MOOmapeamentos

8/3/2019 MOOmapeamentos

1/29

General Facts about the Lidar Technique

Laser Induced and Ranging (LIDAR) is an airborne mapping technique which uses a laser to

measure the distance between the aircraft and the ground. Since the 1970s the application of

airborne LIDAR for topographic and bathymetric mapping has matured at a rapid pace, with

the first commercial Lidar systems appearing in 1993. Much of this growth has directlyfollowed advances in high speed digital and analogue electronics along with increases of

several orders of magnitude in computer memory, storage capacity and processing speed.


2/29

There are two main types of systems operating with different light frequencies:

the first is the topographic Lidar with only one near-infrared (IR) wavelength,

between 1047 and 1540 nm according to manufacturers, the other one is theALB (Airborne Lidar Bathymetry) consisting basically of two rays at different

wavelengths: blue-green (532 nm) and near-infrared. Usually ALB systems are

also geared to survey in dual modes, i.e. topographic and hydrographic.


3/29

Lidar bathymetric technology utilises the reflective and transmissive properties of water andthe sea floor to enable measurement of water depth. When a light beam hits a column of water,

part of the energy is reflected off the surface and the rest, unless absorbed by particles in the

water, is transmitted through the column. As the light travels through the water column and

reflects off the seafloor, scattering, absorption, and refraction all combine to limit the strength

of the bottom return, and therefore the system's maximum extinction depth. This depth is a

function of water clarity, and is generally about 2 to 3 times the Secchi depth (Smith, 2000).

As shown in Figure, for turbid water, the extinction coefficient is smallest in the green part of

the spectrum close to 0.6 nm. The presence of organic matter in the water tends to displace

light penetration towards higher wavelengths.


4/29


5/29

Sea Bottom Topography with Navigation Radar

Sea bottom topography in de Wadden Sea the

North of the Netherlands. The wind was

approximately 10m/sec. Water depth was between 5

and 20 m and total observation/ processing time was2 minutes. The area is about 4 by 2km.

In radar images, changes in sea surface

roughness owing to bottom topography are

mainly affected by wave-current interactions.

The hydro-meteo conditions under whichbottom topography can be observed are both as

high as possible surface current as a wind speed

of between 2 and 4 Bft.

That navigation radars are capable to monitor

sea bottom topography is quit new An example

is given in Figure. In contrast with space-borne

radars these systems are able to monitor a

particular area around the clock. This ability

makes navigation radar attractive foroperational use of sea bottom topography

mapping. Further investigations are needed to

demonstrate the performance


6/29

Side Scan Sonar

Sidescan sonar is based on the same acoustic principles as the single beam echo sounder and the

multibeam echo sounder. All three have transducers which comprise:

a transmitter which emits a sound pulse into the water column down to the sea bed

a receiver which picks up the reflected sound from the sea bed as a vibration which is converted

into a digital or analogue signal and recorded on a survey vessel

Sidescan sonars are characterised by a beam which is

narrow in the horizontal plane and wide in the vertical

plane. This creates a narrow acoustic sweep across

the sea bed at right angles to the track of the towfish.The range of the sweep is governed by the velocity of

sound in water. The longer the range set by the

operator the longer it takes for a sound pulse to travel

out and back to the towfish. Because a sidescan has

two transducers, the sweep coverage of each towfish

is double the range i.e. a typical sidescan set to survey

at a range of 150 m will produce a sweep of 300 m

across its track.


7/29

The sound received and recorded by a sidescan sonar system is a function of two primary

mechanisms which enable sound to return from the sea bed. These are:

1. Reflection. Direct returns of sound bouncing back off features on the sea bed such asrock outcrops, sand waves and wrecks.

2. Backscatter. This is a diffuse and weaker process based on the interaction of sound

energy with the ambient texture and character of the sea bed. The intensity of the

backscattered sound is a function of bottom roughness and angle of incidence.


8/29


9/29


10/29

Multibeam Echo Sounders

Multibeam echo sounders (MBES) determine depth by accurately measuring the angles of

emission, reception and two-way travel time for a pulse of sound energy from the emitting

instrument (transducer) to the seabed and back. MBES systems can achieve full bottom

coverage with beam swath widths of 4 to 7 times the depth of water being surveyed (Figure

81). They are sometimes called beamformers or true multibeam systems, opposed to

interferometric swath systems.

Following data collection, processing is undertaken. This includes offset correction, attitude

correction, tidal offset and cleaning of erroneous echoes present as outliers in the data. The

soundings can then be built into a Digital Terrain Model (DTM) for 3D viewing of the sea

floor, creation of sun-illuminated imagery and contour maps.


11/29

MBES can also measure the amount of acoustic backscatter from the seabed for each acoustic

beam. Backscatter information is perfectly co-located with the seabed bathymetry information

and makes MBES unique in the ability to simultaneously collect bathymetry and backscatterinformation in a single survey. Only part of the acoustic signal emitted will be reflected back

to the receiver from the sea floor, part may be transmitted into the sediment and part scattered

in a different direction by the seafloor. The way the seafloor interferes with the acoustic signal

and the returned echo can be used to characterise the seafloor material. The transmission and

scattering will depend on the frequency of the MBES, the angle of incidence and the type ofsediment its density and porosity (Figure 82).


12/29


13/29

-Spatial scale: Shallow versus deep water survey systems

Acoustic energy emitted from multibeam echo sounder transducers will undergo spreading and

absorption as it propagates through the water column. This propagation loss will limit the range the

acoustic energy may penetrate the water column with high frequency energy being absorbed at a

higher rate than low frequencies and limiting their use to shallow waters. Lower frequencies have

longer wavelengths and will be able to penetrate to deeper seafloor depths. On this basis MBES

systems may be divided into three major categories depending on their operating frequencies,

Deepwater, Shallow and High-resolution systems (Table 81).


14/29

Sound velocity corrections

The speed at which sound travels through the water column (sound velocity profile) must be

known to convert the travel times of acoustic waves into distances. Sound velocity

commonly ranges from 1400 to 1570 ms-1 (Figure 88) approximately four times the speed

of sound through air. This is a function of water density, which is affected by water

temperature, salinity and pressure and therefore varies with the depth. This parameter has a

significant effect upon the calculation of the distance between the seabed and the transducer,and positioning of the footprint of each beam


15/29


16/29

Survey Planning

The time requirements for surveying an area are primarily dictated by the water depths in

question, with shallower areas taking longer to survey owing to smaller footprint size (Figure 8

11). For survey planning one must first gather as much available information as possible on the

bathymetry, from admiralty charts, previous surveys, GEBCO (General Bathymetric Chart of the

Oceans) (BUUHHHHHH) etc.


17/29


18/29

Data cleaning / checking

Data cleaning and checking will apply to vessel navigation and attitude data, as well as the depth

soundings. The process begins with a visual inspection of vessel navigation and attitude data toidentify and remove any invalid measurements by the navigation or MRU instruments.

The suppression of erroneous depths

caused by fish, noise or air in the watercolumn can be performed manually or

automatically through various filters.

Erroneous soundings are also called

outliers or spikes. Manual cleaning can

be performed through several interfacesdepending on the software manufacturer.

The most common incorporate

visualisation of the data ping-by-ping on

a line-by-line basis (also called a

waterfall display) or on a subset of data

in a 2D or 3D view. These interfaces are

very useful for checking the quality of

the data and can reveal problems in the

acquisition settings such as neighbouring

lines not matching and abnormal swath

shape.


19/29


20/29


21/29

What can I do with these data?

Resource inventory Complement swath acoustic techniques

What information lies there?

Measures roughness and hardness of theseabed as well as depth

Can be interpreted (with ground truthing) ashabitats

Good at measuring small acoustic differencesbetween sediment types

What are the limitations?

Coverage is not complete and interpolation

can introduce errorsResolution is coarseMeasurements are specific to each instrumentand every survey should be interpretedindependantly

Single beam Acoustic Ground DiscriminationSystems (AGDS)

To map subtidal facies


22/29

Side Scan Sonar

Acoustic sea bed images as single line records or mosaics. Resolution and coveragefrequency depth dependent: ~0.2 - >1.0 m. Commonly not georeferenced thereforelower resolution for horizontal position: >10 m.


Create mosaic maps of sediment facies,bedforms and rock.

Detect temporal change in bedforms

Guide ground truth sampling


Extent and nature of substrates

Crest and slope orientation of bedformsIndicators of seabed topography

Indicators of human impacts e.g. trawl scars


Towed fish system; positional accuracyimproved by tracking towfish position

Differences between across-track and along-track resolution

Generally not practical in water depths less

than 5-10 metres


23/29

Sub Bottom Acoustic Profiling


Link the occurrence of particular seabedsediments to geological units in the shallowsubsurface.


Distribution of cohesive layers (clay, peat) that

are a suitable substrate for borer shells.Measure of vertical variability, for predictingtemporal seabed-sediment change in case oferosion.


Only top layer is relevant to habitat mapping.

Acoustically acquired continuous profiles of the shallow subsurface. Vertical resolution in decimetre

range, lateral resolution in metre-range.

To map sediment thickness


24/29

Multibeam Echo Sounders


Create detailed seafloor relief maps showingcontours, slope, aspect, rugosity;Hydrographic charting; Benthic TerrainModelling.Create backscatter maps of seabed facies(proxies for habitats)Guide ground truth sampling.


Accurately georeferenced high-resolutionbathymetry & backscatter data.


System requires experienced operatorsRaw data must be processed to eliminate artefacts arising from vessel motion and variability in

tidal height and sound velocity through water column.Swathe width is depth dependant.

Digital Elevation Models of the seabed, obtained from multi-head echo sounder. Resolution in theorder of 2 to 5 metres horizontal, 0.05 m vertical. Co-located backscatter images indicateseafloor roughness and hardness (as for side scan sonar).

To map both subtidal faciesand relief


25/29

Interferometric Sonar

Digital Elevation Models of the seabed with similar quality to sidescan sonar. Resolution in theorder of 2 to 5 metres horizontal, 0.05 m vertical. Swath width to 7 times water depth.


Create detailed seafloor relief maps showing contours, slope, aspect, rugosity, Hydrographiccharting; Benthic Terrain Modelling.Create backscatter maps and infer sediment/substrate types.Guide ground truth sampling.


Accurately georeferenced high-resolution bathymetry & backscatter data.


Requires experience operators and significant post-processing of data (as for multibeam).Generally more applicable in water shallower than 50m.


T b th btid l f i d li f


26/29

Single Beam Echosounders

Echo-integration of acoustic profiles of the seabed. Deployed from light vessels in suitable weather

conditions. Metric alongtrack resolution, vertical resolution 5 cm.


Ground truth swath acoustic techniquesFill gaps in swath data


Identify seabed type, e.g. bare seabed, kelp,seagrassEstimate kelp biomassMeasure depth profiles


Limited coverageMis-interpretation of seagrass/ seaweeds

Photos by Ifremer


T i t tid l d h ll t li f


27/29

Topographic Lidar

Digital Elevation Models of intertidal areas obtained from laser light return time. Dot spacing: metric,vertical accuracy 0.15 m, horizontal accuracy 1 metre.


Precise description of reliefGuide intertidal field workResource inventory (along with facies data)


Highly detailed elevation dataHeight contours, slopesComputation of emersion time


Coverage limited by tidal height, surveysoptimal at LWS tide

Photos by Ifremer

To map intertidaland shallow water relief

T i t tid l d h ll t li f


28/29

Hydrographic Lidar

Digital Elevation Models of seabed areas obtained from laser light return time. Dot spacing: 2-5 m, verticalaccuracy 0.25 m, horizontal accuracy 3-5 m.


Good description of seabed reliefGuide subtidal field work


Highly detailed elevation dataDepth contours, slopesPotential identification of main seabed types(backscatter signature)


Limited by water clarity (3 times Secchidisk)

Coverage limited by tidal height, surveysideal at LWN tide

Photos by Ifremer


To map intertidal and shallow water relief


29/29

Aerial stereo-photography

Digital Elevation Models of intertidal areas obtained from stereo aerial photographs. Grid spacing 2to 5 metres, vertical accuracy 0.6 to 1 metre (with 1:25,000 photos)


A coarse description of reliefA preliminary inventory of main coastal units


Moderately detailed elevation dataCoarse contours and slopes


Low tide survey neededOf limited use for underwater mapping.

Photos by Ifremer


Documents

MOOmapeamentos