Oftentimes, interpretations of shallow water seismic data are unreliable because sections show incorrect or uninterpretable structure caused by effects of near surface anomalies. Reflection statics techniques are used to improve stacked data quality, but they cannot solve the long period anomalies caused by trenches and channels. Refraction statics can be used to solve for long period anomalies and produce sections that show structure correctly.
This paper shows examples of data from the South Timbalier Trench area processed with and without refraction statics.
Explorationists working in shallow water areas with trenches and channels have difficulty obtaining reliable seismic data because these near surface anomalies cause long period static problems. Seismic sections may have areas of poor signal and often show structure incorrectly, resulting in line mis-ties and interpretation errors. Reflection statics techniques are commonly used to improve stacked data quality, but they do not properly solve long period problems. Resulting seismic sections may still show incorrect structure. Refraction statics offers a way to solve long period static problems and help generate optimum stacked sections with correct structure.
South Timbalier Trench is a sediment filled channel entrenchment of the Mississippi River running south through the east half of the South Timbalier area (figure 1). Seismic data from this area 1S generally poor because fill material absorbs much of the energy needed for good reflections, and velocities in the fill are low compared to surrounding material causing time delays in transmitted and reflected ray paths. The first problem can be alleviated by using large airgun arrays, high air pressure, and recording with a drag cable. The second problem must be treated as a long period static.
The survey was designed to provide high quality data beneath the trench and consisted of 27 lines (564 miles) in a 2 × 2 mile grid (figure 2) shot during December 1984 and January 1985. The following acquisition parameters were used: 120 channels, back-down drag cable, offsets 1060 to 10818 feet, 82 foot receiver interval, 164 foot shotpoint interval, 7 second record length. Energy source was a tuned airgun array operating at 4500 psi. Two pops were taken at each shotpoint.
A back-down drag cable operation is one where the boat moves slightly beyond the shotpoint location, then "backs down" until the cable is resting on the sea bottom. When done properly, the boat will stop at the proper shot location. More energy is recorded this way than with a conventional streamer, plus movement of the boat and cable during shooting is minimized, improving resolution and allowing multiple pops per shotpoint to be summed for signal enhancement without smearing the character of the data.
In order to model the effect of the trench, it was essential to pick a refractor below the trench. The back-down drag operation was expensive but considered necessary to maximize energy recorded from beneath the trench and insure that far offset traces had good refraction information.