The RHUM-RUM experiment was one of the first deployments of German Ocean-Bottom-Seismometers, where scientists were especially interested in interpreting the broadband displacement signal of the sea floor. Previous experiments had mainly worked with picked arrival times, where the shortest periods are of main interest. So we were about the first ones to correct the seismograms for instrument response and calculate a spectrum of the ground motion:
The big surprise it the almost linear rise in signal amplitude beyond 10s period (below 0.1 Hz frequency). This is not an earthquake signal, but probably unusable noise. We would have expected the spectrum to look similar to the one of a french instrument:
Here, the noise is below the "new high noise model" (upper black line) over a wide period range. The fact that the German OBS have such a high noise level on longer periods is problematic for us, since we wanted to measure seismograms over a wide period range to measure the dispersion of the seismic signal.
Why the noise?
There has been much speculation over the reasons for this poor performance of the German OBS. An initial suspect was the fact that the seismometer is integrated into the OBS frame for the German instruments (left), while it stands separately for the French ones (right):
The question whether seismometers should be integrated into the OBS frame or not has long been discussed in the OBS community, so a lot of people jumped onto our initial conclusion.
However, for several reasons, we rejected that initial conclusion for reasons that will be discussed in a later blog post. You can read them now in the paper:
Stähler, S. C., Sigloch, K., Hosseini, K., Crawford, W. C., Barruol, G., Schmidt-Aursch, M. C., Tsekhmistrenko, M., Scholz, J.-R., Mazzullo, A., and Deen, M.: Performance report of the RHUM-RUM ocean bottom seismometer network around La Réunion, western Indian Ocean, Adv. Geosci., 41, 43-63, doi:10.5194/adgeo-41-43-2016, 2016.