5.1. Principal Component Analysis
PCA is a multivariate statistical technique (Al-Djazouli et al. 2019), as well as a mathematical procedure, introduced by(Pearson 1901), which allow by orthogonal linear transformation to reduce a large set of data and to transform a number of (possibly) correlated variables into a (smaller) number of uncorrelated new variables called Principal Components (Hermi et al. 2017). Consequently, this transformation removes the irradiance and the redundancy effects of the satellite data(Amer, Kusky, and Ghulam 2010), isolates noise, extracts the most information present in the different bands, maximizes the amount of variance of the input data and then improves the targeted information in the image (Hermi et al. 2017; Abdelouhed, Ahmed, Abdellah, and Mohammed 2021; Abdelouhed, Algouti, et al. 2021). PCA is an efficient technique widely used for identification of lineaments due to its advantage for dimensionality reduction and enhancing multispectral image(Yu et al. 2012; Adiri et al. 2017; Al-Djazouli et al. 2019). Some of these studies have achievedlineament mapping by applying Principle Component analysis method on Landsat 8-Oli multispectral data in order to enhance the structural features(Salui 2018; Farahbakhsh et al. 2020; Aretouyap et al. 2020; Arifin, Adnan, and Rasam 2021; Nedjraoui et al. 2021)
Here, PCA is used to reduce data and to enhance the contrast of the image in order to determine the best principal component of Landsat 8 OLI for extracting lineaments in the Telouet –Tighza area. Six bands of the Principal Components (PCs) were derived from the mosaic of two Landsat8-Oli multispectral images covering the study region. The six principal components obtained are represented in Table 2 with their statistical calculations. The statistical calculations of the resulted Principal Components (PCs) showed that the first component synthesizes around 96% of the original information which mean that this component seems indispensable. The other components seem unnecessary because it contain each one less than 3% of information and decrease in turn. This criterion justifies the choice of the PC1 for the extraction of lineaments and its use to precise the initial structural map in this study. Linear features and edges are better visible and clear on the first principal component PC1 (Fig.6) than on the original mosaic.
5.2. Spectral Band Color Compositions
Multispectral images are often displayed as (RGB) color composites using three spectral bands (Hamdani and Baali 2019). Several studies used composite bands in lineament mapping; different (RGB) color composition were combined through selected bands and then every three combinations bands of spectral bands were examined in order to determine the most combination containing the high amount of information when combined in a composite image (Mhamdi et al. 2017; Aldharab et al. 2018; Farahbakhsh et al. 2020).
OIF is a selection method developed by (Chavez, GL, and LB 1982) which allow selecting the bands containing most of the variance and then to optimally display Landsat 8 data in three color composite images. There are many examples of studies that used OIF technique as a preliminary processing step before the automatic extraction of lineaments (Al-Djazouli et al. 2019; Jellouli et al. 2021). This technique of enhancement, which is often used to better visualize the boundaries between lithological units, was calculated by the following formula (1):
OIF = Σ(Standard deviation (i, j, k) / Σ (CC (i, j), CC (i, k), CC (k, j)) (1)
i: the first selected band
j: the second selected band
k: the third selected band
CC: Correlation coefficient of each couple of bands
In this study we suggest the use of the optimum index factor (OIF) and correlation index to select the best independent band combination useful to achieve the aim of this study. The band combination which has the lowest correlation and highest variance among band pairs is the best one. the 7 Landsat-Oli bands image have been processed using Ilwis 3.3 software and indicated that 6-5- 7 band combination has the highest sum of standard deviations and low level of information redundancy (table 3). ). (Jellouli et al. 2021)suggest the choosing of the three bands with the highest variance because they combine the largest amount of information (high optimum index factor, OIF). In the other hand statistics calculation of PCA provided the correlation matrix shown in table 4. In an effort to improve lineament extraction of this study, we selected the 6-5-7 three bands that present the highest OIF (table 3) whereas the other four bands are eliminated. Furthermore, these same bands display the least amount of duplication (low correlation index see table 4) what’s means that they should give the best lineament mapping. The selected bands are then displayed in red, green and blue to create a colored combination image. The colorful image composed of the three selected bands is shown in Fig. 7 with high visual quality. Beside the geological formations, hydrographic network and vegetation, the chosen compositions composite (RGB) image highlight the most structural information preserved in all bands and display the clearest linear geostructures which were subsequently extracted by automatic method.
Table 2: Statistics of Principal Component Analysis of the used Data set
PC bands EigenValue Percentage % Accumulative Percentage %
|
1 108346304,33 95,21 95,21
|
2 3378111,90 2,96 98,18
|
3 1455480,49 1,27 99,45
|
4 367148,58 0,32 99,78
|
5 208317,32 0,18 99,96
|
6 36794,21 0,03 99,99
|
7 2583,84 0,0023 100
|
Table 3: Statistics of OIF analysis applied to the seven OLI bands.
Option/ Combinaison
|
OIF Highest Index Ranking
|
Percent
|
1
|
B
|
B6
|
B7
|
88,96
|
2
|
B4
|
B5
|
B6
|
83,54
|
3
|
B4
|
B6
|
B7
|
81,99
|
4
|
B3
|
B5
|
B6
|
90,12
|
5
|
B4
|
B5
|
B7
|
79,63
|
6
|
B1
|
B5
|
B6
|
79,62
|
Table 4: Matrix of correlation index derived from PCA analysis of the seven OLI bands.
Bands 1 2 3 4 5 6 7
|
1 1,00000 0,99433 0,95697 0,88663 0,88426 0,95107 0,92914
|
2 0,99433 1,00000 0,98119 0,92415 0,90864 0,87059 0,86737
|
3 0,95697 0,98119 1,00000 0,97407 0,94007 0,93414 0,92953
|
4 0,88663 0,92415 0,97407 1,00000 0,94185 0,96859 0,97273
|
5 0,88426 0,90864 0,94007 0,94185 1,00000 0,82842 0,82633
|
6 0,95107 0,87059 0,93414 0,96859 0,82842 1,00000 0,80018
|
7 0,92914 0,86737 0,92953 0,97273 0,82633 0,80018 1,00000
|
5.3. Band Ratios
Because of their proven ability to enhance the lithological discontinuities which can indicate the presence of lineaments (Hamdani and Baali 2019), different band ratios of Landsat 8- Oli data were used extensively for lithological and lineament mapping in many geological environments, depending on the purpose of the study (Abdelkareem et al. 2021; Saidi et al. 2020). Githenya, Kariuki, and Waswa (2019), deduced that Landsat 8 performed at discriminating structural features using 7/5, 6/4, 4/2 band ratios, while Kolawole (2016)used Color bands ratio of 5/6, 5/4, 4/1 to extract structural lineaments in North Central part of Nigeria. In addition, Hamdani and Baali (2019)showed that Landsat 8-Oli data have important potential for mapping geological lineament, related with karst shapes, by applying band ratios 6/ 7, 3/ 4, 5/ 6 in Moroccan Middle Atlas.
The band ratiois an arithmetical operation used to enhance the spectral differences between bands. Exploiting the variation of reflectance in the spectral signature, this image processing technique consists of dividing Digital Number (DN) of one spectral band by DN values of another one in a multispectral scene (Githenya, Kariuki, and Waswa 2019). In other wise it allows to reduce haze and vegetation cover, to suppress the topographic slope and minimize shadow effects, particularly in mountainous areas where shadow is one of the major problems in satellite imaging (Kolawole et al. 2016).
Based on the results of PCA transformation and OIF compute, it is evident that the Landsat 8-OLI band-ratio image 6/7-6/5-3/2 (Fig. 8) is powerful in distinguishing the lithological boundaries and edges in the study area. Then, it is used as powerful tool to check and validate extracted lineament, eliminate lineaments corresponding to lithological boundaries and distinct structurally significant lineaments.
5.4. Directional Filter
Directional filtering is defined as a spatial domain filtering ( Javhar et al. 2019),Thataccentuates the first derivative between each two adjacent pixels and selectively bring out any feature in the input image having specific direction gradients (Epuh et al. 2020; Jellouli et al. 2021). It consists of changing the values of pixels by replacing each center pixel by the median value within the neighborhood pixels (Farahbakhsh et al. 2020). The resulted image comprises pixels with uniform values while the others present variable values that indicate bright edges (Epuh et al. 2020). Furthermore, the directional nature of structural lineaments give up to the obligation of using directional filtering in order to enhance the perception of the structural features when processing multispectrales images (Javhar et al. 2019; Hermi et al. 2017) displayed in his study that the directional contrast gradient is produced by filtering algorithm on the input image independently of lineaments orientation. It can detect structural lines with horizontal, vertical, and diagonal directions(Hermi et al. 2017). Directional filters were commonly used by several authors in geological applications to enhance and highlight specific linear trends as well as geological (Hermi et al. 2017; Abdelouhed, Ahmed, Abdellah, and Mohammed 2021; Abdelouhed, Algouti, et al. 2021).In the present study, the DF technique was selected for the automatic lineament extraction, since it is considered such a faster and effective way to evaluate lineaments and identify any linear or curvilinear shapes (faults, fractures, contours, roads and defects...) on the image (Nedjraoui et al. 2021; Azman, Ab Talib, and Sokiman 2020). This straightforward method allows the enhancement of lineaments in study area using the Sobel operator(Sedrette and Rebai 2016). This latter is a variety of directional filters where the values of the convolution matrix depend mainly on the distance from the central pixel. Edge enhancing filter is used for detailed identification of lineaments and highlighting any changes of gradient, that are not promoted by the illumination source, by causing an optical effect of shadow on the image as if it were illuminated by oblique light shadow ((Hamdani and Baali , 2019;Azman, Ab Talib, and Sokiman 2020). Accordingly, it is applied to reduce noise and smooth the image ((Bentahar, Raji, and Si Mhamdi 2020; Singh, Arya, and Agarwal 2020).In order to enhance the geological lineaments in all directions, four derived images have been generated from four directional 5x5 Sobel filters in the four principal directions: N-S, E-W, NE-SW and NW-SE (Table 5). This step is performed by applying the directional convolution filter on PC1 and on the selected color composite image (6-5- 7 RGB) via Envi 5.2 software. In addition, the use of the PC1 and 6-5- 7 image is justified as they contain a large amount of information that is manifested by showing most of the features and enhancing linear geological structures in terms of high spectral resolution and contrast. The size of the filters is determined according to the size of the fieldwork; 7x7 matrix kernel size selected in this study would take into consideration all lineaments with small length (Aretouyap et al. 2020). Table 5 display the size and weight of kernels applied in the study area. The outputs resulted from directional filtering of PC1 and 6-5- 7 images are shown respectively in Fig. 9 and Fig.10.
Table 5: Convolution matrix of different directional 7*7 filters.
N0
|
N45
|
-1.0000
|
-1.0000
|
-1.0000
|
0.0000
|
1.0000
|
1.0000
|
1.0000
|
-1.4142
|
-1.4142
|
-1.4142
|
-0.7071
|
0.0000
|
0.0000
|
0.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
0.0000
|
1.0000
|
1.0000
|
1.0000
|
-1.4142
|
-1.4142
|
-1.4142
|
-0.7071
|
0.0000
|
0.0000
|
0.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
0.0000
|
1.0000
|
1.0000
|
1.0000
|
-1.4142
|
-1.4142
|
-1.4142
|
-0.7071
|
0.0000
|
0.0000
|
0.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
0.0000
|
1.0000
|
1.0000
|
1.0000
|
-0.7071
|
-0.7071
|
-0.7071
|
0.0000
|
0.7071
|
0.7071
|
0.7071
|
-1.0000
|
-1.0000
|
-1.0000
|
0.0000
|
1.0000
|
1.0000
|
1.0000
|
0.0000
|
0.0000
|
0.0000
|
0.7071
|
1.4142
|
1.4142
|
1.4142
|
-1.0000
|
-1.0000
|
-1.0000
|
0.0000
|
1.0000
|
1.0000
|
1.0000
|
0.0000
|
0.0000
|
0.0000
|
0.7071
|
1.4142
|
1.4142
|
1.4142
|
-1.0000
|
-1.0000
|
-1.0000
|
0.0000
|
1.0000
|
1.0000
|
1.0000
|
0.0000
|
0.0000
|
0.0000
|
0.7071
|
1.4142
|
1.4142
|
1.4142
|
N90
|
N135
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
0.0000
|
0.0000
|
0.0000
|
-0.7071
|
-1.4142
|
-1.4142
|
-1.4142
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
0.0000
|
0.0000
|
0.0000
|
-0.7071
|
-1.4142
|
-1.4142
|
-1.4142
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
-1.0000
|
0.0000
|
0.0000
|
0.0000
|
-0.7071
|
-1.4142
|
-1.4142
|
-1.4142
|
0.0000
|
0.0000
|
0.0000
|
0.0000
|
-0.0000
|
-0.0000
|
-0.0000
|
0.7071
|
0.7071
|
0.7071
|
0.0000
|
-0.7071
|
-0.7071
|
-0.7071
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.4142
|
1.4142
|
1.4142
|
0.7071
|
0.0000
|
0.0000
|
0.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.4142
|
1.4142
|
1.4142
|
0.7071
|
0.0000
|
0.0000
|
0.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.0000
|
1.4142
|
1.4142
|
1.4142
|
0.7071
|
0.0000
|
0.0000
|
0.0000
|
5.5. Shading relief
The DEM data can be obtained from various remote sensing sources such as ASTER (30 x 30 m) and SRTM (90 x 90m global DEM). In this study, we used SRTM data acquired with the radar C-band at 3 arc second (12 m in resolution), from the Consortium Space Information(Masoud and Koike 2006). SRTM digital elevation model is frequently used to automatically extract the structural features as well as to improve the quality of lineament map by its spectral and resolution properties. In the current study, shaded relief images generated from SRTM digital elevation model will enable the easy recognizing of geological lineaments and evaluating the structural implication of the lineaments resulted from processing of Landsat 8 Oli image. Using ArcGIS 10.2, shaded relief images were produced from SRTM digital elevation model by applying Analytical hill-shading. This latter is used as a common method useful to simulates the topographic illumination under different artificial light directions by introducing both altitude and azimuth parameters(Abdelouhed, Ahmed, Abdellah, Mohammed, et al. 2021). Therefore, this new technique becomes necessary to automatically extract and analyze geologic lineaments using a combination of DEM and remote sensing images. Several studies utilizedshaded relief for the extraction of lineaments and have showed that this can be used as a powerful tool to explore and improve lineament mapping in mountainous areas due to its ability to simulate shadow effects that are provoked by the sun angle and elevation (Saadi et al. 2011)
This step involves selecting some parameters that are necessary for better discrimination between shaded and illuminated areas, namely the azimuth and the altitude angle(Jellouli et al. 2021).
Maintaining solar illumination angle at 30°, four shaded reliefs were created with four contrasting illumination directions 0°, 45°, 90°, and 135° (Fig. 11). According to (Bentahar, Raji, and Mhamdi 2020; Farah, Algouti, Algouti, Errami, et al. 2021; Farah, Algouti, Algouti, Ifkirne, et al. 2021), the boundaries between the shaded and unshaded ground areas show the presence of lineaments that should be validated by using control geological field map.
5.6. Slope
The synergistic use of slope parameter for validation of detected lineament was proposed in many lineaments mapping studies (Abdelouhed, Ahmed, Abdellah, Mohammed, et al. 2021; Adiri et al. 2017; Jellouli et al. 2021).Because of its Availability as an under product easily extractable from digital model data and its higher spectral resolution, slope data appears as important parameter in lineament studies. In this work, the slope derived from SRTM digital elevation model (12m resolution) is performed as an important tool to highlight the Abrupt changes in slope values which are often a key indicator for detecting linear structures(Jellouli et al. 2021). Furthermore, perhaps most importantly, high values in the slope are commonly considered as probable lineaments because they suggest steep slopes.
5.7. Automatic lineaments extraction
In the present work, the automated lineament extraction was involved by using the most broadly used module LINE extraction of PCI Geomatica version 2016 software. The algorithm was designed to detect lineaments from both radar and optical images because of its many advantages in the automatic extraction of lineaments such as: less time-consuming and high ability to extract the linear segments and features when it is compared with manual extraction method (Aldharab et al. 2018).
The process of automatic extraction of lineaments consists of three stages, namely, edge (contours) detection, thresholding and curve extraction (Epuh et al. 2020; Shandini et al. 2020).The edges detection filter identifies the limits of abrupt changes in the values of neighboring pixels (Jellouli et al. 2021).In the second step, PCI Geomatica software detects successively lines which often refer to lineaments. Furthermore, various optional parameters (RADI, GTHR, LTHR, FTHR, ATHR and DTHR) defined by users, control automatic extraction of the lineament process with a view to enhance resulted data (Aretouyap et al. 2020).The selection by users of these parameters values depend mainly on spatial image resolution, lineament density and type of used data (Thannoun et al. 2013).These six parameters must be added simultaneously to LINE module of Geomatica in order to extract lineaments from radar or optical image and then to convert these polylines in vector form (Ibrahim and Mutua 2014). The six parameters used in this are briefly explained in Table 6.
The automatic lineaments extraction was carried out in this paper by involving two steps: the first step consists to select the optimal band from the available datasets (Landsat 8 and STRM DEM) that should be used for automatic lineament extraction. The optimal inputs were tested to select the most suitable input which presents the high ability to identify the linear features over the study area. For more enhancements, the input in this study was the four directional filter (N0°, N45°, N90°, and N135°) generated from the derivatives of the mosaicked OLI images (including 6-5-7 color compositions and PC1 neo component). According to Abdelouhed, Ahmed, Abdellah, Mohammed, et al. 2021, the first principal component image and color composite bands are useful for discrimination of lineaments and delineation of geological features because lineaments are well interpreted in this data. Subsequently, line model was applied to the four shaded relief (N0°, N45°, N90°, and N135°) created from SRTM DEM (with a ground resolution of 12 m) so as to increase the visibility of small features. In the second step, the necessary and appropriate LINE module parameters should be selected through several setting tests and visual inspections of the output in order to get the most credible lineaments related to the tectonic setting of the study area. Since the numbers and lengths of geologic lineaments automatically extracted by this module depends on the values of the input parameters(Aretouyap et al. 2020)different combinations of parameter values provided by literature and ground truth are evaluated in order to select the optimal LINE values that produced a satisfactory result of the lineaments extracted (Saidi et al. 2020).After adjusting the parameters and assessing the outputs, combination of settings values, that gave the reliable and the more appropriate lineament results, was selected. The default values and the values of parameters adopted in this work are provided in Table 7.
After extraction, the lineaments resulted from different input data were converted to Arcgis shapefile format. The sum of lineament automatically extracted from different data-inputs is about 12.283. The Data extracted reveals structural lineaments that were later confirmed by comparing with the corresponding ancillary data.
Table 6: The Parameters of the LINE module.
Step
|
Parameters
|
Significations
|
Range and Units
|
Contours detection
|
RADI (Filter radius)
|
Specifies the radius (in pixels)that will be used in edges detection filter in order to use the lower values to detect more details and high values to minimize noise detection. This parameter roughly defines the small detail in the processed image to be detected values between 3 and 8 are acceptable
|
0-8192 (Pixels)
|
GTHR (Gradient threshold)
|
This parameter specifies the threshold of brightness change and it is defined as a minimum value of gradient that can be considered as an edge during the edge detection
|
0-255 (Unitless)
|
Line detection
|
LTHR (Length threshold)
|
It expounds the minimum length of curve (in pixels) used for mapping curved linear objects THAT are taken as the lineament for further consideration (a value of 10 is suitable)
|
0-8192 (Pixels)
|
FTHR (Line fitting error threshold)
|
specifies the tolerance allowed when fitting arc or line segment to form a (curved) lineament Values between 2 and 5 are ideal
|
0-8192 (Pixels)
|
ATHR (Angular difference threshold)
|
Defines the maximum angle(in degrees) not to be exceeded between two neighboring polyline to be linked Values between 3 and 20 are recommended
|
0-90 (Degree)
|
DTHR (Linking distance threshold)
|
this measurement specifies the minimum distance (in pixels) between the endpoints of a polyline to be linked Values between 10 and 45 are suitable
|
0-8192 (Pixels)
|
Table 7: The defaults and the applied values of line-module parameters that are used in this study.
Line Module Parameters and Units
|
Default Values
|
Used Values
|
- RADI (Pixels)
|
10
|
25
|
2 GTHR (0,225)
|
100
|
90
|
3 LTHR (Pixels)
|
30
|
50
|
4 FTHR (Pixels)
|
3
|
7
|
5 ATHR ( Degrees)
|
30
|
40
|
6 DHTR (Pixels)
|
20
|
30
|