Schlumberger

Technical Paper: Imaging Below Shallow Gas with Full Azimuth Acquisition: A Case Study

Society: SEG
Paper Number: 2012-255
Presentation Date: 2012
 

Abstract

The image of the target in this case study is affected by zones of shallow gas/high Earth attenuation in the overburden. The goal of the study is to determine whether the attenuation of the seismic amplitudes below these areas can be minimized through the application of advanced seismic acquisition designs, e.g. increased azimuth, long offset acquisition.

Following the successful completion of a proof of concept test using a realistic synthetic model, an illumination analysis study was carried out over the survey area to determine the illumination characteristics of the legacy survey and to help understand the impact of the shallow gas bodies on the amplitudes across the target horizons. The illumination characteristics of a circular shooting acquisition strategy were investigated and compared with the legacy data. This non-conventional acquisition approach has the potential to greatly improve upon the azimuthal sampling achievable with the traditional straight line, narrow azimuth survey.

In order to determine the relative benefits of the full azimuth approach when considered against the narrow azimuth legacy data for this specific area, additional target orientated analysis was conducted. This consisted of establishing the full range of reflector dips and azimuths (outside of those in the existing structural model) which could be imaged by each technique combined with simulating local PSDM images for the key areas of interest.

The results of the study indicate that;
a) The existing image of the two target horizons can be improved upon.
b) Increasing the azimuth sampling and fold will increase the amplitudes across the targets.
c) Increasing the azimuth sampling will increase the range of dips imaged and the resolution of the seismic image within the key areas of interest.
d) Circular shooting acquisition was shown to give an improved image of the target horizons. That is, better illumination, better imaging of dips and better image resolution.

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