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This module performs a semblance-based analysis to determine the optimum LMO (Linear Moveout) reference velocity and weathering velocity for a seismic gather. For each combination of reference velocity and weathering velocity within the user-defined search ranges, the module applies a linear time shift to all traces and computes a semblance spectrum over the defined correlation window. The resulting two-dimensional semblance panel allows the user to identify the velocity pair that best flattens the linear events, such as direct waves, head waves, or refraction arrivals. In addition to the semblance output, the module also outputs the gather with the manually specified LMO correction applied using the fixed Reference velocity and Vweath velocity parameters.
Use this module before applying a fixed LMO correction to scan the velocity-space semblance and determine the best-fitting velocity pair for the dataset. This is particularly useful for refraction statics workflows and for preparing gathers for linear noise attenuation with the LNA module.
Container for the standard input data connections, including the input seismic gather, trace headers, and SEG-Y file handle. Connect this to the output of the preceding module in the processing sequence.
The input seismic gather on which the LMO velocity analysis is performed. The gather should be sorted by offset. It typically represents a source or CMP gather containing both refraction arrivals and reflection energy whose moveout is to be analysed.
Selects whether the LMO correction applied to the output gather is Forward (apply the linear time shift) or Reverse (remove a previously applied LMO). The default is Forward. The semblance scan is computed regardless of the selected direction.
The zero-offset reference time (s) used as the baseline for the LMO time shift computation. The default is 0 s. Set this to the two-way time of the refractor you want to flatten if it does not pass through zero offset at time zero.
The time window (s) used for semblance smoothing when computing the LMO semblance spectrum. The default is 24 s (note: this is in native time units as configured in the project). A longer window produces broader but smoother semblance peaks. Reduce this value for high-resolution analysis of rapidly changing refraction velocities.
The fixed reference velocity (m/s) used to compute the LMO time shift applied to the output gather. The default is 2700 m/s. This should be set to the velocity that best represents the apparent velocity of the linear events you want to flatten (for example, the refraction headwave velocity). This parameter does not affect the semblance scan range — use the Lmo Velocity Param range to define the scan.
The fixed weathering layer velocity (m/s) used to account for elevation differences between traces when applying the LMO correction to the output gather. The default is 900 m/s. Set this to the estimated near-surface (weathering) velocity for the survey area.
Container grouping the scan range parameters for the LMO reference velocity axis of the semblance spectrum. Define the velocity range that covers expected refraction velocities in the survey.
The minimum reference velocity (m/s) in the LMO semblance scan range. The default is 1500 m/s. Set this to the lower bound of the expected refraction velocity range.
The maximum reference velocity (m/s) in the LMO semblance scan range. The default is 9500 m/s. Set this to the upper bound of the expected refraction velocity range.
Container grouping the scan range parameters for the weathering velocity axis of the semblance spectrum. Define the velocity range that covers the expected near-surface (weathering) velocities in the survey area.
The minimum weathering velocity (m/s) in the scan range. The default is 500 m/s. Set this to the lowest near-surface velocity expected in the survey.
The maximum weathering velocity (m/s) in the scan range. The default is 1800 m/s. Set this to the upper bound of the near-surface velocity range. For hard-rock surveys, you may need to increase this value.