Description

Statics - tidal computes a per-shot time correction that compensates for the rise and fall of sea level during a marine survey. It reads a tide-gauge record supplied by the user, finds the exact recording time of each shot (from the SEG-Y trace headers or from a separate shot-time log), interpolates the tidal height at that instant, and converts that height into a time shift using the speed of sound in water. Because sea level can move several meters over a tidal cycle, this offset changes the two-way travel time of a shot for reasons that have nothing to do with the subsurface, and removing it before stacking or further processing prevents smearing and misalignment of reflections. This module only computes and stores the tidal static values; a separate module is used to actually apply the correction to the trace data.

Input data

Seismic data

The seismic dataset whose trace headers supply each shot's recording timestamp when shot times are not taken from a separate external file.

Trace headers

The set of traces (one per source point) the tidal statics are computed for; each trace's associated source is what the computed static is keyed to.

Tidal data

A tide-gauge file: a time-stamped table of sea-surface elevation versus date/time, supplied by the user as a text or CSV file. This is the source of the tidal height values that get interpolated and converted into time statics.

Shot time data

An optional external file mapping shot (FFID) number to recording time, used instead of the SEG-Y trace-header timestamp. Only needed when 'Take shot times from external file' is enabled.

Parameters

Water velocity

The speed of sound in seawater used to convert the tidal-height change into a two-way time shift (static equals height divided by velocity). Adjust it, typically within 1480 to 1550 m/s, to match the actual water velocity for the survey area and season (temperature and salinity affect it) if it is known from a sound-velocity profile, since this value directly scales the size of every computed static. Valid range is 1 to 1,000,000 m/s.

Default: 1550 m/s.

Take shot times from external file

When off, the module reads each shot's recording time directly from the raw SEG-Y trace header. Turn this on only if the trace headers do not carry reliable timing, or a separate, more accurate shot-time log is available; a 'Shot time data' file and its format settings must then be supplied, and shot times are looked up by shot (FFID) number instead.

Default: disabled (off).

Tidal data format

Tidal height in feets

Enable this if the tide-gauge file reports height in feet rather than meters; the value is converted to meters before the static is computed. Leaving it off when the source data is actually in feet will produce a static roughly 3.28 times too small.

Default: disabled (off).

Two-digit year

Enable if the year column in the tidal data file is a 2-digit value (for example "24" instead of "2024"); the module infers the century by comparing it to the current year. Use this only for genuinely ambiguous legacy-format files.

Default: disabled (off).

Separator

The column delimiter used in the tidal data file. Must match the actual format of the file (comma-, space-, or tab-delimited text); an incorrect choice causes misaligned columns or a failure to read any records.

Default: Comma.

Header row

The row number of the tidal data file containing the column titles, used to populate the column-selection fields below. Set it to the actual header line number if the file has comment or metadata lines before the header. Minimum value is 1.

Default: 1.

Start row

The row number where the actual tidal data begins. Increase it if there are multiple header or comment lines before the data starts. Minimum value is 1.

Default: 2.

Julian year column

The column in the tidal data file holding the year. This field is auto-populated with the file's actual column names once a tidal data file is selected; together with the day, hour, minute and second columns it forms the timestamp used to interpolate tidal height at each shot time.

Default: None (must be set once a file is loaded).

Julian day column

The column in the tidal data file holding the day of year (Julian day, 1-366), used together with the other time columns to build each tidal reading's timestamp.

Default: None (must be set once a file is loaded).

Julian hour column

The column in the tidal data file holding the hour of day for each tidal reading.

Default: None (must be set once a file is loaded).

Julian minute column

The column in the tidal data file holding the minute for each tidal reading.

Default: None (must be set once a file is loaded).

Julian second column

The column in the tidal data file holding the second for each tidal reading. Together, the year/day/hour/minute/second columns must all be set correctly, otherwise the module cannot build a valid timestamp and reports an error.

Default: None (must be set once a file is loaded).

Tidal height column

The column in the tidal data file holding the sea-surface elevation (tidal height) value. This is the quantity that gets interpolated at each shot time and converted into the tidal static.

Default: None (must be set once a file is loaded).

Shot time data format

These settings are only used, and only shown, when 'Take shot times from external file' is enabled.

Separator

The column delimiter used in the shot time data file. Must match the actual format of the file (comma-, space-, or tab-delimited text).

Default: Comma.

Two-digit year

Enable if the year column in the shot time file is a 2-digit value; the century is inferred by comparison with the current year, as with the same setting in the Tidal data format group.

Default: disabled (off).

Header row

The row number of the shot time file containing the column titles, used to populate the column-selection fields below. Minimum value is 1.

Default: 1.

Start row

The row number where the actual shot time data begins. Minimum value is 1.

Default: 2.

Step row

The row increment used while scanning the shot time file's data rows; for example, set to 2 to read only every other row. Leave at 1 for a normal contiguous shot log. Minimum value is 1.

Default: 1.

FFID column

The column holding the Field File ID (shot/record number), used to match each seismic trace's FFID to the corresponding row in the shot time file. Every trace's FFID must exist in this file or the run fails.

Default: None (must be set once a file is loaded).

Julian year column

The column holding the year of each shot's recording time in the external shot time file.

Default: None (must be set once a file is loaded).

Julian day column

The column holding the day-of-year of each shot's recording time in the external shot time file.

Default: None (must be set once a file is loaded).

Julian hour column

The column holding the hour of each shot's recording time in the external shot time file.

Default: None (must be set once a file is loaded).

Julian minute column

The column holding the minute of each shot's recording time in the external shot time file.

Default: None (must be set once a file is loaded).

Julian second column

The column holding the second of each shot's recording time in the external shot time file. Together with the year/day/hour/minute columns, this defines the exact recording time used to interpolate the tidal height for that shot.

Default: None (must be set once a file is loaded).

Advanced params

Sea bottom receivers

Controls whether the tidal static is applied once or doubled. Leave off for a conventional towed-streamer survey, where both source and receivers float near the tide-affected sea surface and the static is doubled to account for the source-side and receiver-side contribution. Enable it for ocean-bottom node or cable surveys, where the receivers sit fixed on the seafloor and are unaffected by tide, so only the single source-side correction is applied. Using the wrong setting for the acquisition geometry will roughly halve or double every computed static.

Default: disabled (off).

Output data

Statics correction item

A statics-storage object holding one computed tidal time shift per source/shot. This is the standard static-correction data used by downstream modules that actually apply static shifts to the trace data; this module only computes and stores the values, it does not shift the traces itself.

References

Lacombe, C., Schultzen, J., Butt, S. and Lecerf, D., 2006, Correction for water velocity variations and tidal statics: SEG Technical Program Expanded Abstracts, 25, 2881-2885.