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Bathymetric Lidar and Blue Carbon

Bathymetric LiDAR and Blue Carbon

The guest on this show is Andy Waddington, the VP of Bathymetric Services at Hexagon Geosystems. He has an extensive background in the marine industry and hydrographic surveying. In this conversation we will learn about airborne bathymetric LiDAR, and how it is useful in mapping the extent of seagrass. 

Seagrass is a heavy hitter among the marine vegetation that sequesters carbon. Mapping its extent helps in understanding its potential in storing blue carbon, as the extent of seagrass is directly related to the amount of carbon that is deposited in the root system underneath it.

Seagrass and Carbon Sequestration

Observations made in a case study of geo-tagged sharks in the Bahamas sparkled an interest in further studying seagrass.

 Beneath the Waves, a non-profit organization for conservation of ocean life, tags sharks with sensors to study how they behave while monitoring their health. 

From their observations, they have discovered that sharks tend to live in areas with seagrass. This connection between seagrass and sharks kindled an interest in studying seagrass. 

Subsequent studies opened up a lot more discoveries of how this underwater vegetation has a role to play in balancing carbon levels in the ecosystem.

Seagrass plays an important role in carbon sequestration by reducing the amount of carbon in the atmosphere. 

Just like trees, seagrass also stores carbon but the only difference is that while trees store the carbon in wood, seagrass stores the majority of its carbon in its extensive root system.

 Seagrass is a big part of the marine ecosystem that stores blue carbon.

Why Bathymetric Lidar Is an Optimal Solution for Mapping Seagrass

Initially hydrographic surveys were mainly done by echo sounders, but more recently, the advancements in LiDAR technology led to the emergence of bathymetric LiDAR as a useful tool for conducting hydrographic surveys.

Bathymetric LiDAR collects high-definition information relatively quickly compared to acoustic systems, like multi beam echo sounders

LiDAR sensors mounted on aircraft have a larger swath and can cover extensive areas quickly. On the other hand, the swath area in acoustic systems will reduce closer to the surface.

In shallow waters the swath area will be very narrow. Since seagrass naturally exists in shallow waters, data collection with acoustic systems takes a very long time, but green LiDAR is perfect.

Bathymetric LiDAR is a more optimal way to map seagrass than space based systems. Although satellites capture information about very wide areas quickly, they do not have the level of detail that bathymetric LiDAR can collect. 

This does not mean that bathymetric LiDAR has the best accuracy. Multi beam echo sounders have very well-defined processing algorithms that can actually detect seagrass more acutely than bathymetric LiDAR. Still, bathymetric LiDAR is an optimal solution operationally as earth-based UAVs can easily be tasked out for collections.

How Far Down Does Bathymetric LiDAR Go?

The penetration depth of LiDAR in water depends on certain environmental conditions in the water. Suspended sediment, fish, and other materials and properties of the water scatter the laser pulse and cause it not to return to the sensor receivers. The effective depth of most LIDAR systems is about 35 – 40 meters. The actual depth which LiDAR can reach is largely determined by the physics and its interaction with the water.

Temperature, salinity of water, and pressure also have an effect on the wavelength and passage of the pulse through the water, but corrections for these things can be calculated from the waveform. The other thing that can limit the depth of bathymetric LiDAR is the natural reflectance of the seabed. A sandy seabed and areas of rock give off a high reflectance, but areas where the seagrass is greener, thicker, or darker gives poor reflectance, or can totally absorb the pulse, rather than allowing it to be returned to the sensors.

The height of the sensor also has an effect on the depth that bathymetric LiDAR can reach. The higher the flight height, the less penetration as there is less energy available to penetrate the water by the time it has passed through the air.

There is no one fixed height that is optimal for all operations. Different operational needs require adjusted flying heights that gives the maximum performance in meeting the client requirements i.e. density and depth, while also accounting for study area and available equipment.

Why Should We Monitor Seagrass?

Just as the carbon in wood is released when the wood is burnt, the carbon in seagrass is also released when it is teared up or destroyed. The difference is that with seagrass, there is an opportunity to leave the carbon deposited for hundreds of years. If there is no human interference, like dredging, or offshore mineral exploration activities, then there is almost no natural way for the carbon to be released, and so it will stay captured for a very long time.

It is important to measure seagrass and make efforts in its preservation in order to make sure it is healthy and maintain the carbon’s sequestration. A monitoring process can help in understanding the state of the seagrass over time to help know when to take the necessary action.

The Need to Quantify the Economic Value of Seagrass 

There could be an opportunity to use seagrass to give carbon credits, but currently there is no mechanism that attaches monetary value to carbon sequestration. Measuring and quantifying the value of seagrass in cash terms can help when making an assessment for other uses of the space where seagrass exists.

For instance, if someone wants to undertake mineral extraction in the area, it would be possible to compare the worth of the minerals against that of the seagrass to determine whether extraction is a viable use of that space.

Bathymetric LiDAR and AI

There are ongoing efforts at Hexagon to bring to production algorithms that extract information from data captured by bathymetric LiDAR. The current solution relies on manual input, and is slow and costly.

Having AI algorithms will leverage the power of computing to extract information effectively and efficiently. While we may see this take off further in the future, it will take a lot of ground truthing to realize this, since even very subtle differences in the environment can cause differences in the results. Political awareness and the willingness to take risks will also go a long way to making this a reality.

Is Bathymetric Lidar the Silver Bullet?

Bathymetric LiDAR is certainly one of the highly valuable tools in the bag of hydrographic tools, but this is not the silver bullet that can solve everyone’s problem.

Rather, the silver bullet is combining the right hydrographic sensors and tools to the right application, environment, and geographic area to obtain the output that is needed for the next step.

Some tools that complement working with LiDAR data include PDAL and Whitebox Tools.

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