Teal South 4C/3D Survey: A Model for 4C/4D Seismic Data Acquisition
Authors: Roche Steve , Maxwell Pete , Fisseler Glenn 
Published: October 2008

The seismic data acquisition methods, used at Teal South to acquire a four-component (4-C) seismic survey, are intended to be a model for repeated acquisition for time-lapse (4-D) seismic imaging. These methods represent a low-cost approach to 4C/4D acquisition. One critical aspect of the multicomponent seismic data acquisition is determining the orientation of the seismic sensors on the ocean floor. Statistical determination of apparent phone orientations were derived from the seismic data with an accuracy of approximately +/- 5 degrees. Part of the uncertainty appears to be related to source-receiver azimuth. Further analysis of these data and subsequent data will be required to ascertain the cause of the azimuthal variation in apparent phone orientation.

Introduction

During the summer of 1997 (July and August) a data acquisition experiment for low-cost ocean bottom cable (OBC) seismic surveying was undertaken by Texaco EPTD, Input/Output, Inc., Western Geophysical and Digicourse. A four-component (4-C), OBC survey was acquired over the Teal South Field, operated by Texaco. A 4-C seismic survey acquires the pressure and three orthogonal velocity components of the elastic wavefield to be processed, forming a seismic image of the subsurface. Teal South is located in the Gulf of Mexico, Eugene Island Block 354. The water depth in the project area is approximately 90 meters.

Data was acquired using 24, 4-C receiver stations on the ocean floor deployed in four east-west receiver lines spaced 400m apart (Figure 1). Station spacing along a receiver line was 200m. The receiver line cables, with 4-C sensors attached, were deployed under tension to nominally orient the sensors with the H1, or inline horizontal velocity component, aligned approximately east-west and the H2, or xline velocity component, aligned orthogonal in the north-south direction. The individual sensors used were dual axis gimbal assemblies, which allowed the vertical velocity sensor component to be oriented vertical.

A dense source grid, on 25 by 25 meter spacing, was acquired over a 3 by 3 kilometer grid centered on the receiver spread. The receiver spread was stationary for the entire source grid.

One of the Teal South experiment objectives was to provide a low-cost method for OBC seismic data acquisition. One cost-reduction aspect was to place lower-cost analogue sensor and lightweight cables on the sea floor while placing the higher-cost recording equipment at the sea surface in floating buoys (Figure 2). The more-expensive recording electronics are demobilized from the survey area after acquisition. For time-lapse, or 4-D applications, the recording equipment is re-mobilized to the project area for subsequent data acquisition. Geologic framework, acquisition procedures and a more detailed description of the equipment used at Teal South can be found in Ebrom, et al, 1998.

Examples of the field record quality, acquired during the summer of 1997, are shown in Figure 3. These data are from a single receiver station, showing the data recorded along a single north-south source profile. All four components, pressure, vertical velocity and the two horizontal velocity components are shown.

The 4-D aspect of the Teal South experiment has been analyzed within Texaco using comparison with previously acquired marine streamer data (Ebrom, et al, 1998). Currently the Teal South project is active within an industry-sponsored consortium directed by the Energy Research Clearing House (ERCH).