Tuesday, September 30, 2014

Working hard

September 30, 2014

It takes a team of people to get the OBS in the water and back out again. To illustrate the process of deploying a WHOI or SIO OBS, Gary Linkevich has created a time lapse video. The first part of the video captures two WHOI OBS deployments with Peter, Dave, Dylan, Gary, and Kate. The WHOI OBS are the peanut shaped yellow capsules that appear in the background next to the railing. After the WHOI OBS is in the water, we capture an SIO OBS deployment with Mark, Dylan, Gary and Kate. The SIO OBS are the rectangles with a yellow top and white base. Right after we deploy the SIO OBS, we start putting together a new one for deployment. The assembly process involves an instrument test and then attachment of the metal weight, floatation devices, light, and radio together. The deployment of this SIO OBS happened during the midnight crew shift which includes Ernie, Pamela, Afshin and Jenny. Once they pick her up and put her in, they start the assembly process all over again!


Thanks Gary for putting together this time lapse!

See you Later,

Kate Volk aboard the R/V Endeavor

Running the R/V Endeavor

September 30, 2014

One of our assistant engineer, Kurt Rethorn, gave us a tour of the engine room. Here's what we learned:

Kurt is an awesome tour guide!

The Engine
The R/V Endeavor is equipped with a two-stroke (providing more power strokes per engine rpm), diesel engine consisting of 16, 350-cubic inch, cylinders with a maximum output of 3050 horsepower. Also, this bad boy is outfitted with a turbo charger which uses the exhaust to increase pressure in the cylinders and improve the power output from the combustion stroke. The engine is kept lubricated by 500 gallons of motor oil, which is changed when the ship is in port based on the number of engine hours. We burn around 1,000 gallons of fuel a day while on station (between the generators and the main engine) and even more when we are in transit between sites. We left port with around 54,000 gallons of fuel stored beneath the berthing decks, but she can hold up to 56,000 gallons of fuel total (the additional space is left to allow for the expansion of fuel due to temperature changes). A fun fact about the R/V Endeavor is that the propeller only spins one direction, meaning that there is no reverse gear. In order to drive the ship in reverse, the pitch of the propeller blades is switched such that the flow of water is reversed. There is also a powerful bow thruster that can be engaged if necessary.

Kurt starting off the tour (Photo credit: Kate Volk)
The engine with the exhaust manifold above and access to each of the piston heads underneath the latched doors (Photo credit: Kate Volk)
Fuel gauges. Fuel is consumed from both tanks at a relatively similar rate in order to keep the boat properly balanced (Photo credit: Kate Volk)
The Generators
The R/V Endeavor has four generators. Three below the water line and one above (for emergencies only). The generators produce all our electricity directly (i.e. they do not charge any batteries). In a power failure, an emergency generator will kick on in less than a minute.

These are two of the generators, aligned along the centerline of the ship (Photo credit: Kate Volk)
We use about 1000 gallons of water a day between showering, cooking, cleaning, and drinking and the ship can only hold approximately 8600 gallons of fresh water. Therefore, we must produce fresh water throughout the cruise and we have two methods of achieving that. We have a reverse osmosis machine, which takes up salt water and pushes it through a long, blue semi-permeable membrane at 800 psi. The high pressure in the membrane causes the salt to drop out of solution producing fresh water. The second way we have of producing water is an evaporator. This brings in salt water under a vacuum at 711 mmHg (13.75 psi). The water is heated up and the condensation is collected, now free of salt. The reverse osmosis machine  and evaporator can produce around 50 and 80 gallons of water per hour, respectively, so that we can theoretically make 3200 gallons of water everyday. However, the evaporative method is dependent upon the engine heat to turn the water into vapor, which means that it runs at a lower efficiency when the engine is cooler. 

Reverse Osmosis machine. You can see the blue membranes that separates the salt and water (Photo credit: Kate Volk)

Water quality (Photo credit: Kate Volk)
Sea water temperatures in the Gulf Stream are pretty warm (Photo credit: Kate Volk)

See you later, 

Dylan Meyer and Kate Volk aboard the R/V Endeavor

First Look at OBS Data!

September 30th, 2014:

We have our first look at data!! Ernie Aaron merged some of the raw data from the Scripps OBSs with navigation files from the R/V Marcus Langseth such that we can start seeing the seismic waves recorded in the in ENAM project.  In Figure 1, the hydrophone record of OBS 209, which was recovered on Sept. 21st, is shown as a function of space and time. To be clear, this is original seismic data. There are still post-processing methods and inversions to apply to the data back on shore that will help extract the seismic velocity structure down to upper mantle boundary along Line 2 or any of the other seismic lines. Until then, however, here is what we learned thus far.

To remind all, the experimental setup for this study is as follows. We on the R/V Endeavor placed OBSs on the seafloor at a spacing of approximately 15 km along Line 2. The R/V Langseth then cruised along Line 2 from ESE to WNW with airgun shots spaced every 200 meters. The OBSs were then recovered and the hydrophone and geophone data were downloaded.

Figure 1. Traces recorded from OBS 209 (bottom) with various arrivals identified by color. The dashed line shows the multiple of Slope D. Cartoon (top) shows representative raypaths of seismic waves that produced the arrivals indicated in the trace records (Figure Credit: Kate Volk).
The acoustic signal was then segmented into separate traces using the GPS-coordinated time of each shot. Ten seconds of each trace were then plotted, by shot number (Figure 1). In this data panel we see the direct wave from the R/V Langseth shots to the instrument (Figure 1; Slope B), seismic reflections and refractions from the Earth below (Figure 1; Slopes A, C, and D), and a later multiple of these seismic refractions (Figure 1; dashed magenta line), after they bounced between the seafloor and sea surface.  

The direct wave travels directly from the seismic source to the OBS, helping us identify the location of the OBS on the seismic line. Using the time it took for the direct arrival to reach the OBS at this location and the acoustic velocity of water (1500 m/s), we can estimate the depth to the OBS. In the case of OBS 209, the R/V Langseth traveled over the device around shot 2200 and it was deployed in approximately 3000 meters of water (Figure 1).

The general slope of the seismic refractions in the space-time diagram gives an indication of the speed at which these seismic waves travelled at large depth.  The data in Figure 1 have been plotted such that waves traveling with a seismic velocity of 7000 m/s, such as those turning near the crust-mantle boundary, will appear as flat events. Slower seismic waves will dip towards larger time away from the OBS, while faster waves will slope towards smaller travel times.  

The OBSs show seismic arrivals that are recorded over a very wide range of source-receiver distances. The seismic waves recorded close to the instrument (< 10 km), are the direct wave from the airguns through the water column to the instrument (Figure 1; Time 2). As you move farther from the instrument (longer offset from source to OBS), the seismic waves move through deeper materials with faster acoustic velocities and those waves reach the instrument before the direct waves (Figure 1; Times 1 and 3). At longer offsets, the primary response comes from seismic waves that travel along deep materials with very fast seismic velocities (Figure 1; Time 4). When combining all the traces together, the slope between similar acoustic responses in traces can be used to infer the seismic velocity of the seismic wave, which can be used to infer the properties of the Earth.

For example, between 60 – 80 km and 100 – 120 km, we identify acoustic responses that are relatively flat (Figure 1; Slope D), indicating that the sound wave is moving through material with an acoustic velocity of 7000 m/s. This is important because it confirms that we are imaging down to the crust-mantle boundary, which will allow us to get a well-constrained seismic velocity profile throughout the crust beneath the margin of the US East Coast.

Until next time,
Dylan Meyer aboard the R/V Endeavor