Detecting other boundary lines

The Threshold offset algorithm detects in a specified direction, relative to a nominated (base) line, for the first occurrence of a boundary value. The boundary may represent water column features like: transducer ring down, bubbles around the transducer, shadow zones or deadzones.

Prepare an echogram

1. Remove noise from an Sv, TS, Unspecified dB variable.
2. The specification of bad data regions, exclusion lines on the Analysis page and use of the Processed data operator can also clean echogram data.
2. Create and edit a (base) line to use with the Threshold offset operator.
3. Consider the most suitable base line relative to the boundary of interest.
4. Determine a boundary value for the feature of interest.
5. It may be helpful to use a single color gradient color scheme to enhance the contrast between water and the feature.

Create and configure the Threshold offset operator

A feature boundary may be approached from a base line that is an edited line pick representing the acoustic bottom (configure for Detect minimum threshold), or from a base line positioned in empty water (configure for Detect maximum threshold).

Option 1 — Line & Surface tool

Open an echogram. On the Line and Surface tool select New Virtual Line/Time Series…

1. On the New Virtual Line dialog box, select Threshold offset.
2. Click OK.

-OR-

Option 2 — Dataflow Toolbox

From the Dataflow Toolbox, drag the Threshold offset operator to the Dataflow window.

Then configure the operator:

1. On the Operands page of the Time Series Properties dialog box, specify a prepared acoustic variable for Operand 1 and a prepared base line for Operand 2.
2. On the Threshold offset page:
   1. Select Detect minimum threshold or Detect maximum threshold.
   2. Under Threshold (dB), enter a boundary value for the feature of interest.
   3. Select or clear Allow detection from echogram extents. Use this if the base line (Operand 2) extends past Operand 1.
   4. Select or clear Apply line-relative smoothing to the echogram data (a blur convolution) depending on the data.
   5. Under Minimum accepted distance (m), enter the minimum separation between the base line and the Threshold offset line.
   6. Under Maximum detection distance (m), enter a limit for the search relative to the base line.
   7. Select a Direction of detection.
3. Optionally set display and visibility on the Display and Visibility pages.
4. Click OK.

Notes:

• The suitability of the Threshold offset operator is discussed under About the Threshold offset operator: Notes.

See also

About the Threshold offset operator
Detecting a vegetation boundary