Description

SW Modeling reconstructs (forward-models) the ground-roll / surface-wave energy on a common-shot gather using a frequency-versus-phase-velocity dispersion curve picked previously in the 'SW Analysis' module. For each output trace it takes nearby traces within a chosen offset window, phase-shifts each frequency component according to the offset difference and the picked phase velocity at that frequency, and stacks the results to synthesize what the surface wave should look like on that trace. It is used to quality-control a picked dispersion curve by comparing the synthetic surface wave against the real data, or to generate a clean surface-wave-only model that can be subtracted elsewhere in the flow to attenuate ground roll. The module only supports common-shot gathers with traces sorted by absolute offset.

Input data

SW picking item

The per-source frequency/phase-velocity dispersion curve produced by the 'SW Analysis' module (spatially interpolated from nearby picked sources if the current source has no exact pick). This supplies the velocity-versus-frequency relationship the synthesis is built from.

Input gather

A single common-shot gather, sorted by absolute offset, whose surface-wave energy is to be modeled. The module requires exactly one source per gather and rejects gathers with traces out of offset order.

Parameters

Work mode

Selects the gather domain the modeling operates on. Currently 'Common shot' is the only available option, which enforces the requirement that each input gather contain exactly one source and be sorted by absolute offset, mirroring the per-source organization used by 'SW Analysis' so the two modules stay paired shot by shot.

Default: Common shot.

Offset window

For each output trace, only other traces whose absolute offset differs from that trace's offset by no more than this window contribute to the phase-aligned stack used to reconstruct it. Widen it to pull in more traces per output sample (higher fold, a smoother and more stable surface-wave model, but with more risk of smearing across offset ranges where the true dispersion behavior changes); narrow it to keep the model tightly local to true near-offset behavior, at the cost of lower fold and noisier or zeroed output near the edges of the gather. Valid range is 0 to 10,000 m.

Default: 500 m.

Min traces for modeling

Minimum number of contributing traces (fold) within the offset window required before the module will produce a modeled value for an output trace; if fewer traces contribute, that trace is set to zero instead of being built from an unreliable few-trace stack. Raise this value to suppress noisy or unstable model traces at gather edges or in sparsely covered offset ranges; leave it at the minimum to maximize output coverage, accepting more noise-prone results where trace density is low. Valid range is 2 to 10,000 traces.

Default: 2 traces.

Output data

Output gather

The modeled (synthesized) surface-wave gather, with the same geometry as the input, where each trace is the phase-matched stack of nearby traces built from the picked dispersion curve. Traces with fold below the Min traces for modeling threshold are output as zero.

Difference gather

When the module's difference option is enabled, this holds the input gather minus the modeled gather, letting the user visually check how much surface-wave energy the model matches or removes.

References

Haskell, N. A., 1953, The dispersion of surface waves on multilayered media: Bulletin of the Seismological Society of America, 43, 17-34.

Xia, J., Miller, R. D. and Park, C. B., 1999, Estimation of near-surface shear-wave velocity by inversion of Rayleigh waves: Geophysics, 64, 691-700.