Tuesday, September 17, 2013

Seismic Surveys

Ok. Whew. Busy couple of days. Last night the A-Team (some of whom are
at this very moment watching the disappointing movie remake of their
namesake tv show [full disclosure: so am I]) deployed the full monty
of cores, including the rare and elusive casting core, which we'll let
them tell you about later. Yesterday we spent most of the day shooting
a seismic survey for an IODP proposal, which is of course the reason
we're actually here. This is a much lower-key process than coring, in
that we spend most of the time monitoring the survey from a bank of
computers in a nice, air conditioned lab. This doesn't make it any
less complicated or nerve-wracking, however. The last couple sites we
surveyed didn't fit the bill.

First we have to run a multi-beam sonar survey of the ocean bottom. We
want to find a spot where the seafloor is flat, which suggests
flat-lying sediments. Multi-beam sonar is pretty cool, and provides a
detailed bathymetric map of the seafloor (bathymetry is like
topography, but underwater). The map at the top shows the bathymetry
of our survey site yesterday; warm colors are high seafloor (shallower
water) and cool colors are low seafloor (deeper water). On the right
side of the image you can see a broad flat area with our ship track
crossing it in a hatch pattern. That flat area looks like an
attractive drilling target, so we deployed our seismic gear. This is a
long process that mostly involves several of us standing on the fan
tail helping the seismic streamer through the block as it unspools and
attaching "birds," computer-controlled devices with dive planes that
can keep the streamer at a constant depth, which is important when
processing the data. Finally we lower the air guns into the water, and
we're ready to go.

Maybe we should back up and explain how/what seismic survey is. A
seismic survey uses sound to image the sediments under the sea floor.
As you can clearly see in the text-book quality diagram above, we
shoot off an air gun in the water at a set interval of time, and the
sound waves move through the water column and bounce off any surface
where the sonic impedance changes. A long stream full of hydrophones
trailing behind the ship picks up the reflected sound waves. If we
know the interval between the sound waves and the velocity of the
sound through the water column and through the sediment, we can
accurately map features below the sea floor. Some waves bounce off the
sea floor; some of the waves continue through the sediment and are
reflected at places where the sediment changes. These are usually
bedding planes. Seismic can resolve geologic features at a scale of
10s of meters, so large channel fills, onlapping or cross-cutting
bedding, and faults are all features that readily pop out. These are
all bad for scientific ocean drilling. We want flat-lying, boring
sediments with no erosional features so we can get uninterrupted
records of changes through time. This is the opposite of what you'd
look for in petroleum exploration, where complicated stratigraphy can
form traps for oil that make attractive drilling targets.

In the past few days we have completed two seismic surveys. The first
survey was disappointing at first, as it revealed that there was very
little sediment. The reflectors indicated that there was only a little
more than 50 m of sediment bounded below by a large reflector of
basement or volcanic rocks. However, as the survey continued the
sediments began to get deeper, and we ended the first survey with
about 150 m of flat layered sediment. The sediment was good enough for
coring at this location. One problem with this survey was that towards
the end, the marine mammal observer spotted whales close to the ship
(pilot whales and spinner dolphins, I think). In order to shoot
seismics there needs to be at least one observer to make sure that
marine mammals are not harmed by the loud noise the air guns create.
If a marine mammal is spotted within a certain distance of the ship,
the guns must be shut down, as occurred here. After the whales passed,
we were allowed to resume shooting. On top of that problem, the
compressors that create the air for the air gun broke down, which
meant that we had to shut shooting down again and wait for the
pressure to increase. The engineers have been working night and day to
keep the compressors going, yet the compressors seem to be the largest
problem with collecting data so far. Despite the ups and downs of the
first seismic survey, the second seismic survey proved to be a much
better success. There were points where we were able to see almost a
kilometer of sediment, which is great for coring! All aboard were very
happy with the second collection of data. Chief scientist Greg
Mountain even went as far as describing the incoming data as beautiful
and gorgeous! That means it must be good. Hopefully we will have the
same if not better success at our next four seismic survey sites. As
long as the compressors hold up, we should be ready to shoot!

1) multibeam sonar bathymetry of the survey area (note ship track with
crossing lines)
2) super awesome and well-drawn cartoon showing how seismic works

Posted by Chris and Kim