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CRUISE REPORT

RV Ocean Surveyor cruise O1-02-GM; bathymetry and acoustic backscatter of selected areas of the outer continental shelf, northwestern Gulf of Mexico; June 8, through June 28, 2002; Iberia, LA to Iberia, LA

June 8, through June 28, 2002

Iberia, LA to Iberia, LA

Jonathan D. Beaudoin¹, James V. Gardner², John E. Hughes Clarke¹

¹University of New Brunswick, Fredericton, NB
²US Geological Survey, Menlo Park, CA

Open-File Report OF02-410

Conducted under a Cooperative Agreement between the US Geological Survey and the Ocean Mapping Group, University of New Brunswick, Canada

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Use of trade, product, or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Following the publication of high-resolution multibeam echosounder (MBES) images and data of the Flower Gardens area of the northwest Gulf of Mexico outer continental shelf (Gardner et al., 1998), the Flower Gardens Banks National Marine Sanctuary (FGBNMS) and the Minerals Management Service (MMS) have been interested in additional MBES data in the area. A coalition of FGBNMS, MMS, and the US Geological Survey (USGS) was formed to map additional areas of interest in the northwestern Gulf of Mexico (Fig. 1) in 2002. The areas were chosen by personnel of the FGBNMS and the choice of MBES was made by the USGS. MMS and FGBNMS funded the mapping and the USGS organized the ship and multibeam systems through a Cooperative Agreement between the USGS and the University of New Brunswick.

The University of New Brunswick (UNB) contracted the RV Ocean Surveyor and the EM1000 MBES system from C&C Technologies, Inc., Lafayette, LA. C&C personnel oversaw data collection whereas UNB personnel conducted the cruise and processed all the data. USGS personnel were responsible for the overall cruise including the final data processing and digital map products.

Objectives

The objective of the cruise was to map 7 regions of interest (Fig. 2) to MMS and the FGBNMS

The Kongsberg Simrad EM1000 High-Resolution MBES

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, a Kongsberg Simrad EM1000 system was chosen for this cruise because; (1) its operating frequency is compatible with the depths of interest, (2) it has the ability to map large areas at high speed without compromising data quality and, most importantly, (3) it has the ability to simultaneously produce high-resolution, calibrated acoustic-backscatter imagery. For this survey, we used an EM1000 system owned and operated by C&C Technologies, Inc., Lafayette, LA, installed aboard the 102-ft RV Ocean Surveyor (Fig. 3). The transducer is mounted on a rigidly attached boom which was lowered through a moon pool in the hull of the ship (Fig. 4).

An overview of high-resolution MBES systems in general can be found in Hughes-Clarke, et al. (1996). The EM1000 multibeam echosounder (MBES) has an operating frequency of 95 kHz, that is effective in water depths that range from 3 to 800 m. The EM1000 offers several modes of operation that depend on survey requirements; each mode has been designed to operate over a subset of the entire range of depths in which the system can operate. Shallow water modes (3-200 m) are characterized by short pulse lengths (0.2 ms) and an angular sector as great as 150° (providing swath widths up to 7.4 times water depth). Intermediate water depths (200 to 600 m) require a longer pulse length (0.7 ms) and a narrower angular sector whereas deeper waters (600 to 800 m) use a pulse length of 2.0 ms and a narrow angular sector that uses only 48 of the 60 possible receive beams. The specific options used for the northwest Gulf of Mexico shelf mapping 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 EM1000 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 wave front 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 quality control measurements, and uses this method to calculate depth.

The EM1000 also provides quantitative seafloor acoustic-backscatter data that can be displayed in a sidescan-sonar-like image. 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 four times the depth sampling rate (which depends on the water depth and operational mode). The echo amplitudes are sampled at a much faster rate than the projected receive 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 EM1000 software corrects the amplitude time series for gain changes, propagation losses, predicted beam patterns, and for the ensonified 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.

Ancillary Systems

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 of 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.

Attitude Compensation

The northwest Gulf of Mexico shelf mapping 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 Speed in WaterColumn

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. Two additional sound-velocity sensors are installed on the RV Ocean Surveyor to continuously determine the speed of sound in water at the transducer depth (YSI Model 600 temperature and salinity sensor). All the sound-speed data (SVP) are fed directly into the Simrad EM1000 processor for instantaneous beam forming and ray tracing of the individual receive beams.

Data Sources and Type

Raw EM1000 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).

The accurate reduction of swath bathymetric data critically depends on a proper knowledge of the geometry and relative positions of the sonar transducer relative to the motion sensor, the ship, and the positioning-system antennae. C&C Technologies, using standard surveying techniques, measured these values (Table 3) before the survey began. All values are measured relative to the transducer.

Table 1. Kongsberg Simrad EM1000 data stream

Table 2. Ancillary data sources

EM1000 Operational Modes

There are several operational modes available for the EM1000. 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 beam spacing governs the density and spacing of the soundings on the seafloor whereas the angular sector dictates how much of the seafloor is ensonified in one ping.

Although there are numerous operational modes available, the northwest Gulf of Mexico mapping worked primarily in two variations of shallow mode: EA-150 and ED-128. The former is characterized by equiangular beam spacing and a 150° angular sector, whereas the latter uses equidistant beam spacing, with a resulting angular sector of 128°. Equiangular spacing has the distinct advantage that there is minimal electronic beam steering for the receive apertures, thereby reducing errors because of imperfect knowledge of the water column since all beams (except outer beams) are physically steered. The disadvantage of equiangular beam spacing is that the spacing of the beam footprints on the seafloor grows non-linearly with incidence angle. With equidistant beam spacing, the receive apertures are steered in such a manner to achieve a regular footprint spacing on the seafloor. However, the disadvantage is that bathymetric solutions are now contaminated to a greater extent with the effects of refraction in a poorly known velocity structure of the water column. There is an additional drawback to equidistant beam spacing in that 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°, thus a swath width of 128° was used when the sonar was operated in equidistant mode.

Data Transformation

Tides

All soundings are reduced to mean lower low water using predicted tides (World Tides 2002) for the Galveston Pleasure Pier. The range in the tidal data set provided was approximately one meter.

Bathymetry

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) tide corrections, and (2) any unaccounted-for static attitude misalignments. The POS/MV reduced all positions to the transducer, thus there was no requirement to correct for a lever arm between the GPS antennae and sonar transducer in post-processing.

Backscatter

The Kongsberg Simrad EM1000 provides a backscatter-intensity time series for the bottom ensonification period for each of the individual receive beams. The corrections applied by the shipboard recording system are listed in Table 4. The transformations include, (1) conversion of each beam backscatter time series to a horizontal range equivalent, (2) a compilation of all individual beam traces together to produce one full slant-range-corrected trace, and (3) 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.

The data 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. This function is then inverted to minimize the beam pattern and angular variations.

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 empirically 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 can be applied 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.

Table 4. Corrections applied to each beam for backscatter

Patch Test

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, a series of "patch tests" were conducted the first day near the first map area. The ship was run back and forth across a seafloor feature to determine if there were residual roll, pitch, heading, or timing offsets in the MBES data that required correction factors. A full patch test procedure was started prior to data collection on May 16 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

The 1-Hz DGPS and 100-Hz INS navigation data were logged with the Kongsberg Simrad EM1000 software. The Bottom Detection Unit (BDU) time stamps the depth and backscatter telegrams and was slaved to a shipboard SUN Ultrasparc 2 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, Y and Z 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 and not used. On the whole, the navigation was reliable throughout the cruise with only occasional problems (e.g. loss of differential correction).

Refraction Issues

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. Repeated CTD casts allowed us to correct for refraction effects. A representative water-velocity profile is shown in Figure 5. In additional, minor empirical refraction corrections were applied during processing.

Data Processing

Shipboard data processing (Fig. 6) consisted of (1) the editing the 1-Hz navigation fixes to flag bad fixes; (2) examining each beam of each ping to flag outlier beams, bad data, etc.; (3) merging the depth and backscatter data with the cleaned navigation; 4) correcting all depth values to a mean lower low water tide datum; (5) performing additional refraction corrections, if necessary, for correct beam ray tracing; (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 (Table 6).

The Maps

Overview maps of backscatter and shaded relief (Appendix I) were generated for all areas of interest. Shaded-relief and acoustic-backscatter maps were generated at 4 m/pixel resolution.

A shaded-relief map 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.

SI = K * cos2 F (Eq. 1)

The backscatter map 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 EM1000 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.

One of the great advantages of this mapping survey is that every sounding of the bathymetry is accurately georeferenced and coregistered with an acoustic backscatter value. 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 backscatter sonar with bathymetry.

Daily Log

June 8, 2002 (JD 159)
We set out from Port of Iberia, LA in the early afternoon with an expected transit time of 6 hr to reach open water. After passing through the locks, we steamed for approximately 12 hr to the first survey site (Jakkula), arriving Sunday morning.

June 9, 2002 (JD 160)
A patch test consisting of several lines was run in order to find any systematic biases in the alignment angles between the sonar head and POS/MV. Preliminary lines showed a variation of 0.035^o from the previously measured value of 0.452^o. The correction (0.035^o) was applied but with the improper sign and thus the next lines showed twice the variation. This was corrected with the lines being run a third and final time to ensure the correct entry of the alignment parameters. The pitch offset was found to be consistent with the offset determined in the patch test prior to the cruise (May 16). Data collection began shortly after 1200 hr (local).

A GMT rollover issue arose at midnight GMT (1900 local) in that the date did not rollover in the Simrad navigation time stamps until 1 to 2 s after midnight. A bug in the Simard–to-OMG conversion software (RT) compounded this problem by subtracting 10 days from the Julian day after the midnight rollover. This was corrected by modifying the OMG software to detect and correct the midnight rollover in addition to adding a fixed number of days to the date stored in the OMG navigation and bathymetry telegrams (done through two separate programs). These two programs were applied during the conversion stage from this point on for the entire survey.

June 10, 2002 (JD161)
Surveying continued in the Jakkula area, finishing up at 2300 hr (local). A residual beam pattern artifact was noticed in the backscatter mosaics after removal of the mean angular response. Upon investigation of the angular response curves of the lines in question, it was found that nulls appeared at certain angles away from nadir. The artifacts persisted throughout the entire survey, although a software solution was later written to correct the problem. Proceeded to next survey area (Alderdice), with a transit time of 2 hr.

June 11, 2002 (JD162)
Surveying began at 0100 hr. The surface-sound-velocity sensor began to output erroneous data, causing significant refraction artifacts in the outer beams partially through line 31 (swath no. 2714). C&C changed the sensor and the refraction errors fell within tolerable limits. A POS/MV error caused a small data holiday over the feature of interest on line 40; this gap was later filled in during cross-line collection.

June 12, 2002 (JD163)
Another routine day of surveying, this time finishing up the area at approximately 1800 hr. Proceeded to next survey area (Sonnier). Began data collection in Sonnier

June 13, 2002 (JD164)
Artifacts in bathymetry appeared in outer starboard beams as a rising or falling by several decimetres. These were dubbed "roach legs" and usually appeared in heavier sea states. The artifacts manifested themselves in the backscatter as drops in intensity (black across-track bands). The bottom tracking in the outer beams was fooled by multiples while surveying over the high-backscatter/high-relief areas. Once cleaned, these beams continued to influence the data quality during the backscatter conversion from slant range to horizontal range. Long, black across track bands appeared in the holes left by the flagged beams and required the modification of OMG software (makess) to discard shorter "suspicious" footprint lengths during the backscatter generation process. Higher rate of beam dropouts also made for many data holidays in the backscatter maps. Typically, OMG software can handle a few dropouts by linearly interpolating across gaps (glfill). The observed gaps were falling outside of the tolerance of glfill, thus the software was modified to compensate for the larger gaps both inthe across-track and along-track directions.

June 14, 2002 (JD165)
Finished data collection in Sonnier at 1030 hr and proceeded to McGrail Bouma, arriving at 1428 hr. A phone communication with Jim Gardner suggested the removal of the northwest corner of the survey area in hopes of speeding up operations.

June 15, 2002 (JD166)
D. Cartwright noticed a systematic roll bias of 0.15^o in the Sonnier data set (due to the extremely flat terrain) and retroactively "unrolled" the data prior to the current line. It was decided to not change the POS/MV settings such that the entire survey is affected by the same roll bias. Data collected from this point on was unrolled during the conversion to OMG format (in the unravel script).

June 16, 2002 (JD167)
POS/MV dropouts corrupted line 117, so lines 118 and 119 were run to calibrate the unit. Normal operations resumed for following line 119. A change of schedules with C&C made immediate return to port necessary and data collection stopped one quarter of the way through line 124 at 0800 hr. Steamed for Cameran, LA and arrived late in the evening. Refuelled immediately with a partial crew change and tied up for the night.

June 17, 2002 (JD168)
Left Cameron, LA late in the morning and returned to McGrail Bouma area to continue mapping surveying. Arrived at at McGrail Bouma at midnight.

June 18, 2002 (JD169)
Resumed data collection at McGrail Bouma at 0020 hr. It was quickly noticed that there was an inconsistency between the current survey line and the survey lines collected before our return to port. C&C found that the problem consisted of an incorrect transducer draft setting in the Simrad: a draft of 2.75 m was entered since the beginning of the survey whereas it should have been 3.80 m. The draft error was corrected and retroactively applied to previous survey lines as a constant offset during the process of merging tides.

June 19, 2002 (JD170)
Email communication from Jim Gardner gave updated coordinates for Bright area, effectively cutting the large area into two smaller polygons (while trimming away non-essential portions of the survey area).
Line 160 began in the smaller northeastern section, effectively cutting line length in half. At midnight, the RT program lost another 10 days (see note on June 9, (midnight rollover and RT bug). The unravel script was modified to add 20 days to all navigation and bathymetry data from this point on.

June 20, 2002 (JD171)
Finished collection over McGrail Bouma area at 1320 hr and proceeded to Bright area, arriving at 1600 hr. Began in the eastern region of Bright area..

June 21, 2002 (JD172)
Email from Jim Gardner updated coordinates for MacNeil area allowing for a significant time savings.
Noticed abnormally large heave values stored in merged data. Upon further examination, it was found that heave is stored in decimetres when RT converts raw Simrad to OMG format (thus 0.60 m of heave is stored as 60). This is not necessarily an issue since heave is accounted for in the Simrad software, however, it may complicate removal of any long-term heave artifacts from the data set. NOTE: this is consistent throughout the entire data set, even though it was not noticed until this point.

June 22, 2002 (JD173)
Routine day of mapping.

June 23, 2002 (JD174)
Heavy seas make for noisy bathymetry and backscatter. Vessel speed was reduced during line 283 but it only provided minor improvement in data quality. No data holidays resulted but the data became increasingly tedious to edit. Noisy data are characterized by low beam-averaged backscatter.

June 24, 2002 (JD175)
Completed data collection over Bright Complex at 1630 hr and proceeded to MacNeil area arriving at 1800 hr.

June 25, 2002 (JD176)
POS/MV and/or Simrad problems caused delay and intermittent operations through lines 348 through 362 while problem was being corrected. Corrupt lines were stored in a sub-directory such that they would not be included in any gridding process (lines 352 to 354).

June 26, 2002 (JD177)
Developed software tool to clean bathymetry data based on beam-averaged intensities (see June 23, re: noisy data). This tool creates a temporary OMG file that stores the intensities as depths, thus allowing the operator to clean low intensities very easily since they fall outside the typical intensity envelope. This was added as a second cleaning step in addition to normal swath editing (this cleaning step was retroactively applied to areas in which it was required).

June 27, 2002 (JD178)
Completed MacNeil area at 1615 hr and immediately headed for Port of Iberia.

June 28, 2002 (JD179)
We arrived at the dock in Port of Iberia, LA at 1100 L, thus completing the cruise.

Cruise Participants

References

Gardner, J.V., Mayer, L.A., Hughes Clarke, J.E., and Kleinier, A., 1998, High-resolution multibeam bathymetry of East and West Flower Gardens and Stetson Banks, Gulf of Mexico. Gulf of Mexico Science, v. 16, p. 131-143.

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.

Appendix I: DTMs and acoustic backscatter mosaics of the 7 areas mapped.

Alderdice Shaded-Relief Bathymetry

Alderdice Acoustic Backscatter

Sonnier Shaded-Relief Bathymetry

Sonnier Acoustic Backsccatter

East Bright Shaded Relief Bathymetry

East Bright Acoustic Backscatter

West Bright Shaded Relief Bathymetry

West Bright Acoustic Backscatter

Jakkula Shaded-Relief Bathymetry

Jakkula Acoustic Backscatter

MacNeil Shaded-Relief Bathymetry

MacNeil Acoustic Backscatter

McGrail-Bouma Shaded Relief Bathymetry

McGrail-Bouma Acoustic Backscatter