During marine seismic acquisition, obtaining complete subsurface coverage may require combining data from different acquisition dates. The time gaps between the overlapping coverage may vary from hours separating subsequent boat passes, to months when large surveys are acquired in sections. Time-lapse data are an extreme example of overlapping data sets acquired at widely varying acquisition dates. Unfortunately, between the different times of acquisition, changes in physical ocean properties, such as temperature or salinity, can cause variations in water velocity. The result is a dynamic change in recorded traveltimes that makes accurate combination of the data difficult. In shallow water, the distortions are small and do not affect data quality. However, in deepwater, the cumulative distortions can pose a serious impediment to accurate imaging. The existence of water-velocity variations has been documented previously (Barley, 1999). Water temperature changes are the primary cause of velocity variations. Figure 1 shows an area just south of Nova Scotia (coastline in red). The outlined seismic survey area is approximately 3600 km 2. The satellite images show surface temperature variations, with each color contour representing 1°C. In the approximately two-week period shown, surface temperatures varied as much as 10°. The temperature structures evident in Figure 1 are caused by eddies in the Gulf Stream and are indicative of deepwater temperature variations. Significantly, each degree of change causes over 3 m/s of water-velocity variation. The effect of such changes on seismic data collected in deepwater can be significant. Figure 2 shows two midpoint gathers after moveout correction. The gathers are from the Nova Scotia survey outlined in Figure 1. The shallowest event is the water-bottom reflector at approximately 1.7 s. The red line graphed at the top of the sections indicates the relative dates of acquisition. There is a pronounced, dynamic time difference between the data acquired at different dates.

Trinidad and Azerbaijan offshore areas are strongly affected by shallow gas anomalies which greatly attenuate seismic signals. Building velocity models in such areas with shallow water depths and gas can be a difficult task. Here we present two alternative ways to build reliable velocity models in the presence of shallow gas; one that is suitable to very shallow (<100m) and poor data quality areas and the other for deeper water depths. In the first instance, we make use of Diving-Wave refraction tomography method to build shallow velocity models offshore Trinidad and Azerbaijan. Previous use of this method has been limited to processing seismic data to produce a shallow velocity model to determine static corrections in time processing. Our success is in using the velocity model derived from Diving-Wave tomography as a starting model for reflection tomography in depth processing. We show that Diving-Wave method is a robust technique that produces reliable near surface models in the presence of gas and in areas with low signal to noise ratio. In the second case, we show that where data has reasonable offset to work with, reflection tomography can produce fairly accurate and high fidelity velocity models that can be further improved with iterative migration velocity analysis. As a result, depending on available data quality, either Diving-Wave derived shallow velocity model or reflection tomography derived model can be used to improve the ultimate product from iterative pre-stack depth migration and reflection tomography.

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This is a success story of survey design and refraction static correction processing of a large 3D seismic survey in the South Pass area of the Mississippi delta. In this transition zone, subaqueous mudflow gullies and lobes of the delta, in various states of consolidation and gas saturation, are strong absorbers of seismic energy. Seismic waves penetrating the mud are severely restricted in bandwidth and variously delayed by changes in mud velocity and thickness, Using a delay-time refraction static correction method, we find compensation for the various delays, i.e., static corrections, commonly vary 150 ms over a short distance. Application of the static corrections markedly improves the seismic stack volume. This paper shows that intelligent survey design and delay-time refraction static correction processing economically eliminate the historic ‘no data’ status of this area. Introduction Large river deltas present unique problems for seismic exploration. Of particular interest to Gulf Coast geophysicists are the effects of the modern Mississippi delta on seismic data. Variations in the gas content and consolidation of sediments, corresponding to the mud structures of the delta, impose extreme attenuation and traveltime variations on seismic waves. These properties are responsible for designation of the delta area as a ‘no seismic data zone.’ Compensation for these effects is an important part of seismic processing. Deconvolution and spectral balancing compensate for attenuation effects, and refraction and reflection static correction processing compensate variation. Beginning in 1993, Western Geophysical solicited and obtained the participation of several oi I companies in a large 3D survey to be conducted in the area of the delta (Figure 1). This paper highlights the design and refraction static correction processing of Phase 1 of the project. Phase 2 has been acquired and is being processed. From the beginning, we realized that refraction static corrections would be crucial in processing of the survey. Previously, in refraction static correction processing of 2D shallow streamer data, we had produced results similar to those presented by Schatz, et al.] But, although we had been successful in static correction processing of both 3D land and 3D marine-streamer data, we did not know what to expect in this extreme environment. The modern Mississippi river delta has been studied extensively. A particularly useful report on studies by LSU and USGS of the soil properties and structure of the delta was presented by Coleman, et al.z Marathon Oil studied the delta and presented three papers in 1988: Tinkle, et al.3, studied acoustics of the mud structures in the delta; May, et al.4 used acoustic measurements and sediment samples to predict and help correct problems encountered in seismic data; Meeder, et al.s, constructed a velocity and depth model along a 2D profile based on borings and acoustic measurements, and compared static corrections from this model to those generated from refraction and reflection seismic data.