Backscatter
One of the data that is stored by an ADCP is the echo intensity. This parameter can be used to determine the validity of the data in terms of signal-to-noise ratios, see also SNRLimit.
The intensity of the acoustic backscatter depends on the size and abundance of scattering particles in the water column. However, geometrical spreading and attenuation by the water itself cause the strength of the signal to be reduced with increasing distance the acoustic waves travel.
RDI Adcp’s store the acoustic backscatter intensity in counts, a parameter that is linearly proportional to the intensity in dB. For the Explorer DVL this factor is approximately 0.61 dB/count.
where \(I_b\) is the acoustic backscatter intensity expressed in dB, \(K\) the conversion factor from counts to dB, \(N\) the acoustic backscatter intensity in counts and \(N_t\) a threshold count. The acoustic cross section area \(\sigma\) is given by
where \(k_t\) is a device dependent factor, \(r\) the distance to the transducer, and \(\alpha\) the coefficient of attenuation due to sea water. The acoustic cross section area can be interpreted as the backscatter intensity, corrected for the effects of geometrical spreading and attenuation, and can be related to the abundance and size of the scattering particles, see also [1].
The rdi_backscatter module contains a class
- class AcousticCrossSection(self, k_t=1e-8, Nt=45, db_per_count=[0.3852] * 4)
which takes as arguments k_t (\(k_t\)), N_t (\(N_t\)) and
db_per_count as a list of factors, one for each beam. This parameter
corresponds to the factor \(K\).
If an instance of this class is included in the processing pipeline,
each ping is augmented with a field sigma, containing the
variables Sigma1, Sigma2, …, Sigma<nbeams>, and
Sigma_AVG, where the numeric index denotes the beam number, and
the suffix _AVG denotes the cell average of all beams.
References