The development of GPS technology in the past 2 decades has produced a revolution in the scientific community’s ability to monitor deformation of Earth’s surface, including providing direct measurements of the deformation leading up to large plate boundary earthquakes. The radio waves carrying GPS data, however, do not penetrate into the ocean, and many of the most dangerous plate boundaries are located beneath the sea. Development of seafloor geodesy is therefore a high priority for geoscientists, and several different approaches are currently in development.
In December 2015, 23 acoustic geodetic seafloor transponders were successfully installed on the seafloor during GEOMAR’s R/V Sonne cruise SO244. These seafloor transponders are located in 3 arrays to form GeoSEA (Geodetic Earthquake Observatory on the SEAfloor). GeoSEA’s target is the segment of the Nazca-South American plate boundary near 21°S that is in the seismic gap left after partial rupture of much larger seismic gap in 2014 (see PICTURES scientific objectives). Array 1 on the middle continental slope consists of 8 transponders located in pairs on four topographic ridges, which are surface expressions of faults at depth. Array 2 is located on the outer rise seaward of the trench, where 5 stations monitor extension across plate-bending related normal faults. Array 3 is located on the lower continental slope where an array of 10 stations measures diffuse strain build-up.
The seafloor acoustic ranging methods provide relative positioning by using precision acoustic transponders (Autonomous Monitoring Transponder, AMT) that include: pressure sensors to monitor possible vertical movements as well as provide data to correct for tides; tiltmeters in order to measure changes in inclination; and sound velocity (SV) sensors to correct for sound speed variations in the water column. Data are stored internally and can be uploaded to either a High Performance Transducer (HPT) lowered from the side of a ship or an autonomous vehicle developed by Liquid Robotics and controlled via satellite that uses wave action for forward propulsion (the GeoSURF Wave Glider). PICTURES provided the first opportunity to upload data since installation of the array. Array 1 and Array 2 are located within the PICTURES footprint, and data were uploaded to the HPT. The waveglider was deployed to upload data from Area 2. However, although the Wave Glider was able to travel to Array 2 and obeys commands from its operator, communications turned out to be too slow for practical data upload. A future upgrade to the communications system is expected to solve this problem, but in the meantime, we have decided to transit 5 hours to Array 2 to upload data from that site using the HPT if we have unused contingency time near the end of the cruise.
Figure 1: The High Performance Transducer lowered from the side of the R/V Marcus G. Langseth. Photo by Jan Steffen.
Figure 2: The Liquid Robotics GeoSURF Wave Glider. The Wave Glider has two main parts: a float, which contains all sensors and communication units, and a subsurface wing rack, which is connected to the float by a 6-m long flexible umbilical tether. Directional control is accomplished with a rudder on the Glider sub unit. The float is equipped with satellite communication systems (Iridium Satellite LLC) for remote transmission of data, a GPS unit, and a weather station. It also contains batteries that are recharged by a solar panel to provide power at night. The Wave Glider periodically transmits is position via satellite to the “watchkeeper.” During PICTURES, the “watchkeepers” have had to be vigilant to command the Wave Glider (nicknamed “SUZI”) to stay out of the path of other ships. We will be recovering SUZI before returning to port.
– contributed by Florian Petersen, November 2016