A Successful 3D Seismic Survey in the-No-Data Zone,Offshore Mississippi DeltaSurvey Design and Refraction Static Correction Processing.

Abstract

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.