Sound Metrics notes

• Imaging sonar
• Lens mount orientation

Imaging sonar

DIDSON/ARIS imaging sonar data are recorded values of power received. The data are proportional to the reflectance from an object. A frame of imaging sonar data may be understood as a photo with a field of view. A series of frames approximates a video. Pixels/samples in the frame have a calculated “range” (calculated using sound speed) from the transducer. The DIDSON Viewer manual advises that Transmission Loss (TL) is added to the power received. Because the sample value is an uncalibrated reflectance, the chosen TL can be selected by how it improves the sample display.

The derived raw variable “Sv frames” is not Sv data referenced to a standard value with transducer calibration. However, the Sv frames data type is handled as a Multibeam Sv data type and is accepted as an operand in Echoview operators.

Sv Frames = Pr + Nlog10 R +2αR

where:

Pr = Power received (Power units)

R = Range (m)

α = AbsorptionCoefficient (dB/m)

N = TVG range coefficient In the DIDSON Viewer manual, this is known as the "spreading factor with values of 20, 30, 40".

Excerpt from the DIDSON Viewer manual

"Correct for Transmission Loss (Processing menu)

Use this command to correct the displayed image for acoustic transmission loss.  The algorithm adds a range-dependent offset to the data based on the formula

TL = 2*alpha*R + N*log10(R)

where alpha is the sound absorption coefficient, N is the spreading factor (20, 30 or 40) and R is the range in meters.  The offset correction values are normalized to 0 dB at the start of the window, and data values below the display threshold setting are not corrected, to minimize noise “blooming” with range.

You must set the appropriate parameters using Show Fish Parameters for the correction to be valid.  These include settings for fresh or salt water, approximate water temperature (from 5C to 20C) and the desired spreading factor (20, 30 or 40).  While these parameters are accessed from this dialog, there is not requirement for other processing to use this command."

"Show Fish Parameters command (Processing menu)

Use this command to view or change the current processing parameters for the Background Subtraction and Count Fish commands. …

Transmission Loss Parameters

Environment                         Choose Fresh or Salt water environment.

Temperature                        Enter water temperature from 5C to 20C.

N                                        Enter N for N*logR spreading factor. ..."

Lens Mount Orientation

The DIDSON lens mount orientation is important. The DIDSON Viewer software has a setting to preserve the Left-Right orientation of the beams and hence the orientation of the data. The lens may be mounted on TOP or (upside down) REVERSE. A DIDSON logo on the unit provides a visual cue to the orientation of the lens. A top mounted lens corresponds to a right way up logo. The DIDSON.ini file records the setting for Mount Orientation. The DIDSON data file records the state of the Mount orientation under the setting name REVERSE.

The DIDSON Viewer software displays the status of REVERSE as a checkbox.

• A clear checkbox means the lens is TOP mounted.
• A checked box means the lens is REVERSE mounted.

The DIDSON Viewer point of view looks out from the transducer unit and down on the beam fan.

Example

To look across a stream the lens is mounted in Normal position but is tilted about 15 degrees (from the horizontal) to achieve a good field of view – the fish are recorded as if looking from the top down. A steeper angle, decreases the area viewed.

To match the view of the beam fan from the DIDSON viewer, the multibeam echogram uses Flip vertically and Flip horizontally. The Echoview transducer elevation is 75 degrees.

Notes:

• For fish that swim across the beam fan a reliable fish length can be measured from head end to tail end. A known length and known species could help you estimate TS.
• When TS is uncalibrated, it is inadvisable to use the Target length calculator.
• Refer to the Sound Metrics ARIS technical documentation to understand ARIS lens mount orientation settings.
• Echoview transducer elevation allows values 0 to 89.99 and 89.99 to 180 degrees. 0 degrees corresponds to the transducer pointing vertically down. 180 degrees corresponds to the transducer vertically up. Be aware in the 4D window, echogram and scene the 'back' of the beam fan is shown for elevations between 89.99 and 180 degrees. The echogram Display settings Flip vertically and Flip horizontally may be used to restore the view of the front of the beam fan.

See also

Imaging sonar - fish arcing
Transducer geometry illustrated