deApparent

Description

deApparent is a utility designed to “undo” the “apparent depth” correction often applied by SeaBeam sonar systems. SeaBeam systems typically present depths corrected to a constant sound velocity (e.g., 1500 m/s), even when the actual sound velocity profile (SVP) in the water column is different. This tool uses a provided SVP file to convert these “apparent depths” back to their true water depths, effectively removing the 1500 m/s assumption.

This process is critical for accurate hydrographic surveying, as it ensures that depths reflect the actual water column properties rather than a simplified model.

Usage 

deApparent -svp <SVP.file> <mergefile(s)> [-v] [-depscale <value>]

Arguments

Option Description
-svp <SVP.file> Required. Path to the Sound Velocity Profile (SVP) file. This file should contain depth and corresponding in-situ sound velocity pairs.
<mergefile(s)> Required. One or more OMG-HDCS .merged files containing the SeaBeam depth data to be corrected.
-v Enable verbose output.
-depscale <value> (Optional) Manually set a depth scale factor. Defaults to 0.0105.

How It Works

  1. Initialization: Parses command-line arguments to get the SVP file path, input merged files, and optional parameters (verbose_flag, dep_scale).
  2. SVP File Reading:
    • Opens and reads the provided SVP file.
    • Stores depth and in-situ sound velocity pairs into an internal watercolumn structure array.
  3. Harmonic Mean Calculation:
    • Iterates through the read SVP data to calculate the harmonic mean sound velocity for each layer and the cumulative two-way travel time (TWTT) to each depth.
    • From this, it calculates harmonic_SSP (harmonic mean sound speed to a given depth) and a corresponding scale factor (harmonic_SSP / 1500.0) that represents the ratio of the true average sound speed to the 1500 m/s assumed by the sonar.
  4. Merged File Processing Loop:
    • For each input .merged file:
      • Opens the file in read/write mode (r+).
      • Reads the OMG_HDCS_summary_header.
      • Iterates through each profile (ping) in the merged file:
        • Reads the OMG_HDCS_profile_header.
        • Reads the OMG_HDCS_beam array for the current profile.
        • Depth Correction: For each beam that contains an observed depth:
          • Determines the appropriate SVP layer based on the beam’s observed depth.
          • Calculates an interpolated dep_scale factor from the harmonic_SSP values, accounting for the depth range.
          • Applies this dep_scale factor to the beams[i].observedDepth, effectively converting the 1500 m/s-corrected depth back to the true water depth.
        • Writes the modified OMG_HDCS_beam array back to the .merged file.
      • Closes the current .merged file.
  5. Cleanup: Exits.

Output

The input .merged files are modified in-place, with the “apparent depths” converted to true water depths based on the provided SVP.