This software produces 166 proprietary unitless echogram measurements (EMs) on the first seafloor echo for an internal principal components analysis (PCA), and then uses the first three principal components (PCs), generally accounting for more than 95% of the covariance (Ellingsen et al., 2002; Legendre et al., 2002) in K-means clustering, for dividing the first seafloor echoes into acoustically distinct substrate types.
Organizing the echogram measurements along a continuum of measurements or grouping them into a number of acoustically distinct substrate classes is the final step in the process.
Variability and covariance of echogram measurements
Thus significant differences in some of the echogram measurements can be created for the exact same substrate type if users are not careful about ensuring that the 251 sample window of QTC IMPACT matches up well with the first echo length.
Although this analysis demonstrated that there are several strong advantages (gain adjustment, bottom picking, bad data exclusion, and stacking) in using the partially automated echogram classifying software (QTC IMPACT), there are also several potential pitfalls (dependencies among the 166 EMs, lack of standardization, correlation with depth, influence of seafloor slope, and mismatch between 251 sample intervals versus first echo length), such that it does not function as users would expect for distinguishing substrate types.
Instead, our results, with corroborations from independent data sets, indicated the importance of analyzing the echogram measurements before any PCA and K-means analysis so that depth-related and slope-related errors, second echo or echo envelope errors, and variable range or collinearity errors could be caught.
Examination of the data and the variability and covariance of echogram measurements
Any potential effect due to impact angle of echogram reflection, which is a combination of seafloor slope and vessel motion, is not widely addressed in the literature.
Thus accidental analysis of more than one echo with QTC IMPACT can cause strong depth-related influences and can create significantly different echogram measurements such that additional substrate classes could be created.