While mobile oceanographic observing instruments like the Spray glider are good for making measurements over larger areas for extended time periods, scientists on the PEACH project are also deploying observing equipment to make ocean current measurements at fixed locations with high resolution. Acoustic Doppler Current Profilers, or ADCPs for short, are one example of fixed observing equipment employed on the PEACH project. Deployed on the seafloor, ADCPs use acoustic signals generated from a series of four acoustic heads to determine current speed and direction above the device. The acoustic heads work much like a boat depth finder. An acoustic signal is sent out from the head, bounces of an object, and returns back to the transducer head. With an ADCP, the acoustic signals are bouncing off particulates in the water, and the four heads work in concert using the Doppler shift in the ADCP emitted frequency to determine current speed and direction. Current measurements can be made from close to the seafloor to just below the ocean surface, in predetermined increments that depend on the ADCP frequency.
ADCPs log data internally until they are retrieved, which in the case of the PEACH project will be 9 months from deployment for those in shallower water, and 18 months for the deeper deployments. Oceanographers on the PEACH project are deploying nine ADCPs in depths ranging from 30m to 230m. To retrieve the device, an acoustic signal from a topside deck box or through hull transducer is used to trigger a release, which frees the ADCP from its sacrificial ballast, and with the aid of flotation, propels it to the surface. Once on the surface, project researchers can download the data, insert new batteries, and redeploy the ADCP for another 9 months – for the shallower instruments. Deeper ADCPs will remain for the duration of the experiment. In total, the PEACH project will gather 18 months of current measurements from nine ADCP sites off North Carolina.
In addition to the ADCPs deployed on the PEACH project, the UNC Coastal Studies Institute has had an ADCP deployed since 2013 within the Gulf Stream study area as part of the North Carolina Renewable Ocean Energy Program. Cooperation between both projects helps keep expenses lower, and answers questions posed by both. Both project’s combined observation and modeling efforts will help to identify the best location for getting marine hydrokinetic energy from the Gulf Stream for the Ocean Energy program, and will provide an understanding of exchange processes from the shelf to the deep ocean for the PEACH project.
For more information on the PEACH project, please click HERE.