Don’t get upset, I am not bursting YOUR bubble, but rather remote sensing wants to burst surf zone bubbles. Although it may be fun for recreational activities like surfing, fishing, and boogie boarding, the surf zone is an area that makes remote sensing professionals cringe. Why? Because it is difficult to collect relevant and useful data. It would be great if we could just ignore this thin strip of coastline but these nearshore areas are vital ecosystems. What I hope to explain is why this area is so important and why bubbles drive us mad.
Nearshore ecosystems are dynamic, nutrient-rich environments that support a variety of marine life, making them important both environmentally and economically. For example, in Florida, nearshore reefs are considered essential fish habitat. These areas provide permanent habitat plus act as transition area for adolescent fish as they progress from spawning to the open ocean. The health of these ecosystems has a major impact on recreational and commercial fisheries. According to the Florida Fish and Wildlife Conservation Commission’s 2014 facts, saltwater fishing has an annual economic impact of $7.6 billion and provides 109,300 jobs. In order to properly manage these areas, it is important to be able to map them so we can see and understand what we’re managing. Thus the need for remote sensing, and although there have been numerous advances in remote sensing technology, there still remains limitations and difficulties when attempting to collect data in the surf zone.
There are three primary ways to collect remotely sensed data in nearshore environments: acoustics, aerial imagery, and lidar. Acoustic collection systems, such as single and multi-beam echo sounders, emit sound waves to determine depth by utilizing the principle that sound travels well through water and at a rate we can model. Aerial imagery is a tried and true method for studying nearshore areas and can provide some valuable data regarding nearshore bathymetric features. Lidar uses light in a similar fashion to how SONAR uses sound. Taking advantage of high sampling rates, systems are able to collect massive amounts of data points in a given area. As most of you may know, these methods are dependent upon weather and turbidity, but what makes the surf zone especially difficult? The first is depth. Yeah it’s shallow. Most people don’t want to get towed off a beach, swamped by a wave, or grounded on a reef but what really makes the surf zone especially unique is the BUBBLES!
Your first thought might be, well bubbles are clear and you would be correct. But try to imagine what a bubble looks like in the sun and you begin to realize that a bubble has a highly reflective surface. This is a major factor with visual remote sensing systems like lidar and aerial photography. The bubbles actually scatter and attenuate the laser pulse reducing the ability of the sensor to retrieve accurate bathymetric data. Those bubbles create a lot of NOISE!
It is slightly different with acoustic systems. Because they use sound and not light, the reflective surface of the bubble does not cause a problem. As mentioned above, depth plays a major role as acoustic systems are towed or attached to vessels. The surf zone is shallow and often violent due to wave energy. But back to the bubbles. Bubbles can scatter sound but they also have another effect. We know water is considerably denser than air, and sound propagates differently through fluids of varying density. The denser the fluid, the faster the sound travels. However, when you incorporate bubbles into the equation, now the sound wave is travelling through fluids of varying density, negatively impacting returns because we don’t know the correct speed of sound.
BUBBLES are driving us mad, but nearshore areas are vital ecosystems, so managing these areas often relies on more traditional techniques like diver surveys. But how can we as remote sensing professionals at least attempt to overcome this issue? The first would be to take advantage of weather windows. Low winds, minimal surf, and low turbidity rarely occur simultaneously, but the times they do, collections need to occur. The other option is to use tidal energy to our advantage. The location of the surf zone is directly dependent upon the depth of the water. As a wave approaches the shore, friction from the seafloor causes the wave to slow (wavelength decreases) and increase in height. At a known wave height to wavelength ratio (1:7) a wave will break. Over the course of a tidal cycle, the location of the surf zone will vary, moving towards or away from the beach, enabling sensors to capture bathymetric features. In areas where there is enough tidal range, by sampling at high and low tide, remote sensing professionals are able to minimize the impact of the bubbles.
The surf zone is a highly dynamic, often treacherous place to collect data. Remote sensing can provide an avenue for meaningful data collection but, although there have been dramatic advances in the field of remote sensing, the surf zone still remains uncharted territory, so to speak.