A series of experiments were conducted to adjust and normalize the acoustic backscatter acquired by Reson 8111 and 8160 systems. The dependency of the backscatter on the receiver gain, transmit power, pulse width and acquisition mode was analyzed. Empirical beam patterns are calculated as the difference between the backscatter measured by the sonars and the expected backscatter. Expected acoustic backscatter is estimated based on a mathematical model

Calder, Brian R.

Fonseca, Luciano E.

English

UNH Scholars' Repository

University of New HampshireUniversity of New Hampshire Scholars' RepositoryCenter for Coastal and Ocean Mapping Center for Coastal and Ocean Mapping9-2006Experiments for multibeam BackscatterAdjustments on the NOAA Ship FAIRWEATHERLuciano E. FonsecaUniversity of New Hampshire, Durham, luciano@ccom.unh.eduBrian R. CalderUniversity of New Hampshire, Durham, brian.calder@unh.eduFollow this and additional works at: https://scholars.unh.edu/ccomPart of the Computer Sciences Commons, and the Oceanography and Atmospheric Sciences andMeteorology CommonsThis Conference Proceeding is brought to you for free and open access by the Center for Coastal and Ocean Mapping at University of New HampshireScholars' Repository. It has been accepted for inclusion in Center for Coastal and Ocean Mapping by an authorized administrator of University of NewHampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.Recommended CitationL. Fonseca, B. Calder, and M. Wetzler, "Experiments for Multibeam Backscatter adjustments on the NOAA ship Fairweather,"OCEANS 2006, Sep. 2006.Experiments for Multibean Backscatter Adjustmentson the NOAA Ship FairweatherLuciano FonsecaI, Brian Calder', Mark Wetzler2'CCOM- University ofNew Hampshire2NOAA - MCDAbstract- A series of experiments were conducted to adjustand normalize the acoustic backscatter acquired by Reson 8111and 8160 systems. The dependency of the backscatter on thereceiver gain, transmit power, pulse width and acquisition modewas analyzed. Empirical beam patterns are calculated as thedifference between the backscatter measured by the sonars andthe expected backscatter. Expected acoustic backscatter isestimated based on a mathematical model.I. INTRODUCTIONIn the spring of 2005, an experiment was conductedon the NOAA Ship FAIRWEATIiER around Cape Decision,Alaska. The purpose of the experiment was to understandhow changes in acquisition parameters related to recordedvalues of backscatter, so that normalized records can beproduced. The normalized records can be used in enhancedbackscatter mosaics and in methods for remote seafloorcharacterization. Additionally, it is important to normalize theacoustic backscatter acquired by Reson 8111 (100kHz) and8160 (50kHz) multibeam sonars during normal surveyoperations. The survey consisted of five experiments designedto determine the dependency of the backscatter on the mainacquisition parameters, particularly the receiver gain, thetransmit power and the transmit pulse-width. Additionally, weattempted to understand the relationship between the operatingmode of Reson systems (auto-gain onloffand TVG onlofj) andthe modifications made to backscatter data before it isrecorded in the output data stream. Finally we wanted todetermine a methodology for implementation of backscatterprocessing tools in a standard ship-board processing chain,and for the extraction of the transmit beam-pattern of thesonars. The same experiments were conducted with each ofthe sonars.II. EXPERIMENTSThe first experiment tested backscatter dependencyon the receiver gain. The bathymetry chosen for theexperiment consisted of a flat area having a depth of 100m,with identifiable features and a length of 1.8km. The depthwas adequate for operations with both 8111 and 8160 sonars.The sonar acquisition mode was set to TVG on and auto-gainoff and the acquisition parameters of pulse-width and transmitpower were fixed during the duration of the line. The gainsettings were changed at about the middle of the line to allowfor a before-and-after comparison of the backscatter on thesame area. Six data sets were acquired on the same plannedline, testing 12 combinations of gain settings (Figure 1).rig. l- (am experiment. mne tour images snow tmebackscatter mosaics with no compensation for the gainchanges. The gain settings used in the experiment where 7, 9,12 and 14 with the 8111 (two images on the left), and 5, 7, 12and 10 with the 8160 (two images on the right).The second experiment focused on the relationshipbetween backscatter intensity and transmit-power. It wasconducted in the same area as the first experiment. Thesystem settings for the RESONs were: TVG on, auto-gain off,fixed pulse-width and gain, with power settings varied in themiddle of the line. Four additional data sets were acquired onthe same planned line, testing a total of 8 combinations ofpower setting (Figure 2).Fig. 2- Power experiment. the tour images show thebackscatter mosaics with no compensation for the powerchanges. The power settings used in the experiment where 6, 7,8 and 9 for the 8111 (two images on the left), and 5, 6, 7 and 8for the 8160 (two images on the right).1-4244-01 15-1/06/$20.00 ©2006 IEEEAlso in this same area, a third experiment wasconducted to test the backscatter dependency on the pulse-width. For that, the gain and power settings were fixed, TVGon and auto-gain off, and the pulse-width was changed in themiddle of the line. A total of 20 combinations of pulse-widthswere tested, 10 for the 8111 (Figure 3) and 10 for the 8160(Figure 4).Fig. 3- 8111 pulse width experiment. The five images showthe backscatter mosaics with compensation for changes in thepulse width. The pulse width settings used in the experimentwhere 80, 160, 241, 321, 481,642, 802, 1203, 1524 and3048gs. Note that the applied correction was not sufficient at80s.Fig. 4- 86OU pulse width experiment. the tive images showthe backscatter mosaics with compensation for changes in thepulse width. The pulse width settings used in the experimentwhere 396, 583, 692, 791, 989, 1187, 1385, 1978, 5044 and9989ts.The fourth experiment was designed to measure thetransmit beam pattern of the sonars. Bathymetry for thisexperiment was flat and uniform with a depth of 100m. Twoparallel survey lines were laid out with a length of 1.8km suchthat the outer beams of the swaths would overlap 5000. Eachsurvey line was run in the normal and reciprocal directions forboth the 8111 and 8160 systems (Figure 5). The RESONssystem settings for power, gain and pulse width wereoptimized to produce a good return. TVG on and auto-gain offwere selected.Fig. 5- Transmit beam pattern experiment. The two images arethe backscatter mosaics of four overlapping lines. The leftmosaic shows data from the 8111 and the right from the 8160.The blue lines show the navigation track.The last experiment tested the four possiblecombinations of different acquisition modes: auto-gain onloffand TVG onloff. Suitable bathymetry was chosen to be asloping area with identifiable features with depths rangingfrom 40 to 100m and a length of approximately 2.5km (Figure6).Fig. 6 - Mosaic ot corrected backscatter acquired with tourdifferent acquisition modes. The top part of the images is thedeepest area of the survey.III. RESULTSThe acoustic backscatter data were analyzed using aprocessing scheme developed at the CCOM-JHC [1]. Forexperiments 1, 2 and 3, the average backscatter was calculatedinside two small adjacent reference boxes, with areas ofapproximately 40x40m, located before and after the pointwhere acquisition parameters were changed (Figure 7). Inaverage, there was a measured change of 0.94dB per gain stepwith a standard deviation of 0.06dB (Table 1); and 3.23dB perpower step with a standard deviation of 0.06dB (Table 2).correction was not adequate, and the corrected backscatterincreased monotonically with the pulse width (Figure 8). Forlong pulse-widths (200 to 3000gs), the applied correctionswas sufficient, and the corrected backscatter stayed aroundapprox. -24 dB for the 8111 and approx. -29 dB for the 8160.For pulse-widths longer than 3000gs the corrected backscattershowed a non-linear behavior with the pulse-width, decreasingslightly for the longer pulses.Figure 7 - One acquisition parameter is changed in the middleof the line. Four measurements of average backscatter aredone (white boxes) for port and starboard, before and after theparameter change.Sonar G1 w1 c(1 G2 g2 CY2 A8111 7 91.62 0.95 9 93.29 0.90 0.848111 7 90.89 0.84 9 92.70 1.01 0.918111 12 86.51 0.89 14 88.41 0.80 0.958111 12 87.48 0.92 14 89.51 0.96 1.028160 5 80.95 1.03 7 82.74 0.88 0.898160 5 81.29 0.89 7 83.39 0.93 1.058160 12 77.13 0.97 10 78.95 0.73 0.918160 12 77.04 0.83 10 78.99 1.03 0.97Table 1 - Gain experiment. G1 and G2 are the gain settings andg1, GI, and 02 are the average and the standard deviation of thebackscatter in dB of the samples before and after the gainchange. A is the net change in the backscatter (g2-t1).Sonar P1 g1 cT1 P2 g2 G2 A8111 8 87.80 1.03 9 91.16 0.91 3.368111 8 87.37 0.95 9 90.25 0.90 2.888111 6 87.76 0.93 7 91.00 0.87 3.248111 6 89.24 1.07 7 92.14 0.87 2.908160 7 78.72 0.95 8 82.07 1.01 3.358160 7 78.20 0.84 8 81.51 0.99 3.318160 5 78.78 0.73 6 81.99 0.74 3.218160 5 78.53 1.01 6 82.02 0.79 3.49Table 2 - Power experiment. P1 and P2 are the power settingsand g1, GI, g2 and G2 are the average and the standarddeviation of the backscatter in dB for the samples before andafter the power change. A is the net change in the backscatter(RL2-R1l)-For the pulse-width experiment (Table 3), thebackscatter was corrected for receiver gain, transmit-powerand for the area of insonification. The area of insonification iscalculated based on the pulse-width, the range, along-trackand across-track beam widths, sound speed, and angle ofincidence [3]. For short pulse-widths (80 to 200gs) the appliedPW80160241321481642802120315248111-28.67-25.29-23.82-25.25-23.74-24.56-23.36-23.56-24.41cs1.471.440.960.941.101.101.091.101.313048 -24.71 1.65PW39659369279198911871385197850448160-28.54-29.52-28.74-27.87-28.98-29.04-28.70-28.95-30.43cs0.970.980.911.111.041.281.091.370.949989 -30.13 0.93Table 3 - Pulse width experiment. PW is the pulse width in gs.The measurements t and a are the average and the standarddeviation of the corrected backscatter in dB of the port andstarboard samples.-21-23u'-25-2 -27-u)d-29-m-31-33-35- 8160-*- 8111pulse width tsFig. 8 - Corrected backscatter measurements for differentpulse-widths.Empirical beam patterns were calculated as thedifference between the measured and the expected backscatterfor all grazing angles in angle increments of 1 degree.Measured backscatter was calculated as the averagebackscatter angular response of all acquisition lines inexperiment 4. Expected acoustic backscatter was calculatedbased on a mathematical model [2] and assuming a seafloorconsisting of fine sand, grain size 0=2.5, and frequencies of50kHz for the 8160 and 100kHz for the 8111. Beforecalculating the beam pattern, an angle independent offset of60dB was subtracted from the measured backscatter. Thisoffset is the difference in mean backscatter values, averagedover grazing angles from 300 to 600, for both measured andexpected backscatter angular responses. The extracted beampattern for the 8111 is flat from nadir to grazing angles ofapproximately 50 degrees, and then drops -4dB on the outerbeams (Figure 9) . In contrast, the extracted beam pattern forthe 8160 shows a drop of 5dB at nadir, increases to thereference level near grazing angles of 75 degrees, and thendrops lOdB for the outer beams (Figure 10). There is nomajor difference in the extracted beam pattern for the port andstarboard sides.Beam Pattern Correction 8111grazing angle (degrees)Fig. 9-e8111 empirical compensation table for beam pattecorrection.Beam Pattern Correction 8160grazing angle (degrees)Fig. 10 - 8160 empirical compensation table for beam patterncorrection.Finally, the last experiment showed that the auto-gain mode does not affect significantly the backscatter, otherthan the 0.94 dB per step change in the gain. On the otherhand, the TVG off mode requires extra adjustment for thespreading law and for attenuation in the water column. Evenafter applying these adjustments, the backscatter in this modeappears to be affected by more than a 2dB per step of gain, butin a non-linear way. The combination TVG off and auto-gainon is unstable and causes frequent changes in the gain setting(Figure 6). As a result, the mosaic shows artifacts in the formof stripes. In TVG offmode, a non-linearity is also present inthe dependency on the power and pulse-width. The use of thismode was limited to very shallow waters (less than 70m).Figure 6 shows clearly that with the TVG offmode the systemdid not cover the complete range at the deepest part of thesurvey.IV. CONCLUSIONSThe results of these experiments have been used inthe assembly of adjusted backscatter mosaics. Based on theacquired data, recommendations have been made for RESONsystem settings and acquisition procedures where the acousticbackscatter is an important outcome of the survey. Forinstance, the TVG off mode should be avoided, as itcompromises the dynamic range of the logged date, and yieldsnon-linear behavior and saturation in the backscatter. The autogain mode should be preferred, as changes in gain and powerare easily compensated. To avoid saturation, the gain shouldalways be kept above 1 and below 14, when using auto gainmode. This is accomplished by increasing the transmit powerwhen the gain in too high, and decreasing the power when thegain is too low. Very long and very short pulse widths shouldbe avoided, as they also result in non-linear behavior in thebackscatter and, in the case of long pulse, reduce the sonarresolution. The proposed methodology for empirical beampattern extraction can be used to convert the backscatteracquired by different sonars to a common reference level(normalization), provided that a common survey area ischosen as a calibration site.ACKNOWLEDGMENTWe would like to express our appreciation to thecaptain and crew of the NOAA ship FAIRWEATHER for theirdedication and enthusiasm in the conduct of this experiment,and particularly for making available survey time with thelaunches and ship along with expert coxswains and surveytechnicians. This work was support by NOAA grant #NA05NOS4001 153.REFERENCES[1] Fonseca, L. and Calder B. ,"Geocoder: and Efficient Backscatter MapConstructor ", Proceedings of the U.S. Hydrographic 2005, San Diego,CA, 2005.[2] Williams, K., "An effective density fluid model for acoustic propagationin sediments derived from Biot theory," The Journal of the AcousticalSociety ofAmerica., vol. 1 10, No. 5, pp. 2276-2281, 2001[3] Hellequin, L, Boucher, J and Lurton, X., "Processing of high-frequencymultibeam echo sounder data for seafloor characterization", IEEEJournal ofOceanic Engineering, vol. 28, No. 1, January 2003.

Experiments for multibeam Backscatter Adjustments on the NOAA Ship FAIRWEATHER

University of New Hampshire

University of New Hampshire
University of New Hampshire Scholars' Repository
Center for Coastal and Ocean Mapping Center for Coastal and Ocean Mapping
9-2006
Experiments for multibeam Backscatter
Adjustments on the NOAA Ship FAIRWEATHER
Luciano E. Fonseca
University of New Hampshire, Durham, luciano@ccom.unh.edu
Brian R. Calder
University of New Hampshire, Durham, brian.calder@unh.edu
Follow this and additional works at: https://scholars.unh.edu/ccom
Part of the Computer Sciences Commons, and the Oceanography and Atmospheric Sciences and
Meteorology Commons
This Conference Proceeding is brought to you for free and open access by the Center for Coastal and Ocean Mapping at University of New Hampshire
Scholars' Repository. It has been accepted for inclusion in Center for Coastal and Ocean Mapping by an authorized administrator of University of New
Hampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.
Recommended Citation
L. Fonseca, B. Calder, and M. Wetzler, "Experiments for Multibeam Backscatter adjustments on the NOAA ship Fairweather,"
OCEANS 2006, Sep. 2006.
Experiments for Multibean Backscatter Adjustments
on the NOAA Ship Fairweather
Luciano FonsecaI, Brian Calder', Mark Wetzler2
'CCOM- University ofNew Hampshire
2NOAA - MCD
Abstract- A series of experiments were conducted to adjust
and normalize the acoustic backscatter acquired by Reson 8111
and 8160 systems. The dependency of the backscatter on the
receiver gain, transmit power, pulse width and acquisition mode
was analyzed. Empirical beam patterns are calculated as the
difference between the backscatter measured by the sonars and
the expected backscatter. Expected acoustic backscatter is
estimated based on a mathematical model.
I. INTRODUCTION
In the spring of 2005, an experiment was conducted
on the NOAA Ship FAIRWEATIiER around Cape Decision,
Alaska. The purpose of the experiment was to understand
how changes in acquisition parameters related to recorded
values of backscatter, so that normalized records can be
produced. The normalized records can be used in enhanced
backscatter mosaics and in methods for remote seafloor
characterization. Additionally, it is important to normalize the
acoustic backscatter acquired by Reson 8111 (100kHz) and
8160 (50kHz) multibeam sonars during normal survey
operations. The survey consisted of five experiments designed
to determine the dependency of the backscatter on the main
acquisition parameters, particularly the receiver gain, the
transmit power and the transmit pulse-width. Additionally, we
attempted to understand the relationship between the operating
mode of Reson systems (auto-gain onloffand TVG onlofj) and
the modifications made to backscatter data before it is
recorded in the output data stream. Finally we wanted to
determine a methodology for implementation of backscatter
processing tools in a standard ship-board processing chain,
and for the extraction of the transmit beam-pattern of the
sonars. The same experiments were conducted with each of
the sonars.
II. EXPERIMENTS
The first experiment tested backscatter dependency
on the receiver gain. The bathymetry chosen for the
experiment consisted of a flat area having a depth of 100m,
with identifiable features and a length of 1.8km. The depth
was adequate for operations with both 8111 and 8160 sonars.
The sonar acquisition mode was set to TVG on and auto-gain
off and the acquisition parameters of pulse-width and transmit
power were fixed during the duration of the line. The gain
settings were changed at about the middle of the line to allow
for a before-and-after comparison of the backscatter on the
same area. Six data sets were acquired on the same planned
line, testing 12 combinations of gain settings (Figure 1).
rig. l- (am experiment. mne tour images snow tme
backscatter mosaics with no compensation for the gain
changes. The gain settings used in the experiment where 7, 9,
12 and 14 with the 8111 (two images on the left), and 5, 7, 12
and 10 with the 8160 (two images on the right).
The second experiment focused on the relationship
between backscatter intensity and transmit-power. It was
conducted in the same area as the first experiment. The
system settings for the RESONs were: TVG on, auto-gain off,
fixed pulse-width and gain, with power settings varied in the
middle of the line. Four additional data sets were acquired on
the same planned line, testing a total of 8 combinations of
power setting (Figure 2).
Fig. 2- Power experiment. the tour images show the
backscatter mosaics with no compensation for the power
changes. The power settings used in the experiment where 6, 7,
8 and 9 for the 8111 (two images on the left), and 5, 6, 7 and 8
for the 8160 (two images on the right).
1-4244-01 15-1/06/$20.00 ©2006 IEEE
Also in this same area, a third experiment was
conducted to test the backscatter dependency on the pulse-
width. For that, the gain and power settings were fixed, TVG
on and auto-gain off, and the pulse-width was changed in the
middle of the line. A total of 20 combinations of pulse-widths
were tested, 10 for the 8111 (Figure 3) and 10 for the 8160
(Figure 4).
Fig. 3- 8111 pulse width experiment. The five images show
the backscatter mosaics with compensation for changes in the
pulse width. The pulse width settings used in the experiment
where 80, 160, 241, 321, 481,642, 802, 1203, 1524 and
3048gs. Note that the applied correction was not sufficient at
80s.
Fig. 4- 86OU pulse width experiment. the tive images show
the backscatter mosaics with compensation for changes in the
pulse width. The pulse width settings used in the experiment
where 396, 583, 692, 791, 989, 1187, 1385, 1978, 5044 and
9989ts.
The fourth experiment was designed to measure the
transmit beam pattern of the sonars. Bathymetry for this
experiment was flat and uniform with a depth of 100m. Two
parallel survey lines were laid out with a length of 1.8km such
that the outer beams of the swaths would overlap 5000. Each
survey line was run in the normal and reciprocal directions for
both the 8111 and 8160 systems (Figure 5). The RESONs
system settings for power, gain and pulse width were
optimized to produce a good return. TVG on and auto-gain off
were selected.
Fig. 5- Transmit beam pattern experiment. The two images are
the backscatter mosaics of four overlapping lines. The left
mosaic shows data from the 8111 and the right from the 8160.
The blue lines show the navigation track.
The last experiment tested the four possible
combinations of different acquisition modes: auto-gain onloff
and TVG onloff. Suitable bathymetry was chosen to be a
sloping area with identifiable features with depths ranging
from 40 to 100m and a length of approximately 2.5km (Figure
6).
Fig. 6 - Mosaic ot corrected backscatter acquired with tour
different acquisition modes. The top part of the images is the
deepest area of the survey.
III. RESULTS
The acoustic backscatter data were analyzed using a
processing scheme developed at the CCOM-JHC [1]. For
experiments 1, 2 and 3, the average backscatter was calculated
inside two small adjacent reference boxes, with areas of
approximately 40x40m, located before and after the point
where acquisition parameters were changed (Figure 7). In
average, there was a measured change of 0.94dB per gain step
with a standard deviation of 0.06dB (Table 1); and 3.23dB per
power step with a standard deviation of 0.06dB (Table 2).
correction was not adequate, and the corrected backscatter
increased monotonically with the pulse width (Figure 8). For
long pulse-widths (200 to 3000gs), the applied corrections
was sufficient, and the corrected backscatter stayed around
approx. -24 dB for the 8111 and approx. -29 dB for the 8160.
For pulse-widths longer than 3000gs the corrected backscatter
showed a non-linear behavior with the pulse-width, decreasing
slightly for the longer pulses.
Figure 7 - One acquisition parameter is changed in the middle
of the line. Four measurements of average backscatter are
done (white boxes) for port and starboard, before and after the
parameter change.
Sonar G1 w1 c(1 G2 g2 CY2 A
8111 7 91.62 0.95 9 93.29 0.90 0.84
8111 7 90.89 0.84 9 92.70 1.01 0.91
8111 12 86.51 0.89 14 88.41 0.80 0.95
8111 12 87.48 0.92 14 89.51 0.96 1.02
8160 5 80.95 1.03 7 82.74 0.88 0.89
8160 5 81.29 0.89 7 83.39 0.93 1.05
8160 12 77.13 0.97 10 78.95 0.73 0.91
8160 12 77.04 0.83 10 78.99 1.03 0.97
Table 1 - Gain experiment. G1 and G2 are the gain settings and
g1, GI, and 02 are the average and the standard deviation of the
backscatter in dB of the samples before and after the gain
change. A is the net change in the backscatter (g2-t1).
Sonar P1 g1 cT1 P2 g2 G2 A
8111 8 87.80 1.03 9 91.16 0.91 3.36
8111 8 87.37 0.95 9 90.25 0.90 2.88
8111 6 87.76 0.93 7 91.00 0.87 3.24
8111 6 89.24 1.07 7 92.14 0.87 2.90
8160 7 78.72 0.95 8 82.07 1.01 3.35
8160 7 78.20 0.84 8 81.51 0.99 3.31
8160 5 78.78 0.73 6 81.99 0.74 3.21
8160 5 78.53 1.01 6 82.02 0.79 3.49
Table 2 - Power experiment. P1 and P2 are the power settings
and g1, GI, g2 and G2 are the average and the standard
deviation of the backscatter in dB for the samples before and
after the power change. A is the net change in the backscatter
(RL2-R1l)-
For the pulse-width experiment (Table 3), the
backscatter was corrected for receiver gain, transmit-power
and for the area of insonification. The area of insonification is
calculated based on the pulse-width, the range, along-track
and across-track beam widths, sound speed, and angle of
incidence [3]. For short pulse-widths (80 to 200gs) the applied
PW
80
160
241
321
481
642
802
1203
1524
8111
-28.67
-25.29
-23.82
-25.25
-23.74
-24.56
-23.36
-23.56
-24.41
cs
1.47
1.44
0.96
0.94
1.10
1.10
1.09
1.10
1.31
3048 -24.71 1.65
PW
396
593
692
791
989
1187
1385
1978
5044
8160
-28.54
-29.52
-28.74
-27.87
-28.98
-29.04
-28.70
-28.95
-30.43
cs
0.97
0.98
0.91
1.11
1.04
1.28
1.09
1.37
0.94
9989 -30.13 0.93
Table 3 - Pulse width experiment. PW is the pulse width in gs.
The measurements t and a are the average and the standard
deviation of the corrected backscatter in dB of the port and
starboard samples.
-21
-23
u' -25-
2 -27-
u)
d -29-
m
-31
-33
-35
- 8160
-*- 8111
pulse width ts
Fig. 8 - Corrected backscatter measurements for different
pulse-widths.
Empirical beam patterns were calculated as the
difference between the measured and the expected backscatter
for all grazing angles in angle increments of 1 degree.
Measured backscatter was calculated as the average
backscatter angular response of all acquisition lines in
experiment 4. Expected acoustic backscatter was calculated
based on a mathematical model [2] and assuming a seafloor
consisting of fine sand, grain size 0=2.5, and frequencies of
50kHz for the 8160 and 100kHz for the 8111. Before
calculating the beam pattern, an angle independent offset of
60dB was subtracted from the measured backscatter. This
offset is the difference in mean backscatter values, averaged
over grazing angles from 300 to 600, for both measured and
expected backscatter angular responses. The extracted beam
pattern for the 8111 is flat from nadir to grazing angles of
approximately 50 degrees, and then drops -4dB on the outer
beams (Figure 9) . In contrast, the extracted beam pattern for
the 8160 shows a drop of 5dB at nadir, increases to the
reference level near grazing angles of 75 degrees, and then
drops lOdB for the outer beams (Figure 10). There is no
major difference in the extracted beam pattern for the port and
starboard sides.
Beam Pattern Correction 8111
grazing angle (degrees)
Fig. 9-e8111 empirical compensation table for beam patte
correction.
Beam Pattern Correction 8160
grazing angle (degrees)
Fig. 10 - 8160 empirical compensation table for beam pattern
correction.
Finally, the last experiment showed that the auto-
gain mode does not affect significantly the backscatter, other
than the 0.94 dB per step change in the gain. On the other
hand, the TVG off mode requires extra adjustment for the
spreading law and for attenuation in the water column. Even
after applying these adjustments, the backscatter in this mode
appears to be affected by more than a 2dB per step of gain, but
in a non-linear way. The combination TVG off and auto-gain
on is unstable and causes frequent changes in the gain setting
(Figure 6). As a result, the mosaic shows artifacts in the form
of stripes. In TVG offmode, a non-linearity is also present in
the dependency on the power and pulse-width. The use of this
mode was limited to very shallow waters (less than 70m).
Figure 6 shows clearly that with the TVG offmode the system
did not cover the complete range at the deepest part of the
survey.
IV. CONCLUSIONS
The results of these experiments have been used in
the assembly of adjusted backscatter mosaics. Based on the
acquired data, recommendations have been made for RESON
system settings and acquisition procedures where the acoustic
backscatter is an important outcome of the survey. For
instance, the TVG off mode should be avoided, as it
compromises the dynamic range of the logged date, and yields
non-linear behavior and saturation in the backscatter. The auto
gain mode should be preferred, as changes in gain and power
are easily compensated. To avoid saturation, the gain should
always be kept above 1 and below 14, when using auto gain
mode. This is accomplished by increasing the transmit power
when the gain in too high, and decreasing the power when the
gain is too low. Very long and very short pulse widths should
be avoided, as they also result in non-linear behavior in the
backscatter and, in the case of long pulse, reduce the sonar
resolution. The proposed methodology for empirical beam
pattern extraction can be used to convert the backscatter
acquired by different sonars to a common reference level
(normalization), provided that a common survey area is
chosen as a calibration site.
ACKNOWLEDGMENT
We would like to express our appreciation to the
captain and crew of the NOAA ship FAIRWEATHER for their
dedication and enthusiasm in the conduct of this experiment,
and particularly for making available survey time with the
launches and ship along with expert coxswains and survey
technicians. This work was support by NOAA grant #
NA05NOS4001 153.
REFERENCES
[1] Fonseca, L. and Calder B. ,"Geocoder: and Efficient Backscatter Map
Constructor ", Proceedings of the U.S. Hydrographic 2005, San Diego,
CA, 2005.
[2] Williams, K., "An effective density fluid model for acoustic propagation
in sediments derived from Biot theory," The Journal of the Acoustical
Society ofAmerica., vol. 1 10, No. 5, pp. 2276-2281, 2001
[3] Hellequin, L, Boucher, J and Lurton, X., "Processing of high-frequency
multibeam echo sounder data for seafloor characterization", IEEE
Journal ofOceanic Engineering, vol. 28, No. 1, January 2003.


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Experiments for multibeam Backscatter Adjustments on the NOAA Ship FAIRWEATHER

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