3D High Resolution Seismic: Case Study Deepwater West Africa
Authors: Wood G.A. , Orren R.J. , Conway A.M. 
Published: October 2008

This paper describes a trial site survey performed in deep water offshore West Africa, utilising the 'short offset' form of 3D high-resolution (HR3D) seismic survey technique. The initial premise for the trial was that application of the HR3D technique could improve geohazard assessments in deep water, and improve the resolution of shallow exploration targets at a marginal cost increase over traditional HR2D. The objectives of the survey were to derive the optimum acquisition and processing parameters for HR3D, and to compare the results with traditional 2D high-resolution (HR2D) seismic and 3D exploration seismic data over the same area. The results obtained demonstrate the benefits of the technique over both HR2D and re-processed 3D exploration seismic data, at marginal additional cost over HR2D data acquisition.

Introduction

In recent years, oil companies have increasingly used exploration 3D seismic data to assess geohazards at a chosen location at the expense of the traditional HR2D-gas survey. Lower costs, and the fact that drilling gas charged units is not considered by some drilling engineers to be hazardous in deep water, have driven this current trend. However, the site survey industry argues that using exploration seismic data alone vastly decreases confidence in interpretation, as the data is significantly lower in terms of resolution, therefore potential hazards will go undetected.

However, traditional HR2D methods are stretched to their limits in deep water, especially when the seabed topography is steep and complex. To address the problem, Fugro Geoteam Limited performed a trial using an HR3D, short offset acquisition technique to collect data in a deep water environment, offshore Ivory Coast, West Africa after Newman5. The main objectives were to acquire HR3D data that could match the resolution of the traditional HR2D seismic data, but increase confidence in the interpretation by improving lateral resolution. The main driving force was to provide HR3D data at an affordable cost and improve geohazard assessment in the deep-water environment building on the ideas of de Vellier etal1. An added incentive was to demonstrate the power of the technique with regards to exploration aspects by achieving penetration down to potential target depths, whilst maintaining resolutions akin to site survey specification. This could provide invaluable information about the target zone that could not be supplied by exploration 3D data, at a very reasonable cost.

This paper will describe the trial acquisition parameters, then discuss the extensive seismic processing trials, before comparing the results of the HR3D data set against HR2D data and exploration 3D data from the same area. Finally, implications with regards to site surveys in deep water will be discussed.

3D High-resolution Seismic Data Acquisition Trial

High-resolution (HR) 3D acquisition trials were performed over an area previously surveyed for Ocean Energy Inc. off the Ivory Coast. The survey vessel MV Geo Prospector was used to acquire data over a 1km × 2.5km grid with 12.5m line spacing using an 80cu in sleeve gun cluster and the vessel's existing single streamer configured for 200m.