Filling a Dredged Channel With Refractions

ABSTRACT The time structure obtained from a reflection seismic survey is believed to be distorted by extremely slow velocity fill material in a sea-floor channel. The refraction data from the reflection seismic survey is used to compute short-wavelength refraction residual statics and to estimate a near-surface model. The improvement in the definition of the time structure on the reflection seismic data is demonstrated, as well as ancillary benefits associated with incorporating these corrections at early stages of reflection data processing. INTRODUCTION Seismic reflection data were collected in the South Timbalier area offshore Louisiana in August, 1984. The data were collected using 120 220 foot groups in a split-spread mode, yielding a 13,420 foot far offset at each side of the spread. The cable was laid on the water-bottom in approximately 30 foot water depth. Conventional reflection seismic data processing techniques indicated that an area of large-magnitude, short-wavelength statics degraded data quality in a particular area. This area was interpreted as being a sea-floor channel, filled with extremely slow velocity material (Figure 1). The statics caused by this velocity anomaly were too large to be corrected using conventional reflection residual statics. Interactive refraction modeling was chosen as a technique to resolve these large magnitude statics. The results of applying refraction residual statics, refraction modeling, and reflection residual statics showed an improvement in data quality which was not expected. We then displayed the data at each stage of the processing to analyze where the benefits occurred and how the improvement at one stage benefited the succeeding stages. REPROCESSING SEQUENCE EDITING The shot records were loaded to an interactive refraction modeling workstation after applying gain recovery edits derived from paper record displays. The first step in the interactive refraction modeling sequence is to pick the first breaks and perform any additional editing required. A combination of automatic and manual picking was used. As the data is displayed for picking, the interpreter has various display options. Typically, a dual-polarity display is used which slows peaks as a blue var and troughs in a red var in the opposite direction. This improved display cosmetic adds dynamic range to the data, making editing a much easier task. The data editing on this line amounted to removing as much as half the data in some areas, reducing the common depth point fold from 60 to 30. The stack with these edits applied (Figure 2) shows improved continuity throughout the line, as well as improved signal-to-noise ratio. REFRACTION STATICS AND MODELING The next step in the interactive refraction modeling sequence is to interpret the times derived from picking the first breaks. The refraction residual statics are those statics which best align the first-break picks on a refractor. These statics are most accurate at high spatial frequency and less accurate at long wavelengths, so a one spread length filter was applied to the statics.