HOBITSS2:

Last night we towed a buoy behind the ship to make GPS-Acoustic measurements. Here is a description of the method from Kido et al., Geophysical Research Letters, 2011:

“The GPS/acoustic technique, which was proposed and

developed by the Scripps Institution of Oceanography

[Spiess , 1985], consists of two essential components: the

kinematic GPS of a surface platform, and acoustic ranging

between the surface platform and an array of seafloor

transponders. As introduced by Kido et al.  [2006], we used a

towed buoy navigated near the center of the transponder

array as the surface platform during a campaign (hereafter

called a stationary survey). The shape of the transponder

array is assumed to be rigid through repeated campaigns

over the years because of its relatively short baseline. With

the assumptions that sound speed in the ocean is laterally

stratified and that the seafloor moves only in the horizontal

direction, the horizontal position of the array and the mean

sound speed from the sea-surface to the bottom can be

determined with a single acoustic ping of simultaneous

ranging to all the transponders [Kido et al. , 2008]. The

temporal position of the array is then obtained as a time

series for thousands of pings. The final position of the array

in a campaign is determined by averaging the fluctuations in

these time series, which are presumably due to undesired

spatio-temporal variation in the sound speed beyond the

stratified approximation. The resultant accuracy of array

positioning in a single campaign is typically 5 cmdepending

on the ocean conditions and duration of the survey.”

With this method we can make very precise measurements of a position on the seafloor, and by repeating measurements over several years and comparing them with sites on land, can measure the motions of the tectonic plates and motions associated with large earthquakes. Similar seafloor GPS-acoustic measurements were made before and after the 2011 Japan Tohoku-Oki earthquake, and found displacements of the seafloor by 31 meters horizontally, which is huge.