Wednesday afternoon, October 19th – Off the coast of Washington at the edge of the continental margin
We just deployed another Cascadia seismometer. It will take 65 minutes to reach the ocean floor, 2630 meters below. It will take another hour to conduct the acoustic survey, as the Wecoma cruises in a kilometer and ½ circle overhead.
The weather has deteriorated somewhat, but is still serviceable. We deployed two seismometer last night and another one this morning. Since we are in deeper water, it takes longer for the seismometer to reach the bottom. A acoustic survey is immediately conducted to determine its exact location in three dimensions; latitude, longitude and depth. The Wecoma circles the seismometer location as the acoustic release sends signals to the ship.
We have traveled north to test two Cascadia seismometers that were deployed earlier during the first leg during July. A specific frequency of sound (between 9 and 13 kilohertz) is sent down to check if the instrument package is working. Sometimes a fishing trawl can flip over the seismometer, rending it useless. If the instrument is working, it responds with a series of audio chirps that are picked up here on the Wecoma.
So far we have deployed five Cascadia and four Abalone seismometers. Another five are scheduled for deployment through the night.
CTD stands for conductivity, temperature and depth. These terms refer to the changing seawater characteristics that the CTD array encounters as it descends. It electrically sends continuous measurements back to the ship.
Conductivity is an electrical measurement of salinity. The salinity varies near the surface, but is a reliable 34 to 35 parts per thousand for most of the descent. The seawater temperature drops to about 3 degrees Celsius, about 37 degrees Fahrenheit.
We dropped the cabled CTD to just above the seafloor depth of 1500 meters (4921 feet). The pressure increases about one atmosphere for every 10 meters (or ~33 feet) of depth. We attached to the CTD array two nylon bags filled with Styrofoam cups, decorated by students from the Lincoln County School District. When subjected to pressure 150 times greater than standard atmospheric, the air is squeezed out from the Styrofoam.
Yesterday, we deployed three seismometers, two “Abalones” from Scripps Institute of Oceanography with the trawl resistant design and one from the Lamont-Doherty Earth Observatory (LDEO) called “Cascadian” which also has an absolute pressure gauge and hydrophone. These seismometers are generally deployed deeper along the continental margin.
There are two sets of Ocean Bottom Seismometers (OBS), 10 were built by Lamont-Doherty Earth Observatory (LDEO) and include pressure gauges. The pressure gauges are for sensing vertical seafloor uplift and will be deployed on the edge of the continental margin to detect strain accumulation between the North American and Juan de Fuca plates. The remaining 15 OBS were built by Scripps Institute of Oceanography and are designed to be trawl resistant. Called “Abalones” they are pyramidal shaped and can be deployed from 100 m to 6000 meters depth.