Multi-Azimuth and Wide Azimuth Towed Streamer Acquisition for Subsalt Imaging in the Gulf of Mexico and Egypt
Authors: Michell Summers , Summers Tim , Shoshitaishvili Elena , Etgen John , Regone Carl , Barley Brian , Keggin Jim , Benson Mark , Rietveld Walter , Manning Ted 
Published: October 2008

Introduction

Marine seismic surveys have been acquired in fairly standard configurations for approximately the last 12 years. A conventional towed streamer seismic survey configuration consists of towing 6-10 cables that are usually 4-10 km long with a hydrophone sensor located every 25m along the cable. In this conventional configuration two seismic sources are deployed directly behind the vessel. The seismic source consists of an array of canisters that release high pressure air to produce the acoustic energy, the seismic signal. These air guns fire once every 8 to 15 seconds, depending upon the goals of the survey. This particular configuration of towed streamer data, where the boat sails across the area in one particular direction (azimuth), has been very effective in imaging areas with fairly benign geologic settings.

As the industry has moved into areas with more complicated geologic settings these surveys no longer meet the imaging objectives for risk reduction in exploration and production. In particular BP, and to varying degrees the oil and gas industry, has moved into exploration and development areas with large lateral velocity contrasts above the target. These velocity contrasts produce strong multiples and they distort the seismic signal in ways that can not be recovered by using a single azimuth towed streamer survey. Two examples that will be highlighted in this paper are the large diapiric salt bodies in the deep water Gulf of Mexico and the interbedded anhydrites found in the Nile Delta, offshore Egypt.

The generally accepted approach to solving imaging problems such as these is to put greater effort into processing, improving the seismic noise attenuation and imaging algorithms. BP determined that processing enhancements alone could never meet the business objectives, and instead we designed seismic acquisition programs that use conventional seismic acquisition hardware deployed in novel ways. Figure 1 illustrates the multi-azimuth acquisition design used to image the sub-evaporite plays, Nile Delta, Egypt and the Wide Azimuth Towed Streamer Survey (WATS) that was deployed to image the Mad Dog reservoir that lies beneath a thick salt in the Gulf of Mexico.

Figure 1: Acquisition designs giving a wide range of sourcereceiver azimuths: from left to right, Multi-azimuth towed streamer (MAZ) and Wide Azimuth Towed Streamer (WATS). Note that one shooting direction in a MAZ survey is a standard single azimuth towed streamer survey.

Subsalt, Gulf of Mexico

The Mad Dog field, discovered in November, 1998, is located approximately 140 miles south of the Louisiana coastline in the southern Green Canyon area. Water depths over the field vary from 4100 feet to more than 7000 feet and vary most quickly along the Sigsbee Escarpment (figure 2). Due to a complex salt canopy, which overlies a large portion of the field, seismic data quality has been generally considered to be very poor. BP has improved the Mad Dog image by applying advanced imaging techniques including waveequation migration and by implementing a dual azimuth imaging effort (Michell 2004). Each effort improved the image of the reservoir but the image still had gaps.