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|HE 279 /// HE 269 /// AL 278
HE 279 (October 2007)
Airgun Reflection seismic and Scholte-wave tomography analysis
In October 2007 an airgun reflection seismic survey was conducted aiming at the structure of the salt dome "Erika" which is located to the West of the FINO3 building site. The survey included parallel airgun profiles heading East-West seperated by 200m, two profiles heading North-South and six boomer-source reflection profiles to image the shallow intersection domain with high resolution.
Furthermore, the survey included a Scholte-wave tomography covering an area of 500m square around the location of future FINO3. Three specially constructed Ocean Bottom Seismometers were deployed for this measurement.
HE 269 (May 2007)
In May 2007 a further survey was conducted for seismic pre-site investigation. A hydrophone array was designed for the 3D seismic survey and was employed during the measurements together with an ocean bottom seismometer (OBS). The seismic survey was carried out with RV Heincke from May 20-25.
The purpose of this survey was to obtain a higher resolution of the planned FINO3 platform location (55° 11.7' N, 007° 09.5' E) using 3D seismics and Scholte wave tomography in order to verify geological subsurface structures and to resolve these on a smaller scale. Additionally, the survey was supposed to help determine the distribution of shear wave velocities at the future platform location. Another profile was shot across the planned FINO3 platform location using airgun and streamer to map deeper structures.
AL 278 (May 2006)
- Boomer reflection seismic
The structural geology in the immediate vicinity of the planned FINO3 platform location was investigated with a high resolution 2.5D reflection seismic survey. The sediment structure thus obtained not only helps giving a starting model for the Scholte wave inversion, but will also lead to a suggestion for the platform location.
The reflection seismic pre-site investigation was a standard single-channel boomer survey. A 2x2km area was mapped in north-south and east-west direction, with parallel profiles at 50m separation. Acquisition parameters were: 3Hz shot rate, 100kHz sampling rate and 200ms record length, using a 24-bitdata logger. Navigation data was acquired using a radio corrected single-frequency D-GPS system. Reference station was Heligoland. The GPS antenna was mounted midships.
- Physical parameter study using Scholte waves
The shallow subsurface shear wave velocity distribution of the FINO3 platform building site is investigated by exploiting the dispersive properties of surface waves. A low frequency airgun system towed behind RV Alkor was used to excite so called Scholte waves which are elliptically polarized waves, comparable to Rayleigh waves, except that wave energy is traveling in the water layer and the subsurface.
Scholte wave energy was recorded with Ocean-Bottom-Seismometers (OBS: 3C-geophones and hydrophone). Airgun shots were fired with approx. 5m shot separation along three different profiles. The profiles lie within the boundaries of the 2x2km area. Two OBSs were placed near each intersection point of the profiles. The data were compiled to build a common receiver gather for each OBS. From the data obtained one can invert the vertical shear wave velocity distribution of the subsurface.
The inversion is based on a comparison of observed phase slowness-frequency dispersion spectra with artificial spectra resulting from a shear wave velocity model (Bohlen et al. 2005). The inverted shear wave velocities will then be related to the shear stiffness of the soil. This first investigation also provides helpful information for further studies of the area. It gives for example information about the Scholte wave frequency content of the soil and provides data for a start model for further inversion of tomographic exploration in the vicinity of FINO3.
Created on 2006-07-04 13:24:39 by dwilken
Updated on 2007-12-11 11:54:55 by swoelz