29 October 2013, Karin Sigloch
Swell on the open ocean (from last year’s cruise).
The tropical storm to our north was downgraded to “a tropical depression without development potential” – uplifting news to most of us. We did experience substantial winds and especially waves over the past days. Especially during meal times in the bow of the ship, we felt the blows to our stomachs.
Interestingly, the waves originated mostly not from the nearby storm but were “swell”, i.e., far-travelled wave trains generated by distant storms, mainly in the Southern Ocean. Experiencing the swell first hand is interesting to a seismologist because it causes most of what we consider “noise” in seismograms: unwanted signal that obscures “useful” features, like earthquakes. Through a non-linear coupling process that still awaits full explanation, the shallow water waves of the swell couple into the solid earth along coastlines and rough seafloor topography. Thus a small amount of water wave energy is converted into seismic waves that travel through the earth’s interior to seismological stations worldwide.
This “microseismic noise” is present on any kind of seismological recording, even in the middle of continents, but it is particularly pronounced on ocean-bottom sensors, which are sitting in the middle of the action, so to speak. As we have perused through our newly acquired data, it has been slightly sobering to realize the signal-processing challenge that this noise will present. The swell is a big nuisance to as seismologist, but going to sea can teach something like acceptance: if the swell is so real on my stomach, no wonder it is real on my data…