ACCESS NUMBER: 30660-19910 

STUDY TITLE: Digital High Resolution Acoustic Data for Improved Benthic 

Habitat/Geohazard Evaluations 

REPORT TITLE: Improved Geohazards and Benthic Habitat Evaluations: Digital 

Acoustic Data with Ground Truth Calibrations 

CONTRACT NUMBER: 14-35-0001-30660-19910 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Central Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 1993 - 2001 

COMPLETION DATE OF REPORT: June 2001 

COST(S): FY 1993: $64,772; FY 1994: $52,372; FY 1995: $50,165; FY 1997: $225,225 

CUMULATIVE PROJECT COST: $392,534 

PROJECT MANAGER: H. H. Roberts 

AFFILIATION: Louisiana State University, Coastal Studies Institute 

ADDRESS: Howe/Russell Geoscience Complex, Baton Rouge, LA 70803 

PRINCIPAL INVESTIGATOR: Harry H. Roberts, Ph.D. 

KEY WORDS: Geohazards; hydrocarbon seepage; chemosynthetic communities; high 

resolution acoustic data; 3D-seismic surface attribute data 

BACKGROUND: 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. 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. 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. 
ACCESS NUMBER: 30660-19910 

OBJECTIVE(S): The objective of this study is to apply state-of-the-art digital acoustic 

data, both high resolution seismic and lower resolution, but deeper penetrating 3Dseismic 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 geohazards 

evaluation and 3D-seismic surface amplitude data. 

DESCRIPTION: 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 from 

3D-seismic data has added a new dimension that 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. 

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 

wipeout 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 wipeout zones to be the product of bubble phase gas in 

surface and shallow subsurface sediments. 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 wipeout 

zones can be associated with many different types of features and sea floor types. 

They range from mud volcanoes to mud mounds containing gas hydrates to areas of 

lithified sea floor and extinct expulsion sites. Through manned submersible inspection 

of these sea floor types and features that 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 wipeout 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. 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. 
ACCESS NUMBER: 30660-19910 

SIGNIFICANT CONCLUSIONS: 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. 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. Mud-prone features such as mud vents, mudflows, 

and mud volcanoes do not generally support complex chemosynthetic communities, 

and hydrocarbons reaching the seafloor are only slightly biodegraded. Mineral-prone 

features such as mounded carbonates, hardgrounds, or barite-carbonate-encrusted 

areas also do not support densely populated and complex chemosynthetic 

communities. 

STUDY RESULTS: 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). A shallow subsurface acoustic wipeout zone on high resolution seismic 

represented each one of these sites. 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. 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 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. 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. 

STUDY PRODUCT(S): 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. 179 pp. 

Roberts, H.H., B. Kohl, D. Menzies, and G.D. Humphrey, 1999, Acoustic wipeout zones 

- A paradox for interpreting seafloor geologic/geotechnical characteristics (An example 

from Garden Banks 161). Proceedings 31st Annual Offshore Technology Conference, 

OTC 10921:1-6. 

Roberts, H.H., and E.H. Doyle. 1998. Seafloor calibration of high-resolution acoustic 

data and amplitude rendering of the “diapiric hill”, Garden Banks 427. Proceedings 

Offshore Technology Conference; OTC Paper’ 8592:19-30. 
ACCESS NUMBER: 30660-19910 

Roberts, H.H. and R. Carney. 1997. Evidence of episodic fluid, gas and sediment 

venting on the northern Gulf of Mexico continental slope. Economic Geology, 92:863-

879. 

Roberts, H.H., E.H. Doyle, J.R. Booth, B.J. Clark, M.J. Kaluza and A. Hartsook. 1996. 

3D-seismic amplitude analysis of the seafloor: An important interpretive method for 

improved geohazards evaluations. Proceedings Offshore Technology Conference; 

OTC 7988:283-292. 

Roberts, H.H. 1995. High-resolution surficial geology of the Louisiana middle-to-upper 

continental slope. Gulf Coast Association of Geological Societies Transactions 45:501-

508. 

Roberts, S.J. and J.A. Nunn. 1995. Episodic fluid expulsion from geopressured 

sediments. Marine and Petroleum Geology 12:195-204. 

Roberts, H.H. and P. Aharon. 1994. Hydrocarbon-derived carbonate buildups of the 

northern Gulf of Mexico continental slope: A review of submersible investigations. Geomarine Letters, 14:135-148.