Merging Ocean Bottom Cable And Marine Streamer Surveys
Authors: Starr J.G. , Ralph J.G. 
Published: October 2008

Towed streamers are a highly productive method of acquiring seismic data in the marine environment. They are limited, however, in their ability to operate near obstructions, shallow water and have limited versatility with acquisition geometry. The ocean bottom cable method (OBC) of marine seismic data acquisition maintains consistent offset and azimuth distribution near obstructions, has versatile acquisition geometry capabilities and can function in water depths ranging from the very shallow to 100 meters.

A technique has been developed to incorporate the advantages of the two acquisition types by merging them to produce a single data volume. The method involves acquiring marine streamer and OBC data separately according to compatible acquisition parameters, then merging the two data sets during the processing stage. This method was used to produce a survey over the ship shoal area of the Gulf of Mexico and is described in this paper.

INTRODUCTION

OBC involves recording seismic data from receivers stationary on the water bottom in water depths out to 100 meters. A boat towing only a short air gun array, independent of the receiver cable, is used as the seismic source. This allows for minimal interference from cultural obstacles and boat traffic (Barr et al 1992). The receivers can be laid up to and in some cases, as in cat walks, underneath obstacles. On Board Receiver Location (OBRL) utilizes first break information from the production data and source navigation to locate each receiver after deployment. The recording boat is anchored on the prospect and linked to the receivers by a telemetry cable.

An OBC operation can be thought of as a land survey in the marine environment with the shots and receivers transposed. Land acquisition uses many receivers and as few shots as possible while OBC uses as few receivers an possible and many shots. Thus, OBC can make use of a variety of acquisition geometries depending on the survey objectives. Split spread or patch geometries allow for greater azimuth distribution; this in conjunction with positive receiver locations canbe utilized to improve statics solutions.

Dual SensorSM is used by OBC to remove the water layer reverberation or "receiver ghosts!. The receiver ghost occurs when upward travelingenergy reflects off the water-air boundary back down to the receiver. The ghost energy causes notches in the amplitude spectrum of therecorded data. An amplitude spectrum of the frequency response of a hydrophone and geophone at depth Figure 1(as shown in full paper)show the ghost notches of the hydrophone are located where the geophone as peaks and visa versa. By summing data recorded by hydrophones and geophones at the same location the receiver ghost is removed and the recorded band width is expanded (Barr et al 1990).

The marine streamer method involves towing the source and receiver array behind a vessel. Streamers are typically 3000 m to 4500 m long. Multiple source and streamer shooting can acquire large amounts of data relatively inexpensively compared to other acquisition Techniques.