SUMMARY Two tomographic methods for assessing velocity models obtained from wide‐angle seismic traveltime data are presented through four case studies. The modelling/inversion of wide‐angle traveltimes usually involves some aspects that are quite subjective. For example: (1) identifying and including later phases that are often difficult to pick within the seismic coda, (2) assigning specific layers to arrivals, (3) incorporating pre‐conceived structure not specifically required by the data and (4) selecting a model parametrization. These steps are applied to maximize model constraint and minimize model non‐uniqueness. However, these steps may cause the overall approach to appear ad hoc, and thereby diminish the credibility of the final model. The effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide‐angle data set: the first‐arrival times. For data sets with Moho reflections, the tomographic velocity model can be used to invert the PmP times for a minimum‐structure Moho. In this way, crustal velocity and Moho models can be obtained that require the least amount of subjective input, and the model structure that is required by the wide‐angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data. The tomographic models are not intended to supersede the preferred models, since the latter model is typically better resolved and more interpretable. This form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models. Four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above. This was followed by conventional first‐arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth, minimum‐structure models. The case studies are from the SE Canadian Cordillera across the Rocky Mountain Trench, central India across the Narmada‐Son lineament, the Iberia margin across the Galicia Bank, and the central Chilean margin across the Valparaiso Basin and a subducting seamount. These case studies span the range of modern wide‐angle experiments and data sets in terms of shot–receiver spacing, marine and land acquisition, lateral heterogeneity of the study area, and availability of wide‐angle reflections and coincident near‐vertical reflection data. The results are surprising given the amount of structure in the smooth, tomographically derived models that is consistent with the more subjectively derived models. The results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide‐angle traveltime data.

The operational condition that dominates the survey planning and implementation is the presence of major shipping transit fairway to and from the Suez Canal. This shipping thoroughfare covers about 70% of the survey area. Operational considerations necessitate a shooting orientation that closely parallels the shipping lanes, which approximates the strike direction of the subsurface target. Shooting in the dip direction, across the shipping lanes, was not considered to be operationally feasible for a 3D spread or operation. Wave equation simulation was used in preparation of the acquisition program specifications. The technique was utilized to produce synthetic seismogram shot records that were in turn examined, interpreted and processed to assess the impact of various parameter combinations on meeting technical, operational and economic requirements set by the exploration staff.

A methodology has been developed for quantitative prediction of shallow-hazard zones based on 3D high-resolution (3DHR) seismic and shallow-log data. The main goal of the integrated workflow is to produce an optimal structural image, conduct an AVO feasibility study and, if applicable, identify AVO trends within the depositional sequences of the shallow overburden at the Azeri field offshore Azerbaijan. Shallow, near-surface geologic-engineering hazards and features are not always visible on seismic volumes typically processed for reservoir targets. The 3D high-resolution seismic data provide good definition of shallow stratigraphy, indication of buried mudflows and landslides, and good fault delineation. AVO feasibility from rock-property analysis was affected by uncertainties arising from lack of or poor log data quality. With limited amounts of log data increasing the uncertainties, the use of trends based on data provided offered some initial understanding of shallow-seismic sensitivity. In depths < 300 m below mud line (BML), there is greater uncertainty because rock-physics models are not well understood in a generally unconsolidated section. However, where log data are available, there is an opportunity to use the integrated workflow in combination with geohazard interpretation to reduce uncertainty in subsurface description.