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A feasibility study was undertaken on a 20km section of 2D seismic data from the Makassar Straits, offshore eastern Kalimantan. The study was designed to test the suitability of using prestack depth migration in an area dominated by overthrust geology and at water depths in excess of 2000m.
Thrusting stacks rocks of higher seismic velocity over lower velocity material. Such a velocity contrast results in strong wavefront attenuation, culminating in poor sub-thrust illumination. It is shown that imaging of the footwall is highly dependent on the thrust interpretation and on the offset range of the dataset. Near offset fault shadowing produced by strong velocity contrasts inhibits sub-thrust illumination.
Lateral positioning of the thrust faults can vary by up to 1000m depending on the derivation of interval velocities within the hanging wall. Travel time inversion is used to constrain this velocity variation, and combined with velocity focusing analysis provide sufficient resolution to accurately migrate the data. Strong water bottom and related multiples exist, which although encountered at a later time than the target, have a dramatic effect on the imaging as they are overmigrated to shallower depths, coincident with the objective.
Prestack depth imaging provides greater detail of the footwall section which on time data was defocused or absent. The additional resolution shows that the footwall section contains several secondary thrusts which rotated the strata into opposing dips. The resulting structure is a potential trap and changes the original geological interpretation of the area.
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