The Use of Four-Component Data for Imaging, Lithology Prediction, and Fluid Discrimination: Three Case Studies
Authors: Fromyr Eivind , Randazzo Santi 
Published: October 2008

In recent years the development of marine Four-Component acquisition and processing technology has made it possible to apply the technique in most geographical areas and in a wide range of waterdepth. Due to higher cost compared to conventional streamer surveys, a key to future success is to identify the proper geophysical use and demonstrate that results can be interpreted and used in ways that still make the technology cost efficient. In this paper we will not address processing and interpretation issues on a detailed technical level but discuss the results associated with three different case histories, each showing a different application of marine 4C data. The applications cover both Imaging and Characterization.

Introduction

A conventional marine source gives rise secondarily to the full complement of wave types through conversions, chiefly upon reflection at any impedance contrast. Four-component acquisition makes it possible for to record the entire vector wav-field. Imaging of structures beneath gas clouds is a well-established application of this technology. Additionally, acquisition with stationary receiver systems on the seabed enables us to image complex structures and beneath better, due to improved aperture, and application of dual sensor summation to remove the water-born multiple.

Generally speaking, recording and processing a shear-wave section in addition to a P-wave section gives us a second independent set of information from the subsurface which enables us to map S-wave velocity (VS) and amplitude, which can be combined with data on P-wave velocity (VP) to yield Poisson's ratio, or simply the velocity ratio VP/VS. This gets us closer to an image of lithology and pore saturants. We cannot usually do this with P-wave data alone because many different rocks with different saturants have similar P-wave responses. An overview of the various aspects of theory and applications can be found in Tatham and McCormack (1991).

Multiple Removal and Subsalt Imaging

Seabed sensors offer semi stationary acquisition with a free geometry with optional wide azimuth and a full range off offset, including true zero offset. It is well known from shallow water OBC data that the water-born reverberations can be removed by combining the wave-field recorded by the hydrophone and the vertical geophone, see Barr et.al (1989). In deepwater seabed acquisition the effective aperture for a given nominal source offset range increases relative to a streamer geometry. These three aspects of marine 4C are the focus of this case history.

Figure 1 demonstrates how a 2D4C line can prove additional reserves due to improved imaging beneath a salt structure. The data is collected in the Gulf of Mexico. The field is covered by a 3D streamer survey and a comparison of the 2D4C line and an extracted 2D line from the 3D cube is shown.

We believe that the illumination advantage combined with the multiple suppression technique (no destructive FK or Radon) are responsible for the improvement which allows us to map the reservoir reflector further beneath the salt structure.