Description

SW Analysis builds a frequency-versus-phase-velocity dispersion image (a MASW-style spectrum) for every source in a survey by phase-shifting and stacking the surface-wave (ground-roll) energy recorded across offset. It lets the user pick, automatically or by hand, the ridge of maximum energy in that image to obtain a dispersion curve (phase velocity as a function of frequency) for each shot, optionally smoothed and cleaned of outliers with a polynomial regression. The resulting per-source dispersion curves are the standard first step toward near-surface shear-wave velocity characterization used for statics corrections or shallow engineering studies; this module produces the picks, it does not perform the depth inversion itself.

Input data

Seismic data

The seismic dataset containing the raw shot gathers (typically dominated by ground-roll energy) that the dispersion spectra are computed from.

Trace headers

The trace header and geometry information used to group traces by source location and to compute each trace's source-to-receiver offset.

Gather index

The list of gathers the module iterates over as it builds the location map and computes a spectrum for each source in turn.

Parameters

Min frequency

Lower bound of the frequency band over which the dispersion spectrum is computed and displayed. Surface-wave energy relevant to near-surface work is usually concentrated at low frequency, so narrow the band to where coherent ground-roll energy is actually visible on the gather; this reduces computation time and removes clutter from frequencies with no useful dispersion trend. Valid range is 0 to 1000 Hz.

Default: 0 Hz.

Max frequency

Upper bound of the frequency band the dispersion spectrum covers. Set it high enough to include all frequencies where surface-wave energy shows a clear moveout trend on the gather; pushing it too high adds computation and can dilute the spectrum with noise or higher-mode energy that is not part of the fundamental-mode trend. Valid range is 0 to 1000 Hz.

Default: 70 Hz.

Min phase velocity

Lower bound of the phase-velocity search window scanned when building the dispersion image. This should bracket the slowest apparent surface-wave velocity expected for the near-surface materials at the site; setting it too high can clip the true dispersion trend off the bottom of the spectrum. Valid range is 1 to 10,000 m/s.

Default: 80 m/s.

Max phase velocity

Upper bound of the phase-velocity search window. This should bracket the fastest apparent surface-wave velocity expected at the site; setting it too low clips the true dispersion trend off the top of the spectrum, while setting it far higher than needed wastes computation and makes the true peak harder to see. Valid range is 1 to 10,000 m/s.

Default: 220 m/s.

Step phase velocity

Velocity-axis sample spacing of the dispersion image, i.e. the resolution of the resulting curve. A finer (smaller) step produces a smoother, more precise dispersion curve at the cost of more computation, especially over a wide Min/Max phase velocity range; a coarser step is faster but gives a blockier curve and less precise auto-picks. Valid range is 0.01 to 1000 m/s.

Default: 0.5 m/s.

Calc offset by

Chooses how the source-to-receiver offset used in the phase-shift stacking is computed. 'Offset' takes the value stored in the trace header directly; 'Coordinates' computes the offset geometrically from source and receiver X/Y coordinates. Use 'Coordinates' when header offsets are missing, unreliable, or inconsistent with the actual survey geometry (for example on crooked-line or 3D layouts); use 'Offset' when header offsets are trusted, since it is the faster option.

Default: Offset.

Picking parameters

Auto picking

When enabled, every time a source's dispersion spectrum is computed the module automatically extracts a dispersion curve by picking the strongest-energy velocity at each frequency, subject to the Threshold below, without requiring the user to click. Turn this off to build the curve entirely by hand, or if automatic picks are consistently noisy for the current dataset and a manual review is preferred.

Default: enabled.

Threshold

Fraction of the spectrum's overall peak amplitude that a frequency column's strongest velocity peak must reach before it is accepted as an automatic pick. Raising the threshold suppresses weak or noisy picks near the edges of the analysis band, giving fewer but more reliable points; lowering it extends picking further into frequencies where surface-wave energy is weak, at the risk of picking noise or higher-mode energy. Valid range is 0 to 1 (a fraction, e.g. 0.85 = 85% of the peak).

Default: 0.85.

Use regression

When enabled, a polynomial trend is fitted through the picked frequency/velocity points and used to smooth the displayed curve and to reject outlier picks that lie too far from the trend (see Velocity deviation). This is the main quality-control step for keeping the dispersion curve as a single clean fundamental-mode trend. Turn it off to keep the raw, unsmoothed picks exactly as picked, whether automatic or manual.

Default: enabled.

Velocity deviation

Maximum distance a picked point's velocity may lie from the fitted regression trend at that frequency before it is rejected as an outlier (active only when Use regression is enabled). Tighten this value to aggressively remove scattered or noisy picks and enforce a very smooth curve; loosen it to retain more of the original picks and allow more curve wiggle, for example on sites with genuinely rapid lateral or depth velocity variation. Valid range is 0 to 10,000 m/s.

Default: 5 m/s.

Regression pow

Controls the order of the polynomial curve fitted to the picked points (active only when Use regression is enabled). A low value fits a simple, gently curving trend appropriate for smoothly varying dispersion; a higher value lets the fitted curve bend more to follow more complex dispersion shapes, but risks overfitting to noise if set too high. Valid range is 1 to 9.

Default: 2.

Output data

Output matrix

The raw frequency-versus-phase-velocity dispersion spectrum (amplitude image) for the currently selected source, as displayed in the spectrum view.

SW picking item

The consolidated dispersion-curve database: for every source in the survey, the final set of frequency/phase-velocity pick points (after regression filtering, if enabled). This is the deliverable used downstream, typically for near-surface shear-wave velocity inversion or statics work, and can be exported to or imported from a text file using the module's Save/Load actions.

References

Park, C. B., Miller, R. D. and Xia, J., 1999, Multichannel analysis of surface waves (MASW): Geophysics, 64, 800-808.