Nedimovic, Mladen R.
Permanent URI for this collectionhttps://hdl.handle.net/10222/22105
Browse
Recent Submissions
Item Open Access Crooked-line 2D seismic reflection imaging in crystalline terrains: Part 1, data processing(2003-01) Nedimovic, Mladen R.; West, GFFor cost and access reasons, most of the seismic reflection data collected in crystalline terrains have been acquired by 2D crooked-line profiling. When the survey geometry is significantly irregular and the geologic structures have cross-profile dip, several standard 2D imaging procedures severely underperform. As a result, reflection signal is poorly aligned across individual common midpoint (CMP) gathers, and much is lost during the CMP stack. To improve imaging, either the methods used to align signal before stack need to be modified or more tolerant methods of combining trace signals than the standard CMP stack need to be applied. Because a high-fold 2D crooked-line profile is really a 3D survey of a swath of terrain around the processing line, better signal alignment before CMP stacking may be achieved by revisiting the traveltime equation and including the cross-dip terms into the moveout calculations. Therefore, in addition to the correction of NMO and in-line dip moveout (DMO), we also locally compute and subsequently remove cross-dip moveout (CDMO). This requires a procedure for estimating the amount of cross-dip associated with each local reflection event. Stacking after the successful removal of the CDMO yields what we call an optimum cross-dip stack-a seismic section that is significantly more complete and informative than the standard stack. Alternatively, amplitude stacking appears to be more robust to residual time anomalies. When little or no cross-dip information can be extracted from the 2D crooked-line data, we use it as a last resort to obtain a section that contains more structural information than the standard stack.Item Open Access Crooked-line 2D seismic reflection imaging in crystalline terrains: Part 2, migration(2003-01) Nedimovic, Mladen R.; West, GFSeismic reflection surveys are frequently conducted over very complicated geological structure, but surveying often must be confined to existing crooked roads or tracks. Typically, data from such 2D crooked-line surveys are processed using standard common midpoint (CMP) stacking techniques to obtain a 2D time section which is then 2D migrated. In Part I, we show that a reflector dip component across the processing line can cause serious problems for standard CMP stacking. We also propose a supplementary processing step in which cross-dip is determined locally and cross-dip moveout (CDMO) is removed from data to form an optimum cross-dip stack. However, a crooked-line survey is really a swath 3D survey, and ideally we would like to obtain a 3D image of reflectivity surrounding the profile. Here we investigate the potential of 3D prestack Kirchhoff migration to directly image all observed reflections; i.e., we attempt to construct a 3D image volume of all reflectors viewed by the survey. Because reflectors that face away from the acquisition line cannot return much wave energy from available sources to available receivers, they cannot be imaged even if they lie directly beneath the survey profile. Tests that the cross-profile spread of trace midpoints usually is sufficient to provide a useful degree of cross-line positioning of reflection points. A very helpful image volume is thus obtained. Kirchhoff 3D prestack migration is computationally laborious. A much quicker but less complete method is to create the 3D migrated image volume from the 2D optimum cross-dip stack and the associated set of cross-dips. Robustness of migration methods to time errors in the prestack data traces such as poorly corrected statics is also an issue. Tests show that in difficult cases, particularly where only 2D processing is warranted, migration of trace absolute amplitude rather than standard phase data may lead to a superior result.Item Open Access 2D waveform tomography applied to long-streamer MCS data from the Scotian Slope(2011-07) Delescluse, Matthias; Nedimovic, Mladen R.; Louden, Keith E.Detailed velocity models of the earth's subsurface can be obtained through waveform tomography. The accuracy of the long-wavelength component of such velocity models, which is the background velocity field, is particularly sensitive to modeling low-frequency refracted waves that have long paths through target structures. Thus, field examples primarily have focused on the analysis of long-offset wide-angle data sets collected using autonomous receivers, in which refractions arrive at significantly earlier times than reflections. Modern marine acquisition with long streamers now offers the ability to record refracted waves with high spatial density and uniform source, both in shallow and deep water. We used 2D multichannel seismic (MCS) data acquired with a 9-km-long streamer over the Scotian Slope in water depths of similar to 1600 m. The refracted arrivals, although mostly restricted to far-offset receivers, provided sufficient information to successfully invert for a high-resolution background velocity field. Using a frequency-domain acoustic code over frequencies from 8 to 24 Hz on two crossing profiles, we found that the limited refracted waves can constrain the velocity field above the depth of the turning waves (similar to 1.5 km below seafloor). Several important features were resolved by the waveform velocity model that were not present in the initial traveltime model. In particular, a high-velocity layer at 300 m below the seafloor, interpreted as gas hydrates, was imaged even where a characteristic bottom-simulating reflector was not visible. At 750-m depth, a strong velocity increase of 300 m/s existed beneath a gently dipping reflector along which low-velocity zones, possibly related to gas, were present. Velocity models were highly consistent at the crossing point between the two profiles. The depth extent of the MCS waveform tomography constrained by refractions could be extended by even longer streamers (e. g., 15 km) or by joint inversion with data from ocean-bottom seismographs.Item Open Access Recent Advances in Multichannel Seismic Imaging for Academic Research in Deep Oceanic Environments(2012-03) Canales, Juan Pablo; Carton, Helene; Mutter, John C.; Harding, Alistair; Carbotte, Suzanne M.; Nedimovic, Mladen R.No abstract available.Item Open Access Recent Seismic Studies at the East Pacific Rise 8 degrees 20 '-10 degrees 10 ' N and Endeavour Segment Insights into Mid-Ocean Ridge Hydrothermal and Magmatic Processes(2012-03) Carbotte, Suzanne M.; Canales, Juan Pablo; Nedimovic, Mladen R.; Carton, Helene; Mutter, John C.As part of the suite of multidisciplinary investigations undertaken by the Ridge 2000 Program, new multichannel seismic studies of crustal structure were conducted at the East Pacific Rise (EPR) 8 degrees 20'-10 degrees 10'N and Endeavour Segment of the Juan de Fuca Ridge. These studies provide important insights into magmatic systems and hydrothermal flow in these regions, with broader implications for fast- and intermediate-spreading mid-ocean ridges. A mid-crust magma body is imaged beneath Endeavour Segment underlying all known vent fields, suggesting that prior notions of a tectonically driven hydrothermal system at this site can be ruled out. There is evidence at both sites that the axial magma body is segmented on a similar 5-20 km length scale, with implications for the geometry of high-temperature axial hydrothermal flow and for lava geochemistry. The new data provide the first seismic reflection images of magma sills in the crust away from the axial melt lens. These off-axis magma reservoirs are the likely source of more-evolved lavas typically sampled on the ridge flanks and may be associated with off-axis hydrothermal venting, which has recently been discovered within the EPR site. Clusters of seismic reflection events at the base of the crust are observed, and localized regions of thick Moho Transition Zone, with frozen or partially molten gabbro lenses embedded within mantle rocks, are inferred. Studies of the upper crust on the flanks of Endeavour Segment provide new insights into the low-temperature hydrothermal flow that continues long after crustal formation. Precipitation of alteration minerals due to fluid flow leads to changes in P-wave velocities within seismic Layer 2A (the uppermost layer of the oceanic crust) that vary markedly with extent of sediment blanketing the crust. In addition, intermediate-scale variations in the structure of Layers 2A and 2B with local topography are observed that may result from topographically driven fluid upflow and downflow on the ridge flanks.