Ba k a e -A aly e
14
Hydrographische Nachrichten
Fteferences
Föote, kfenneth G; Dezhang
Chu; Terence R Ham mar;
kenneth Baldwin;Larry
A. Mayer; Lawrence C.
HufnagleJr.; Michael
Jsch ( 005): Protocols for
calibrating multibeam
9anar;TheJournä of the
Acoustical Sbdety of
America, \£>l. 7 No. 4, pp.
03-0 7
Lamarche,Geo roy; Xavier
Lurton;Anne LaureVferdler;
Jean Marie Augustin
(0 ^Quantitative
characterisation of sea oor
substrate and bedforms
using advanced processing
of multibeam backscatter-Application
to Gx>k 3rät,
New Zealand;Continentä
Snelf Ffesearch,Vbl.3 ,No. ,
pp.93- 09
Lan zon I, Jose G; Thomas
G V\feber( 0 0). High
resolution cäibratlon of a
multibeam echo sounder;
Oceans 0 0,1^^ Seattle,
pp. -7
Fig. 6: Decision of the angular
compensated mean BSIn
the 5to60degreesangular
interval for all operational
Transmission Modes of the
BVI30 of OOGS»Amundsen«
the Kcngsberg real time data reduction process
from the angular compensated BSimage in order
to obtain an angular uncompensated BSimage.
9ep consists in modelling the BS angular
dependence visible in the angular uncompen
sated BS image. The Generic Sea oor Acoustic
Backscatter (GSM^ model (Lamarche et al. ) is
used for that purpose.
In step , the new »best« modelled estimate of
the BS angular dependence is applied and the
Kcngsberg default transmission sector pattern
compensation is removed. This process isolates
the sector patterns, which can be »seen« in the
backscatter image (Rg. a).
Rnally,step consistsin modelling thetransmis
sion sector patterns as rst order polynomials, an
approximation of the mathematically correct sine
function. Three parameters constitute the vari
ablesof the polynomial functionsthe source level,
the transmission angle and the sector opening
angle. Rg. b shows the corresponding angular
response of the residual colour coded transmis
sion sector beam patterns The points represent
the mean source level residual per ° bin and the
continuous lines represent the modelling of the
transmission sector patterns
Ftesults
The newly modelled transmisson sector patterns
can be applied to the survey datato assessthe qual
ity of the calibration. Rg. ashowsthe angular com
pensated BS image in ping vs beam geometry for
the Shallow, Dual, CW Transmission Mode with the
new transmisson sector patterns and GSAB models
applied. Rg. b isthe correqconding mean BSangu
lar reqeonse. Wiile some modulation is still viable in
the BSimage, espedally at the sector boundaries, the
overall predsion of the mean BSangular response is
signi cantly improved. Rg. illustrates the predson
improvements of the angular compensated mean
BSinthe to degreesangular interval for all op
erational Transmisson Modes The latter interval is
chosen so asto exdude the nadir and grazing angle
zones, which alwsyscomprise less reliable data
The modelled transmission sector patterns re
suiting from the calibration can be utilised in two
ways Rrst, the valuesof the modelscan be written
inthebs .x le and injected to the EM echo
sounder. This will ensure that all newly collected
data will be exempt from any modulating e ect
due to improperly calibrated transmission sector
patterns Second, the models can be applied to
existing backscatter data in order to mitigate the
intra line artefacts Rg. a illustrates the identical
mosaic as in Rg. b with the transmission sector
patterns properly calibrated. Aside from a BSmini
mum in the nadirarea, no further along track arte
facts are visible.
Several attempts were undertaken to discover
the reason forthe remaining inter linebiasin Rg. a
This dB bias between the survey line collected
on September nd and the one collected on Sep
tember th could not be explained by changes to
environmental factors(changesto the seabed type
or to the water column propertied The only con
elusive explanation stems from the software insta
bilitiesexperienced during the surveys These were
correlated to an incompatibility between hardware
and software versionswhich necessitated the com
pletere installation of the data collection software
after the rst and before the second survey. This
type of problem cannot be accounted for in post
processing other than by applying a statistical an
gular correction whose result isillustrated in Rg. b.
Although the backscatter mosaic now appearsuni
form, one cannot speak of absolute BSIevelssince a
bias has been introduced.
Discussion and future improvements
Wiilethe proper calibration of transmission sector
patterns is a step towards absolute and properly
referenced backscatter data, transmission sector
patterns are not the only factor that needs to be
addressed. Other factors include:
• proper compensation for absorption of the
acoustic signal in thewater column;
• accounting for the topography of the sea
bed when calculating the ensoni ed area;
• possible compensation for the receive beam
patterns;
• the need for a bathymetry surface except of
outliersand biases
Several of these factorscould not beorweresim
ply not fully addressed during the calibration survey
and subsequent data analysis Logistical and time
constraints help explain why the calibration survey
could not be performed under ideal conditions
Gven the multi disciplinary nature of the research
conducted on OOGS »Amundsen«, this is to be ex
pected. Limitations in the work ow used to transfer
the raw data collection measurementsto abathym
etry processing software and nally to SbnarStxipe
are also a root cause for sub optimal results
However, the single greatest weakness of the
calibration procedure isthe reliance on the mod
elled BSangular response from the empirical data.
