Impact of Irregularities of 3D Seismic Geometry on Subsurface Imaging

3D seismic data acquisition brought a sea change in subsurface imaging. The bridge between geology and seismic is P/S wave velocity of different subsurfaces/lithology. The technique has acquired prime importance due to its inherent strength of determining seismic velocity/amplitudes of the different medium. Lithological variations estimations is possible due to proper Vp/Vs ratio or either velocity alone. Success of advance seismic studies solely depends on accurate subsurface mapping. 3D seismic subsurface image is biased to a great extent by the acquisition geometry, as it affects lateral & vertical velocities. Thus a proper evaluation of the influence of the acquisition geometry on image quality, in areas with complex geology, needs to be carried out. The acquisition geometry footprint is the cumulative effect of azimuthal fold variations across the near offsets, source-receiver directivity, and subsurface illumination irregularities. The ideal 3D acquisition geometry called “full 3D” is the one in which the whole sub-surface area should be covered with regular source positions, and recorded in receiver positions covering the whole acquisition area. Each receiver position is to be covered by source position as well. In 3D seismic survey the choices of acquisition geometry is governed by geological objectives. The optimization of acquisition geometry arises out of resource constraints / cost. The impact of offsets, azimuths, inherent acquisition footprint needs to be understood while optimizing these acquisition parameters/geometry. Obstructions, environmental objectives, economic constraints and many other factors cause irregularity and sparse data sampling. This needs to be studied before geometry optimization as demography is altering at a rapid pace with urbanization. The irregularities are observed in the form of variations in the fold coverage, irregular offset which can manifest itself as an acquisition footprint on pre-stack data or even the stacked image. This study deals with the various irregularities introduced by the choice of geometry. Thus assessing the environmental / logistic /economic resource constraints, design of geometry should not sacrifice the basic objective of the 3D survey.