September 30th, 2014:
1437
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.
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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).
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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
