Oceanographic and Geophysical Surveys

The Ocean Floor Mapping System Using Sea Beam
Sea Floor Seismic Reflection Soundings (Generation of a Local Velocity Model)
Seismic Monitoring (The Generation of a Regional Velocity Model)
Seismic Activity and Monitoring within the Kos - Yali - Nisyros - Tilos Volcanic Field
Gravity and magnetic surveys within the Kos - Yali - Nisyros - Tilos volcanic field
The GPS network

Sea Floor Seismic Reflection Soundings (Generation of a Local Velocity Model)

Deep seismic soundings and active tomography provided the data needed for developing a velocity-depth model of the volcanic structure, the crust and the upper mantle. This could only be achieved by sub-sea floor seismic investigations, which is a new method used for the first time world-wide.

Tomographic inversion and modeling of the magmatic intrusions and the sedimentary and crustal structures in 2-D and 3-D has resolved the velocity structures at shallow depth and provided the means for a 3-D block presentation of the subvolcanic intrusions and their spatial distribution.

Gravity data constrained by the velocity models in 2-D and 3-D has been used to develop a density model of the volcanic structures, which has been converted in lithostatic pressure maps as functions of depth.

Ocean Bottom Seismographs (OBS)

The installation of ocean bottom seismographs (OBS) on the sea floor for seismic recording yielded unique, one of a kind information. Ocean bottom seismographs (OBS) for this experiment not only correspond to the latest state of the art but are technologically unique world-wide and allow very accurate active seismic experiments if deployed in a dense network.

They have a capacity of 4,2 gigabyte, operate for one month under any water depths and environmental conditions, and are retrieved by acoustic or time release mechanisms (Figs. 8 and 9).

They record seismic signals using three geophones and one hydrophone and already have been widely deployed for marine seismological observations as well as active seismic experiments. This knowledge can be used for further technological developments and for application in seismic sub-sea floor investigations.

Fig. 8 Ocean bottom seismograph (OBS). GeoPro GmbH Hamburg (GeoPro)
(Click on image to enlarge).

Fig. 9 Deployment of OBS from R/V AEGAEO
(Click on image to enlarge).

Fig. 10 Ship tracks of "Wide Aperture Reflection/Refraction Profiling (WARRP) seismic surveys" and deployment of OBS within the Kos-Yali-Nisyros-Tilos volcanic field. University of Hamburg (UHIG), GeoPro, NCMR and NOAIG
(Click on image to enlarge).

Wide Aperture Reflection/Refraction Profiling (WARRP)

Three Wide Aperture Reflection/Refraction Profiling (WARRP) seismic surveys were carried out within the GEOWARN project, using 36 OBS (Ocean Bottom Seismographs) and 22 seismic land stations on the islands of Astipalea, Kalimnos, Pserimos, Kos, Nisyros, Tilos, Simi and Rhodos. 7,000 airgun-shots were fired along 8 profiles, along a total length of 1,000 km and covering the area of 120 x 120 km for the purpose of obtaining a 3D tomography model.

OBS spacing was in the range of 12 km to 14 km. A 49 litre airgun (Fig. 11) array was used as the main energy source, with a shot spacing of 130 metres.

Fig. 11 The 49 litre airgun for "Wide Aperture Reflection/Refraction Profiling (WARRP)"
(Click on image to enlarge).

A total of 42,000 seismograms were recorded. The data recovered form OBS and land stations were extracted, processed and written in SEG-Y format on DAT tapes.

The modeling sequence involved picking and inversion of first arrivals, definition of interfaces, identification of picks, non-linear inversion and kinematic and dynamic forward modeling to produce a final model. Five velocity models were compiled along different profiles, permitting tomographic modeling and interpretation (Figs. 12, 13 and 14).

Fig. 12 Local velocity model down to 15km depth, shown as a tomographic cross-section of the Kos-Yali-Nisyros volcanic field. University of Hamburg (UHIG) and GeoPro
(Click on image to enlarge).

Fig. 13 Tomographic cross-section through Nisyros volcanic island down to 6,000 m. New "Tomographic Software Tool" developed by the Institute of Cartography, Swiss Federal Institute of Technology Zurich (ETHZ)
(Click on image to enlarge).

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