# Characterisation of Coastal Sediment Properties from Spectral Reflectance Data
## 3 Results
### Spectral Analysis of Sediment Samples
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\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Wavelength (nm)** & **CaCO\({}_{3}\)** & **Fine Sand** & **Span** \\ \hline
2338 & 0.1530 & 0.8069 & 0.7031 \\ \hline
2350 & 0.7137 & 0.0696 \({}^{\text{-}}\) & 0.4356 \\ \hline
2352 & 0.2443 & 0.0253 \({}^{\text{*}}\) & 0.9282 \\ \hline
2370 & 0.3290 & 0.2500 & 0.7780 \\ \hline
2400 & 0.3596 & 0.3150 & 0.0041 \({}^{\text{**}}\) \\ \hline
2420 & 0.3237 & 0.1892 & 0.2231 \\ \hline
2435 & 0.2855 where \(R_{852}\), \(R_{952}\), \(R_{2252}\), \(R_{2300}\), \(R_{2350}\), \(R_{2400}\), and \(R_{2447}\) are reflectances at the 852, 952, 2252, 2300, 2350, 2400 and 2447 nm wavelengths, respectively. Fine sand and _span_ are not well predicted using single wavelength models (see Table 4), whereas CaCO\({}_{3}\) shows a much stronger relationship. The models were then validated using 40 independent samples that were not used in the model development (Figure 7). It is notable that a multilinear model leads to a better fit between measured and estimated values. Figure 7: Relationship between estimated and measured (a) CaCO\({}_{3}\), (b) fine sand %, and (c) _span_ using a linear single wavelength model (model 1) and multilinear model (model 2). Cross-validation of the output of the linear regression model (Table 4) through comparison between predicted and measured samples is shown in Figure 8. R2 values, adjusted for the number of variables considered in each model, are higher for CaCO3 with decreasing values for fine sand and _span_. An increased degree of scatter reflects the inability of the model to describe all of the sample points, and this is particularly the case for _span_ (see Figure 7). Thus, CaCO3 and fine sand values show the most robust statistical relationships to the spectral measurement data. Figure 8: Cross validation results of (**a**) CaCO3 values, (**b**) fine sand and (**c**) _span_. Vertical axes are measured values. Small symbols show cross-validation prediction values. The spectral characteristics of shell samples 1-5 are presented in Figure 9. Throughout, this shows higher absorption values in the SWIR with consistent dips between all samples at the wavelengths \(\sim\)1100, 1600 and 2000 nm. The latter may correspond to the water absorption wavelength at \(\sim\)1950 nm. There is also a very slight jump in reflectance at the water absorption wavelength at \(\sim\)1450 nm. In addition, the individual shell samples show some variability in the VNIR bands in particular, because of the different shell colours present (Figure 2). In order to consider whether there are any spectral differences between individual sediment samples with different values of CaCO\({}_{3}\), fine sand and _span_, the samples with the highest and lowest values of these parameters (Table 1) are compared to each other (Figure 10). All these samples reflect the aggregated patterns shown in Figure 5a, in which there are decreases in reflectance in the water absorption bands. The samples also show that, irrespective of individual values of CaCO\({}_{3}\), fine sand and _span_, there are similar reflectance values in the range 1400-1950 nm (see Figure 6). In more detail, comparison of the CaCO\({}_{3}\) values within individual samples shows that shorter wavelengths have a higher reflectance where higher CaCO\({}_{3}\) values are present, but that the sample with the lowest CaCO\({}_{3}\) values has a higher reflectance at longer wavelengths (Figure 10a). This is mirrored by the results for fine sand (Figure 10b), where the signature for the sample with the lowest amount of fine sand (i.e., the coarsest sample) is very similar to the sample with the highest amount of CaCO\({}_{3}\). The reason for this is that broken marine shells (as the source of CaCO\({}_{3}\) in the sample) give rise to coarse rather than fine sediment [46].