Acquisition of 3D, 4C seismic data for offshore Gulf of Mexico time lapse studies
Authors: Ebrom Dan , Krail Paul , Scott Larry 
Published: October 2008

A 3D, 4C seismic survey acquired in the summer of 1997 has proven the feasibility of acquiring offshore muiticomponent data in the Gulf of Mexico. The survey, acquired by Western Geophysical over Texaco's Teal South field, utilized new technology from Input/Output consisting of permanently deployed receivers. The motivation for acquiring the survey was to aid in reservoir development and to prove the new acquisition technique. This first survey provided an initial 3D "snapshot" of the reservoir. This initial "snapshot" will be used as a baseline against which to compare later 3D surveys. An unusual aspect of this survey was the capture of the horizontal components of the seismic wavefield. These horizontal components show clear evidence of mode converted P-S energy. These mode conversions were of high data quality (i.e. highly energetic and highly coherent). The P-S events provide extra information about the lithology of the reservoir and seal units in the field. The P-S events can be incorporated into the reservoir model generated from the P-wave data, and can also be used for direct differencing as subsequent surveys are performed.

Introduction

Earlier seismic surveys at Teal South had utilized towedstreamer hydrophone technology, wherein a single vessel acts as both source vessel and recording vessel. Multicomponent recording is not feasible with towed streamers because shear waves cannot propagate in a fluid medium. In order to acquire P-S events, some form of ocean bottom cable (OBC) is required to allow receivers to be in contact with the ocean floor. The cables constructed by Input/Output contained sensors with both a pressure hydrophone and also a three-component geophone (i.e. a vertical component plus two orthogonal horizontal components). Unlike usual OBC operations, the cables were not connected to a recording vessel. Instead, each cable was connected to a recording buoy, which stored the seismic data in nonvolatile solid-state memory. Twice a day, Zodiac boats would travel to the recording buoys to download the seismic data ("data harvesting"). Once the downloaded data were ferried back to the mother vessel, the data were transcribed onto standard 3480 tapes in SEGD format.

The Survey

The seismic survey took place during calm weather at the end of July, 1997 through early August, 1997. Four cables were deployed in 85 meters of water, with each cable running due east-west. The cables were spaced 400 meters apart in the north-south direction. The detector groups (i.e. a hydrophone plus 3-component geophone) were spaced 200 meters apart inline, with six detector groups per cable. The receiver effort covered an area of 1.2 kilometers by 1.0 kilometers. Overall, deployment of the four cables took less than 18 hours. Subsequent polarization analysis of the horizontal component data by Steve Roche of Input/Output indicated that deviations of the horizontal component geophones from perfectly east-west orientation were less than 6 degrees. Each detector group was enveloped by a sandbag for enhanced coupling of the geophones to the seafloor. Each detector group had an associated acoustic transponder for positioning. The positioning of the receivers was performed both in realtime by acoustics and in processing by firstbreak analysis.