getSnippetVariance

Description

getSnippetVariance is a utility designed to analyze the variance of backscatter snippet data from multibeam sonars, primarily focusing on Reson systems. It processes merged files, extracts snippet intensities, and calculates the mean and standard deviation of these intensities after applying a calibration.

The tool uses a Digital Terrain Model (DTM) to estimate grazing angles and calculate ensonified areas, which are crucial inputs for the calibrate_reson function. It outputs the mean and standard deviation for each beam.

Usage 

getSnippetVariance <filename.merged> -dtm <dtm.r4> [-debug]

Arguments

Option Description
<filename.merged> Required. The path to the input OMG-HDCS merged file (with associated .ss_data file).
-dtm <dtm.r4> Required. Path to the DTM file in .r4 (float grid) format.
-debug Enable debug output.

How It Works

  1. Initialization: Initializes sum, sum_squares, and count arrays for statistical accumulation per beam. Sets debugging flags.
  2. DTM Loading: Opens the DTM file (-dtm), reads its header, and loads the DTM data into memory.
  3. File Opening: Opens the input merged file and its corresponding .ss_data file.
  4. Summary Header Reading: Reads the summary header and loads sonar parameters from the .param file.
  5. Profile Iteration: Loops through each profile (ping) in the merged file:
    • Reads the current profile header and raw beams.
    • Grazing Angle and Ensonified Area Calculation: Calls get_dtm_grazing_angles (to estimate grazing angles using the DTM) and calc_ensonified_area (to calculate ensonified areas). These are crucial for the backscatter calibration.
    • Snippet Calibration: For each beam with valid data:
      • Seeks to the beam’s snippet data in the .ss_data file and reads IScans (raw snippet intensities).
      • Calls calibrate_reson (an external function, likely from Echo_calib.h) to apply calibration to these IScans, producing Scans (calibrated snippet intensities). This function uses grazing angles, ensonified areas, and profile/beam data.
      • Extracts the calibrated snippet intensity at the beams[j].centre_no (the central sample of the snippet).
      • Accumulates the squared value of this snippet intensity in sum_squares[j] and the raw value in sum[j], incrementing count[j].
  6. Statistical Output: After processing all pings:
    • For each beam, if count[i] is greater than 1, it calculates the mean and std_dev (standard deviation) of the snippet intensities using the accumulated sum, sum_squares, and count values.
    • Prints beam_index, mean, and std_dev to standard output.
  7. Cleanup: Closes all open files and frees allocated memory.

Output Files

The tool prints a three-column ASCII listing to standard output:

  • beam_index
  • mean of calibrated snippet intensity
  • std_dev of calibrated snippet intensity

Dependencies

  • OMG_HDCS_jversion.h: For OMG-HDCS data structures.
  • array.h: For JHC_header structure and DTM data handling.
  • support.h: For general utility functions and error handling.
  • grazing_angle.h: For get_dtm_grazing_angles function.
  • ensonification.h: For calc_ensonified_area function.
  • Echo_calib.h: For calibrate_reson function.

Notes

Analyzing snippet variance provides insight into the homogeneity of the seabed and the quality of the backscatter measurements. High variance might indicate a heterogeneous seafloor, noise, or an ineffective calibration. This tool is useful for quality control and seabed characterization, particularly for Reson multibeam systems. The calibration step uses a DTM to ensure accurate grazing angles and ensonified areas, which are critical for meaningful backscatter values.