A
MAPPING SIDE SCAN SONAR MOSAIC
AND
BATHYMETRIC STUDY
OF A
SELECTED PORTION OF
DEADMAN
BAY
OFFSHORE
OF
Submitted to
the Office of Coastal and Aquatic Managed Areas
Florida
Department of Environmental Protection
Daniel C. Phelps (P.G. # 1203), Ronald W.
Hoenstine Jr.,
James Sparr, and David S. Anderson
Department of Environmental Protection
Summary
This Florida Department of Environmental Protection (FDEP)
investigation, conducted by the Florida Geological Survey’s (FGS) Coastal
Research Program, examines,
via the use of side scan sonar mosaics and bathymetric analysis, an
approximately 15 square-mile (38.85 square-kilometers) area of seabed in
The project involves mapping seabed features lying in state submerged
lands in a selected portion of Deadman Bay using a Marine Sonic side scan sonar
system operating at 600 (kilohertz) kHz, and a Garman 168 global positioning
and bathymetric system operating at 50 kHz.
The side scan sonar “tow fish”, which houses the transducers, was towed 15.2
feet (4.6 meters) behind the global positioning system antenna at an average
height of 8 feet (2.4 meters) above the seabed.
The side scan sonar system was operated at a 164 foot (50 meter) range
and imaged a 328 foot (100 meter) swath.
Individual lines within the survey grid were approximately 5 miles (8
kilometers) long and spaced at 275 foot (83.8 meter) intervals from center line
to center line. This line spacing
provides 53 feet (16.2 meters) of swath overlap between lines. The grid was oriented parallel to the
shoreline. Over
300 line-miles (482.8 line-kilometers) of side scan sonar and bathymetric data
were collected.
An
analysis of the above referenced data is provided within this report
(DVD). The
side scan sonar data are provided as a low resolution (9.8 foot (3 meter)) mosaic
tied to individual mosaic tiles created at essentially full resolution (0.66
foot (0.2 meter)) of these data.
A review of
the side scan sonar data indicates that bottom characteristics within the study
area show little evidence of features elevated significantly above the sea
floor. Seabed features identified on
individual survey line segments, and on the side scan sonar mosaics created
from them, are primarily attributed to differences in bottom sediment
reflectance. A number of narrow and
relatively linear features lying generally perpendicular to the coast,
exhibiting reflectance’s either higher or lower than the average, were
noted. Of these features, those which
exhibit lower reflectance are interpreted to be the surface expression of
drowned channels associated with lower sea level stand antecedents of the
Introduction
The survey area is located in the
northeastern Gulf of Mexico offshore of the west coast of Florida in Deadman
Bay approximately 5 miles (8 km) due west of the mouth of the Steinhatchee
River. It extends offshore of Dixie and Taylor Counties, Florida, in a 2.98 by
5.47 miles (4.8 by 8.85 km) area approximately 3 to 6 miles (4.8 to 9.7
kilometers) seaward of the coast. Water
depths in the survey area range from approximately 12 feet (3.7 meters) to
approximately 22 feet (6.7 meters) below mean sea level (MSL).
In addition to the bathymetric data
collected, this project examines, via the use of side scan sonar mosaics, a
seabed area of approximately 15 square-miles, (38.85 square-kilometers) at a
frequency of 600 kHz, and at resolutions not usually attempted for such large
size areas. Examinations of such large
areas are typically accomplished at lower frequencies with associated lower
resolutions in deeper waters than those of this study area. Lower frequencies allow for wider swaths to
be imaged via wider grid spacings and thus more area is covered per survey
hour. Twitchell et al. (2000), Donahue
et al. (2000) and Harrison et al. (2000) produced broad area lower resolution
side scan sonar mosaics in deeper waters offshore of
The primary
goal of this project is the continuation of a FDEP program, initiated in 2005, mapping features exhibited on the
seabed of state submerged lands offshore of
Survey
Plan and Geophysical Instrumentation
Field data collection took place aboard
the FGS’s 40-foot research vessel, R/V
GeoSearch, during 16 cruise days
between May 7, 2006 and June 2, 2006.
The survey plan consisted of a grid of northwest-southeast oriented,
shore parallel, track lines approximately 5 miles (8 kilometers) in length with
a line to line spacing of 275 ft (83.8 m).
Including reshoots, approximately 300 miles (482.8 kilometers) of survey
data were collected during this investigation.
The data collected consists of 58 lines. Each individual line took
approximately one hour of ship time to acquire.
Two geophysical
systems were utilized in the study, a Marine Sonic Sea Scan System operating at
600 kHz, and a Garman 168 global positioning system containing a sonar depth
sounder unit operating at 50 kHz.
Side Scan Sonar
The side scan sonar device utilized to generate sonic pulses
and record their reflections, typically referred to as the “tow fish”, was
towed at approximately 5 knots (5 nautical miles per hour) at a side look range
of 164 feet (50 meters) thus imaging a swath 328 feet (100 meters) wide. The system’s effective resolution is a
function of both its range and axial resolution. According to the instrument manufacturer
(Marine Sonics Inc.) this 600 kHz “tow fish”, when set at a 164 feet (50 meter)
side looking range, achieves resolutions of 3.8 inches (9.7 centimeters)
lateral (range) and 12 inches (30.5 centimeters) parallel (axial) to its axis
of travel.
As the distance between track lines for the survey was 275
feet (83.8 meters), an overlap of 53 feet (16.15 meters) of adjacent side scan
sonar swaths was built into the survey grid.
In practice this produced sufficient overlap for the purposes of
building side scan sonar mosaics. However, due to the necessity of steering
around sometimes abundant buoyed crab traps and the effects of currents, wind
and sea state, some minor gaps between adjacent lines were, albeit infrequently,
experienced.
The water depths in the survey area ranged from less than 15
feet (4.6 meters) to greater than 20 feet (6.1) meters). The “tow fish” was towed an average height of
8 feet (2.4 meters) above the seabed with a consistent layback from the GPS
antenna of 13.1 feet (4 meters). Due to
the water depths found in the survey area and equipment constraints, the “tow fish”
was, by necessity, towed proximal to both the sea surface and the survey vessel. In certain
infrequent sea conditions, due to the “tow fish’s” proximity to the sea
surface, objectionable reflections from the sea/air interface were manifested
in these data. The “tow fish’s”
proximity to the vessel resulted in some minor degradation of the center
portion of individual records due to the effects of prop-induced
cavitations. The
quality of side scan sonar data obtained in this survey was also occasionally impacted
by marginal or adverse sea conditions. The “tow fish’s” tow cable was
rigged with a mechanical compensator to minimize the effects of those sea states
on the moment-to-moment attitude of the “tow fish” relative to the seabed. In addition to imaging the seabed, the side
scan sonar system also intermittently imaged sea life in the water column above
the sea floor. Vessel
maneuvers to avoid crab trap floats and the occasional entanglement of the “tow
fish” with crab trap floats locally affected data quality as well. The occasional incidental crossing of another vessel’s fresh
wake also degraded the instrument’s ability to image the sea floor in the
immediate vicinity of that vessel’s path.
When
data quality on extensive line segments was deemed marginal, it was reacquired
on subsequent days. The
above noted caveats not withstanding, the quality of the side scan sonar data collected
is on the whole quite good. All field records were digitally
recorded, and retained for long term storage.
They are available for general distribution by contacting the Florida
Geological Survey.
Echosounder
The echosounder data collected was fed as a data stream to the side scan
sonar computer and saved as a text file.
The depth
sounder transducer utilized was mounted below the dive platform of the survey
vessel. Its depth below working water
level was determined under still water conditions (i.e., no wave activity) and found to be -0.417 feet (12.7
centimeters).
Data Reduction, Computer Processing
and Interpretation of Geophysical Data
Processing of the side scan sonar data was accomplished
using the Sonar Web Pro/SonarWiz.MAP software package developed by Chesapeake
Technologies Inc. The
side scan sonar data is provided as a low resolution (9.8 foot (3 meter)) mosaic
tied to individual mosaic tiles created at the essentially full resolution
(0.66 foot (0.2 meter)) of these data. These resolutions were selected because multiple features typically
present in such survey areas are best observed using different resolutions.
Side Scan Sonar Data Interpretation
On the macro
scale over most of the survey area, there is little evidence of features
elevated substantially above what is a relatively flat sea floor. Numerous bottom features spread across the
survey area related to differences in sediment reflectance can be seen on the
mosaics, the nature of which is not always immediately apparent on individual
lines. A
number of relatively narrow linear features, exhibiting both higher and lower than
average reflectance, with an orientation generally perpendicular to the shore
line were noted. This trend in
orientation is shown to good advantage in the low resolution mosaic. The lower reflectance linear features are
interpreted to be the surface expression of drowned channels associated with
lower sea level stand antecedents of the
On selected high resolution side scan sonar
mosaic tiles, most notably in those proximal to the northwestern edge of the
survey area, features indicative of the presence of submerged aquatic
vegetation were noted. At near the
limits of resolution, small scale sand waves of limited extent infrequently
occur within the survey area. The crests
of these features generally exhibit an east/west lineation. While the seabed over much of the area is
relatively smooth, occasional patches of what appear to be rough bottom were
noted. These features do not appear to have
significant relief and are interpreted to be rock outcroppings.
Bathymetric Analysis
Review of the bathymetric data reveals a number
of interesting features which correlate with those seen on the side scan sonar
data. Low reflectance linear features best
seen on the low
resolution side scan sonar mosaic strongly correspond to linear bathymetric
low trends while generally high reflectance features correspond to bathymetric
high trends as seen on the bathymetric
display. Interpretations of the origins of the linear features
observed, based on both the low resolution side scan sonar mosaic and the
bathymetric display, were made.
Conclusions
In this study, approximately 300 miles (482.8 kilometers) of
side scan sonar and bathymetric data were collected within a survey area totaling
over 15 square miles (38.85
square kilometers) in size.
All geophysical data has been processed and side scan sonar mosaics at two
resolutions as well as a bathymetric
display have been prepared. The results of this study indicate that the
side scan sonar system utilized will adequately image bottom features at
relatively high resolutions. The seabed
in the study area, while relatively gentle in gradient, exhibits a number of
features, identified via their lower reflectance characteristics and supported
by the bathymetric display,
that are conjectured to be related to past sea level low stands. Evidenced as
linear features oriented toward the present mouth of the
Recommendations
Interpretations made through the use of
remote sensing tools should be verified through extensive ground truthing using
bottom sampling and diver reconnaissance as well as video and still
photography. It is thus recommended that such components be part of
future studies. It is also recommended
that seismic reflection profiler data be collected as part of future studies to
determine the influence of underlying stratigraphy on bottom topography. Baseline mapping studies such as this, which collects
and interprets ge
References Cited
Donahue, B.T., Hine, A.C., Tebbens,
S.F., Locker, S.D.,
Locker, S.D., 1999, Geological
framework and classification of bottom environments off
Locker, S.D., 2005, Establishing
baseline benthic habitat coverage in Faka Union and
Locker, S.D., 2006, Mapping
submerged aquatic vegetation using sidescan sonar, Cape Romano Shoals,
Twitchell, D.C., Cross, V.A.,
Paskevich, V.F., Parolski, K.F., Brooks, G.R., Gelfenbaum, G.R., Hine, A.C.,
and Locker, S.D., 2000, Sarasota side-scan sonar mosaic map: U.S. Geological
Survey, Open File Report 99-443, CD-ROM.