Bottom_hardness_normalized

Bottom_hardness_normalized (dB re 1m2/m3) is a bottom feature that may be extracted during bottom classification. A Bottom_hardness_normalized analysis variable can be selected under Bottom on the Export page of the EV File Properties dialog box. Bottom_hardness_normalized can be included with onscreen analyses and exported. Within a data set, hardness values are relative to each other. A higher hardness corresponds to a harder substrate.

Bottom_hardness_normalized represents the mean of the depth normalized hardness index E2 in the Feature extraction interval for a bottom classification.

Where:

Siwabessy et al (1999) discuss the quantities E1 and E2.

E1 is known as a roughness index. E2 is known as a hardness index.

The following is an excerpt from their paper:
“…E1 is derived from an integration of the tail of the first acoustic bottom return and E2 is derived from an integration of the complete second acoustic bottom return. The rationale of this is that the energy in the tail of the first acoustic bottom returns (E1) arises from the roughness of the seabed and that of the entire second acoustic bottom returns (E2) arises from the acoustic impedance mismatch of the seabed and the water column (Chivers et al., 1990; Chivers and Burns, 1992). This parameter is customarily taken to represent the “hardness” of the seabed. Heald and Pace (1996) provide theoretical expressions derived from scattering theory for E1 and E2. …”

Echoview uses a variation of Equation (1) from Siwabessy et al (1999) to determine E2 for the first bottom echo of the pings in the Feature extraction interval.

Extent of the second bottom echo

The depth of the start of the second bottom echo is given by twice the Bottom line depth. Echoview finds the first three samples below the Bottom echo threshold value and deems the first of these samples to be the end of the second bottom echo. For more detail refer to Bottom classification algorithms: Extent of the second bottom echo.

j	=	Bottom point in a bottom points variable. Bottom point number j is assigned sequentially in time. The pings of the echogram are partitioned according to the number of pings in a specified Feature extraction interval.
i	=	Ping i in the Feature extraction interval j.
k	=	Sample k of the second bottom echo in Ping i in the Feature extraction interval j.
n	=	Number of pings in Feature extraction interval j.
m	=	Number of samples in the second bottom echo of ping i.
Bottom_hardnessDepth normalized i,j	=	Depth normalized hardness index for ping i in Feature extraction interval j. The depth normalization equation is:   Refer to depth normalization algorithms for definitions of OffAxisPulseLengthRef, OffAxisPulseLengthActual.
ESecondEcho i, j	=	Hardness index E2 for ping i in Feature extraction interval j. Where: sv(i, k) is the linear value for Sv for sample k in ping i. Note: Hamilton 2001 observes that although E2 is treated as a hardness index, it may also been seen as an acoustic reflectivity index which has components of roughness and hardness (p 11).

See also

About bottom classification
Bottom classification algorithms
EV File Properties dialog box