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COMPARISON OF SEABEAM 2112 AND SCAMP BATHYMETRY DATA
ALONG THE GAKKEL RIDGE: PRELIMINARY MAPPING RESULTS FROM THE
HEALY0102 ARCTIC CRUISE.
G.J. Kurras1, M.H. Edwards1, R.M. Anderson1, P. Michael2, J.R. Cochran3
and B.J. Coakley4
1
Dept. of Marine Geology and Geophysics, School of Ocean Earth Science and Technology, University of
Hawaii, Honolulu, Hawaii 96822; 808-956-3593; email: [email protected]
2
Dept. of Geosciences, University of Tulsa, OK 74104
3
Dept. of Geology and Geophysics, Lamont-Doherty Earth Observatory, Palisades, NY 10964
4
Dept. of Geology, Tulane University, New Orleans, LA 70118
Arctic Ocean by a factor of 30 during approximately 2
months of mapping [2]. At the time this manuscript is being
prepared, the USCGC Healy is further expanding the Arctic
Ocean sonar database, collecting acoustic data using a hullmounted Seabeam 2112 system. The total contribution of the
USCGC Healy to the Arctic bathymetric database won't be
known before this article goes to press; however, through
one-third of the duration of the Healy0102 survey the
Seabeam 2112 system has collected a large volume of highquality data despite the ubiquitous presence of surface ice in
the field area. Some of the Seabeam 2112 data coverage
overlaps with SCICEX/SCAMP data coverage, making it
possible to compare the performance of these two disparate
systems. Qualitative comparisons of bathymetric charts
emailed from the USCGC Healy to shore demonstrate general
agreement between depths in the two datasets although the
locations and shapes of prominent features are noticeably
different. We anticipate that by the time of the Oceans 2001
conference, the differences in system performance will be
quantified for presentation.
Abstract-As part of an international effort to investigate
geological processes at Earth's slowest spreading mid-ocean
ridge and to map the largely unexplored Arctic Ocean, the US
Coast Guard Cutter Healy and the German research vessel
Polarstern surveyed and sampled the Gakkel Ridge in the
summer/fall of 2001. The expedition was planned using
bathymetry and sidescan data acquired by the SCAMP
interferometric sonar mounted on the hull of the nuclearpowered submarine USS Hawkbill in 1998 and 1999. In addition
to seismic surveys, dredging and underway oceanographic data
collection, both icebreakers participating in the 2001 program
acquired multibeam sonar data: Seabeam 2112 from the USCG
Healy and Hydrosweep from the R/V Polarstern.
The
acquisition of these two new datasets presents the opportunity to
compare GPS-navigated data collected using multibeam systems
operating in ice-covered waters with interferometric data
collected aboard a nuclear-powered submarine operating
beneath the permanent ice pack. Comparisons reveal that even
in 7/10 and 8/10 ice conditions, the quality of the hull-mounted
bathymetry data are equivalent or even superior to the
interferometric bathymetry data. Similar features detected in
both datasets allow the GPS-navigated Seabeam 2112 data to
resolve navigational ambiguities in data collected using the USS
Hawkbill's inertial navigation system.
II. SCAMP VS. SEABEAM SONAR SYSTEMS
The SCAMP and Seabeam 2112 systems have different
configurations and are susceptible to variable environmental
conditions in the Arctic that directly affect performance.
Both systems operate at approximately the same frequency,
but despite being "hull-mounted" on a Sturgeon-class nuclear
submarine, the SCAMP SSBS is more analogous to a towed
interferometric sonar than a multibeam system; the Seabeam
2112 is a conventional multibeam system with significantly
longer fore-aft and athwartship arrays than the SCAMP
SSBS. As nuclear submarines are quiet by design and
significantly larger than conventional towed sonar vehicles,
they provide an exceptional survey platform for hull-mounted
interferometric systems. The quiet platform and submarine
stability, combined with the ability to survey below the
thermocline and unconstrained by the presence of ice, yield
excellent acoustic data. Unfortunately, submarine navigation
is possible only using inertial guidance systems with
occasional satellite fixes upon surfacing through the ice. In
contrast, sonars mounted on the hulls of surface vessels have
I. INTRODUCTION
Year-round ice cover in the Arctic Ocean has, until very
recently, hampered efforts to collect the comprehensive,
high-resolution, wide-swath bathymetry data currently
available for the rest of Earth's oceans. The recently released
International Bathymetric Chart of the Arctic Ocean [1],
included ca. 1.5 million depth single-beam soundings for the
entire Arctic Basin. During the 1998 and 1999 SCience ICe
EXercise (SCICEX), a collaboration between the United
States Navy and U.S. National Science Foundation-sponsored
researchers, a Sidescan Swath Bathymetry Sonar (SSBS) was
mounted on the hull of the nuclear-powered submarine USS
Hawkbill as part of the Seafloor Mapping and
Characterization Pods (SCAMP).
The SCAMP SSBS
increased the total number of soundings acquired in the
The National Science Foundation sponsored this research
(grants OPP-9619251 and OPP-0122387).
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the navigational advantage of being able to use the Global
Positioning Satellite (GPS) system to determine ping
location; however, data collected by the hull-mounted sonars
are more susceptible to noise because of surface interactions
and because the system must transmit and receive sound
through the thermocline. In general, multibeam systems
produce bathymetry data with better vertical resolution than
towed sonars because of the data beam steering permitted by
their longer arrays; however, it was unknown whether severe
noise during icebreaking operations and the occasional
covering of the Seabeam 2112 arrays by ice trapped under the
ship would affect the quality of the USCG Healy's data.
Many operational aspects of the Seabeam 2112 and the
SSBS are similar. The SCAMP SSBS is an interferometric
system with operating frequency of 12 kHz, with estimated
sidescan (imagery) swath width of ~160° and a bathymetric
swath width of up to 140° (Table 1). In the region of the
Gakkel Ridge, where water depths range from 1000 to 6000
meters, observed sidescan widths were up to 20 km, with
bathymetric swath widths of up to 10 km. Spatial resolution
varies as a function of altitude above the sea floor and sample
position relative to nadir. Along-track resolution of SCAMP
varies from 20m (at nadir in 1000m depth) to about 160m (at
swath edge in 4000-6000m depth). Cross-track resolution is
a function of pulse length and angle of incidence to the sea
floor, and varies from about 5m at the outer edge of swath to
230m at nadir in deep water. The along-track sampling
interval is 99 meters at the submarine’s survey speed of 16
knots. The bathymetric data presented in this paper was
gridded at 100m cell size; sidescan imagery was gridded at
33m cell size.
The Seabeam 2112 multibeam sonar also operates at a
frequency of 12 kHz, with a bathymetric swath width of 100°
to 120° (Table 1). In water depths of 1000 to 6000 meters,
the observed swath width is up to 12 km. With 2° beamwidth
in both the along-track and across-track directions, the spatial
resolutions along- and across-track are identical, and vary
from 35m (at nadir in 1000m depth) to 300m (at outer edge
of swath in 4000-6000m depth.) The along-track sampling
interval is 6 to 37m at USCG Healy’s survey speeds of 1 to 6
knots. The Seabeam 2112 bathymetry presented in Fig. 1
was gridded at 50m cell size.
III. DATA COMPARISON
Comparison between the SCAMP and Seabeam 2112
bathymetry focuses on the region around a single geologic
feature [85oN, 10oE] to evaluate the relative quality of
bathymetric data, feature location and resolution (Fig. 1).
Identical processing algorithms were used to convert from
geographic latitude, longitude and depth to polar
stereographic grids using the WGS-84 ellipsoid. The top
panel in Fig. 1 shows Seabeam 2112 multibeam bathymetry;
the bottom panel depicts SCAMP interferometric bathymetry
for the same region. The color palettes, sun illumination
angles (275o) and latitude/longitude boundaries for each chart
are identical.
A. Navigation
TABLE I
For the purpose of this discussion, internal navigation
errors are defined as positioning errors resulting in
inconsistencies between adjacent survey lines, which distort
the true shape and character of seafloor features. Global
navigation errors are defined as positioning errors resulting in
a seafloor feature being incorrectly located on the Earth. It is
possible to have global navigational errors without having
internal navigation errors; however, internal navigation
always results in at least small global navigational errors. In
the case of the SCAMP data, the magnitude and inconsistent
nature of the inertial navigation errors result in both global
and internal navigational errors.
The comparative charts (Fig. 1) demonstrate obvious
differences in the location of features between the two
datasets. The shallow topographic ridge near the center of
the Seabeam 2112 chart is offset approximately 2km to the
east in the SCAMP chart. Because GPS data have been
acquired consistently during the Healy0102 survey, the offset
in the SCAMP positional data is almost certainly in error.
Internal navigational errors in the SCAMP data have also
produced noticeable variations in the shapes of topographic
features. For example, the SCAMP data depict en echelon
steps in most of the SW-NE trending linear features;
however, in the Seabeam map these same features are more
contiguous albeit curvilinear. These kinds of distortion in
topography can lead to erroneous geologic interpretation.
TECHNICAL SPECIFICATIONS FOR THE SCAMP AND SEABEAM SONAR SYSTEMS.
SCAMP Phase-Array Bathymetric Sonar
Frequency:
12 kHz
Pulse Length:
83 microseconds to 10
milliseconds
Gakkel Ridge = 6 ms
Modulation:
CW (tone burst) and FM (chirp)
Gakkel Ridge = CW
Repetition Rate:
2 to 20 seconds
Gakkel Ridge = 12 s
Source level:
233 dB re 1 mPascal @ 1 meter
Power
115 VAC
Full Backscatter Swath Width
~160o
Full Bathymetry Swath Width
~140 o
Along Track Beam Width
~1.2 o
RMS depth accuracy
~1.0% of water depth
Seabeam Multibeam Bathymetric Sonar
Frequency:
12 kHz
Pulse Length:
3 milliseconds to 20 milliseconds
Gakkel Ridge = 10-14 ms
Modulation:
CW (tone burst)
Gakkel Ridge = CW
Repetition Rate:
2 to 20 seconds
Gakkel Ridge = 12 s
Source level:
234 dB re 1 mPascal @ 1 meter
Power
115 VAC
Full Bathymetry Swath Width
up to 120 o (121 beams maximum)
Along Track Beam Width
~2 o
RMS depth accuracy
<0.5% of water depth
2
B. Bathymetry
Bathymetry data processing for the SCAMP SSBS [3] and
the Seabeam 2112 system are quite different. Minimal
processing is performed on the Seabeam data prior to
incorporation into the gridded and filtering scheme. For the
SCAMP bathymetry data, empirical tables for converting
phase angle differences to geometric angles are repeatedly
developed, applied and evaluated before latitude, longitude
and depth triples are input into the gridding and filtering
algorithms. SCAMP data processing is thus much more
operator intensive than Seabeam 2112 bathymetry
processing.
Despite the different processing steps and efforts needed to
produce bathymetry for the two systems, the depths depicted
in the SCAMP SSBS and Seabeam 2112 datasets are fairly
comparable. Depths range through approximately the same
spectrum in both maps although the SCAMP data have a
blurry appearance due to the large grid cell size. Artifacts in
the Seabeam 2112 depths are manifested as checkerboard
patterns on the slopes of steep features and where swaths
appear to turn. In the SCAMP data bathymetric artifacts
appear as parallel SE-NW trending bands of bumpy terrain;
these occur along the edges of overlapping swaths.
The primary advantage of the SCAMP bathymetry data is
the 100% data coverage in the survey area. Because the USS
Hawkbill operated independently of the ice conditions, the
SCICEX-98 and SCICEX-99 surveys were accomplished
very quickly and systematically. The USCGC Healy, on the
other hand, is not able to acquire good quality data when
breaking heavy ice and therefore is occasionally unable to
map specific sites. Also, the slow survey speed of the
USCGC Healy relative to the USS Hawkbill reduces the
amount of data that can be collected during surveys of equal
duration by a factor of 3-10+.
C. Sidescan and Amplitude data
At press time no examples of sidescan or amplitude data
for the Gakkel Ridge acquired by the USCGC Healy's
Seabeam 2112 have been made available largely because the
quality of the data are greatly inferior to the SCAMP SSBS
sidescan data. In the case of the amplitude data this is not
surprising - amplitude values for the Seabeam 2112 are
acquired for every beam and thus have an order of magnitude
lower across-track resolution than the SSBS sidescan data.
Comparisons of the Seabeam 2112 and SCAMP sidescan
data are currently underway.
IV. CONCLUSIONS
The Healy0102 survey of the Gakkel Ridge in summer/fall
2001 collected a Seabeam 2112 dataset well-suited for
comparison with the SCAMP SSBS data collected during
SCICEX-98 and SCICEX-99 aboard the Sturgeon-class
nuclear submarine USS Hawkbill. Preliminary comparisons
of overlapping regions in the two sonar datasets has provided
a rare opportunity to contrast the performance of two
Fig. 1. Comparison of Seabeam 2112 versus SCAMP bathymetric data
over a single geologic feature on the Gakkel Mid-Ocean Ridge.
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different types of sonars in ice-covered waters. Qualitative
comparisons of the bathymetry and navigation data reveal:
• GPS navigation data collected aboard the USCGC Healy
can be used to recognize and correct navigational offsets
in the SCAMP data that result from the submarine's
inertial guidance system.
• The vertical and horizontal resolutions of the Seabeam
2112 bathymetry are at least as good and frequently
better than analogous resolutions for the SCAMP
system. However, the Seabeam 2112 data quality is
highly dependent on ice conditions and good quality data
cannot be acquired while breaking heavy ice.
Additionally, ice occasionally gets trapped under the hull
of the USCGC Healy, blocking the transducers and
making it impossible to acquire Seabeam 2112 data even
in open water.
• Similar gridding and filtering approaches can be used for
the Seabeam 2112 and SSBS bathymetry; however,
processing of the SCAMP SSBS data to reach this step is
significantly more labor intensive than processing the
Seabeam 2112 data.
• The quality of the Seabeam 2112 amplitude data is
inferior to the SSBS sidescan data.
• The presence of ice occasionally prevents the USCGC
Healy from reaching specific survey sites and reduces
the amount of data that can be acquired relative to a
nuclear-powered submarine that can operate largely
without being constrained by ice. The presence of
permanent pack ice makes it virtually impossible to
undertake systematic "mowing the lawn" surveys from
an ice breaker.
[2] Edwards, M.H., G.J. Kurras, M. Tolstoy, D.R.
Bohnenstiehl, B.J. Coakley, and J.R. Cochran, Evidence of
recent volcanic activity on the ultra-slow spreading Gakkel
Ridge, Nature, vol. 409, pp. 808-812, 2001.
[3] Davis, R.B., M.H. Edwards, M.R. Rognstad, T.B.
Appelgate, and D.N. Chayes, New Tools for Processing
Sidescan and Interferometric Bathymetry Data, Sea
Technology, vol. 42, pp. 21-27, 2001.
ACKNOWLEDGMENTS
SCAMP data: We thank the captain, Robert Perry, officers
and crew of the USS Hawkbill and the scientists and
engineers who sailed during SCICEX-98 and SCICEX-99.
Thanks to D. Chayes, M. Rognstad and their associates for
building and maintaining SCAMP. R. Davis developed the
SCAMP processing software; B. Appelgate and P. Johnson
led development of the GMT-based processing scheme used
to produce the bathymetry charts. GMT software was
developed by P. Wessel and W. Smith.
Seabeam 2112 data: We thank Captain David Visneski
and the officers, crew and scientists who participated in
Healy0102. D. Chayes, R. Davis and P. Lemmond provided
systems and software support for the Healy Seabeam and
computer systems. MB-System software was developed by
D. Caress and D. Chayes.
SOEST contribution number 5481.
REFERENCES
[1] Jakobsson, M., Cherkis, N., Woodward, J., Coakley, B.J.
and Macnab, R., A new grid of Arctic bathymetry: A
significant resource for scientists and mapmakers. EOS
Trans. Am. Geophys. Union, vol. 81, pp. 89-96, 2000.
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