added value brought by auv’s in the surveillance of exploited marine sand banks

Nice, 17-18 December 2001

Underwater Robotics for Ocean Modelling and Monitoring

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I3S

IST

TMS

MUMM

Added value brought by AUV’s in the surveillance of exploited marine sand banks

G.Pichot, A.Norro, J.Ozer

MUMM



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MUMMPrecision of thedata

Type (on the scaleof a profile)

Tidal reduction ±13 cm systematic

Static draught ±5.0 cm systematic

Var. in stat. draught ±5.0 cm systematic

Speed of sound ±2.5 cm systematic

Subtotal systematic ±25.5 cm

Echosounder ±4.0 cm random

Error in navigation ±8.5 cm random

Heave ±2.5 cm random

Presence of slopes +8.0 cm random

Subtotal random - 15 cm / +23 cm

Total error -40.5 cm/+48.5 cm

Sources of error



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MUMM International Hydrographic Norms

In coastal areas for 20 meters depth

- horizontal accuracy : 6 meters - depth accuracy : 0.564 meters



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MUMM The tidal reduction problem

H = H mes. - H T.



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MUMM

Transects over the Kwintebank

(provided by MUMM) rG19 - rH01

-3000 -2000 -1000 0 1000 2000 3000-3000

-2500

-2000

-1500

-1000

-500

0

-3000 -2000 -1000 0 1000 2000 3000-2600

-2400

-2200

-2000

-1800

-1600

-1400

-1200

meters meters

cent

imet

ers

cent

imet

ers



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MUMM The I3S approach

- Shape of the bank is stable

- Profiles are re-aligned wrt first profile

- A constant offset (vertical) and a constant horizontal shift are determined

- Criteria : minimisation of the distance wrt first profile



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MUMM-150

-100

-50 0 50 1000

5

10

15

20

25

-50 0 50 100 1500

5

10

15

20

Offset in cm

Alignment correction : histogram of the observed vertical offset

Compatible with the systematic part of the error budget



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MUMMDeviation wrt the mean profile (rG19)

-50 0 50 100

0150

0

500

1000

1500

2000

2500 Error max 100 cm

compatible with the random part of the error budget

-100 -50 0 50 100 (cm)



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rG21 error max 250 cm rH01 error max 150 cm

MUMM’s random error budget explains only 100 cm

200 100 0 100 200 -200 -150 -100 -50 0 50 100 150 2000

100

200

300

400

500

600

700

800

900histogram of variations wrt to average profile

-400

-300

-200

-100

0 100 200 3000

200

400

600

800

1000

1200

Deviation wrt the mean profile (rG21 and rH01)

-200 -100 0 100 200 (cm) 150 100 50 0 50 100 150 (cm)



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MUMM Volume consideration

Volume variation

MUMM I3S

RG19 Yes No

RG21 Yes Yes

RH01 Yes Yes

Should a part of the volume variation be extracted from a trend in the alignement ?



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MUMM

Do we need a tidal reduction ?



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MUMM Offset with non tidally reduced data

0

1

2

3

4

56

7

8

9

10

-29

0

-25

0

-21

0

-17

0

-13

0

-90

-50

-10

30

70

11

0

15

0

19

0

23

0

27

0

Offset (cm)

# o

f p

rofi

les



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MUMM Conclusion

With non tidally reduced data

Win one third of the systematic error

Increased offset making the trend identification more difficult

Interest to compare or to combine the two approaches



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Volume decrease

5060708090

100110120130140150

10 40 70 100

130

160

190

220

250

280

310

340

370

400

# of profiles

Ret

riev

ed t

ren

d (

m3/

m)

Need more data

Conclusion(2)

added value brought by auv’s in the surveillance of exploited marine sand banks

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