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NOTE: The sample image presented on this disc is the 2 degree, quadrant 13
image (Latitude 26 - 28 degrees North, Longitude 86 - 88 degrees West).
Contact Bonnie McGregor, USGS, Reston, VA, to obtain the Gloria CD-ROM
for complete Gulf of Mexico coverage.
FOREWORD
Dallas L. Peck
Director, U.S. Geological Survey
An act of Congress in 1879 established the U.S. Geological
Survey (USGS) as a scientific research organization and charged
it to conduct an "examination of the geological structure,
mineral resources, and products of the national domain." Not
until 1962, however, were these examinations extended into the
marine realm, when Congress first appropriated funds for offshore
investigations by the USGS. During the late 1960's and 1970's,
offshore geologic surveys conducted by the USGS rapidly expanded
as the result of national and international events such as
increased domestic oil and gas exploration and the oil embargo by
the Organization of Petroleum Exporting Countries.
On March 10, 1983, President Reagan proclaimed that the ocean
area out to 200 nautical miles off the coast of the United
States--including the Commonwealths of Puerto Rico and the
Northern Mariana Islands, and the island territories of the
United States--was the Exclusive Economic Zone (EEZ) of this
Nation. The proclamation increased the area of offshore Federal
lands to approximately 3.4 million square nautical miles, an area
about 30 percent larger than the total onshore area of the United
States. As noted by the National Advisory Committee on Oceans
and Atmosphere, considering the rate of depletion of the earth's
natural resources on land and the potential that the oceans are
believed to have for addition to our resource base, the
significance of the EEZ to the future of our country may well be
greater than that of the Louisiana Purchase of 1803. That act
(during the Presidency of Thomas Jefferson) doubled the size of
our country and brought with it a vast territorial and resource
base on which the United States grew during the 19th century.
In establishing the EEZ, this country has gained access to
potentially large energy and mineral resources that may lie on or
below the surface of the sea floor. Accordingly, the resources
of the EEZ are a potential means of ensuring national economic
security by allowing the Nation to become more self-sufficient in
strategic and critical minerals. However, the United States must
establish a framework for the orderly exploration and development
of the EEZ and provide sufficient information regarding the EEZ
to allow resolution among competing alternative uses. Private
ventures in the EEZ will be encouraged and enhanced both by
demonstrations of substantial potential energy and mineral
resources and by the provision of regulations that allow normal
competitive market forces to operate. In view of the substantial
resources that may exist in the EEZ, management of the EEZ will
also require adequate protection of the environment during future
exploration and development activities.
The EEZ, by comparison to long-studied onshore areas, clearly
is a new frontier. The exploration, characterization,
understanding, management, protection, and utilization of this
frontier present an exciting challenge to all elements of the
U.S. marine community including academia, industry, and
government. In meeting this challenge, the USGS has the
important role of developing an integrated, comprehensive
scientific understanding of the EEZ as a basis for formulation of
Government policies. In his State of the Union address of
January 25, 1984, President Reagan said "The Department of the
Interior will encourage careful, selective exploration and
production of our vital resources in an exclusive economic zone
within a 200-mile limit off our coasts . . . .". Under the
direction of the Secretary of the Interior and the Assistant
Secretary for Water and Science, the USGS assumed responsibility
for developing and coordinating an EEZ program of national scope.
As part of this effort, the USGS marine program in the EEZ
provides for the orderly exploration necessary to develop a
geologic understanding of these new Federal lands. The EEZ
program of the USGS serves the national need by developing or
extending our understanding of where mineral or petroleum
resources occur, the geologic framework in which such resources
may exist, the geologic environmental conditions that may be
encountered during their future exploitation, and how their
formation in ocean areas can aid in the search for analogous
onshore deposits of economic significance.
A logical first step in the exploration of a new frontier is
to map it. In April 1984 the USGS, in cooperation with the
Institute of Oceanographic Sciences (IOS) of the United Kingdom,
initiated Program EEZ-SCAN as a first effort to expand our
geologic understanding of the EEZ. This program was created to
map the EEZ at a reconnaissance scale using a unique sidescan
sonar system developed by IOS. This revolutionary system, known
as GLORIA (Geological LOng-Range Inclined Asdick), is capable of
mapping large areas of the sea floor on a single pass of the
ship. In 1984, GLORIA was used to map the EEZ off California,
Oregon, and Washington with spectacular and significant results.
During 1985, GLORIA was used to map the EEZ in the Gulf of Mexico
and off Puerto Rico and the U.S. Virgin Islands. The results of
those surveys are presented in this publication. Over the next
four years, EEZ-SCAN will be extended to the EEZ of the Atlantic
coast, Alaska, and Hawaii. We believe these surveys will provide
the critical "road maps" for future EEZ research.
This publication provides a graphic overview of the EEZ in the
Gulf of Mexico and the eastern Caribbean, but it represents only
a fraction of the information that must be collected and analyzed
in the exploration of the new frontiers. It illustrates how
cooperative efforts between governments can successfully deal
with the challenges of exploration. Most important, perhaps, the
many discoveries illustrated in the atlas remind us that this is
a beginning, not an end, to understanding our marine heritage.
INTRODUCTION
PLEASE NOTE: The sonar mapping technology described in
this document is applicable to the Atlas of the Exclusive
Economic Zone, Gulf of Mexico and Eastern Caribbean Areas as well
as the GLORIA CD-ROM Disc. The Atlas is a separate USGS product
and is available for $45.00 per copy from the following source:
Map Distribution Section
U.S. Geological Survey
Box 25286, Federal Center
Denver, CO 80225
USGS Map I-1864A,B
Introduction
On March 10, 1983, President Reagan signed a proclamation
establishing an Exclusive Economic Zone (EEZ) extending 200
nautical miles seaward from the coasts of the United States, the
Commonwealths of Puerto Rico and the Northern Mariana Islands,
and the U.S. territories and possessions. Within this zone, the
United States claimed jurisdiction over the seabed and its
resources (fig. 1). As part of its mission to map the federal
lands and to determine their resource potential, the U.S.
Geological Survey (USGS) began a program in 1984 to provide maps
of the EEZ. The reconnaissance-scale mapping tool that the USGS
selected was the long-range sidescan-sonar system, GLORIA
(Geologic Long-Range Inclined Asdic), owned and operated by
British colleagues at the United Kingdom's Institute of
Oceanographic Sciences (IOS).
Sidescan sonar was selected as the mapping tool because it
can be used to obtain information on geologic processes. The
intensity of the back-scattered sound from the sea floor is a
function of the gradient or slope of the sea floor, of the
microtopography or surface roughness, and of the sediment
characteristics such as texture or induration. Because sidescan
sonar provides information from a swath of sea floor, large areas
can be mapped quickly.
In the summer of 1984 the USGS began its sidescan-sonar
survey of the EEZ off the coasts of Washington, Oregon, and
California. In 100 days, the EEZ was mapped from the Canadian to
the Mexican border, extending from the continental shelf edge
(approximately 200 meters (m) water depth) to the seaward
boundary. Results of this survey were published by the USGS in
the Atlas of the Exclusive Economic Zone, Western Conterminous
United States (EEZ-SCAN 84 Scientific Staff, l986) (fig. 2).
Continuing the mapping effort in the late summer and fall of
1985, the USGS conducted surveys of the EEZ in the Gulf of Mexico
and around Puerto Rico and the U.S. Virgin Islands.
1
DATA COLLECTION, l982 AND l985 SURVEYS
Sixty-seven days in 1985 divided into three cruise legs were
spent mapping in the Gulf of Mexico (fig. 3). This study abutted
an area surveyed with the GLORIA system in 1982 as part of Outer
Continental Shelf geohazards work that focused on the Texas-
Louisiana continental slope. Also in 1982, preliminary work for
the Deep Sea Drilling Project (DSDP) Leg 96 site surveys was done
on the Mississippi Fan in the eastern Gulf. Combining the data
from the 1985 and 1982 surveys provides sidescan-sonar coverage
of the EEZ in the Gulf of Mexico from just seaward of the shelf
edge to a maximum water depth of 3,600 m. In addition to the
sidescan imagery, seismic-reflection profile data and total-
magnetic-field data were collected along the ship tracks during
much of the surveys.
Legs l, 2, and 3 of the 1985 survey focused on the western,
central, and eastern Gulf of Mexico respectively (fig. 3). Leg 1
departed from Miami, Fla., crossed to the western Gulf, and ended
in New Orleans, La. Leg 2 began in New Orleans and ended in
Tampa, Fla.; and Leg 3 started from Tampa and finished in Key
West, Fla. The 1982 survey, begun in New Orleans, ended in
Miami.
Primary navigation for the 1982 and 1985 data collection was
with loran-C, except on Leg 3 when transit satellite and Global
Positioning System (GPS) were used. Transit satellite fixes,
and, in 1985, GPS data, were also logged for comparison on Legs 1
and 2. Our estimate of the positional accuracy of the loran-C
navigation is expected to be no worse than + 200 m.
Tracklines of the MV Farnella, the ship used for the survey
in the Gulf of Mexico, are shown on p. A6 and A7 (figs. l0 and
ll). The Julian day (consecutive day of the year, starting with
January l as Julian day l) is annotated on the tracklines twice
each day, the time is Greenwich Mean Time and is annotated every
l2 hours (hr), and every hour is marked by an arrowhead that
gives the direction of profiling. The l982 survey was conducted
from Julian day 33 to 55, and the l985 survey from Julian day 2l9
to 295 (fig. 3). Orientation of the tracklines is in general
parallel to the trend of the bathymetric contours. Trackline
spacing was determined so that overlap of sidescan data from
adjacent tracks was achieved. The pronounced thermocline in the
Gulf of Mexico during the summer months reduced the swath width
of the 1985 survey, which required a closer trackline spacing
than that chosen during the l982 survey. Because of the reduced
swath width in shallow water, the sidescan imagery coverage in
the western and eastern Gulf begins seaward of the shelf edge.
GLORIA II SIDESCAN-SONAR SYSTEM
The GLORIA system is a long-range sidescan-sonar tool
developed by IOS specifically to map the morphology and texture
of sea-floor features in the deep ocean. Sidescan-sonar images
(sonographs) are a record of the acoustic backscatter properties
of the sea floor. These images of the sea floor are formed by
transmitting sound pulses from two sets of transducers in a towed
vehicle which look to port and starboard, respectively. The
transducers are tuned so that their beams form a narrow arc
(2.7o) in the horizontal plane and a broad arc in the vertical
plane. Each transmitted sound signal thus insonifies a narrow
band of sea floor from directly beneath the towed vehicle out
perpendicular to the ship's track to the maximum range the
acoustic signals travel to both sides. By varying the interval
between the emission of pulses (20, 30, or 40 seconds (s)), the
widest possible swath of sea floor mapped is 30, 45, or 60
kilometers (km), respectively. As the ship moves, successive
bands of sea floor are insonified, and in this way an acoustic
map of the sea floor is recorded.
A few of the important technical features of the GLORIA
system are provided here. For more detailed information and
specifications, the reader is referred to Somers and others
(1978). The sidescan vehicle, or "fish", is 8 m long, weighs
2.25 tons in air, and is almost neutrally buoyant. The sonar
arrays consist of a total of l20 transducers, 30 to a row, 60 to
each side. The vehicle is towed about 400 m behind the ship with
no active depth control, but at the normal survey speed of 8
knots (kts), or l5 km/hr, the vehicle depth is about 50 m (fig.
4). The operating frequency of the GLORIA system is about 6.5
kiloHertz (kHz), with the port array at 6.8 kHz and the starboard
array at 6.2 kHz to eliminate cross-talk between the two sides.
Each array is 5.3 m long by 40 centimeters (cm) high, a
configuration that gives a horizontal acoustic beam 2.7o wide and
a vertical beam of 35o. The beam width is specified between
half-power points, and considerable energy actually radiates
outside these limits. The arrays are designed to confine the
energy as nearly as possible to the plane perpendicular to the
track and to fill the quadrant from nadir (the point on the sea
floor directly beneath the towed vehicle) up to near horizontal.
The maximum swath width largely depends on the prevailing
acoustic propagation conditions of the water column. For GLORIA
the swath width can be as great as 30 km on each side of the
track. Under normal conditions, however, it is usually somewhat
less. If acoustic conditions are unfavorable and the water depth
is less than about 1,500 m, then the range may be less than 10 km
on each side of the track. In the Gulf of Mexico during August
through October, the pronounced thermocline reflected the far-
range, low-incident-angle sound waves, restricting the swath
width. The maximum range for the data reported in this atlas is
15 km to either side (30 km total swath). In the shallow water
of the continental slope the maximum range obtained was
considerably less.
The acoustic energy reflected from the sea floor is recorded
in digital format on magnetic tape. Each pixel of the image has
a size, measured along the track, that is proportional to the
range and that increases to hundreds of meters at extreme range
because the sound beam diverges at 2.7o. The recording system
was designed so that one complete scan is subdivided into 1,000
pixels. The cross-range pixel size represents about 50 m, which
is smaller than the along-track pixel size; thus features in the
raw data are elongated parallel to the track, particularly at
extreme range.
OTHER GEOPHYSICAL DATA
Collected simultaneously with the sidescan-sonar data was a
suite of geophysical data, which can be used to aid in the
geologic interpretation of the sidescan imagery. Seismic-
reflection profile data were collected along the ship's track by
means of three systems: a 10-kHz echo sounder, a 3.5-kHz high-
resolution subbottom profiler, and an airgun system.
The airgun system consisted of a 160-cubic-inch airgun (80 cubic
inch in the eastern Gulf on Leg 3) fired every 8 to l0 s using
air compressed to l500 pounds per square inch (psi); and, as a
receiver, a two-channel hydrophone with 48 elements in each of
two active sections. Each type of data was recorded on analog
recorders.
A proton-precession magnetometer was towed to collect total
magnetic field values of the Earth along the ship's track during
the 1985 survey.
ATLAS FORMAT AND KINDS OF DATA PRESENTED
This atlas is composed of three data sections. The first
section presents the sonar-imagery mosaics of the EEZ sea floor,
along with generalized geologic interpretations and bathymetry.
Following the mosaics is a section providing seismic-reflection
data collected during the surveys. The third section presents
data on bathymetry and residual magnetic anomalies throughout the
l985 survey area.
MAP PROJECTION AND SCALE
The sidescan-sonar imagery is displayed on l6 sheets, which
cover the northern half of the Gulf of Mexico. Each sheet covers
two degrees of latitude from top to bottom and two degrees of
longitude from side to side, except for five sheets near the
edges of the survey, which cover only one degree or one-and-one-
half degrees of latitude or longitude. An Albers Equal-Area
projection was used for the sheets of imagery data and their
companion overlays of bathymetry and geologic interpretation.
The standard parallels used were 29.5o and 45.5o N. An Albers
projection has some shape distortion with increasing distance
from the standard parallels.
Maximum distortion in the Gulf of Mexico is in the southernmost
sheets and is l.5 percent in both the east-west and north-south
directions. The imagery sheets are printed at a scale of
1:500,000. At this scale one centimeter of distance on the sheet
represents 5 km on the Earth's surface or 1 inch equals
approximately 7 nautical miles. The Albers projection was
selected for the imagery data display to be compatible with the
USGS Continental Margin Map Series (fig. 2). This map series is
a compilation of offshore geologic data into a digital data base
(Escowitz, 1985).
SONAR-IMAGERY MOSAICS
The sidescan-sonar data were mosaicked to produce 16 sheets,
which provide continuous imagery of the sea floor of the EEZ in
the Gulf of Mexico (p. A10 and A11). Each of the imagery sheets
has a complementary facing page showing the imagery screened
(subdued) under an overlay of generalized geologic interpretation
and bathymetric contour data. Each of the sheets is composed of
data segments approximately 6 hr in length that were image
processed by computer, then mosaicked end-to-end along the ship's
track, producing swaths. Adjacent swaths were stenciled together
also by computer, resulting in a digital 2o by 2o (or smaller)
sheet. A regular pattern of bands is visible on the imagery and
represents the location of the ship's track. Because major
spatial corrections are necessary in the area directly beneath
the sidescan vehicle, individual pixels (digital picture
elements) from this zone are enlarged, thus creating the pattern
that shows the ship's path. Being able to identify the location
of the ship's track is important in interpreting features,
especially shadows. Identification of the ship's track also
allows correlation of profile data (for example, seismic
reflection and magnetic anomaly) with the imagery.
Each mosaic sheet is a halftone black and white print of the
acoustic reflectivity of the sea floor. White represents the
strongest acoustic reflectivity and black represents the weakest
acoustic return. The darkness or lightness of a feature or an
area on the mosaics, therefore, is a function of how much sound
is reflected from the sea floor. Reflectivity, in turn, is
controlled by the relief of the sea floor (height and gradient),
by the microtopography and roughness of the sea floor (for
example, sediment waves, and pressure features on submarine
landslide deposits), and by the physical properties of the sea
floor (such as sediment type, compaction, and induration). When
viewed from the trackline, positive-relief features such as domes
and escarpments (Sigsbee or Florida Escarpments) usually appear
as a bright zone when facing the slope, whereas when looking down
the slope, dark zones (shadows) result (fig. 5). Negative-relief
features, such as canyons or basins, usually appear as a dark
zone (the near wall is shadowed) followed by a bright zone (the
far wall is facing the sonar beam). Not all bright regions are
related to topographic relief; many are caused by sedimentation
patterns.
Dark regions also have a variety of causes, of which shadows and
certain sediment facies are the most common. Contained within
the sidescan imagery is a wealth of information on both the
geology of the sea floor and the processes by which sediment is
deposited.
BATHYMETRY
The bathymetric contours are printed in blue directly onto
each imagery sheet along with the geologic interpretation.
Geographic names, also printed in blue, correspond to features
identified on the airgun records in the seismic section of the
atlas and are approved names from the Gazetteer of Undersea
Features (Defense Mapping Agency, 1981). The bathymetric
contours are in meters, with a 250-m or 500-m contour interval,
except for a 100-m interval in areas where the water depth
exceeds 3,000 m. The bathymetry was digitized from the National
Oceanic and Atmospheric Administration's (NOAA) National Ocean
Service (NOS) Bathymetric Map Series of l:250,000 and l:l,000,000
scale (see references). The quality and accuracy of the
bathymetric data vary from sheet to sheet, and under no
circumstances should the bathymetric maps be used for navigation.
The continental slope seaward of Texas and Louisiana has a very
complex topography resulting from salt deformation. In this area
the bathymetry may not be a good representation of the topography
and may not align with the imagery. The bathymetry should be
used as a guide for interpretation and not as a precise rendition
of the topography.
SEISMIC-REFLECTION PROFILE DATA
Three seismic-reflection profile systems were used: a 10-
kHz echo sounder, a 3.5-kHz high-resolution subbottom profiler,
and an airgun system. Depth values were digitized from the 10-
kHz echo sounder every 6 minutes (min) along with intermediate
peak, trough, and inflection-point values. These values, in
meters, were corrected for sound velocity and merged with the
navigation data. The bathymetry values were used in the image
processing to remove the water column for the sonographs. They
are shown as profiles plotted along the ship's track at the
beginning of the seismic-reflection profile section of the atlas
(p. A46 and A47) as a key to aid in correlating the seismic-
reflectionprofile data with the imagery. The bathymetry data are
also shown in the magnetics section of the atlas for comparison
with the magnetic anomaly data.
The 3.5-kHz data provide high-resolution, shallow-
penetration seismic-reflection information that was used to help
interpret the sidescan-sonar data. Examples of the 3.5-kHz data
are shown to clarify features that are identified as part of the
geologic interpretation of the mosaics.
The airgun data were recorded on analog recorders at
different sweep rates (2 and 4 s in 1982 and 5, 8, and 2 or 10 s
in 1985). The data were filtered between 20 and 200 Hz. Shown
in the seismic-reflection profile section are photographs of the
analog records recorded at a sweep rate of 4 s for the 1982
survey and 5 s for the 1985 survey. The photographs are printed,
however, so that the vertical depth scale is consistent
throughout the section. The vertical scale is annotated in two-
way traveltime, in seconds. As an approximation, each second of
two-way traveltime equals 750 m of depth (assuming a sound
velocity in sea water of 1500 m/s). Within the sediments, the
velocity of sound may be less, but generally it is greater, which
means that each second within the sediments represents more than
750 m (for example, 800 to 900 m).
The Julian day and time are annotated along the profiles, as
are course and speed changes. The l982 survey was conducted
during Julian days 33 to 55, and the l985 survey from Julian day
2l9 to 295. The data are broken into segments to draw attention
to major course changes, in order to facilitate correlation with
the track map at the beginning of the seismic-profile section and
with the imagery sheets. Occasional gaps occur in the records as
a result of repairs being made to the airgun system; such gaps
are identified with the words "No data". In some cases there is
an actual gap in the data coverage, and in other cases the vessel
turned off the track course for repairs but returned to allow
complete coverage. By checking times on the records against the
ship's tracks, one can determine if a real data gap exists.
Handwritten annotations, visible on some of the records, are the
shipboard remarks for post-cruise analysis.
MAGNETIC-ANOMALY DATA
Total Earth's magnetic field values were obtained along the
ship's track. No magnetic field data were collected during the
1982 survey. Magnetic field data were collected continuously
during the 1985 survey beginning on Julian day 224. Only minor
gaps occur in the data; they were caused by system malfunction
and when the system was secured in preparation for port stops.
Total magnetic field values were logged by computer and merged
with the navigation and bathymetry information. Residual
magnetic anomaly values were calculated by subtracting the
International Geomagnetic Reference Field updated to 1985 (IAGA,
Division 1, Working Group I, 1986) from the total Earth's
magnetic field values measured. These residual anomaly values
are displayed in the magnetic anomaly profile section of the
atlas as continuous profiles plotted with bathymetric data for
each Julian day. The anomaly data are plotted in gammas (one
gamma represents one nanotesla) against time, and are shown as
profiles plotted along the ship's track at the beginning of the
magnetic-anomaly section of the atlas (p. A82 and A83). The
residual magnetic anomalies are subdued in the Gulf of Mexico
except for several hundred-gamma anomalies near the Florida
Escarpment in the eastern Gulf.
DIGITAL PROCESSING TECHNIQUES
The imagery in this atlas is made from computer-processed,
digitally collected sidescan sonographs. In order to process the
digital sonar data, computer software had to be designed that
would correct for both geometric and radiometric distortions that
exist in the original "raw" data. This section describes the
techniques developed by the USGS to correct and enhance GLORIA
digital sonar images. A more detailed explanation of the digital
processing is given by Chavez (1986).
Sonographs are a record of the acoustic backscattering
properties of the sea floor; those from GLORIA represent the
backscatter of the sea floor produced by the 6.5-kHz frequency.
The strength of the acoustic backscatter is a convolution of
several functions, the four major ones being: (1) the slope angle
of a feature relative to the incident sonar signal (topographic
characteristics); (2) the sea-floor roughness factor, the minimum
being 4 cm of relief for the GLORIA system (determined by the
wavelength of the sonar and the grazing angle of the sonar ray to
the sea floor; Sabins, 1978); (3) the variation in physical
properties of the upper few tens of centimeters of the sea floor;
and (4) the water column--the distance of the vehicle above the
sea floor--which attenuates the strength of the sonar signal as
well as produces background noise. The backscatter was recorded
as a digital number (DN) with a range of 0 to 255 discrete values
(8-bit data generated from a pulse of sound every 30 s to give a
maximum range of 22.5 km on each side of the ship's track). The
reflected sound waves were recorded on a time basis so that the
data are in a slant-range rather than a ground-range (true
geographic) geometry. Also, the variations in the ship's speed
generated variations in the size of the footprint (the area on
the sea floor) of each pixel in the along-track direction. These
distortions, as well as others discussed below, were corrected so
that the sonograph images represent orthorectified and true plan
views (assuming a flat sea floor) of acoustic backscatter
patterns on the sea floor.
GEOMETRIC CORRECTIONS
The major sources of geometric distortions in the sidescan-
sonar data are: (1) water-depth offset; (2) slant-range
geometry; (3) aspect- or anamorphic-ratio distortion; and (4)
changes in the ship's speed. Each of these major distortions was
eliminated from the data presented in this atlas. Below is a
brief explanation of the procedures used to correct for these
geometric distortions.
The GLORIA system starts recording data as soon as the
transmitted acoustical wave is terminated, and therefore the
original images include pixels to both sides of the nadir (the
projection directly beneath the ship) that contain water-column
data.
The number of these pixels varies along the trackline as a
function of the water depth directly beneath the ship. The
processing software merges the navigational and bathymetric data,
recorded every four sonar scan lines, with the image data.
Included as part of the header information for each pixel in the
across-track direction is the water depth directly beneath the
ship. The water-depth offset can be calculated in pixels to
predict the location on each side to which the nadir pixel was
mapped.
The slant-range distortion is a consequence of the range to
features being measured relative to the vehicle, not to the
projected location of the trackline on the sea floor below the
vehicle. In trigonometric terms, GLORIA measures the length of
the hypotenuse of a triangle (the distance a feature is from the
vehicle). From the bathymetric data the vertical side of the
triangle is known, and, therefore, the horizontal side of the
triangle can be determined to properly locate pixels in the
across-track direction. Thus, for example, in the raw data the
point on the sea floor directly below the trackline--the nadir--
is plotted the distance the vehicle is above the sea floor away
from the nadir line on both the port and starboard sides. The
water-depth and slant-range corrections remove the water column,
thus drawing the nadir into the zero range (the point directly
beneath the ship); and in the same way they properly locate other
features in the across-track direction.
The third major geometric distortion present in GLORIA
images is the aspect ratio, or anamorphic ratio, that exists
between the along- and across-track directions. The sampling
interval in the across-track direction for the 30-s pulse-
repetition rate generates pixels with an approximate resolution
of 45 m. The computer program that corrects for the water-depth
and slant-range distortions generates 50-m-resolution pixels in
the across-track direction. However, the resolution in the
along-track direction is dependent not only on the 30-s pulse-
repetition rate but also on the ship's speed. The average
resolution in the along-track direction is approximately 125 m at
a speed of 8 kts (15 km/hr), which produces images with an
aspect-ratio distortion of about 2.5. This generates a raw image
that is distorted or stretched by a factor of about 2.5 in the
along-track direction. The fourth, and related, source of
geometric distortion is introduced by any change that occurs in
the ship's speed while it is collecting the image data. The
ship's speed is influenced mainly by the direction and strength
of current and wind relative to the ship's course and by whether
the ship is heading in a straight line or is in a turn. During
the surveys the speeds varied from about 7 to 10 kts (13 to 18
km/hr), which caused the pixel resolution in the along-track
direction to vary from approximately 110 to 140 m. This
introduced an "accordion" effect into the geometry of the image
in the along-track direction (Chavez, 1986).
The aspect-ratio distortions discussed above were removed by
using the latitude and longitude values extracted from the header
of each record to compute the distance traveled by the ship every
30 min (unless a turn is detected, in which case the program used
a 10-min interval). Given the distance traveled and the desired
pixel size, the number of pixels required for the particular 30-
min segment was calculated. To simultaneously correct for the
aspect-ratio distortion, a 50-m-resolution pixel size was
generated on the output image in the along-track direction. This
pixel size was selected so that information in the across-track
direction would not have to be omitted. This procedure corrects
the image data for aspect-ratio distortions and for any
distortion introduced by changes in the ship's speed.
RADIOMETRIC CORRECTIONS
Radiometric corrections, the second major category of
processing steps, change the DN value of a pixel rather than its
spatial location, as is the case with geometric corrections.
Four different corrections were used for the GLORIA data in this
atlas: (l) a shading correction to correct for the attenuation
of the sonar energy in water as a function of range; (2) a power
correction for very-near-nadir data because of slow buildup; (3)
a speckle-noise correction; and (4) removal of striping noise.
The value of making these corrections is that, in the case of
noise, the corrections remove artifacts associated with the data
acquisition, and in the case of attenuation, they normalize all
data so that pixel values from the across-track direction can be
compared directly with each other.
The loss of power due to attenuation and the power buildup
problems were both corrected by using a two-pass algorithm.
During the first pass through the data, the averge DN value was
computed for each column of pixels of the digital image in the
along-track direction. These values were then normalized by the
average of all the column averages (the overall average of the
image) to generate correction coefficients for each column. The
correction coefficients were then applied to each pixel during
the second pass through the data. The coefficients are nonlinear
and are a function of range, so they effectively removed
attenuation in the across-track direction. This technique has
the characteristics of a spatial filter that removes large
horizontal low-frequency patterns that are present because of the
radiometric problems introduced by the imaging system (Chavez,
1986). By normalizing to the average of the image rather than to
a set DN value, backscatter comparisons could be made between
different areas or different images. The correction also allowed
areas within the image with lower or higher backscatter
characteristics to be properly identified and mapped. This was
not possible before the correction because the DN values were
strongly modified as a function of their range position.
Profiles of different areas in the across-track direction could
then be used for backscatter comparison.
Speckle noise was removed by applying a small smoothing
filter (2 samples by 2 lines) to the entire image. This approach
was found best with GLORIA data, because in addition to removing
speckle noise, it helped smooth the blocky appearance that the
image otherwise would have had--caused by the stretching (by a
factor of 2.5) in the along-track direction by pixel
duplication--that was introduced when correcting for the aspect-
ratio distortion.
Another radiometric or noise problem commonly present in
data collected by scanning devices is striping in the scan
direction. A combination of high- and low-pass spatial filtering
was used to remove the striping. Two separate images were
generated from the input data; one composed of the high-frequency
components minus the noise frequency, and the other composed of
the low-frequency components minus the noise. The two resultant
images were then combined to produce an image similar to the
original but without the noise (Chavez and Soderblom, 1974). The
filter shapes used to remove the striping noise from the GLORIA
data were a 1-line by 71-sample high-pass filter and a 9-line by
71-sample low-pass filter (except for the 1982 data, for which a
21-line by 71-sample low-pass filter was used).
DIGITAL MOSAICKING
After the sidescan-sonar data underwent the initial
processing to correct them geometrically and radiometrically,
they were ready for the digital mosaicking steps that result in
the 2o x 2o (or smaller) imagery sheets shown in the sonar mosaic
section of the atlas.
The initially processed segments of data, approximately 6 hr in
length, were spliced end-to-end to make a continuous line segment
for portions of the tracklines where the ship's heading remained
generally constant. The segments were tone matched by adjusting
the contrast stretch of each to minimize the seam where they were
joined.
Navigational information (latitude and longitude) was
determined along the center line of the continuous segment of
data at the start, end, and two intermediate points. These four
control points, with additional pairs of points for each located
along the edges of the image, were used to position the
continuous line segment within the 2o x 2o sheet. After the map
projection and latitude and longitude boundaries for the sheet
were selected, a transformation file was created to position the
12 control points and then the complete image within the sheet.
The next step was to stencil adjacent line segments
together, providing a continuous mosaic covering the 2o x 2o
sheet. Interactively a line was drawn on a video display
outlining the portion of each image to be retained. This line
was smoothed and then converted from vector to raster format.
The rasterized mask was superimposed on the sidescan image and
all pixels outside the area of the mask were converted to zero
values, thereby retaining only that portion of the image desired.
After this process of stenciling, each line segment was mosaicked
to adjacent segments, sequentially building the composite map.
In this way a digital file for each of the 2o x 2o sheets was
created with the desired map projection. The scale of the sheets
was determined by output of film negatives from the Scitex
scanner.
GEOLOGIC INTERPRETATION
The GLORIA sidescan-sonar images provide a unique view of
the sea floor in the deep water of the Gulf of Mexico. A better
understanding of the morphology, surficial geology, and
sedimentary processes of the continental slope and rise in the
Gulf of Mexico is important for evaluating and developing energy
and mineral resources, and for siting sea-floor structures. The
GLORIA data provide information on depositional environments and
geologic processes that is important in developing depositional
models useful as analogs in understanding the rock record.
The Gulf of Mexico, a small ocean basin, is geologically
diverse. The EEZ in the Gulf of Mexico can be divided into
three major sedimentary provinces: a salt deformation province
in the western section, the Mississippi Canyon and Fan system in
the central section, and a carbonate province in the eastern
section, which is separated from the terrigenous Mississippi Fan
by the Florida Escarpment.
WESTERN GULF OF MEXICO
The most striking feature on the imagery in the western Gulf
is the Sigsbee Escarpment, which marks the seaward edge of the
salt deformation province. The 3000-m contour roughly outlines
the base of this escarpment (fig. 6). Vertical relief on the
escarpment is approximately 500 m, and piles of debris are
visible on the sonographs along its base, suggesting that
sediment is transported down the escarpment. Seaward of the
escarpment, patches of highly reflective (bright) sea floor with
numerous lineations are interpreted as bedform fields formed by
the reworking of debris from the escarpment. Seismic-reflection
profiles across the escarpment suggest that a wedge of salt is
overriding sediments that were deposited in the deep waters of
the Gulf (Amery, 1969).
Landward of the escarpment, the continental slope has a very
complex morphology, formed in response to intrusion by the salt.
The continental shelf in the Gulf of Mexico prograded seaward
during the Tertiary as a series of depocenters migrated eastward
from the Rio Grande River area of Texas to the presently active
Mississippi River area in the north-central Gulf (Humphris,
1984).
Loading of these Tertiary sediments onto an underlying salt layer
of Jurassic age has resulted in diapiric intrusion by the salt.
Diapirs have created numerous domes and isolated basins on the
slope that have significantly influenced the paths of submarine
canyons crossing the continental slope. Two remnant pathways
appear to be traceable on the GLORIA imagery from the upper
slope, tying into the two major reentrants in the Sigsbee
Escarpment. The reentrant near long. 92o W. has a leveed channel
emanating from it and meandering seaward (to the southeast) for
approximately 160 km to the edge of the survey area. Identifying
salt domes and basins on the slope based on imagery alone is
difficult. In some cases the salt domes are highly reflective,
because of the inclination of the flanks or the microtopography
on the crests, whereas in other instances basin floors are
reflective, possibly because of differences in sediment texture.
An integration of bathymetric, seismic-reflection profile, and
imagery data is necessary for a detailed interpretation of the
sea-floor morphology.
The migrating depocenters in the Gulf have consisted of a
series of deltas, some of them located near the edge of the
continental shelf (Suter and Berryhill, 1985). On these deltas,
as on the modern Mississippi River Delta, mass wasting resulted
in slumps and slides, and was an important process in
distributing sediments seaward. One such Pleistocene-age shelf-
edge delta is located in the northwestern corner of the Gulf in
the East Breaks area. Lehner (1969) described a major submarine
slide that originated from the edge of this delta. The extent of
the slide and the influence that salt diapirs had on its path are
both revealed on the GLORIA images. The brightness of the
backscatter from the surface of the slide is related to
microtopography having relief of approximately 10 m (fig.7)
(McGregor and Twichell, 1985). The continental slope seaward of
the Rio Grande River has also undergone extensive mass wasting,
as indicated by highly reflective sea floor classified as
irregular sea floor (ISF) in the interpretation. Part of the Rio
Grande Fan also has high surface reflectivity. The 3.5-kHz
profiles (fig. 8) show that this region is characterized by a
hummocky sea floor. Regions mapped as being hummocky sea floor
have been interpreted not only from the imagery but also from the
3.5-kHz data, with characteristics of irregular surface relief
similar to that shown in figure 8.
CENTRAL GULF OF MEXICO
The Mississippi Canyon and Fan system is the dominant
morphologic feature in the central Gulf. The Sigsbee Escarpment
cannot be identified because it is buried by sediments from the
Mississippi Canyon. Diapirs influence the location of the
Mississippi Canyon on the continental slope and are present on
the slope northeast of the Mississippi Canyon over to De Soto
Canyon. On the imagery they have high backscatter in comparison
to the surrounding sea floor, which is very dark.
The 3.5-kHz subbottom data show this dark area to be well
laminated; it is characterized as such in the explanation for the
geologic interpretation of the sonar-imagery mosaics (p. A8, fig.
l3).
The Mississippi Canyon shows as a highly reflective region
on the imagery because it is filled with debris-flow deposits.
The seismic-reflection profile records show this canyon fill to
be composed of acoustically transparent units. Where the flows
have spilled over the canyon walls at meanders, the deposits have
an arcuate pattern on the imagery. As the filled Mississippi
Canyon is traced seaward onto the middle part of the Mississippi
Fan, a meandering channel becomes apparent. Adjacent to this
channel on the eastern fan the imagery shows a highly reflective
region marked by very intricate flow patterns. This region is
interpreted to be debris-flow deposits emanating from the
Mississippi Canyon and covering most of the fan. On the 3.5-kHz
subbottom data across most of these deposits, there is no
subbottom penetration (fig. 9). Walker and Massingill (1970)
identified submarine slump features on the mid-fan based on
seismic-reflection profile data. Prior to collection of the
GLORIA data, the immense area covered by material emplaced by
mass-wasting processes was not imagined. The mid-fan channel is
overrun and buried by these debris-flow deposits. The channel
appears different acoustically upstream of the blockage because
sediments have filled it. Downstream, the channel floor is not
obvious. The surface of the fan to the west of the channel has a
different acoustic character than to the east. It is also
covered by a series of debris flows, some of which can be seen to
originate from the bank overflow features on the upper fan.
South of the channel on the mid-fan are pronounced south-trending
lineations that are associated with erosional processes on this
part of the fan. Some areas of the fan are characterized as
having an irregular surface, which may represent partly buried
material related to older mass-wasting events.
EASTERN GULF OF MEXICO
In the northeastern Gulf, a highly reflective area on the
imagery marks submarine debris-flow deposits in the De Soto
Canyon area. A meandering channel shown as a bright sinuous line
on the images emerges from the deposits and trends southward,
parallel to the Florida Escarpment. Another, less reflective,
channel (perhaps abandoned) lies closer to the base of the
escarpment, and a tongue of the debris-flow deposits from De Soto
Canyon extends into this channel. The bright meandering channel
is part of an elevated channel and levee system (levee ridge).
The highly reflective debris-flow deposits from the Mississippi
Canyon area are dammed by this elevated channel until the flow
eventually overtops the levee and buries the channel. The
southern extent of this channel and its deposits is masked by the
debris-flow deposits.
The dominant feature in the eastern Gulf is the Florida
Escarpment, which has a gradient of as much as 40o and relief
ranging from 1,000 m in the north near De Soto Canyon to 2,500 m
west of the Florida Keys. The escarpment consists of Lower
Cretaceous to Miocene carbonates and is part of a reef that
extends north from Mexico through Texas, southeast from
Mississippi, around Florida and the Bahamas (Bryant and others,
1969; Antoine and others, 1967), and north under the eastern U.S.
Atlantic continental margin (Schlee and others, 1979).
On the basis of seismic-reflection profiles, the relief and
morphology of the Florida Escarpment are attributed to the
vertical growth of a reef on the gradually subsiding continental
margin (Antoine and others, 1967; Corso and Buffler, 1985).
Samples recently collected by dredging and by submersible dives
indicate that only a fine-grained back-reef lagoonal facies is
exposed on the escarpment (Freeman-Lynde, 1983). The GLORIA
imagery shows that the erosional morphology varies along the
escarpment. North of lat. 27o N. the escarpment is dissected by
a series of closely spaced canyons with tributary gullies. South
of lat. 27o N., large box canyons with nearly vertical headwalls
have been cut as much as 15 km into the escarpment. Numerous
scarps having up to 250 m of relief are present in the carbonate
sediments above the escarpment. These scarps are the product of
mass wasting of the carbonate sediments. Some of the scarps
align with the canyons, suggesting that the canyons are conduits
for mass-wasting products from the continental slope above.
Massive slumps of the Tertiary sequence have resulted in the
deposition of carbonate material in the deep water of the Gulf,
interbedded with clastic material of the Mississippi Fan.
SUMMARY
Mass wasting was found from the GLORIA survey to be much
more extensive in the Gulf of Mexico than previously thought.
The survey data revealed that movement of terrigenous sediments
and both lithified and unconsolidated carbonates has occurred in
a variety of styles and volumes. Mass-wasting processes have
been a major contributor to the sediments in the deep water of
the Gulf of Mexico. Four submarine channels can also be
identified meandering across the floor of the Gulf, providing
pathways for the transportation and distribution of sediments.
PARTICIPANTS IN THE GULF OF MEXICO SURVEYS
l985 l985 l985
1982
SCIENTIFIC PERSONNEL Leg 1 Leg 2 Leg 3
Dr. Bonnie McGregor, USGS, co-chief scientist..........X.......X
Mr. Guy Rothwell, IOS, co-chief scientist..............X
Mr. Neil Kenyon, IOS, co-chief scientist.......................X...............X
Mr. David Twichell, USGS, co-chief scientist...................X.......X.......X
Dr. Lindsay Parson, IOS, co-chief scientist............................X
Dr. Louis Garrison, USGS, co-chief scientist...................................X
Dr. Michael Somers, IOS, GLORIA engineer...............X.......................X
Mr. Malcolm Harris, IOS, GLORIA engineer.......................X
Mr. Derek Bishop, IOS, GLORIA engineer.................X...............X
Mr. Jon Campbell, IOS, GLORIA engineer.................................X.......X
Mr. Ross Walker, IOS, GLORIA engineer..................X.......X
Mr. John Cherriman, IOS, GLORIA engineer...............................X
Mr. Eric Darlington, IOS, GLORIA engineer......................X
Mr. Andrew Harris, IOS, GLORIA engineer................................X
Dr. Robert Mattick, USGS, geologist....................X
Dr. John Schlee, USGS, geologist...............................X
Dr. Page Valentine, USGS, geologist....................................X
Dr. William Sweet, MMS, geologist......................X
Mr. Dann Blackwood, USGS, photographer.................X.......................X
Mr. Kim Benjamin, WHOI, engineer.......................X...............X
Mr. Quentin Huggett, IOS, geologist....................X
Mr. Christopher Jackson, RVS, shipboard computing......X
Mr. Edward Cooper, RVS, shipboard computing............X
Mr. Alan Gray, IOS, airgun technician..................X
Dr. Stephen Williams, IOS, geologist...................X
Mr. Ron Circe', USGS, geologist................................X
Mr. John Wagner, LSU, geologist................................X
Mr. Martin Beney, RVS, shipboard computing.....................X
Mr. Robert Wallace, IOS, airgun technician.....................X
Mr. Robert Commeau, USGS, geologist....................................X
Dr. Charles Paull, SIO, geologist......................................X
Mr. Robin Bonner, IOS, airgun technician...............................X.......X
Mr. Collin Jacobs, IOS, geologist-photographer.................X.......X
Ms. Doriel Jones, RVS, shipboard computing.............................X
Mr. Derek Lewis, RVS, shipboard computing..............................X.......X
Mr. Steve Mateus, USGS, geologist......................................X
Dr. Mahlon Ball, USGS, geologist...............................................X
Dr. Edward Escowitz, USGS, geologist...........................................X
Mr. Gareth Knight, RVS, shipboard computing....................................X
Ms. Kathryn Scanlon, USGS, geologist..........................................X
Mr. Richard Sylwester, USGS, electronics technician............................X
Mr. John West, USGS, airgun technician.........................................X
Ms. Sarah Eisner, USGS, geologist..............................................X
Mr. Steve Wolf, USGS, geologist................................................X
USGS, U.S. Geological Survey
IOS, Institute of Oceanographic Sciences (U.K.)
MMS, Minerals Management Service
WHOI, Woods Hole Oceanographic Institution
RVS, Research Vessel Services (U.K.)
LSU, Louisiana State University
SIO, Scripps Institution of Oceanography
The efforts of many people, from the data collection to
atlas preparation, are greatly appreciated. We thank Dr. Gary
Hill and Dr. Terry Offield for their insight into the importance
of and their enthusiastic support for the EEZ mapping program.
We express our appreciation to Dr. Anthony S. Laughton,
Director of IOS, and the superb technical staff of IOS who made
this mapping effort possible with the design, construction, and
operation of the GLORIA system. Their professionalism and
expertise contributed greatly to the quality of the data. Capt.
Roy Hadgraft and the crew of the MV Farnella, with their
seamanship and cheerful assistance, contributed much to the
success of the cruise.
Our appreciation is also extended to our colleagues at NOAA
who made their ship base in Miami, Fla., available to us as a
staging area. The personnel at the ship base readily assisted in
every way possible, including standing by with hurricane
information during the survey. Special appreciation goes to
Robert Mattos, chief engineer of the NOAA ship Researcher, whose
invaluable knowledge of ship services expedited repairs so that
the cruise could begin on schedule.
