sea

level when the photic zone coincided with mound depths.

Figure 2. (a) High resolution sparker profile across a dome top illustrating the mounded seafloor, acoustic wipe-out

zones beneath these mounts, and truncated bedding on the dome flanks. (b) Large clasts of authigenic

carbonate are common constituents of the sediments on the dome top and around the mounded carbonates.
7

2. Dome-Top Erosional Features

Studies of currents active on the upper continental slope (e.g. Hamilton, 1990) indicate

that intrusions of the Loop Current and its westward-moving eddies have associated currents at

upper slope depths sufficient to transport sand-sized sediments. Regional relief features,

specifically dome tops, represent zones where much of this energy is naturally focused.

Certainly, the coarse sediment lags, as described above in associations with carbonate mounds,

and the commonly occurring truncated beds associated with the dome-top settings reflect the

existence of persistent and on-going erosive forces. In isolated cases, coarse sediments, largely

composed of biogenic grains (shell hash) and nodules/clasts of diagenetic origin, are organized

into migratory bed forms (Roberts 1995). This association is direct evidence of strong bottom

currents on the upper slope and helps explain truncation of bedding and missing sedimentary

sections associated with the sea floor over salt structures.

3. Gas Hydrate Outcrops and Associated Mounds

Because of the strong fault-related vertical flux of both gas and water to the sea floor, as

described above, gas hydrates are able to exist at or near the sea floor in water depths greater

than about 500 m (Figure 3). Gas hydrates are ice-like substances composed of rigid cages of

water molecules that enclose molecules of hydrocarbon gases, primarily methane.

Figure 3. A high resolution seismic profile (15-in3 water gun) across a known gas hydrate complex in Green

Canyon Block 185. Note the acoustic turbidity or acoustic wipe-out zone associated with the feature and

its mound-like shape.
8

They occur under special conditions of temperature and pressure where the supply of

hydrocarbon gas is sufficient to stabilize the molecular architecture of the hydrate. Abundant

deep-seated hydrocarbons, numerous and complex fault systems that function as transport

pathways, nearly continuous fault adjustments related to salt tectonics, and a myriad of surface

hydrocarbon seeps, makes the Louisiana continental slope an ideal setting for hydrate

accumulation. Recently, they have been observed as “outcrops” on the sea floor by the author

and others (McDonald et al. 1994). In most cases, however, hydrates occur below the surface

with an overlying and insulating layer of fine-grained sediment. Collectively, the process of

vertical flux of water and gas up faults results in mound-like accumulations. These features

characteristically display acoustic “wipe out” zones beneath their surfaces on high resolution

seismic profiles. In addition to affecting local topography/geology, gas hydrates associated with

larger mound-like buildups function as a rather constant trophic resource for chemosynthetic

communities. Also, as interpreted from piston cores and direct submersible sampling, they are

intermeshed with authigenic carbonates. These carbonates frequently take the form of small

nodular masses to ledge-like outcrops. Both types are usually very 13C-depleted, but may have

varied carbonate mineralogies.

4. Mud Vents and Mud Volcanoes

When the transport of fluids, gases, and fine-grained sediment up-fault planes is

rapid, cone-shaped accumulations of mud often develop. As Neurauter and Bryant (1990) and

Kohl and Roberts (1995) point out in studies on the northern Gulf of Mexico continental slope,

these features are not only common in this setting, but they have been described from many

submarine environments throughout the world oceans (Hovland and Judd, 1988). From

submersible observations, Neurauter and Roberts (1992) noted that active mud volcanoes have

caldera-like depressions that contain fluid mud bubbling with gas that is frequently mixed with

globules of crude oil. As the pool of fluid mud upwells over the lip of the crater, sheets of

sediment-rich fluid flow down slope, add a new accretion unit to the cone’s flank, and extend

the diameter of the mud volcano’s base. Features of this description occur on a variety of scales

from small cones less than 1 m (Figure 4a) diameter to large features with over 30 m relief and

bases of over 1 km in width (Figure 4b). Kohl and Roberts (1994) demonstrate that the process

of fluid mud extrusion results in displaced microfaunas and inversion of biostratigraphic marker

horizons in slope sediments. For example, they show that surface sediments at four vent sites

contained microfossils yielding age dates of Pleistocene to early Miocene. In some cases, fluid

mud extrusion takes place without the formation of a cone-shaped vent. For example, smallscale sheets of mud are extruded on the surfaces of gas hydrate mounds, perhaps an expulsion

product during the hydrate-forming process. In other cases, thick sheets of mud flow kilometers

down slope from extrusion sites such as those in the Garden Banks, Block 338 area, a case study

discussed later in this report.
9

a b

Figure 4. (a) A small mud volcano from Green Canyon Block 272 shows all the features typical of larger forms that

reach heights of over 30 m above the surrounding seafloor. The field of view in this photograph has a

width of approximately 5 m. Note the gas-induced micro-pockmarks around the mud volcanoes. (b) This

high resolution seismic profile of a mud volcano with two distinct vents is located in Green Canyon Block

97. Note the stratified units that form the core flank and the concentric form on the underlying side-scan

sonar record.

5. Slope Instability Features

Rapid deposition of sediment at the shelf edge, faulting, and vertical migration of shallow

salt create instabilities primarily by over-steepening of slopes. A wide range of failure features

results, from massive shelf edge evacuation features (Winker and Edwards 1983) to small-scale

slumps along fault faces and on the sides of diapirs (Figure 5). Depending on scale, massive

volumes of sediment can be transported downslope in association with subaqueous mass

movement processes (Coleman et al. 1986). These processes currently pose a considerable risk

to man’s activities on the northern Gulf’s continental slope. Even thin deposits of hemipelagic

highstand sediments that drape topography of the slope display a tendency to fail (Doyle et al.

1992). Sediments displaced by slumps, submarine landslides, and other mass movement

processes tend to have chaotic-to-acoustically opaque internal reflectors on high resolution

seismic data and commonly produce small-scale irregular surface topography. Some intraslope

basins contain fill-sequences of repeated and stacked chaotic units that are interpreted as the

products of massive failures. These deposits likely originated at or near the shelf edge during

periods of lowered sea level and failed during the sediment loading process. Vertical movement

of salt is also very important in creating oversteepened slopes that lead to slope failures. The

rugged surfaces of these deposits and chaotic internal reflectors mimic similar slumped units

currently found at the sea floor in many places on the present continental slope.
10

Figure 5. This picture shows submarine slump deposits that have originated from the flank of an adjacent salt diapir

in East Breaks Area, Block 211. Note the acoustically opaque nature of the displaced material and the

irregular surface profile evident on this 3.5 kHz record.

C. Data Base

Our present understanding of the surficial geology of the Gulf’s northern continental

slope has resulted primarily from the interpretation of geohazards data sets acquired in support of

petroleum company activities. Early data sets consisted of high resolution seismic profiles from

a variety of acoustic sources (sparkers, minisleeve exploders, small air guns, etc.) deployed in

surface tow mode. These initial data sets were also collected as analog records which meant that

the operator played a large part in record quality and there was no chance of post data collection

processing. In addition to seismic and echo sounder profiles, most geohazards surveys had sidescan sonar data, also initially in analog format. After the mid-1980s most side-scan data were

digital and corrected for slant range and boat speed. From these data sets it was discovered that

the sea floor at slope depths in the Gulf was covered with mounds, pinnacles, depressions, and

other sea floor features of unknown origin. Many of these features were accompanied by zones

of “acoustic wipeout” or “acoustic turbidity” where no internal reflection events were visible

beneath the surface reflector on high resolution seismic records. Reflective gas plumes in the

overlying water column were also found to be common to some of these areas. As interest in

deeper and deeper parts of the slope increased as the search for hydrocarbons moved toward the

middle and lower slope, deep-tow instrumentation was developed to compensate for the lack of

resolution of data collected in surface-tow mode. Deep-tow data collection systems became
11

generally available in the mid-to-late 1980s, but are still not universally used for geohazards

surveys primarily because of cost. These systems typically contain a conventional 100 kHz sidescan sonar and 3.5 kHz sub-bottom profiler. They produce excellent data collected at a constant

height above the bottom. Now, autonomous underwater data collection vehicles (AUVs) are

replacing the deep-tow system.

In most geohazards surveys, if ground truth is provided it is by piston or gravity cores.

However, since the mid-1980s a variety of research submersibles have been used to improve the

“calibration” between acoustic data and actual conditions at the sea floor (Kennicutt et al. 1985;

Roberts et al. 1989; Doyle et al. 1992; McDonald et al. 1994; Roberts and Aharon 1994).

This project has utilized geohazards data sets collected for the petroleum industry of

many different vintages and acquisition modes (Appendix A). In addition, high resolution

acoustic data, primarily seismic profiles with some supporting side-scan data have been collected

for this project using state-of-the-art seismic sources and digital acquisition and processing

software (DELPH 2). The seismic sources include a Geopulse Boomer, Seismic Systems S-15

(15 in3

) water gun, and a Seismic Systems GI (50 in3) air gun. The water gun and air gun are

supported by a Hamworthy 50 cfm seismic compressor (see Appendix 2 for equipment details).

The advantage of collecting seismic data in a digital format for the project is that data processing

can be accomplished both during and after acquisition. This option, not available with analog

data, provides a method for improving image quality in a post-cruise laboratory setting through

applying deconvolution, stacking, a swell filter, and other routines.

A relatively new and important source of data for application to practical engineering

problems on the slope sea floor as well as for meeting regulatory requirements is 3D-seismic

data. This new trend of utilizing 3D-data for engineering and regulatory applications is being

driven by the widespread coverage of the continental slope as a product of industry’s accelerated

pace for development of deepwater exploration and production projects. Although resolution at

the sea floor and in the shallow subsurface is not equivalent to high resolution seismic used for

standard geohazards surveys, there are some clear advantages. For example, 3D-data once

loaded on a workstation, provide a rapid and accurate method for linking subsurface geology to

sea floor configuration. In addition, the mapping of surface attribute data in conjunction with

phase data can quickly identify areas with hard, reflective surfaces from those where surface and

near-surface sediments are charged with gas. The main limitation of 3D-data is reduced

definition when compared to good quality high resolution geohazards survey data. Utilization of

3D-seismic data on this project, specifically surface amplitude maps, has provided a separate and

important line of evidence for making the best possible interpretation of sea floor geology and,

indirectly, biology.

Perhaps the most important data for this project have been the sea floor observations

made from research submersibles. These data in conjunction with results of analyses on samples

collected have been used to calibrate remotely sensed acoustic data to real world surficial

conditions on the continental slope. The PI as well as colleagues from other institutions have

been collecting data (personal observations, video, 35mm photos, cores, and grab samples) for

over a decade in an effort to develop a better understanding of surficial geology and biology of

the continental slope, particularly in conjunction with those features linked to hydrocarbon
12

seepage or venting. The Johnson Sea-Link from Harbor Branch Oceanographic Institute in Fort

Pierce, Florida has been the vehicle of choice and the one most used. This vehicle is limited to a

diving depth of 1000 m. Therefore, all examples selected for presentation in this report occur in

water depths shallower than 1000 m.

In order to build the best possible understanding of sea floor features that display acoustic

wipeout zones on high resolution seismic records and may or may not have associated

chemosynthetic communities, four quite different data sets were utilized: (1) standard high

resolution geohazards data (surface-tow and/or deep-tow), (2) digital acoustic data (acquired as

part of this project) showing the same feature in different perspectives, (3) 3D-surface attribute

data, and (4) direct observation or sea floor verification using a research submersible. Sites for

study were selected only if these four data sets were available or could be made available within

the time-frame of the study. All data sets reviewed for the study are listed in Appendix 1. Sites

identified for study are listed in Table 1 and Figure 6. Survey tracks over critical features

indicating data collected by the PI in support of this project are given in Appendix B.

Table 1. Sites and features that were the focal point for data collection in support of this study.

Area and Block Feature Area and Block Feature

Ship Shoal 286 Mud Mound Garden Banks 189 Diapiric Mound

Green Canyon 18 Large Buildup Garden Banks 201 Mounded Seafloor

Green Canyon 53 Mud Volcano Garden Banks 215 Gas-prone

Green Canyon 53 Collapse Depression Garden Banks 260 Mounded Carbonates

Green Canyon 53 Mounded Carbonates Garden Banks 304 Mud Vent

Green Canyon 140 Mounded Carbonates Garden Banks 304 Gas Vent

Green Canyon 143 Mud Volcano Garden Banks 304 Mounded Carbonates

Green Canyon 152 Fluid Expulsion Garden Banks 338 Expulsion Center

Green Canyon 180 Mounded Seafloor Garden Banks 382 Expulsion Center

Green Canyon 185 Hydrate Complex Garden Banks 427 Diapiric Hill

Green Canyon 193 Hydrate Mounds Mississippi Canyon 709 Hydrates

Green Canyon 232 Hydrate Mounds Mississippi Canyon 843 Hydrates

Green Canyon 234 Hydrate Mississippi Canyon 885 Grassy Sediments

Green Canyon 237 Brine/Hydrate Mississippi Canyon 929 Mineralized Cones

Garden Banks 171 Mounded Carbonates

The criteria for evaluting the acoustic properties of specific sea floor features, field of

features, and sedimentary units flanking features/areas of interest are listed in three tables: (1)

side-scan sonar image characteristics, (2) seismic facies, and (3) surface reflection

characteristics. The criteria for determining the characteristics of side-scan sonar records and the

character of surface reflections on seismic records are straight forward and deal with simple

geometries or qualitative measures of reflection signal strength. No detailed explanation of the

terms used in these tables was considered necessary for this report. However, terms used to

describe the subsurface (seismic facies) are defined in the following section of the report.

Following these definitions are seven cases studies of sites that are critical to the overall

conclusions drawn from this investigation. Each case study is represented by excellent data sets
13

that present some important variations on the theme of acoustic wipe-out zones and sea floor

response to hydrocarbon venting/seepage.

Figure 6. Map of the Louisiana Texas continental slope, deep water leasing areas, and the locations of case history

sites.

1. Seismic Facies

a. Gently Divergent and Parallel Layered This reflection pattern is very common on the

continental slope and the seismic expression is a series of even-layered, parallel, or gently

divergent reflections. These reflection horizons tend to be rather continuous and show little

amplitude variations laterally. The uniformity of these reflections suggests deposition in uniform

layers over large areas. When data sets permit, these parallel or slightly divergent reflectors are

usually stacked and can be traced over distances of many kilometers. Units are measured in

meters to 10s of meters. This facies is common in areas of low relief.

b. Strongly Divergent This reflection pattern is common on the flanks of diapiric

structures where sediments have obviously been eroded from the regional topographic high and

deposited on the diapir flanks. Although the reflection horizons tend to be rather continuous,

they clearly diverge downslope over distances of a few kilometers and sometimes are

accompanied by distinct amplitude changes over short lateral and vertical intervals. These types

of reflection horizons tend to occur in stacked groups, each group having an upper truncation

surface. This seismic facies is usually found on the flanks of regional diapiric structures.
14

c. Onlapping Onlapping facies represent sedimentary units that fill a depression, such as

a small basin or trough, and reflectors of the fill truncate against the reflector that defines the

geometry of the depression or basin of deposition. Onlapping units are generally parallel and

relatively continuous reflector events. For the slope environment, these units can typically be the

products of turbid flows. Not all onlapping seismic facies are well-behaved parallel and

continuous reflection horizons. Chaotic reflection events representing debris flow and other

deposits created by mass wasting processes can also be onlapping.

d. Layered or Acoustically Turbid Drape The drape facies forms a uniform blanket of

parallel reflections or an acoustically turbid unit that conforms to an underlying topography.

There is little evidence of thinning over topographic highs except on the upper slope (<1000 m).

There also is very little evidence of onlapping in this type of seismic facies. The modern

condensed section or hemipelagic blanket that occurs at the surface is 3-7 m thick, covers most

topography on the modern slope, and constitutes a drape seismic facies. This deposit has been

the product of a relatively slow and carbonate-rich hemipelagic rain of suspended sediments

(pelagic clays and calcareous foraminifera tests primarily) since the retreat of fluvial systems

from the shelf edge following the last glacial maximum (~ 18 kyr BP). Thicker drape deposits

are common deeper in the sedimentary section and are frequently observed on high resolution

seismic records.

e. Sigmoid-Progradational This seismic facies has a sigmoid configuration in dip

sections and represents progadation, both on the shelf and slope. In a strike direction this pattern

can appear either roughly parallel or divergent. This type of seismic facies pattern is clearly

progradational when viewed in dip orientation. Downlap is common at the base of these

reflection events. Amplitudes may vary in the basal areas and sometimes these sigmoid

reflectors interface with chaotic units.

f. Oblique-Progradational The oblique-progradational seismic facies also denote

outbuilding, usually at or near the shelf edge. Both oblique-progradational and sigmoidprogradational seismic facies are common in shelf edge deltas that form sediment sources for

slope deposition during periods of lowered sea level. Downlap occurs at the base of these

reflection events and chaotic reflections of limited extent occur at the bases of obliqueprogradation reflection events.

g. Chaotic This seismic facies occurs on a variety of scales in high resolution seismic

facies. Wavy-discontiuous reflection events between boundaries defined by rather continuous

reflectors are typical of this facies. Reflection amplitudes also vary considerably within the unit

of discontinuous reflectors. This seismic facies generally defines units that have been deposited

by gravity-driven density flows and mass movement events. These units can display erosion at

their base, but evidence of erosion is not always present. They commonly have irregular upper

surfaces.

h. Contorted-Discordant Mounded remnants of bedding interfingered with contorted

units, wavy-discordant reflectors, and hummocky reflector events, particularly at the upper

surface of the entire unit, are typical of this seismic facies. Mass transport processes are

probably responsible for most units that fit this description. The mounded components with
15

internal bedding probably represent large blocks of sediment that have failed from oversteepened

slopes and did not completely degrade during the transport process.

i. Acoustically Turbid A lack of clear reflection horizons is common to the subsurface

of many features on the continental slope. This response is most commonly associated with

bubble-phase gas in the near surface sediments resulting in an acoustic wipe-out zone. However,

some areas of acoustic turbidity appear to be stacked and separated by well-defined reflectors.

Others contain discontinuous reflectors, some of which may be parallel and reflect bedded

sediments, while others are more chaotic with overlapping parabolic reflectors. Still, in other

cases, parallel and parabolic reflectors are mixed in a matrix of acoustically turbid seismic facies.
17

II. Case Studies

The body of this report consists of individual case studies of features or groups of

features that were selected for study based on their characteristics on high resolution acoustic

data, as well as accessibility of data to the project. As stated in the introductory parts of this

report, the focus of this research effort is to develop criteria for better assessment of geohazards,

particularly those associated with fluid and gas seepage/venting, and habitats for protected

chemosynthetic communities. Features characterized by acoustic wipe-out zones have presented

considerable problems regarding definitive interpretations. The task of interpreting highly

complex sea floor types, particularly those that incorporate acoustic wipe-out zones, is truly a

difficult one and is the reason no widely-used interpreatation schemes are available at present.

One of the problems has been the availability of actual ground tuth data to verify remotely

sensed approximations of seafloor characteristics. Echo sounder traces, seismic profiles, 3Dseismic attribute data, and swaths of side-scan sonar data are only proxies of real conditions at

the seafloor. For the first time, this study attempts to use both high resolution acoustic data

(seismic and side-scan sonar data) with exploraiton scale 3D-seismic data (primarily surface

amplitude and phase data) in conjunction with direct sea floor observation-sampling to assess sea

floor and shallow subsurface characteristics. The ultimate goal is not an improved description of

specific sea floor characteristics but to develop a better understanding of sea floor feature

evolution, the fundamental processes responsible for feature development, and an improved

method for evaluating these features from remotely sensed acoustic data.

The following section of this report presents selected case studies of a wide variety of

different sea floor types and features. After reviewing many data sets, these case studies were

selected from a group of 29 features (Table 1) on which industry had already acquired high

quality acoustic data sets and field verification data had been or could be collected as a part of

this project. All the sites are located on the upper continental slope in water depths less than

1000 m. This depth-restricted zone is necessary because of the depth limitation of manned

submersibles readily available to the Gulf of Mexico for scientific work.

A. Case Study: Large Complex Mound, Garden Bank Block 161

1. Introduction

In 1985 a high resolution geophysical survey was conducted in GB161 for the purpose of

evaluating the block for geohazards and a prospective drilling site (Figure 7). Most of this block

is occupied by a large, complex mound (location 27°49.83'N; 92°30.24'W) the base of which

extends to the south to neighboring GB205. The geomorphology of this mound is one of a large

positive feature comprised of numerous smaller mounds giving the mound surface a “lumpy”

appearance on seismic and bathymetric profiles. The crest of the mound reaches a water depth

of approximately 200 m while the mound base in GB205 is in approximately 380 m of water

(Figure 8).

In October-November of 1997 Fugro-McClelland Marine Geosciences, Inc. was

contracted to conduct a geotechnical site investigation. This investigation consisted of a primary

soil boring that was completed in two stages because of weather. The final penetration depth
18

was 128 m. The first portion of the boring penetrated to 16.5 m. Core sampling and piezocone

penetrometer testing was conducted in the first portion and core sampling was continued to the

final depth. Water depths at the soil boring locations were 297.5 m and 298.5 m respectively.

Extensive laboratory testing was conducted on the samples from these borings.

Figure 7. Map showing the location of Garden Banks Block 161.

Figure 8. Bathymetric map illustrating the large mound-like feature that nearly occupies the entire GB161 Lease

block.
19

The GB161 data set from this geotechnical investigation is interesting and unusual because it

affords a rare opportunity to probe the interior of a feature represented both on high resolution

and 3D-seismic records as an acoustic wipe-out zone. Generally, industry avoids features

represented as acoustic wipe-out zones, so little in the way of subsurface sampling has been

carried out on this class of features. Therefore, the GB161 mound was selected as a case study

for this report because it provided insight into the subsurface character of a mounded sea floor

area that is acoustically opaque in seismic data.

2. Geologic Setting

The study site is at the transition between the continental shelf and continental slope

south of the eastern chenier plain of western Louisiana. According to Berryhill et al. (1987),

GB161 is located in an area of late Wisconsinan deltaic deposition at the shelf edge and on the

upper continental slope. In the south additions of the East Cameron, Vermillion, and western

South Marsh Island lease block areas, the shelf is entrenched with complex channel networks

that point to or attach themselves to an irregular and elongate shelf edge depocenter. Zones of

diapiric uplift surround this shelf edge delta complex and appear to have responded to the rapid

sediment loading during and after the relatively short period of deposition. Deltaic deposits in

the shelf edge depocenter are well over 100 m thick on sparker lines used to map the shelf-slope

transition. Shelf edge deltaic deposition and diapirism (salt and shale diapirs) are

penecontemporaneous and interacting processes as demonstrated by the mapping of deformed

late Wisconsinan deposits by Berryhill et al. (1987). Their maps of the principal area of late

Wisconsinan deltaic deposition for the Garden Banks region shows a deltaic lobe extending into

the lease block to the north of GC161 and that GC161 fits into a region of diapiric uplift. When

one studies the rounded plan-view shape of this feature which has a diameter of over 5 km, the

presence of an acoustic wipe-out zone that extends deep into the subsurface, upturned

stratigraphy along the feature flanks, and a concentric structure on 3D-seismic surface amplitude

data these characteristics are compatible with the interpretation of perhaps a salt-cored diapir.

The sediments from the geotechnical boring, however, suggest another origin.

3. Available Data

In 1985 COMAP Geosurveys Inc. was commissioned by Pennzoil Exploration and

Production Company to conduct a standard geohazards survey of GB161. The geophysical

systems used to collect high resolution acoustic data from this block were (a) precision

hydrographic echosounder, Raytheon DSF-6000 dual frequency (24 kHz and 200 kHz) digital

survey fathometer, (b) a side-scan sonar and subbottom profiler, Edo Western integrated 4055

deep-tow system for simultaneous acquisition of high resolution surface and shallow subsurface

data, and (c) a multichannel Seismic System (EG&G model 402-7 nine-electrode sparker). The

combined 100 kHz side-scan and subbottom profiler system was found to produce the best

subbottom data at the elected frequency of 3.5 kHz. The subbottom profiler data as well as the

side-scan data were displayed on an EDO Model 706 mapping recorder. The sparker (8

kilojoules) data were recorded with a Texas Instruments DFS V recorder system. This system

recorded 24 channels of data in SEG B format. Survey positioning was accomplished using the

SYLEDIS BETA System, a short-to-medium range radio positioning system using frequencies in

the 420-450 MHz band.
20

Additionally, in 1997, Fugro-McClelland Marine Geosciences, Inc. was engaged to

conduct a seismic and geotechnical field investigation involving the acquisition of a boring (total

length 128 m) as well as in situ and laboratory testing. Field and laboratory tests, including

classification and strength tests, were performed on the sediment samples to evaluate pertinent

index and engineering properties of the sediments encountered. An advanced laboratory testing

program was conducted to evaluate sediment stress history and normalized strength behavior

using samples still in their shelby core tubes. Due to unusual sediment characteristics additional

chemical and geological testing was performed on selected samples. As part of the FugroMcClelland geotechnical evaluation of the site John Chance and Associates collected a coarse

grid (300 m line spacing) of additional high resolution multichannel seismic data which also

confirmed the lack of organized reflectors from the mound interior. Finally, 15 in3

 water gun

profiles were digitally acquired as part of this study to further investigate the seismic

characteristics of the GB161 mound.

4. Feature Characteristics

Figure 8 illustrates that the study area is characterized by a local topography feature with

considerable positive relief above the surrounding sea floor. As was clearly demonstrated in the

original 1985 COMAP Geosurveys sparker data, the mound-like feature has an acoustically

opaque interior. The 3.5 kHz subbottom profiler data displayed little penetration and the sidescan sonar images of the mound surface lacked sufficient crispness and feature difinition to add

to a reasonable interpretation of the mound and its possible origin. Figure 9 illustrates the most

recent attempt to image this feature with a high resolution acoustic source. Note the sharp

truncation of bedded sedimentary units against the interior of this feature, its acoustically opaque

interior, and the multimound character of its surface and near-surface morphology.

Figure 9. An E-W profile of the GB161 mound digitally acquired as part of this investigation using a Seismic

Systems S-15 water gun as an acoustic source.
21

Analysis of 3D-seismic surface amplitude data over this feature and at depth (Figure 10)

illustrates that sea floor amplitudes are in the same range as the surrounding “background” areas.

Furthermore, amplitude slices through the mound into the subsurface indicate that the circular

shape is maintained and that minor reflectors above background within this feature describe a

concentric pattern. However, there is little amplitude signature within this feature even at

considerable subsurface depths.

Figure 10. (a) A map of 3D-seismic surface amplitude data from the GB161 mound area. Note the lack of

amplitude expression over the mound. Slightly higher amplitudes on the mound flanks are not

considered real since these zones do not translate into the subsurface. (b) An amplitude slice made

through the subsurface extension of the feature at the 1 sec (TWTT) horizon.

Observation and sampling (short cores) transects made across the GC161 mound in 1998

using a manned submersible (Johnson Sea-Link, JSL-1, Dive 4064) confirmed the rather smooth

and nondescript nature of the sea floor. During this dive no rock outcrops or white bacterial mats

(Beggiatoa sp.) were found during the dive which lasted nearly 3 hours. The surface sediments,

upon later analysis, turned out to be largely of hemipelagic origin. However, mixed microfossil

assembledges were encountered suggesting mixing with older sediment.

The boring taken at the apex of the mound provided critical data for a confident

interpretation of the origin and formational conditions associated with this feature (Roberts et al.,

1999). The results of the geotechnical testing program revealed unusual soil characteristics at

the site. With exception of approximately 5 m of soft clay at the top of the boring the remainder

of the sedimentary section (to a depth of 128 m) provided contradictory results. Undisturbed

shear strength values indicated that the sediment was overconsolidated while effective vertical

stress measurements suggested that the sedimentary sequence is underconsolidated below a

subsurface depth of about 23 m (Roberts et al., 1999). The reason for these differences was that

test results were affected by the inclusion of shale clasts and chemical cementation of the clay

matrix (Figure 11). Cementation was the result of chemical precipitation in the presence of the

high concentrations of salt and other chemical compounds present within the sediments (Roberts

et al., 1999).
22

Figure 11. X-ray radiographs of two cores from Boring 1 (exposure 5888 from 16.7 m and exposure 5890 from 66.9

m) illustrate (a) a massive nonstratified structure and (b) suites of unusual coarse-grained components in

a fine-grained matrix. The coarse components include shale clasts, calcite rhombs, and odd-shaped

nodular diagenetic masses (modified from Roberts et al., 1999).

Measurements of the pore fluids extracted from selected sediment samples, revealed a

salt content level at least eight times greater than normally found in marine soils. The pore fluid

salt concentration level for the Boring 1 profile is shown in Table 2. High pore fluid salt

concentration levels influence basic soil index properties like water content and unit weight

measurements. Measured water content values for Boring 1 appear up to 70% lower and

effective unit weight values as much as 20% greater than corrected values based on corrections

suggested by Norrany (1984). These high salt concentration levels approach or may have

exceeded a supersaturated level.

Table 2. Pore fluid salt concentration for Boring 1, Garden Banks Block 161.

________________________________________________________________________________________________________________

Sample No. Depth (m) Mg (ppm) Ca (ppm) Na (ppm)

 S-37 16.6 1,138 1,867 90,052

 S-65 45.6 869 1,991 110,798

 S-84 66.9 887 1,830 110,350

 S-111 95.5 1,149 2,760 116,476

 S-127 115.8 1,033 1,714 108,664

Avg. Sea Water 1,290 411 10,760

________________________________________________________________________________________________________________

The pore water analysis clearly indicates that sodium and calcium levels in the porewater

of these samples are considerably elevated above those levels expected in normal seawater.

Magnesium levels, in contrast, appear to be near normal seawater levels or below. The calcium

concentrations are 4 to 5 times higher than one would expect if the pores of these sediments were

filled with deep marine water. Sodium level are even higher. They range from 9 to 11 times
23

higher than values for normal seawater, suggesting perhaps that interstitial fluids may have

derived from dissolution of salt.

Analysis of selected samples from Boring 1 by X-ray radiography revealed an unusual

suite of sedimentary particles comprising the sediment column represented by this boring.

Figure 11 illustrates two X-ray radiographs from Boring 1 that display the salient characteristics

of sediments analyzed from eleven samples representing the stratigraphic interval 7.5 - 125.0 m

(Roberts et al., 1998). All samples analyzed by this technique were characterized by a lack of

stratification and an abundance of shale clasts as well as diagenetic inclusions in the form of

nodular masses, replaced burrows, altered shell material, and cement-filled microfossil tests. On

a freshly cut core, to the unaided eye, very few of these inclusions are visible. However, using

the X-ray radiography technique enhances density contrasts between inclusions and the matrix

which makes the two distinguishable, one from the other. For example, the sample acquired at a

subsurface depth of 57.2 m contains an abundance of diagenetic material from gravel-to-silt size.

These inclusions, and others like them in the remaining core samples, were found to be

composed of mostly Mg-calcite and pyrite as determined from both X-ray diffraction and

microscopic examination. Close inspection of all X-ray radiographs indicates the presence of

rhombic grains in the coarse sand to fine gravel size ranges. Analysis of the rhombs indicates

that they are composed of calcite.

Inclusions that produce more subtle images on X-ray radiographs are scattered

throughout every sample analyzed. These particles are rounded-to-subrounded, occur in the

course sand-to-gravel size range, tend to be composed of fine-grained sediment, and have a

density that is slightly higher than the surrounding clay-rich matrix. Close inspection of these

inclusion types under the binocular microscope indicates that they are “shale clasts” that appears

to have been transported from deeper subsurface horizons and partially rounded before

deposition. Analysis of the shale clasts for microfossils yielded fauna from late Pleistocene to

Eocene, indicating that many of the shale clasts originated deep within the sedimentary basin

beneath the GC161 mound.

5. Feature Synopsis and Interpretation

The GB161 mound has the same apparent attributes as many mounds of the upper

continental slope which are built of low strength gas-charged sediments derived from

considerable depths in the subsurface. A cone-shaped exterior and an interior characterized by

an acoustic wipe-out zone supports this interpretation. However, when all data sets are

collectively analyzed, including those generated from a 128 m boring into the center of the

mound, it is clear that this feature has unusual characteristics. The 3D-seismic surface amplitude

map (Figure 10) illustrates an interesting and unexpected characteristic of this feature, it has no

surface amplitude expression that deviates from background. Considering the multiple vent-like

topography of the mound top and acoustically opaque interior, one would expect that venting gas

and perhaps fluid mud would be present at the surface and perhaps chemosynthetic communities

and authigenic carbonate outcrops. None of these sea floor conditions and features were

observed during site verification data collection using a manned submersible. Direct

observations made during the field verification phase were further supported by the lack of hard

targets on side-scan data and by the persistent reflection strength of high resolution seismic
24

surface reflectors. However, profiles and amplitude slices through this feature using 3D-seismic

data indicate a rather chaotic pathway from deep within the subsurface to the mounded sea floor

feature. Close inspection of the amplitude slices indicates that there are slight variations in

amplitudes that describe a concentric pattern within the general subsurface extension of the

surface mound which is an acoustic wipe-out zone in profile view (Figure 12).

Figure 12. This 3D-seismic profile of the GB161 mound illustrates the complex subsurface configuration, the

chaotic and acoustically turbid zone beneath the mound, and the position of Boring 1.

Data acquired from analysis of samples from Boring 1 help finalize an interpretation of

this feature. Geotechnical data were confusing and appeared rather contradictory. That is, shear

strength measurements suggested the sediments were overconsolidated while sensitivity test

results indicated sensitivity lower than most marine clays in the Gulf of Mexico. A

sedimentological evaluation of selected cores from Boring 1 helped explain these and other

unusual properties of sediments beneath the GB161 mound. Numerous diagenetic products (e.g.

pyrite, halite, calcite, gypsum) were found in sediments throughout Boring 1 (Figure 11). In

addition, pore waters throughout the boring were 10 X normal marine salinity. In association

with the diagenetic inclusions were well-defined, semi-lithified shale clasts that displayed

rounding, presumably from vertical transport. Careful analysis of these shale clasts indicated

that they contained microfossils ranging in age as far back as Eocene. Therefore, when taken as

a whole, the data assembled on the GB161 mound suggest that this feature was constructed by

fluidized sediment expulsion originating from deep within the subsurface. The fact that (a) the

present sea floor feature has no surface amplitude expression that is different from the regional

background, (b) transported and displaced shale clasts as well as a diverse group of diagenetic

products are part of the sediment suite, and (c) that hemipelagic sediments cover the mound

surface suggests an extinct sediment extrusion feature and supports the interpretation of a

dormant expulsion feature. No chemosynthetic communities were found in association with this

feature even though it is a classic mound-like buildup with an acoustic wipe-out zone beneath it,

like “Bush Hill” (GC185). Acoustic characteristics of the GB161 mound are tabulated in Tables

3 to 5.
25

Table 3.Side-scan sonarimage characteristicsforGB161 case study.

 Feature Feature Field Flanking Areas

Acoustic Backscatter

 Strong

 Moderate

 Weak X X

 Mixed

Target Shapes

 Bumps

 Cones

 Mounds

 Pinnacles

 Depressions

 Irregular

Target Surface

 Smooth X X

 Irregular

 Variable X

Feature Occurrence

 Isolated

 Asymmetric Groups

 Linear

 Circular

 Elliptical

 Polygonal

Table 4.Seismic faciesforGB161 case study.

Seism ic Facies Feature Feature Field Flanking Units

Gently Divergent-Parallel X

Strongly Divergent

Onlapping

Downlapping

Drape X

a. Layered

b. Acoustically Turbid

Sigm oid Progradational

Chaotic

Contorted-Discordant

Acoustically Turbid X

a. Com plete X

b. Stacked Zones

c. Chaotic Discontinuous Reflectors

(1) Parallel

(2) Parabolic

(3) M ixed

Table 5.Surface reflection characteristicsforGB161 case study.F = feature;FF = feature field;FA = feature adjacentfield.

3.5 kH Pinger Interm ediate Source 3D Seism ic

F FF FA F FF FA F FF FA

Surface Reflection Strength 

Strong

M oderate X X XX

W eak X X

Variable

W idth ofSurface Reflectors (M S)

0-2 XX

2-5 X X

>5 X

Variable X

Reflector Characteristics

Sim ple Doublet X X

Stratified X X

Prolonged X X X

Chaotic

W indowed

BrightSpots

Surface Reflection Geom etries

Planar X X X

W avy

M ounded X X X

Scarped

Isolated Pinnacle/s

Isolated Depression/s

Parabolics

Isolated

M ultiple

Overlapping

Phase Inversion
26

B. Case Study: Mound, Garden Banks Block 189

1. Introduction

While conducting a high resolution geophysical survey of Garden Banks Block 189 in

preparation for construction of an oil and gas production platform for Texaco USA, a large

mound of unknown origin became important because of its proximity to the proposed platform

site. The location of this mound (27°46.48’N; 93°17.74’W) is approximately 225 km south of

the town of Cameron, along Louisiana’s western coast (Figure 13). The lease block containing

this feature (GB189) spans water depths ranging from 165 m to 256 m and is located just below

the shelf edge on the upper continental slope. This part of the continental slope displays

complex bathymetry resulting from a long geologic history of sedimentary loading and salt

tectonics.

Figure 13. Location map for GB189.

2. Geologic Setting

It is now generally known and accepted by geoscientists that the delivery of sediments to

present deep water areas of the Louisiana-Texas continental slope has been closely tied to

frequent sea level changes, particularly during the Plio-Pleistocene time interval (Poag 1973). It

is also accepted that sea level fell to approximately 120 m during the last glacial maximum,

approximately 18 kyr BP (Aharon and Chappell 1986). Since the top of the GB189 mound is

currently at 182 m water depth, this feature would have been in slightly over 60 m of water at the

latest maximum lowstand of sea level.

The upper slope region near GB189 is characterized by the occurrence of salt masses in

the shallow subsurface. Berryhill et al. (1986), through the mapping of shelf-edge and upper
27

slope depositional sequences as revealed on high resolution seismic profiles, discovered that

diapiric structures just south of GB189 were actively moving during the late Pleistocene and

have remained active into the Holocene. As Berryhill et al. (1986), Sydow and Roberts (1994),

Roberts (1995), and others have stated in slope-related research, the movement of salt on short

geologic time scales is probably related to the rapid introduction of large volumes of sediment

during periods of falling-to-low sea level. At these times, fluvial systems entrench themselves as

they migrate across the shelf. They build depocenters at the edge of the shelf where sediments

are introduced directly to the slope or are transported downslope as the product of mass wasting

processes.

Shelf edge deltas that formed during the Pleistocene have been mapped by Berryhill et al.

(1986), Sydow and Roberts (1994), Anderson et al. (1996) and Winker (1996). According to the

work of Berryhill et al (1986), the nearest area of major deltaic deposition during the late

Pleistocene was about 19 km upslope and to the northwest of GB189, which was probably in the

downslope sediment pathway of this depocenter. Piston cores from GB189 analyzed by the

author revealed sands around the base of the mound as well as on the mound flanks. Since the

sand is currently exposed at the modern sea floor, it is reasonable to assume that its input to the

slope occurred during the latest Pleistocene sea level lowstand period.

3. Available Data

Original geohazards data sets were acquired by John Chance and Associates, Inc. of

Lafayette, Louisiana. A narrow beam EDO Western 24 kHz bathymetric profiling system was

used to establish water depth charges and sea floor topography within GC189. Shallow subbottom data were obtained with an ORE 3.5 kHz pinger profiler. This acoustic source acquired

usable information to subsurface depths of approximately 75 ms (~50-60 m). Intermediate

penetration was accomplished with a high resolution sparker system. The sparker system was

used to fill the data gap between sub-bottom profilers, like the 3.5 kHz O.R.E profiler used in

this survey, and low frequency exploration-scale seismic sources. Subbottom penetration of the

sparker used on the GB189 survey was over 1000 m with a bed resolution of about 2 m. The

frequency range of this sparker was 25-2000 Hz with an adjustable energy range from 100-

24,000 joules. Sparker data were collected in single channel as well as multi-channel modes.

All geophysical survey data collected by John Chance and Associates, Inc. were spatially

controlled by STARFIX navigation system and collected in a surface-tow configuration.

Additional high resolution data were collected as part of this project. Seismic profiles across the

mound were acquired using a Seismic Systems (model S-15)15 in3

 water gun supported by 50

cfm Hamworthy seismic compressor. This acoustic source produces useful data to a subsurface

depth of over 200 ms (~ 180 m) with a bed resolution of less than 2 m. An EG&G Model 260

digital side-scan system was used to study surficial features. These acoustic instruments were

used in a surface-tow configuration with GPS-based navigation. TEXACO made available 3Dseismic surface amplitude data to the project for comparison with other data sets. Research

submersible Johnson Sea-Link, JSL-1, Dive 3306, provided ground truth observations for

calibration of acoustic data to sea floor conditions.
28

4. Feature Characteristics

Bathymetric data (Figure 14) show that the GB189 mound is circular in plan-view, has a

diameter of ~ 760 m and maximum relief above the surrounding sea floor of approximately 60

m. On low power pinger data the mound appears acoustically opaque (Figure 15).

Figure 14. Bathymetric map of the GB189 area showing the symmetrical mound in the south-central part of the

block and the graben-like trough that trends to the south away from the mound.

However, both water gun (Figure 16) and sparker data (Figure 17) show that the mound

interior is characterized by a single prominent internal reflection horizon in addition to being

otherwise acoustically opaque. The opaque internal nature of the mound was originally

interpreted to be the product of bubble-phase gas in the sediment (Fugro-McCelland, Marine

Geoscience Inc, personal communication). Whelan (1977) demonstrated that even very low gas

concentrations could attenuate seismic energy resulting in an acoustically turbid signature or

acoustic wipe-out zones.
29

Figure 15. A 3.5 kHz profile oriented north-south across the feature. Note the distinct mound-like profile, the

highly reflective top surface, and acoustically opaque interior.

Figure 16. This north-south oriented 15 in3 water gun profile reveals important characteristics about the GB189.

Note elevated rims of the feature, the extremely reflective upper surface, and the distinct reflector below

an acoustically opaque upper interior of the feature. Also note the domes adjacent beds, onlap on the

youngest sedimentary units and the patchy acoustic wipe-out zones in the shallow subsurface.

The mound is situated at the northern end of a north-south trending graben defined by

bathymetry (Figure 14) and bound by well-developed faults. Side-scan sonar records indicate

that the flanks of the mound are relatively smooth, but clusters of low-relief hard targets

surround a central depression on the mount top. This depression is clearly visible on the water
30

gun profile (Figure 16). Sediments of the top also exhibit a strong backscattering of acoustic

energy, suggesting that they have different characteristics from those of the flanks.

Figure 17. This processed multichannel sparker profile oriented north-south across the GB189 mound clearly

illustrates the high amplitude internal reflection event as well as the very reflective upper mound surface.

This profile also shows that bedded units terminate against the acoustically opaque center of the larger

feature.

Submarine observations indicate that the flanks of the mound are covered with rather

featureless hemipelagic sediments while sediments of the top are coarser and have a substantial

content of broken shell debris. A rim of scattered, low relief buildups of primarily coralline alga

is present (Figure 18). These buildups are scattered over the top of the mound and on the upper

flanks of the mound and produce the hard targets on side-scan as well as create a very reflective

surface on seismic profiles.

Figure 18. A bottom photograph from JSL-1, Dive 3306, which shows the small coralline algal buildups that occur

on the upper mound. These buildups form a rim that is clearly visible in the water gun profile.
31

5. Feature Synopsis and Interpretation

Initital interpretations of this feature suggested that it originated as a “gas heave

structure” that mounded the sea floor (J. Hooper, Fugro-McClelland Marine Geosciences, Inc.,

personal communication). However, new data sets and further investigations indicate that it

currently is not gas-charged and that its present upper surface is mantled with small coralline

algal buildups and coarse bioclastic sediments. The biogenic carbonates that occur on the mound

top are probably the result of colonization of this positive relief feature during the sea level

lowstand following the latest Pleistocene glacial maximum. On seismic records, this surface is

an efficient reflector of acoustic energy leading to a high amplitude sea floor reflection event.

Surface amplitude maps generated from 3D-seismic data over GC189 indicate an amplitude

associated with the mound that is similar to but slightly higher than the response of the

surrounding sea floor (Figure 19). Both 3D-sesimic profiles and digital high resolution seismic

data acquired as part of this project demonstrate a mound-top phase consistent with that of the

surrounding sea floor. The fact that no phase reversal is associated with the surface return from

the mound indicates that the mound surface is relatively hard and that surficial sediments are not

gas-charged.

Figure 19. This 3D-seismic amplitude map indicates that this feature has an acoustic impedance similar to the

surrounding sea floor with no evidence of a phase inversion over the feature which is generally

associated with gas-charged sediments.

Independent of the ultimate origin of this feature, it can now be stated that it has

stabilized and at least since the last glacial maximum, approximately 20 kyr BP, this feature has

been dormant and has not been a source of gas, fluid, or sediment expulsion. Surficial

carbonates are biogenic and do not have light carbon isotope values (unpublished data, H.
32

Roberts). Shale clasts in cores from the dome flanks (unpublished Fugro-McClelland report)

suggest possible expulsion of fluid mud before the final phase of biogenic carbonate mantling of

the feature. Recent appraisal of 3D-seismic data suggest the feature may be salt-cored and that

the distinct internal reflector represents an acoustic impedance boundary between overlying

sediment and underlying salt. The lack of 13C-depleted authigenic carbonates and remains of

chemosynthetic organisms, plus the absence of surrounding stratigraphy generally related to

mud-extrusion during the mound-building process supports a diapiric salt origin for this feature.

The acoustic characteristics of this feature as revealed in geophysical records are tabulated in

Tables 6 to 8.
33

Table 6. Side-scan sonarim age characteristics forGB189 case study.

Feature Feature Field Flanking Areas

Acoustic Backscatter

Strong X

M oderate X

W eak

M ixed X

TargetShapes

Bum ps X

Cones

M ounds

Pinnacles

Depressions

Irregular X

TargetSurface

Sm ooth X

Irregular X

Variable

Feature O ccurrence

Isolated X

Asym m etric G roups

Linear

Circular X

Elliptical

Polygonal

Table 7. Seism ic facies forGB189 case study.

Seism ic Facies Feature Feature Field Flanking Units

G ently Divergent-Parallel

Strongly Divergent x

O nlapping

Dow nlapping

Drape

a. Layered

b. Acoustically Turbid X

Sigm oid Progradational

O blique Progradational X

Chaotic

Contorted-Discordant

Acoustically Turbid

a. Com plete

b. Stacked Zones

c. Chaotic Discontinuous Reflectors X

(1) Parallel

(2) Parabolic X

(3) M ixed

Table 8. Surface reflection characteristics forGB189 case study. F = feature;FF = feature field;FA = feature adjacentarea

3.5 kH Pinger Interm ediate Source 3D Seism ic

F FF FA F FF FA F FF FA

Surface Reflection Strength 

Strong X X X

M oderate X X

W eak X

Variable

W idth ofSurface Reflectors (M S)

0-2

2-5 XX X

>5 X XX

Variable

Reflector Characteristics

Sim ple Doublet X X X X

Stratified

Prolonged X X

Chaotic

W indow ed

BrightSpots

Surface Reflection G eom etries

Planar X XX

W avy

M ounded X X X

Scarped

Isolated Pinnacle/s

Isolated Depression/s

Parabolics

Isolated X

M ultiple

O verlapping

Phase Inversion
34

C. Case Study: Expulsion Centers, Garden Banks Blocks 338 and 382

1. Introduction

Upslope of the prominent diapiric mound in GB427 are two areas in GB338 and GB382

(Figure 20 ) from which downslope transport of large volumes of sediment has occurred (Figure

21).

Figure 20. Location map for Garden Banks Blocks 338 and 382 (GB338 and GB382).

Figure 21. A bottom features map of the GB338-427 area illustrating the expulsion centers in GB338 and GB382,

gravity-driven deposits that have been transported downslope, and the large diapiric mound in GB427.
35

These features were selected for investigation because of the obvious slope failures,

fluidized sediment flows, and current near-dormant state of the expulsion centers. It is clear that

in the recent past that both expulsion centers were actively extruding large volumes of fluidized

sediment onto the sea floor where this extruded sediment traveled toward the a large mound in

GB427. Some flows traveled a distance of over 15 km downslope.

The expulsion centers in GB338 (27° 37.80'N; 92° 28.12'W) and GB382 (27° 36.49'N;

92° 29.05'W) clearly are visible on regional bathymetry as mound-like, rounded areas on top of a

regional and elongated bathymetric high oriented in a NE-SW direction (Figure 21). Both

expulsion centers are in water depths of approximately 500 m. These areas are on the fringes of

an excellent deep-tow high resolution geohazards survey conducted in support of Shell’s Auger

Prospect development. The mudflow deposits originating on the ridge upslope of the proposed

drilling site generate concern because flow patterns on the sea floor appear relatively fresh on

side-scan sonar data. It is apparent from analysis of the subsurface that large blocks of sediment,

obviously derived from slope failures, have been transported into GB426 and GB427 not far

from the proposed platform site. Analysis of all data sets from the site survey show that the time

frame for the slope failures and expulsion events provide an adequate margin of safety for the

project and that additional flows are not to be expected over the production life of the Auger

Field. However, it is clear from the surficial geology that the expulsion centers have been very

active in the past and now have essentially shut down. This demonstrated history of change in

activity level provides an important opportunity to investigate the remotely sensed character of

these sites and the actual sea floor conditions that exist there today.

2. Geology Setting

The ridge on which the expulsion centers are located (Figure 22) is held up by regionally

distributed shallow salt. The average off-ridge slopes in the area surrounding GB427, where the

diapiric mound is located, are around 2°.

Figure 22. Bathymetric map for the GB338-382 area illustrating the NE-SW ridge-like topography which is a

reflection of underlying shallow salt.
36

However, local slopes associated with the ridge where the expulsion centers are located reach as

much as 12°. Berryhill et al. (1987) indicate that the northern part of this salt-cored ridge was

completely surrounded by thick late Wisconsinan deltaic depocenters that must have provided

abundant sediments to the downslope sedimentary basins both north and south of the ridge.

Stratigraphic relationships in high resolution seismic profiles used to construct a synthesis of

shelf edge deposition during the late Pleistocene (Berryhill et al., 1987) indicate late falling stage

and early rising stage deposition up slope of the GB338 and GB382 areas. Deposition of shelf

edge deltas and sediment loading of the slope adjacent to the shelf edge depocenters has been

shown to take place in the time frame ~ 23,000 - 16000 yr BP around the latest glacial maximum

which occurred 20,000-18,000 yrs BP (Sydow and Roberts, 1994). It is reasonable to assume

that his intense sedimentary loading of the basins to the N and S of the salt ridge on which the

GB338 and 382 expulsion centers are located triggered salt movement and activated faults that

breach the deep subsurface overpressure zone. These events, in turn, would have initiated uplift

of parts of the salt ridge, slope failures, expulsion of fluids (including fluidized sediment) and

gases at the modern sea floor, and perhaps destabilization of gas hydrates. Sediment samples

acquired by manned submersible at the GB382 expulsion center indicate that sediments were

brought to the surface from great depth. These sediments were rich in early-middle Miocene

microfossils which represent an age of at least 14.6 Ma (Kohl and Roberts, 1994).

The sediment flows that originated from the GB338 and GB382 expulsion centers are

spectacularly displayed on the sea floor as viewed on high quality deep-tow side-scan sonar

(Figure 23).

Figure 23. Side-scan sonar mosaic of flows originating from the GB338 expulsion center. Note both the flow as

well as irregularities within flows indicating large blocks of sediment associated with debris flows.
37

Flow patterns are easily distinguished from surrounding sediments which suggests a relatively

young flow that has not been covered by thick hemipelagic drape deposits typical of some parts

of the slope. Most flows do, however, have 1-3 m of hemipelagic drape deposits over them

which suggests they were deposited several thousand years ago (hemipelagic depositional rate ~

1 m/3000 yrs). Close inspection of these flows with both side-scan sonar and subbottom profiles

indicates that some are debris flows containing large blocks of sediment while others appear to

be highly fluidized. The interpretation of geologic events leading to the creation of these flows

and then their abandonment is clearly related to sea level change and upper slope sediment

loading by shelf edge deltaic depocenters. As sea level fell prior to the latest Pleistocene glacial

maximum, a large shelf edge delta complex developed upslope of the area of interest and loaded

the underlying salt causing adjustments. At the same time a decrease in hydrostatic loading

forced by falling sea level could have destabilized gas hydrate deposits and perhaps helped

initiated slope failures. Sediments continued to be applied till the early stage of sea level rise.

At the same time, salt was adjusting to this load by deforming. These events activated existing

faults and probably created new ones. Vertical salt movement combined with expulsion of fluids

and gases at the sea floor initiated slope failures, fluidized sediment, and downslope transport.

Salt adjustment to late Pleistocene sediment loading appears to have waned significantly in the

last few thousand years, effectively terminating the mudflow/debris flow episode.

3. Available Data

Original geohazards data for the Auger Prospect area were acquired by FugroMcClelland Marine Geosciences, Inc. with a deep-tow high resolution acoustic system for

providing details of both the sea floor and shallow subsurface. This system incorporates a 3.5

kHz subbottom profiler with a 100 kHz side-scan sonar. A narrow beam precision depth

recorder was used for bathymetry. A high resolution multichannel seismic system was used for

moderately deep subsurface penetration. The Tri-Cluster seismic source was used with a 1⁄4

millisecond sampling rate. The data were initially analyzed by Fugro-McClelland personnel and

a site evaluation report submitted to Shell Offshore. This report contained side-scan sonar

mosaics of the mudflows from expulsion centers in both GB338 and 382 and the diapiric mound

in GB427. In addition, the subsurface characteristics of the flows and other important shallow

stratigraphic units were summarized. As part of this MMS-CMI project, additional high

resolution seismic profiles were acquired over the expulsion centers using both a Seismic

Systems S-15 water gun and a 50 in3

 air gun (GI gun). Both of these sources were supported by

a 50 cfm Hamworthy seismic air compressor. Data were acquired in both digital and analog

formats using the Delph II seismic data acquisition and processing software. Digital data were

archived on Exobyte tape and analog records were printed on an EPC 9800 graphic recorder.

Ground truth observations were made and bottom samples taken using the Johnson Sea-Link

manned submersible. Two dives have been made on the mound in GB382 (JSL-1, Dive 3307;

JSL-2, Dive 2898) while five dives have been made on the mound in GB338 (JSL-1, Dives 3308,

3565, 3566; JSL-2, Dives 2897 and 2905). The dive made by the author or September 7, 1992

was the dive that discovered the first large and natural barite deposits observed on the northern

Gulf of Mexico continental slope. These deposits constitute an interesting part of the expulsion

history of this site.
38

4. Feature Characteristics

As shown in the bottom features map of Figure 21 and the side-scan sonar mosaic of

Figure 23 the two expulsion centers in GB338 and 382 have played an important role in

modifying sea floor geology at these sites and downslope where debris flows and more fluidized

sediment flows impact the character of the bottom. High resolution seismic profiles (3.5 kHz)

acquired across these expulsion features, Figures 24 and 25, indicate that they are acoustically

opaque features with a thin layer of rather acoustically transparent material (hemipelagic drape

sediments) at the surface.

Figure 24. A 3.5 kHz profile across the expulsion center (mound) in GB338.

Figure 25. A 3.5 kHz profile across the expulsion center (mound) in GB382.

This surficial sedimentary unit is much more apparent over the GB382 expulsion center as

compared to its counterpart in GB338. As is clear from the 3.5 kHz profiles, the GB382 feature

is a symmetrical mound whereas the feature in GB338 is relatively flat with evidence of smallscale (<2 m) bottom roughness across the relatively flat area of the mound. Where bedding on

the flanks of these two features can be imaged, the sedimentary section is highly faulted. In the

case of the GB382 mound (Figure 26), flanking stratigraphy is represented by cyclic units that

are probably composed of extruded sediments that were deposited during the active expulsion
39

phase of this feature. They thicken away from the mound and merge with debris flow and

fluidized mudflow deposits downslope. A water gun profile across the GB338 feature suggests

that bedded sediments on the mound flank appear to be upturned and truncated (Figure 26). The

surface of this mound is highly reflective and its interior is an acoustic wipe-out zone even when

imaged with more penetration provided by the more powerful water gun.

Figure 26. A 15 in3 water gun profile across the expulsion center in GB338.

Side-scan sonar data from the two mounds appears quite different. The smaller of the

two expulsion centers (GB382) displays a rather smooth, nonreflective surface with only small

relief features (Figure 27). These side-scan sonar targets do not appear as features with highly

reflective surfaces at the 200 m swath scale. Instead, they appear as rather soft targets, mounds

of various sizes, that are possibly draped with hemipelagic sediments.

Figure 27. An E-W oriented side-scan sonar swath across the crestal area of the GB382 expulsion center mound.

The 3.5 kHz profile of Figure 25 suggests a thin (1-2 thick) and discontinuous hemipelagic drape

across the expulsion center mound. The surface of the GB338 mound has quite a different

signature on side-scan sonar records. This mound exhibits numerous highly reflective targets,

considerable reflective surface texture changes over short distances, and relief features that cast
40

significant shadows across the side-scan records (Figure 28). Many of the targets are lineated or

they are organized into linear patterns suggesting fault control. Other side-scan sonar records

indicate flow patterns that originate in the acoustically opaque area of the mound crest and are

gravitationally oriented downslope where a blocky bottom topography (debris flow surface) is

encountered.

Figure 28. A NW-SE oriented side-scan sonar swath across the crestal area of the GB338 expulsion center mound.

5. Feature Synopsis and Interpretation

High resolution seismic profiles clearly distinguish the two mound-like features in

GB383 and GB382 from surrounding sediments by their reflective surfaces, mounded shapes,

and acoustically opaque interiors. Analysis of accompanying side-scan sonar data indicate that

each of these mounds has functioned as a focal point for the expulsion of fluidized sediment that

has been transported downslope many kilometers from the source. In addition, this expulsion

process coupled with possible uplift of the subsurface salt ridge on which the expulsion centers

are located plus possible destabilization at gas hydrates caused slope failures leading to fluidized

debris flows.

When this general area of the slope is analyzed using 3D-seismic data, a very interesting

picture evolves. The expulsion centers as well as the fluidized flow deposits have a much higher

amplitude than the surrounding background (Figure 29). The expulsion centers themselves are

exceptionally high with the GB338 mound being of a higher amplitude than its smaller GB382

counterpart. An interesting aspect of these amplitudes is that they reflect a hard bottom except

for a small central area of the GB382 mound and a slightly larger area central to the GB338

mound. Seismic profiles across these features emphasize strong surface reflectivity suggesting a

hard bottom and a central zone characterized by a positive to negative polarity shift suggesting

the presence of sedimentary gas (Figure 30).

The two manned submersible dives made on the GB382 mound and five dives made on

the large GB338 mound were extremely revealing with regard to real sea floor conditions being

reflected in the remotely sensed acoustic data. Dives to the smaller of the two expulsion features

(GB382) revealed a highly bioturbated sea floor covered with sediments rich in calcareous

microfossil tests (hemipelagic sediments). This character of much of the mound surface

correlated well to both the high resolution seismic data (3.5 kHz), both of which show a thin

acoustically transparent layer across most of the mound. The general lack of hard reflectors on

side-scan sonar data was also consistent with much of the mound surface as viewed directly.
41

However, not all of the mound surface fits this description. Near the mound’s apex bacterial

mats became common bottom features and isolated tube worm colonies with associated

authigenic carbonate crusts and larger rocks were present. Although gas was never observed to

be escaping from the bottom when the bottom was disturbed in the bacterial mat areas, gas

bubbles were vented into the water column. These gas-prone sites occurred in the vicinity of the

area circled shown on the side-scan sonar record of Figure 27.

Figure 29. A 3D-seismic surface amplitude map of the GB338 and 382 area illustrating the amplitude deviations

from background.

Figure 30. A 3D-seismic profile through the GB382 expulsion center illustrating the acoustically opaque interior of

this feature which extends deep within the subsurface, the strong surface reflector defining the mound,

and the positive to negative phase reversal over the mound crest.
42

Surface data collection from the GB338 mound provided a much different picture of

processes and response features associated with these two expulsion centers. On the first dive to

GB338 it became obvious that this mound-like feature was still a marginally active area

regarding the flux of fluids and gases to the sea floor. The side-scan sonar data (Figure 28)

suggested that there were a number of distinct small-scale buildups, some with relief of up to 3

m. These features turned out to be small mud-rich mounds which had several spectacular

volcano-shaped cones near their crests (Figure 31).

Figure 31. Barite cones found near the apex of the GB338 mound.

Upon laboratory analysis (Roberts and Aharon, 1994) these multicolored and unusual cones were

composed of barite. Cones are not the only barite features on the mound top. A large area of the

mound surface is occupied by small vents and chimneys resulting from the slow vertical flux of

barium-rich fluids (Figure 32).

Figure 32. Small barite chimneys common to a large area of the GB338 mound surface.

The largest features of importance are mud volcanoes which are extruding fluidized sediment

rich in barium. Barite crusts form on the sides of these features which produces a semi-lithified

internal structure. Gas freely escapes from the mud volcanoes and occasionally from the

chimneys and cones. Chemosynthetic mussels (Bathymodiolus sp.) and bacterial mats are

common to some parts of the mound, but tube worms are scarce. As Figure 33 illustrates,

benthic communities are cemented with barite as barium-rich fluids seep to the seafloor over a

large area of the GB338 mound surface.
43

Figure 33. Chemosynthetic mussels (Bathymodiolus sp.) cemented together with barite.

Analysis of the surface geology of the GB338 and GB382 sites clearly indicates a history

of rapid expulsion of fluidized sediment followed by a distinct reduction in the rate of flux to

conditions we observe there today. The smaller of the two expulsion centers GB382 has nearly

become dormant and a drape of hemipelagic sediments has been deposited over the mound

surface and is largely undisturbed except for bioturbation and a crestal site where gas is in the

surface sediments. By contrast, the GB338 mound is still actively fluxing gas, fluidized

sediment, and barium-rich fluids to the modern seafloor. Analysis of the mud extruded from this

area indicates that microfossils as old as Miocene are being extruded onto the modern seafloor

(Kohl and Roberts, 1994). The acoustically opaque nature of the mound’s interior, its highly

reflective surface on both high resolution and exploration-scale seismic, the general hard

signature on 3D-surface amplitude data, and the localized polarity shifts over the mound crest are

consistent with the characteristics observed on side-scan sonar data and direct observations of the

mound surface. The acoustic characteristics of the GB338 feature are tabulated in Tables 9 to

11.
44

Table 9. Side-scan sonarim age characteristics forGB338,case study.

Feature Feature Field Flanking Areas

Acoustic Backscatter X

Strong

M oderate X

W eak

M ixed

TargetShapes

Bum ps

Cones

M ounds X

Pinnacles

Depressions

Irregular X X

TargetSurface

Sm ooth

Irregular X X

VariableXX

Feature O ccurrence

Isolated

Asym m etric G roups X

Linear X

Circular X

Elliptical

Polygonal

Table 10. Seism ic facies forGB338 case study.

Seism ic Facies Feature Feature Field Flanking Units

G ently Divergent-Parallel X

Strongly Divergent

O nlapping

Dow nlapping

Drape X X

a. Layered

b. Acoustically TurbidX X

Sigm oid Progradational

Chaotic X

Contorted-Discordant

Acoustically Turbid X

a. Com plete

b. Stacked Zones

c. Chaotic Discontinuous Reflectors X

(1) Parallel

(2) Parabolic

(3) M ixed

Table 11. Surface reflection characteristics forGB338 case study. F = feature;FF = feature field;FA = feature adjacentar

3.5 kH Pinger Interm ediate Source 3D Seism ic

F FF FA F FF FA F FF FA

Surface Reflection Strength 

Strong X X X X

M oderate X X

W eak X

Variable

W idth ofSurface Reflectors (M S)

0-2 XX

2-5 X

>5 X XX

Variable X

Reflector Characteristics

Sim ple Doublet X X

Stratified X X

Prolonged X X

Chaotic

W indow ed

BrightSpots

Surface Reflection G eom etries

Planar X XX

W avy X

M ounded X

Scarped

Isolated Pinnacle/s

Isolated Depression/s

Parabolics X X

Isolated

M ultiple X X

O verlapping

Phase Inversion X X
45

D. Case Study: Diapiric Mound, Garden Banks Block 427

1. Introduction

Within the Garden Banks Lease area, block 427 (Figure 34) is dominated by a large

mound that occupies most of the southern two-thirds of this block (Figure 35).

Figure 34. Location map for Garden Banks Block 427 (GB427).

Figure 35. Bathymetric map for GB427 and surrounding areas. Depth contours are in meters.
46

This impressive feature stands out on regional bathymetric maps, computer-enhanced multibeam

images of the upper continental slope, and seafloor renderings of this region of the Garden Banks

Lease area (Doyle et al., 1996). The mound (27° 33.59’N; 92°25.53’W) rises over 200 m above

surrounding topography and has a base diameter of about 3.8 km. Initial evaluations of this

feature for a geohazards assessment of Shell’s Auger Prospect area were problematic. Many

questions remained, especially concerning details of the sea floor, even after the best high

resolution acoustic data and 3D-seismic were acquired over the feature and analyzed to

determine its characteristics.

High resolution acoustic data and 3D-seismic renderings indicate areas of extremely high

backscatter on the upper flanks and top of the feature. In addition, the cone exhibits linear

patterns radiating away from the crest area. A highly reflective surface on the cone flanks

highlight the radial patterns that tend to converge at the apex of the mound. These radial patterns

were originally interpreted as tensional faults radiating away from a salt-cored and diapiric

structure. Attribute mapping using a data base of 3D-seismic, as discussed by Campbell (1997),

Hill (1996), Roberts et al. (1996) and Trabant (1996), initially indicated that the mound surface

was extremely variable and that making a detailed geologic-sedimentologic interpretation of this

variability was difficult without sea floor verification data.

2. Geologic Setting

The area of interest, GB427, is located on the northwest Gulf of Mexico continental slope

approximately 300 km southwest of the modern Balize lobe of the Mississippi River delta

(Figure 34). Within this general slope area, the combined effects of massive sediment input

(Suter and Berryhill, 1985), primarily at periods of lowered sea level, and the compensating

deformation of salt has created a complex slope topography characterized by canyons, troughs,

hills, ridges, and intraslope basins. The hills and ridges are, in most cases, a direct reflection of

underlying salt. These features have sea floor gradients of 10-12° which are approximately 10X

the average gradient of the average continental slope and approximately 30X the gradients of

basins (0.2°- 0.4°) that occur as flat areas between the erratic topography created by salt

structures. Faults and fluid expulsion features accompany most areas where salt adjustment has

created positive sea floor topography.

To the north of GB427 is a topographic ridge oriented roughly NE-SW which is a

reflection of underlying and relatively shallow salt masses. Unlike GB427, this area displays the

impressive effects of massive fluid expulsions that has been discussed separately in the previous

case history. Within this area N-NW of GB427 impressive debris flows and fluidized mudflows

have originated from two faulted and acoustically opaque fluid expulsion centers located on a

northeast-southwest trending salt ridge upslope of the GB427 diapiric hill.

The diapiric hill in GB427 shares several important characteristics with the fluid

expulsion centers to the north. The shallow reflection character over the expulsion area is

amorphous on high resolution seismic profiles and there are large areas of patchy high amplitude

backscatter on side-scan sonar records. Although the expulsion centers in GB382 and GB338

are also acoustically amorphous and have a few high amplitude backscatter areas on side-scan

sonar records, the highly reflective backscatter areas on the top and flanks of the diapiric hill
47

appear darker on side-scan data. These dark areas are much more widespread than in the cases

of the upslope expulsion centers. They also function as prevalent reflective surfaces on high

resolution seismic profiles. In addition, the diapiric hill exhibits numerous patterns that tend to

radiate away from the crest. These features generally have small displacements and therefore do

not dramatically offset the seafloor. Vertical displacement is usually 1 m or less.

3. Available Data

The best possible high resolution acoustic data were acquired for investigation of the

Auger Prospect area. Original data were acquired by Fugro-McClelland Marine Geosciences,

Inc. using a Tri-Cluster 1/4 millisecond sampled multi-channel digital data for deepest

penetration into the subsurface, a deep-tow 3.5 kHz subbottom profiler for configuration of the

shallow subsurface, 100 kHz side-scan sonar for seafloor feature detection, and a narrow beam

precision depth records for bathymetry. Mosaics of side-scan sonar data and maps of the

subsurface were compiled by Fugro-McClelland personnel from geohazards survey data.

Ground truth observations and samples were provided by two manned submersible dives on the

GB427 diapiric hill using the Johnson Sea-Link research submersible. The first dive (JSL-1,

Dive 3305) was made on September 6, 1992 and concentrated primarily on the top of the feature

(touchdown at 613 m leave bottom at 608 m). The second dive (JSL-2, Dive 2896) was

conducted on August 4, 1997, starting at a depth of 817 m and ended at a depth of 610 m. The

objective of this second dive was to traverse the western flank of the hill where high amplitude

zones appears on the side-scan sonar data from the base of the hill to its crest. During both

dives, video documentation of the seafloor, 35 mm photographs, rock samples, and sediment

grab samples were collected. Samples were analyzed for mineralogy using the X-ray diffraction

technique (Cook et al., 1995). Stable isotope ratios of carbon (δ13C) and oxygen (δ180) were

provided by a commercial laboratory using PDB as a standard.

4. Feature Characteristics

The symmetrical shape of the GB427 mound, as reflected on a bathymetric map, is

mimicked by a strong internal reflector approximately 50-100 ms beneath the surface of this

feature (Figure 36).

Figure 36. Migrated multichannel seismic profile (TriCluster data) across the GB427 mound illustrating the strong

internal reflector which is less than 100 ms from the sea-floor at the mound’s apex.
48

Beneath this strong reflector there are no coherent reflection horizons (acoustically opaque)

which suggests either a rather homogeneous substance or perhaps the presence of gas. The

strong and rather uniform acoustic impedance boundary that defines the mound interior is

characteristic of an interface between terrigeous sediments and salt. Original interpretations of

this feature suggest that it is salt-cored which certainly is consistent with the geophysical data

(personal communication with Fugro-McClelland Marine Geosciences personnel who worked on

the Auger Prospect project). Being a salt-cored diapiric structure suggests that existing

sediments have been uplifted, faulted, and perhaps forced to fail under the force of gravity

creating submarine landslides or slumps. Analysis of the multichannel seismic data indicate that

numerous faults with small offsets are present in the sedimentary section above the salt in the

crestal part of the mound.

When a somewhat more detailed view of the surface and near-subsurface sediments is

analyzed (Figure 37), even the sediments between the salt core and the sea floor have an

acoustically opaque nature, especially those sediments directly over the apex of the salt and on

the slightly steeper western side of the mound. A side-scan sonar mosaic of the mound

emphasized the complexity of the sea floor associated with this feature. During the initial

analysis of the deep-tow side-scan and subbottom profile data, the dark reflective areas as well as

the radial patterns originating near the mound top were difficult to interpret. Even with 3Dsurface amplitude data, analyzed in conjunction with other remotely sensed (acoustic) data sets, a

detailed interpretation was not possible till direct sea floor verification data were collected and

integrated with other data sets. Additionally, 3D-seismic profiles across the GB427 mound did

not indicate a phase inversion over the crest which identifies gas within the near-surface

sediments (Figure 38).

Figure 37. High resolution seismic (3.5 kHz) profile across the GB427 mound illustrating the acoustically opaque

mound interior, bedded sediments on the mound flank, remnants of a hemipelagic drape deposit over part

of the feature, and its irregular surface of variable reflectivity.
49

Figure 38. A 3D-seismic profile from the GB338 expulsion center to the GB427 diapiric mound. Note the polarity

shift over the expulsion center, but no similar change in phase over the diapiric mound.

5. Feature Synopsis and Interpretation

Analysis of 3D-seismic surface amplitude data in conjunction with the deep-tow sidescan sonar and subbottom data produced a reasonably good correlation between the dark and

complex patterns on side-scan data (Figure 39) and patterns of high amplitude on the subbottom

profiles and 3D-seismic amplitude map (Figure 40).

Figure 39. A deep-tow side-scan sonar mosaic of the GB427 mound illustrating important surface characteristics of

this feature such as the irregular and highly reflective areas and the linear patterns that are radial to the

mound crest area.
50

Direct observation and sampling of these areas using a manned submersible provided the

necessary data to make an improved interpretation of the processes that impacted the mound

surface and produced a set of response features identifiable on high resolution acoustic data sets.

Figure 40. A map of 3D-seismic surface amplitude map of the GB427 mound. The high amplitude zones correlate

to areas of lithified seafloor.

Sea floor verification data indicated that high amplitude areas on the 3D-seismic surface

amplitude data correlate to regions of the mound top that are hard bottoms. These hard bottom

areas resulted from the combined effects of previous fluid-gas expulsion and seafloor

lithification. Expulsion centers at the crest of the mound did not appear to be presently active.

The two expulsion centers that were directly observed were not degassing or forcing fluids to

the surface and appeared to have been dormant for a long time. They were represented as slight

depressions with obvious flow pathways oriented away from these depressions and down the

mound flanks. The depressions were filled with fine-grained sediment covered with numerous

disarticulated lucinid-vesycomyid clams (Figure 41). The fact that no phase reversal was

observed on 3D-seismic profiles across the GB427 mound suggests that the vents are currently

inactive, the near-surface sediments are not gas-charged. This observation suggests that if gas is

still being delivered to the underlying sediments, it is at a slow rate, perhaps enough to cause the

near surface sediments to be acoustically opaque on high resolution seismic data.
51

Figure 41. Disarticulated and scattered lucinid-vesycomyid clams at one of the dormant vent sites near the crest of

the GB427 mound.

In close proximity to the dormant expulsion centers, the sea floor is lithified in the form

of slabs, boulders, and gravel-sized debris. Samples of the shallow subsurface using short cores

indicate that nodular masses of lithified shallow sediment are forming beneath the surface

(Figure 42).

Figure 42. (a) massive rocks of authigenic carbonate are found on the surface and (b) nodular masses of host

sediment cemented with 13C-depleted Mg-calcite are found in the subsurface at the mound crest as well as

along its flanks, especially near the flow pathways from upslope expulsion centers.

Lithification is not confined to the crestal areas of the mound, but lithified slabs, boulders, and

gravel-sized masses are present on the sides of the gullies that radiate down the mound flanks

(Figure 43). These radial features were originally interpreted as faults, but with ground truth
52

data, they link directly with fluid expulsion centers at the top of the mound and therefore appear

to be erosional in origin and the products of flows originating from the expulsion centers. The

lithified surface and shallow subsurface material is composed of fine-grained host sediment

(terrigenous clay and quartz silt) cemented with Mg-calcite and dolomite (trace amounts) that are 13C-depleted (δ13C range from -31.2‰ to -45.2‰ in a total of 4 samples tested). Light carbon

isotope values indicate a link with hydrocarbons as a carbon source. These results are consistent

with similar analyses of other hydrocarbon seep/vent-related carbonates from the northern Gulf

of Mexico continental slope.

Figure 43. Lithification on the flank of the GB427 mound.

Finally, rapid expulsion of fluids and sediment, appears to have been a dominant process

associated with the GB427 diapiric mound at sometime in the past. However, because the flows

are episodic, an environmental setting that provides limited trophic resources to sustain complex

and widespread chemosynthetic communities results. In addition, rapid sedimentation during

expulsion events is detrimental to fixed bottom dwelling organisms. Only disarticulated lucinidvesycomyid clam shells scattered on extruded mud surfaces mark the sites of sediments that were

once hydrocarbon-charged. The delivery of hydrocarbon-charged fluids and fluidized sediments

to the modern seafloor is an episodic process that apparently does not occur often enough to

sustain viable and diverse chemosynthetic communities. Migration pathways at the top of the

mound are short between the mound surface and underlying salt. Discharge events are

apparently not large enough to initiate slope failures. No slumps or larger failure scarps were

observed during field verification data collection. Acoustically, the surface of the GB427 mound

forms a strong reflector that does not demonstrate a phase inversion that would suggest gascharged sediments in the shallow subsurface (Figure 44). The acoustic characteristics of the

GB427 mound are presented in Tables 12 to 14.
53

Figure 44. Site-specific areas of side-scan sonar data as extracted from the mosaic of the GB427 mound, illustrated

in Figure 38.
54

Table 12. Side-scan sonarim age characteristics forGB427 case study.

Feature Feature Field Flanking Areas

Acoustic Backscatter

Strong X

M oderate

W eak X

M ixed X

TargetShapes

Bum ps

Cones

M ounds

Pinnacles

Depressions

Irregular X

TargetSurface

Sm ooth X

Irregular X

Variable X

Feature O ccurrence

Isolated

Asym m etric G roups

Linear X

Circular

Elliptical

Polygonal X

Table 13. Seism ic facies forGB427 case study.

Seism ic Facies Feature Feature Field Flanking Units

G ently Divergent-Parallel

Strongly Divergent

O nlapping

Downlapping X

Drape X X

a. Layered

b. Acoustically TurbidX X

Sigm oid Progradational

Chaotic

Contorted-Discordant

Acoustically Turbid X

a. Com plete

b. Stacked Zones

c. Chaotic Discontinuous Reflectors X

(1) Parallel X

(2) Parabolic

(3) M ixed

Table 14. Surface reflection characteristics forGB427 case study. F = feature;FF = feature field;FA = feature adjacentarea.

3.5 kH Pinger Interm ediate Source 3D Seism ic

F FF FA F FF FA F FF FA

Surface Reflection Strength 

Strong X X X

M oderate X X

W eak X X

Variable

W idth ofSurface Reflectors (M S)

0-2

2-5 X XX

>5 X X

Variable

Reflector Characteristics

Sim ple Doublet

Stratified X X X

Prolonged X X

Chaotic

W indowed X

BrightSpots

Surface Reflection G eom etries

Planar X XX X

W avy X X

M ounded

Scarped X

Isolated Pinnacle/s

Isolated Depression/s

Parabolics

Isolated

M ultiple

O verlapping

Phase Inversion
55

E. Case Study: Three Separate Features and Distinct Bottom Types (Mounded

Carbonates, Extinct Mud Volcano, and Collapse Depression), Green Canyon Block 53

1. Introduction

The first high resolution geophysical survey of the GC52-53 area was conducted in 1983

by Racal Geophysics, Inc. This initial survey found that much of the sea floor in these blocks

was mounded and that the mounds had very little internal acoustic character and highly reflective

surfaces. Although there were areas of relatively smooth sea floor outside the mounded zones,

some smooth areas had numerous pockmarks. The seafloor geology of GC53 was found to be

more complex than that of GC52. In addition to the large areas of hard bottom mounds down the

central and western parts of the block, GC53 had a large mound, interpreted as a mud volcano, in

the northeast quadrant and a rounded depression in the southwest that showed a signature of gas

moving through the water column above it on 3.5 kHz profiles. These three different bottom

features of GC53 will be discussed in this case history.

2. Geologic Setting

The lease block GC53 is located at the shelf edge and at the transition between the

continental shelf and slope (Figure 45). Water depths in GC53 range from about 100 m in the

most northern parts of the block to approximately 220 m in the south (Figure 46).

Figure 45. Location map for Green Canyon Block 53 (GC53).

A raised area of sea floor bounded by regional faults trends NW-SE through the northern half of

the block. This area and an additional region to the SW are characterized by mounded bottom

topography where some individual mounds have over 10 m relief above the surrounding sea

floor. Early studies of these shelf edge banks and mounds that characterize much of the shelf-
56

slope transition in the northern Gulf found that some banks (e.g. the Flower Garden Bank of the

NW Gulf) supported reef-building communities while most others were veneered with a spongecoralline algal assmbledge (Bright and Pequegnat, 1974; Rezak and Bright, 1981; Rezak et al.,

1985). These reefs, carbonates banks, and mounds seem strangely out of place in a sedimentary

province dominated by large volumes of terrigenous clastic sediments transported to the outer

shelf and slope during periods of falling to low sea level, especially during the Pleistocene. As

Rezak et al. (1985) have pointed out, most of these reefs, banks, and mound complexes are

located over shallow allochthonous salt that in recent geologic history has been emplaced near

the modern sea floor.

Figure 46. Bathymetric map for GC53 and surrounding areas. The locations of large regional faults have been

superimposed on the bathymetry.

Recent studies of structure of the shelf and slope (Diegel et al., 1995; Rowan et al., 1999)

emphasize the interplay between application of large volumes of sediment to a system containing

thick units of deformable salt. Research by Suter and Berryhill (1985), Roberts and Sydow

(1994), and Anderson et al. (1996) clearly demonstrate the relationship between sea level cycles

and fluvial-deltaic deposition at the shelf edge and on the adjacent continental slope. During

periods of falling-to-lowstands of sealevel the shelf edge becomes a focal point for deposition.

Suter and Berryhill (1985) demonstrate that local depocenter development at the shelf edge

causes a rapid response in vertical salt migration. These events seem to be responsible for the

numerous near-surface salt masses at or near the shelf edge that now support overlying reefs,
57

carbonate banks, and mound complexes. Salt movement also creates faults and reactivates

existing faults which can provide a setting for fluid and gas migration to the seafloor.

Roberts (1992) suggested that some of the reefs, banks, and mounds of the shelf edge

may be seated on a substrate of authigenic carbonate created as a by-product from the microbial

utilization of the hydrocarbons. Even the well-studied Flower Garden Banks may have been

initiated on a hard substrate created by this process. Although acknowledged and certainly

studied in detail (Bright et al., 1980; Rezak et al., 1985), the hydrocarbon and brine seeps

associated with these banks were initially evaluated with ecological impacts to the reef

community as a focal point. The significance to reef and bank locations was not realized until

studies of seeps from many different settings (Hovland and Judd 1988), and specifically the Gulf

of Mexico continental slope (Roberts et al., 1987, 1988), provided a clear association between

authigenic carbonates and hydrocarbon seeps. These observations of seeps suggested that the

authigenic carbonates provided rather extensive hard substrates on which reef-building biota

attached and created buildups in an environmental setting that was otherwise characterized by an

abundance of fine-grained siliciclastic sediment, and thus a poor area in which to initiate reef and

bioherm growth. Rezak et al. (1985) pointed out that gas plumes have been detected in the water

column on many high resolution seismic profiles taken across banks of the northern Gulf.

The GC53 area is at the shelf edge and has a large area of carbonate mounds that are

currently veneered with encrusting coralline algae, various types of sponges, and other sessil

organisms. Gas has been observed escaping through the mound complex in several locations as

well as in off-mound sites. The inactive mud volcano to the east of the mounded seafloor area is

evidence of the fact that fluidized sediment and gas are delivered to the sea floor in greater

quantities and at higher rates in the recent past than we are able to observe today. Observational

data suggest that a collapse depression in the SW portion of the block is still an active gas

venting site.

3. Available Data

The original high resolution geophysical survey by Racal Geophysics, Inc. of Houston,

Texas was conducted for the GC52-53 area in the summer of 1983. Data were generated from

four different surface-tow acoustic sources: a 3.5 kHz subbottom profiler, an 8.4 kilojoule

sparker system, a side-scan sonar system, and a precision echosounder (for baythymetry).

Survey data were located using the Lorac AA net noting the setback distances of sources and

receivers from the Lorac antenna atop the survey vessel’s mast. Data from this initial survey

confirmed the mounded, bank-like character of GC53 and the presence of gas in the water

column at several sites. Later, McClelland Engineers, Inc. conducted an engineering geology

and geotechnical assessment of the GC 52-53 area which included the acquisition of high

resolution geophysical data as well as piston cores for laboratory testing. Interpretations of sea

floor geology for these blocks were based primarily on data from a precision echosounder (Edo

Western Model 4077) for bathymetry, surface-tow side-scan sonar (EG&G SMS 960) for

geomorphology of the bottom, and an ORE 3.5 kHz subbottom profile plus mini-sleeve exploder

for subsurface evaluations. Later, deep-tow side-scan sonar data and 3.5 kHz profiles were

collected as part of a pipeline survey between the Jolliet Platform site in GC184 to the Marquette

Field in the GC 52-53 area. Finally, both digital high resolution seismic and side-scan sonar data
58

were collected from GC53 as part of this MMS-CMI study. Three high resolution seismic

sources were used to image selected targets, and EG&G Uniboom profiler, a Seismic Systems

Model S-15 water gun (15 in3), and a Seismic Systems GI air-gun (50 in3). Surface-tow sidescan sonar data were provided by and EG&G Model 260 digital imaging system. These

geophysical data were acquired in both analog and digital formats. DELPH 2 acquisition and

processing software was used for acquiring the seismic data. Survey planning was accomplished

through the hydrographic software program HYPACK. Location data were supplied through

HYPACK using differential GPS (Magnavox M4 200d). Seafloor verification data from GC53

on the mud volcano was provided by JSL-1, dive 2582. Mounded carbonates were investigated

on JSL-1, Dives 3111, 3112, 3113, 3114. The collape depression was observed and sampled on

JSL-1, Dive 2583 and JSL-2, Dive 2894 and 2912.

4. Feature Characteristics

a. Mounded Carbonates Much of the area within GC53 falls within a bottom type

category dominated by mounds of various sizes. Topographically, elevated areas in the NW,

central, and SW parts of the block are characterized by a sea floor composed of hard, carbonate

mounds, some of which have over 10 m relief (Figure 47).

Figure 47. High resolution seismic profile (3.5 kHz) across the mounded area in the SW sector of GC53 (see Figure

45). This profile illustrates the individual and fused mounds of this area.

In the northern Gulf of Mexico both reefs and bioherms have been identified as contributors to

the province of mounds and banks along the shelf-slope transition zone (Rezak and Bright, 1981;

Rezak et al., 1985). The term reef refers to linear or mound-like buildups that have been

constructed by frame-building organisms (e.g. corals) that are both organically and inorganically

cemented into a structure that is resistant to significant erosion by physical processes (waves and

currents). These types of carbonate buildups are generally associated with the photic zone of

warm, tropical oceanic settings. However, special oceanographic conditions in the northwestern

Gulf of Mexico allow true coral reefs of the Flower Garden Banks to survive in a subtropical

latitudinal setting (Gittings et al., 1992; Lugo-Fernandez, 1998). Bioherms, in contrast, are

mound-like buildups that contain the remains of a limited variety of calcareous organisms that
59

are not frame-builders, (Cunnings and Shrock, 1928). Bioherms have no latitudinal restrictions

like reefs. Lithoherms are deepwater (below the photic zone) mounded structures formed by

submarine lithification of sediment that may contain some skeletal debris (Neumann et al.,

1977). A subset of the general class of lithoherms are those mounded carbonates that develop as

by-products from the microbial utilization of hydrocarbons. Aharon et al. (1993) and Roberts

and Aharon (1994) have informally introduce the term chemoherm to distinguish these special

mounded carbonates that are the by-products of chemosynthesis.

The shelf edge mounds and banks of the northern Gulf, of which the GC53 mounds are a

part, display rough topography superimposed over shallow salt. Numerous faults accompanied

by sea floor scarps are common to this setting. Hydrocarbon and brine vents are commonly

associate with these features (Rezak et al., 1985; Roberts et al.,1989). Figure 48 illustrates the

shallow salt, numerous faults that intersect the modern sea floor, and the variety of sea floor

features that are associated with these faults.

Figure 48. A 3D-seismic profile oriented NE-SW across GC53 showing the relationships between salt, faults, and

important surface features.

Direct inspection of the GC53 carbonate mounds indicates that all of the mounds, ranging

in depth from approximately 80-200 m, are veneered by actively growing communities of

sponges, coralline algal encrustations, leafy algae, antipatharians, crinoids, and hydroids. Figure

49 illustrates typical views of biogenic veneers of the GC53 mounds. Although 13C-depleted
60

authigenic carbon ates were found in local sites with the zones of mounded sea floor, the living

biogenic veneer obscures the character of the mound interiors. Therefore, the contention that

biogenic growth could have originated on a hard substrate provided by hydrocarbon seep-derived

carbonates (Roberts, 1992) cannot be easily tested. Mounds occur as individual features as well

as fused groups, as is illustrated on the high resolution seismic line of Figure 47. In many places,

sediments immediately surrounding the mounds are coarse and consist of both shell debris and

rhodoliths (Figure 49). These coarse sediments, especially associated with the shallowest

mounds of the central GC53 area, appear to be the products of physical winnowing of finegrained components leaving a coarse shell lag. Loop Current intrusions and tropical storm

waves are the two most probable physical process agents.

Figure 49. Photographs of the GC53 mounded area in the middle of the block illustrating: (a) a large mound surface

encrusted with crinoids sponges, hydroids, and antipatharians on a hard substrate of encrusting coralline

alga, (b) a small mound-like buildup of coralline alga with attached sessile organisms, and (c) the edge of

a mound with a field of rhodoliths at its base. Water depths range from 105-120 m for these pictures.

Although high resolution seismic profiles of the mounded areas generally indicate that

the mounds display hard, reflective surfaces and acoustically opaque interiors, bedding can

sometimes be seen in the mound interiors, suggesting possible seep-related lithification of

faulted blocks of sea floor as the starting point for biogenic accretions (Figure 47). On side-scan

sonar images (Figure 50), mound surfaces are rough and reflective.

Figure 50. Side-scan sonar image of the central mounded area in GC53. Note the complex nature of the seafloor in

this area and the various sizes and shapes of individual mounds. This line is coincident with the 35 kHz

profile of Figure 47.
61

Many mounds merge to make composite features formed from the process of growth and fusion

of several smaller mounds to make larger ones. Mound sides appear to be steep on side-scan

data, even though the surface-tow high resolution seismic data suggest less steep sides because

of parabolic reflector shapes. The steep sides of many mounds have been confirmed by direct

observation. Gas has been found escaping through some of the mounds, especially in the SW

sector of the block. Analysis of gas samples from the mounded area of GC53 indicates that the

gas was methane of biogenic origin (personal communication, Mark Beunas, Chevron).

b. Extinct Mud Volcano In the NE sector of GC53 the bathymetric contours of Figure

45 clearly define a cone-shaped feature with a base diameter of approximately 600 m and with

over 30 m relief above the surrounding sea floor. The base of the feature occurs at a depth of

approximately 160 m while the crest is slightly less than 130 m. The 3D-seismic profile that cuts

through this feature (Figure 48) suggest that it is directly the product of expulsion of fluidized

sediment by way of a deep cutting fault that extends down the flank of a shallow salt mass under

the GC53 area. A high resolution seismic profile (3.5 kHz) across this feature illustrates an

acoustically opaque interior, but with a distinctive unit having little internal acoustic character

(approximately 5 m thick) draping across the entire feature (Figure 51).

Figure 51. A 3.5 kHz profile across the inactive mud volcano in the NE section of GC53. Note the hemipelagic

drape deposit across this feature.

Seismic profiles indicate wedge-shaped sedimentary units flanking the cone-shaped feature

suggesting vertical accretion from a sediment source originating from the feature itself. Upon

direct visual inspection using a manned submersible, the flanks of the cone are well-burrowed

with distinct fluting on the SW side. These few grooves originated near the apex of the structure

and extended downslope to the feature base. Ridges between furrows are partially lithified with

plate-like slabs of carbonate-rich cemented sediment (Figure 52). These furrows suggest the

downslope movement of fluids or fluidized sediment which eroded pathways through the

hemipelagic sediments that drape the core flanks. Furrow walls expose thin lithified and thicker

unlithified units suggesting alternating periods of sedimentation and exposure resulting the

lithification. Cements in these crusts were mostly Mg-calcite that are extremely 13C-depleted,

indicating a link to microbial degradation of hydrocarbons (Neurauter and Roberts, 1994). These
62

hydrocarbons were probably incorporated in the muds and deposited with them during the

mound-building process.

Figure 52. Bottom photograph of a gully on the flank of the GC53 inactive mud volcano along its SW side. This

gully is flanked by slabs of lithified sediments while the gully floor is relatively free of this lithification.

Observation of the mound crest revealed a flattened sea floor with no indication of fluid

or gas expulsion and no chemosynthetic organisms except one small area which supported a

patch of white Beggiatoa sp. (approximately 1 m in diameter). Feature-top sediments were

highly burrowed with a few localized areas of hard substrate that supported anemones and a few

sea fans (gorgonians). Because of its cone shape, hint of sediment accretion structure on the

cone flanks, coincidence with a deep-cutting fault, and acoustically opaque subsurface character

far below the modern seafloor, this feature is interpreted as an inactive mud volcano. The

furrowed side is interpreted to be a response to the waning phase of activity when only sedimentfree fluids welled up from the central vent of the feature and ran down the south side eroding

sediment in the process. The lack of a central depression relates to the apparent long period

since this feature has been active. The hemipelagic drape over the entire cone suggests it has

been inactive for several thousand years since these deposits accumulate at a rate of about 30

cm/1000 years (unpublished data).

c. Collapse Depression On the 3D-seismic profile shown in Figure 48, this unusual

feature appears as a graben. The sea floor associated with it is visibly lower than surrounding

areas. An early high resolution seismic profile (3.5 kHz) taken by Racal Geophysical in 1983

in the GC53 area crossed this feature and found the small and highly reflective area of depressed

sea floor venting gas into the overlying water column (Figure 53). Side-scan sonar data collected

as a part of this project indicate that the floor of the collapse depression is very smooth except for

a central area characterized by two distinct vents, extruded mud sheets, and semi-concentric lowrelief scarps related to local sea floor subsidence (Figure 54).
63

Figure 53. A 3.5 kHz profile across the collapse depression in the SW quadrant of GC53. Note the highly reflective

seafloor within the depression and the distinct gas plume in the overlying water column.

Figure 54. This side-scan sonar record, acquired as part of this project, illustrates the venting area within the

collapse depression, newly deposited mud flows, and a set of roughly circular low-relief scarps indicating

local subsidence.
64

Direct observation and sampling of this feature on three occasions confirmed the smooth

extruded mud floor of the depression, fresh unoxidized light gray mud sheets originating from

the vents illustrated in the side-scan sonar image (Figure 54), and the venting of gas into the

water column. On all three visits to this site, Beggiatoa mats were observed on the older mud

surface of the collapse depression. These sediments were dark and reducing beneath the thin

oxidized surficial layer. No other chemosynthetic organisms were observed at this site.

Escaping gas was also found at the central venting site on all three dives on this feature.

However, the dive in 1997 (JSL-2, Dive 2894) encountered a dynamic event characterized by

violent outgassing (Figure 55). This event resulted in gas bubbles traveling from the sea floor to

the sea surface and large pieces of mud (up to ~ 15 cm diameter) being propelled into the water

column along with an abundance of fine-grained suspended sediment. This event emphasized

the episodic and potentially dynamic nature of sea floor outgassing that has never been directly

measured, but may have important practical consequences to man’s activities in deep water. A

follow-up dive at this site five days after the active venting event indicated that the vents were

still active but at a much decrease level of intensity.

Figure 55. This figure represents the cross section of a combined gas and suspended sediment plume acquired by a

precision depth recorder prior to a manned submersible dive on this feature in the summer of 1997. This

exceedingly energetic episode of venting resulted in bubble-phase gas reaching the sea surface.

5. Feature Synopsis and Interpretation

High resolution geophysical data clearly distinguish the mounded sea floor, the inactive

mud volcano, and the collapse depression as fundamentally different bottom types and features.

On the 3D-seismic surface amplitude map of GC53 (Figure 56) sea floor areas representing these

three features all display surface amplitudes that significantly deviate from background.
65

Figure 56. This 3D-seismic surface amplitude map of GC53 clearly identifies that three bottom types and features

discussed in this report: (a) mounded carbonates, (b) inactive mud volcano, and (c) gas-emitting collapse

depression.

The areas of mounded sea floor are presently covered by a living veneer of organisms,

some of which help accrete these features with biogenically precipitated Ca-Mg carbonate

(primarily coralline alga). The mounds are hard carbonate structures with a complex surface

comprised of small-scale surface roughness elements (local buildups) of 1-2 m and topographic

lows containing pockets of sediment. The large complex mounds tend to have relatively steep

sides that commonly interface with a shell-rich sediment around the mound base and in pockets

across the mound surface. Side-scan sonar images of these mounds corroborate their complex

surface structure, steep sides, and the variability of their relief (Figure 52). High resolution

seismic profiles across the mounded areas of GC53 confirm the acoustically opaque interiors of

these features, the sometimes clear relationship with bedded sediments as a substrate, and the

lack of “pull-down” or “pull-up” of reflectors (bedded units) interfacing with the acoustically

opaque mound interior. Surface amplitude data derived from 3D-seismic over the GC53 area

clearly distinguish the mounded areas as having a moderate amplitude deviation above

background. Even though a few small gas seeps were observed by manned research submersible

within the areas of mounded seafloor, no phase inversions were noted in the 3D-seismic data.

That is, the mounds produced a response in phase with the surrounding sea floor. This result

coupled with the evaluated amplitude response suggests a hard bottom that is not associated with

sediments or carbonates that are gas-charged. Individual acoustic characteristics of this bottom

type as determined on high resolution seismic and side-scan sonar records are summarized in

Tables15 to 17.
66

The isolated and bathymetrically distinct mound in the NE quadrant of GC53 was

interpreted as an inactive mud volcano based on its seismic character, geomorphology, and

associated surface feature. Although seismic profiles from several acoustic sources indicate an

acoustically opaque interior for this feature, a hemipelagic drape appears to have been deposited

over this cone-shaped mound. High resolution seismic data also define flanking wedge-like

accretion units that suggest vertical growth even through interior stratigraphy of the feature

cannot be readily identified. The surface of this cone is relatively smooth, as determined on sidescan sonar records as well as with direct observation (manned submersible), except for limited

gully-like fluting of the surface on the cones south side. This fluting is interpreted as a product

of sediment-free fluid expulsion during the final stages of mud volcano activity. No “pull-up” or

“pull-down” of reflectors interfacing with this feature was observed. The 3D-seismic surface

amplitude data indicate that the overall mud volcano deviates slightly above background, but a

small area near the apex of the feature displays a high deviation from background (Figure 56). A

questionable phase reversal is observable in both the digital high resolution data acquired for this

project and on 3D-seismic profiles that cut the apex of the cone. Although there was very little

surface expression of a gas seep (one small area of bacterial mat development), perhaps gascharged sediments still exist in the subsurface feeder system of the cone interior. Individual

acoustic characteristics of this feature as determined from high resolution seismic and side-scan

sonar data are summarized in Tables 18 to 20.

The collapse depression in the SW quadrant of GC53 is not defined in the regional

bathymetry (Figure 46), but is clearly visible in the 3D-seismic surface amplitude data (Figure

56). This feature is depressed 3-4 m below surrounding topography, is roughly circular, and is

filled with dark reducing mud covered with bacterial mats (Beggiatoa sp.). No other

chemosynthetic organisms were observed within the confines of this feature. Although the

surface of this sediment is smooth, side-scan sonar data confirm an expulsion area in the northcentral area of the depression which shows sheets of recently extruded fluidized mud and

concentric scarps caused by local collapse. The surface amplitude data outline the circular

nature of this feature and indicate a clear moderate-to-strong deviation from background (Figure

56). Two strong anomalies are present within the circular confines of the feature. The

northernmost amplitude anomaly corresponds to the vent areas shown on side-scan sonar (Figure

54). A second vent was not identified on high resolution geophysical data or by direct inspection

of the feature using a manned submersible. Perhaps this anomaly is responding to a shallow

subsurface gas accumulation which has little or no surface expression. A notable characteristic

of the overall feature is that on high resolution seismic flanking reflectors interfacing with the

acoustically opaque center of this feature tend to exhibit slight pull-down which is consistent

with the presence of gas in the sediments. Individual acoustic characteristics of this feature from

high resolution seismic and side-scan sonar data are summarized in Tables 21 to 23.
67

Table 15. Side-scan sonarim age characteristics forGC53,m ounded carbonates,case study.

Feature Feature Field Flanking Areas

Acoustic Backscatter

Strong X X

M oderate X

W eak

M ixed X

TargetShapes

Bum ps

Cones

M ounds X X

Pinnacles X X

Depressions

Irregular

TargetSurface

Sm ooth X

Irregular X X

Variable XX X

Feature O ccurrence

Isolated X

Asym m etric G roups X X

Linear

Circular X

Elliptical

Polygonal X X

Table 16. Seism ic facies forGC53,m ounded carbonates,case study.

Seism ic Facies Feature Feature Field Flanking Units

G ently Divergent-Parallel

Strongly Divergent X

O nlapping

Downlapping

Drape

a. Layered

b. Acoustically Turbid XX

Sigm oid Progradational

Chaotic

Contorted-Discordant

Acoustically Turbid

a. Com plete

b. Stacked Zones

c. Chaotic Discontinuous Reflectors X

(1) Parallel

(2) Parabolic X

(3) M ixed

Table 17. Surface reflection characteristics forGC53,m ounded carbonates, case study. F = feature;FF = feature field;FA = feature adjacentarea.

3.5 kH Pinger Interm ediate Source 3D Seism ic

F FF FA F FF FA F FF FA

Surface Reflection Strength 

Strong X X X X X X

M oderate X XX

W eak

Variable

W idth ofSurface Reflectors (M S)

0-2 X

2-5 XX XXX

>5 XXX

Variable

Reflector Characteristics

Sim ple Doublet XXX

Stratified X X

Prolonged X X X X

Chaotic

W indowed X X X X

BrightSpots

Surface Reflection G eom etries

Planar X XX

W avy

M ounded X X X X X X

Scarped

Isolated Pinnacle/s X

Isolated Depression/s

Parabolics

Isolated

M ultiple

O verlapping

Phase Inversion
68

Table18. Side-scan sonarim age characteristics forGC53,extinctm ud volcano,case study.

Feature Feature Field Flanking Areas

Acoustic Backscatter

Strong

M oderate X

W eak X

M ixed

TargetShapes

Bum ps

Cones

M ounds

Pinnacles

Depressions

Irregular

TargetSurface

Sm ooth XX

Irregular

Variable X

Feature O ccurrence

Isolated X

Asym m etric G roups

Linear

Circular

Elliptical

Polygonal

Table 19. Seism ic facies forGC53,extinctm ud volcano,case study.

Seism ic Facies Feature Feature Field Flanking Units

G ently Divergent-Parallel X

Strongly Divergent

O nlapping

Downlapping

Drape X

a. Layered

b. Acoustically Turbid X

Sigm oid Progradational X

Chaotic

Contorted-Discordant

Acoustically Turbid X

a. Com plete

b. Stacked Zones

c. Chaotic Discontinuous Reflectors

(1) Parallel

(2) Parabolic

(3) M ixed

Table 20. Surface reflection characteristics forGC53,extinctm ud volcano, case study. F = feature;FF = feature field;FA = feature adjacentarea.

F = feature;FF = feature field;FA = feature adjacentarea.

3.5 kH Pinger Interm ediate Source 3D Seism ic

F FF FA F FF FA F FF FA

Surface Reflection Strength 

Strong

M oderate X XX X X X

W eak

Variable X XX

W idth ofSurface Reflectors (M S)

0-2

2-5 X XX X

>5 X X

Variable

Reflector Characteristics

Sim ple Doublet X X

Stratified X X X X

Prolonged

Chaotic

W indowed

BrightSpots

Surface Reflection G eom etries

Planar X X X X X X

W avy

M ounded X XX

Scarped X XX

Isolated Pinnacle/s

Isolated Depression/s

Parabolics

Isolated

M ultiple

O verlapping

Phase Inversion X X
69

Table 21. Side-scan sonar image characteristics for GC53, collapse depression, case study.

 Feature Feature Field Flanking Areas

Acoustic Backscatter

 Strong

 Moderate X

 Weak X

 Mixed

Target Shapes

 Bumps

 Cones

 Mounds

 Pinnacles

 Depressions

 Irregular

Target Surface

 Smooth X X

 Irregular

 Variable X

Feature Occurrence

 Isolated X

 Asymmetric Groups

 Linear

 Circular

 Elliptical

 Polygonal

Table 22. Seismic facies for GC53, collapse depression, case study. 

Seismic Facies Feature Feature Field Flanking Units

 Gently Divergent-Parallel X

 Strongly Divergent

 Onlapping

 Downlapping

 Drape X

 a. Layered

 b. Acoustically Turbid X

 Sigmoid Progradational X

 Chaotic

 Contorted-Discordant

 Acoustically Turbid X

 a. Complete

 b. Stacked Zones

 c. Chaotic Discontinuous Reflectors

 (1) Parallel

 (2) Parabolic

 (3) Mixed

Table 23. Surface reflection characteristics for GC53, collaspe depression, case study. F= feature; FF = feature field; FA = feature adjacent area. 

 3.5 kH Pinger Intermediate Source 3D Seismic

F FF FA F FF FA F FF FA

Surface Reflection Strength 

 Strong

 Moderate X X X X X X

 Weak

 Variable X XX

Width of Surface Reflectors (MS)

 0-2

 2-5 X X X X

 >5 X X

 Variable

Reflector Characteristics

 Simple Doublet X X

 Stratified X X X X

 Prolonged

 Chaotic

 Windowed

 Bright Spots

Surface Reflection Geometries

 Planar X X X X X X

 Wavy

 Mounded X XX

 Scarped X XX

 Isolated Pinnacle/s

 Isolated Depression/s

 Parabolics

 Isolated

 Multiple

 Overlapping

Phase Inversion X X
70

F. Case Study: Mounded Carbonates, Green Canyon Block 140

1. Introduction

In preparation for establishing a pipeline from Jolliet Field in Green Canyon Block 184

(GC184), the upslope and adjacent block (GC140) became an obstacle between the Jolliet Field

and a central processing platform in GC52, the Marquette Field. The Jolliet platform, in a water

depth of 536 m, was the world’s first tension leg well platform. It was placed to the south of

higher sea floor terrain in GC140 (Figure 57).

Figure 57. Location map (lease block areas) showing the position of Block 140 in the Green Canyon leasing area.

While acquiring high resolution acoustic data in 1985 for planning a pipeline route, it became

clear that GC140 contained sea floor topography that could make pipeline routing difficult.

Figure 58 illustrates the regional pattern of relief related to a shallow salt intrusion beneath

GC140 creating a broad structural high upwarped by salt movement. The area of mounded sea

floor occurs in the southwest part of GC140 (27° 48.17’N; 91° 33.24’W) where the dome-shaped

regional feature is shallowest. These mounds were observed on an earlier 1980 high resolution

geophysical survey of GC184 and the southern part of GC140 by Racal-Decca Survey, Inc.

Water depths at the crest shallow to slightly less than 240 m. Superimposed on regional

bathymetric high are numerous mound-shaped buildups that cannot be discriminated at the

mapping resolution of Figure 57. This area is located about 16 km from the shelf edge and

approximately 240 km southwest of the active Balize lobe of the Mississippi River delta (Figure

57).
71

Figure 58. Bathymetric map of the GC140 dome and adjacent areas.

2. Geologic Setting

The dome and basin topography of the Gulf of Mexico’s northern continental slope is the

result of sediment loading and salt tectonics, processes that created a variety of salt geometries

(Jackson and Talbot 1989). The GC140-184 area is underlain by a saddle-shaped salt mass that

penetrates to within about 400 m of the sea floor under GC140, but another mass beneath the

Jolliet Field in GC184 is much more deeply buried (Cook and D’Onfro 1991). It is now widely

accepted that throughout Pleistocene times ancestral Mississippi and other northern Gulf Coast

rivers delivered sediments to the outer shelf and slope by propagating across the shelf during

periods when the Gulf fell to levels somewhat lower than today. Canyons and feeder channels

fed sediment from the shelf edge to downslope depositional sites in intraslope basins as well as

onto the Gulf basin floor (Pulham 1993; Winker 1993). Intraslope basins are filled with cyclic
72

deposits that are the products of rapid input of sediments during lowstands and slow deposition

of hemipelagic deposits as fluvial sediment sources retreat from the shelf edge as sea level rises.

Cook and D’Onfro (1992) indicate that the Jolliet Field occurs in such a basin filled with sandrich stacked submarine fans deposited as a product of sediment input during sea level lowstands.

Twenty-three such coalescing fans were intitially targeted for production in the Jolliet Field

(Cook and D’Onfro 1991). As the sedimentary basin between GC140 and GC184 subsided and

filled, the salt beneath GC140 remained very close to the sea floor while the salt beneath the

Jolliet Field was buried progressively deeper with time during the Pleistocene. Fault pathways

over the Jolliet Field developed into deep-cutting networks that tapped hydrocarbon-rich

subsurface horizons. The large mound with known gas-hydrates and abundant chemosynthetic

communities (“Bush Hill”, GC185) is fed by this fault system. By contrast, the thin sedimentary

column above the salt mass in GC140 was broken by numerous faults with limited access to

overpressured zones of the deep subsurface and a source of hydrocarbons.

3. Available Data Sets

The early (1980) Racal-Decca Survey was conducted with a 7 kHz subbottom profiler, a

8.4 kilojoule sparker, and a magnetometer using Lorac A navigation. Later, geohazards data sets

were acquired by John Chance and Associates, Inc. of Lafayette, Louisiana. A precision echo

sounder (EDO Western Model 4077) was used to generate a high quality bathymetric map of the

GC140-184 area. This system employed a narrow beam (5-10° beam width) output, which helps

minimize the problem of side echoes that give false impressions of seafloor features. Shallow

subbottom data were obtained with an ORE 3.5 kHz pinger profiler. This system is employed

because of the low vertical exaggeration, good resolution of surface cutting faults, and

identification of hemipelagic drape deposits deposited over topographic irregularities and fault

scarps. Penetration of 50-60 m for this acoustic source is common in slope sediments. Mediumrange subsurface penetration (to ~300 m) was provided by a mini-sleeve exploder system. This

acoustic source helped define fault planes, bedded as opposed to nonbedded and gas-prone

sediments, and buried deposits from mass movement processes. A sparker was used both for

collection of single channel and multi-channel data. The frequency range of this acoustic source

was 25-2000 Hz with an adjustable energy range from 100-24,000 joules. An SMS 960 EG&G

side-scan sonar system, which compensates for slant-range and boat speed distortions, was used

to make seafloor mosaics in order to map features that create difficulty in pipeline routing. All

geophysical data were spatially controlled with STARFIX navigation which was a high-accuracy

(<5 m) satellite positioning and message transmission system provided by John Chance and

Associates, Inc. Later, deep tow side-scan sonar and 3.5 kHz data were collected between the

Jolliet Platform site in GC184 to the Marquette Field in GC52 in support of pipeline routing.

As part of this research project, high resolution seismic profiles were acquired across the

GC140 mound using three sources, and EG&G Uniboom profiler, a Seismic System Model S-15

water gun (15 in3

), and a Seismic systems GI air-gun (50 in3). These data were acquired in both

analog format on EPC Model 9800 seismic recorder and in digital format through the DELPH 2

data acquisition and processing software. Survey planning was accomplished through the

hydrographic software program HYPACK and DELPH 2 software programs through a

Magnavox M4200d differential GPS system. Using differential GPS navigation data points were
73

located with an accuracy of 3-5 m. Sea floor verification data were provided by JSL-1, Dives

2950, 2592 and 3118.

4. Feature Characteristics

Bathymetric and high resolution seismic profiles, as well as swaths of side-scan sonar

data, clearly indicate that the crestal areas of the GC140 dome are characterized by numerous

mound-like buildups that vary in relief from a few meters to over 20 m (Figures 59 and 60).

Figure 59. A 50 in3 air gun profile run N-S across the GC140 dome. This profile illustrates the high amplitude

surface return from the mounded carbonates on the dome top as well as the bedded sediments on the

dome flank. Areas beneath the mounded surface appear as acoustic wipe-out zones.

Figure 60. This figure represents a side-scan sonar mosaic of the GC140 dome top area. Note the complex array of

hard targets that possess sizable acoustic shadows indicating significant relief above the surrounding

seafloor. It is also interesting to note that many of these mounds appear to be aligned, possibly indicating

fault control.
74

To the south of the crestal areas, the regional slope gradient is less than 5%, but in the southern

part of GC184 the gradient increases to between 5-10% in a southeasterly direction. However,

the fault-controlled slopes to the east and southeast of the dome crest have gradients that vary

from 10-25%, with local slope gradients across fault scarps reaching as high as 50%. The flanks

of the GC140 dome are draped with fine-grained hemipelagic sediment varying in thickness

from approximately 2 m to over 6 m. On the top of the dome, however, the hemipelagic drape is

missing, and the sea floor is characterized by complexes of mounds that are clearly visible on

high resolution acoustic data sets.

Surface-tow high resolution seismic profiles cannot resolve individual mounds when

mounds are closely spaced It is clear that there are many mounds on the crest of the GC140

dome. Some occur in high density clusters when mound distribution is viewed in plan-view on

side-scan data (Figure 60). The end result on seismic profiles is a series of overlapping parabolic

reflectors (Figure 59). However, the cumulative effect of this reflection response is a high

amplitude or strong and irregular surface reflector. On exploration-scale 3D-seismic, the surface

mounds are below the resolution of this low-frequency source and are therefore not clearly

resolved (Figure 61). Although it is sometimes difficult from seismic and bathymetric profiles to

determine if the dome-top mounded surface is composed of a series of lithified buildups or

multiple mud volcanoes, the complex structure of individual buildups as viewed on side-scan

sonar data suggests that they are not smooth-sided mud extrusion features.

Figure 61. A 3D-seismic profile oriented in a northwest-southeast direction across the GC140 dome to the mounds

that are known to contain gas hydrates in GC185. Note that the mounds on the crest of the GC140 dome

are not clearly resolved.
75

Submersible observations from the crestal area of the GC140 dome (JSL-1, Dives 2590,

2592, and 3118) clearly indicate that the mounds occupying this area are composed of authigenic

carbonates (Figure 62).

Figure 62. A photograph of the base of an authigenic carbonate mound in GC140 showing large blocks of welllithified Ca-Mg carbonate forming this feature.

Laboratory work on these carbonates shows that they are extremely 13C-depleted, identifying an

origin linked to the microbial utilization of hydrocarbons (Roberts et al. 1992a, b). The

mineralogy of the carbonates in GC140 is primarily Mg-calcite with some dolomite. Close

inspection of the carbonates reveals the presence of extremely biodegraded crude oil in isolated

pores. However, carbon isotope values from carbonate cements suggest that methane was the

main contribution of isotopically light carbon (12C) to the carbonates rather than biodegradation

of crude oil (Roberts and Aharon 1994). The buildups are composed of chaotically oriented

blocks and clasts that are 20-100 m in diameter, up to 20 m high, and most have relief in the 10-

15 m range. Surrounding sediments can be quite coarse, containing a high proportion of

diagenetic clasts and shell debris. The mounds are cut by fractures and crevices probably related

to post-mound movement of underlying salt. Only a single rudimentary community of

chemosynthetic organisms was found at this site (Figure 63), living on products expelled through

a deep crevice.

Figure 63. A crevasse in a GC140 authigenic carbonate mound. Note the rudiments of a small chemosynthetic

community living in a fracture through the mound.
76

Evidence of gas or crude oil leaking into the water column was not present in the GC140 area,

although the presence of both is indicated by the authigenic carbonates. This area appears to

have been one of very slow seepage over a long period of time. Radiometric dates of the

buildups (Roberts and Aharon 1994) indicate some mounds date at nearly 200 kyr BP.

5. Feature Synopsis and Interpretation

The mounded character of the GC140 dome top is clearly displayed on side-scan sonar

data. The array of numerous hard targets with highly reflective and irregular surfaces

distinguishes these mounds from a field of fluid mud expulsion features. In addition, the highly

reflective seafloor, as viewed on high resolution seismic records, tends to support an

interpretation of a hard bottom, possibly lithified. This response is in contrast to that of a gasprone mud-rich sea floor, that would likely exist at the site of an active field of mud volcanoes or

other types of fluid and gas explusion features. A key source of supporting data in this regard is

3D-seismic surface amplitude data. Figure 64, a map of 3D-seismic surface amplitude data from

the GC140 dome, shows a close relationship between the greatest deviation of amplitudes from

background and the distribution of carbonate mounds on the dome top (Figure 60).

Figure 64. A 3D-seismic surface amplitude map of the GC140 area.

Profiles of 3D-seismic data across this area, such as the one in Figure 61, do not show a phase

reversal across the mound crest as in areas of gas-charged sediments in adjacent areas of GC185.

The (a) significant surface amplitude shift away from the character of the surrounding sea floor,

no phase reversal across the dome crest on the 3D-seismic profiles, (b) a highly reflective and

mounded surface on high resolution seismic, and (c) discrimination of complex mounds with

irregular surfaces and well-defined and steep sides on side-scan sonar data identifies these

features as lithified buildups as opposed to mud mounds or other sediment-derived topographic
77

features. Submersible groundtruth and subsequent laboratory work on mound samples confirms

that these features are by-products of microbial degradation of hydrocarbons delivered to the

surficial sediments slowly in small amounts. As a general statement, these features in GC140 are

not associated with chemosynthetic communities other than small bacterial mats (Beggiatoa sp.).

Acoustic characteristics, seismic and side-scan sonar, of the GC140 mounds are tabulated in

Tables 24 to 26.
78

Table 24. Side-scan sonarim age characteristics forGC140 case study.

Feature Feature Field Flanking Areas

Acoustic Backscatter

Strong X

M oderate

W eak X

M ixed

TargetShapes

Bum ps

Cones

M ounds X

Pinnacles

Depressions

Irregular

TargetSurface

Sm ooth X

Irregular X

Variable X

Feature O ccurrence

Isolated X

Asym m etric G roups X

Linear

Circular

Elliptical

Polygonal X

Table 25. Seism ic facies forGC140 case study.

Seism ic Facies Feature Feature Field Flanking Units

G ently Divergent-Parallel X

Strongly Divergent X

O nlapping

Drape

a. Layered

b. Acoustically Turbid

Sigm oid Progradational

O blique Progradational

Chaotic X

Contorted-Discordant

Acoustically Turbid X

a. Com plete

b. Stacked Zones

c. Chaotic Discontinuous Reflectors X

(1) Parallel

(2) Parabolic

(3) M ixed X

Table 26. Surface reflection characteristics forGC140 case study. F = feature;FF = feature field;FA feature adjacentarea.

3.5 kH Pinger Interm ediate Source 3D Seism ic

F FF FA F FF FA F FF FA

Surface Reflection Strength 

Strong X X

M oderate X XX

W eak X

Variable

W idth ofSurface Reflectors (M S)

0-2

2-5 X X

>5 X X XX

Variable

Reflector Characteristics

Sim ple Doublet

Stratified XXX

Prolonged X

Chaotic XX

W indowed X

BrightSpots

Surface Reflection G eom etries

Planar XXX

W avy

M ounded X X X

Scarped

Isolated Pinnacle/s

Isolated Depression/s

Parabolics

Isolated X

M ultiple X

O verlapping X X

Phase Inversion
79

G. Case Study: Mound with Gas Hydrates and Chemosynthetic Communities, Green

Canyon Block 185

1. Introduction

As part of a drilling hazards survey for the GC184 Jolliet field conducted by McClelland

Engineers, Inc. in 1985, the mound-like feature on the block border between GC184-185 (27°

46.97’N; 91° 30.47’W) was initially surveyed (Figure 65).

Figure 65. Location map for Green Canyon Block 185 (GC185).

Later in 1985 (November) a high resolution survey, including deep-tow data, was conducted by

John Chance and Associates in support of Pipeline survey from GC184 to the Marquette Field

(GC52-53). This feature is usually referred to as a single mound, but in fact it consists of two

mounds with the northern mound being the larger of the two. The tops of these mounds occur in

water depths of about 540 m to 560 m respectively (Figure 66). The larger of the two mounds

has a relief above the surrounding seafloor of over 30 m. These mounds occur to the southeast

of the large GC140 dome which rises over 300 m above them. The mounds are located in a

trough-like regional bathymetric low that is oriented roughly north-south. Roberts et al. (1999)

have shown that currents in the vicinity of the GC185 mounds are topographically steered and

therefore are biased in north-south directions. The GC185 area is very close to the shelf edge

(approximately 20 km) and was a region of rapid deposition from shelf edge sediment sources

during the Pleistocene (Cook and D’Onfro, 1991).
80

Figure 66. Bathymetric map of the GC185 mound complex indicating that two distinct mound-like features exist

along the GC184-185 border.

2. Geologic Setting

Cook and D’Onfro (1991) clearly illustrate that the GC140 dome is related to the shallow

emplacement at allochthonous salt and that faulting associated with basin formation and salt

migration accounts for the mounded feature on the GC184-185 boundary which is the subject of

this case history.

During the Nebraskan and Kansan stages of the Pleistocene, lowstand deposition of large

volumes of sediment in this area triggered underlying salt sheet deformation and movement by

differential sediment loading. Salt began to rise in the sedimentary section while salt withdrawal

promoted interslope basin formation. (Figure 67). Faults started forming in association with the

rising diapirs which probably initiated the flux of fluids and gases to the seafloor. Throughout

the Pleistocene, fluctuations of sea level promoted cyclic filling of the rapidly subsiding basins

with coarse facies during lowstands and fine-grained and carbonate-rich hemipelagic sediments

during highstands. During the final stages of the Pleistocene (Illinoisan) to present the diapir

under GC140 continued to rise to a near seafloor position while subsidence continued in the

GC185 basin. Faulting occurred in the tensional environment of the rising GC140 dome

resulting in an area over the salt characterized by numerous faults of very limited subsurface
81

penetration. The eastern GC185 area, however, was broken into several fault blocks. A

prominent reverse fault (Figure 67) created the conduit that fed fluids, sediment, and gas to

construct the mound complex that is the subject of this case study.

Figure 67. Schematic figure representing the present salt and sediment subsurface configuration of the GC140, 184,

and 185 area. Faults and resulting seep-related seafloor geology is shown on this diagram modified from

a study by Cook and D’Onfro (1991).

3. Available Data

As in the case of the GC140 dome, the original high resolution geohazards data sets were

acquired by John Chance and Associates, Inc. using a precision echo sounder (EDO Western

Model 4077) for bathymetry and a shallow subbottom profiler (ORE 3.5 kHz pinger) for

evaluating the near-bottom subsurface. In sediments not charged with gas, penetration of this

profiler commonly reached 60 m. Medium-range subsurface data were acquired using a minisleeve exploder which had the acoustic energy to penetrate up to 250-300 m. A sparker system

was used for deeper penetration and was used for the collection of both single channel and multichannel data. Side-scan sonar data were initially collected with an SMS 960 EG&G digital sidescan system. All geophysical survey data were spatially controlled with STARFIX navigation

provided by John Chance and Associates with a spatial accuracy of < 5 m. A 3D-seismic survey

was acquired by CONOCO that covers both the Jolliet (GC184) and Marquette (GC52) areas.

This project was given access to parts of this 3D-seismic data set for incorporation in this report.

In association with this project, both 50 in3

 air gun (Seismic System GI gun) and 15 in3

water gun (Seismic Systems Model S-15) profiles were acquired by the author across the GC 185

mound complex. The data were collected in digital format using the DELPH 2 high resolution

seismic data acquisition and processing software system. Analog records were recorded on an

EPC Model 9800 seismic recorder while the digital data were written to Exobyte tapes.

Hydrographic Surveying software used on this project for survey planning was HYPACK by

Coastal Oceanographics, Inc. Navigation data for input to the program were provided from

differential GPS, a Magnavox M4200d which has an accuracy of 3-5 m.
82

4. Feature Characteristics

The mound complex on the border of GC184 and GC185 occurs southeast of the large

GC140 dome. The mounds are clearly visible in the regional bathymetry where the larger

northern-most mound (commonly referred to as “Bush Hill”) rises over 30 m above the

surrounding seafloor to a depth of approximately 540 m (Figure 66). This mound is slightly less

than 1 km wide at the base and has a roughly conical shape when viewed in cross section, as is

illustrated in the 15 in3

 water gun profile shown in Figure 68.

Figure 68. High resolution seismic profile run E-W across the northernmost GC185 mound. Note the mound’s

acoustically opaque interior, up-turned bedding on its flanks, and highly variable surface return.

Originally, Neurauter and Bryant (1990) attributed the origin of this mound to heave or

diapiric processes rather than vertical accretion as in the case of mud volcanoes. This

interpretation was based primarily of high resolution seismic data which illustrated an

acoustically opaque interior and upturned bedded sediments on the east and southeast side of the

feature. Inspection of 3D-seismic profiles through the mound complex suggest a long history of

vertical accretion, with the acoustically opaque mound of today being just the latest chapter.

However, clear evidence of sediment accretion as a product of mound growth is difficult to
83

identify in high resolution profiles like the one shown in Figure 68. Although deep-tow data

were not available for this feature, surface-tow seismic profiles demonstrate three important

mound characteristics, (1) the mound has an acoustically opaque interior, (2) surface reflectivity

is highly variable, and (3) bedded sediments that truncate against the acoustic wipe-out zone tend

to be upturned and thin away from the feature. Side-scan data from the mound surface

(MacDonald and Schroeder, 1993; MacDonald et al., 1996) display a high degree of spatial

variability in reflectance patterns and reflectance strengths. Hard targets are of a scale that can

easily be detected within the typical 100 m swath of a single side-scan channel. Most highly

reflective targets have cross-sectional dimensions that fall within the 10-50 m range.

Seafloor verification data of the GC185 mounds were acquired through JSL-1, Dives

3300, 3301, 4063 and JSL-2, Dives 2640 and 2647. The GC185 mud vent site was the subject of

data on JSL-2, Dive 2787 and 2899 and JSL-1, Dive 4062.

5. Feature Synopsis and Interpretation

At first inspection of high resolution seismic profiles across the prominent northern

GC185 mound, it appears simply as an acoustically opaque feature, the same as many such

features observed in geohazard surveys from the northern Gulf’s upper slope. However, when

viewed in different cross sections and with different acoustic sources it is apparent that this

feature exhibits a high degree of surface reflectance variability, even on surface-tow data. Sager

et al. (1999) confirm this variability of acoustic response from 25 kHz profiles acquired near the

bottom using a submarine (NR-1). Side-scan sonar data (Figure 69) corroborate the variations in

surface reflectance observed on profiles generated form various seismic sources. Fields of hard

targets cluster around the apex of this feature which occurs at water depths ranging between 550-

540 m. These hard targets are scattered throughout the crestal area of the mound which is

steeper on its northwestern flank. Therefore, this field of hard targets is biased toward the

northwest portion of the overall feature.

Figure 69. A mosaic of 77 kHz side-scan sonar data collected from the northern GC185 mound (Bush Hill) using

the manned submersible NR-1 (MacDonald et al., 1995). Note the complex field of hard targets imaged

in this mosaic and the scales of these features.
84

Extraction of surface amplitude data from a 3D-seismic survey over the area indicates

that the main GC185 mound and surrounding region exhibit three areas where amplitude strength

is far above the surrounding seafloor (Figure 70).

Figure 70. This 3D-seismic surface amplitude map of the GC185 mound complex clearly identifies areas that

deviate significantly from the background of seafloor surrounding the mounds. Note that the highest

deviations correlate with the highest topography except for the western anomaly which correlates with an

active mud vent that constantly leaks gas into the water column.

The northernmost high amplitude zone correlates well with the zone of variable reflectance on

high resolution seismic and side-scan sonar records. Analysis of both digital high resolution

profiles and 3D-seismic profiles of the main GC185 mound indicates a phase inversion over the

highest amplitude zone as compared to the seafloor surrounding the mound (a positive to

negative polarity shift), Figure 71. The same phase inversion occurs for the other two high

amplitude zones. The one to the south represents the crestal area of a smaller mound

(27°46.14’N; 91° 30.32’W) and the western anomaly is an active mud vent (27° 46.17’N; 91°

30.38’W). All of these zones represent areas where gas is trapped in the shallow subsurface and

is also being actively vented from the seafloor.
85

Figure 71. A 3D-seismic profile across the northern GC185 mound (Bush Hill) illustrating (a) the phase reversal of

the surface reflector, (b) subsurface accretion wedge on flank of the acoustic wipe-out zone, (c) a

prolonged acoustic wipe-out zone, and (d) apparent “pull-up” of north side reflectors entering the

acoustic wipe-out zone.

Over a decade of direct observation and sampling of the GC185 mound, using a variety

of manned submersibles, has produced a sizeable data based on surface features and conditions

(Brooks et al., 1985; Kennicut et al., 1985; Roberts et al., 1992; MacDonald et al., 1996). The

GC185 mound has an extremely variable suite of surface features, as the high resolution

geophysical data suggest. In the north and northwest sectors where the highest parts of the

mound are located, the bottom is characterized by bush-like colonies of tube worms, mussel

beds, outcrops of authigenic carbonate from slabs several meters in diameter to small nodular

masses in and on the surface of the sediments, and mound-like outcrops of gas hydrates (Figure

72).

Figure 72. These three photographs are representative of seafloor conditions and communities typical of the high

topography of both the northern and southern GC185 mounds: (a) symmetrical colonies of

vestimentiferan tube worms (approximately 1.5 m high), (b) bathymodiolid mussels, vestimentiferan

tube worms, and slabs of authigenic carbonate are commonly associated at sites of thriving

chemosynthetic communities, and (c) an outcrop of gas hydrate.
86

Areas of the mound which have these characteristics correlate with (a) areas of high reflective

and variable scale targets on side-scan sonar records, (b) variable reflectivity of the seafloor on

seismic, and (c) a very high amplitude response as well as phase inversion on 3D-seismic surface

amplitude maps and seismic profiles suggesting the presence of gas at the seafloor and in the

shallow subsurface. Numerous areas have been identified where both gas and crude oil are being

vented into the water column. Flanks of the mound and the saddle area between the larger

northern mound and its smaller southern counterpart are represented on the 3D-seismic surface

amplitude data as slightly deviating from surrounding seafloor amplitudes. However, high

amplitude areas associated with the apex of the smaller mound, Figure 70, have similar seafloor

characteristics as the larger mound with the exception of gas hydrate outcrops. Figure 72

illustrates typical seafloor scenes in the gas-prone high amplitude areas of the mound.

The strong amplitude anomaly to the west of the mound complex is different (Figure 70).

This area consists of an active gas and fluidized mud venting site. Since its discovery in 1992

based on 3D-seismic surface amplitude data, the site has been visited several times. Each dive

on this site (mostly with the Johnson Sea-Link) has confirmed both active gas venting and

expulsion of very light-gray fluid mud (Figure 73). No diverse chemosynthetic communities are

associated with this site. Although, a few disarticulated lucinid-vesycomyid clam shells can be

found on old flows with oxidized brown surfaces. On a high resolution seismic profile acquired

through this feature, it appears as a graben-like depression.

Figure 73. This bottom photograph of the mud vent at the base of the GC185 mound illustrates the active escaping

gas and mud extrusion taking place at the site. The field of view is about 3 m wide.

In summary, the mound has been confirmed as a site of scattered, but abundant

chemosynthetic communities that become more closely spaced toward the apex areas of both the

large and small mounds. Within these areas expulsion of gas as well as crude oil into the water

column can be observed. High resolution acoustic data reflect the scattered seafloor features

(chemosynthetic communities, outcrops of authigenic carbonates, partially lithified bottom

sediments, and gas hydrates). The 3D-seismic surface amplitude data match these areas
87

beautifully, but appear to be responding to surface and near-surface gas since a phase inversion

on 3D-seismic profiles is coincident with these zones. The mud vent area on the west side of the

mound complex produces the same amplitude and phase response. The phase inversion over

these areas is also clearly visible in the digital high resolution seismic profiles taken as a part of

this study when analyzed in “wiggle trace” format. Since this feature has an acoustically opaque

interior, the interface of bedded sediments with the mound may hold information about the

interior character of this feature. Gas-charged sediments may cause local “pull down” because

acoustic energy travel slightly slower through this less dense medium. A mound interior

characterized by gas hydrate and/or lithified sediments may cause “pull-up” in the interface zone

because of slightly faster travel times through this denser medium. A slight pull-up is suggested

on the profiles illustrated in Figures 68 and 71. Acoustic characterisitc of the GC185 mound

area summarized in Tables 27 to 29.
88

Table 27. Side-scan sonarim age characteristics forGC185 case study.

Feature Feature Field Flanking Areas

Acoustic Backscatter

Strong X

M oderate X

W eak

M ixed X

TargetShapes

Bum ps X

Cones

M ounds X

Pinnacles

Depressions X X

Irregular X

TargetSurface

Sm ooth

Irregular X

Variable X

Feature O ccurrence

Isolated X

Asym m etric G roups

Linear

Circular

Elliptical

Polygonal

Table 28. Seism ic facies forGC185 case study.

Seism ic Facies Feature Feature Field Flanking Units

G ently Divergent-Parallel X

Strongly Divergent

O nlapping

Downlapping

Drape

a. Layered

b. Acoustically Turbid

Sigm oid Progradational

O blique Progrdational X

Chaotic X

Contorted-Discordant

Acoustically Turbid X

a. Com plete X

b. Stacked Zones

c. Chaotic Discontinuous Reflectors

(1) Parallel

(2) Parabolic

(3) M ixed

Table 29. Surface reflection characteristics forGC185 case study. F= feature;FF -feature field;FA = feature adjacentarea.

3.5 kH Pinger Interm ediate Source 3D Seism ic

F FF FA F FF FA F FF FA

Surface Reflection Strength 

Strong X

M oderate

W eak X

Variable

W idth ofSurface Reflectors (M S)

0-2

2-5 XX

>5 X X X X

Variable XX

Reflector Characteristics

Sim ple Doublet X X

Stratified X X

Prolonged X X

Chaotic

W indowed

BrightSpots X

Surface Reflection G eom etries

Planar X X X

W avy

M ounded X X X

Scarped

Isolated Pinnacle/s

Isolated Depression/s

Parabolics

Isolated

M ultiple

O verlapping

Phase Inversion X X
89

III. Summary of Project Results

Through the analysis of numerous data sets taken for geohazards surveys or engineering

projects, review of 3D-seismic data, and seafloor verification from research submersible

observations it has become clear that the rate of supply of fluids (including fine-grained

sediment) and gases to the seafloor largely controls geologic and biologic responses. In this

section of the report, a summary of project results will be presented. This section of the report

will first will summarize the general observations concerning geologic and biologic responses to

various hydrocarbon venting and seepage settings. This interpretive section is designed to

provide a simple framework for understanding the geologic and to a lesser extent the biologic

variability of hydrocarbon seepage and venting sites. Along with this summary characteristic of

each major part of the feature spectrum will be presented for interpreting this variability from

high resolution acoustic data (seismic and side-scan sonar) and 3D-seismic surface attribute data.

A. Geologic and Biologic Responses to Venting and Seepage

As previously discussed in the introductory parts of this report, the geology of the

northern-northwestern Gulf’s continental slope is complex and sometimes bewildering when

viewed in a high resolution perspective. Although domes, ridges, and basins dominate regional

slope topography, at a smaller scale slope failures, mounds, depressions, and scarps become

apparent. When a focus is placed on those seafloor features judged to be products of fluid and

gas expulsion they always appear to be situated over or near well-defined faults and

accompanying subsurface acoustic wipe-out zones (Behrens, 1988; Roberts et al., 1990). On

high resolution seismic and side-scan sonar records it is common to observe alignments of pock

marks, mud volcanoes, small mud flows, and a variety of mounded features. This linearity in the

spatial distribution of these features caused by fluid and gas expulsion can usually be directly

correlated to a fault that intersects the modern seafloor (Roberts, 1995). Figure 74 illustrates the

alignment of authigenic carbonate mounds along a fault crossing Garden Banks Block 304.

Figure 74. Authigenic carbonate mounds are aligned along a fault that is clearly visible in high resolution seismic

data. This relationship confirms the importance of faults as conduits for the transport of fluids and gases

to the modern seafloor.
90

This close relationship between faults and venting/seepage-related features emphasizes the

importance of faults as conduits for fluids and gases moving upward through the sedimentary

column to the seafloor. The actual mechanics of this process are poorly understood and this

subject is currently under investigation (e.g. Roberts and Nunn 1995; Roberts et al. 1996).

Studies of reservoir recharge by hydrocarbons migrating vertically along faults have

demonstrated that fluid and gas movement up faults can involve an extremely complex set of

geologic conditions including pressure release from overpressured zones at depth (Anderson et

al. 1991; Anderson 1993). Considering our present state of understanding concerning fluid and

gas transport along faults, there is little argument that faults function as conduits. It is also clear

that formation waters, hydrocarbons, fine-grained sediments, and gases are transported from

depth at various rates to the modern seafloor. Kohl and Roberts (1994, 1995) have convincingly

demonstrated that relic microfossil assemblages are being actively spread over the continental

slope in some areas through the extrusion of fluid muds that originated at considerable depths

beneath the present ocean bottom. Even though these vent-seep features are commonly and

clearly associated with faults, the controls on frequency of fault movements and flux rates are not

known. The consistent presence of oil slicks over the continental slope, as viewed from satellite

imagery (MacDonald et al., 1993, 1994) suggests that the expulsion of crude oil and gas is an ongoing process that may be episodic on the short term, but is consistently impacting the geology

and biology of the Gulf’s continental slope.

1. Feature Spectrum

From approximately a decade of direct observations of the hydrocarbon-impacted seafloor using

research submersibles (e.g. Brooks et al., 1985; Roberts et al., 1988; MacDonald et al. 1995)

coupled with the systematic analysis of remotely sensed data of the slope surface and shallow

subsurface associated with this study, it is clear that geologic and biologic responses are highly

variable. Compilation, review, and interpretation of data assembled in support of this MMSCMI study have shown that there is a relationship between the qualitative rate at which ventingseepage products are delivered to the seafloor and both geologic and biologic responses.

Although rates of delivery of these products have never been satisfactorily quantified, episodic

releases of fluids and gases from deep overpressured subsurface environments have been

recorded in Gulf Coast Basin mineral deposits (Cathles and Smith 1983; Cathles 1990) and have

been predicted from numerical modeling (Roberts et al. 1996). Perhaps the best example that

relates to a discussion of Louisiana’s continental slope comes from the occurrence of thin sulfide

layers in the anhydrite caprock of Winnfield salt dome (Ulrich et al. 1984). These sulfides are

interpreted as products of the episodic release of reducing waters from geopressured zones deep

in the stratigraphic section. Sulfides are precipitated at the salt-anhydrite interface when

reducing fluids are released from the subsurface by way of faults along the flanks of the dome

(Hallager et al. 1990). Paleomagnetic dating of these sulfides suggests that expulsion events may

have occurred at a frequency of around 300 years (Kyle et al. 1987). These results are

compatible with recent numerical modeling solutions that suggest that fluid expulsion from

geopressured sediments of the northern Gulf of Mexico occurs in short-lived events (Roberts and

Nunn 1995). This and other studies (Ranganathan and Hanor, 1989; Roberts and Nunn 1995)

suggest that expulsion events create local, transient anomalies in overlying sediments in short

periods of time, perhaps less than 100 years. A recent two-dimensional finite element model of a

South Eugene Island Block 330 area sedimentary basin (Roberts et al. 1996) suggests that excess
91

fluid pressure in abnormally pressured sediments would drop to approximately half its original

value after about 10,000 years of expulsion. They also estimate by variable fault permeability

simulations, where compaction of fault zone sediments closes the fracture network, that fault

permeability decreases by 1-2 orders of magnitude within 200 years after fluid expulsion begins.

In order to produce the thermal and baric anomalies observed in the subsurface overlying the

South Eugene Island area geopressured zone Roberts et al. (1996) suggest that faults must

remain open for 20-30 years. These model results support episodic expulsion of fluids and gases

from geopressured zones and for the first time provide insight into timing for these events from

numerical modeling.

Even though only qualitative data concerning flux rates and durations of delivery events

exists for the Gulf’s hydrocarbon seeps and vents, it is clear some areas are dominated by rapid

venting of hydrocarbons and associated fluid plus sediment while other areas are affected only

by very slow seepage. The terms venting and seepage are used in this report as qualitative

expressions of comparative rates at which hydrocarbons, fluidized sediment, and other carrier

fluids are delivered to the modern slope surface. Venting implies rapid delivery while seepage

suggests slow flux rates. Figure 75 summarizes, in a schematic way, the geologic responses to

the flux rate spectrum. On one end of the spectrum are those features that result from the rapid

expulsion of large volumes of fluid mud frequently containing crude oil and hydrocarbon gases.

On the other end of the spectrum are features related to the very slow seepage of hydrocarbon

gas or hydrocarbon-charged (gas and/or crude oil) waters. In this setting, precipitation of

calcium and magnesium carbonates, as well as exotic minerals like barite takes place (Roberts

and Aharon 1994). While some features clearly have been produced in a single flux rate

environment (rapid flux, slow flux, or somewhere between the two extremes), it is also clear that

some areas of seafloor have evolved from one extreme to the other because of a change in the

rate of the fundamental forcing process, vertical flux of fluids and gases. Enough comparative

work has now been completed, as a part of this investigation between acoustic response on high

resolution seismic profiles and direct observations of the seafloor, to know that most areas of

hydrocarbon venting-seepage are represented as zones of acoustic wipe-out or acoustic turbidity

(Roberts 1995, 1996). A goal of this MMS-CMI research is to build a better understanding of

the geologic and biologic variability that these zones represent and develop criteria for

interpretations using a high resolution acoustic and 3D-seismic data base.

Figure 75. A schematic illustration of the relationship between rate of delivery of fluids and gases to the seafloor

and geologic/biologic response.

Table 30 summarizes the general relationships between vent-seep features and

chemosynthetic communities with delivery rate of hydrocarbons plus carrier fluids (including
92

fine-grained sediment). End member and transition cases are discussed below starting with the

rapid delivery, mud-prone case.

Table 30. General relationships between delivery rate of fluids and gases to the seafloor and response in seafloor

geology, biology, and hydrocarbon degradation.

Flux Rate Rapid (Venting) Transitional Slow (Seepage

Feature Spectrum Mud Prone ––––––––––––––– Mineral Prone

Dominant Mud Volcanoes Gas Hydrate Mounds Authigenic Carbonate

 Feature Types Mud Flows (Isolated Authigenic •Mounds/Hardgrounds

Gas Expulsion Features Carbonates) •Crusts/Nodules

Small-scale Gas/Fluid Mineralized Cones/Chimneys

 Expulsion Features

__________________________________________________________________________________________________

Chemosynthetic Localized Dense, Diverse, Widespread Very Localized

 Communities •Bacterial Mats •Bacterial Mats •Bacterial Mats

•Lucinid Vesycomyd Clams •Lucinid/Vesycomyid Clams •Lucinid/Vesycomyid Clams

•Pogonophoran Tube Works •Pogonophoran Tube Worms •Vestimentiferan Tube Worms

•Bathymodiolid Mussels Bathymodiolid Mussels

___________________________________________________________________________________________________

Hydrocarbons and Non-biodegraded Moderately Biodegraded Highly Biodegraded

 By-product* •Cruide Oil •Crude Oil •Crude Oil

•Thermogenic Gases •Thermogenic Gases •Thermogenic Gases

•Biogenic Methane •Biogenic Methane •Biogenic Methane

HS and H2S HS and H2S (limited)

____________________________________________________________________________________________________

* The types of hydrocarbons found at any given site depend on the numbers and types of sources being tapped by the delivery system(s).

a. Mud-Prone Cases Hedberg (1974) recognized that the vertical migration of gas and

fluids through unconsolidated-to-semiconsolidated sediments may create a slurry-like mixture of

sediment, gas, and water that can produce mud volcanoes and other fluid expulsion features at

the seafloor. Mud volcanoes and extruded mud sheets have been observed from many different

geological settings (Hovland and Judd, 1988) ranging from areas where accretionary prisms are

being degassed and dewatered by the process of subduction (e.g. Barber et al. 1986; Breen et al.

1986; Brown 1990; Henry et al. 1990; Orange and Breen 1992; Zhao et al. 1986) to areas of

active salt deformation like the Gulf of Mexico (Neurauter and Bryant 1990; Neurauter and

Roberts 1992, 1994). They have also been reported from Trinidad (Arnold and Macready 1956)

the Caspian Sea (Hedberg 1974), the Black Sea (Ivanov et al. 1989) Beaufort Sea (Pelletier

1980), offshore Panama (Reed et al. 1990), the Arabian Sea (Hovland and Judd 1988), the

Northeast shelf of New Zealand (Nelson and Healy 1984), as well as many other places.

An example of a mud-prone feature from the northern Gulf of Mexico slope that has

formed in a rapid flux regime is the active mud volcano in the north central part of Green

Canyon, Block 143 (Neurauter and Roberts,1994). This feature (Figure 76) has formed in a

water depth of ~ 335 m, is cone-shaped, has built to a height of 35 m above the surrounding

seafloor, and has a base of about 450-500 m diameter. Short cores into the sides of this feature

indicate that it is composed of fine-grained sediment (clay to fine silt sized mud) with no
93

indication of post depositional cementation at the surface. However, a similar but dormant mud

volcano in GC53 (Neurauter and Roberts 1994), discussed as a case study in this report, displays

lithification, as exposed in an eroded gullies on the cone flank. This lithification occurs in

multiple layers as if the deposition of cone-building mud was episodically turned-on and turnedoff allowing time for surficial lithification, at least in localized areas. Both cases, GC53 and

GC143 clearly indicate demonstrate that these features are acoustically opaque on high quality

geohazards-scale seismic profiles.

Figure 76. (a) A high resolution seismic profile (15 in3 water gun) across a mud volcano in Green Canyon Block

143. (b) This submersible-acquired picture shows gas escaping from the central crater of the GC143 mud

volcano. The white streaks on the fluid mud surface are Beggiatoa bacteria.

The GC143 mud volcano has been observed and sampled on four occasions (JSL-1,

Dives 2587, 2588, 3115, and 3298). On a 1992 dive (JSL-2, Dive 3298), fluid mud saturated

with crude oil and gas was actually spilling from the central caldera-like crater (approximately

50 m diameter) onto the flanks of the core. During other observation periods, gas was escaping

from the central crater. The fluid mud in the crater was being convectively mixed as interpreted

from the swirl-like patterns of thin orange and white bacterial mats (Beggiatoa sp.) on the mud

surface. Escaping gas probably forced the convective mixing and caused fine-grained sediment,

gas, and droplets of crude oil to be entrained in the overlying water column. Terrace-like

features around the mud-filled central depression indicate the dynamic nature of the fluid mud

level. No macroscale chemosynthetic organisms were observed in the central crater or on the

cone flanks. Kohl and Roberts (1994, 1995) have demonstrated that this process causes fossil

foraminifera to be spread over considerable areas on the modern slope. In the GC 143 mud

volcano case, late Pleistocene foraminifera are being deposited. At other sites, for example the

mud expulsion area in GB 338, discussed as a case study in this report, microfossils as old as

early-to-middle Miocene are being deposited at the surface (Kohl and Roberts, 1994). At this

site large quantities of extruded mud and associated slope failures have transported sediment

nearly 10 km downslope (Figure 21). The case studies of the GC53 and GC185 mud vents are

similar in that gas and fluidized sediment are being forced to the modern seafloor where the

sediment spreads into well-defined sheets.
94

In areas where the seafloor is impacted by a rapid flux of fluidized sediment, complex

chemosynthetic communities have problems becoming established because of unstable substrates

and high sedimentation rates, even though the necessary trophic resources may be available. In

fact, sediments from rapid flux sites commonly contain hydrocarbons that have experienced

minimal biodegradation. The soft sediments of venting and seep environments may be inhabited

by chemosynthetic lucinid-vesycomyid clams and occasional tubeworms. Both employ bacterial

symbionts to derive food from the oxidation of hydrogen sulfide, and both require relatively firm

sediments in order to maintain a stable life position. It is easy to appreciate that life position,

literally how an animal is positioned so as to survive in an environment, is especially important

for species exploiting the gradients of two toxins, oxygen and hydrogen sulfide. Lucinids are

typically infauna, living within sulfide-rich sediments and maintaining respiratory contact with

oxygenated water via a burrow and siphon. The vesycomyid live at the sediment-water interface,

extending a foot down into sulfide-rich sediment while a short siphon keeps the body

oxygenated. Pogonophorans (tube worms) dwell in a tube which extends into the sulfide rich

sediments, leaving the anterior end exposed a few centimeters above the seafloor. In rapid flux,

mud-rich environments, lucinids are the most common members of the infauna that exploit

rapidly deposited sediments from hydrocarbon vents while Beggiatoa sp inhabit the surface.

Sheets of crude oil soaked fluid mud extruded from mud volcanoes and other types of expulsion

sites commonly display scattered lucinid shells on the surface. Cores through fluid mud deposits

reveal concentrated layers of lucinid-vesycomyid clam shells separated by mud containing few

infauna constituents. These observations suggest episodic influx of fluid muds burying the

lucinid shells that probably represent the remains of a community that had depleted its trophic

resource, hydrogen sulfide.

Sites of rapid venting with fine-grained sediment deposition both from suspension and as

fluid mud are extremely hostile environments for all fauna. Suspended sediment deposition can

restrict respiratory organs and blanket bacterial mats. Fluid muds make life positioning

impossible except for bacteria which might float at the interface. These rapid flows may come

from reservoirs with little sulfide and flow rates preclude sulfide-producing anaerobic oxidation

of hydrocarbons. These conditions are consistent with the observations of Sassen et al.(1994)

that hydrocarbons in such venting are not highly biodegraded. Infaunal lucinids could not

survive in the fluid muds, but might colonize some areas of greater sediment stability on the

periphery of the feature so long as a sulfide source is present. Therefore, areas of the continental

slope where fluid mud is being rapidly forced to the surface by pressure release in the deep

subsurface, buoyant forces created by gas pressure, and other poorly understood mechanisms are

regions where chemosynthetic communities are absent or poorly developed.

b. Transitional Cases - Figure 75 schematically identifies this class of feature and the

processes that produce them between the rapid venting mud-prone end numbers and mineralprone features produced by slow seepage on the other end of the feature spectrum. When

venting rates decrease from the mud-prone rapid venting case, substrates and trophic resources

become available and the seafloor environment becomes favorable to the establishment of

complex local chemosynthetic communities. Conversely, when seepage rates increase in

mineral-prone settings of slow seepage, trophic resources increase and local conditions become

favorable to complex community support. Chemosynthetic species at environmentally suitable

Gulf of Mexico vent/seep sites are vestimentiferan tube worms, bathymodiolid mussels, and
95

vesicomyid clams along with lucinid clams and pogonophoran worms (Carney, 1994). These

groups of specialized organisms constitute the complex chemosynthetic communities referred to

in this report. Along with the occurrence of chemosynthetic communities, there is evidence of

seafloor lithification, which takes a variety of forms ranging from nodular masses in sediment to

hardgrounds, slabs, and/or mound-like buildups (Roberts et al. 1992a, b).

Vent-seep related features and communities that fall into the transitional category

generally occur in water depths below ~ 500 m and are coincident with the occurrence of

surface-to-near surface gas hydrate deposits. These unusual deposits are stable-to-metastable at

temperatures and pressures associated with water depths below about 400-500 m in the Gulf of

Mexico (Brooks et al. 1985). The stability window shifts with temperature, pressure, and types

(mixtures) of hydrocarbon gases (Sloan 1990). As observed by Roberts and Carney (1997),

viable chemosynthetic communities in the Gulf’s upper-to-middle continental slope province

seem to be concentrated in the same depth ranges and at known gas hydrate sites. Only

infrequent occurrences of chemosynthetic organisms have been reported shallower than the gas

hydrate window (Roberts et al. 1990). This association is probably not fortuitous since gas

hydrate areas store the trophic resources necessary to sustain long-term and complex

chemosynthetic communities. Such communities require sustainable sources of methane and

sulfide. As Carney (1994) suggests, “solid-phase gas hydrates of methane and water, which are

stable at the pressure and temperature conditions below 400-500 m of water depth in the Gulf of

Mexico, may be that unique aspect of deep ocean methane and sulfide chemistry that allows for

chemosynthetic community development.” Although slow migration of methane through the

sediment column will cause gas-hydrates to form at the lower boundary of the

pressure/temperature stability zone (Hyndman and Davis 1992), the many faults on the Gulf of

Mexico continental slope introduce boundary instabilities causing methane to be transported

vertically to the seafloor where gas-hydrate formation takes place near the sediment-water

interface. Once at the seafloor, methane comes in contact with sea water sulfate and microbial

communities, and sulfide is generated by methane oxidation-sulfate reduction. Sassen et al.

(1993) suggest that there is a strong coupling between microbial mats (Beggiatoa sp.) common

to the seafloor over gas hydrates and hydrocarbon concentrations in the sediment. The mats

function as semi-permeable membranes that trap vertically migrating hydrocarbons beneath.

Bacterial oxidation of hydrocarbons depletes oxygen which encourages bacterial sulfate

reduction to produce hydrogen sulfide needed for Beggiatoa growth and mat development.

Sassen et al. (1993) also re-emphasize the point made by other researchers that bicarbonate and

carbon dioxide from microbial oxidation of hydrocarbons is probably responsible for initiating

the precipitation of authigenic carbonates, nodular masses in the sediment as well as slab-like

hardgrounds and more massive rocks.

The observations of gas hydrates outcropping on the seafloor (Figure 77), first made by

the author in 1991 (GC232) and later observed by other researchers (MacDonald et al., 1994),

suggest that they are localized along fault trends such as those crossing GC 232, 233, and 234, as

well as mounds like the one in GC 185 (discussed as a case study in this report). Within either of

these areas of known gas hydrate exposure, individual outcrops are only a few meters across.

This localization of surface gas hydrates reflects the high spatial variability of seafloor types that

occurs over gas hydrate areas. Within a few tens of meters one can move from a newly formed

area of outcropping gas hydrates covered with bright orange and white bacterial mats (Beggiatoa
96

sp.), Figure 78, to complex and densely arranged chemosynthetic communities of mussels and

tube worms mixed with outcrops of authigenic carbonate (Figure 78) to small areas of seafloor

where fluid muds have been extruded and lucinid clam shells litter the surface (Figure 79).

Variability of benthic communities and seafloor geology is characteristic of transitional areas

that fall between the rapid venting and slow seepage end members of the seafloor features

spectrum. Areas characterized by transition fluid and gas expulsion environments are

acoustically opaque with variable surface reflectivity on high resolution seismic profiles, and

reflect a variety of hard target spacings and geometries on side-scan sonar.

Figure 77. A small gas hydrate mound in Green Canyon Block 232 showing outcropping ledges of gas hydrate at its

base and along its flanks and orange bacterial mats on the surface. This mound is about 2.5 m in

diameter.

Figure 78. (a) This outcrop of authigenic carbonate with associated tube worms and mussels occurs in Green

Canyon Block 232. Gas was observed escaping the mussel bed in this field of view. (b) Tube worm

colonies and bacterial mats on a known gas hydrate area in Green Canyon Block 185.
97

Figure 79. Small fluid mud expulsion areas in Green Canyon Block 232 exhibit sheets of extruded mud on which

lucinid clam shells litter the surface.

Known gas hydrate features geomorphically may be associated with mounds, such as

“Bush Hill” in GC 185, oblate in plan-view but flat (GC 180-181), or elongate along a fault line

(GC 232-234). Direct observations of gas hydrate areas indicate: (a) abundant complex

chemosynthetic communities usually associated with hardgrounds and more massive authigenic

carbonate rocks; (b) occasional low-relief carbonate mounds (usually <2-3 m relief); (c)

abundant bacterial mats (both white and orange); (d) occasional outcrops of gas hydrate

breaching the surface (MacDonald et al. 1994); and, (e) small-scale fluid and gas expulsion

features. The localized authigenic carbonates (Figure 78) that are present at the best-studied sites

of known gas hydrates (GC 185, GC 232, GC 234, GC 272, and MC 709) are composed

dominantly of Mg-calcite and aragonite and frequently occur as imbricated slabs suggesting an

episodic formational history. High reflectively on both high resolution seismic profiles and sidescan sonar swaths across gas hydrate areas occur in well-defined zones characterized by seafloor

lithification and complex chemosynthetic communities.

c. Mineral-Prone Cases When rates of delivery of hydrocarbons and carrier fluids to the

ocean floor are very low, either through primary supply or by a systematic decrease from higher

rates, mineralization within surface and near-surface sediments is an important geologic process.

Positive regional relief features (domes) that occur above shallow salt masses of the upper

continental slope are common sites of mineral-prone features that result from a slow seepage

environment. Most observations are limited to < 1000 m by the maximum diving depth of

commonly used research submersibles. These sites generally exhibit complex fault networks

that have limited depth penetration because of shallow salt. Unlike carbonate buildups at the

shelf edge that have similar morphologies but are covered with thick biogenic carbonate veneers,

seep-derived authigenic carbonate buildups below depths of ~ 200 m display little encrustation

by carbonate-secreting organisms. As suggested in an earlier paper (Roberts 1992), the

carbonate buildups at the shelf edge may be seated on substrates of seep-related authigenic
98

carbonates. However, during late Pleistocene periods of lower sea level, reef-building organisms

colonized these sites because they were more environmentally suitable than under today’s

conditions, particularly because of more light for photosynthetic zooxanthellae and the absence

of a continental shelf where shallow water becomes cold during the winter because of limited

heat storage capacity.

As summarized by Roberts and Aharon (1994), most of the mineralization associated

with transitional and mineral-prone areas of the Gulf’s continental slope is represented by the

deposition of Mg-calcite, aragonite, and dolomite. These mineral phases are distinguished as

being hydrocarbon-related by their 13C -depleted compositions inherited from microbial

oxidation of hydrocarbons (both crude oil and gas) and incorporation of abundant 12C into the

various mineral phases. Similar carbonates have been reported from other seep environments

like the base of the Florida Escarpment (Neumann et al. 1988; Paull et al. 1992) and the

subduction zone off the coasts of Oregon and Washington (Ritger et al. 1987). Although

carbonates of mixed mineralogies occur at nearly every site where carbonate cementation occurs,

there is usually a clear dominance of one phase over another. The most common carbonate

mineralogy found in seep sites of northern Gulf slope is Mg-calcite (Roberts and Aharon 1994).

Unlike the mineralogy, stable carbon and oxygen isotope compositions do not occur in well

defined patterns. For example, 13C values can range 1 ‰ to values more negative than -55‰.

Values for 180 also have quite a range (1°/00 to ~ 6 ‰). This range in stable isotope values and

carbonate mineralogies presents a complex picture of formational processes.

One area, previously discussed as a case study, that demonstrates both transitional and

mineral-prone responses to various venting and seepage rates is the GC 140-185 area. Cook and

D’Onfro (1991) indicate that important faulting in this area resulted from salt movement in the

lower Pleistocene. At this time a major thrust fault was initiated by salt movement responding to

lowstand deposition of sand-rich gravity flows on the continental slope. Over Pleistocene times

these types of deposits and intervening condensed sections filled a basin created by salt

withdrawal. Figure 67 of the GC140 case study illustrates the present subsurface configuration

of salt, sediments, and major fault networks. Seafloor features associated with the two major

fault zones shown on this figure are quite different, one over a shallow salt diapir and the other

where normal and antithetic faults intersect a deep-cutting thrust fault. Numerous faults cutting

the thin sedimentary sequence over the shallow salt mass of GC 140 appear poorly connected to

the deep subsurface and therefore have provided a slow seepage environment that has produced a

complex field of authigenic carbonate mounds, as discussed above. In contrast, the antithetic

fault that has intersected a family of eastward dipping normal faults, which connect with a deepcutting reverse fault, has produced a complex of mounds containing gas hydrates. They are

interpreted as features arising from moderate rates of fluid and gas flux to the seafloor that are

intermediate between mud volcanoes and the aforementioned authigenic carbonate mounds.

The top of the regional dome expressed primarily in GC140 has been to a slow seepage

environment provided by numerous shallow-cutting faults that are connected to a deep

subsurface hydrocarbon source by a complex plumbing system. This seafloor area is

characterized by many distinct carbonate mounds ranging in relief of a few meters to over 20 m.

These mounds are composed of authigenic carbonates (Mg-calcite and dolomite primarily) that

are 13C -depleted (Figure 60) and clearly the by-product of microbial utilization of hydrocarbon
99

gas, primarily methane (Roberts et al. 1992a, b). Although a very localized occurrence of dead

tube worms and mussels was reported in a crevice (water depth ~ 290 m) in one of these features

(Roberts et al. 1990), the general absence of fossil shell debris and tube worm molds from the

mound-building carbonates suggests that seepage has been too slow to sustain a complex

chemosynthetic community, but carbonate precipitation was mediated by methanotrophic

bacteria.

The mineral-prone features of GC 140 have not only developed in a slow seepage

environment but are interpreted to have formed over a relatively long period of time.

Radiometric dates from six samples acquired from a carbonate mound in Green Canyon Block

140 range from 194.5 to 13.3 ka (Roberts and Aharon 1994). In contrast, samples from

carbonate slabs exposed at the surface of the largest gas hydrate mound (transitional between

mud-prone and mineral-prone features) in GC 185 yielded ages ranging from 3.2 to 1.4 ka

(Roberts and Aharon 1994). In addition to developing in an environment of slow seepage,

dome-top carbonate mounds of the upper slope have also developed under conditions of erosion.

Seismic profiles across these areas clearly indicate truncation of stratified units and general

sediment stripping from dome crests. Processes of erosion are poorly understood in slope-depth

environments, but expanding knowledge of the Gulf’s Loop Current indicates that currents

capable of transporting sand-sized sediment occur to depths of approximately 600 m (Hamilton

1990). Surface sediments of the GC 140 dome top contain sand and gravel-sized diagenetic

clasts and shell debris interpreted as a long-term lag deposit created by the dome-top erosional

setting. Seismically, the GC 140 area is an acoustic wipe-out zone that has a strongly reflective

and topographically variable surface.

Some areas that now are in slow seepage environments illustrate geologic and

sedimentologic characteristics that suggest a progressive evolution from a rapid venting area to a

present state of slow seepage. Good examples of areas evolving from one stage to another are

found within the previous discussed case studies from GB 338 and 382, which overly a

subsurface salt ridge. There are well-defined sites within each lease block area from which vast

quantities of mud have been expelled, creating mud flows and fluid debris flows that are clearly

recognizable on side-scan sonar data. These flowed sediments originated from two circular

expulsion centers and have been deposited over hummocky landslide-debris flow deposits that

probably developed as a result of slope instability related to movements of the underlying salt

ridge. Although no absolute dating exists for the late stage mud flows, geomorphology of these

deposits suggests they are very young. Sediment transport pathways and boundaries of

depositional lobes are still clearly visible on side-scan sonar images. In addition, the thicknesses

of the mudflow deposits diminish stratigraphically upward, suggesting a slowing of the

expulsion process. A thin (< 3 m thick) hemipelagic sediment drape occurs over much of the

present seafloor in this area but is thin to indistinguishable on high resolution seismic records

over the latest flow channels and deposits, suggesting a very young age. The thickness of

hemipelagic drape deposits outside the mudflow deposit areas is comparable to the drape on

other parts of the continental slope (Doyle et al. 1992). These highly calcareous hemipelagic

drape deposits have developed as the sources of fluvial sediments have retreated landward

following the latest Pleistocene glacial maximum, leaving the slope without a direct source of

terrigenous sediments. Therefore, the thin drape over the latest mudflows suggests that they

were deposited sometime after sea level started to rise from the last glacial maximum.
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The fluid mud expulsion centers are mounded areas that appear acoustically amorphous

on high resolution seismic profiles, but the one in GB 338, which is slightly larger than its

counterpart in GB 382 (~ 825 m vs 700 m diameter), has a flattened top. Direct observation and

sampling of these two areas has revealed that the expulsion of large volumes of fluid mud has

stopped. A few small mud volcanoes have been observed in GB 338, but they appear to only

extrude small volumes of fluid mud. At present, the seafloor of these areas is in the initial stages

of mineralization by both Ca-Mg carbonates, but primarily by more exotic mineral species such

as barite (Roberts and Aharon 1994; Fu et al. 1994). Figure 32 illustrates barite cones and

chimneys that occur on top of the GB 338 mound. Pyrite is also a common constituent in

mound-top sediments, as well as a minor component in barite and carbonate deposits (Fu et al.

1994). Equivalent features have not been found on the GB 382 mound, but Ca-Mg carbonate

crusts and slabs are abundant. These areas have evolved from producing mud-prone features to

producing mineral-prone features as the pressure drive responsible for venting of large volumes

of fluid mud has progressively waned in favor slow seepage of mineral-rich fluids. The time

frame for this transition has not been quantified. However, it seems reasonable to suggest that

the rapid expulsion of mud followed the latest Pleistocene lowstand sediment loading event. The

onset of a mineral-rich phase appears to relate to a slowing of salt movement as sediment sources

have retreated landward with rising sea level and the lowstand sediment load has largely

equilibrated with underlying salt. Seismically, both GB 338 and 382 exhibit an acoustic wipeout signature on high resolution profiles.

2. Criteria for Making Better Interpretations of Seep-Vent Related Seafloor

Features from Acoustic Data

This summary section of the report is based on the criteria used in each case study to

characterize and identify each feature used in this investigation. These criteria include (a) high

resolution seismic reflection characteristics, (b) seismic facies of the feature and surrounding

units, (c) acoustic backscatter and target shapes from side-scan sonar data, (d) 3D-seismic

surface amplitude and phase relationships, and finally (e) ground truth observations provided by

submersible work. These data are summarized for each major feature group ranging from

rapidly deposited mudflows and mud volcanoes to slowly accreted authigenic carbonate mounds.

a. Mud-Prone Cases Rapid extrusion of sediment as a part of carrier fluid and gas

expulsion tends to create a rather smooth seafloor since many small-scale bottom roughness

elements are buried by the process. Depending on the local setting, fluidized sediment extrusion

can result in sheet-like flows as discussed with regard to GC53 collapse depression, GC185 mud

vent, or the GB338, 382 mudflow case studies or in mound or cone shaped mud volcanoes as

investigated in GC53 and 143. As a general statement these features have a smooth surface

texture on side-scan sonar records with a low level of backscattered energy. Localized mud

flows tend to have rather concentric patterns since they generally originate from a central vent or

set of vents. Only in cases like the large-scale GB338, 382 flows where slope collapse is also

obviously involved would side-scan sonar records contain highly apparent flow patterns and hard

targets related to blocks of displaced sediment. Since only a few chemosynthetic communities

have been found in association with mud-prone settings, a high degree of small-scale surface
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roughness and textured changes associated with these communities is not to be expected. This

relationship is true of mud volcanoes and both small-scale and larger-scale mud sheets or flow.

Seismically, the thickness of individual mud-prone sheets is generally below the

resolution limit of commonly used high resolution seismic sources and they are positioned over a

zone that has little or no acoustic character (acoustic wipe-out zone). However, larger scale

features like mud volcanoes commonly exhibit clear downlap of mud-prone deposits onto the

surrounding slope surface because of vertical accretion from a central venting site. Even though

most mud volcanoes have an acoustically turbid character, the flanks of these features frequently

exhibit stratigraphy and downlap onto the adjacent slope surface. Reflector strength over mudprone features is moderate-to-strong with a prolonged character over obvious gas-charged areas.

On 3D-seismic surface amplitude data active mud vents and extruded mud sheets strongly

deviate from background and display a positive-to-negative polarity shift. In the case of the

GB338 and GB382 expulsion centers with associated flows, the extensive barite deposition over

the mound crests and incorporated in the fluidized flows makes them appear as hard reflective

surfaces on surface amplitude data.

b. Transitional Cases Transitional settings are characteristically variable regarding local

seafloor topography, types and numbers of small-scale bottom roughness elements, and presence

or absence of gas in surface sediments. This spatial complexity is reflected in side-scan sonar

records as groups of scattered hard targets separated by areas of seafloor with varying “texture”

or backscatter response. This high degree of backscatter and feature variability can be separated

from areas characterized, for example, characterized by multiple small mud mounds in that the

side-scan targets associated with transitional areas are generally more reflective. They consist of

hard bottoms (authigenic carbonate slabs or nodular masses in the sediments), upturned

carbonate slabs or small cemented mound-like features, outcrops of gas hydrate, numerous tube

worm colonies, small-to-large fields of mussels, small mud extrusion features with lucinidvesycomyid clams, and scattered areas of bacterial mats. Grouped in this matrix of diverse

bottom features are active gas and crude oil expulsion sites where trains of bubbles and crude oil

droplets can be observed in the water column. Roberts et al. (1998) have shown that some of this

gas derives from decomposition of surface and near-surface gas hydrates as a product of thermal

loading from the water column. In general, transitional areas at or below the gas hydrate stability

zone (~ 500 m depth in the Gulf) can be characterized as having a broad spatial array of irregular

hard targets that frequently occur in linear patterns, presumably along faults. Acoustic

backscatter strength is generally strong and “texture” changes from area-to-area are usually

moderate to high with high spatial variability. Feature outlines are irregular and usually of

limited relief (usually < 2 m high).

Seismically, the subsurface of transitional areas is basically represented as an acoustic

wipe-out zone with occasional chaotic reflection events. However, careful inspection of high

resolution seismic profiles across these areas demonstrates extremely variable surface reflection

strength, with localized zones being highly reflective. Reflector characteristics vary from

prolonged and chaotic to bright spots with windowed reflections in transitions to flanking areas.

Reflector geometries are irregular to isolated and multiple small parabolic reflectors. Sediments

flanking transitional features are generally well-stratified, commonly demonstrate divergent

facies, and exhibit both “pull-up” and “pull-down”. Areas where flanking reflectors are up-
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turned may represent areas where gas hydrates (relatively dense medium) may be present while

down-turned reflectors may reflect the slower velocities associated with free gas in the

sediments. Analysis of 3D-seismic surface amplitude data over these areas indicates the

presence of bright spots or high amplitude zones, which suggest gas in near-surface sediments.

Profile data support the interpretation of gas-charged sediments by demonstrating a positive-tonegative polarity shift or phase inversion over these areas.

c. Mineral-Prone Cases The best examples of mineral-prone cases are characterized by

areas of seafloor with a high degree of surface roughness. In the most dramatic cases, such as

the GC140 case study presented earlier in this report, surface topography is dominated by

mounds of various dimensions that are composed of 13C-depleted authigenic carbonates. On

side-scan sonar records these mounds present clear targets with sharp edges, irregular surfaces,

and variable relief. They cast well defined acoustic shadows which can indicate relief of many

meters above the surrounding seafloor. Intermound areas demonstrate a variety of backscatter

patterns, but reflectance values are much lower than the mounds themselves. Sediments

between the mounds vary from fine-grained hemipelagic muds to coarse lags of shell debris and

diagenetic clasts. On high resolution seismic profiles these mounded areas are represented as

highly reflective surfaces that are organized into isolated and overlapping parabolic reflectors.

The subsurface beneath this highly reflective seafloor is represented dominatly by acoustically

turbid or somewhat chaotic signature. Because of the interpreted low quantity of bubble phase

gas in the near subsurface of many mineral-prone sites, particularity the mounded sites, acoustic

windows occur that reveal subsurface stratigraphy. Frequently, these windows occur at

locations where considerable distance occurs between mounds or groups of mounds. Sediments

flanking mounded areas are generally upturned, eroded, and highly faulted since most mineralprone sites are located over shallow salt. On 3D-seismic surface amplitude data mounded

mineral-prone seafloor is highly reflective and stands out as compared to surrounding seafloor.

However, 3D-seismic and digital high resolution profiles across these areas show no phase

reversal and therefore indicate a hard bottom with no substantial amounts of bubble-phase gas

in the shallow sediment column.
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IV. Project Conclusions

During the course of this project detailed data sets were collected on 29 features of the

northern Gulf of Mexico upper continental slope (<1000 m water depth). Each one of these sites

was represented by a shallow subsurface acoustic wipe-out zone on high resolution seismic. In

addition to high resolution seismic, side-scan sonar, 3D-seismic, and direct seafloor verification

data sets were analyzed for each feature and a synthesis of feature characteristics derived. This

program has led to an improved understanding of hydrocarbon seep/vent-related phenomena on

the Louisiana-Texas continental slope by helping clarify the relationships between rate of

delivery of fluids and gases to the seafloor and geologic as well as biologic response. Specific

conclusions from this project are as follows:

1. Not all acoustic wipe-out zones are related to hydrocarbon seepage and venting sites.

Areas of seafloor that are extremely compacted, coarse grained (gravel lag), mounded, or

cemented (diagenetically altered) are highly reflective and scatter acoustic energy resulting in

acoustic wipe-outs on high resolution seismic records. In addition, areas where allochthonous

salt is near the seafloor with only a thin sediment drape over it can appear as an acoustic wipeout

zone, but these areas are not prone to have hydrocarbon seeps/vents and the chemosynthetic

communities that are commonly associated with them.

2. Areas of continental slope seafloor impacted by hydrocarbon venting and seepage are

nearly always represented on high resolution seismic profiles as acoustic wipe-out zones. These

zones correlate with the occurrence of gas in surface and near-surface sediments. Through an

appraisal of the variability of seafloor features within these zones, feature types can be

conveniently grouped into those that are the result of rapid delivery of fluids and gases at one

end of the spectrum to those that are the result of slow seepage on the other. The rapid delivery

features are mud-prone while the slow delivery features are mineral-prone.

3. Considering the data reviewed for this project, no areas shallower than a water depth

of ~500 m and characterized by acoustic wipe-out zones were accompanied by macro-scale

chemosynthetic communities.

4. Through an appraisal of the variability of seafloor features within areas characterized

by acoustic wipe-out zones plus fluid and gas expulsion, feature types can be conveniently

grouped into those that are the result of rapid delivery of fluids and gases at one end of the

spectrum to those that are the result of slow seepage on the other. The rapid delivery features are

mud-prone while the slow delivery features are mineral-prone.

5. Mud-prone features such as mud vents, mud flows, and mud volcanoes do not

generally support complex chemosynthetic communities, and hydrocarbons reaching the seafloor

are only slightly biodegraded. Bacterial mats and lucinid clams are the most common

chemotrophic forms. On high resolution seismic records these mud-prone features usually have

a vertical acoustic wipe-out zone (“gas chimney”) that connects the surface form to the

subsurface. Gas in the water column, as interpreted by “plumes” on high resolution seismic

records, is common. The flanks of mud volcanoes, as well as stacked mudflows, may have

internal stratigraphy detectable on high resolution seismic mounds. These mound-flanking
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sediments commonly downlap onto adjacent slope sediments. The surfaces of mud volcanoes

and mud flows are not highly reflective on side-scan sonar records. Vent areas may be identified

on side-scan sonar records by concentric patterns that reflect former fluid levels or localized mud

flows. Flow lines are sometimes visible on mud flows. Active mudflows, mud vents, and mud

volcanoes will appear “bright” on 3D-seismic surface amplitude maps, and a phase change or

positive to negative polarity shift is present when compared to the surrounding seafloor

(indicating a slower velocity caused by gas-charged sediment). Ground truth verification of

areas where fluidized sediments, fluids, and gases are being rapidly delivered to the seafloor

indicates that both regional and small-scale fluid/gas expulsion features are present and that

sediments in expulsion areas are highly charged with gas and frequently mixed with crude oil.

Evidence of chemosynthetic organisms is limited to scattered bacterial mats and lucinid clams.

6. Mineral-prone features such as mounded carbonates, hardgrounds, or barite-carbonate

encrusted areas do not support densely populated and complex chemosynthetic communities.

Hydrocarbons that reach the seafloor in these slow seepage areas, especially crude oil and

associated products, are badly biodegraded. On high resolution seismic records across these

mineral-prone areas are represented as acoustic wipe-out zones with highly reflective surfaces

(high amplitude). Commonly, complex interfingering of parabolic reflectors characterizes the

seafloor on surface-tow high resolution seismic profiles. On deep-tow records it is clear that

these surfaces are mounded. Small “plumes” of gas are occasionally observed on high resolution

seismic records in the water column above these areas, but the general case is for no gas to

appear on acoustic records. Shallow subsurface seismic returns are variable from total acoustic

wipe-out zones to alternating acoustic wipe-out and chaotic zones to windows of stratified

sediments that are usually faulted and inclined. Side-scan sonar data indicate discrete moundlike buildups that have complex and reflective surfaces. Data from this project suggest that these

buildups vary in relief above the surrounding seafloor from less than 1 m to over 20 m. Outside

the mounded seafloor areas variable backscatter intensities on side-scan records suggest variable

sediment types. On 3D-seismic surface amplitude data mineral-prone areas are highly reflective

and have no phase inversion as compared to surrounding seafloor areas. Research submersible

verification of these areas indicates the presence of authigenic carbonate buildups, coarse surface

sediment lags of diagenetic clasts, and little evidence of chemosynthetic communities or

escaping hydrocarbons. Isolated tube worm, clams, and mussels can be found in these areas, but

they are certainly not plentiful and occur in isolated patches. Most mineral-prone areas that have

thus far been studied have only marginal chemosynthetic communities. The general case is for

no macro-scale chemosynthetic communities to be present.

7. The best examples of areas of seafloor that fall between the two end members of rapid

versus slow delivery of fluids (including fluidized sediment) and gases and are in water deeper

than ~500 m. These areas are the most diverse with regard to surficial geology and biology.

High resolution acoustic data reflect this variability in spatially varying patterns of reflection

strength and backscatter. Areas of known gas hydrate occurrence at or near the modern seafloor

best fit this intermediate or transitional case. Many areas impacted by the formation of gashydrates appear as simple mounds to larger low-relief mounded complexes. Most of these areas

are characterized by complete acoustic turbidity in the shallow subsurface. Occasionally,

discontinuous and sometimes chaotic high amplitude reflection events can be observed in the

shallow subsurface. The strength of surficial returns on high resolution seismic profiles varies
105

over short distances from weak to extremely strong. Surface amplitude maps from 3D-seismic

data demonstrate a surface variability characterized by scattered “bright areas” that correlate to

phase reversals (positive to negative polarity shift) as compared to background seafloor areas.

On side-scan sonar data these areas are characterized by scattered, low-relief hard targets and

variable seafloor backscatter outside the hard target areas. Hard targets are usually of low relief

(< 2 m), have irregular shapes, and commonly occur in lineated patterns. Research submersible

verification of the surface of these intermediate flux cases verifies that the surficial variability in

geology and biology reflected in amplitude variations on seismic and numerous hard targets on

side-scan records is related to scattered tube worm colonies, mussel beds, low-relief outcrops of

authigenic carbonate, and mounded gas-hydrates.

8. The quality of interpretation of seafloor geology using remotely sensed acoustic data

is dependent on a wide variety of variables, including frequency and firing rate of the source,

towing configuration, filtering, and recording-data storage modes. Results of research associated

with this project clearly identify the benefits of digitally acquired high resolution acoustic data as

opposed to analog. These benefits primarily fall in the realm of post-data collection processing

and play-back. Digital acquisition preserves the frequency content of the data, allows for postacquisition filtering, deconvolution, stacking, and other manipulations of the data to help

understand the time character of the seafloor. Not only does digital data provide the benefits of

processing for higher quality images, but polarity changes in the data can be easily recognized

when data are displayed in appropriate format.

9. Surface-tow data derived from high resolution acoustic seismic sources are depthlimited. Digital acquisition and processing enhances surface-tow data, and this project produced

excellent data to water depths of about 850 m. However, it is the author’s opinion that 1000 m is

the outside limit for surface-tow data adequate for reliable appraisals of seafloor geology. Below

this depth, deep-tow technologies must be employed for reliable high resolution data sets. In

extremely complicated topographic and geologic settings the maximum depth for reliable high

resolution seismic data is even shallower than 1000 m. This reservation is especially true for

side-scan sonar data, which become difficult to acquire because of cable length and fish location

problems.

10. Utilization of 3D-seismic surface amplitude and phase data provides a powerful

additional element for interpreting seafloor geology and, to some extent, biology when used in

conjunction with good quality high resolution seismic and side-scan sonar data. The 3D-seismic

amplitude and phase data can help discriminate hard and soft seafloor types, surface and nearsurface sediments that are gas-charged and geohazards like subtle faults that may be difficult to

image with other acoustic methods.
107

V. Recommendations

As one of the deliverables for the final report, recommendations are to be made to the

Minerals Management Service regarding feature detection, upgrades for surface-tow data

acquisition, and merits of digitial processing of acoustic data, and the role for 3D-seismic in

geohazards evaluations. The accumulation of data for this project has lead to the development of

a convenient scheme for describing areas and features that are either impacted by or have

developed from the delivery of fluids and gases to the modern seafloor. This spectrum of mudprone to mineral-prone features helps us understand the variability of seep/vent-related

phenomena and it introduces a level of predictability not only for seafloor geology, but also for

hydrocarbon vent/seep biology. The characteristics of seafloor areas that have acoustic wipe-out

zones and fall into these areas are discussed in the project conclusions.

Regarding surface-tow acquisition of high resolution seismic data, computer programs

are now readily available for digital acquisition and processing. Analog data should no longer be

acceptable because of the post-processing options available to upgrade record quality. However,

surface-tow data have water depth limitations. In the author’s opinion, surface-tow data are not

reliable proxies for the seafloor below water depths of 1000 m. In extremely complex seafloor

settings, this maximum depth can be somewhat shallower. Deep-tow high resolution data

represent the seafloor better in continental slope water depths than any other option. This option

is best for both side-scan sonar and high resolution seismic.

New options using 3D-seismic data for seafloor evaluations should be used in

conjunction with higher resolution data sets. Surface amplitude and phase data are extremely

valuable for interpreting areas of hard and soft bottom, as well as gas-prone sediments.

However, even the best 3D-seismic has resolution problems when one is interested in small-scale

features like mounds, small slumps, gas hydrate outcrops, chemosynthetic communities as well

as other features common to the northern Gulf of Mexico continental slope. Therefore, multiple

data sets are required for the best interpretations. In the author’s opinion 3D-seismic profiles

coupled with seafloor renderings incorporating multibeam bathymetry and 3D-seismic

amplitude data are sufficient for clearing a block for areas that are probable sites for potential

geohazards or protected benthic communities. However, for determining pipeline routing and

platform locations more detailed data sets are required.

Finally, the northern Gulf of Mexico continental slope is such a complex and diverse area

of ocean bottom that more work must be focused on understanding its geologic-sedimentologic

variability and how this variability impacts slope biology. It is the author’s opinion that as the

oil and gas industry moves deeper, more seafloor verification of acoustic data must be

accomplished before geologists,geotechnical engineers, and regulatory personnel can be

confident of seafloor interpretations based on remotely sensed acoustic data (both highresolution data including Seabeam bathymetery and low frequency exploration-scale seismic).
109

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Appendix A

Industry Data Sets Reviewed for MMS-CMI Project

Data sets acquired by individual companies in the petroleum industry formed the data base from

which this MMS-CMI funded project evolved. Some data sets were given to Coastal Studies Institute

for research purposes and others were either loaned or the PI was provided access. After an initial

review of the data, a determination was made by the PI as to data quality and interest level in the area

regarding project goals and objectives. Within any given area that fit the project guidelines, features of

interest were selected , and the site was put on a high priority list. All data sets were eventually evaluated, and a high priority list was compiled. In order to make this list, acoustic wipe-out zones had to be

present in the data set and data had to be of high quality. The next level of selection was based on the

willingness of the participant company to share the data for publication and the availability of complimentary data sets, such as 3D-seismic and ground truth observations. When selected early enough,

ground truth was planned for the following year’s submersible program.

A-1
AREA GREEN CANYON

BLOCK 6 9 10 11 17 18 19 25 26 3 4 3 5 4 0

CONTRACTOR MTS RGI RGI JCA RM ME MTS JCA GSI GSI JCA

COMPANY Texaco Conoco Conoco Chevron Shell Shell Conoco Texaco Amoco Amoco Texaco

HAZARD REPORT X X X X X X

ENGINEERING REPORT X X X X X X X X X X X

BATHYMETRIC MAP X X X X X X X X X X X X

DEEP TOW

SURFACE TOW X

3.5 kHz X X X X X X X X X X X X

Boomer

Sparker X X X X X X X X X X

Minisleeve X

Air-Gun

Water-Gun X

Side-Scan X X X X X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 85-271 79-274 79-274 160-245 329-805 215 262-501 253-457 553-663 538-601 564-488

CORES X

SUBMERSIBLE OBSERVATIONS X

FM=Fugro-McClelland

ME=McClelland Engineers, Inc.

JCA=John E. Chance & Associates, Inc.

OOS= Odom Offshore Surveys

IRC=Intersea Research Corporation

ST=Sea Tales

CGI=Comap Geosurveys, Inc.

AREA GREEN CANYON

BLOCK 41 52 52 53 53 54 59 60 6 1 68 72 73

CONTRACTOR JCA M E RGI M E RGI RGI RGI JCA JCA RGI JCA JCA

COMPANY Texaco Conoco Conoco Conoco Conoco Conoco Conoco Sohio Chevron Gulf Mobil Shell

HAZARD REPORT X X X X

ENGINEERING REPORT X X X X X X X X X X

BATHYMETRIC MAP X X X X X X X X X X X

DEEP TOW

SURFACE TOW

3.5 kHz X X X X X X X X X X X X

Boomer

Sparker X X X X X X X X

Minisleeve X X X

Air-Gun

Water-Gun

Side-Scan X X X X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 625-823 110-235 79-274 110-235 79-274 79-274 214-303 232-348 343-625 511-635 530-799

CORES X X

SUBMERSIBLE OBSERVATIONS

FM=Fugro-McClelland

ME=McClelland Engineers, Inc.

JCA=John E. Chance & Associates, Inc.

OOS= Odom Offshore Surveys

IRC=Intersea Research Corporation

ST=Sea Tales

CGI=Comap Geosurveys, Inc.

RDS=Racal-Decca Survey, Inc.

A-2
AREA GREEN CANYON

BLOCK 75 79 80 81 89 91 97 98 102 103 104

CONTRACTOR GSI GSI JCA JCA CGI CGI CGI RDS/CAGC CI JCA CCS

COMPANY Amoco Amoco Texaco Texaco Exxon Eddon Arco Conoco Mobil Sohio Marathon

HAZARD REPORT X X X X X X X

ENGINEERING REPORT X X X X X X X X X X

BATHYMETRIC MAP X X X X X X X X X X X

DEEP TOW

SURFACE TOW

3.5 kHz X X X X X X X X X X

Boomer

Sparker X X X X X

Minisleeve X X

Air-Gun

Water-Gun

Side-Scan X X X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 577-681 567-703 549-923 549-923 110-238 98-262 170-320 243-564 284-363 273-360 305-392

SUBMERSIBLE OBSERVATIONS

FM=Fugro-McClelland

ME=McClelland Engineers, Inc.

JCA=John E. Chance & Associates, Inc.

OOS= Odom Offshore Surveys

IRC=Intersea Research Corporation

ST=Sea Tales

CGI=Comap Geosurveys, Inc.

RDS=Racal-Decca Survey, Inc.

AREA GREEN CANYON

BLOCK 105 110 111 112 116 117 133 134 138 140

CONTRACTOR CCS RGI RGI RGI JCA JCA IRC RDS CGI CI

COMPANY Marathon Marathon Gulf Gulf Shell Shell Gulf Exxon Exxon CAGC

HAZARD REPORT X X X X

ENGINEERING REPORT X X X X X X X X X

BATHYMETRIC MAP X X X X X X X X X X

DEEP TOW X

SURFACE TOW

3.5 kHz X X X X X X X X X X

Boomer

Sparker X X X X X

Minisleeve X

Air-Gun X X X

Water-Gun X

Side-Scan X X X X X

3D-SEISMIC SURFACE AMPLITUDES X

WATER DEPTH (m)

CORES

SUBMERSIBLE OBSERVATIONS

FM=Fugro-McClelland

ME=McClelland Engineers, Inc.

JCA=John E. Chance & Associates, Inc.

OOS= Odom Offshore Surveys

IRC=Intersea Research Corporation

ST=Sea Tales

CGI=Comap Geosurveys, Inc.

RDS=Racal-Decca Survey, Inc.

A-3
AREA GREEN CANYON

BLOCK 141 142 143 144 146 147 148 152 153 154 158

CONTRACTOR RGI RDS/CAGC AGO JCA JCA JCA OOS OOS RGI RGI M E

COMPANY Conoco Conoco Arco Mobil Sohio Texaco Marathon Marathon Marathon Marathon Shell

HAZARD REPORT X X X X X

ENGINEERING REPORT X X X X X X X X X X X

BATHYMETRIC MAP X X X X X X X X X X X

DEEP TOW X

SURFACE TOW

3.5 kHz X X X X X X X X X X

Boomer

Sparker X X X X X

Minisleeve X X

Air-Gun

Water-Gun X X X X X

Side-Scan X X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 320-823 243-564 131-564 307-492 349-574 348-412 357-488 441-549 396-762 396-762 671-1055

CORES

SUBMERSIBLE OBSERVATIONS

RGI-Racal Geophysics Inc.

ME=McClelland Engineers, Inc.

JCA=John E. Chance & Associates, Inc.

OOS= Odom Offshore Surveys

AGO- Arco Geoscience Operations

CAGC-C.A.G.C. Marine Region

AREA GREEN CANYON

BLOCK 179 180 181 180 181 182 184 184 184 (Boring 1) 184 (Boring 2 184 (Boring 3)

CONTRACTOR CGI JCA JCA OI ME RDS/CAGC ME ME ME

COMPANY Exxon Shell Shell Shell Shell CAGC Conoco Conoco Conoco Conoco Conoco

HAZARD REPORT X X X

ENGINEERING REPORT X X X X X X X X X X

BATHYMETRIC MAP X X X X X X

DEEP TOW X X X

SURFACE TOW

3.5 kHz X X X X X X

Boomer

Sparker X X X

Minisleeve X X

Air-Gun X

Water-Gun

Side-Scan X X X X

3D-SEISMIC SURFACE AMPLITUDES X X

WATER DEPTH (m) 262-543 280-883 280-805 335-732 274-640 243-762

CORES X X

SUBMERSIBLE OBSERVATIONS

CGI=Comap Geosurveys, Inc.

ME=McClelland Engineers, Inc.

JCA=John E. Chance & Associates, Inc.

RDS-Racal-Decca Survey, Inc.

IRC=Intersea Research Corporation

CAGC-C.A.G.C. Marine Region

RDS=Racal-Decca Survey, Inc.

A-4
AREA GREEN CANYON

BLOCK 185 190 191 193 196 197 198 202 204 228 232 233 237

CONTRACTOR RGI JCA JCA JCA FM FM CCS ME JCA RGI IRC IRC JCA

COMPANY Conoco Sohio Sohio BHP Shell Shell Marathon Shell Texaco Conoco Placid Placid BHP

HAZARD REPORT X X X X X X

ENGINEERING REPORT X X X X X X X X X X X

BATHYMETRIC MAP X X X X X X X X X X X X X

DEEP TOW X X X X X X

SURFACE TOW

3.5 kHz X X X X X X X X X X X X

Boomer

Sparker X X X X X X X

Minisleeve

Air-Gun

Water-Gun X X X

Side-Scan X X X X X X X

3D-SEISMIC SURFACE AMPLITUDES X X

WATER DEPTH (m) 320-823 383-574 394-531 457-963 457-963 498-914 671-1055 766-1008 450-823 476-914 475-914 560

CORES

SUBMERSIBLE OBSERVATIONS X X

FM=Fugro-McClelland

ME=McClelland Engineers, Inc.

JCA=John E. Chance & Associates, Inc.

RGI- Racal Geophysics, Inc.

IRC=Intersea Research Corporation

AREA GREEN CANYON

BLOCK 234 235 241 509 863 864 907 908

CONTRACTOR IRC RGI FM FM/JCA FM FM FM FM

COMPANY Placid Conoco Shell Taxaco Shell Shell Shell Shell

HAZARD REPORT

ENGINEERING REPORT X X X X X X X X

BATHYMETRIC MAP X X X X X X X X

DEEP TOW X X X X X X

SURFACE TOW X X

3.5 kHz X X X X X X X X

Boomer

Sparker X X X X X XX

Minisleeve

Air-Gun X X

Water-Gun X X X

Side-Scan X X X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 476-914 495-884 457-963 1158-13591347-1719 1347-1719 1469-1719 1469-1719

CORES

SUBMERSIBLE OBSERVATIONS

FM=Fugro-McClelland

JCA=John E. Chance & Associates, Inc.

RGI-Racal Geophysics, Inc.

IRC=Intersea Research Corporation

A-5
AREA GARDEN BANKS

BLOCK 21 65 84 104 135 147 148 152 161 189

CONTRACTOR JCA JCA GSI JCA GSI JCA JCS JCA CGI FM

COMPANY Chevron Chevron Amoco Chevron Amoco Chevron Chevron Sohio Pennzoil Texaco

HAZARD REPORT X X X X X X X X

ENGINEERING REPORT X X X X X X X X

BATHYMETRIC MAP X X X X X X X X X X

DEEP TOW

SURFACE TOW

3.5 kHz X X X X X X X X

Boomer

Sparker X X X X X X X X

Minisleeve

Air-Gun

Water-Gun X

Side-Scan X X X X X X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 131-187 131-187 107-192 152-234 23-125 152-234 152-234 134-226 198-413 171-238

CORES

SUBMERSIBLE OBSERVATIONS

FM=Fugro-McClelland

GSI-Gardline Surveys, Inc.

JCA=John E. Chance & Associates, Inc.

OOS= Odom Offshore Surveys

CGI=Comap Geosurveys, Inc.

AREA GARDEN BANKS

BLOCK 189 191 192 193 195 196 201 215 236 237 240 241

CONTRACTOR JCA JCA JCA JCA JCA JCA JCA JCA JCA JCA JCA JCA

COMPANY Texaco Chevron Chevron Chevron Sohio Sohio Chevron Hess Chevon Chevron Sohio Sohio

HAZARD REPORT X X X X X X X X X X X X

ENGINEERING REPORT X X X X X X

BATHYMETRIC MAP X X X X X X X X X X X X

DEEP TOW X

SURFACE TOW

3.5 kHz X X X X X X X X

Boomer

Sparker

Minisleeve X X X

Air-Gun

Water-Gun X X X

Side-Scan X X X X X X X X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 165-258 152-234 126-219 126-219 134-226 148-498 304-615 350-545 126-219 126-219 248-498 248-498

CORES

SUBMERSIBLE OBSERVATIONS

JCA=John E. Chance & Associates, Inc.

A-6
AREA GARDEN BANKS

BLOCK 244 255 256 257 260 298 300 304 343 359 368 369

CONTRACTOR FM JCA JCA JCA JCA GSI GSI JCA GSI OOS AOSS AOSS

COMPANY Shell Shell Shell Shell Hess Amoco Amoco Hess Amoco Amoco Conoco Conoco

HAZARD REPORT X X X X X X X X X

ENGINEERING REPORT X X X X X X X X X

BATHYMETRIC MAP X X X X X X X X X X X X

DEEP TOW

SURFACE TOW

3.5 kHz X X X X X X X X X X X

Boomer

Sparker X X X X X X

Minisleeve

Air-Gun

Water-Gun X X X X X X X

Side-Scan X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 239-689 442-561 430-586 421-696 412-518 341-555 383-668 400-670 337-637 344-498 230-425 230-425

CORES

SUBMERSIBLE OBSERVATIONS

FM=Fugro-McClelland

AOSS-Alpine Ocean Seismic Surveys, Inc.

JCA=John E. Chance & Associates, Inc.

OOS= Odom Offshore Surveys

GSI-Gardline Surveys, Inc.

AREA GARDEN BANKS

BLOCK 413 426 427 470 471 499 500 543 544 545

CONTRACTOR JCA FM FM FM FM CGI CGI CGI CGI CGI

COMPANY Chevron Shell Shell Shell Shell Exxon Exxon Exxon Exxon Exxon

HAZARD REPORT X X

ENGINEERING REPORT X X X X X X X X X

BATHYMETRIC MAP X X X X X X X X X

DEEP TOW X X X X

SURFACE TOW X X X X

3.5 kHz X X X X X X X X X X

Boomer

Sparker X

Minisleeve X X X X

Air-Gun

Water-Gun

Side-Scan X X X X X X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 396-601 472-1005 472-1005 472-1005 472-1005 506-601 518-853 536-793 536-853 655-893

CORES

SUBMERSIBLE OBSERVATIONS

FM=Fugro-McClelland

JCA=John E. Chance & Associates, Inc.

CGI=Comap Geosurveys, Inc.

A-7
AREA MISSISSIPPI CANYON

BLOCK 20 538 539 582 583 709 713 714 757 758 762

CONTRACTOR WCO ME ME ME ME ME KCO KCO KCO KCO FM

COMPANY Sohio Shell Shell Shell Shell Arco Texaco Texaco Texaco Texaco Shell

HAZARD REPORT X X X X X

ENGINEERING REPORT X X X X X X X X X

BATHYMETRIC MAP X X X X X X X X X X X

DEEP TOW X X X X X

SURFACE TOW X

3.5 kHz X X X X X X X X X X X

Boomer

Sparker X X X X X X

Minisleeve

Air-Gun X

Water-Gun

Side-Scan X X X X X

3D-SEISMIC SURFACE AMPLITUDES X

WATER DEPTH (m) 49-168 482-945 482-945 482-945 482-945 685 943-1066 943-1066 943-1066 943-1066 884-1036

CORES X X

SUBMERSIBLE OBSERVATIONS X

FM=Fugro-McClelland

ME=McClelland Engineers, Inc.

WCO-Woodward Clyde Oceaneering

KCO-KC Offshore, L.L.C.

AREA MISSISSIPPI CANYON

BLOCK 736 806 807 810 854 885 929

CONTRACTOR FM FM FM SCI SCI KCO

COMPANY Shell Shell Shell Shell Shell Texaco BHP

HAZARD REPORT X X

ENGINEERING REPORT X X

BATHYMETRIC MAP X X X X X X X

DEEP TOW X X X

SURFACE TOW X X X X

3.5 kHz X X X X X X X

Boomer

Sparker X X X X X X X

Minisleeve

Air-Gun

Water-Gun X X

Side-Scan X X X X X

3D-SEISMIC SURFACE AMPLITUDES X X

WATER DEPTH (m) 884-1036 884-1036 884-1036 994-1331 994-1331 580 640

CORES X X

SUBMERSIBLE OBSERVATIONS X X

FM=Fugro-McClelland

SCI-Shell Offshore, Inc.

KCO-K.C. Offshore, L.L.C.

A-8
AREA VIOSCA KNOLL

BLOCK 786 786 864 (Boring 1)

CONTRACTOR FM KCA ME

COMPANY Taxaco Texaco Conoco

HAZARD REPORT X X

ENGINEERING REPORT X X X

BATHYMETRIC MAP X X

DEEP TOW X

SURFACE TOW

3.5 kHz X X

Boomer

Sparker X

Minisleeve

Air-Gun

Water-Gun

Side-Scan

3D-SEISMIC SURFACE AMPLITUDES X

WATER DEPTH (m) 378-908

CORES X

SUBMERSIBLE OBSERVATIONS

FM=Fugro-McClelland

ME=McClelland Engineers, Inc.

KC-K.C. Offshore, L.L. C.

AREA EWING BANKS

BLOCK 305 (Plat. A) 305 (Boring a1) 305 (Boring a2 305 (Boring a2) 305 (Boring 1)

CONTRACTOR M E WWC WWC WWC M E

COMPANY Conoco Conoco Arco Conoco Conoco

HAZARD REPORT

ENGINEERING REPORT X

BATHYMETRIC MAP X

DEEP TOW

SURFACE TOW

3.5 kHz

Boomer

Sparker

Minisleeve

Air-Gun

Water-Gun

Side-Scan X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 81-87

CORES

SUBMERSIBLE OBSERVATIONS

WWC-Woodward Clyde Oceaneering

ME=McClelland Engineers, Inc.

A-9
AREA EWING BANKS

BLOCK 305 (Boring 2 302 (Boring 3 (305 (Boring 4) 500 543 544 545

CONTRACTOR M E M E M E CGI CGI CGI CGI

COMPANY Conoco Conoco Conoco Exxon Exxon Exxon Exxon

HAZARD REPORT

ENGINEERING REPORT X X X X X X X

BATHYMETRIC MAP X X X X

DEEP TOW

SURFACE TOW

3.5 kHz X X X X

Boomer

Sparker

Minisleeve

Air-Gun

Water-Gun

Side-Scan X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 518-853 536-793 537-853 655-893

CORES

SUBMERSIBLE OBSERVATIONS

ME=McClelland Engineers, Inc.

CGI=Comap Geosurveys, Inc.

AREA EWING BANKS

BLOCK 869 870 871 875 909 910 912 913 914 915 916

CONTRACTOR JCA JCA JCA ME ME JCA JCA JCA JCA JCA

COMPANY Sohio Mobil Sohio Exxon Exxon Sohio Sohio Mobil Mobil Mobil

HAZARD REPORT X X X

ENGINEERING REPORT X X X

BATHYMETRIC MAP X X X

DEEP TOW

SURFACE TOW

3.5 kHz X X X X X X X X X X

Boomer

Sparker X X X X X X X X

Minisleeve X X

Air-Gun

Water-Gun

Side-Scan X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 297-360 146-311 146-311

CORES

SUBMERSIBLE OBSERVATIONS

ME=McClelland Engineers, Inc.

JCA=John E. Chance & Associates, Inc.

A-10
AREA EWING BANKS

BLOCK 932 933 937 938 953 958 959 966 991 994

CONTRACTOR JCA JCA ME ME ME JCA JCA JCA ME

COMPANY Sohio Sohio Exxon Exxon Exxon Sohio Sohio Texaco Exxon

HAZARD REPORT X X X X X X X

ENGINEERING REPORT X X X X X X X

BATHYMETRIC MAP X X X X X X X

DEEP TOW

SURFACE TOW

3.5 kHz X X X X X X X X X

Boomer

Sparker X X X

Minisleeve X X X X X X

Air-Gun

Water-Gun

Side-Scan X X X X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 114-169 14-169 128-183 128-183 146-311 518-579 155-430

CORES

SUBMERSIBLE OBSERVATIONS

ME=McClelland Engineers, Inc.

JCA=John E. Chance & Associates, Inc.

AREA EWING BANKS

BLOCK 1003 1005 1006 1010 1011

CONTRACTOR JCA JCA JCA JCA JCA

COMPANY Sohio Mobil Mobil Texaco Texaco

HAZARD REPORT X X X X X

ENGINEERING REPORT X X X X X

BATHYMETRIC MAP X X X X X

DEEP TOW

SURFACE TOW

3.5 kHz X X X X X

Boomer

Sparker X X X X

Minisleeve X

Air-Gun

Water-Gun

Side-Scan X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 421-610 480-606 480-606 488-625 518-655

CORES

SUBMERSIBLE OBSERVATIONS

JCA=John E. Chance & Associates, Inc.

A-11
AREA EAST BREAKS

BLOCK 118 120 122 165 165 166 170 206 207 208

CONTRACTOR CGI CGI JCA ME JCA JCA CCS JCA JCA JCA

COMPANY Gulf Gulf Texaco Sohio Sohio Texaco Amoco Sohio Sohio Sohio

HAZARD REPORT X X X X X X X X X X

ENGINEERING REPORT X X X X X X X X

BATHYMETRIC MAP X X X X X X X X

DEEP TOW

SURFACE TOW

3.5 kHz X X X X X X X X X X

Boomer

Sparker X X X

Minisleeve X X

Air-Gun

Water-Gun

Side-Scan X X X X X X X X

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 128-177 85-378 146-351 146-351 85-378 180-446 264-434 264-434 258-439

CORES

SUBMERSIBLE OBSERVATIONS X

ME=McClelland Engineers, Inc.

JCA=John E. Chance & Associates, Inc.

CGI=Comap Geosurveys, Inc.

AREA EAST BREAKS

BLOCK 211 244 245 247 248 250 251 288 289 302

CONTRACTOR IRC ST ST RDS RDS JCA JCA ST ST JCA

COMPANY Texaco Exxon Exxon Tenneco Tenneco Sohio Sohio Exxon Exxon Texaco

HAZARD REPORT X X X

ENGINEERING REPORT X X X X X X X X X X 

BATHYMETRIC MAP X X X X X X X X X X 

DEEP TOW

SURFACE TOW

3.5 kHz X X X X X X X X X X 

Boomer

Sparker X X X X

Minisleeve X X X X

Air-Gun

Water-Gun X X X

Side-Scan X X X X X X X X X X 

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 234-449 366-488 366-488 372-463 372-463 264-434 264-434 366-488 366-488 320-625

CORES

SUBMERSIBLE OBSERVATIONS

JCA=John E. Chance & Associates, Inc.

IRC=Intersea Research Corporation

ST=Sea Tales

RDS=Racal-Decca Survey, Inc.

A-12
AREA EAST BREAKS

BLOCK 303 339 340 341 342 383 384 386 425 428

CONTRACTOR CGI CGI CGI CGI CGI FM FM CGI IRC FM

COMPANY Exxon Pennzoil Pennzoil Pennzoil Shell Shell Shell Shell Conoco Shell

HAZARD REPORT

ENGINEERING REPORT X X X X X X X X X X 

BATHYMETRIC MAP X X X X X X X X X X 

DEEP TOW X X X 

SURFACE TOW

3.5 kHz X X X X X X X X X X 

Boomer

Sparker X X X

Minisleeve X X

Air-Gun X X

Water-Gun

Side-Scan X X X X X X X X X X 

3D-SEISMIC SURFACE AMPLITUDES

WATER DEPTH (m) 305-526 321-626 302-610 463-617 281-732 402-853 402-853 281-732 654-910 402-853

CORES

SUBMERSIBLE OBSERVATIONS

FM=Fugro-McClelland

IRC=Intersea Research Corporation

CGI=Comap Geosurveys, Inc.

AREA EAST BREAKS

BLOCK 469 684 685 728 729 920 921 964 965

CONTRACTOR IRC FM FM FM FM FM FM FM FM

COMPANY Conoco Shell Shell Shell Shell Shell Shell Shell Shell

HAZARD REPORT

ENGINEERING REPORT X X X X X X X X X 

BATHYMETRIC MAP X X X X X X X X X 

DEEP TOW X X X X 

SURFACE TOW

3.5 kHz X X X X X

Boomer

Sparker X X X X

Minisleeve X

Air-Gun

Water-Gun X X

Side-Scan X X X X X

3D-SEISMIC SURFACE AMPLITUDES 654-910 975-1280 975-1280 975-1280 975-1280 305-610 305-610 305-610 305-610

WATER DEPTH (m)

CORES

SUBMERSIBLE OBSERVATIONS

FM=Fugro-McClelland

IRC=Intersea Research Corporation

A-13
APPENDIX B

Equipment Used On MMS/CMI Project

A. Field Data Collection Phase

1. Navigation

The navigation control program, “M4200DT”, is designed to acquire data from the Magnavox

MX4200d differential GPS, provide steering information to a user-designated way point, and output fix

marks with annotation to several survey instruments. Way point annotation is currently supporting the

EG&G Model 260 side scan sonar, the DELPH 2 seismic system and the Raytheon (Ocean Data

System) DSF-6000 precision depth recorder.

A provision has been made to allow the user to input a table of way points that can be

incremented or decremented as necessary during a survey. The user can also manually input way point

coordinates. The software will provide the user with navigation information updates at a one second

interval. Fix marks and associated fix mark data are output at 1-, 2-, 4-, 6-, or 10-minute intervals at

the user’s preference. The fix mark data can be written to a disk file as ASCII text. Steering information is provided at one second intervals reporting course over ground, speed over ground, distance to

way point, bearing to way point, distance from survey line in meters, and which side of the vessel the

survey line is located (i.e., to port or starboard).

This system requires an IBM 80386 or 80486 compatible computer with MS-DOS operating

system, 640K memory, hard disk, floppy disk, at least one serial port, and a least one 8-printer port.

Up to four serial ports and two printer ports are supported. An interface was designed and built to

work with this system. It provides an analog output (0-5 volts DC) for a steering meter, a digital event

mark out, and a switch closure event mark out for up to four recorders. This interface will connect to

an IBM PC or compatible computer with a serial port and a printer port.

2. Seismic

a. ORE GeoPulse Boomer This resilient high resolution acoustic system employs an electromechanical sound source that delivers increased energy over a broader frequency spectrum than other

similar conventional systems. It provides increased penetration and resolution over a wide range of subbottom features in deep water and in high noise environments. The out-going pulse is short, powerful,

and damps rapidly to provide unusually high resolution (< 1 m bed resolution). A CSI-built surf board

towing arrangement was found to produce less noise on the records than the conventional catamaran

sold with the GeoPulse system. A convenient acoustic receiver unit provided gain controls filter combinations, hydrophone preamp power, and other necessary programming for collection of excellent

seismic profiles. We consistently acquired mappable data to subsurface depths of 100-200 ms. Both

the GeoPulse and water gun data were acquired by a newly designed hydrophone produced by Innovative Transducers/Source Technology of Houston, Texas. This hydrophone was found to be extremely

efficient with regard to eliminating extraneous sea surface and water column noise. The GeoPulse

system worked well throughout the survey till a power supply problem was encountered near the end of

the project. An EG&G power supply was substituted and worked well until Phase 1 was complete. In

general, the GeoPulse records were of outstanding quality.

B-1
b. Water Gun Maximum subsurface penetration in our survey was provided by an S-15 (15

in3) water gun acoustic source manufactured by Seismic Systems, Inc. of Houston, Texas. This water

gun is a pneumatic seismic source that uses compressed air (2000 psi-operating pressure) as air guns

do. A Hamworthy seismic compressor capable of delivering 50 cfm at 3000 psi during 1 second shots

was used for the survey. The water gun is an implosive source creating an acoustic pulse which is free

of bubble oscillations and has a broad spectrum extending to high frequencies. Another important

characteristic of this acoustic source is the repeatability of both shape and amplitude of the outgoing

signal. The water gun was used primarily for the deep water parts of our survey where GeoPulse

penetration was minimal. Bed resolution for the water gun is slightly over 1 m and subsurface penetration was routinely over 500 ms. Water gun profiles have been run to water depths of about 1000 m

with good success.

c. Air Gun A seismic System 50 in3 (GI Gun) will be used during the Phase 2 “fill-in” seismic

acquisition program. It operates with the same compressor as the water gun but is a more powerful and

lower frequency source. Bed resolution has been estimated at ~ 2m from previous field data collection

programs. It will be used to penetrate hard bottom areas where the high resolution sources may suffer

from signal attenuation.

3. Side-Scan Sonar

The EG&G Model 260 sea floor imaging system was used in conjunction with an electric winch

containing nearly 800 m of cable. The basic Model 260 unit is coupled to a digital recorder module

(Model 380) so that in addition to analog copies, side-scan data can be archived on Exabyte tape. A

100 kHz fish is used to image the sea floor and navigation fixes are written into the records directly from

our GPS positioning system. Side-scan sonar data were used to help interpret bottom features observed on seismic profiles especially these related to fluid and gas vents/seeps. Both data sets are

calibrated in real-time with bottom sediments using an underway sampler designed to take short surface

cores while in survey mode. In general, side-scan data are of high quality at the depth to be used.

4. Digital Data Acquisition System (DELPH 2)

Both GeoPulse boomer and water gun data are acquired in analog as well as digital format. The

analog data are printed on EPC recorders while the digital data are acquired using the dual channel

DELPH 2 high resolution seismic data acquisition/processing system. The DELPH 2 program provides

synchronization of both seismic channels to avoid interference, dual channel triggering, and independent

sampling frequency on each channel. The DELPH 2 system is configured to enable quality control of

data and monitoring of source characteristics while data are being acquired. Data can also be processed in real time. The processing consists of filtering (low pass and high pass linear phase filter),

automatic gain control, time variable gain, swell filter, horizontal stacking, predictive deconvolution, time

variant filtering, and spectral analysis. The DELPH 2 program is loaded on a dedicated 486 DX

computer and a Goulton plotter is used to print out the seismic data. Two Exabyte tape drives are used

to transcribe the data from the disk to an 8 mm tape format. A dedicated computer program converts

the Elics format data produced by the DELPH 2 system to SEG-Y.

B-2
B. Technical Capability

1. The CSI Field Support Facility and Personnel

Coastal Studies Institute maintains a field support facility staffed by experienced, professional

research associates. This seven-man team of support personnel has a combined sixty-nine years of

experience in conducting the Institute’s research studies. Their professional training, skills, and experience span a wide range of expertise, i.e., electronic and mechanical engineering and fabrication, seamanship, instrument deployment and recovery techniques, data collection and diving. Unique to this

team is that, although each member is specialized in at least one of these disciplines, each member has a

practical working ability in all of them.

The Field Support Group (FSG) is located at LSU in the CSI Field Support Building. This is a

dedicated 8000 ft2 building housing separate office space for each team member, maintenance and

calibration laboratories, fabrication and machine shops, and equipment storage areas. The design,

fabrication, and calibration of the instrument systems described in this proposal will be fabricated and

tested within this facility. Specific FSG capabilities relative to this task are as follows:

a. Electronics

• a laboratory for design, prototype testing and printed circuit layout.

• a laboratory for general electronic maintenance.

• a shop for printed circuit etching, fabrication, assembly and testing.

• a shop for fabrication of underwater cables.

b. Electronic test equipment on hand

• dual trace storage scope

• two general purpose dual trace scopes

• two precision voltage standards

• numerous digital and analog mulitmeters

• two frequency counters

• two frequency generators

• numerous other electronic support instruments

c. Calibration

• dual temperature controlled salinity baths

• pressure calibration to 0.015 % accuracy up to 100 psi. and 0.1% up to 1500 psi

• temperature calibration to 0.02 deg. C. accuracy

• salinity calibration to 0.003 ppt accuracy

d. Calibration instruments on hand

• Guildline Autosal

• Hewlett/Packard Quartz thermometer

• Amtek precision dead weight tester

• Amtek hydraulic dead weight tester

B-3
e. Mechanical

• a complete machine shop for precision machine work and large scale fabrication

• gas, rear gas and electrical welding

• plastic composite fabrication

• mooring wire fabrication

f. Machine tools on hand

• two Bridgeport mills

• two laths, 40" x 10" dia. capacity and 50" x 16" capacity

• electric welder and rare gas welder

• gas welder

• two drill presses

• band saw

• two grinders

• two pressure test vessels:

• 10" diameter by 60" length at 1500 psi.

• 24" diameter by 38" length at 100 psi.

• Fenn rotary swedger

• numerous other small machine tools

g. Inventory of Field Equipment Relevant to this Proposal

• 4 each Aanderra RCM7 Solid state, vector averaging recording current meters (copy

of specifications attached)

B-4
APPENDIX C

A Summary of Sites Surveyed under MMS/CMI Support

and Types of Data Collected

High resolution geophysical data were collected in support of this project on two cruises, one in

July 1995 and the other in May 1997. A May 1996 cruise was aborted because of problems with a

seismic air compressor. The 1995 cruise was conducted from the M/V Sea Gull rented from LaSalle

Marine, Inc. of Lockport, Louisiana. Data were collected on a 24-hour schedule for five days. The

1997 cruise was conducted from the R/V Pelican of Louisiana Universitites Marine Consortium

(LUMCON). This ship proved to be an extremely quiet vessel, which promoted a favorable signal-tonoise ratio on our records, a factor that improved our record quality as compared to records collected

during the 1995 cruise. Sea state was excellent for both cruises and in general, data quality was exceptional. The tables and plots that follow summarize the sites where data were collected, general characteristics of the features being investigated, and the survey tracts associated with each feature. All data

collection was spatially controlled with differential GPS navigation through a software program written

and implemented by Mr. Walker Winans of the Coastal Studies Institute Field Support Group. All data

were stored on 8 mm magnetic tape while on board ship and then transferred to CD ROM in the

laboratory at Coastal Studies Institute as a means of permanently archiving the data.

C-1
CMI/MMS Cruise 1

Ships Track, Target 1

Ship Shoal 286

28.225

28.230

28.235

28.240

28.245

28.250

28.255

28.260

28.265

-90.895-90.890-90.885-90.880-90.875-90.870-90.865-90.860-90.855-90.850

Longitude

Latitude

CMI/MMS Cruise 1

Ships Track, Target 2

Mud Vent NE GC 53

27.900

27.905

27.910

27.915

27.920

27.925

27.930

27.935

-91.495-91.490-91.485-91.480-91.475-91.470-91.465-91.460-91.455

Longitude

Latitude

C-2
CMI/MMS Cruise 1

Ships Track, Target 3

Mounded Area GC 53

27.86

27.87

27.88

27.89

27.90

27.91

27.92

-91.52 -91.51 -91.50 -91.49

Longitude

Latitude

C-3
Ships Track, Target 5

Mud Volcano GC 143

27.815

27.820

27.825

27.830

27.835

27.840

27.845

27.850

27.855

-91.380-91.375-91.370-91.365-91.360-91.355-91.350-91.345-91.340-91.335

Longitude

Latitude

CMI/MMS Cruise 1

Ships Track, Target 4

Collapsed Depression GC 53

27.865

27.870

27.875

27.880

27.885

27.890

27.895

27.900

-91.515-91.510-91.505-91.500-91.495-91.490-91.485-91.480-91.475

Longitude

Latitude

C-4
CMI/MMS Cruise 1

Ships Track, Targets 6 & 7

27.750

27.760

27.770

27.780

27.790

27.800

27.810

27.820

27.830

-91.580-91.570-91.560-91.550-91.540-91.530-91.520-91.510-91.500-91.490-91.480

Longitude

Latitude

East/West, Target 6

Carbonate Mounds GC 140

North/South, Target 7

Bush Hill & Bush Lite

GC 185

CMI/MMS Cruise 1

Ships Track, Target 10

Garden Banks 382

27.610

27.615

27.620

27.625

27.630

27.635

27.640

27.645

27.650

27.655

27.660

-92.490-92.485-92.480-92.475-92.470-92.465-92.460-92.455-92.450-92.445-92.440

Longitude

Latitude

C-5
CMI/MMS Cruise 1

Ships Track, Last Line

GC 234

27.720

27.725

27.730

27.735

27.740

27.745

27.750

27.755

27.760

27.765

-91.225-91.220-91.215-91.210

Longitude

Latitude

C-6
CMI/MMS Cruise 2

Mobile Delta Survey

29.00

29.20

29.40

29.60

29.80

-88.80 -88.60 -88.40 -88.20 -88.00 -87.80 -87.60 -87.40 -87.20

Longitude

Latitude

CMI/MMS Cruise 2

Mississippi Canyon Blocks 929, 931

27.98

28.00

28.02

28.04

28.06

28.08

28.10

28.12

-89.78 -89.76 -89.74 -89.72 -89.70 -89.68 -89.66 -89.64 -89.62 -89.60 -89.58

Longitude

Latitude

MC-929

MC-931

C-7
CMI/MMS Cruise 2

Mississippi Canyaon Block 843

28.08

28.09

28.10

28.11

28.12

28.13

28.14

-89.68 -89.67 -89.66 -89.65 -89.64 -89.63 -89.62 -89.61 -89.60 -89.59 -89.58

Longitude

Latitude

CMI/MMS Cruise 2

Mississippi Canyon Block 709

28.14

28.16

28.18

28.20

28.22

28.24

28.26

28.28

-89.76 -89.74 -89.72 -89.70 -89.68 -89.66 -89.64

Longitude

Latitude

C-8
CMI/MMS Cruise 2

Mississippi Canyon Block 709

28.14

28.16

28.18

28.20

28.22

28.24

28.26

28.28

-89.76 -89.74 -89.72 -89.70 -89.68 -89.66 -89.64

Longitude

Latitude

CMI/MMS Cruise 2 

Mississippi Canyon Block 539

28.38

28.39

28.40

28.41

28.42

28.43

28.44

28.45

28.46

-89.45 -89.44 -89.43 -89.42 -89.41 -89.40 -89.39 -89.38 -89.37

Longitude

Latitude

C-9
CMI/MMS Cruise 2 

Green Canyon Blocks 180, 181

27.75

27.76

27.77

27.78

27.79

27.80

27.81

27.82

27.83

-91.76 -91.75 -91.74 -91.73 -91.72 -91.71 -91.70 -91.69 -91.68 -91.67 -91.66

Longitude

Latitude

CMI/MMS Cruise 2 

Green Canyon Block 18

27.90

27.91

27.92

27.93

27.94

27.95

27.96

-91.07 -91.06 -91.05 -91.04 -91.03 -91.02 -91.01 -91.00

Longitude

Latitude

C-10
CMI/MMS Cruise 2

Green Canyon Block152

27.79

27.80

27.81

27.82

27.83

27.84

27.85

-90.97 -90.96 -90.95 -90.94 -90.93 -90.92 -90.91 -90.90

Longitude

Latitude

CMI/MMS Cruise 2 

Garden Banks Blocks 260, 304

Area 4

27.68

27.69

27.70

27.71

27.72

27.73

27.74

27.75

27.76

-92.01 -92.00 -91.99 -91.98 -91.97 -91.96 -91.95

Longitude

Latitude

C-11
CMI/MMS Cruise 2 

Garden Banks Blocks 260, 304

Area 3

27.80

27.81

27.82

27.83

27.84

27.85

-92.04 -92.03 -92.02 -92.01 -92.00 -91.99 -91.98

Longitude

Latitude

Gas Charged Area

CMI/MMS Cruise 2 

Garden Banks Block 260, 304

Area 2

27.76

27.77

27.78

27.79

27.80

27.81

27.82

27.83

-92.04 -92.03 -92.02 -92.01 -92.00 -91.99 -91.98

Longitude

Latitude

C-12
CMI/MMS Cruise 2

Garden Banks Blocks 260, 304

Area 1

27.67

27.68

27.69

27.70

27.71

27.72

27.73

27.74

27.75

-92.0

0

-91.9

9

-91.9

8

-91.9

7

-91.9

6

-91.9

5

-91.9

4

-91.9

3

-91.9

2

Latitude

Longitude

2nd Run GB-1A GB-1

CMI/MMS Cruise 2 

Garden Banks Block 201

27.76

27.77

27.78

27.79

27.80

27.81

27.82

-92.75 -92.74 -92.73 -92.72 -92.71 -92.70 -92.69 -92.68 -92.67 -92.66 -92.65

Longitude

Latitude

C-13
CMI/MMS Cruise 2

Tuning Run in Garden Banks Blocks 260, 304

Using Air Gun Model GI-90

27.72

27.74

27.76

27.78

27.80

27.82

-92.00 -91.98 -91.96

Longitude

Latitude

CMI/MMS Cruise 2

All Sites with Recorded Digital Data

27.5

28.0

28.5

29.0

-93.0 -92.5 -92.0 -91.5 -91.0 -90.5 -90.0 -89.5 -89.0

Longitude

Latitude

GB-201

Tuning Run GB-260, 304

GB

260, 304 GC-180 & 181

GC-18

GC-152

MC-929 & 931

MC-834

MC-709

MC-539

C-14
CMI/MMS Cruise 2

All Sites with Recorded Digital Data

27

28

29

30

-94 -93 -92 -91 -90 -89 -88 -87

Longitude

Latitude

Mobile Delta

Tuning Run

GC-180 & 181

GC-18

GC-152

GB-201

GB-260

GB

260 & 304

MC-539

MC-709

MC-834

MC-929 & 931

CMI/MMS Cruise 1

Sites Surveyed

27.40

27.50

27.60

27.70

27.80

27.90

28.00

28.10

28.20

28.30

28.40

-93.50-93.40-93.30-93.20-93.10-93.00-92.90-92.80-92.70-92.60-92.50-92.40-92.30-92.20-92.10-92.00-91.90-91.80-91.70-91.60-91.50-91.40-91.30-91.20-91.10-91.00-90.90-90.80-90.70

Longitude

Latitude

Target 1

Ship Shoal 286

Target 2

Target 3 Mud Vent NE GC 53

Mounded Area

GC 53

Target 4

Collapsed Depression

GC 53 Target 5

Mud Volcano

GC 143

Target 6

Carbonate Mounds

GC 140

Target 7

Bush Hill & Bush Lite

GC 185

Target 8

Hydrate Venting Site

GC 234

Target 10

Garden Banks 382

Target 9

Garden Banks 189

C-15
Appendix D: Glossary

acoustic impedance: The product of seismic velocity and density. At normal incidence, the reflection coefficient

depends on changes in acoustic impedance.

acoustic transparency: This condition results from a sedimentary medium that has constant acoustic impendance so

that no reflectors appear within it. A thick and remolded submarine landslide deposit has these properties.

acoustic turbidity: Chaotic seismic reflections caused by the presence of something (commonly gas) within the

sediments that scatters and/or absorbs the acoustic energy. Commonly used as a synonym for acoustic wipeout.

acoustic wipeout: A term used for describing zones on a seismic record that have little or no reflector character. This

lack of reflectors is commonly attributed to the presence of bubble phase gas that settles and/or absorbs the acoustic

energy.

airgun: An energy source extensively used in marine seismic surveys. Air under high pressure is abruptly released

into the water to generate a seismic wave. Air guns are also used in boreholes, and in modified form on the land

surface. Arrays of airguns of different sizes are often used in marine surveys so that a broad frequency spectrum will

be generated.

ALVIN: a manned submersible operated by the Woods Hole Oceanographic Institute (Massachusetts, USA). This

vehicle has been used for most of the deep ocean surveys during which major scientific discoveries, such as

hydrothermal vents, black and white smokers and the associated ecosystems, have been discovered.

amplitude: The maximum deviation of a wave from the average value. Commonly used to describe reflection

strength.

amplitude anomaly: A local increase or decrease of seismic reflection amplitude. Amplitude anomalies may be

caused by geometric focusing, velocity focusing, interference, processing errors, etc. Amplitude analysis is commonly used to locate hydrocarbons at the seafloor as well as in the subsurface.

amplitude mapping: A methodology used primarily in conjunction with 3D-seismic data sets where amplitudes are

mapped in plan-view on a given stratigraphic horizon, e.g., amplitude mapping of the seafloor to identify areas of

hydrocarbon venting/seepage.

amplitude variation with offset (AVO): The variation in the amplitude of a seismic reflection with source-geophone

distance. This technique is used as a hydrocarbon indicator for gas.

anomaly: A deviation from uniformity in physical or acoustic properties. As used in acoustic surveys, this term may

apply to unusual shapes or reflection characteristics of targets on side-scan sonar records or features on seismic

profiles.

anoxia: A state of having no free oxygen in the system, e.g. in anxoic sediments.

apparent polarity: A convention that relates a peak or trough of a seismic reflection to the sign of the reflection

coefficient, assuming the reflecting interface is an isolated one.

assemblage density: The count per unit area of all fauna in a particular assemblage of organisms.

authigenic carbonates: Carbonates that have formed or been generated in place. As applied to hydrocarbon seep/

vent-related carbonates, these are carbonate minerals (ususally Mg-calcite, aragonite, or dolomite) that have formed

as a by-product of microbial utilization of hydrocarbons (both crude oil and gas) and are therefore C13-depleted.

autonomous underwater vehicle (AUV): a non-tethered self-propelled vehicle used for collecting data on the sea

floor, shall subsurface, and water column. These vehicles are currently replacing deep-tow technologies for high

resolution acoustic data collection used for geohazards assessment.

backscatter: the scattering of radiant energy into the hemisphere bounded by a plane normal to the direction of the

incident radiation and lying on the same side as the incident ray; the opposite of a forward scatter. Atmospheric

D-1
back scatter depletes 6 to 9 percent of the incident solar energy before it reaches the Earth’s surface. In radar usage,

backscatter generally refers to the microwave radiation scattered back toward the antenna.

bacteria mats: As applied to hydrocarbon seep/vent areas, these are filamentous accumulations of the

large bacterium Beggiatoa sp. that live at the sediment-water interface in areas where hydrocarbonds

have permeated the near-surface sediments.

bandwidth: the range of frequencies over which a given devise is designed to operate within certain limits

barite: A white, yellow, or colorless orthorhombic mineral: BaSO

4

. Strontium and calcium are often

present. Barite occurs in tabular crystals, in granular form, or in compact masses resembling marble, and

it has a specific gravity of 4.5. It is used in paint, drilling mud, and as a filler for paper and textiles, and is

the principal ore of barium. Cores and chimneys of barite form on the Gulf of Mexico slope as a product

of the flux of barium-rich fluids to the modern seafloor.

bedded: [ore dep]: Formed, arranged, or deposited in layers or beds, or made up of or occurring in the

form of beds; especially said of a layered sedimentary rock, deposit, or formation. The term has also

been applied to nonsedimentary material that exhibits depositional layering, such as the “bedded deposits”

of volcanic tuff alternating with lava in the mantle of a stratovolcano.

Beggiatoa: The generic name for a large (to 0.2 mm in diameter and several cm in length) white or

pigmented filamentous bacterium that commonly forms mats on the seafloor over areas of hydrocarbon

seepage. Beggiatoa is an H

2

S oxidizing bacteria in the sulfur cycle of marine sediments.

bioherm: Mound-like buildups that contain the remains of a limited variety of calcareous organisms that

are not frame-builders like the corals in a coral reef. Bioherms have no latitudinal restrictions like true

coral reefs.

benthic assemblage: A multi-species group of organisms found in a restricted location on the ocean

bottom, with no assumptions being made about the cause of the grouping or functional interactions.

Incorrectly as a synonym of community.

benthic community: A multi-species group of organisms found in a restricted location on the ocean

bottom. It is assumed that the group has been caused to exists due to a common set of factors, and the

organisms in the community interact in ways that regulate the community. Discovery of the causal factors

and the interactions is often the main purpose of community research. Incorrectly used as a synonym of

assemblage.

biogenic gas: gas produced by the biological degradation of organic matter within shallowly buried

sediments. The principal biogenic hydrocarbons are methane, ethane and propane.

biogenic veneer: As used in reference to seep/vent-related geology, a biogenc veneer is a non-chemosynthetic biologic community that now covers a once active seep/vent-related feature, such as a hard

substrate community covering an outcrop of C13-depleted authigenic carbonate.

blow-out: In the context of deep marine settings, a low-out is a general term used to describe sauver-,

cup-, or trough-shaped depressions in the seafloor usually related to the winnowing effets of gas and/or

fluid expulsion. This class of features can be small, less that 1 meter in diameter, to features tens of

meters in diameter. Pockmarks, gas craters, and other terms have been used to describe these features.

D-2
boomer: A type of high resolution seismic profiler that produces a very clean out-going pulse that can

achieve penetration in most marine sediments of a few 10s of meters with a bed resolution of <1 m. The

boomer acoustic source consists of capaitors that are charged to high voltage and then discharged through

a transducer in the water (charged plates sealed with a rubber diaphram).

bottom simulating reflector (BSR): high amplitude reflection event on a seismic profile that may cut

across normal stratigraphy. This relector originates from the high acoustic impedance at the phase

boundary between gas hydrates and free gas at the base of the hydrate stability zone.

box corer: a type of corer that retrieves relatively undisturbed and quantitative sediment samples in a

block rather than in a cylinder.

bright spot: A local increase of amplitude on a seismic section. A hydrocarbon accumulation is one way

a bright spot can be generated.

bubble effect: The result of bubble oscillations (bubble noise) on a seismic record. This effect commonly manifests itself as a repeat of the first arrivals and all other source-generated events.

bubble train (plume): A verticaly iriented set of reflectors in the water column portion of a high resolution seismic record resulting from venting of gas from the seafloor to the water column.

calcareous: containing a substantial proportion of calcium carbonate minerals, especially calcite (or

aragonite).

cap rock: A dense limestone, anhydrite, and gypsum rock that occurs above a salt diapir that results from

a variety of processes that include leaching of soluble minerals in the salt body and re-precipitation, as well

as precipitation of C13-depleted carbonates as a by-product of bacterial degradation of hydrocarbons.

chaotic facies: as applied to slope geology, a chaotic facies has not organized internal reflection horizons

or seismic profiles and is generally the product of sediment remolding associated with mass transport

processes.

character: When applied to seismic data, character refers to the recognizable aspect of a seismic event,

usually a frequency or phasing effect. When used in conjunction with side-scan sonar data, character

refers to the recognizable aspect of a target or field of targets.

chemoherm: a mound-shaped feature composed primarily of Ca-Mg carbonates that have been precipitated at a hydrocarbon seep site as a product of microbial utilization of hydrocarbons

chemosynthesis: The synthesis of carbohydrates via fixation of inorganic carbon (carboxylation) through

a pathway which derives energy from a chemical oxidation rather than light.

chemosynthetic: An organism with carries out chemosynthesis; limited to prokaryotes. The term is also

applied to host metazoans which harbor symbiotic chemosynthetic organisms.

chemotrophic: An prokaryote organism which derives it primary nutrition from chemosynthesis, or a

metazoan host deriving primary nutrition from chemosynthetic symbiotes.

D-3
chimney: a columnar mass of rock formed by precipitation of minerals as miner-charged fluids are

expelled at the seafloor. Various carbonates and barite comprise most chimneys on the northern Gulf of

Mexico slope.

chirp: a signal of continuously varying frequency (a linear change of frequency with time). Broad

frequency subbottom profilers (chirp sonars utilize this technology.

chirp sonar: a very high resolution subbottom profiling instrument that utilizes a broad frequency range

clathrate: a hydrate of natural gas (biogenic, thermogenic or mixed origin). A frozen gas and water mixture

found in permafrost regions and in continental margins below water depths of about 500 m.

coherent reflectors: These are responses on a seismic record that are produced when seismic reflections line up to represent a seismic reflector.

collapse depression: A seabed depression morphologically similar to a pockmark, but formed by the

collapse of the sediment as a result of dewatering or degassing, and not by the erosion of the sediment

material.

columnar disturbance: A vertical column seen on seismic reflection profiles in which the seismic layering is absent. Migrating fluids are considered to be responsible for them.

cone : [geomorph] A depositional form shaped like a cone, having relatively steep slopes and a pointed

top. In context of expulsion topography, the term cone is generally applied to mud volcanoes of various

sizes.

diagenesis: The chemical, physical and biological changes, modifications or transformations undergone

by a sediment after its initial deposition, during and after its lithification.

diapir: A geological structure formed when a sediment of low density which is overlain by a denser

sediment flows upwards, bending or piercing the overlying layers. These are most commonly produced by

salt (halite) to produce “salt domes,” but diapirs may also be produced by mud. In the Gulf of Mexico,

both types occur.

dome: A positive relief feature characterized by bowing-upwayrd of the seafloor as seen in cross section

on seismic records. This process can result from gas pressure or diapiric intrusions of salt or shale.

drape deposit: hemipelagic sediments that drape topography of the Gulf of Mexico continental slope.

This unit varies in thickness between about 1-5 m and is eroded from many positive relief features of the

upper slope. Sediments of these drape deposits are highly calcarious because of a high perentage of

pelagic foraminifera tests. Drape deposits appear “transparent” or only weakly stratified on high resolution seismic profiles largely because they are highly bioturbated.

echo-sounder: a high frequency seismic instrument for determining water depth and profile configuration

of the water bottom

eel: an array of hydrophones enclosed in tub to be towed behind a vessel when acquiring high resolution

seismic profiles

effective permeability: this property of fluids depends on their relative saturations; that is, the presence

of one fluid effectively changes the permeability to another fluid

electric log: the general term used for all electical borehole logs (e.g, SP, normal, induction,

microsensitivity logs)

endobenthos: Animals dwelling within the bottom. Contrasted with epibenthos, animals on the surface

of the bottom.

D-4
epicenter: the location of the earth’s surface below which the first motion is an earthquake occurs

eustacy: this term refers to worldwide sea level change. In stratigraphy the interplay of eustatic changes

with sediment supply, isostatic subsidence, and thermal uplift produces sequence boundaries

eustatic cycle : the time interval over which a global rise and fall of sea level occurs

exosystem: Lying outside a restricted system.

exploration: in the Gulf of Mexico, this term is generally applied to the search for commercial deposits of

hydrocarbons

exploration geophysics: the application of geophysics and the equipment associated with this discipline

for exploration pruposes

exploration 3D: a widely spaced form of 3D-seismic acquisition that relies on interpolation to give the

data sampling required for migration

exploratory well: a well drilled to an objective not previously known to be productive

facies: the aspect, appearance, and characteristic of a rock or sedimentary unit, usually reflecting its origin

facies change: a lateral or vertical variation in lithologic or paleontologic characteristics of contemporaneous sedimentary deposits

facies map: a general term for a map sowing the gross areas variation in observable attributes of different sedimentary units within a given stratigraphic interval

fairway: the region within which effort is concentrated (e.g. drilling fairway) or a trend of hydrocarbon

accumulations

fathometer: a device for measuring water depth by timing sonic reflections from the water bottom

fault: a fracture or a zone of fractures along which there has been displacement of the sides relative to

one another parallel to the fracture. Faults are known to be avenues for fluid and gas migration.

fault scarp: (a) a steep slope formed directly by movement along a fault and representing the exposed

surface of the fault before modification by erosion

filter: the part of a seismic data acquisition system that discriminates against some of the information

entering it, usually frequency

fish: with regard to side-scan sonar, the fish is the sensor towed in the water

fluidization: the mixing process sediment and fluid so that there is a breakdown of sediment structure so

that grains become entrained in moving pore fluids

fluid mud: highly fluidized sediment of clay and silt sized particles

fluid expulsion: in the context of the Gulf of Mexico continental slope geology, fluid expulsion refers to

the release of formation fluids and fluidized sediment at the seafloor. This release usually occurs in

association with faults.

fluid expulsion feature : in the process of fluid expulsion, features such as mud vents, mud volcanoes,

mud flows, brine pools, etc., are created. These are fluid expulsion features.

foraminifera: any single-celled organizm belonging to the subclass Sarcodina, order Foraminifera. They

are characterized by calcareous tests that contribute greatly to the calcareous nature of hemipelagic

deposits (drape deposits as the are commonly called in the deep Gulf)

D-5
formation: a lithologic unit with characteristics that allow it to be distinguished from other lithologic units

frequency: the repetition rate of a periodic wave form measured in cycles per second or hertz

gain: an increase (or change) in signal amplitude (or power) from one point in a circuit to another. Gas in

used to compensate for variations in input signal strength (i.e., in seismic or side-scan sonar data acquisition)

gas chimney: on seismic profiles, this is a vertical region of poor-to-no-data assocaited with bubble phase

gas in the sediments. Often there

gas front: the upper or lateral limit of gas-charged sediments as indicated by acoustic blanking and/or

acoustic turbidity on shallow seismic profiles.

gas hydrates (or clathrates): crystalline, ice-like compounds composed of water and natural gas (most

commonly methane but other gases can also be involved)

gas seep: an area on the seafloor where gas is escaping at a slow rate compared to a gas vent. Gas

seeps and vents can be sometimes be identified on acoustic profiler data s vertical or near-vertical

“plumes” in the water column. Acoustic wipe-out zones in the subsurface usually accompany gas seeps.

gas vent: a location on the seafloor where gas is released at a vigorous rate as opposed to a gas seep

which exhibits a relatively slow rate of gas release

geophone : an instrument used transform seismic energy into an electrical voltage

geophysical exploration: making and interpreting physical property measurements of the earth to

determine subsurface conditions, usually for economic purposes such as the serach for hydrocarbons

geophysics: the study of the earth by quantitative physical methods, especially seismic refelection and

refraction as well as other methods such as gravity and magnetics

geopressure : subsurface formation pressure that differs significantly from normal hydrostatic pressure

geothermal gradient: the rate of change of temperature with depth in the earth (average is about 30°C/

km at shallow depths)

geothermal heat flow: heat flow from the earth’s interior per unit area per unit time (the product of

thermal conductivity and thermal gradient)

giant gas mounds: large mud mound seabed features, up to 2km across and 100m high, reported from the

Gulf of Mexico that some have attributed to diapiric processes. Another interpretation is that these large

features are actually the product of accretionary processes are actually mud volcanoes

global positioning system (GPS): a U.S. government system of 24 satellites that permit determination

of latitude, longitude and elevation by trilateration

grab sampler: an ocean-bottom sampler that commonly operates by enclosing material from the seafloor

between two jaws upon contact with the bottom.

graben: as used in a marine setting, a down-dropped block of seafloor bounded by normal faults, often

long and narrow. Grabens are tensional features that commonly form over vertically migrating salt masses

gravity corer: a device for obtaining a solid sediment core of seafloor solely under its own weight

ground truth: as applied to marine studies, ground truth refers to data acquired directly from the seafloor

(cores, direct measurements, etc.) as compared to indirect data like acoustic data (seismic, side-scan

sonar, echo sounder, etc.). Ground truth data are commonly used to “calibrate” remotely sensed data sets

to real seafloor conditions

D-6
hard target: a highly reflective target as identified on side-scan sonar imagery or other type of sonar

images

hemipelagic: sediments formed by the slow accumulation on the sea floor of biogenic and fine terrigenous particles deposited through the water column to the seabed

hertz (Hz): the unit of frequency that is equivalent to cycles per second (cps)

hiatus: an interval of time that is not represented the sedimentary record (sediments were removed by

erosion or never deposited)

high resolution seismic: seismic frequencies above the normal, lower frequency exploration range.

Frequency ranges for this type of seismic is commonly 500 Hz or higher and it is used to improve resolution of the near-sruface sediment column for geohazards assessments, engineering purposes, and basic

science. Penetration is highly dependent on power of the system used and sediment type, but most

systems used in the Gulf of Mexico will penetrate well over 100 m in clay-rich soils

horst: a structural term used in marine settings to identify a block of seafloor raised, by normal faulting,

above neighboring blocks of seafloor

hot spot: in conjunction with hydrocarbon exploration and seep research, a region on a seismic profile or

30-amplitude slice that records an abnormal amplitude that probably represents hydrocarbons

hydrocarbon indicator: a response in seismic data (bright spot, dim spot, phase change, flat spot,

shadow zone, or velocity sag) that may indicate the presence of hydrocarbons

hydrocarbon seep: the slow seepage of hydrocarbons, a diverse group of chemical compounds of

hydrogen and baron that may occur in gas or liquid (crude oil) form, to the seafloor

hydrocarbon vent: as opposed to a hydrocarbon seep, a hydrocarbon vent displays a very vigorous

delivery of hydrocarbons (as well as formation fluid and sometimes fine-grained sediment) to the seafloor

and into the water column

hydrothermal: pertaining to hot water; especially with respect to the process whereby water is circulated

through hot rocks and during which it may leach metals from the rocks. These metals are precipitated

when the water is returned to the seawater and cooled.

hypoxia: A state of having little free oxygen.

impedance: the product of density and velocity as applied to accoustics

incident angle : the angle which a raypath makes with a perpendicilar to an interface

interslope basin: as applied to the Gulf of Mexico, this term refers to small sedimentary basins formed

on the continental slope by rapid sedimentation is a setting of salt deformation

isobath: a line of equal water depth

isopod: Any malacostracan crustacean belonging to the order Isopoda, characterized by absence of a

carapace, serial segmentation, a single podia type (iso-poda), and dorsoventral flattening.

Johnson Sea-Link: a manned submersible with a maximum diving depth of 1000 m, owned by Harbor

Branch Oceanographic Institution of Fort Pierce, Florida

Jurassic salt: a regional salt unit, the Louzanne Salt, deposited in the early stages of formation of the Gulf of Mexico.

the Jurassic is a period of geologic time ranging between 190-136 million years ago.

kerogen: organix matter disseminated throughout many sedimentary rocks from which hydrocarbons (cruise oil and

gas) are formed

D-7
layered: terminology referring to the sedimentary configuration of the seafloor, consisting of a number of horizontal

to subhorizontal sedimentary units

liquid limit: the water-content boundary between the semiliquid and the plastic states of a sediment, e.g. a soil. It is

one of the Atterberg limits.

liquifaction: the abrupt breakdown of grain-to-grain contacts in a sedimetn so that the grains become temporarily

suspended in the pore fluid until grain structure is re-established

lithification: the conversion of a newly deposited, unconsolidated sediment into a coherent and solid rock.

lithoherm: a deep-water mound (can be up to several hundred meters long by 50 m high) of limestone, apparently

formed by submarine lithification of carbonate mud, sand, and skeletal debris; e.g., in the Florida Straits.

lithologic log: a log illustrating lithology as a function of depth

lithostatic pressure: the pressure produced by the weight of overlying sediment or rock

low velocity layer: a layer whose velocity is lower than units above and below. This term can also be appleid to a

surface layer that has a lower velocity signature than deeper units

lowstand system tract: the earlies (lowermost) systems tract within a sequency with a lower boundary defined as a

Type-1 sequence boundary where sea level falls below the pre-existing shelf edge. A lowstand systems tract is

deposited during a rapid eustatic sea level fall. It can be separated into units by downlap surfaces: lowstand fan,

slope fan, and lowstand wedge

lucinids: Bivalve molluscs of the family lucinidae. Common in anoxic and hypoxic muds at all depths and known to

be sulfide detoxifying and possibly chemosynthetic.

magnetometer: an instrument for measuring magnetic field strength

marine flooding surface: a surface above which sediments were deposited in appreciably greater water depth than

those below

mass movement: a unit movement of a portion of the seabed; specifically mass wasting or the gravitative transfer of

material down a slope. These processes are especially important in the Gulf of Mexico because they produce

potential geohazards.

meiofauna: Organisms passing through a 0.63 micrometer seive; size class between macrofauna and microfauna.

microbial mats: Filamentous mats of the large bacterium Beggiatoa sp. that occur on the seafloor in areas of

hydrocarbon seepage or venting.

migration (seismic): an inversion operation involving re-arrangement of seismic information elements so that

reflections and diffractions are plotted at their true locations

millisecond (ms): a thousandth of a second

mixed assemblage: an assemblage composed of some specimens representing a fossil community, plus others

representing one or more transported asssemblages brought into the locality where found

mosaic (side-scan sonar): an assemblage of side-scan swath images whose edges have been matched or overlapped

to form a continuous presentation

mud clasts: semilithified or compacted angular-to-rounded fragments of fine-grained sediment (mud) created by mass

movement or expulsionof sediment at the seafloor cometimes along with formation fluids, hdrocarbons, and finegrained sediment

D-8
mudflow: a term applied to a mass-movement geomorphic form and process characterized by a gravity-driven flowing

mass of fine-grained sediment with a high degree of fluidity. In the context of fluid and gas expulsion at the seafloor,

mudflows are highly fluidized fine-grained sediments extruded from a localized vent and transported down-slope

under the influence of gravity

mud volcano: a volcano-shaped structure composed of mud that has been forced above the normal surface of the

sediment, creating a positive relief feature

multiple (seismic): seismic energy that has been reflected more than once. Multiples may obscure or “overprint”

primary reflection information (i.g., stratigraphy)

mussel: Any of the common marine bivalves belonging to the family Mytilidae. Not to be confused with fresh water

“mussels”, the clam superfamily Unionacea.

natural gas: a highly compressible mixture of hydrocarbons occurring naturally in gaseous form. The most common

components are methane (usually over 80%) ethane, propane, isobutane, butane, pentane, and others.

neritic: relating to water depths between low tide and 200 m

nose: a plunging anticline with structural closure in three out of four directions

onlap: successive landward termination of strata at the base of a depositional unit. In seismic terms onlap refers to a

reflection termination at the base of a unit where the reflection is flat or dips away from the termination.

outcrop (marine): formations, rocks or structures that appear at the seafloor

overburden (seismic): the sedimentary section above a given reflector

penetration (seismic): the greatest depth from which seismic refletors can be picked with reasonable certainty

permeability: a measure of the ease with which a fluid can pass through the pore spaces of a sediment or rock

petroleum: natural gas, crude oil and related substances, such as asphalt, formed by the thermal cracking of plant and

animal remains (kerogens).

phase (seismic): the angle of lag or lead of a sine wave with respect to a reference

phase velocity (seismic): the velocity of any given phase (such as a trough) or a wave of single frequency. It may

differ from group velocity because of dispersion.

phytoplankton: planktonic plants (usually microscopic)

pinger: A transponder or device that emits an acoustic signal, ususally used for marking sites or equipment on the

seafloor. A type of shallow seismic profiling system producing a high frequency acoustic signal, achieving a

penetration of a few tens of metres and very good (<1m) resolution.

piston core: sediment sample obtained by dropping a dart-like corer (a hollow metal tube topped by a heavy weight)

into the seabed. As the corer penetrates, sediment is forced into a plastic liner inside the metal barrel. An internal

piston moves up the core tube as sediment intrudes. The piston reduces distortion of the sedments both during the

coring process and during post-coring handling.

Pleistocene: an epoch of geologic time immediately preceding the Holocene (e.g., 1.8 million years before present)

Pliocene: an epoch of geologic time within the Tertiary period (7-1.8 million years before present)

pockmark: shallow seabed depression typically a few meters to

several tens of meters across and up to a few metres deep; generally formed in soft, fine-grained seabed sediments by

the winnowing effects of escaping fluids and gases of fluids into the water column.

D-9
pogonophora: A phylum of tube-dwelling marine worms distinguished by a crown of tentacles and the lack of a gut

and now generally regarded as chemosynthetic. The status as a phylum separate from the Annelida Is now under

debate.

porosity: pore volume per unit gross volume

positioning (marine): determining the location of a ship, data points, samples, etc., using reference beacons whose

geodetic locations may not be exactly known or by the Global Positioning System (GPS), which can provide extremely

accurate estimates of positions expressed in latitude and longitude

primary reflection (seismic): energy that has been reflected only once and hence is not a multiple

profiler: a high frequency marine seismic refelction system usually involving a low power source. this class of

seismic tools is used for geohazards and engineering studies (e.g., 3.5 kHz profiler)

prograding: deposition of sediment that builds progressively into deeper and deeper water

pull-apart structure: an extensional feature usually accomodated by normal faulting. In the Gulf of Mexico pull-apart

features are common to sediments over rising salt structures.

push core: a short sediment core taken by forcing a tube into surface sediments. Most push cores have a vlave

assembly that allows water to be forced out of the tube as sediment enters. The valve closes, creating a vacuum, as

the core tube is extracted from the bottom.

Quaternary: a period of geologic time incorporating the Pleistocene and Holocene epochs

radar: a system in which short electromagnetic waves are transmitted and the energy scattered back by reflecting

surfaes is received and imaged to detect objects. In the marine environment radar is ued by ships to detect and range

other ships, coasts, and buoys, etc.

Recent: in geologic time, the Holocene epoch, the last 10,000 years to present

reef: a carbonate buildup that contains the remains of both framework and cementing organisms, e.g., a coral reef. In

general nautical terms, a reef can be any positive relief feature than can be a hazard to navigation.

reflection configuration (seismic): the pattern is which relfections group

reflection event (seismic): a well-defined reflector or set of reflectors on a seismic record

reflection strength: amplitude of the envelope of a seismic wave

reflector: a contrast in acoustic impendance that gives rise to a seismic reflection

refraction: the change in direction of a seismic ray upon passing into a medium with a different velocity

relief (marine): the difference between the highest and lowest elevation of a feature above or below the surrounding

seafloor

reservoir geophysics: the use of geophysical methods to assist in delineating, describing or monitoring a hydrocarbon reservoir

reverse polarity: having polarity opposite to normal convention. Seismic sections are often plotted with both normal

and reverse polarity.

rim syncline: a ring of depressed sediments surrounding a salt diapir caused by subsidence following salt withdrawal

ROV (remotely operated vehicle): unmanned submersible (submarine) commonly equipped with video and stills

cameras, manipulator arm, etc.; extensively used for the inspection of and maintenance work on offshore structures.

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salt diapir (dome): a salt mass that intrudes on the sedimentary column at a high angle

scarp: (a) a line of cliffs produced by faulting or by erosion. The term is an abbreviated form of escarpment, and the

two terms commonly have the same meaning, although “scarp” is more often applied to cliffs formed by faulting. (b)

a relatively straight, cliff like face or slope of considerable linear extent, breaking the general continuity of the land by

separating surfaces lying at different levels, as along the margin of a plateau or mesa. A scarp may be of any height.

The term should not be used for a slope of highly irregular outline.

scattering: the irregular and diffuse dispersion of energy caused by inhomogeneities in the medium through which

the energy is traveling

secondary porosity: additional porosity in a sediment or rock created by subsequent diagenetic (geochemical)

changes/solution rugs, porosity caused by a mineralogical change like calcite to dolomite, fractures, etc.)

sedimentary basin: a confined area of thick sediment acumulation as compared to surrounding areas. In the continental slope province of the northern Gulf of Mexico these basins are found by salt withdrawal related to sedimentary

loading.

sedimentary facies: a term used by Moore for a stratigraphic facies representing any areally restricted part of a

designated stratigraphic unit (or of any genetically related body of sedimentary deposits) which exhibits lithologic

and paleontologic characters significantly different from those of another part or parts of the same unit. It comprises

“one of any two or more different sorts of deposits which are partly or wholly equivalent in age and which occur side

by side or in somewhat close neighborhood”.

seepage: escape of fluids (gas or liquids) from the seabed (or the land surface). Seabed seepages take the form of

macro-seepages where gas bubbles are large enough to be visible to the naked eye, and micro-seepages, where

individual bubbles are of microscopic size, or the seeping fluid is in liquid form (e.g. gases and solids dissolved in

pore water).

seep lithoherm: a deepwater mound that has been formed by precipitation of dominantly Ca-Mg carbonates as a byproduct of microbial utilization of hydrocarbons.

seismic: having to do with elastic waves. Energy may be transmitted thorugh the body of an elastic solid by body

waves of two types: P-waves (compressional waves) or S-waves (shear waves).

seismic exploration: the use of seismic techniques to map subsurface geology with the focus on finding economically viable products, particularly hydrocarbons

seismic profiler: geophysical instrument akin to an echo-sounder but emitting an acoustic signal of lower frequency

in order to penetrate the seabed. Seismic profiles show the configuration of sediment layers beneath the seabed.

seismic reflection: acoustic signal reflected from a boundary (seismic reflector) between sediments of different

physical properties, most particularly density and elasticity. The greater the difference between the sediments, the

stronger (darker) the reflection. Coherent seismic reflections make up a seismic reflector.

seismic reflector: a sediment boundary which causes a seismic reflection. Differences in physical properties, most

particularly density and elasticity (demonstrated by a difference in seismic velocity), are required for a seismic

reflection to occur. These differences are not necessarily represented by a change in lithology or the visual appearance of the sediment.

seismic section: a plot of seismic data long a line

seismic stratigraphy: methods of determining the nature and geologic history of sedimentary rocks and their

depositional environment from seismic evidence

seismic (or acoustic) velocity, Vp: the speed of propagation of an acoustic signal (or other pressure wave); dependent upon the physical properties of the medium, most particularly density and elasticity.

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sequence analysis: the procedure of picking unconformities and correlative conformaties or seismic sections so as to

separate out the packages involved with different time depositional units

sequence stratigraphy: the study of genetically related strata bounded by unconformities or their correlative

conformities

sessile: living attached to the sub-strate.

shallow gas: bubble-phase gas within unlithified sediments. Pockets of shallow gas under high pressure present a

hazard to drilling.

shear strength: resistance of a sediment (or soil) to failure by shear; normally measured in kPa (kilopascals: 1 kPa =

1kN/m2).

side-scan sonar: geophysical instrument designed to acoustically image the surface topography and roughness of

the seafloor. this instrument is typically towed behind a ship or smaller boat. It pulses energy in the 100-500 kHz

range to produce a swath of data rather than a profile like seismic systems.

sideswipe: evidence or a seismic record of a feature which lies off the side of the seismic profile

slant range (side-scan sonar): a distance measurement that involves both horizontal and vertical components (e.g.,

distance from the side-scan fish to an object on the bottom)

slump: (a) a landslide characterized by a shearing and rotary movement of a generally independent mass of rock or

earth along a curved slip surface (concave upward) and about an axis parallel to the slope from which it descends,

and by backward tilting of the mass with respect to that slope so that the slump surface often exhibits a reversed

slope facing uphill. (b) the sliding-down of a mass of sediment shortly after its deposition on an underwater slope;

especially the downslope flowage of soft, unconsolidated marine sediments, as at the head or along the side of a

submarine canyon. (c) the mass of material slipped down during, or produced by, a slump.

slump deposits: a depositional unit caused by flope failure (slumping)

smokers (black and white): hydrothermal vents at ocean spreading centers from which high temperature mineralized

water (black smokers) and lower temperature water believed to contain abundant bacteria (white smokers) is exhaled.

In both cases chimneys of precipitated minerals may form on the seabed around the vent.

sniffer: a geochemical survey tool for measuring the concentration of hydrocarbon compounds in seawater.

(‘Sniffer’ is a trademark of the Interocean Corporation.)

sonogram (side-scan sonar): an acoustic image btained by side-scan sonar

sparker: a type of seismic profiling system. Conventional sparkers achieve a penetration of over 100 m, while deeptowed sparkers, which operate at a higher frequency, do not penetrate so far, but achieve better resolution.

species diversity: A statistically and mathematically complex means of expressing the variety of species found in an

assemblage or community. The simplest form is species richness, the number of species present. The utility of

species diversity is severely limited by sample-size effects and the quality of taxonomic identification.

streamer: a marine cable incorporating pressure hydrophones designed for towing through water while acquiring

seismic data

sub-bottom profiler: an instrument that produces a cross-sectional view of the near-surface sediment column. The

3.5 kHz subbottom profiler is commonly used in the Gulf of Mexico for geohazards and engineering studies.

subsurface: n. (a) the zone below the surface, whose geologic features, principally stratigraphic and structural, are

interpreted on the basis of drill records and various kinds of geophysical evidence. (b) rock and soil materials lying

beneath the Earth’s surface. —adj. Formed or occurring beneath a surface, especially beneath the Earth’s surface.

surficial sediments: sediments exposed at or very near the modern seafloor

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systems tract: a subdivision within a sequence: lowstand or shelf-margin, transgressive, and highstand. System

tracts are characterized by geometry and facies associations

target (side-scan sonar): an object that creates a well-defined reflectance pattern on a side-scan sonogram

thermocline: a discontinuity in water temperature and density in the ocean. Many thermoclines may be encountered

as one descends from the surface to depth in the ocean.

thermogenic (or petrogenic) gas: petroleum gas produced by the thermal cracking of organic material within deeply

buried sedimentary rocks.

three-dimensional (3D) seismic: seismic data collected over an area with the objective of determining spatial

relationships in three dimensions, as opposed to determining components along separated data acquisition lines.

Three-dimensional (3D) seismic data collection is structured on a grid so that data are uniformly distributed overn an

area.

timing lines (seismic): marks at precise intervals of time such as used on seismic profiles

tube worms: A general term for tube-dwelling worms of the phylum Vestimentifera. These worms are chemotrophic

and have affinities with the annelids and pogonophorans which challenge the validity of a separate phylum. Many

types of worms are tube-dwelling, and the term is easily misleading.

turbidity current: a bottom-flowing current resulting from a fluid that has higher density because it contains

suspended sediment

unit pockmarks: small (<5 m diameter), shallow pockmarks.

vent: site of escape of seeping fluids; most particularly used in describing the source of hydrothermal fluids, for

example at ocean spreading centers.

vent communities: faunal and micro-faunal communities found at vents, particularly the hydrothermal vents of, for

example, ocean spreading centers.

vented gas column: a stream of gas bubble above the seafloor (in the water column) that is the product of expulsion

of gas from the subsurface

vesycomiyids: Clam-like bivalves of the family Vesycomyidae. These are known to be chemotrophic.

water gun: a seismic source that propels a slug of water into the water mass, producing an explosive effect

wavelet: a seismic pulse usually consisting of only a few cycles

wipe-out zone: a region on a seismic record without internal reflections, commonly representing grassy sediments or

zones of thoroughly remolded sediments

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The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility

for most of our nationally owned public lands and natural resources. This includes fostering

sound use of our land and water resources; protecting our fish, wildlife, and biological diversity;

preserving the environmental and cultural values of our national parks and historical places;

and providing for the enjoyment of life through outdoor recreation. The Department assesses

our energy and mineral resources and works to ensure that their development is in the best

interests of all our people by encouraging stewardship and citizen participation in their care.

The Department also has a major responsibility for American Indian reservation communities

and for people who live in island territories under U.S. administration.

The Minerals Management Service Mission

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS)

primary responsibilities are to manage the mineral resources located on the Nation's Outer

Continental Shelf (OCS), collect revenue from the Federal OCS and onshore Federal and Indian

lands, and distribute those revenues.

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

administers the OCS competitive leasing program and oversees the safe and environmentally

sound exploration and production of our Nation's offshore natural gas, oil and other mineral

resources. The MMS Minerals Revenue Management meets its responsibilities by ensuring the

efficient, timely and accurate collection and disbursement of revenue from mineral leasing and

production due to Indian tribes and allottees, States and the U.S. Treasury.

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being

responsive to the public's concerns and interests by maintaining a dialogue with all potentially

affected parties and (2) carrying out its programs with an emphasis on working to enhance the

quality of life for all Americans by lending MMS assistance and expertise to economic

development and environmental protection.