The OBS noise due to deep ocean currents
- 1Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal
- 2CEIIA, Av. D. Afonso Henriques, 1825, 4450-017 Matosinhos, Portugal
- 1Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal
- 2CEIIA, Av. D. Afonso Henriques, 1825, 4450-017 Matosinhos, Portugal
Abstract. Ocean bottom seismometers (OBS) are usually deployed for seismological investigations but these objectives are impaired by noise resulting from ocean environment. We split the OBS recorded seismic noise into three domains, short-period, microseisms and long-period, also known as tilt-noise. We show that the first and third domains are controlled by bottom currents but these are not always a function of the tidal forcing. Instead we suggest that the ocean bottom has a flow regime resulting from two possible contributions, the permanent low frequency bottom current and the tidal current. The recorded noise displays the balance between these two currents along the full tidal cycle, between neap and spring tides. In the short-period noise band the ocean current generates harmonic tremors that corrupt the dataset records. We show that, in the analyzed cases, the harmonic tremors result from the interaction between the ocean current and mechanical elements of the OBS that are not essential for sea bottom recording and thus have no geological origin. The data from a new Broadband OBS type, designed and built at Instituto Dom Luiz (University of Lisbon)/CEIIA, hiding no essential components from current flow, shows how utmost of the harmonic noise can be eliminated.
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Carlos Corela et al.
Status: open (until 28 Aug 2022)
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RC1: 'Comment on nhess-2022-196', Wolfram Geissler, 30 Jul 2022
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Dear authors,
thank you for sharing your interesting results and innovations about the influence of environmental/oceanic parameters and technical limitations in the acquisition of ocean-bottom seismological data and ways to improve the quality of the recordings. The manuscript provide a highly valuable documentation how technical developments can lower the impact of instrument-generated noise on one side, but keep the handling of the instruments at the open sea still easy enough. Your contribution focuses on two important frequency bands that are influenced by instruemt-generated noise, that is originally caused by the action of deep geostrophic ocean bottom currents and temporally varying currents forced by the tides.
I think the manuscript needs some moderate revisions before it should get published. I provide most of the comments, questions and suggestions in the annotated manuscript. I will list the most important ones below. I hope they are helpful and constructive to improve the manuscript.
General comments:
The focus of the manuscript is on the noise that is generated by imperfect ocean bottom seismometers that were and still are used, and how to design better instruments to reduce that noise. I think you provide clear evidence that the new instrument designed at IDL has a much better performance regarding the noise impact of bottom and tidal currents. You should better focus on this aspect throughout the manuscript. It sometimes reads still like two separate manuscripts about the noise recorded previously with the standard OBS and the newly designed OBS. In that sense also, the title of the manuscript does not fully meet the content yet.
The manuscript needs in parts some restructuring. I would move several paragraphs to other chapters, e.g., lines 135-145 reads to me more like Introduction but not Data and Methods. Maybe the complete section 3 (Results and discussion) needs a better structure. Also results and discussion could/should be separated.
Also the model of Voet et al. (2020), which seems to be a fundamental base for the study should be explained in more detail. You take the amplitude of the noise in certain frequency/period bands as a proxy for current speed. I do not fully get that yet from the manuscript. Also, you should indicate later in the manuscript that you always interpret the proxy but not directly measured current speeds!
You discuss the influence on permanent geostrophic and tidal components of bottom currents. However, you never introduce the existing knowledge of currents in the study region. At least a map and short introduction to existing knowlegde on bottom (and tidal) currents is necessary as a base for your interpretations.
The instrument-sediment coupling was not at all explained and discussed but appears in the conclusions. Either extend that topic or leave it out.
Could you a bit more elaborate on the differences between NT OBS 01 and NT OBS03, with respect to their different locations (topography, bottom current regime).
Specific comments:
I miss a short statement about potential effects caused by the use of different types of seismometers.
Use abbreviations only, if you really need them. And if you use abbreviations all should be explained within the manuscript text and within the figure captions, separately.
How do you know about the noise frequencies caused by the antennas? Why the noise is not excited by the radio and flash light beacons?
What are "flow patterns" to you? (e.g., line 175)
What is a "energetic phenomenon"? (Line 221)
The language should be checked carefully at the end.
The Conclusions should be shortened. (but the discussion could be extended)
I do not fully get how you judge on flow directions. Can you also discuss briefly the influence of the distance of the studied stations to your tide reference station? Could you also show examples of stations that are not influenced that strong? Why these stations provide less noisy data? Is it all related to the currents or could there be other causes of noise (tremors etc.)?
How the flow gets turbulent? Is it the complete flow that gets turbulent, or do just the OBS components cause turbulences at a certain flow speed?
The figure should be marked with important information discussed in the manuscript. I put my specific comments next to the figures. It would be great if you could fully describe the plots in the captions or the manuscript text. There are sometimes features, that are not clear to the reader (related to the discussed problem or just something else not relevant to the study).
Why do you use different color scales throughout the manuscript?
Be consistent in using terms like "band" or "domain". (e.g., line 266)
With best regards, Wolfram Geissler
Carlos Corela et al.
Carlos Corela et al.
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