Attenuated Signal Removal

This operator identifies and adjusts pings which show an attenuated signal when compared to surrounding pings. Attenuated signal can arise from air bubble attenuation within the water column below the transducer.

The operator is based on the “attenuated signal (AS) filter” algorithm and definitions described in Ryan et al. (2015).

It accepts operands with the following data types:

Settings

The Attenuated Signal Removal Variable Properties dialog box pages include (common) Variable Properties pages and these operator pages:

Operands page

Attenuated Signal Removal page

Detection

Setting

Description

Exclude above

Specifies the exclude above line.

Ryan et al observed that the attenuated signal algorithm is optimized when working with a deep scattering layer where signal to noise is high and ping overlap is high (assuming no vessel rotation). Generally, such a layer provides a relatively homogeneous region in which to assess pings with attenuated signal. In the case where a deep scattering layer is weak or non-existent, the sea floor region may provide a useful reference layer.

The use of exclusion lines can improve operator performance.

Exclude below

Specifies the exclude below line.

Ryan et al observed that the attenuated signal algorithm is optimized when working with a deep scattering layer where signal to noise is high and ping overlap is high (assuming no vessel rotation). Generally, such a layer provides a relatively homogeneous region in which to assess pings with attenuated signal. In the case where a deep scattering layer is weak or non-existent, the sea floor region may provide a useful reference layer.

The use of exclusion lines can improve operator performance.

Context window size (pings)

Specifies the extent of the surrounding pings on either side of a center ping. Attenuated pings can occur as single pings or a group of successive pings. Therefore, the value of Context window size affects the detection of attenuated signal pings and the replacement of samples in identified pings.

Ryan et al have found that the value of Context window size 'is a compromise and is chosen based on an inspection of the echogram to match the typical duration of contiguous attenuated pings.' If the size is too small the percentile of samples in the center ping becomes too similar to the percentile of the samples in surrounding pings. If the size is too large and the signal in the deep scattering layer is too variable or too weak, the window sample percentile may not be a useful local reference value.

Echoview requires odd values for the Context window size and will round up for even numbers.

The default value for Context window size is 31 pings.

Percentile

Percentile specifies the value for the Nth Percentile algorithm for the context pings.

Threshold (dB)

Specifies the attenuated signal threshold. The current ping is deemed to be an attenuated signal when the difference between the percentile score of samples in the surrounding pings and the percentile score of samples in the current ping is greater than the Threshold.

Replacement

Setting

Description

Replacement value

Specifies the replacement method used for attenuated signal samples.

No data

The ping is replaced by no data.

Percentile

The ping is replaced by the percentile score of samples at the same range in neighboring pings.

Percentile

Percentile specifies the value for the Nth Percentile algorithm for the Percentile Replacement method.

Attenuated signal removal algorithm

This algorithm identifies and adjusts sample values that are significantly lower than those of surrounding samples.

You can specify settings on the Attenuated Signal Removal page of the Variable Properties dialog box.

Notes:

Context window for attenuated signal detection and sample replacement

Attenuated signal window

A Context window consists of a center ping and surrounding pings. The detection of attenuated signal pings tests a statistic of the samples in the center ping against a statistic of samples in surrounding pings. A difference greater than a Threshold indicates that the center ping is attenuated. Samples in identified pings are replaced using a specified Replacement value.

  1. Calculate the percentile score for the samples in the center ping of the Context Window (between Exclude lines if specified).
  2. Calculate the percentile score for the samples of the Context window pings surrounding the ping of interest.
  3. If the central ping's percentile score is lower than the Context window percentile score by more than a specified Threshold then consider the center ping to be attenuated.
  4. Attenuated signal test
  5. Samples in an attenuated ping are replaced by the specified Replacement value.
    1. When the Replacement value is No data, all samples in the center ping are set to a no data.
    2. When the Replacement value is Percentile, the sample in the center ping is set to the percentile score of the horizontal surrounding samples in the Context window.

Context Window percentiles

The Context Window Percentile Lower percentile of samples in the context windowrepresents the percentile score of the sample values surrounding the center ping.

Let vp be the value of the Context Window Pth-Percentile of an ascending order data set containing N elements with values. The N elements are the samples values of the surrounding pings within the Context Window, and exclude center ping sample values. The element values are ordered as follows:

Percentile set

  1. Calculate:

center ping percentile

Where P is specified by Context Window Percentile on the Attenuated Signal Removal page of the virtual variable.

The center ping percentile vij represents the percentile score of samples in the center ping of the Context Window.

Replacement value percentile

The Replacement value Percentile vp is used to replace a center ping attenuated sample value.

  1. Let vp be the value of the Sample Pth-Percentile of an ascending order data set containing N elements with values. The N elements are the horizontal samples values on either side of the center sample within the Context Window. The element values are ordered as follows:

Percentile set

  1. Calculate:

center ping percentile

Where P is specified by Replacement Percentile on the Attenuated Signal Removal page of the virtual variable.

Percentile algorithm

General percentile

Where:

k'

=

Percentile rank where:

Percentile rank

P is the Pth-percentile.

N is the number of elements in the ascending order set of values.

k

=

floor function of k dash where Floor function k under a floor function, that gives the largest integer less than or equal to k'.

See also

About virtual variables
Background noise in Echoview
Background noise calculation on the Analysis page
Transient Noise Removal operator
Impulse Noise Removal operator
COM EvAttenuatedSignal