Walt Lynn

Coherent noise in marine seismic data

Three mechanisms that might produce noise patterns on stacked sections are examined: direct and trapped waves that propagate outward from the seismic source, cable motion caused by the tugging action of the boat and tail buoy, and scattered energy from irregularities in the water bottom and sub-bottom. Different noise patterns can be observed on shot profiles and common-midpoint (CMP) gathers; these patterns can be diagnostic of the dominant mechanism in a set of data. Field data suggest that the dominant noise is from waves scattered within the shallow sub-bottom. This type of noise is enhanced by CMP stacking. Moreover, it can be as strong as surface wave noise on stacked land seismic data as well. Moveout filtering is best for suppressing the noise while preserving signal. Since the scattered noise does not exhibit a linear moveout pattern on CMP-sorted gathers, moveout filtering must be applied either to traces within shot records and common-receiver gathers or to stacked traces. Our data example demonstrates that although it is more costly, moveout filtering of the unstacked data is particularly effective because it conditions the data for the critical data-dependent processing steps of predictive deconvolution and velocity analysis.--Modified journal abstract.

The zero‐velocity layer: Migration from irregular surfaces

Seismic data acquired in areas with irregular topography are usually corrected to a flat datum before migration. A time‐honored technique for handling elevation changes is to time shift the data before application of migration. This simple time shift, or elevation‐static correction, cannot properly represent wide‐angle or dipping reflections as they would have been recorded at the datum. As a result, when elevation varies significantly, accuracy in event positioning may be compromised for migration and other wave‐equation processes, such as dip moveout processing (DMO). Traditionally, such over‐ and under‐migration artifacts have been dealt with by increasing or decreasing the migration velocity. However, simple adjustment of the migration velocity cannot undo the wave‐field distortions induced in seismic data acquired over varying elevations. More sophisticated and accurate solutions such as wave‐equation datuming are too computationally demanding for routine use. Here, we propose an efficient and accura...

Experimental investigation of interference from other seismic crews

In a study of the contamination of reflection seismic data by interfering noise from other seismic crews, controlled experiments were performed in the Gulf of Mexico and the North Sea. In each experiment, a survey ship traversed a line several times collecting both data free of and data contaminated by interfering crew noise. In the Gulf of Mexico experiment, the “noise” ship followed a prescribed course about 11 km from the survey ship. In the North Sea experiment, the noise ship was positioned at stationary locations 10 and 40 km broadside to the survey line. Recorded interference noise in both experiments had peak amplitudes well above the 0.5 to 1.5 Pa (5 to 15 μbar) limit beyond which crews typically must agree on time‐sharing. Despite recorded crew noise that was three to eight times higher than levels typically considered acceptable, the conventionally processed common‐midpoint stack of the contaminated Gulf of Mexico data shows only slight evidence of the interference noise; in contrast, the North...

Uncertainty implications in azimuthal velocity analysis

Commercial methodologies of azimuthal velocity analysis fall into two categories: 1) azimuth sectoring and 2) no azimuth sectoring. A wide-azimuth seismic survey was processed independently by two processing contractors – one using azimuthal sectoring, the other using no azimuth sectoring. A comparison of the corresponding azimuthal interval velocity maps revealed little similarity between the two approaches. Several possible reasons were examined to account for the differences. I conclude that the azimuth sectoring approach for the data and target zone considered here yields seemingly plausible, but meaningless results. The non-azimuth sectoring approach is statistically far more robust and provides azimuthal velocity data that are consistent with other observations. Azimuthal velocity analysis via azimuthal sectoring should be avoided except for very thick layers, and then should only be considered after careful modeling of the impact of rms velocity uncertainty.

Cascaded f-k migration; removing the restrictions on depth-varying velocity

Stolt’s frequency‐wavenumber (f-k) method is computationally efficient and has unlimited dip accuracy for constant‐velocity media. Although the f-k method can handle moderate vertical velocity variations, errors become unacceptable for steep dips when such variations are large. This paper describes an extension to the f-k method that removes its restrictions on vertical velocity variation, yielding accuracy comparable to phase‐shift migration at only a fraction of the computational time. This extension of the f-k method is based on partitioning the velocity field, just as in cascaded finite‐difference migration, and performing a number of stages of f-k migration. In each stage, the migration‐velocity field is closer to a constant—the ideal situation for the f-k migration method—than when the migration is done conventionally (i.e., in just one stage). Empirical results and error analyses show that, at most, four stages of the cascaded f-k algorithm are sufficient to migrate steep events as accurately as by...

Efficient migration through complex water‐bottom topography

An efficient means of imaging structures beneath complex water‐bottom topography is obtained using a conventional time‐migration algorithm with a simple modification to the migration‐velocity field. The process consists of two migration steps: one with the migration velocity set to zero below the water bottom and the other with the migration velocity set to zero above the water bottom. Between the two steps the data are vertically time shifted to account for the lateral velocity variations between the water‐sediment interface. The time shifts used are equivalent to the so‐called “thin‐lens” term used in depth‐migration algorithms. Efficiency is obtained by applying the thin‐lens term only once and by using computationally optimized time‐migration algorithms. Results obtained from this technique are nearly identical to more costly wave‐equation, layer‐replacement, and depth‐migration techniques.

Efficient Migration Through Irregular Water-bottom Topography

An efficient means for imaging structure beneath complex water-bottom topography can be obtained using conventional time-migration algorithms and a simple modification to the migration-velocity field. The process consists of two migration steps: one with the migration velocity set to zero below the water bottom and the other with the migration velocity set to zero above the water bottom. Between the two steps the data are time-shifted to account for the so-called “thin-lens” term used in depth-migration algorithms. Efficiency is obtained by applying the thin-lens term only once and by using existing computationally optimized time-migration algorithms. Results obtained from this technique are nearly identical to more costly layer-replacement and depth-migration techniques.

Extended stolt F‐K migration

Despite our understanding that depth migration is a more powerful imaging tool than time migration, time migration still constitutes the majority of migration done today. In selecting a time-migration algorithm, three primary criteria are of concern: accuracy in imaging steep dips, accuracy in the presence of vertical velocity variation, and computational effort. The ideal algorithm would be efficient and unlimited in its ability to image steep dips in arbitrary vertical velocity structures. It would also accommodate gentle lateral velocity variations deemed acceptable for time-migration methods.