Sea Technology

FEB 2014

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12 st / February 2014 www.sea-technology.com tems typical of most seismic explora- tion programs. The advent of ultrahigh-resolution digital multichannel systems, such as the GeoEel (2D) and P-Cable (3D) systems built by Geometrics (San Jose, California), has revolutionized seismic refection techniques for ma- rine engineering and construction ap- plications. These systems are among the highest-resolution and utilize the highest-fdelity offshore hydrophones available and offer immense fexibility for use in shallow-water environments and small areas of interest. The cables are smaller than previous generations of digital streamers, so they can be de- ployed from smaller vessels with shal- lower drafts that have lower operating costs. The exceptional signal-to-noise ratio also maximizes data quality when there are limits imposed on the maxi- mum source signal power (such as in California). Seismic refection data is not a di- rect observation of the physical subsur- face, but rather a measurement of the two-way travel time of seismic energy to distinct acoustic impedance inter- faces. Interpretation software is used to map out these horizons of signifcantly different acoustic properties (which correspond to different material types) to create a 3D geological model that represents the bounds of different soil layers. To calculate the actual verti- cal positions (depths) of stratigraphic changes, this model must be calibrat- ed, typically using geotechnical bor- ings, cone penetration testing (CPT) and velocity profling. Geotechnical Exploration. Geo- technical exploration provides es- sential information for engineering analyses (e.g. slope stability, sediment transport and scour, etc.), constraining geophysical data interpretation, devel- oping geologic models and providing samples for testing. Geotechnical in- vestigations are typically conducted from foating barges, jack-ups, lift boats or drilling ships. Exploration typically involves drilling or in-situ testing (e.g., CPT). Drilling using marine techniques offers many advantages over land- based drilling techniques (i.e., using a truck-mounted drill rig on a barge and drilling using rod-based techniques). Marine drilling techniques utilize wireline systems that collect higher quality data and are much faster than rod-based, land techniques. Assessment in GIS Integration and interpretation of the various data types are often managed within the GIS environment. A GIS da- tabase is populated with the "layers" of information collected during the various site investigation techniques. The datasets can be integrated, allow- ing the development of a 3D ground surface and subsurface model. This 3D model can be used to conduct compre- hensive hazard and risk assessments. Various interpretations of faults, slope instability features, sediment transport features (e.g., sand wave felds), fuid escape and environmentally sensitive areas are cataloged and incorporated into the GIS database. Relatively re- cent developments have demonstrated signifcantly improved effciencies in site condition assessments when GIS has been employed to this effect. For surface data, bathymetric and topographic terrain data may be fused to produce the topmost layer in a 3D model. The imagery data (sonar back- scatter/refectance or aerial imagery onshore and/or in shallow water) can then be draped onto this top layer. For subsurface, geophysical and geotechnical data are incorporated to produce models. Geophysical data is too vast to import and manipulate di- rectly in a GIS environment. Instead, it is generally interpreted in specialized software packages (such as Kingdom Suite). However, after layer strata are identifed, representation and further analysis in GIS is typically ideal. This is due to the ability to integrate data from other sources and the presentation ca- pabilities through planimetric maps, cross sections and oblique scenes. The 3D subsurface model is imported into Esri's (Redlands, California) ArcGIS as multiple gridded surfaces represent- ing the boundary between each soil layer. However, once again the seismic refection data do not provide a true spatial representation. To calculate the actual vertical positions (depths) of stratigraphic changes, the acoustic properties of each layer of material must be known and the model cali- brated. ArcGIS can be used to import geotechnical log data, which are then matched to the interpreted seismic sur- faces. The GIS analyst works with the geophysicist to correct the subsurface model by ftting the interpreted layers to the borings data. Once calibrated, the 3D subsurface model is added to the surface model Feb2014.indd 12 2/11/14 1:08 PM

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