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New insights into image analysis applied to beach grainsize
variability
I. Bosnic (1, 2), H. Sousa (3), J. P. Cascalho (4), R. Taborda (2), M. Ribeiro (1, 2) e C. Lira (3)
(1) Instituto Hidrográfico, Rua das Trinas, 49, 1249093 Lisboa. [email protected] (2) Instituto Dom Luiz, Universidade de Lisboa, Campo Grande, 1749016 Lisboa (3) Centro de Geologia da FCUL, Campo Grande, 1749016 Lisboa (4) Museu Nacional de História Natural (UL), Rua da Escola Politécnica 56/58, 1250102 Lisboa
Abstract: In the last years, image analysis emerged as an efficient tool to perform sediment textural analysis. This work aims to improve and simplify both image acquisition (insitu) and processing techniques of beach textural data, supporting the study of the textural variability. Research efforts have focused on the development of a simplified lightweight hardware (SAND BOX©) and on the adaptation of an autocorrelation based software algorithm that can process a large set of images expeditiously. As a final product, it was created a standard procedure that provides the sediment median grainsize directly from the acquired images without any type of calibration. The validation process shows that the median grain sizes obtained from image analysis are highly correlated with the sieving results (R2=0.90 using 169 samples), which attests the method reliability.
Keywords: median grain size, image analysis, autocorrelation, validation
1. INTRODUCTION
Image analysis (IA) has been successfully applied to the study of grainsize analysis and has become an efficient tool to perform this task expeditiously on unconsolidated sediments (Carbonneau et al., 2004; Buscombe et al, 2008; Pina and Lira, 2009; Rubin et
al., 2010). As this technique can be applied in the field, it provides not only almost realtime grainsize analysis but can also extend the conditions where the use of grainsize analysis is performed. In fact, since the requirements for sediment sampling are no longer needed it is possible to measure grainsize at the bed surface (the part that interacts directly with a flow), thus replacing the greasedplate technique for sampling only surficial sediment. Other advantage is to make possible to measure vertical profiles in grainsize at a submillimeter resolution (Rubin, 2004). The first generations of image analysis methods, using digital images, were designed to be automated. These algorithms (e.g. Butler et al., 2001; Sime and Ferguson, 2003; Graham et al., 2005) use image segmentation and thresholding which give a robust estimate of grain size distributions for coarse gravels. However, problems remain with these methods, such as those related with segmenting grains from samples containing a large range of colors and minerals. Due to this restriction, Rubin (2004) proposed a new method for estimating grainsize based on the spatial distribution of image intensity (spatial autocorrelation). The author suggested that, throught the use of careful calibration correlograms for images containing known sediment sizes, reliable estimates of mean grainsize could be found by comparing the correlogram from a sample image with the calibration catalogue, through a simple leastsquares
problem. The technique has been validated for use on sandsized sediment by Rubin et al. (2007) and Barnard et al. (2007). Nevertheless, the application of this autocorrelation method still have limitations because it relys on the necessary calibration catalogues, on a highcost device for taking standardized sediment images (Chezar and Rubin, 2004; Rubin et al., 2007) and also on the lack of consistency of the developed algorithm to obtaining grainsize distribution curves. In order to overcome the limitations outlined above, this work describes an improved methodology, designed to obtain exclusively the median grainsize (D50) of sandy samples in the field, without the need of any type of calibration. This procedure includes both image acquisition and processing techniques (for surficial images and vertical sequences) and targets portability and usability.
2. METHODS
2.1. Hardware development
This stage consisted on the development of an “inhouse” image acquisition system (SAND BOX©) where a compact 14 megapixels digital camera is attached to a portable wooden box at a fixed distance from the sediment (7.5 cm). The device is equipped with a LED illumination system that aims a homogeneous light distribution over the target area. The SAND BOX© is made by two modules. The upper one contains the camera holder outside and the LED lights inside and it is used for acquiring images from surficial sediment (Fig. 1 A). Within the lower module there is a support for a PVC core that, after being sectioned, slide between the two modules for obtaining a succession of images from the vertical sediment sequence (Fig. 1 B).
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Fig. 1. SAND BOX. A) Upper module. B) Lower module
2.2. Software development
Image processing was based on the algorithm
proposed by Barnard et al. (2007), which has been
improved, mainly, in what concerns the computation
of the sediment median grainsize without any type
of calibration and the adaptation of the algorithm to
analyze images acquired from vertical sequences.
To allow the computation of D50 directly from
sediment images without the need of calibration
catalogues, a conceptual model that relates the lag
size or offset (associated with a predefined
autocorrelation value) with the sediment median
grainsize was developed. To define the optimal
autocorrelation value it was performed a systematic
analysis of several autocorrelation curves from
various sieved grainsize fractions. This analysis
revealed that the best discrimination between grain
size fractions is found when the autocorrelation is
equal to 0.5. Independently, Buscombe et al. (2010)
also found a similar relation on their work.
The software development started with the
autocorrelation computation of images from the
sieved fractions. Then, the obtained offsets and the
corresponded sieving median grain sizes have
supported the creation of the following model:
D50= 0.0204 × offset1.2982 Eq. 1
A fluxogram representing the algorithm of this
model is shown in figure 2 .
The Matlab® scripts are freely available on
http://sandcode.fc.ul.pt/ImageAnalysis.html.
Fig. 2. Work flowchart representing the software development
and its application in order to obtain the sample median grain
size (D50).
2.3 Method extension to sedimentary vertical
sequences
For analyzing images from vertical sediment
sequences, the same existing algorithm was revised
in order to create a specific routine to this kind of
samples.
In the IA process of vertical sediment sequences, the
images are firstly cut to restrict the target area to be
filled with sediment. After that, the autocorrelation
computation is made for each set of 32 lines (pixels)
using a moving average. As a final result a median
grainsize value, for each centimeter layer, is
obtained. Vertical resolution was chosen considering
sample representativeness and the scale at which
beach sedimentary processes occur (Bosnic et al.,
2011). However, the layer thickness to be analyzed
is totally flexible and depends only on the digital
camera resolution.
A
B
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The figure 3 shows the IA process applied to the
analysis of a short beach core.
Fig. 3. Example of image analysis of a small core performed at
the beach face.
3. VALIDATION
Method validation was supported by a set of 169
samples of beach sediment collected in the scope of
a coastal monitoring program being developed for
the coast under the jurisdiction of the Administração
da Região Hidrográfica do Tejo, I.P. (Andrade et al.
2010). The sampling was conducted in order to
collect sediment only from the most surficial layer.
These samples were sieved and photographed in
laboratory. The acquired images were processes
according to the model application descripted on
figure 2. After that, both median grain size results
were represented on a scatterplot (Fig.4).
Fig. 4. Scatterplot of 169 median grain sizes obtained throught
image analysis and sieving (R2 = 0.90).
The method validation revealed an excellent
correlation between the image analysis and sieving
results, with a determination coefficient (R2) of 0.90
and a standard error of the estimate (SEE) of 0.2 .
Nevertheless, within the scatter plot we can observe
that some samples are anomalously deviated from
the trend line. The deviated samples were identified and its
statistical parameters obtained from sieving data
were analyzed in order to inspect if there is any
relation between sediment sorting and the error of
the image analysis method. Nonsystematic visual
observation showed that samples with high
deviations were often poorly sorted . Based on that,
both grainsize parameters were represented in a
scatterplot (Fig. 5).
Fig. 5. Scatterplot of absolute deviation between sieved and image
analysis D50 and the sorting coefficient.
As expected, there is a positive correlation between
both sorting and the absolute deviation, so the
proposed method performs better in moderate to
moderate wellsorted sediments. Futher investigation
is needed to improve method application to poorly
sorted sediments.
4. CONCLUSIONS
This work describes a lowcost technique for
studying the textural variability of sandy beach
environments. Hardware improvement consisted on
the development of a portable, lowcost, image
analysis device that allows the acquisition of images
from both vertical sequences and horizontal insitu
sediments. Concerning software development,
several adaptations were carried out into existing
algorithms that optimize and simplify image
processing. The major advantage of the new
algorithm is related to the fact that it does not
require any kind of calibration.
Method validation was based on the comparison of
the median grainsize obtained through image
analysis and standard sieving methods, using a set of
169 sediment samples. Results show a high
determination coefficient (0.90) between both
methods which attest the reliability of the proposed
methodology.
Acknowledgements
This work was undertaken as part of the project
Beach Sand Code (PTDC/CTEGEX/64592/2006).
The authors would like to thank the CISML program
for the availability of data which was fundamental in
this article.
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