Beam Angle, Grazing Angle

You can improve certain aspects of your data imagery by applying beam angle compensation and grazing angle compensation.  BAC and GAC are found in the Isis > Tools Menu, there must be an active waterfall to be able to invoke BAC and GAC corrections.

Overview

After sonar imagery has been corrected for slant-range distortion and the water column has been removed, prominent across-track variations in signal intensity generally remain in the imagery. These variations result in track-parallel stripes of high and low intensity, commonly referred to as beam pattern. You can use Isis to eliminate, to a large degree, the unwanted beam pattern that sometimes appears in your imagery.

The actual causes of these variations are the transmit and receive characteristics of the transducers, increased attenuation of the acoustic signal at far range, and the grazing angle between the acoustic ray path and the seafloor. Most sonars include time-varied gain (TVG) amplifiers in the towfish that accurately correct for signal attenuation. These corrections are helpful in reducing the dynamic range of signals received by the topside equipment.

The Isis beam angle and grazing angle compensation feature (BAC and GAC) goes further by allowing independent compensation for beam pattern and grazing angle distortions in record and playback modes. Isis can apply either type of compensation to as many as four channels in a single Waterfall window.

These compensations can only be applied to slant-range-corrected data. Both compensations require reliable towfish altitude information and are much more accurate when a reasonable bottom slope is entered (i.e., from a regional chart or detailed bathymetry data).

Beam angle and grazing angle compensations act independently for each waterfall window. Therefore, it is important to select the desired waterfall window first before selecting Beam Angle, Grazing Angle.

Beam Angle Compensation

Each transducer array has a distinct beam pattern whose intensity varies with the angle off the acoustic axis, or boresite. If far-field transducer calibration patterns are available for a sonar’s transducers, Isis can use these to derive a beam angle compensation (BAC) from them. BACs are stored and read by Isis as beam patterns from which compensations are created while Isis is running. Refer to Appendix B for a description of the BAC file format.

If no calibration patterns are available, Isis can derive an empirical BAC from statistics collected over at least several hundred pings. This is generally the approach taken since transducer beam patterns are rarely available.

If the empirical approach is to be taken, select a featureless piece of seafloor within the survey area with a well-known slope where at least 200 pings can be or have been collected. The greater the number of pings used, the more successful the compensation will be.

When the statistics are being collected, the towfish altitude should be varied over the entire range of altitudes expected for the survey. This will help Isis to eliminate the effect of grazing angle intensity variations from the beam angle compensation statistics. It is also helpful to collect statistics at altitudes lower than the expected minimum for the survey in order to increase the information available at high beam angles. It is very important that the towfish altitude is well known during the collection of statistics and during the entire survey for which beam angle compensation is applied.

Grazing Angle Compensation

At far range, the angle at which the acoustic energy strikes the seafloor shrinks toward zero. As this grazing angle gets smaller, less energy is scattered back in the direction of the sonar, thereby creating another across-track intensity variation. By collecting statistics over well-known and well-behaved seafloor, Isis can derive a grazing angle compensation for each bottom type and general track heading in a given survey. You can then load the GAC from a file or collect new statistics for each run.

When calculating a GAC from collected statistics, select a featureless piece of seafloor of known slope where at least 200 pings can be collected. As with statistics collection for BAC, the towfish altitude should be varied between the estimated upper and lower altitude limits for the survey. This will allow the most thorough sampling of grazing angle independent of beam angle.

It is also helpful to collect statistics at altitudes lower than the expected minimum for the survey in order to increase the information available at low grazing angles. It is very important that the towfish altitude is well-known during the collection of statistics and during the entire survey for which grazing angle compensation is applied.

It is important to realize that Isis will only calculate a GAC outside of boresite since distortions inside of boresite are dominated by effects of the beam pattern. If, therefore, due to the tilt angle of the sonar, the boresite falls outside of the collected statistics for the GAC, the compensation will be completely flat.

 

Using Beam Angle and Grazing Angle corrections

You must have a waterfall window open and selected before you can have Isis give you angle corrections. When you choose Beam Angle, Grazing Angle, the system displays a dialog box for this function.

In the figure, the green line (the higher line) represents data coming from the starboard channel, and the red line (lower line) represents data coming from the port channel. To understand the color coding of any additional channels that may be present in your data, click Color key... to reveal the color schemes.

Conceptually, you can choose any of three progressively richer “levels” of operation to get the angle compensation needed for your survey environment. These operational levels are:

·         Basic Level — You use no extra input to influence angle compensation calculations.

·         Middle Level — You specify some basic additional input to influence angle compensation calculations.

·         Full Level — You have full control over all factors that can influence angle compensation and what you will see in your imagery.

Although the operational levels are not labeled as Basic, Middle, and Full in the dialog box, you use the dialog box’s Options button to choose a level you want. You then choose Do It to have Isis use your chosen options for the level you selected.

Working at the Basic Level of Compensation

Basic level provides you with the most automated way of deriving angle compensation.

To acquire beam angle compensation from the basic level

1.       Click in a waterfall window.

2.       Choose the Beam Angle, Grazing Angle dialog box from the Tools menu.

The BAC and GAC dialog box initially appears without any options displayed. In basic level, only BAC is available.

3.       In the waterfall window that is to have the compensation applied to it, left-click the trackball.

4.       In the Beam Angle, Grazing Angle dialog box, click Do it. If you have not yet clicked in a waterfall window, Isis reminds you that you must “First click in the waterfall window to be corrected. Your actions will then apply to that window.”

Isis automatically calculates your beam angle compensation over 200 pings and applies the compensation to your data imagery without waiting for any more input from you.

After the beam angle compensation is calculated, it stays on for the selected window until the dialog box is closed.

Working at the Middle Level of Compensation

Middle level gives you more control than basic level but not as much control as full level in the task of collecting statistics and calculating a beam angle compensation.

To acquire angle compensation from the middle level

 1.       Click in a waterfall window.

2.       Choose the Beam Angle, Grazing Angle dialog box from the Tools menu.

3.       Click Options once.

4.       Choose Collect Statistics or Correct Image or Off.

·         Collect Statistics. The Collect Statistics button initiates the collection of statistics for imagery channels displayed in the selected waterfall window. As the statistics are collected, Isis displays a graph of the running average of voltage versus angle off vertical. In addition, the minimum and maximum encountered altitude are updated as collection proceeds, (you may allow Isis to run for as long as you wish collecting statistics).

·         Correct Image. This causes Isis to use the statistics collected thus far to calculate a compensation for the imagery channels displayed in the selected waterfall window. Click on this button after the voltage curves have smoothed and the minimum and maximum altitudes span the range of altitudes encountered within the file or survey.

·         Off. The Off button turns the current compensation off. When all compensations are off, the Status box will display the Idle message.

5.       Choose Do It

The system produces the beam angle compensation using whatever parameter you specified in step 3.

Working at the Full Level of Compensation

Full level gives you the most control over angle compensation. It is also the only level that gives you access to grazing angle compensations. It is strongly recommended that beam angle compensation be applied prior to the application of any grazing angle compensation.

To acquire angle compensation from the full level

1.       Click in a waterfall window.

2.       Choose the Beam Angle, Grazing Angle dialog box from the Tools menu.

3.       Click Options twice.

4.       Choose any of the visible buttons except for the Do it button: Collect Statistics, Correct Image, Off, Compensation, Across Track Slope, Gain Adjust, Save Corrections, Load Corrections, Color Key, Beam Width, Tilt Angle, or Cutoff.

5.       Choose Do it.

The system produces the beam angle compensation using whatever parameters you specified in step 3.

Meaning of the options when you are applying full level beam angle, grazing angle compensation is shown in Table 9.4.

Collect Statistics, Correct Image and Off:

These are the same for full level as they are for middle level (see ‘Working at the Middle Level of Compensation’)

Compensation:

Choose between Beam Pattern (Beam Angle) and Grazing Angle compensations.

Across-Track Slope:

In order to calculate accurate beam angle and grazing angle compensations, the across-track slope must be specified. Zero slope is at the center of the slide bar. The direction of the arrows on the slide bar indicates the up direction. The distance from the center corresponds to the size of the slope angle.

Gain Adjust:

Use the Gain Adjust box to adjust the relative gain of port and starboard channels. You can do this manually or let Isis do it automatically. Any adjustment is incorporated into the BAC.

Automatic:

The relative average intensity of any pair of port and starboard channels is tracked for beam angle compensation statistics. When the BAC is calculated, any necessary gain adjustment is included in the compensation.

Manual:

Move the slide bar to the right to increase gain or to the left decrease gain of either the port or starboard channel. Any gain adjustment that is set at the time that the beam angle compensation is calculated is incorporated into that compensation.

Save Corrections:

Once a beam angle or grazing angle compensation has been calculated, the compensation can be saved in a BAC or GAC file for later use in VISTA or with another imagery data file. Use the Save Compensations button for this purpose.

Load Corrections:

If you wish to use a stored beam pattern file or previously calculated BAC or GAC, the path and name of that file can be specified using the Load Compensations button.

Color Key:

The Color Key button displays a box indicating the color assignments for graph traces displayed in the Beam Angle / Grazing Angle dialog box.

Tilt Angle:

Enter the transducer tilt angle for the current sonar. Isis will use this information for extrapolating the BAC and GAC at far ranges where collected statistics are sparse. Since GAC is only calculated for far ranges outside of boresite, the Tilt Angle is used by Isis to determine the range outside of which a GAC will be applied.

Cutoff:

By adjusting the Cutoff, you can eliminate sparse far-range statistics which cause a discontinuous transition between the empirical and extrapolated compensation.

Isis extrapolates beam angle and grazing angle compensations for far-range angle bins where no statistics have been collected. Any angle bins having fewer than 20 contributors are considered empty by Isis. In order to extrapolate, Isis searches from far range inward for the first non-empty angle bin. From this point, the extrapolation point is moved inward by the number of bins (degrees) specified in the Cutoff box.

Parts of the Dialog Box Common to All Levels

Regardless of the level of operation you choose to use to derive an angle compensation, the Do it button and the Status box apply to all three levels.

Do it Button

For any operational level (basic, middle, or full), the Do it button tells Isis to calculate the angle compensation and apply it to your data imagery in your selected waterfall window. Isis will collect statistics over 300 pings and display a graph indicating a running average of voltage versus angle from vertical. After 300 pings have been displayed, Isis will calculate a compensation based on the collected statistics.