Garbage Patch Detection and Assessment in 3D Ocean Medium with Hydrographic Lidar
Over 5 trillion pieces of plastic currently litter the world’s oceans. The trash accumulates in 5 main ocean garbage patches, the largest one being the Great Pacific Garbage Patch (GPGP) located in the North Pacific Ocean between Hawaii and California. The Ocean Cleanup Foundation, a not-for-profit organization that develops technologies to rid the world’s oceans of plastic, needs a reliable method to understand GPGP’s dimensions to know how many tons of trash are to be extracted in the coming years. Their Aerial Expedition project in 2016 used aerial surveys to quantify the amount of large plastic debris in the GPGP, and Teledyne Optech’s CZMIL (Coastal Zone Mapping and Imaging Lidar) was proposed to fulfill The Ocean Cleanup team’s goal. The reasoning for using the airborne hydrographic system was based on:
• Earlier experimentally obtained environmental data (very similar densities of all ocean plastics in known patches, moderate depth of the subsurface water layers that contain most of the garbage, and extremely clear water in the test region);
• Theoretical analysis carried out to estimate the spatial resolution available with CZMIL as a characteristic of the system’s ability to detect the submerged objects in question.
During the two-day mission (October 2 and October 6, 2016) over the GPGP, the expedition scanned hundreds of square kilometers with Optech CZMIL’s suite of imaging and data collection instruments, including its lidar, RGB camera, and integrated ITRES SASI SWIR (short-wave infrared) sensor. The C-130 aircraft, named Ocean Force One, transited five hours out over the Pacific Ocean at approximately 20,000 feet, and then dropped to an altitude of 400 m (1,300 feet) and a survey speed of 140 knots while mapping the area, following CZMIL’s survey requirements.
In post-processing, we used Optech’s HydroFusion software to gather and fuse data from the lidar, camera, and spectral sensors to estimate the size of the GPGP (e.g., object detection – amount of garbage per square kilometer of sea surface). First, the aerial photos were analyzed to detect floating plastic in 2D automatically. The automatic debris detection technique used adaptive thresholding combined with morphological operators. The second step applied CZMIL waveforms to transform the 2D estimates of plastic abundance into 3D estimates of plastic volume (e.g., m3 of plastic per km2 of sea surface) and included the bathymetric lidar point cloud for progression to a 3D debris map.
The results of the program tests in this expedition opened up the possibility of using the Optech CZMIL for the effective assessment of the vertical distribution of marine debris in the water column. The data-fusion approach produced the first detailed 3D visualization of several pieces of debris within the GPGP for post-expedition research. The information obtained also provides the basis for development of an optical model of plastic debris (distribution along the water surface and at depth, size distribution of plastic particles, and reflectance range), which will be used for the theoretical prediction of garbage accumulation and distribution.