Quick Start Guide to Sub-Bottom Processing

This guide will show you how to import and process sub-bottom data in SonarWiz

For more information about the individual commands, please refer to the User Guide found in the Help Menu of SonarWiz. You can download a PDF version of this guide here.

By following this procedure, you will learn how to:

Import sub-bottom data into SonarWiz
Apply bandpass filters to sparker and boomer data
Bottom Track sub-bottom profiles
Topographically correct your SBP data with tides or bathymetry
Compute Intersections
Digitize Acoustic Horizons
Create thickness reports between acoustic reflectors
Create isopach maps of sub-surface layers
Export profiles with fixed vertical and horizontal scales

Download Demonstration Data
You can download a copy of the data set used here and follow along on your own computer.
Dataset to Download.

Instructions

Click the New Project Icon to open the Create New Project Dialog
Enter a Project name and Project Folder, then set the approximate position of the data. This can be done either by entering in a known location or Get from File – navigate to a single file from the project (note: at this point, only one file is needed to set the geodesy, regardless of the number to be processed).

Click the Automatically Select CRS button to allow SonarWiz to select a UTM coordinate system for you or use the Browse Coordinate Systems button to set the project coordinate system yourself.
Open the Program Properties (click the green “chicken-foot” button)
Select the Advanced Settings page and review the Default Water Column Sound Velocity and Default Sediment Sound Velocity.
In the program, go to the second tab -- Post Processing -- and select Import.
Select data files to import. Multiple files can be selected at once.
In the File type Specific Options… button, you can select particulars to your file format and system. This is often necessary for SEG-Y import to properly extract position and depth.

Click OK to close the File Type Specific Options panel.
In the Import dialog, check Sub-Bottom to import Sub-Bottom from your files. Click the Options button next to sub-bottom to set the import channels you would like to import.
Select one or more SBP files to import and click Import
The sub-bottom data will be loaded into the program. The next few steps get the data adjusted with gains, bottom track and other tools to create a nice image.
To view the sub-bottom profile, you can double-click the line in the map window or in the Project Explorer. (If your profile doen’t match the image below, don’t worry, we’ll fix it in the next step)
To adjust the colors in the profile, open the Color window by clicking the last button in the toolbar

Adjust the Scale Mode to Manual and use the mouse to adjust the color mapping in the histogram widget until the profile image looks good.

As you adjust the gains in the following steps this will change the histogram of intensity values in the project. You will probably need to re-adjust the colors to make the data look good again.
Broadband sources such as Sparkers, Boomers and Air-Guns (not chirp systems) may need bandpass filtering applied to remove low frequency noise that overprints the entire profile. This should be applied now before proceeding with bottom tracking. If the digitizer window is not open, Double-click on a SBP file and select the Appearance tab of the digitizer. Click the Show Spectrograph icon.

Clicking the Appearance button on the Spectrograph window will allow you to switch to a logarithmic frequency display as shown in the figure below.
In the Spectrograph window, turn ON the bandpass filter. Set the High-pass slider to the upper cutoff frequency (start with 2500 Hz), set the Low-pass slider to the lower cutoff frequency (start with 250 Hz). Set the Window to Blackman and set the Number of taps to 500 or more.

As you sweep your mouse across the data in the SBP Digitizer, the spectrograph window will display the raw data in blue and the filtered data in red. You want red only in your pass band window. If it isn’t clipping at your pass band, increase the number of taps.
In the Spectrograph window, Click the Apply to File button to test out your bandpass filter. When you’ve got the pass-band set up properly, you can see a dramatic reduction in low-frequency broadband noise as shown in this screen shot:
If the digitizer window is not open, Right click on a file and select the Bottom Track... option, otherwise, select the Bottom Tracking tab of the digitizer. Set the blanking zone (distance from ping to an area below any apparent noise) and threshold (a value of change detection) then click the Track All button. Typically, the lower number finds smaller changes, and could track noise/fish, etc in the water column. A number too high will go past the bottom into the data. In this example, I use a blanking of 10 and threshold of 1.
Go through all of the files, using the next and previous buttons until you reach the end of the files. (See also the Batch Bottom Track command)
Gain controls. Select the Appearance Tab in the SBP Analysis Window and click on the Gain Settings Icon. This will bring up the gain control window. Note: you only need to do these settings on a single file, as we will use the “Make Like Others” option to copy over the settings later.

Here you will find the Bandpass filter, Ping Stacking, Water Column Blanking option and AGC. Try different settings to see which makes the image look best.
Result:
If this is the setting that is to be used, then we will make the others just like this one. Select the file, and go to Make Others Like This.

A dialog box will show which files to use, and which settings to make. You can select ALL files and Apply All settings, which will take in the gain and other changes made to the first file. Bottom track is not an option, which is why we did that initially.
The next step is to vertically align the sub-bottom data to a bathymetric surface. Right-click the Grids branch of the Project Explorer and select Add/Import Grid Files… to load your grid into the program.

You can adjust the shaded relief effect of the grid in the View menu.
Click on the Post Processing tab > SBP Tides and Vertical Offsets… command

Check to enable Bathy Grid Separation and select the bathymetry grid from the drop-down menu and select all of the sub-bottom files as shown.

Click OK.
The next step is to Compute Intersections between our SBPs. This allows SonarWiz to visualize intersecting profiles and their feature markers in the digitizer. It also sets up navigation jumps between the intersecting lines (double-click the intersection line in the digitizer).
Open the Intersection Manager from the Post Processing ribbon

Click the Compute button

Click OK to close the Intersection Manager
To visualize how your datum alignment worked, Right-Click an intersection marker in the map view (or in the Project Explorer) and select Quick View 3D command

This will open the 3D viewer in intersection mode where you can examine the vertical alignment of your intersecting profiles.
In the Sub-bottom Analysis window, select the Bottom Track tab and convert the bottom track to a seafloor reflector by clicking on the Make Reflector button.

Set the Downsample bottom track points value to 1 (All) and click OK.
Switch to the Acoustic Reflectors tab and note that the new seafloor reflector has been added to the drop down list:

It is also listed in the Project Explorer under the Features branch:
To digitize an Acoustic Reflector in the sub-bottom analysis window, click the New button. This will open the Create New Reflector dialog where you can set the Name, sound velocity and line properties of the reflector. The sound velocity entered here is used to compute the depth of this reflector below the seafloor.
Note: One or more features that have the same Name are implicitly considered observations of the same horizon. Later, you can export features by name and all of the observations will be collected together and treated like a single unit.

Click OK to start digitizing the new reflector.

Your cursor will turn into a pen and you use the left mouse button to trace the reflector in the profile. Right-Click 1 time to lift the pen and move to a new location to start tracing the same reflector again. Right-Click 2 times to finish drawing and close the feature.

When you close the feature drawing (Right-Click 2 times), you will see your feature drawn in the display. The feature will also be listed in the Features branch of the Project Explorer and in the Feature drop-down list of the SBP Analysis Window.
To Edit a feature. Select the feature with the mouse and use the toolbar icons in the Acoustic Reflector ribbon menu to modify the vertices.
To Compute the Thickness between two features. Select the Compute Thickness icon in the Acoustic Reflector ribbon menu. Then specify the Top and Bottom reflectors. Remember you can convert your bottom track to a reflector (See Step 17 above).

This will produce a Thickness feature which you can export later in the workflow.
Once you have digitized all of your acoustic horizons and computed thicknesses between horizons, you can export these features to spreadsheets and 3D vector files from the Feature Manager. Open the Feature Manager from the Post Processing Ribbon menu.
To Export a Thickness feature as a table of positions and thickness values click the Export button from the Feature Manager.
- Select Export by Feature name
- Export Format: ASCII Simple Thickness
- SBP Datum Separation Checked
Then Select the THK (thickness) feature you want to export. Make a note of the output location.
The program will export a CSV file containing the thickness estimate down the profile.
To Create a Depth Grid of an Acoustic Horizon click the Export button in the Feature Manager.
- Select Export by Feature name
- Export Format: XYZ Text [DEP] XYZ
- Feature Name: RF0
- Use the Settings button to open the Annotation Options and turn Topo Correction ON
Then Select the horizon feature you want to export. Make a note of the output location (defaults to the SHP folder of your project).
The program will export a CSV file containing the (easting,northing,depth) coordinates of the horizon observations along all profiles where it was recorded.
To Create a Grid from the XYZ file. Right-Click Grids in the Project Explorer and select the Create New Grid… command.
In Gridding Setup dialog:
- Select External XYZ File, then enter the path to the XYZ file you exported above.
- Select Grid Cell Size fairly large. (I recommend a spacing about 1/10th the distance between tracklines to start)
- Select Natural Neighbor as the gridding algorithm. This is best for smooth interpolation. But it is very slow, so adjust the cell size accordingly!
The resulting grid of the horizon is added to the project:
Create contours of the depth horizon grid by Right-Clicking the grid and selecting the Contour Grid… command

Select the contour interval and smoothing parameters you want

The resulting vector map is overlayed on top of the grid:
To Visualize your SBP and Horizon grids in the 3D Viewer, Go to Post Processing > Export > Export Files to 3D Viewer > Export SBP Files to 3D…

Set the output format to Chesapeake XYZ format, and check Launch 3D viewer when export is complete, and Make water column transparent. Select the files you want to view and click OK

The Chesapeake 3D Viewer will open with your profiles displayed. Use the Green Handle to control the vertical exaggeration.
You can add elevation grids to the 3D display as well, including the Acoustic Horizon grid we made earlier. In the 3D viewer, go to File > Load grid… and select the grid you want to add to the 3D display. SonarWiz stores grids in the Grids folder of your project.

The grid will appear vertically aligned with the sub-bottom profiles
To export sub-bottom profiles as images use the Post Processing > Export > Batch Image Export command.

This opens the Batch Image Export dialog where you can set the output format and resolution of the profiles.

Note that you control the vertical and horizontal resolution of SBP Images in the Program Properties > SBP Options page which can be accessed from the green chicken-foot menu or from the Preferences icon in the Appearance tab of the SBP Analysis window.

The resulting images (here scaled with 10x VE)

Instructions

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