Seabed Classification

Question 1

Give 3 industries that use seabed classification data?

Answer 1

1. Nautical Charting
2. Commercial and Environmental Surveys
3. Military

Question 2

Give 4 direct methods

Answer 2

1. Divers
2. Grabs
3. Corers
4. Dredges

Question 3

Give 3 remote methods

Answer 3

1. Underwater video/photography
2. Acoustics
3. LIDAR

Question 4

Give 5 disadvantages of direct methods

Answer 4

1. Inefficient data collection
2. Expensive per unit area covered
3. Environmental Impacts
4. Specialised sample analysis
5. Time consuming & labour intensive

Question 5

Remote sensing acoustic seabed classification is the organisation of seabeds into discrete units based on acoustic response which includes what 2 things?

Answer 5

1. Acoustic Hardness

2. Physical Roughness

Question 6

What are the 3 commonly used acoustic methods?

Answer 6

1. Quester Tangent (analysis of first echo)
2. Roxann Method (analysis for first and second echo)
3. Statistical Analysis using SSS and MBES

Question 7

What are the 3 stages of the Roxann Method?

Answer 7

1. Echo Digitisation
2. Echo Description
3. Echo Classification

Question 8

What does E1 determine?

Answer 8

Roughness Echo

Question 9

What does E2 determine?

Answer 9

Hardness Index

Question 10

Give 3 points about the Reference Catalogue

Answer 10

1. It is developed by obtaining information on the seabed using visual and mechanical methods
2. Seabed observations are matched to echo responses
3. Can be time-consuming - requires groundtruthing and is often unique to the location

Question 11

What does SSS use?

Answer 11

Raw backscatter and statistics (mean and SD) in cells to determine backscatter characteristics which are compared to the reference catalogue

Question 12

What is the problem with using backscatter?

Answer 12

The angular dependance on backscatter strength

Question 13

What is AVG?

Answer 13

Angle-Varying Gain: aims to compensate for across-track variation in backscatter due to physical dependance of the seafloor response with the incidence angle

Question 14

What does AVG require?

Answer 14

Each data sample to be associated with its angle of incidence to the seabed

Question 15

How are corrections determine using AVG?

Answer 15

It is looked up in a table that is organised by angle

Question 16

Give 5 benefits of acoustic seabed classification systems

Answer 16

1. Provides remote, real time data acquisition without interfering with normal vessel operations
2. Doesn’t impact the seabed itself
3. Basic units are comparatively cheap
4. Easy to set up and use and are portable
5. Can cover large areas efficiently

Question 17

Give 7 disadvantages of acoustic seabed classification systems

Answer 17

1. Constant vessel speed required to provide a constant noise background (usable signal:noise ratio)
2. Depth may change results even if bottom type is the same
3. Prone to bottom slope effects (especially for 2nd echo methods)
4. Affected by bubbles, wakes, acoustic noise and sea state
5. For the first and second echo method the sonar range must be set to twice the maximum depth in order to get the 2nd echo
6. During data acquisition the sonar settings must remain constant
7. Ground thruthing can be time consuming and difficult