Recovering Uniform Coverage in a 3D Survey: Case Study from Onshore Southern India

K Shukla,P Jaiswal,C S Singh

doi:10.1155/2014/987605

Abstract

We present a case study on 3D seismic acquisition under inaccessible ground conditions and heavy head-loads. The novelty in this case study is designing novel shot recovery strategies. The study area is in a densely populated region in Southern India, where a 3D survey was conducted over an area of 700 km2to illuminate a target formation at 3000 m depth. The survey was designed in a brick pattern with six shots in a round, a line repeat interval of 2, and offset and skid grid of 300 m×300 m. Only 40% of full fold could be obtained with conventional shot recovery methods. To increase the survey fold, two new shot recovery strategies were attempted. First, the original offset and skid grid was increased to 1100 m×1200 m. Second, if the recovery shot did not fall in this grid, it was relocated along the swath from its original location to a distance equal to half the inline offset. The two strategies, employed together, increased the fold to 90% of full fold at the target area maintaining its uniformity. Although the illumination of the target zone was adequate for interpretation in this case, we see a need for adaptation if these strategies are to be applied in other surveys. In general, issues related to offset-limit, shot-density, and grid-spacing are a matter of continual optimization in 3D surveys.

Highlights

3D seismic acquisition has had a substantial impact on hydrocarbon exploration [1]. 3D surveying provides many benefits over 2D but it presents challenges that are unique and often require solutions based on local geology and resource availability. 3D acquisition cannot be treated as a mere extension of 2D; it is distinguished by simultaneous data recording along multiple, closely-spaced broadside profile which enables reconstruction of the true nature of the seismic wavefield
As opposed to 2D, 3D acquisition creates a data volume from which lines, planes, or probes can be extracted in any orientation [2]
The most critical parameter in a 3D survey design is the bin size, which determines the maximum wavenumber sampled in the field which sets the threshold for spatial aliasing [6, 7]

Summary

Introduction

3D seismic acquisition has had a substantial impact on hydrocarbon exploration [1]. 3D surveying provides many benefits over 2D but it presents challenges that are unique and often require solutions based on local geology and resource availability. 3D acquisition cannot be treated as a mere extension of 2D; it is distinguished by simultaneous data recording along multiple, closely-spaced broadside profile which enables reconstruction of the true nature of the seismic wavefield. The most critical parameter in a 3D survey design is the bin size, which determines the maximum wavenumber sampled in the field which sets the threshold for spatial aliasing [6, 7]. 3D design needs to address ground realities—costs versus survey size versus structural dip at the depth-of-interest, need of more complex static solutions, and higher susceptibility to culture to name a few [11, 12] The latter in particular, which is the focus of this paper, becomes more pronounced in urban areas. We discuss the current limitations and potential improvements in the offset-recovery method

Study Area and Survey Design

Recovery Strategy and Results

Discussions

Conclusions