The study area is located in 58 km northeast of Isfahan in Central Iran (Figure 1(a)). Due to proximity to the desert, the climate of this area is warm temperature and dry in summer and cold temperature and dry in winter. The most important natural phenomena in the study area are hot spring systems, which have a significant impact on regional mineralization and creation of travertine and bentonite mines. Mainly the area consists of Devonian to Quaternary sedimentary units. Remote sensing instruments can provide detailed information on the mineralogy and geochemistry of the rock types comprising the Earth’s surface, and have been used for decades to map rocks, mineral assemblages and weathering characteristics  -  .
The type of land cover or mineral composition of the surface of the earth is determined using multispectral data based on the spectral effects. The Landsat and Terra satellites are of the most successful earth’s surface observation systems. Regarding huge advancements in sensors technology, the spectral and spatial resolution of remote sensing data has been improved. The VNIR spectral range is used by nearly all the broadband reflective multispectral sensor systems. The SWIR spectral range is regarded as the most effective for lithological and mineral mapping because most rock types have the most reflectance in the 1.55 - 1.75 µm range (Figure 2) and hydrous (clay) minerals (often products of hydrothermal alteration) have diagnostic absorption features in the spectral range 2 - 2.4 µm  . The advanced spaceborne thermal emission and reflection radiometer (ASTER), launched on board NASA’s Terra spacecraft in December 1999, were developed based on the success of airborne Thermal Infrared Multispectral Scanner (TIMS). ASTER consists of three separate subsystems with a total of 14 spectral bands across the VNIR, SWIR and TIR with three, six and five bands in each part of the spectrum respectively  . Several recent studies have focused on lithological mapping using ASTER data including all VNIR, SWIR and TIR wavelength regions      . ASTER sensor has a
Figure 1. (a) Satellite image (Landsat 8) of the study area; (b) The band combination RGB: 652 from the study area.
good combination of spatial resolution in the VNIR, SWIR bands and multispectral thermal data bands which are very useful in geological applications (Figure 2).
The VNIR bands with a resolution of 15 m are adequate for distinguishing broad categories of land surface such as vegetation, water, soils, urban areas, superficial deposits and general rock outcrops while the six narrow SWIR bands of 30 m resolution have the potential to map major mineral assemblages (rock forming and alteration) and lithological unites  . Landsat 8 is the latest satellite in this series of Landsat satellites picking up images using two sensors. The first sensor is the operational land image (OLI), that takes the image from 9 short wavelength bands with a resolution of 30 meters for the VNIR and SWIR bands and the second sensor is a thermal infrared sensor (TIRS) that takes the image from two bands of thermal wavelength and includes a panchromatic band with a resolution of 15 meters  . In this study, we used the 1T Level of OLI sensor data with it’s the panchromatic band and also VNIR, SWIR bands of 1B level of ASTER sensor data. Geological maps on the scale of 1:25,000 are the most important tools for presenting geological surveying results and regionally display geological unit, especially their distribution, composition, structural relation and age. Collecting the existing geological data, preparing topographical maps for field surveys, providing necessary equipments for field work, informing the authorities, gathering samples and recording of field observations are required to prepare geological maps in field method. In field works, expenses such as cost of travel, vehicle, equipment and assistance are the most costly part of the program and providing and supplying of the items mentioned above is necessary for any of these field acquisitions to control the costs of the mapping project  . This research aimed to use satellite data in the preparation of geological map, which will lead to saving time and reducing coast, as well as increasing the accuracy of mapping operations. So, in this method, access to geological maps is more cost effective.
2. The Geological Setting
The study area is tectonically considered as sedimentary part of the Sanandaj-
Figure 2. The spectral range comparison for Landsat 8 (OLI), Landsat 7 (ETM +) and ASTER, this band distribution as shown in the USGS documents  .
Sirjan Zone. The Sanandaj-Sirjan Zone is located between the Urumieh-Dokhtar Magmatic belt and the Zagros Fold-Thrust Belt  . The Urumieh-Dokhtar Magmatic belt is composed of a series of massive volcanic and pyroclastic rocks associated with it that extends from Sahand to Bazman and have seen wide activities in Cenozoic. In the Eocene, the most intense volcanic activities have occurred in this zone so that pyroclastic rocks in the south in form of Urumieh?Dokhtar belt and parallel to Sanandaj-Sirjan belt have been developed and andesite and dacite rocks have covered a vast part of the area  . The Qom-Zefreh fault with the approximate NW-SE trend divided the Sanandaj-Sirjan Zone and The Urumieh-Dokhtar Magmatic belt. The eastern part of the fault Qom-Zefreh includes intrusive and volcanic rocks and its Western part includes high fractured sedimentary units that our study area is located in this section Figure 1(a). In economic terms, hot springs have formed the travertines of the region and these mines are currently active. In terms of stratigraphy, the most important rocks and sediments covering the area can be classified according to the age and the time of the appearance in the following order from old to new accordingly Figure 3.
The oldest rock units (Paleozoic period) of the study area are the Bahram and Jamal formations, which consists of fossiliferous limestone, dolomite and shale.
The Sorkh shale, Shotory and Naiband formations are formed in the Mesozoic period, which contains red ferruginous quartzite, shale, dolomite and grey limestone.
In the Jurassic and Triassic periods, sedimentary units such as shale, sandstone, red conglomerate, and limestone are formed in the region. The lower red formations (upper and lower members) and Qom formation are the most important formations in the Cenozoic period, which consists of conglomerate, red sandstone and yellow fossiliferous limestone. In the Quaternary period, the most important lithology in this area is travertine from hot springs  .
3.1. Data Preprocessing
In this study VNIR and SWIR bands OLI data and the panchromatic band taken
Figure 3. Geological map of the study area  .
in 2015 and VNIR and SWIR bands (LIB) ASTER data taken in 2002 were used. Digital image preprocessing is a set of techniques that provide using more precision and less noise data. First, both data has been corrected atmospherically and radiometrically, The Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes (FLAASH) model was applied to the VNIR, SWIR dataset to correct the atmospheric influence and convert radiance data to reflectance images  . The negative effects of atmosphere and topography on the data were removed and finally the data with less noise were processed.
3.2. Data Processing
Color image is made by three different bands combination and assigning a color of the three primary colors, red, green, and blue to each band. Since most satellite images are available in multi-band formats, the study of a band does not provide much information. By combining multiple bands, we can investigate details of the features better than before. Awareness of the interactions between different wavelengths is important for identifying the features and different types of land cover. In making false color combinations it is better to use bands with less correlation  . In this research, the False Color Composite (FCC) with RGB: 652  was used to visualize the formations. This combination highlighted the geological features of the region with greater clarity and resolution.
Minimum Noise Fraction (MNF) is a modified linear algorithm  , which determines the inherent dimensions of image data. This method is used to separate data from noise and reduces the dimensions of data information; it helps reducing the spectral and noise dependence of image bands and the amount of error existing in detection of the target features  . MNF is a useful algorithm which applies two cascaded principal components analyses for reducing the dimensionality of multispectral data and minimizing the noise in the imagery. The two-step transformation could separate the noise from useful information  ,  . So in this conversion, a graph of eigenvalue representing the values for each band is obtained. The MNF algorithm divides the data in two categories: The first part is the data with eigenvalues above 1, which includes useful and without noise information and the second part is the data with eigenvalues of less than or equal to 1 that includes noises  . MNF eigenimages could be directly used for vegetation and lithological mapping  . In this study, the VNIR and SWIR data were used for MNF transformation.
The Matched Filtering (MF) is the optimal linear filter for maximizing the signal-to-noise ratio (SNR) that was originally known as a North filter. MF relies on correlating the received signal (a vector) with a filter (another vector) that maximizes the enhanced signal parallel to the signal  . The results of MF appear as a series of grey scale images for each selected endmember. Floating-point provides a mean of estimating the relative degree of match to the reference spectrum and approximate sub-pixel abundance, where 1.0 is a perfect match. In histogram the background material’s data is centered on 0 and the target (endmember) appears in the upper tail of the histogram  . In this research, the trains were provided by Region of Interest (ROI) method and ASTER data. The images obtained from the MF method signified the prospective lithology units much better than the Mixture Tuned Matched Filtering (MTMF) method.
The Spectral Analogues tool maps the occurrence throughout an image of some desired material. Spectral Analogues uses spectral matching algorithms that compare the spectrum of each pixel to the mean spectrum of user-specified training pixels. You must know at least one location in the image where the desired material exists, usually through visual observation or prior knowledge, so that it can be used for training. The Spectral Analogues tool is designed for use with multispectral data  . After applying this method, we have three types of outputs: MF, SAM, and MF/SAM. The MF/SAM output Suppresses false positives that may be present in one method, but not the other, while enhancing true positives. For example, a pixel containing water has a high MF value and low SAM value. A high value divided by a low value results in a very large value, therefore enhancing the positive result  . In this research, the Trains were provided with OLI data, and according to the results of three types of outputs, in most cases, the MF/SAM ratio is very good for lithological detection Figure 4.
4.1. Matched Filtering (MF) Method
MF is used to find the abundances of user-defined endmembers using a partial unmixing. This technique maximizes the response of the known endmember and suppresses the response of the composite unknown background, thus matching the known signature. It provides a rapid means of detecting specific materials based on matches to library or image endmember spectra  . In this research, the selected trains were provided by ROI method and ASTER data. These selected trains were applied in the MF method. Outputs include gray scale images which the target pixels are bright pixels Figure 5 and Table 1.
4.2. Spectral Analogues Method
Spectral Analogues uses spectral matching algorithms that compare the spectrum of each pixel to the mean spectrum of user-specified training pixels  . In this research, the selected trains were provided using ROI method and OLI data, these selected trains were exerted in the Spectral Analogues method. The outputs consisted of three types of data, SAM, MF, and MF/SAM. In the MF, MF/SAM outputs, the images are Gray Scale that the target pixels are the bright ones. In SAM output the target pixels are dark. Best image for drawing each lithology was determined by comparing the three images Figure 6 and Table 2.
Geological formations were determined by examining the images obtained from
Figure 4. The flowchart of ASTER and OLI data processing steps.
the Matched Filtering and Spectral Analogues methods and also, the exact boundary of each formation was drawn through comparing these images to the image obtained from the FCC with RGB (652) (Figure 1(b)).
Assessing the Accuracy of the Classification Map
Maps help us to measure the location and distribution of the resources, to analyze their interactions, to find suitable locations for specific actions and plan for future events. To obtain better results for the map data, the determination of their accuracy is mandatory.
For accuracy assessment of the formation’s map, the thematic accuracy method was done. The thematic accuracy deals with labels or descriptive properties of the map’s features and modules the difference between phenomena’s label and names on map and their actual nature on Earth.
The thematic accuracy done in two ways:
Figure 5. The images from MF method in ASTER data.
Non-Site-Specific Accuracy evaluation and the Site-Specific Accuracy evaluation which in former, only the total area of classes is calculated and their location is not considered. However, in the Sites specific accuracy evaluation, in addition to calculating the total area of classes, their location is also considered. In this project, the sites specific accuracy assessment method was used  .
After drawing, the classification map (Figure 7) and reference map (Figure 3) were examined using ENVI software and the Confusion Matrix method in terms of location and the area of the formations. Use Confusion Matrix to show the accuracy of a classification result by comparing a classification result with ground truth information  . The Confusion Matrix report shows the overall accuracy, kappa coefficient, errors of commission (percentage of extra pixels in class), errors of omission (percentage of pixels left out of class), producer accuracy, and user accuracy for each class  .
Figure 6. The images from Spectral Analogues method in OLI data.
The kappa coefficient (K) “Equation (1)” is another measure of the accuracy of the classification. It is calculated by multiplying the total number of pixels in all the ground truth classes (N) by the sum of the confusion matrix diagonals ( ), subtracting the sum of the ground truth pixels in a class times the
sum of the classified pixels in that class summed over all classes ( ), and dividing by the total number of pixels squared minus the sum of the ground truth pixels in that class times the sum of the classified pixels in that class summed over all classes  .
Figure 7. The map drawn from applying MF and Spectral Analogues methods.
Table 1. The properties of images from the MF method (for image name see Figure 5).
Table 2. The properties of images from the Spectral Analogues method (for image name see Figure 6).
The overall accuracy is calculated by summing the number of pixels classified correctly and dividing by the total number of pixels “Equation (2)”. The ground truth image or ground truth ROIs defines the true class of the pixels. The pixels classified correctly are found along the diagonal of the confusion matrix table which lists the number of pixels that were classified into the correct ground truth class. The total number of pixels is the sum of all the pixels in all the ground truth classes  .
In this regard ( ) is sum of diagonal elements and (N) the total number of pixels  .
The output error images are mask images, one for each class, where all correctly classified pixels have a value of 0 and incorrectly classified pixels have a value of 1. The last error image band (Figure 8) shows all the incorrectly classified pixels for all the classes combined  . Using the Kappa index value, the accuracy rate for the selected formations is 86%, so this map’s accuracy is acceptable (Table 3).
In this research, the ASTER and OLI data were used.. The noise data value reduced significantly by applying the atmospheric, geometric and topographic corrections. The SWIR and VNIR bands were used in the ASTER and OLI data for features detection. For a better detection of the region’s formations using the Region of Interest method the selected train was taken in both data, considering that each formation consisted of several sedimentary units and it was affected by recent sediments. These selected trains were used for OLI data and ASTER data in the
Figure 8. Image of Total Class Error.
Table 3. The accuracy of the prepared map.
Spectral Analogues and the MF method, respectively. At last, by comparing the formations obtained from these two methods and the RGB (652) band composite in OLI data, the formation’s map of the region was drawn. In the Spectral Analogues method, the MF/SAM output images have the most adaptation to the region’s formations according to examining the SAM, MF, MF/SAM output images. Due to higher radiometric resolution, OLI data is more efficient than ASTER data in detecting the region’s formation. The thematic accuracy method was used for formation’s map accuracy assessment. In terms of area and location, each formation in the drawn map was compared with the reference map and finally, the map accuracy 86% was confirmed.
ASTER: Advanced Space borne Thermal Emission and Reflection Radiometer
ENVI: Environment for Visualizing Images
ETM+: Enhanced Thematic Mapper Plus
FCC: False Color Composite
FLAASH: Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes
MF: Matched Filtering
MNF: Minimum Noise Fraction
MTMF: Mixture Tuned Matched Filtering
NASA: National Aeronautics and Space Administration. www.gsfc.nasa.gov
OLI: Operational Land Imager
RGB: Red Green Blue
ROI: Region of Interest
SAM: Spectral Angle Mapper
SNR: signal-to-noise ratio
SPOT: System Pour Observation de La Terre
SWIR: Short Wavelength Infrared
TIMS: Thermal Infrared Multispectral Scanner
TIR: Thermal Infrared
TIRS: Thermal Infrared Sensor
USGS: United States Geological Survey. www.usgs.gov
VNIR: Visible and Near Infrared
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