removeAngular

Description

removeAngular is a specialized utility designed to correct for angular variations in backscatter intensity within OMG-HDCS merged files. It first calculates the mean backscatter response for each beam (or beam number, which serves as a crude proxy for grazing angle) across an entire dataset. Then, it uses this average response to flatten the backscatter, aiming to produce a more “angle-independent” backscatter measurement.

The tool can either re-calculate backscatter values (for EM1000/EM100_depth_ss/Seabat9001 systems) or apply a correction to existing reflectivity/calibrated backscatter values.

Usage 

removeAngular <OMG_HDCS_datafile(s)> [OPTIONS]

Arguments

Option Description  
<OMG_HDCS_datafile(s)> Required. One or more paths to OMG-HDCS merged files to be processed. These files are modified in place.  
-v Enable verbose output.  
-recalc Re-calculates calibratedBackscatter for each beam based on system-specific algorithms (e.g., EM1000, EM100_depth_ss, Seabat9001). This option often requires associated .ss_data files for EM1000.  
-use_calib Use the calibratedBackscatter field instead of reflectivity for averaging and correction.  
-nocorrect Do not apply the correction (only calculates and dumps beam averages if recalc is not used).  
-draft <val> Specifies the draft (in meters) to use in amplitude conversion calculations. 4.0
-signal <min_dB> <max_dB> Defines a signal range (in dB) for backscatter values (used during recalculation for EM1000). 120.0 to 180.0

How It Works

  1. File Processing (Loop 1 - Calculation): The tool iterates through each provided merged file:
    • Summary Reading: Reads the summary header to determine sonar type.
    • Recalculation (-recalc): If specified, it iterates through each profile and beam, recalculating the calibratedBackscatter based on the sonar toolType. For EM1000, this involves reading raw sidescan traces from an .ss_data file and applying a 1994 algorithm considering incidence angle. For other types, it uses EM_convert_amplitude.
    • Beam Average Accumulation: If correct_flag is enabled, it accumulates the sum of reflectivity or calibrated backscatter values and the count for each beam number (sum[j], count[j]) across all pings in all input files.
  2. Beam Average Output: After the first loop, if correct_flag is enabled, it calculates the average backscatter for each beam number and dumps these averages to a file named Beam.Averages.
  3. File Processing (Loop 2 - Correction): If correct_flag is enabled, the tool re-opens each merged file:
    • Overall Average: It calculates an overall average of the mean beam responses.
    • Correction Application: For each beam in each profile, it calculates a correction factor: (Overall_Average - Mean_Beam_Response[beam_number]). This correction is added to the beam’s reflectivity or calibrated backscatter.
    • Pseudo Angle Independent Backscatter: The corrected value is stored in the pseudoAngleIndependentBackscatter field of the beam structure, and clipped to 0-255.
  4. In-Place Update: The modified beam records (specifically the pseudoAngleIndependentBackscatter field) are written back to the input merged files.

Output Files

  • The input merged files are modified in-place.
  • Beam.Averages: An ASCII file containing the mean backscatter response for each beam number.

Dependencies

  • OMG_HDCS_jversion.h: For OMG-HDCS data structures.
  • support.h: For general utility functions and error handling.
  • Echo_calib.h: For EM_convert_amplitude function.

Notes

This tool is used to generate angle-independent backscatter measurements, which are more indicative of the true physical properties of the seafloor. By removing the angular dependence caused by beam patterns and geometry, the resulting backscatter can be used more effectively for seafloor classification. The tool modifies merged files in place, so backups are recommended.