Extending offshore limits with joint streamer and ocean bottom cable operations

Abstract

Towed streamers are a highly productive method of getting 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. Other methods such as buoyed radio telemetry systems also have limits related to water depth, currents and 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. The two acquisition types may be merged to a single data volume if proper acquisition and processing techniques are used. This 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 airgun 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 against and occasionally, 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 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 with positive receiver locations can be utilized to improve statics solutions. *Dual techniques are used by OBC to remove the water layer reverberation or “receiver ghost.” The receiver ghost occurs when upward traveling energy reflects off the water-air boundary back down to the receiver. The ghost energy cancels some of the subsequent primary energy and thus causes notches in the amplitude spectrum of the recorded data. An amplitude spectrum of the frequency response of a hydrophone and geophone at depth (Figure 1) shows the ghost notches of the hydrophone are located where the geophone energy 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, inexpensively compared to other acquisition techniques. Normally, streamer acquisition is restricted to offend shooting. Merging OBC and streamer data into a single product takes advantage of the versatility of OBC and the economics of streamer operations to produce high quality seismic surveys in areas where OBC or streamer, by themselves, would prove impractical. OBC can be used in areas of localized obstructions, statics anomalies, or shallow in water while marine streamer acquires data in the unobstructed deeper water areas. ACQUISITION The Ship Shoal area block 269 of the Gulf of Mexico was used to demonstrate the validity of merging OBC and streamer data types. Figure 2 is a map of the area showing the subsurface coverage of OBC and streamer data. Coverage lines for both streamer and OBC were northsouth. The streamer data was collected on the north and west portion of the prospect and OBC data was collected over the southeastern portion where several well heads existed. The subsurface coverage of the two acquisition types overlapped 12 CDP lines in the crossline direction and one cable length in the inline direction. Water depths ranged from 30 to 70 meters across the survey area. CDP bin spacing was 12.5 meters in the inline direction and 40 meters in the cross line direction. The streamer data was recorded with a Titan-1000 recording system and the OBC data was recorded with an MDS-18 system. The shot interval of the streamer data was 50-m and the receiver group