RV MOANA WAVE CRUISE M1-01-GM
THE BATHYMETRY AND ACOUSTIC BACKSCATTER
OF THE MID SHELF TO UPPER SLOPE OFF PANAMA
CITY, FLORIDA, NORTHEASTERN GULF OF MEXICO
September 3, through October 12, 2001
Panama City, FL to Panama City, FL
James V. Gardner 1, Larry A. Mayer 2, John E. Hughes Clarke 3, Peter Dartnell 1, and Kenneth J. Sulak 4,
1 - US Geological Survey, Menlo Park, CA
2 - University of New Hampshire, Durham, NH
3 - University of New Brunswick, Fredericton, NB
4 - US Geological Survey, Gainesville, FL
USGS Open-File Report
Conducted under a Cooperative Agreement between the US Geological Survey and the Center for Coastal and Ocean Mapping, University of New Hampshire
Precisely georeferenced high-resolution mapping of bathymetry is a fundamental first step in the study of areas suspected to be critical habitats. Morphology is thought to be critical to defining the distribution of dominant demersal plankton/planktivores communities. Fish faunas of shallow hermatypic reefs have been well studied, but those of deep ahermatypic reefs have been relatively ignored. The ecology of deep-water ahermatypic reefs is fundamentally different from hermatypic reefs because autochthonous intracellular symbiotic zooxanthellae (the carbon source for hermatypic corals) do not form the base of the trophic web in ahermatypic reefs. Instead, exogenous plankton, transported to the reef by currents, serves as the primary carbon source. Thus, one of the principle uses of the morphology data will be to identify whether any reefs found are hermatypic or ahermatypic in origin.
Community structure and trophodynamics of demersal fishes of the outer continental of the northeastern Gulf of Mexico presently are the focus of a major USGS reseach project. A goal of the project is to answer questions concerning the relative roles played by morphology and surficial geology in controling biological differentiation. Deep-water reefs are important because they are fish havens, key spawning sites, and are critical early larval and juvenile habitats for economically important sport/food fishes. It is known that deep-water reefs function as a key source for re-population (via seasonal and ontogenetic migration) of heavily impacted inshore reefs.
The deep-water reefs south of Mississippi and Alabama support a lush fauna of ahermatypic hard corals, soft corals, black corals, sessile crinoids and sponges, that together form a living habitat for a well-developed fish fauna. The fish fauna comprises typical Caribbean reef fishes and Carolinian shelf fishes, plus epipelagic fishes, and a few deep-sea fishes. The base of the megafaunal invertebrate food web is plankton, borne by essentially continuous semi-laminar currents generated by eddies, spawned off the Loop Current, that periodically travel across the shelf edge.
A few, sidescan-sonar surveys have been made of areas locally identified as Destin Pinnacles, Steamboat Lumps Marine Reserve (Koenig et al., 2000; Scanlon, et al., 2000; 2001), Twin Ridges (Briere, et al., 2000; Scanlon, et al., 2000), and Madison-Swanson Marine Reserve (Koenig et al., 2000; Scanlon, et al., 2000; 2001). However, no quantitative and little qualitative information about the geomorphology and surficial geology can be gained from these data. Existing bathymetry along the northwestern Florida shelf suggests the existence of areas of possible isolated deep-water reefs. NOAA bathymetric maps NOS NH16-9 and NG16-12 show geomorphic expressions that hint of the presence of reefs in isolated areas rather than in a continuous zone. There has been no systematic, high-resolution bathymetry collected in this area, prior to this cruise.
After the successful mapping of the deep-water reefs on the Mississippi and Alabama shelf (Gardner et al., 2000; in press), a partnership composed of the USGS, Minerals Management Service, and NOAA was formed to continue the deep-reef mapping to the northwest Florida mid shelf and upper slope. This cruise is the first fruit of that partnership.
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Our objective was to map the region between the 50 to 150-m isobaths south from the eastern edge of De Soto Canyon as far as Steamboat Lumps using a state-of-the-art multibeam mapping system (MBES). The cruise used a Kongsberg Simrad EM1002 MBES, the latest generation of high-resolution mapping systems. The EM1002 produces both geodetically accurate georeferenced bathymetry and coregistered, calibrated, acoustic backscatter. These data should prove extremely useful in relating dominant species groups (which display highly specific biotope affinities) to the geomorphology (e.g., reef flattop, forereef crest, reef wall, reef base, circum-reef talus zone, circum-reef, high-reflectivity sediment apron, etc.).
The Kongsberg Simrad EM1002 High-Resolution Multibeam Mapping System
There are several different brands of high-resolution MBES systems that are appropriate for shallow-water surveys. After a review of the currently available systems, we chose to use the Kongsberg Simrad EM1002 system for this cruise because; (1) it is the latest generation of MBES with a frequency compatible with the depths we were interested in, (2) it is based on the highly successful EM1000 system, (3) it has the ability to map large areas at high speed without compromising data quality and, most importantly, (4) it has the ability to simultaneously produce high-resolution, calibrated acoustic-backscatter imagery. For the northwest Florida shelf survey, we used an EM1002 system owned and operated by C&C Technologies, Inc., Lafayette, LA, installed aboard the 220-ft RV Moana Wave (Figure 2, 28kb).
An overview of high-resolution MBES systems in general can be found in Hughes-Clarke, et al. (1996). The Simrad EM1002 system operates at frequencies of 98 kHz (inner ±50° swath centered at nadir) and 93 kHz (the outer ±20°) from a semi-circular transducer (Figure 3, 17kb) mounted on a rigidly attached boom on the bow of the ship (Figure 4, 18kb). The system was designed to operate in several modes through a range of water depths from 5 to approximately 800-m. The shallow (ultrawide) mode, used to maximum depths of about 200 m, forms 111 receive apertures (used interchangeably with "beams") with a spacing of 2 distributed across track and 2 wide along track. The beam geometry generates a 150 swath that can cover as much as 7.4 times the water depth. The other two modes, wide mode, and deep mode, are for depths of greater than 200 m and were not used for the Northwest Florida shelf mapping. There are options within each mode for beam distribution (equiangular or equidistant) and pulse lengths (0.2, 0.7, and 2 ms). The specific options used for the Northwest Florida shelf survey are discussed in the data processing section below.
Most conventional vertical-incidence echo sounders determine the time of arrival of the returned pulse (and thus the depth) by detecting the position of the sharp leading edge of the returned echo, a technique called amplitude detection. In multibeam sonars, where the angle of incidence increases to either side of the vertical for each consecutive receive beam, a returned echo loses its sharp leading edge and the depth determinations become inaccurate. To address this problem, the Kongsberg Simrad EM1002 MBES system uses an interferometric principle in which each receive aperture is split, through electronic beamforming, into "half beams" and the phase difference for each received signal for each aperture is calculated to provide a measure of the angle of arrival of the echo. The point at which the phase is zero (i.e., where the wavefront of the returned echo is normal to the receive-beam bore) is determined for each aperture and provides an accurate measure of the range to the seafloor. Both amplitude and phase detection are recorded for each aperture and the system software picks the "best" detection method for each aperture, based on a number of qualitycontrol measurements, and uses this method to calculate depth.
The EM1002 also provides quantitative seafloor acoustic-backscatter data that can be displayed in a sidescan-sonar-like image (see Maps section below). The backscatter images can be used to gain insight into the spatial distribution of seafloor properties. A time series of echo amplitudes from each beam is recorded at 0.2- to 2.0-ms sampling rate, depending on the water depth. The echo amplitudes are sampled at a much faster rate than the projected aperture spacings and can be processed from beam-to-beam to produce a backscatter image with the theoretical resolution of the sampling interval (15 cm at 0.2 ms). The amplitude information can be placed in its geometrically correct position relative to the across-track profile because the angular direction of each range sample is known. The EM1002 software corrects the amplitude time series for gain changes, propagation losses, predicted beam patterns, and for the insonified area (with the simplifying assumptions of a flat seafloor and Lambertian scattering). Subsequent processing (see Processing section below) uses real seafloor slopes and applies empirically derived beam-pattern corrections to produce a quantitative estimate of seafloor backscatter across the swath.
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In addition to the multibeam sonar array, a MBES survey requires a careful integration of a number of ancillary systems. These include: (1) differentially corrected Global Positioning (DGPS) to aid an inertial navigation system (INS); (2) an accurate measure the heave, pitch, roll, and heading of the vessel, all to better than 0.01° and the transformation of these measurements to estimates of the motion of the transducer at the times of transmission and reception (motion sensor); (3) a method to precisely determine the sound-speed structure of the water column, using measurements of temperature and salinity with depth or directly measuring sound speed versus depth.
The Northwest Florida shelf survey was navigated with a TSS Applanix POS/MV model 320 (version 2) INS inertial motion sensor (IMU) as well as dual Trimble model 4000 DGPS with a commercial Satloc satellite differential station. Spatial accuracy (positions) for the mapping is ±0.5 m. In addition, the POS/MV records vehicle motion (pitch, roll, heading, and heave) at 100 Hz with an accuracy of 0.02° for roll, pitch, and heading, and 5% of heave amplitude or 5 cm, which ever is greater.
Sound-velocity profiles were calculated several times each day so that ray-tracing techniques could be used to correct refraction of the acoustic wave through the water column. A SeaBird model 19-02 CTD (conductivity and temperature vs depth) was used to measure temperature and salinity versus depth and sound speed vs depth was calculated from these measurements. An additional sound-velocity sensor is installed on the RV Moana Wave to continuously determine the speed of sound in water at the transducer depth. All the sound-speed data (SVP) are fed directly into the Simrad EM1002 processor for instantaneous beamforming and raytracing of the individual receive beams.
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Data Sources and Type
Raw EM1002 data telegrams were acquired over a shipboard Ethernet network. The data stream is outlined in Table 1. In addition, a number of ancillary data sources were also acquired by C&C Technologies (Table 2).
Table 1. Kongsberg Simrad EM1002 data stream.
|Entered static sonar alignment parameters.|
|Applied sound velocity profiles.|
|External navigation data (1-Hz DGPS)|
|Ship-relative bathymetric profile data.|
|Beam-relative backscatter intensity data|
|Transducer temperature, conductivity and (derived) sound speed data.|
|POS/MV 1-Hz position and attitude data .|
|Independent serial record of DGPS data stream (GPGGA format).|
|Original SeaBird SVP data|
EM1002 Operational Modes
There are several operational modes available for the EM1002. The differences in the modes are a function of pulse length, beam spacing, and angular sector. The pulse length controls the amount of energy transmitted into the water column. The system can be operated in an "equiangular" (EA) mode in which the beams are spaced at equal angles apart, resulting in a non-linear (increasing spacing away from nadir) spacing of sonar footprints on the seafloor. The system can also be operated in an "equidistant" (EDBS) mode in which the beams are spaced such that the sonar footprints are equally spaced in the across-track profile. The EDBS geometry is achieved by generating variable beam-angular spacings. Although EDBS has advantages in data handling (i.e., provides even sounding density), there are two limitations. The beams in the 140° and 150° modes are spaced wider than their beam widths and results in incomplete coverage that produces a striping close to nadir. This problem disappears as the swath width closes to ~120°. However, the second limitation occurs because of attitude uncertainties and imperfect refraction models that can result in sounding errors that grow with angle from the vertical. Because these limitations render the outermost beams less reliable than for the EA mode, we used the EA mode.
The Northwest Florida shelf survey was run in the EA mode and was operated with a 0.2 ms pulse length in waters less than 200 m deep and restricted swath widths of 350 to 500 m, depending on water depth. We restricted the swath width to about 4 times the water depth, with a 25-m overlap of adjacent swath so as to assure no data gap were generated by the large range in water depths.
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At sea two separate tide corrections were used. During the major portion of the survey, initial processing used predicted 6-min tides from the Panama City Beach, FL tide station (872-9210). The predicted tide amplitudes were reduced by 4% with no time corrections, based on the suggestion by NOAA's Office of Tides. The corrections compensate for the distance to the operating area from the two tide stations. About 24 hr after the data collection, measured 6-min tides from the Panama City Beach tide station were emailed to the ship by NOAA. Periodically throughout the cruise, predicted and measured tides for a 24-hr period were compared. The phase between the two tide data never differed by more than 1 minute and the maximum tide amplitudes differed by 12 cm. The predicted and measured tides were very similar over the time of the survey (Figure 6, 27kb). The tide corrections for the survey of Steamboat Lumps used the same tide station but a tide height reduction of 41% was used.
All bathymetric data were adjusted through Kongsberg Simrad
software for (1) transducer draft, (2) static roll, pitch and
gyro misalignments, (3) roll at reception, (4) refracted ray path,
and (5) beam steering at transducer interface. Post-logging transformations
included (1) transformation of navigation from antenna to transducer,
(2) correction for positioning to sonar time shifts, (3) tide,
and (4) any unaccounted-for static attitude misalignments.
Table 3. Offsets to sensor alignments on the RV Moana Wave
|Echosounder to IMU mounting angles|
|X = 0.00°|
|Y = 0.00°|
|Z = 0.00°|
|Ship to IMU mounting angles|
|X = 0.00°|
|Y = 0.00°|
|Z = 0.00°|
|IMU to Echosounder lever arms (m)|
|X = 0.00|
|Y = 0.00|
|Z = 0.00|
|IMU to GPS lever arm (m)|
|Y = -1.01|
|Z = -7.90|
|GAMS parameter setup|
|2-antenna separation: 1.9988 m|
|heading calibration threshold: 1.000°|
|heading correction: 0.00°|
|Simrad installation parameters|
|Motion sensor delay: 0.0 ms|
|Pitch installation angle -2.3°|
|Waterline 1.52 m|
|Outer beam angle offset 0.00°|
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The Kongsberg Simrad EM1002 provides a backscatter-intensity time series for the bottom insonification period for each of the 111 individual beams. The corrections applied by the shipboard recording system are listed in Table 4.
A set of required backscatter data transformations is performed by specialized software written by the Ocean Mapping Group at the University of New Brunswick. The transformations include conversion of each beam backscatter time series to a horizontal range equivalent, compiling the 111 beam traces together to produce one full slant-range-corrected trace, and removal of residual beam-pattern effects. Although the system software corrects for average beam pattern, there are ± 2 dB ripples in the average beam pattern that vary from transducer to transducer.
Our processing approach to backscatter was to stack several thousand pings to view the angular variation of received backscatter intensity as a function of beam angle. Inherent in this function is both the transmit and receive sensitivities, as well as the mean angular response of the seafloor. We then invert this function to minimize the beam pattern and angular variations.
Table 4. Corrections applied to each beam for backscatter.
|Source power adjustments.|
|Spherical spreading compensation.|
|Attenuation compensation (using operator entered 30 dB per km.).|
|Designed beam-pattern compensation.|
|Calculation of insonified area (assuming a flat seafloor at the nadir depth).|
|Application of a Lambertian model using flat seafloor equivalent grazing angles) to reduce the dynamic range of the data (stored at 8 bit (0= -128dB, 255 = 0 dB.).|
Kongsberg Simrad uses a variable gain within 15° of vertical to reduce logged dynamic range at nadir and near-nadir. The sidescan data at this stage have had a Lambertian response backed out and the beam pattern has been corrected with respect to the vertical and all receive beams have been roll stabilized. Consequently, corrections have been made for variations in the beam-forming amplifiers but not variations in the stave sensitivities of the physical array. Additional transformations were required to produce calibrated backscatter measurements. These include (1) removal of Lambertian model, (2) true seafloor slope correction, (3) refracted ray-path correction, (4) residual beam-pattern correction, and (5) aspherical-spreading corrections.
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Despite the careful measurements of transducer alignments and
offsets, the true geometry of the installed system can only be
determined through the determination of the self-consistency of
seafloor measurements. To facilitate such a determination, we
conducted a series of "patch tests" the first day of
the Northwest Florida shelf survey whereby the system was run
back and forth across a seafloor feature to determine if there
were residual roll, pitch, heading, or timing offsets that required
A full patch test procedure was started prior to data collection on September 3 to calibrate any time delay, and gyro misalignment. The static adjustments were estimated from the patch test are listed in Table 5.
Table 5. Adjustments to shipboard alignments used for the Northwest Florida shelf survey for Legs 2 and 3.
|Tme delay 0.4 s|
|Gyro misalignment 0.0°.|
|Roll misalignment 0.0°.|
|Pitch misalignment -0.0°.|
|The time delay was entered in to the UNB post-processing software.|
|The roll offset was entered into the Simrad software.|
|The gyro misalignment was entered into the Simrad OPU.|
|Pitch misalignment entered into Swathed post-processing software|
The 1-Hz DGPS and 100-Hz INS navigation data were logged with the Kongsberg Simrad EM1002 software. The Bottom Detection Unit (BDU) time stamps the depth and backscatter telegrams and was slaved to a shipboard SUN Sparc 20 workstation that was synchronized to the GPS 1 PPS. The navigation telegrams were externally stamped by the Trimble 4000 GPS receiver. The receiver antenna positions were shifted to the transducer position according to the X and Y offsets using the POS/MV output (Table 3). Every 1-Hz navigation fix was checked for gross time and/or distance jumps by graphical examination during data processing. Outliers were interactively interrogated for time, flagged and rejected (or re-accepted). All navigation jumps greater than 20 s were automatically flagged as uninterpolatable.
The single biggest limitation on the quality of sounding data is water-column refraction. Refraction-related anomalies grow non-linearly with beam angle and the resulting artifacts can create short-wavelength topographic features that may be misinterpreted as seabed relief. There was some fear prior to the cruise that suspected strong water stratification would present a problem for the beam steering and ray tracing of individual beams. Although a strong thermocline was measured, repeated CTD casts allowed us to correct for refraction effects. A representative water-velocity profile is shown in Figure 7, 22kb. In additional, minor empirical refraction corrections were applied during processing. If all of the alignments are correctly determined, Kongsberg Simrad states that the depth resolution of the EM1002 is 30 cm or 0.1% of water depth, whichever is larger.
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Shipboard data processing (Figure 8, 39kb) consisted of (1) the editing the 1-Hz navigation fixes to flag bad fixes; (2) examining each ping of each beam to flag outlier beams, bad data, etc.; (3) merging the depth and backscatter data with the cleaned navigation; (4) correcting all depth values to mean lower low water tide datum; (5) performing additional refraction corrections, if necessary, for correct beam raytracing; (6) separating out the amplitude measurements for conversion to backscatter; (7) gridding depth and backscatter into a geographic projection at the highest resolution possible with water depth; (8) regridding individual subareas of bathymetry and backscatter into final georeferenced map sheets; (9) gridding and contouring the bathymetry; and (10) generation of the final maps. Nearly finalized maps were completed in the field during the transit to port and the final maps (Appendix I) were completed one week after the end of the cruise.
Overview and section maps of backscatter and shaded relief (Appendix I) were generated from 45 larger-scale subarea maps (Figure 9, 26kb and Table 6). The 4-m-resolution subarea maps were regridded at 8 m to produce the series of section maps of the entire area (see examples Figure10, 39kb and Figure 11, 38kb). This regridding sacrifices resolution in the shallower areas but allows the entire area to be mapped. The detailed subarea maps are 3750 columns (14 km) by 2500 rows (10 km) in size and were produced at the maximum resolution as determined by water depths and beam angle.
A shaded-relief map (Figure 10 , 39kb and back of report) is a pseudo-sun-illumination of a topographic surface using the Lambertian scattering law (equation 1), where SI is the pseudo-sun intensity, K is a constant that allows for even background, and F is the angle between the pseudo sun and the bathymetric surface.
The backscatter map (Figure 11, 38kb and back of report) is a representation of the amount of acoustic energy, at ~95 kHz, that is scattered back to the receiver from the seafloor. The Kongsberg Simrad EM1002 system has been calibrated at the factory and all gains, power levels, etc. that are applied during signal generation and detection are recorded for each beam and are used to adjust the amplitude value prior to recording. Consequently, the backscatter is calibrated to an absolute reflectance of the seabed. However, the amount of energy, measured in decibels (dB), is some complex function of constructional and destructional interference caused by the interaction of an acoustic wave with a volume of sediment or, in the case of hard rock, the rock material. The backscatter from sediment represents volume reverberation to at least 5 cm caused by seabed and subsurface interface roughness above the Rayleigh criteria (a function of acoustic wave length), the composition of the sediment, and its bulk properties (water content, bulk density, etc.). Although, it is not yet possible to determine a unique geological facies from the backscatter value, reasonable predictions can be made from the backscatter based on the known local geology.
Table 6. Map numbers, resolution in meters/pixel and size
One of the great advantages of this survey is that every sounding of the bathymetry is accurately georeferenced and coregistered with the backscatter. Each pixel on the map has a latitude, longitude, depth, and backscatter value assigned to it. Consequently, there is no ambiguity in correlating the backscatter to the bathymetry, as is often the case when correlating sidescan sonar with bathymetry.
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September 3, 2001 (JD 246)
We departed Panama City 0800 hr local and headed for the northern part of the survey area to commence the patch test. By 1400 hr we were at the patch test area and began to survey of seafloor targets that would provide a reference point for the testing. Seafloor targets were hard to come by so after two hours of searching we headed for an increase in slope at about 100 m water depth. During the search, we noticed that the quality of both the bathymetry (Figure 12, 60kb) and backscatter (Figure 13, 53kb) was poor (Figure12, 60kb, Figure13, 53kb, Figure14, 57kb, Figure15, 37kb). The backscatter showed pronounced sector boundaries and beam-pattern striping along track. Although these can be cosmetically corrected by processing, the underlying problems that create these artifacts were troubling. However, the more vexing problems were with the bathymetry. The sector boundaries generated 30 to 50-cm artifacts that paralleled the track, In addition, the inner sector appeared to be tilted about 0.1° relative to the two outer sectors. An internal check of the transducer reported that 24 elements failed in 7 of the128 staves. Potentially, this was a serious problem and the afternoon and evening were spent checking out the electronics and formulating a message to Simrad/Norway for assistance. The night was spent running a small survey outside the area of interest, in a region of flat seafloor, to have an area of data to analyze the various artifact.
September 4, 2001 (JD 247)
Simrad/Norway contacted the ship at 0900 L and suggested a series of diagnostic tests for us to perform. The first test was to disconnect and reconnect the 16 electrical cables that lead directly form the transducer to the Simrad box (Figure 16, 32kb). This did not improve the data. The second test was to swap the four Simrad transmit/receive boards into various slot patterns (i.e. 1-2-3-4; 2-3-4-1; 2-4-1-3, etc) to see if the artifacts remained or disappeared. Once the boards were switched, the system was turned on and a short line was run for comparison to the previous series of lines. The artifacts persisted throughout these test. The third test was to measure the transmit voltages at the back of each board. The voltages measured correctly. The fourth test was to check each of the 128 stave leads for continuity. They all checked out. This left us with the dilemma that the problem resided in the transducer itself. A call to Simrad/Norway confirmed our conclusion that the transducer was bad. All this testing took until late in the afternoon. The last test of the MBES showed that the total system failed and it could not acquire a bottom lock.
September 5, 2001 (JD 248)
We arrived at the Panama City dock at 0600 L and John Hughes Clarke and Mike Annis departed the ship at 0800. The day was spent fixing various computers and improving various software programs.
September 6, 2001 (JD 249)
The day was spent at the dock in Panama City.
September 7, 2001 (JD 250)
The day was spent at the dock in Panama City. The transducer arrived at New Orleans International Airport in the morning and was cleared through US Customs by noon. C&C personnel picked up the transducer and drove it to the ship, arriving at 2030 L. The transducer could not be tested in air so nothing further could be accomplished.
September 8, 2001 (JD 251)
The transducer was mounted on the bow ram and attached to the bow in 1.5 hr. John Hughes Clarke was picked up at the airport at 1400 L and we were underway for testing by 1545 L. We headed for a location on the northwestern region of the area to be mapped, hoping to find suitable targets for a complete patch test.
We arrived at the patch test area at 2100 L and commenced a full patch test, starting with a CTD cast to get the sound-speed profile (SVP). Patch testing continued through the night. A good patch-test target was found that appears to be the impressions in the seafloor of a jack-up rig (Figure17, 29kb). A timing and roll patch test found a 0.4 s timing shift and a offset 0.8° roll bias. The timing offset was applied at swathed and the roll bias was entered in the Hydromap acquisition software.
September 9, 2001 (JD 252)
The patch test was completed and mapping began at 0830 L. The mapping began at the northeast corner of the planned area (Destin Pinnacles area), at the 50-m isobath, and progressed to the southeast. Small problems with erratic beam pattern, poor phase detection of the bottom, and a highly variable sound-speed profile cropped up during the day.
Several small bathymetric features with relief of <2 m and high backscatter were mapped in the northwest area. The features are similar to the smaller patches of hardgrounds mapped in the Pinnacles area. The weather was hot and humid and the seas were calm.
September 10, 2001 (JD 253)
The weather was hot and humid and the seas were calm. Routine data collection with excellent data quality, although a persistent artifact began to appear. A series of beams, but not always the same beams, would track between 20 and 50 cm below the average seafloor on a random schedule. The artifact is about 0.5% of the water depth so it falls at the limit of resolution of the system. Also, a moving 500 beam-average technique was tested to remove some of the beam pattern that cosmetically degrades the backscatter image. The technique appeared to work so it was continued throughout the day.
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September 11, 2001 (JD 254)
Routine data collection. Today we were shocked by the news of the attacks on New York and Washington DC. The reaction aboard ship was one of total disbelief. We rigged up an antenna for a portable radio to get continuous radio reports.
The wind freshened to 35kt late in the evening and continued throughout the night. The wind and seas picked up to the point that bubble wash-down over the bow and transducer rendered very poor quality data. We altered the ship speed to reduce pitching but to no avail.
The survey began to produce results almost immediately. An extensive barrier reef complex was mapped with a summit at about the 62-m isobath (Figure18, 33kb). The seafloor on the north side of the reef is about 3 m shallower than the seafloor on the south side. Also, what appears to be an erosional moat occurs immediately to the south of the reef front. The reef is composed of a series of at least 3 ridges; the main reef front rising 4 m, and at least two low ridges rising about 1 m that lay in front (to the south) of the main reef.
September 12, 2001 (JD 255)
The wind and seas persisted at 35 kts and 6 to 8 ft, respectively. The data quality was poor so we altered the plan and headed for the western edge and commenced running lines at the 125±-m isobath but parallel to the survey headings (NW-SE). Because the ship was rolling ±4° on the transit to the western edge, a series of roll tests were run to check for roll bias. The weather forecast showed a deep low-pressure disturbance about 200 m south of us and moving to the NNW. The heavy seas and wind continued throughout the day and evening causing poor-quality data on the SE lines and marginally acceptable data on the NW lines.
September 13, 2001 (JD 256)
The weather worsened during the night but the data on the NNW course continued to be OK. We received weather reports from C&C home office every 3 hours and watched as tropical disturbance Gabrielle developed into a tropical storm. Although the seas and wind abated during the day, the wind came around to the SW and built up during the early evening. The predicted path of the storm was about 180 km south of us (Figure 19, 67kb) so we decided to stay at sea and continue mapping.
The data continued to have random "pull downs" with an amplitude of about 40 cm of several adjacent beams. The shallow depth of the transducer resulted in severe bubble wash down during the SW course. Even with the storm-degraded data, we were able to map a large field of bedforms on a slope along the 100- to 112-m isobaths.
September 14, 2001 (JD 257)
The seas and wind continued throughout the day but the forecast was for a moderation late in the day and evening as Gabrielle moved across the Florida peninsula and out into the Atlantic Ocean. The data were still of poor quality on the SE heading but acceptable on the NW heading. We continued to define a series of sediment covered reef-like features in the Destin Pinnacles area that are concentrated between the 85- and 120-m isobaths with bedforms on their west- and south-facing slopes. We switched to a 500-m fixed swath with a 400-m line spacing when the water depths shoal to about 100 m. Deeper than 100 m, we used a fixed 600-m swath and 500-m line spacing.
September 15, 2001 (JD 258)
The seas and wind laid down and ideal mapping conditions prevailed. The day was spent routine mapping. A ship wreck (Figure 20, 21kb) was found at 29.91806°N 86.58228°W in 100 m water depth
September 16, 2001 (JD 259)
The day was spent routine mapping with excellent data quality. The data continued to be plagued by "wobbles" (Figure 21, 29kb) random drop downs of 40 to 60 cm, the cause of which were unknown. The wobbles occur even when we have no roll or pitch.
September 17, 2001 (JD 260)
We broke off mapping at 0300 L and headed for Panama City to exchange some of the scientific staff. We arrived at the dock at 0730 L and we underway at 1130 L after loading groceries and picking up spares.
We arrived back at the map area at 1700 L, took a CTD cast, and were mapping by 1730L. Three lines were required to complete the northern section of the survey area.
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September 18, 2001 (JD 261)
The day was spent routine mapping with seas calm and slight breezes. The data quality was excellent. The central section of the survey area was begun and immediately located several areas of interesting relief. We received an email from C&C/Lafayette that it one of the artifacts, the outer sectors being deflected up relative to the middle sector (Figure 22, 25kb), might be caused by not compensating for the change in beam-forming characteristics versus water temperature at the transducer. A Simrad software upgrade added a parameter "outer beam angle offset" in the setup parameters. This parameter was set at 0 during the initial setup and had not been changed. We ran a small test by taking a sound-velocity profile with the CTD, entered the new sound-velocity profile into the software and ran two 10-minute lines; one with the outer beam angle offset at 0.95° and the second at 0.60°. The 0.60° setting produced acceptable results so we stayed with it
September 19, 2001 (JD 262)
Routine day of mapping with excellent data quality. The day was hot with calm wind and a flat sea. Porpoise playing around the bow caused numerous data dropouts throughout the day. Late in the day the water depths varied between 85 and 130 m so the system was switched to automatic mode to see if the system would automatically change from shallow (0.2 ms) to medium (0.7 ms) mode and back without operator intervention. The system stayed in shallow mode.
September 20, 2001 (JD 263)
Routine day of mapping with excellent data quality. The day was hot with calm wind and a flat sea.
September 21, 2001 (JD 264)
Routine day of mapping with excellent data quality. The day was hot with calm wind and a flat sea. At 1800 L the data got very noisy, probably caused by a school of porpoise playing off the bow. The porpoise blocked out much of transducer for about 10 minutes until they moved away from the ship.
The Swathed software seemed not to be adding -3 dB during mosaicking the acoustic backscatter when the pulse length changed from 0.2 to 0.7 ms. The shifts will have to be reprocessed back in the lab.
September 22, 2001 (JD 265)
Routine day of mapping with excellent data quality. The day was hot and humid with flat seas.
September 23, 2001 (JD 266)
Routine day of mapping with excellent data quality. The day was hot and humid with flat seas. We discovered that the bottom-tracking gate on the Simrad runtime parameters had been set at 40 to 180 m, causing the bottom signal to get very noisy in water depths deeper than 180 m. The southern end of the central area is deeper than 180 m and the data quality in this region is poor because of this setting. We opened the tracking gate to 250 m and the data quality immediately improved.
The NOAA measured tide data stopped coming to the ship via email. The last day's measured tides were on Sept. 21. A comparison was made between the measured and predicted tides for this day and found to differ by only 11 cm at the maximum deviation. So, we continued to use predicted tides while querying NOAA to continue to send measured tide data.
We completed the western extent of the central area at about 1300 L and ran an east-west cross line along the southern border of the north area for calibration. A sound-velocity cast was run just prior to extending the central area to the east and line 384 was began. However, after only 10 minutes of running the line we noticed a pronounced refraction problem so we terminated the line and took another sound-velocity cast. The second cast showed a strange sound-velocity profile (Figure 23, 21kb), especially in the upper 50 m.
Porpoise continued to interfer with the transducer at random intervals of about 5 minutes at a time, scattered throughout the day. The effect is to cause data dropouts across the transducer range (Figure 24, 46kb). Weather closed in on us late in the afternoon with the wind picking up to 20 kts, heavy overcast sky and squalls.
We completed the middle of the central section at about midnight and ran a cross line (line 395) to begin filling in the shallower eastern portion of the central section. Refraction became a persistent problem in the shallow area. This required taking numerous CTD casts throughout the lines to correct for changing water-column conditions.
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September 24, 2001 (JD 267)
Routine day of mapping with excellent data quality. The ship's gyro crashed in the night and it required 1.5 hr to repair it. The weather continued to be overcast, breezy, and rainy. The sea developed a chop that induced a slight pitch to the ship and adversely affected the data. The slight pitch, coupled with the fact that the ship is riding several feet higher out of the water because of having burned three weeks of fuel, has brought the transducer even closer to the water surface and potential bubble wash. We slowed the ship from 10.5 kts to 8.5 kts and the data significantly improved. The depths began to shoal to 60 to 80 m so we reduced the swath width from 450 to 250 m to reduce the effect from the outer beams flapping.
The DGPS unit within the POS/MV started losing its lock on satellites. The result was that the yaw accuracy started drifting about 0.5% (~ 1.5 m). This was still well within our 4-m pixel size but it alerted us a potential problem.
September 25, 2001 (JD 268)
Routine day of mapping. The weather was overcast but began to clear in the afternoon. The wind calmed and the seas came down. The data quality continued to look good but the persistent artifact at the sector boundaries (about 30 cm) continued to show in the shaded-relief images. The artifact is at or below Simrad's minimum resolution so nothing much can be done about it at this stage.
The SGI processing computer crashed 5 times within 30 minutes during the afternoon. Each crash powered down the CPU but the disks still had power. All cables and UPSs were checked and found fine. All disks had plenty of room and the system disk appeared OK. We monitored the system disk usage and could see that the system was taking 100% of the available RAM for short intervals. However nothing relevant to the crashes was apparent in the system log. The entire afternoon and evening was spent trying to determine the cause of the crashes. It appeared to crash when more than one process was running at the same time. Typically, RT, Cube, makess, and weigh_grid were all running at the same time. This had not been a problem during the cruise up to this afternoon. When only one process was run, the system crashed only every 5 to 8 hrous. Although this slowed the processing, we went into the single-process mode and continued processing data. Cube was stopped on line 450.
September 26, 2001 (JD 269)
The day was partly cloudy, cool with lumpy seas, mostly coming abeam of the ship causing rolls of about ±3°. The data quality continued to look good. The SGI almost stayed alive all night with only one crash. A mirror of the SGI directory structure was constructed on the spare SGI (caspian) and realtime RT was transferred to Caspian. Cube was not restarted on caspian because of the limited disk space.
The Simrad data-acquisition software (Hydromap) crashed during Line 470. Evidently the navigation input was corrupted and the software could not read it. It took about 45 minutes to get the software reloaded and accepted.
The sky cleared and the seas became calm in the evening.
September 27, 2001 (JD 270)
Routine day of mapping with excellent data quality. The day was warm, breezy, and clear. We finished the central section at 2330 L and commenced the southern section. To this point, we were on schedule.
September 28, 2001 (JD 271)
We completed one line along the shallow (east) side of the southern section and continued southeast to the Steamboat Lumps area. We began mapping the Steamboat Lumps area at 1000 L. The day was overcast with a persistent 25 to 30 kt wind out of the north that generated a 6-ft sea.
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September 29, 2001 (JD 272)
The wind, swell, and seas worsened during the night and by morning we had 10-ft swell, 10-ft seas and 35-kt winds. The seas and swell were about 10° from one another giving the ship a cork-screw ride. The ship's motion, rapid ±7° of roll and ±5° of pitch, badly degraded the data. During the morning, acoustic backscatter from the starboard outer sector suddenly went black and the overall starboard signal strength dropped to unacceptable levels, as if the one or more of the amplifiers failed. The acoustic backscatter in the middle of Line 544 dropped to 50 dB. At first the signal-strength drop appeared to be isolated to the starboard outer sector, but a series of tests changing the swath width revealed the problem to occur across the entire starboard swath. The Simrad system was powered down and restarted before the beginning of Line 546 to see if the system would reset itself. The problem persisted so we ran a reciprocal course to put the seas on our opposite side to see if the lack of signal strength was related to the rough seas. The starboard signal strength remained unacceptably low so the problem was not sea related. The final test was to switch the transmit/receive circuit boards in the Simrad unit. When board 1 and 4 were switched, the low-signal strength switched from the starboard to port side. This test confirmed that a Simrad transmit/receive board had failed. There were no spare Simrad boards aboard ship so the C&C office was contacted to send a new board to Panama City for us to pick up. We terminated the mapping at 0900 L and steamed for Panama City.
September 30, 2001 (JD 273)
We reached the dock at Panama City, FL at 0730 L. The C&C office was unable to obtain fuel at the Port of Panama City because it was the weekend. The concern for fuel was to ballast the ship down because at the present fuel load the ship was riding at least 0.8 m higher that with a full load. The present draft of the transducer is only 0.5 m, much too shallow for ideal data collection.
The replacement Simrad computer boards could not be shipped from Seattle, evidently because of security concerns, so a Simrad employee had to hand carry the boards from Seattle to Lafayette then a C&C employee drove the 8 hours from Lafayette to Panama City. The board was due in Lafayette at midnight, causing at least another 24-hr delay in the cruise.
October 1, 2001 (JD 274)
The circuit board arrived at 0800 L and the Simrad system was checked out and was performing to specifications. We departed the dock at 0845 L and steamed to the north border of the central area while checking out the multibeam system performance. We arrived at the start point at 1330 L, took a CTD cast, and commenced mapping on Line 546.
October 2, 2001 (JD 275)
Routine day of mapping. The weather was clear, warm, and breezy with light seas. The water-velocity structure at the Steamboat Lumps MPA is very complicated with the eastern end having a different vertical structure from the western end. This cause considerable refraction problems that required a lot of effort with the empirical refraction tool. Numerous CTD casts were made but the water structure continuously changed.
During the afternoon the data quality started to be degraded on west headings. We slowed to 8.5 kts and the data improved. Consequently, we could map on east headings at 9.5 kts and westward headings at 8.5 kts. This improved the data quality. The predicted tides, however, proved unacceptable. The NOAA Tides Group suggested we use the predicted tides from the Clearwater Beach, FL station for the Steamboat Lumps survey. These tides produced tide artifacts in excess of 1 m, especially in the eastern part of the Steamboat Lumps area. Rather than change the tide model half way into a map, we continued with the predicted Clearwater Beach tides and generated new tides corrections for Steamboat Lumps from the measured tides after the cruise.
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October 3, 2001 (JD 276)
Routine day of mapping. The weather was partly cloudy, warm, and breezy with light seas. The Steamboat Lumps survey took longer than expected because the existing bathymetry is not too accurate. The water depths of the top of the reef complex is at about 70 m requiring a 350-m swath width throughout almost the entire area. Finally, in the early evening, the average water depths approached 90 m and we expanded the swath to 400 m.
October 4, 2001 (JD 277)
Routine day of mapping with excellent data quality. The weather was partly cloudy and mild with a fresh wind out of the northwest, creating a 2- to 3-ft chop. This did not affect the data quality. The mapping of Steamboat Lumps Marine Protected Area was completed at 1700 L and we ran a cross line north through the entire area on our transit back to the South area. We collected 69,516,500 depth measurements and at least 5 times that number of acoustic backscatter measurements in the Steamboat Lumps survey.
October 5, 2001 (JD 278)
We began mapping in the South area at 0300 L. The weather was cloudy, humid, and calm. The seas were running a 2-ft swell. We increased the line spacing to 350 m with a 400-m swath and ran at 9 kts to the northwest and 9.5 kts to the southeast. The data quality was excellent at these speeds.
The seas picked up to 3+ feet in the late afternoon and produced ±6° rolls that did degrade the data somewhat. The Simrad computer unexpectedly crashed at 1930 L but was back up and running in a few minutes, leaving only a small data gap. Simrad crashed again at 2000 L.
October 6, 2001 (JD 279)
The seas stayed lumpy all day with 4- to 5-ft seas quartering our course producing ±4° to 6° rolls and ±3° of pitch. The data are degraded somewhat with a strong correlation of ship roll with a pitch artifact. The wind came around in the evening and the seas moderated a bit.
October 7, 2001 (JD 280)
The seas continued to be 3 to 5 ft in height all day, although the data continued to be of good quality.
October 8, 2001 (JD 281)
The seas built to 6 to 9 ft during the night and continued large during the day. The seas were coming out of the east which put us in a quartering following sea on the northwest course and the data were of good quality. However, on our southeast course, we were quartering into the sea, rolling ±10° and pitching ±6°. In addition, the ship was running light because of all the fuel consumed during this cruise. This motion brought the transducer out of the water on the southeast course, causing loss of signal and intense cavitation when the bow fell back into the sea. The data quality on the southeast course is poor. Conditons worsened during the evening and rolls of ±12 to 15° with ±6° of pitch were common.
October 9, 2001 (JD 282)
The seas continued to be 6 to 10 ft throughout the day, producing large rolls and pitches on the southeasterly courses. The data quality from the southeasterly course are marginal because of the numerous data dropouts. Data quality on the northwest course are good.
October 10, 2001 (JD 283)
The seas began to subside by noon, although they still were 5 to 8 ft. The wind died down to 10 to 15 kts. The data quality immediately began to improve.
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October 11, 2001 (JD 284)
The South area was completed at the start of the day. We ran a cross line across the bottom of the South area and commenced running lines on the east (shallow) side of the South area. Once one long line was run along the east side, the remainder of the time was spent running shallow lines on the east side of the Madison-Swanson MPA. The seas continued to be 6 to 8 ft and the wind came up to 35 kts, producing poor data-quality conditions on the southeast courses but good-quality conditions on the northwest courses.
At 1730 L the POS/MV IMU crashed. The crash caused the loss of vehicle motion sensing so the MBES system was shut down. The the IMU was fixed at 2115 L and the line was resumed. The cruise was terminated at 2300 L and we transited to Panama City, FL.
October 12, 2001 (JD 285)
We arrived at the dock in Panama City, FL at 0600 L, thus completing the cruise.
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The maps on the following links, plus Figure 10, 39kb and Figure 11, 38kb, are a summary of the mapping accomplished on this cruise. Because the area is so large (i.e., the file sizes are huge), the area was subdivided into North, Central, and South regions. The Overviews are gridded at 16 m and the regions are gridded at 8 m.
Figure 25, 38kb. Overview colored
shaded-relief map of entire mapped area. Red labeled boxes outline
sections gridded at 8-m resolution.
Figure 26, 27kb. Overview colored acoustic-backscatter map of entire area mapped.
Figure 27, 24kb. Colored shaded-relief map of North section. See Figure 25 for color code. Note map is rotated 90° counter clockwise.
Figure 28, 26kb. Colored acoustic-backscatter
map of North region. See Figure 26 for color code. Note map is
rotated 90° counter clockwise.
Figure 29, 24kb. Colored shaded-relief map of Central region. See Figure 25 for color code. Note map is rotated 90° counter clockwise.
Figure 30, 27kb. Colored acoustic-backscatter map of Central region. See Figure 26 for color code. Note map is rotated 90° counter clockwise.
Figure 31, 25kb. Colored shaded-relief map of South region. See Figure 25 for color code. Note map is rotated 90° counter clockwise.
Figure 32, 24kb. Colored acoustic-backscatter
map of South region. See Figure 26 for color code. Note map is
rotated 90° counter clockwise.
|USGS Team||Leg 1: Sept. 3 thru 4||Leg 2: Sept. 8 thru 17||Leg 3: Sept. 17 thru Oct. 11|
|James V. Gardner, USGS, Chief Scientist||X||X||X|
|Peter Dartnell, USGS||X||X||X|
|Larry A. Mayer, UNH||X||X|
|Brian Calder, UNH||X||X|
|John E. Hughes Clarke, UNB||X||X|
|Anya Duxfield, UNB||X||X|
|Mark Paton, IVS||X||X|
|Michael Annis, NOAA||X|
|Garret Duffy, UNB||X|
|Art Kleiner, C&C, Party Chief||X||X|
|David Fitts C&C, Party Chief||X|
|Dave Everhart, Ship Captain,|
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Anonymous, 1999, Northeastern Gulf of Mexico coastal and marine ecosystem program: Ecosystem monitoring, Mississippi/Alabama shelf, 3ed annual interim rept., Minerals Management Service, 210p.
Briere, P.R., Scanlon K.M., Fitzhugh, Gary, Gledhill, C.T., and Koenig C.C., 2000. West Florida Shelf: Sidescan-sonar and sediment data from shelf-edge habitats in the northeastern Gulf of Mexico, U.S. Geological Survey Open-file Report 99-589. CD-ROM.
Gardner, J.V., Sulak, K.J., Dartnell, P., Hellequin, L., Calder, B., and Mayer, L.A, 2000, The bathymetry and acoustic backscatter of the Pinnacles area, northern Gulf of Mexico. US Geological Survey Open-File Rept. 00-350, 35 p.
Gardner, J.V., Dartnell, P., Sulak, K.J., , and Calder, B., and Hellequin, L., in press. Physiography and Late Quaternary-Holocene Processes of Northeastern Gulf of Mexico Outer Continental Shelf off Mississippi and Alabama, Gulf of Mexico Science.
Hughes-Clarke, J.E., Mayer, L.A., and Wells, D.E., 1996, Shallow-water imaging multibeam sonars: A new tool for investigating seafloor processes in the coastal zone and on the continental shelf. Marine Geophysical Researches, 18: 607-629.
Koenig, C.C., Coleman, F.C., Grimes, C.B., Fitzhugh, G.R., Scanlon, K.M., Gledhill, C.T., and Grace, M., 2000. Protection of fish spawning habitat for the conservation of warm temperate reef fish fisheries of shelf-edge reefs of Florida. Bulletin of Marine Science, v.66, no.3, p.593-616.
Ludwig, J.C and Walton, W.R., 1957, Shelf-edge calcareous brominences in northeastern Gulf of Mexico. Bull. Amer. Assoc. Petroleum Geologists, v. 41, p. 2054-2101.
Scanlon K.M., 2000. Surficial Seafloor Geology of a Shelf-edge Area off West Florida. in: Briere, P.R., Scanlon K.M., Fitzhugh, G., Gledhill, C.T., and Koenig C.C., 2000. West Florida Shelf: Sidescan-sonar and sediment data from shelf-edge habitats in the northeastern Gulf of Mexico, U.S. Geological Survey Open-file Report 99-589. CD-ROM.
Scanlon, K.M., Koenig, C.C., Coleman, F.C., and Rozycki, J.E. (2001). Paleoshorelines, drowned reefs, and grouper habitat in the northeastern Gulf of Mexico. Geology of Marine Habitat Session, Geological Association of Canada Annual Meeting, 2001, St. Johns, vol. 26, p. 132.
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