Harald Westerdahl

Multicomponent ocean bottom and vertical cable seismic acquisition for wavefield reconstruction

In ocean-bottom seismic and vertical-cable surveying, receiver stations are stationary on the sea floor while a source vessel shoots on a predetermined x-y grid on the sea surface. To reduce exploration cost, the shot point interval often is so coarse that the data recorded at a given receiver station are undersampled and thus irrecoverably aliased. However, when the pressure field and its x - and y -derivatives are measured in the water column, the nonaliased pressure field can be reconstructed by interpolation. Likewise, if the vertical component of the particle velocity (or acceleration) and its x - and y -derivatives are measured, then this component can also be reconstructed by interpolation. The interpolation scheme can be any scheme that reconstructs the field from its sampled values and sampled derivatives. In the case that the two fields’ first-order derivatives are recorded, the total number of components is six. When also their second-order derivatives are measured, the number of components is ...

Development and testing of a prototype seabed coupled shear wave vibrator

Summary A prototype seabed coupled seismic vibrator source has been developed at the Norwegian Geotechnical Institute (NGI). The suction anchor based electro-hydraulic source was tested in June 2003 at five different locations, penetrating the 2.5m diameter skirt more than 1meter into the sandy seabed at the Gullfaks field test site. The 9 tons heavy source was run in sweep mode, 2-45Hz, with a dynamic horizontal ground force amplitude of 100kN, and in pulse mode at one location. Different orientations, both radial and azimuthal polarization were tested. The test was conducted within a 24 hours window during a conventional 4C seabed seismic survey. The OBC seismic service company recorded the seismic signals produced by the shear wave source on four 4C ocean bottom cables. The test showed that the suction anchor based shear wave source concept is operationally feasible and promising in the context of generating polarized S-wave in a survey where a limited number of shot points are needed, for example as add-on in a 4C seabed seismic survey, for local monitoring in instrumented oil fields or for shear wave VSP. Preliminary data analyses show that the source generates very little P-wave “noise”, and events interpreted as S-S reflections can be seen in some of the shot gathers down to about 3-4 seconds (TWT). In the deeper part of the section, continuous S-S reflections cannot be easily identified. The 2D-PS-section generated from the conventional 4C data also indicates very low reflectivity and lack of continuous reflectors at the deeper part of the section. The vertical resolution of the S-S data seems similar or better than the 2D-PS data in the shallower parts where it is possible to correlate the two data sets. As expected, the data quality varies between shot locations as the seabed properties vary, and the best quality is obtained in the stiffer sites. The repeatability is excellent for the stiffer sites, from first shot to last. At the softer sites, a few shots are needed before the source signature stabilizes. A list of actions for improvements is identified and will be pursued in continuing development work.

Ocean Bottom Node Seismic - Present Challenges And Future Solutions

Statoil has since 1997 acquired an extensive catalogue of 3D Ocean Bottom Seismic (OBS) surveys on the Norwegian and British continental shelves (Thompson et al., 2007) where the acquisition geometries of these surveys have encompassed both wide and full azimuth concepts. These surveys have primarily been cable based, but Statoil also has experience from node based acquisition from the Volve field, through the IMPREDO project and later at Heidrun where surveys incorporating both cables and nodes concurrently were carried out. Through geometric emulation and direct observation of node data trace density and proper source sampling are identified as challenges for node acquisition. A new interpolation technique is identified that has the potential to solve some of the sampling issues for node technology in the future.

Multicomponent OBS and VC acquisition for wavefield reconstruction

In ocean-bottom seismic (OBS) and vertical-cable (VC) surveying, receiver stations are stationary on the sea floor while a source vessel shoots on a predetermined x− y grid on the sea surface. To reduce exploration cost, the shot point interval often is so coarse that the data recorded at a given receiver station are undersampled and thus irrecoverably aliased. However, when the pressure field and its xand y-derivatives are measured in the water column, the non-aliased pressure field can be reconstructed by interpolation. Likewise, if the vertical component of the particle velocity (or acceleration) and its xand y-derivatives are measured, then also this component can be reconstructed by interpolation. The interpolation scheme can be any scheme that reconstructs the field from its sampled values and sampled derivatives. In the case that the two field’s first-order derivatives are recorded the number of components are six. When also their second-order derivatives are measured, the number of components is ten. The properly interpolated measurements of pressure and vertical component of particle velocity from the multicomponent measurements allow proper up/down wavefield decomposition, or deghosting. New wavefield reconstruction methods as those suggested here are of significant interest since, presently, the seismic industry is in the process of developing multicomponent cables or streamers, and is in the process of carrying out research on new multicomponent sensors.