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September 14, 2010

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Acoustic seafloor mapping systems

Delft Institute of Earth Observation and Space Systems

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Acoustic seafloor mapping techniques

• Single-beam echosounder (since 1920s)

• sidescan sonar (since 1960s)

• multi-beam echosounder (since 1970s)

• …

3

The single-beam echosounder (SBES)

2

ctH

• Signal duration T : ~0.1-1 ms

• Beam width (angular aperture) : ~10

• Frequency f : ~10-500 kHz

• Source level SL : 200-230 dB

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Transducer directivity pattern Beam width

d

65

2,1

2,1 50L

Circular:

Rectangular:

5

Modern digital single beam sounder: effect of frequency choice (38 kHz or 200 kHz)

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Single-beam echo sounder Transmission sequence

TTc

HTR

2

TTc

nHTR

2

or

7

• Vertical resolution: ,

• Horizontal resolution:

Single-beam echo sounder Measurement resolution

2

cTz

B

cz

2

HHx

2tan2

1)(

2

0

2

0

2

0

2

z

xx

z

z

8

Horizontal resolution shallow water

9

Horizontal resolution - continued shallow water

= 10

10

Horizontal resolution - continued deeper water - same beam width

= 10

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Single-beam echo sounder Echo formation

Assumptions:

• A flat and horizontal seafloor

• A conical directivity pattern

• Only echo from water-seafloor interface

a) : impact point

b) From to :

c) From to :

d) After

with

cHt /20

0tt Ttt 0

)()( 0

222 ttHcHRrA e

Ttt 0

HcTcT

RRrrA ie

2

222

2)(

maxt

812/tan1

2

0

2

0max

ttt

)( 22

max, ie rrA

2/)2/tan(max, HHre

12

Single-beam echo sounder Echo formation, continued

short-pulse or pulse-limited regime:

Ttt 0max

long-pulse or beam-limited regime:

Ttt 0max

Modified expressions for

active area, see lecture notes!

September 14, 2010 13

Exercise handed out last lecture Consider the following situation: We are taking measurements with a multi beam echo sounder system. The sound speed

profile in the water column is as follows:

Take for the density in water column of 1g/cm3 The sediment is a clay sediment (sound speed of 1470 m/s, density in the sediment of 1.2 g/cm3)

a. Calculate for a sound ray emitted at an angle of 22.1 degrees with the horizontal, the angle of the ray at the bottom. b. What is the reflection coefficient? c. What happens to the sound if it arrives at the bottom, taking your above answers into account.

0 m

100 m

Depth (m)

Sound speed (m/s) 1460 m/s

1500 m/s


Exercise handed out last lecture, contn’d

• Step 1: make a sketch of the situation c1 = 1460 m/s, 1 = 1 g/cm3

c2 = 1500 m/s, 1 = 1 g/cm3

c3 = 1470 m/s, 1 = 1.2 g/cm3

1=22.1

2

3

• Step 2: Determine angles

22 1

1

c 1500a cos cos a cos cos 22.1 17.8

c 1460

33 2

2

31

1

c 1470a cos cos a cos cos17.8 21.1

c 1500

c 1470or a cos cos a cos cos 22.1 21.1

c 1460



• Step 3: make a sketch of the reflection coefficient

What is the situation, do we have an angle of intromission or a critical angle? -> The sound speed in the sediment is lower than that in the water column directly above the sediment: no critical angle -> , i.e., we have an angle of intromission

2 2 3 31500 1470 1.2 1764c c x

2

3

2

0 2

3 3

2 2

1

arctan 17.8

1

c

c

c

c



• Step 4: What is the reflection coefficient

Based on the previous analysis we expect a reflection coefficient of 0. Check:

3 3 2 2

3 2

3 3 2 2

3 2

1.2 1470 1500sin sin sin 21.1 sin17.8 0

1.2 1470 1500

sin 21.1 sin17.8sin sin

c c x

Rc xc


Exercise

• SBES with an opening angle of 10 degrees

• 4000 m of water depth

What is the horizontal resolution?


2 2

e

4000 10349 m

2 2 180

x=698 m

A= r 0.4 km

e

H x xr

x

Beam footprint

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Single-beam echo sounder Maximum operating range

• Water depth: 5 km

• Dominant noise source: self noise

• Receiving bandwidth: 500 Hz

• Ship speed: 15 knots

• Echo-sounder: circular transducer with an area of 400 cm2

• SNR should be at least 10 dB

Required specifications for the echo sounder, its frequency f in particular?

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BGLELSNR

Single-beam echo sounder Maximum operating range, continued

PLSLEL

DINLBGL W

2

10 4log10

ADI

, with A = 400 cm2, DI = 19.5 dB at 20 kHz

DIPSL log108.170 10

For P = 50 Watt: SL = 207 dB re 1 Pa at 20 kHz.

with

12log208.133 10 f

vNLWNLNLW log1010 with

NLw amounts to 82.6 dB at 20 kHz

21


BLHHPL 2)2log(2060 10

is 4.1 dB/km

1122

112210 log20cc

ccBL

Clay : BL = 22 dB

Coarse sand : BL = 7.7 dB

22


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The sidescan sonar system

beam footprint signal footprint

Footprint:

Transmission sequence:

c

RTR

max2

• Signal duration T : ~less than 0.1 ms

• Beam width (angular aperture) : ~1 in

horizontal plane, ~85 in vertical plane

• Frequency f : several 100 kHz

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Sidescan sonar Image reconstruction

Measurements consist of backscattered

intensity as a function of time.

For a flat seafloor:

222

22

4H

tcHRy

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Sidescan sonar Echo formation

Ft

tHh

Obstacle height:

t

tf

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Sidescan sonar resolution

Across track y

Along

track x

sin2

cTy Across track y:

Along track x: Rx

2/cTy

HcTy

for → 90

for = 0

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Sidescan sonar coverage

To ensure 100 % coverage:

Hx (insonified strip is narrowest near vertical)

RTvx (sonar distance between two pings)

c

RTR

max2

max

max

cos22

c

R

cHv

(i.e., 100 % coverage condition independent of H)

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Sidescan sonar Examples

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Multi-beam echo sounders (MBES) simultaneous measurement of bathymetry and

backscatter

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• Deep water systems: operating at f ~ 12 kHz. The large dimensions of the transducer array limits their installation to deep-sea vessels.

• Shallow water systems: operating at f ~ 100-200 kHz. Mapping of the continental shelf.

• High-resolution systems: operating at f ~ 300-500 kHz. Local studies (shipwreck location, inspection of underwater structures). Small size allows for installation on small ships, tow fishes or autonomous underwater vehicles (AUV).

MBES MBES types

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MBES transmission and reception arrays

Transmission

(determines along-track

resolution x)

Reception (using beamforming)

(determines across track

resolution y)

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MBES bathymetry (topography) measurement

cos2

cos)(

sin2

sin)(

cttRz

cttRy

(when ssp is constant, otherwise ray tracing is

necessary)

Joint measurement of time t and angle

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MBES imaging

Combining time signals after beamforming:

Compared to SSS:

backscatter pixels are now at the correct position because of bathymetry

measurement capability !!!

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MBES bathymetry measurement errors

cos2

cos)(

sin2

sin)(

cttRz

cttRy

Errors in range R:

Errors in angle :

sin

cos

Ry

Rz

R

R

HRy

HRz

cos

tansin

Prediction of bathymetry measurement accuracy with MBES

is an important research item !!!

Basis:

Types of errors:

• Errors in the acoustic measurement itself (acoustic noise, non-stable backscattered signal);

• Movements of the support platform (finite accuracy of the attitude sensors);

• Inaccuracies in sound speed corrections.

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MBES Refraction correction

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MBES Refraction correction, continued

zgczc 1)( with c(H) = c2

)(zc

dst

gzc

z

zc

z

c

11

1 )(cos

)(

)(coscos Snell’s law:

d

g

cdz

1

1

cos

sin

2

1

2

1

2

2/tan

2

2/tan

ln1

cos

11

cos

sin

sin

1

cos

cos1

1

2

1

11

1

gd

gd

g

c

ct

, with 22 dzdyds and dydz tan sin/dzds :

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MBES Refraction correction, continued

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MBES resolution

LRx for bathymetry & imaging

sin2

cTy For imaging:

T

Hy

2cosFor bathymetry:

HcTy 0( for )

Along track resolution

Across track resolution

-

sin2

cTy

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MBES Repetition period and coverage

maxcos2

Lc

v

Global mapping of the deep oceans (H = 4 km)

max = 75, L = 1.5

v: 5 m/s or 10 knots (100 % coverage)

Swathe:

Area covered per second: 13000 km2 per day

Surface of Earth’s deep oceans: 350x106 km2

km30tan2 max HL

sec/km15.0 2vL

Required for global mapping: 2.7x104 days = 74 years

With 50 deep water MBES systems (situation in 2002): ~1 year

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A typical MBES result

acoustic seafloor mapping systems - tu delft ocw · pdf fileacoustic seafloor mapping systems...

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