Description

The VSP 2D Modeling module generates synthetic Vertical Seismic Profile (VSP) shot records for a 2D survey geometry using a depth-domain velocity model. It simulates wave propagation through the velocity model using the full elastic wave equation, placing sources at surface positions and receivers either in the borehole (for classical VSP geometry) or at the surface (for walk-away or 2D survey configurations). The module outputs synthetic seismograms that can be used for velocity model QC, wavelet estimation, or planning VSP acquisition geometry.

Use this module to validate a near-surface or depth velocity model by comparing synthetic VSP arrivals with field VSP data, or to generate reference data for time-depth conversion. The output is saved directly to a GSD file.

Input data

Output file name

Path to the output GSD file where the synthetic VSP seismograms will be written. This file is created or overwritten each time the module executes.

Depth velocity

Link to the depth-domain velocity model used for forward wave propagation. The model must be defined on a depth axis (in metres). Provide a spatially smooth velocity field for stable numerical modelling; sharp velocity contrasts may require a finer modelling grid.

Parameters

Output DT

Sample interval (s) of the output synthetic seismograms. This is independent of the internal modelling time step. Default: 0.004 s (4 ms). Set this to match the sample rate of your field VSP data.

Detect topography

Controls how the module represents the near-surface air layer above topographic relief. No topo — flat surface assumed. By air velocity — cells above the surface are filled with the Air velocity value. By headers — surface elevation is taken from trace headers. Default: By air velocity.

Air velocity

Velocity (m/s) assigned to model cells above the topographic surface. Active only when Detect topography is set to By air velocity. Default: 310 m/s (approximate speed of sound in air at standard conditions).

Datum

Reference datum elevation (m) relative to which depths in the velocity model are interpreted. Default: 2000 m. Set this to the elevation datum of your velocity model.

Additional apperture

Extra lateral padding (m) added beyond the outermost source and receiver positions when building the modelling grid. This prevents model edge effects from interfering with the synthetic seismograms. Default: 500 m.

Distribution type

Controls the spatial layout of sources and receivers. Selects between standard surface-to-borehole VSP geometry and alternative acquisition configurations.

Source start

Index of the first source position to model. Default: 1.

Source end

Index of the last source position to model. Default: 1000000 (effectively all sources). Reduce this to limit modelling to a subset of the source array.

Source step

Decimation step between modelled source positions. A value of 1 models every source; larger values skip intermediate sources to reduce computation time. Default: 5.

Receiver start

Index of the first receiver (depth level) to include in the output. Default: 1.

Receiver end

Index of the last receiver (depth level) to include in the output. Default: 1000000 (all receivers).

Receiver step

Decimation step between output receiver levels. A value of 1 includes every receiver depth; larger values thin the borehole receiver array. Default: 5.

Sources count around rcv left

Number of surface source positions to the left of each receiver used for constructing horizontal shot gathers. Default: 10.

Sources count around rcv right

Number of surface source positions to the right of each receiver used for constructing horizontal shot gathers. Default: 10.

Sources step horizontal

Spacing (source index step) between consecutive surface sources used in horizontal shot gather construction. Default: 5.

Receivers count vertical

Total number of borehole receivers distributed along the vertical receiver array. Default: 10.

Receivers step horizontal

Vertical spacing (m) between adjacent borehole receivers. Default: 200 m.

Padding size

Number of absorbing boundary cells added around the modelling grid to prevent artificial reflections from the model edges. Default: 20 cells. Increase this if you observe spurious late-time reflections in the output synthetic data.

Free surface

When enabled, applies a free-surface boundary condition at the top of the model, allowing surface-related multiples and ghost reflections to be included in the synthetic output. Disable this option to produce primary-only (no free surface) synthetics. Default: disabled.

Frequency

Dominant frequency (Hz) of the source wavelet used in the modelling. Default: 20 Hz. Set this to the dominant frequency of the field source wavelet for the most accurate comparison with field VSP data.

Calculate params

When enabled, the module automatically calculates the optimal internal modelling time step and number of samples from the model dimensions and maximum velocity, overriding the manually specified Time step and Samples count values. Use this option to ensure numerical stability before running with custom parameters.

Time step

Internal modelling time step (s) used in the finite-difference wave propagation. This value must satisfy the Courant-Friedrichs-Lewy (CFL) stability condition: it must be smaller than the Model resolution divided by the maximum velocity. Default: 0.0005 s (0.5 ms).

Samples count

Total number of internal modelling time steps. Together with Time step, this defines the total simulation duration. Default: 10000. Set this so that the simulation runs long enough to capture all arrivals of interest.

Max t

Maximum recording time (s) for the output synthetic seismograms. Traces in the output GSD file will be truncated to this length. Default: 5 s. Set this to slightly longer than the two-way travel time to the deepest reflector of interest.

References

Virieux, J., 1986, P-SV wave propagation in heterogeneous media: Velocity-stress finite-difference method: Geophysics, 51, 889-901.