Putting PIESs in a Greenland Fjord
Our team has been using pressure-sensor equipped inverted echo sounders (PIESs) to study the heat content and iceberg dynamics in Sermilik Fjord. A PIES rests in a rigid anchor stand on the seafloor (well out of reach of iceberg keels) and emits 12 kHz pings, typically at a rate of 24 per hour (in bursts of 4 at ten-minute intervals). The PIES listens for each ping’s first echo and records the round-trip acoustic travel time. In addition PIESs measure bottom pressure and bottom temperature.
Part of a typical PIES travel-time record from Sermilik Fjord showing reflections from the air-sea interface and from several iceberg keels.
Interpreting PIES Data
In ice-free environments this first echo usually comes from the air-sea interface directly above the instrument. In this case, the bottom-to-surface round-trip acoustic-travel-time serves as an excellent proxy for the heat content (vertically-integrated temperature) of the overlying water column. This is due to the temperature dependence (and to a lesser extent salinity dependence) of sound speed in seawater.
Icebergs and Sea Ice
If ice is present, the first echo may reflect from the seawater-ice interface rather than the air-sea interface. In this case, the PIES is often able to track the iceberg keel as it transits through the PIES’s field of view.
When land-fast sea ice is present over the PIES, the standard deviation of the 4 pings in a burst is typically very low since the ice suppresses surface gravity waves. Sermilik Fjord is typically open (not covered by land-fast sea ice), but our mid-fjord PIES detected a 2-week period of sea ice cover in March 2012.