Seafloor Geodesy
Key research question: Can we establish underwater a high accuracy network of geodetic control?
The Earth’s tectonic plates are separated by faults and are constantly grinding and scraping against one another at slow moving rates that are measured at a millimeters-per-year (mm/yr) scale. We only feel this motion when the energy locked between two plates is released as moderate earth tremors, or sometimes as earthquakes. The deformation process prior to the release of energy is defined by geologists as “strain.” For plate boundaries over land, like the San Andreas Fault system, it is possible to use leveling and Global Navigation Satellite Systems (GNSS) stations on land that can measure surface deformation to an accuracy of a few millimeters per year (mm/yr). However, many of the tectonic plate boundaries are underwater. As such, NOAA’s National Geodetic Survey is collaborating with academic partners to learn more about different GNSS-Acoustics (GNSS-A) approaches that can monitor three-dimensional changes of the seafloor at the several centimeter level, also known as “Seafloor Geodesy.” Outcomes of these studies will produce a better understanding of the surface deformation of plate boundaries and their rates of change in a three dimensional space. This information can be used as a source layer in advance warning systems for earthquake and tsunami monitoring programs.
The radio frequencies used by GNSS are strongly attenuated by seawater. As an alternative way to monitor land deformation underwater, acoustic transponders are deployed to the seafloor and become underwater “active benchmarks,” transferring ranging and location information from the seafloor to the sea surface. Over the years, there have been several configurations tested for seafloor geodesy using different water surface platforms with a GNSS receiver, such as ships, autonomous (uncrewed) marine systems, and buoys. The GNSS receivers on the surface platform statistically calculated the position of the transponder on the seafloor and is able to reference it to a global reference system. Some configurations include the deployment of several acoustic transponders placed relatively close to each over (hundred of meters to several kilometers). Periodic surveys (typically, once or twice per year) involving ranging between the sea floor transponders and a GNSS-equipped acoustic receiver on the surface define the position and time-averaged displacement of each transponder. NOAA’s National Geodetic Survey is currently investigating approaches to extend geodetic land networks into the ocean. Potential outcomes are providing better regional geodetic control and the ability to define plate boundaries, and developing better deformation models as a dynamic component for the US geospatial infrastructure.
