RV OCEAN SURVEYOR CRUISE O1-02-GM
BATHYMETRY AND ACOUSTIC
June 8, through June 28, 2002
Iberia, LA to Iberia, LA
Jonathan D. Beaudoin1, James V. Gardner2, John E. Hughes Clarke1
New Brunswick, Fredericton, NB
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.
The Kongsberg Simrad Systems
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.
The objective of the cruise was to map 7 regions of interest (Fig. 2) to MMS and the FGBNMS
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.
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.
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.
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.
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.
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.
Corrections applied to each beam for backscatter
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).
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.
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).
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.
June 8, 2002 (JD 159)
June 9, 2002 (JD 160)
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 Simardto-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.
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June 23, 2002 (JD174)
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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.