1. High-resolution bathymetric mapping with the new broad-bandwidth, split-beam, scientific, multibeam sonar installed on the new NOAA FSVs
G. R. Cutter Jr.1
D. A. Demer1
L. Berger2
1NOAA NMFS Southwest Fisheries Science Center, La Jolla, CA
2IFREMER, Département NSE, Plouzané, France.
I’m here today to talk to you about a new multibeam sonar system, the Simrad ME70.
U.S. Hydro 2009 Conference
Submitted abstract:
Comparisons of seafloor bathymetry from the ME70 in bathymetric and fisheries modes Cutter, G. R.1, Demer, D. A.1, Berger, L.2 The ME70 is a relatively new multibeam echosounder system that was designed by Simrad and Ifremer for quantitative fisheries research and is installed on the new, acoustically quiet NOAA Fisheries vessels.  The ME70 can provide calibrated, full-water-column acoustic scattering data for fisheries surveys, or can be operated in bathymetric mode to collect soundings that are expected to meet IHO S-44 Order 1 standards.  The bathymetric mode requires an additional processor unit that is not currently part of the system on the NOAA Fisheries vessels.  In order to acquire bathymetric data for seafloor habitat studies using the NOAA ME70 systems, bathymetric data must be derived from fisheries mode operation.  Here, we describe a method to process the angular coordinate data from multiple split-beams formed in fisheries mode to estimate seafloor range, within-beam seafloor slope, acoustic dead zone height, and surface scattering strength.  We then compare bathymetry resulting from the split-beam method for a fisheries mode survey to a bathymetric mode survey of the same area in the Bay of Biscay. 
1 NOAA, Southwest Fisheries Science Center, 8604 La Jolla Shores Dr., La Jolla, CA. 2 Ifremer, Département NSE, Plouzané, France.
Comparisons of seafloor bathymetry from the ME70 multibeam in bathymetric and fisheries modes
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High-resolution bathymetric mapping with the new broad-bandwidth, split-beam, scientific, multibeam sonar installed on the new NOAA FSVs
Cutter, G. R. (1), Demer, D. A. (1), and Berger, L. (2)
The Simrad ME70 is a new multibeam-echosounder system that was designed for quantitative fisheries research and is installed on each of the new, acoustically-quiet, NOAA Fisheries survey vessels (FSVs). The ME70 has configurable beams and transmits in the range of kHz to provide calibrated, acoustic backscattering data throughout the detection range (Fisheries Mode). With hardware and software add-ons, the ME70 can also collect soundings that are expected to meet IHO S-44 Order 1 standards (Bathymetric Mode). Furthermore, with custom algorithms and software, bathymetric data can be obtained from the ME70 operating in Fisheries Mode, and volume backscatter can be sampled from the ME70 operating in Bathymetric Mode. This flexibility may allow data to be concurrently and efficiently collected on fish and their seabed habitat. Here, we describe a method to process the echo amplitude and phase data from multiple split-beams formed in Fisheries Mode to estimate seabed range, slope, roughness, and normalized surface scattering strength (a hardness metric). We compare the resulting bathymetry to that collected with the ME70 operating in Bathymetric Mode in the same area of the Bay of Biscay.
1 NOAA, Southwest Fisheries Science Center, 8604 La Jolla Shores Dr., La Jolla, CA. 2 Ifremer, Département NSE, Plouzané, France. Correspondence to:

2. NOAA ME70s New NOAA Fisheries Survey Vessels (FSVs)
NOTE: SWFSC methods & IFREMER methods differ slightly but result in statistically the same FM bathymetry. Therefore, we present SWFSC results.
New NOAA Fisheries Survey Vessels (FSVs)
Oscar Dyson, Bigelow, Pisces, Shimada, FSV5, FSV6
Equipped with the Simrad ME70
Quiet vessels (ICES spec.)
NOAA FSVs
Mount: ME70 transducer in centerboard
Nav/Pos: POS/MV
I’m here today to talk to you about a new multibeam sonar system, the Simrad ME70.
I’m not trying to sell you an ME70. …
In fact, NOAA already owns several ME70s, they are part of the outfit of the new NOAA Fisheries Survey Vessels (FSVs).
Six new DYSON-class FSVs are part of NOAA’s ship recapitalization plan.
Two of these new FSVs are already in service, these are: Oscar Dyson and Bigelow.
Another (Pisces) is to be delivered and in service by this fall.
Another, the Shimada, will be delivered by end of this year.
FSV6 is to replace the David Starr Jordan (by FY14), with home port in San Diego.
FSV5 is a shallow draft version, and will be built after FSV6.
>>>>
NOAA FSVs
Oscar Dyson, Bigelow, Pisces, …
Quiet survey vessels
Each is equipped with a new, scientific multibeam echosounder for fisheries research, the Simrad ME70
DYSON (homeport Kodiak AK) and BIGELOW (homeport Woods Hole MA) are already in service.  PISCES will be delivered this summer and expected to be operational after shakedown and post-delivery work in the fall.  She will work out Pascagoula MS.  SHIMADA will be delivered Sep-Nov and will be operational Q4 FY10.  Her homeport is designated as Seattle, but we will be using her for several of our surveys.  FSV6 to replace to JORDAN will be homeported in San Diego and is expected to be operational in late FY13 or FY14.  Although it's referred to as FSV6 it will be the 5th in the DYSON class.  FSV5 will be a shallow draft version to be built after FSV6 - the numbering system is an artifact of an old ship-replacement plan.  We have some stuff on the new FSVs here The full ship recapitalization plan can be viewed here Go to page iii for a concise time table. - Roger
Mount: ME70 transducer in centerboard
Nav/Pos: POS/MV
Description and demonstration of seafloor characterization results available from ME70
(following methods developed for EK60)

3. ME70 Scientific multibeam echosounder Developed by Simrad and Ifremer
Principally designed for fisheries research
Two operational modes: Fisheries Mode (FM) and Bathymetric Mode (BM)
Either Mode
800 element array
Split-aperture processing for all beams
Motion-compensated to ±10° roll, ± 5° pitch, and heave
Calibration (by standard sphere)
NOTE: SWFSC methods & IFREMER methods differ slightly but result in statistically the same FM bathymetry. Therefore, we present SWFSC results.
NOAA FSVs
Mount: ME70 transducer in centerboard
Nav/Pos: POS/MV
ship p s
Description and demonstration of seafloor characterization results available from ME70
(following methods developed for EK60)
The  ME70 was designed by Simrad and Ifremer (Trenkel et al. 2008) as a multibeam sonar for quantitative acoustic surveys of fish or other organisms.
The ME70 forms split-beams (like the EK60s), but produces a fan of beams athwartship, like a typical multibeam sonar. So, if the fish are not directly beneath the vessel where the EK60s are looking, because of avoidance or natural distribution, then the ME70 may still receive echoes from the fish.
The ME70 has two operational modes: Fisheries Mode (FM) and Bathymetric Mode (BM). The only vessel currently equipped with the ME70 with FM and BM processors is Ifremer’s Thalassa (Trenkel et al. 2008).
In either mode (BM or FM), the ME70:
Uses an 800 element array used to form beams
All beams can be split
Is motion-compensated to ±10° roll, ± 5° pitch, and heave (requires instantaneous IMU data input)
Can be calibrated (by standard sphere).
-30°
30° 0°

4. ME70 Operational Modes Fisheries mode FM
Records from entire water-column
USER-CONTROLLED CONFIGURATION:
Number of beams (3 to 45)
Beam directions, swath span and overlap
Beam opening angles (min. 2°)
Beam frequencies
WIDE-BAND ( kHz) frequency transmission
Two-way sidelobe suppression
Adjustable sidelobe levels (to -70 dB)
Calibrated Sv, TS, and single target detections over entire water column
Depth estimation, by built-in amplitude bottom detection (with backstep)
Control and interface:
ME70.exe software
The configuration used for the calibration and SAT is: SAT Config 2.
This config forms 17 beams in the fan, and 2 reference beams.
The frequency of the beams in the fan range from 70 to 120 kHz.
(We implement bottom-detection for FM data using amplitude and phase)

5. ME70 Operational Modes Bathymetric mode BM
Requires additional processor machines (bathymetric module)
FIXED CONFIGURATION
Equidistant or equiangle beams, *2 possible pulse options.
80 narrow beams (up to 200 soundings per swath)
Beams formed during reception
One-way sidelobe suppression (-35 dB)
SINGLE FREQUENCY transmission
Bottom-detection using amplitude near normal-incidence and phase for more oblique angles
Control and interface:
Standard Simrad EM processor station and SIS software (as for EM series multibeams)
BM is like a typical seafloor-mapping multibeam
The configuration used for the calibration and SAT is: SAT Config 2.
This config forms 17 beams in the fan, and 2 reference beams.
The frequency of the beams in the fan range from 70 to 120 kHz.
Integrates ancillary data, and produces .ALL format files.

6. NOAA ME70s
NOTE: SWFSC methods & IFREMER methods differ slightly but result in statistically the same FM bathymetry. Therefore, we present SWFSC results.
NOAA FSVs
ME70s with Fisheries Mode only
Q1 - In addition to water-column fisheries survey data, can the ME70 FM provide bathymetric data for hydrographic or habitat studies?
For this, we need FM mode to deliver:
High-resolution bathymetry, comparable to BM & standard MBES
Backscatter data for seafloor characterization
Q2 - Do we really need the BM?
This study: derives and compares bathymetry collected with and without the bathymetric processor unit (BM & FM)
Quiet vessels (ICES spec.)
NOAA FSVs
Mount: ME70 transducer in centerboard
Nav/Pos: POS/MV
Description and demonstration of seafloor characterization results available from ME70
(following methods developed for EK60)

7. ME70 Flowchart .ALL Files Bathymetric Mode (fixed configuration)
SIS work station
Hydrographic-data
processing software
(e.g. Caris HIPS, Hypack)
IMU
GPS CT
Seabed bathymetry
and classifications
.RAW
ME70 workstation
Simrad ME70
Custom software
Fishery Mode
(custom configuration)
Fishery-data
(e.g. Movies 3D, Echoview)
Animal classifications,
abundances, distributions,
and 3D images

8. Survey Conducted by Ifremer Bay of Biscay, west of France R/V Thalassa+
Survey Area*
Survey
Conducted by Ifremer
Bay of Biscay, west of France
R/V Thalassa
Currently, the only vessel with ME70 with FM & BM
Date: 19 March 2008
Vessel: R/V Thalassa
Survey speed: ~10 knots (BM)
ME70_Ifremer_UTM_OverviewMap.png
Date: 19 March :03:26 GMT
Survey speed: ~10 knots
Thalassa
See Ifremer articles for vessel and equipment details,
Or get from L. Berger.
Particularly, Nav/Pos
ME70 on blister on hull
Pic in Berger paper

9. Study
Common coverage from BM and FM
Show swath with depth > range
Comparisons using ME70 data collected in bathymetric and fisheries mode from overlapping coverage
Range setting for FM: 232 m
Common coverage
betw BM and FM
>250 m
>240 m
>230 m
Drop these?
Range setting for FM used during survey of data4: 232 m

10. Bathymetry from Bathymetric Mode
Processing of Simrad ME70 Bathymetry Mode data using CARIS HIPS
BM Bathymetry
Standard hydrographic data processing methods and software:
CARIS HIPS
Only gross outliers were removed
Other options: Ifremer software, Hermes, Movies3D, and perhaps Hypack, or Triton
BM Bathymetry
Standard hydrographic data processing methods and software:
We used CARIS HIPS
Processing of Simrad ME70 Bathymetry Mode data using CARIS HIPS included:
Defined vessel configuration (If accurate, only additional data needed is tide elevation, other ancillary data are in the .all file)
Imported .ALL file
Removed outliers (filter, then editor)
ONLY GROSS OUTLIERS WERE REMOVED (important to note for comparisons).
Examined motion data
Applied tidal elevation compensation
Compensated for refraction using sound speed data (in .all file)
Merged data
Created surfaces
Exported results for use in GIS
Ifremer uses other software, Hermes and Movies3D,
Perhaps the BM data could have been processed using Hypack, or Triton Suite
Notice the few bad detections from sidelobes,
Auto-removed by filter.
Import .ALL
Remove outliers (editor or filter)
Examine motion data
Apply tide
Merge
Create surface
Export

11. Bathymetry from Fishery Mode
Revise fig w/ swath where sf is not so flat,
And aligning the ray with an actual beam
Post-proc tools for ME70 FM data are lacking, so we have to implement ourselves.
This bucket is your ship.
The orange Tdcr is the sonar transducer....
The tdcr transmits a pulse of sound.
It travels through the water and reflects or scatters off organisms, density interfaces, and the seafloor, and some of it travels back to the receiver,
We use the time of travel to estimate the range to the targets (in this case the sf); that is r(theta_a) where theta_a is the angle of arrival. We know the angle of arrival for each formed beam, and use that to estimate the depth
relative to the tdcr (giving z_uncorrected)
If we know tdcr depth, heave (we do), and estimate water elevation due to tide (we did), then we can compensate for those, resulting in an estimate of corrected depth, by beam direction and time, z_corrected(t,beamdir).
FM Bathymetry
Custom Matlab code
Other options: Myriax Echoview, Ifremer software
Depth estimation:
Not compensated for refraction
SWFSC Estimation of Depth from ME70 FM .RAW files
ztide
Tdcr zθ
rθ  zθ θ rθ
heave
Tdcr depth
Tdcr θa
r(θa) zu
seafloor
FM Bathymetry
Post-processing tools for ME70 FM data are lacking, so we implemented this ourselves, by developing custom Matlab code. Other options include: Myriax Echoview, Ifremer software. However, we did not have Ifremer’s software, and Echoview currently has limited processing capabilities for the ME70 data.
Depth estimation is done (as depicted in the figure to the right):
The blue trapezoid is the ship.
The orange rectangle labelled “Tdcr” is the sonar transducer.
The tdcr transmits a pulse of sound.
It travels through the water and reflects or scatters off organisms, density interfaces, and the seafloor, and some of it travels back to the receiver,
We use the time of travel to estimate the range to the targets (in this case the sf); that is r(theta_a) where theta_a is the angle of arrival. We know the angle of arrival for each formed beam, and use that to estimate the depth
relative to the tdcr, giving uncorrected depth (z_u).
If we know tdcr depth and heave (we do), and estimate water elevation due to tide (we did), then we can compensate for those, resulting in an estimate of corrected depth, by beam direction and time (z_corrected(beamdir,t)).
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Estimating depth
Range r(θa) is estimated from a bottom-detection method.*
Depth (z) was estimated using a geometric solution, not ray-tracing, and assumed that the ME70 has compensated for pitch and roll by actively steering beams—meaning that angle of arrival θa is true. Then, uncorrected depth (depth below transducer plane) zu is:
zu(t) = r(θa)*cos(θa)
Compensating for transducer depth (draft), heave, and water elevation due to tide gives corrected depth zcorr:
zcorr(t) = zu(t) + ztdcr + heave(t) – ztide(t)
Solving for athwartship distance y, (assume x = 0) and rotating x,y,zcorr by the heading gives x’,y’,zcorr.
Translating local coordinates of the bottom detection position by global position of transducer gives global coordinates E,N,zcorr (after converting from geographic to projected coordinates, UTM Eastings and Northings; m).
zcorr(t) = zu(t) + ztdcr + heave(t) – ztide(t)
Trigonometric solutions for local x’,y’ and then conversion to global coordinates E, N  E, N, z
Heave from the IMU and Tdcr depth are recorded in the raw file.
Theta(a) is recorded in the raw file (beam direction, compensated for motion).
Range r(θa) is estimated from a bottom-detection method using amplitude or phase;
converted to uncorrected depth (z_u); then corrected for transducer depth, heave, and tide, giving corrected depth (z_corr); and its associated local horizontal coordinate values (x,y) are converted to global values (E, N; UTM Zone 30 north).

12. Bottom detection * Amplitude e or Sv Center of mass of e or Sv
Clarify
What BM uses, What Ifremer uses, What SWF uses
Bottom detection
Amplitude e or Sv
Center of mass of e or Sv
Differs from peak if noisy
Sv (dB)
Thr *
Differential phase
Alternatives
CUBE (Calder et al.)
Bayesian model including amplitude and phase information from several beams and transmissions (Bourguignon et al., 2009)
More robust phase-differencing (Demer et al. 2009)
Various methods can be used to estimate the range to the target or seafloor.
Some are more efficient for detections at or near normal-incidence, and some more effective for oblique angles.
For near normal-incidence, the range to the seafloor can be estimated accurately using the travel-time of the peak or center of mass of the amplitude (or volume backscatter) values of the received echo, after filtering or thresholding to avoid spurious detections from the initial pulse or strong water-column targets.
Peak amplitude is accurate for normal incidence from hard or flat seafloors, for near-normal incidence, or rough seafloors COM can be more accurate, because of the effects of roughness or slope on the amplitude that can cause COM and peak locations to differ.
For oblique angles, the phase differences between wavefronts received by different elements or portions of the receiver array can be used to accurately determine the range to the seafloor. Where the phase difference is equal to zero is the range to the seafloor at the center of the beam.
There are some more elaborate, alternative methods for estimating range and seafloor bottom-detections, but typical seafloor mapping multibeam systems use a combination of amplitude and phase detection methods to estimate the range to the seafloor for different portions of the multibeam swath. Amplitude-detection is used near normal incidence (usually corresponding to vertical beam direction) and phase-detection is used for oblique angles.
Some of the alternative methods for bottom detection and estimation of bathymetry use information from multiple beams and models to determine the statistical probability of the detection (e.g. CUBE, Bourguignon) or within-beam information to provide more robust estimation of range from phase differences (e.g. Demer et al., 2009).
>>>>>
Clarify
What BM uses, What Ifremer uses, What SWF uses
Alt
Calder et al. ()
Bourguignon et al. 2009
Demer et al. 2009
>>>>>>>>>>
Bourguignon, S., Berger, L., Scalabrin, C., Fablet, R., and Mazauric, V Methodological developments for improved bottom detection with the ME70 multibeam echosounder. – ICES Journal of Marine Science, 66: 000–000.
Multibeam echosounders and sonars are increasingly used in fisheries acoustics for abundance estimation. Because of reduced side-lobe levels in the beam-array pattern, the new Simrad ME70 multibeam echosounder installed on board Ifremer’s RV "Thalassa" has been designed to allow improved detection of fish close to the seabed. To achieve this objective, precise and unambiguous detection of the water-bottom interface is required, which raises the issue of bottom detection, especially in the outer beams. The bottom-detection method implemented in the ME70 is based on the amplitude of the reverberated echo. Such an approach is efficient for vertical beams, but less accurate for beams with higher incidence angles, typically 30°–40° for the beam configurations used on RV "Thalassa", where the incidence angle, the beam opening, and the nature of the seabed contribute to weakening the backscattered signal. Therefore, the aim of this study was twofold. First, we proposed to improve the current bottom-detection method based on the amplitude of the echo. Thanks to the split-beam configuration being available for all beams of the ME70, we also proposed to use the phase information in the backscattered signals of the outer beams, as is more commonly done with multibeam systems dedicated to seabed mapping. Then, we set a Bayesian estimation framework that takes into account the spatial continuity between adjacent echoes, giving more robustness to the bottom estimation itself. Results using data collected at sea for various bottom types are presented here. θ°
Range (m)
Show waves arriving at 2 tdcrs
That allow phase diff meas

13. Soundings Sounding locations resulting from ME70 BM & FM
Mean depth in this subregion: 220 m
FM: 21 beams, each ~ 3°
BM: 80 beams
Up to 200 soundings per ping
(~150 soundings per ping with valid bottom detections for this dataset)
Here, we see the spatial distribution and extent of the sounding locations for seafloor returns detected in BM and FM operation, for part of the study area.
Red points are BM soundings, and black are FM soundings.
This range and swath width in FM were selected by the users and could have been the same as BM, but for other purposes, the FM configuration was different from BM. The FM beams were steered to form a 60 deg swath.
The number of soundings per 10 by 10-m area, were approx 2 – 3 for BM, and 1 – 2 for FM.
# per 10-m cell
Number of soundings per 10 by 10 m grid cell, for a) BM, and b) FM.
Seafloor grid models (5 by 5 m grid cell size) from FM Zpzc and BM ZBM plotted using identical colormaps.

14. Note In BM, the ME70 swath spans 120°
In FM, the user specifies the swath span for the ME70
…or wider
For this study, the ME70 swath span in FM was chosen to be 60°.

15. Bathymetry Results BM FM FMampl FMampl&phase
These are maps of bathymetry for this 2 by 2-km area
where common coverage between BM and FM exists.
On the left is plan view with bathy from bathy mode.
On the right is bathy from fishery mode.
The depth ranged from 205 to 240 m in this area, as indicated by the colors on the surfaces.
In general there is good agreement (same color scale).
BM has a 120 deg swath.
This range and swath width in FM were selected by the users and could have been the same as BM, but
For other purposes, the FM configuration was different from BM. The FM beams were steered to form a 60 deg swath.
The effects of the depth exceeding the recorded range in FM is evident as the sf data trail off and no detections were available from outer beams.
FMampl
FMampl&phase

16. Comparison Perspective view from NW Interpolated surfaces
Explain the two datasets, the viewpoint,
The colorscale and interval, the similarity, and differences,
Replace FM results from phase-detection only
With FM results from combined ampl/phase detec.
If outer beam artifacts apparent, show a swath where depth > range.
Maybe, enhance boundaries in 3d ?
Comparison
Perspective view from NW
Interpolated surfaces
FM (ampl. detect only)
205
240
Depth (m)
Explain the two datasets, the viewpoint,
The colorscale and interval, the similarity, and differences,
Replace FM results from phase-detection only
With FM results from combined ampl/phase detec.
If outer beam artifacts apparent, show a swath where depth > range. FM
This shows a perspective view of the bathymetric surfaces viewed from the northwest at an elevation of 35 degrees.
The FM data here resulted from using amplitude-detection only for all beam directions.
The mean difference is near zero, and the principal differences here are due to using only amplitude detection for the FM data.
I will present quantitative differences later.
The common area is quite flat.
But, that can be beneficial to comparisons and optimization of FM bot det method selection.
>>>>>
Explain the two datasets, the viewpoint,
The colorscale and interval, the similarity, and differences,
Replace FM results from phase-detection only
With FM results from combined ampl/phase detec.
If outer beam artifacts apparent, show a swath where depth > range.
Differences are practically zero BM
Diffs for normal inc beams and where bottom exceeded range (outer beams in deep water)

17. Comparison Perspective view from NW Interpolated surfaces
Explain the two datasets, the viewpoint,
The colorscale and interval, the similarity, and differences,
Replace FM results from phase-detection only
With FM results from combined ampl/phase detec.
If outer beam artifacts apparent, show a swath where depth > range.
Maybe, enhance boundaries in 3d.
Comparison
Perspective view from NW
Interpolated surfaces
FM (ampl. & phase detect)
205
240
Depth (m)
Differences are practically zero FM
Again, this shows a perspective view of the bathymetric surfaces viewed from the northwest at an elevation of 35 degrees.
The FM data here resulted from using amplitude and phase-detection.
Amplitude-detection was used for beam directions within 8 degrees of vertical (normal incidence) and phase-detection was used for beam directions > 8 degrees (i.e. 8 to ~ 30 deg. in this configuration).
The mean difference is near zero, and the principal differences here are due to remaining motion artifacts in the BM results.
Of course this area is quite flat.
But, that can be beneficial to comparisons and optimization of FM bot det method selection. BM
Diffs for normal inc beams and where bottom exceeded range (outer beams in deep water)

18. Path: C:\Cutter\ME70\Collaboration\IFREMER\GIS\map_images
Comparison
% Difference
% difference in depth between BM and FM grids
Difference
BM – FMpSv
Difference
BM – FMpzc_with_outliers
The difference between BM and FM depths was < 0.5 % for … of the area, for this region with a mean depth of > 200 m.
-0.5 to +1 m
Measured diffs
Betw BM, FM grids
ME70_comparisons_BMtc-FMpzctc.png
Path: C:\Cutter\ME70\Collaboration\IFREMER\GIS\map_images

19. Path: C:\Cutter\ME70\Collaboration\IFREMER\GIS\map_images
Comparison
Calc proportion of diffs.
Maybe Combine with previous slide
Difference in depth between BM and FM grids
Green:
-0.5 < dz < 0.5 m
Mean difference:
-0.5 to +1.5 m
for a depth range from 200 to 230 m
Overall mean difference (SD): (0.40) m
Differences may be due to refraction or tide
Difference
Difference
BM – FMpSv
dz (m), BM-FM
Difference
BM – FMpzc_with_outliers
but not stationary
We calculated the difference in depth between the results from BM and FM, and produced a grid where each cell’s color is indicative of the difference.
There are two shades of green shown in this figure. The difference in depth for any cell colored green is within 50 cm of zero.
Yellow indicates differences of one-half to one meter. And light orange indicates differences from one to one and a-half meter.
Systematic differences that seem correlated with increasing depth may be explained by lack of compensation for refraction of FM data, or changes of water elevation due to tide that were not predicted exactly .
* The FM depths were not compensated for refraction.
-0.5 to +1 m
Measured diffs
Betw BM, FM grids
ME70_comparisons_BMtc-FMpzctc.png
Path: C:\Cutter\ME70\Collaboration\IFREMER\GIS\map_images

20. Seafloor slope BM FM
Here is shown slope of the seafloor in plan-view maps from BM and FM grids.
Slopes were estimated by depth diffrences between neighboring grid cells.
This is a very flat seafloor in the common area, with slopes less than 2° throughout this portion (FM map).
Occasional values of 2 – 3 degrees occur, and are associated with bottom-detection mode artifacts, and some motion residual, and the shape of the seafloor as it gets deeper toward the southeastern portion of the common coverage area.
The largest slopes (5 to 14 degrees) for this study area are evident in the BM data as the seafloor smoothly transitions from shelf to slope.

21. Seafloor roughness BM FM
Local roughness was estimated from the std. dev. Of depth values for neighborhoods of 15-m square, and results are shown here for the FM and BM surfaces.
BM and FM results indicate that large-scale slope is controlling roughness in this area.
The local roughness is on the order of less than 10 cm for most of the common area, increasing to 1 m for the large depressions.
Some minor variation of slope due to residual motion artifacts is evident in the BM results.
Minor variation of slope in FM results are due to the fixed angle used to switch from amplitude to phase detections.

22. Seafloor backscatter Note outline of FM on BM Map of FM BS.DN
Ss (dB)
Calibrated Seafloor BS (from FM)
Sv  Ss
Normalized for slope
Note outline of FM on BM
Seafloor backscatter data are shown here in plan-view maps. Left: BM, right FM.
The BM backscatter are computed by Simrad during acquistion, and onl compensated for incidence angle assuming a flat seafloor.
Lack of adequate compensation for beam pattern and normal-incidence scattering is evident in the BS from BM, with higher values occurring near nadir beams.
Correlation with seafloor slope is evident in the spatial pattern of backscatter values [, and the plot of BS vs slope]. FM
Calibrated Seafloor BS (from FM), as surface scattering strength (Ss)
SF backscatter strengh (dB) shown on right, was estimated from integration of volume backscatter coefficients, compensation for ensonified area and for incidence angle,
Normalized for slope
Values ranged from ~ -20 to -30 dB for the this study area, but Ss was fairly homogeneous in the FM study area, reinforcing the interpretation of a flat, sedimented seafloor, evident from the bathymetry.
Map of FM BS.
Plot of BS vs Slope
205
240
Depth (m)

23. Seafloor classification
SSID
Demer et al. (2009)
Roughness & hardness index from backscatter and seafloor slope
See: Demer et al ICES J. Mar. Sci, 66.
Relies on incidence angle of each beam
The backscatter and seafloor slope data can be used for seafloor classification and characterization, and habitat mapping.
We are in the process of implementing a technique that uses the backscatter and seafloor slope data from the ME70 FM data and results in roughness hardness index. An example of the results from applying this technique to EK60 echosounder data (SSID; Demer et al. 2009) is shown here. The roughness/hardess index values are clearly associated with seafloor slopes, e.g. the steep sides of the bank and flat top, and rocky region on top of the bank where rockfish aggregate, as shown in the image derived from the EK60 data (Figure b) and seen by the ROV cameras (not shown).
Figures are from Demer et al
Statistical-spectral method for target identification and seafloor characterization (SSID).
SSID uses multiple frequencies, beam inc angle, and sf backscatter, to roughness/Hardness index from backscatter and seafloor slope
Caption:
Figure 6. (a) The acoustically sensed distribution of rockfish on the 43 Fathom Bank; (b) a grey-scale image of the seabed bathymetry
(courtesy of Chris Goldfinger and Chris Romsos, Oregon State University and Mary Yoklavich, SWFSC) colourized with a roughness metric
[20 log(pd/p0); dB]. The high-relief rocky areas on the top of the bank are indicated by higher values (darker colours). Voids indicate
incomplete bathymetry data.

24. Conclusions
Accurate and precise bathymetry can be obtained from ME70 operating in FM and BM
Differences between bathymetry surfaces from BM and FM* were < 1% of depth at over 200 m for > 90% of the common coverage area
Differences were < 0.25% for more than 40% of the common coverage area
By accounting for refraction for FM solutions differences could be reduced
Possible limitations of FM
Reduced ping rate
Reduced angular span of the swath
Reduced number of beams
Some new methods have promise to overcome these limitations
Lack of post-processing software
Advantages of FM
Two-way beamforming and lower sidelobe levels
Water-column data
Multiple-frequencies
Do we need the Bathymetric Mode?
Accurate and precise bathymetry can be obtained from ME70 operating in FM and BM
BM – FM: dz < 1% of depth at over 200 m for > 95% of the common coverage area
Differences were < 0.25% for more than 40% of the common coverage area
By accounting for refraction, using alternative bottom detection methods, for FM solutions differences could be reduced
Possible limitations of FM
Reduced ping rate
Reduced number of beams
Some new methods have promise to overcome these limitations
Lack of post-processing software
Advantages of FM
Two-way beamforming and lower sidelobe levels
Water-column data
Multiple-frequencies
Do we need the Bathymetric Mode?
Consider the quality of FM bathymetry
Resources for FM data processing could be a better investment
Accurate and precise bathymetry can be obtained from ME70 operating in FM and BM
Recall BM surfaces from Ifremer
BM – FM: dz < 1% of depth at over 200 m for > 95% of the common coverage area
Differences were < 0.25% for more than 40% of the common coverage area
By accounting for refraction for FM solutions differences could be reduced
Possible limitations of FM
Reduced ping rate
Reduced number of beams
Some new methods have promise to overcome these limitations
Lack of post-processing software
Advantages of FM
Two-way beamforming and lower sidelobe levels
Water-column data
Multiple-frequencies
Do we need the Bathymetric Mode?
*It is evident that the ME70 in BM can generate high-resolution bathymetric data and detailed surfaces that represent the seafloor accurately (e.g. Slide 4c, 4d). However, FM can also produce bathymetry data of the same quality and not significantly different from BM results, using standard seafloor-detection methods.
In addition, alternative methods are being developed that promise to produce better bathymetry from FM than BM.