Explorations | Bonaire: Exploring Coral Reef Sustainability with New Technologies | Logs

Bathycache- The Truth is in the Ground

January 17, 2008

Arthur Trembanis
Expedition Co-Principal Investigator
Assistant Professor, University of Delaware
AUV Operations and Marine Geologist

In addition to the shape of the seafloor, we also gather information from the AUVs about the nature of the composition of the bottom, e.g. whether it is sandy or coral covered. To do this, we use a technology called side-scan sonar resulting in images like that seen in the following image. This shows the intensity of the sound that bounces off the seafloor and back to the robots. Imagine what your living room would look like at night illuminated only by the light and shadow of a flashlight and you will begin to get a sense of what kinds of images side-scan sonar is providing. Light areas indicate hard acoustic returns such as what you would expect from a large brain coral while darker areas indicate shadow or zone with more subdued topography or possibly fine sand. Notice that I said rather ambiguously what might be causing the light and dark patches. The reason is that there are many combinations that can make similar patterns, and it is only through careful ground-truthing (that is taking close up visual or physical samples) that scientists are able to differentiate one bottom type from another. The task of ground-truthing is strenuous, repetitive, and thankless work but critical to habitat mapping such as we are doing here in Bonaire. Many students came to our aid recently to help us conduct sonar groundtruthing here at the science compound by swimming out to document various acoustic targets on the seafloor and noting the kind of bottom-type and marine organisms associated with the feature(s).

We have managed to conduct seafloor-mapping surveys in six separate locations acquiring many tens of square kilometers of seafloor mapping data in addition to water column data regarding the conditions within the water (salinity, temperature, oxygen levels, etc).

AUV operations especially on windy islands such as Bonaire are not without their challenges. Sometimes we have to call on various support equipment to help us locate the robots amidst the choppy surface waves. One such instrument we use is called and acoustic pinger and in the following image Bernard is demonstrating its use. The operator wears a set of headphones like you might use for listening to an MP3 player while the other end is composed of a hydrophone, essentially a microphone for picking up underwater sounds. By changing the orientation of the hydrophone “bell” the operator can get a sense of the direction from which the sound is coming from. The intensity, or loudness, of the sound is an indicator of the distance that the hydrophone is from the sound source. Each robot luckily has its very own distinctive sound pattern thus allowing us to tell each robot apart even if they are many hundreds of meters away underwater.