**2. Radiometric spectra**

The radiometric aspects of the island materials are approached from direct spectrum determinations with a radiometer and from the numerical analysis of the selected Landsat 8 image. To obtain the reflectance of the sampled surfaces, we used an Ocean Optics USB2000+VIS+NIR radiometer with a bandwidth ranging from 350 to 1050 nm. These reflectances are then compared to those of the corresponding bands of the Landsat 8 image. The rocks measured radiometrically were analyzed by X-ray fluorescence obtaining their elemental compositions, which also allowed for comparison with the spectral signature of the corresponding database of the US Geological Survey [5].

Landsat 8 images have 11 bands; of them, we used coastal/aerosol, blue, green, red, Near Infra-Red (NIR), and the 2 Short Wavelength Infra-Red (SWIR) bands. The atmospheric correction was performed with the Histogram Minimum Method [6], and resampling of the digital numbers was made to fit into the 8-bit radiometric resolution format. The spatial resolution was not changed. **Figure 2** shows a color composition (RGB-432) of the image after preprocessing was accomplished. At this scale, the individual pixels of 30 × 30 m begin to show, giving the image a fuzzy appearance.

### **2.1 Radiometric stations**

Ten stations were sampled across the island; at each station, several radiometric measurements were made, and rock samples were collected for subsequent composition analyses. **Figure 3** shows the location of each station and **Table 1** summarizes pertinent information. Owing to space reasons, data obtained in some stations will be omitted in this report.

## *2.1.1 Station 1*

This station corresponds to a basalt outcrop, and the reflectance spectra are shown in **Figure 4**. The four curves follow the same tendencies, with the same variations in percentage reflectance, indicating a consistent response at the station. The different reflectance baselines are attributed to different collection angles. Reflectance is fairly low in Station 1; the four spectra show a wide maximum between 440 and 640 nm.

#### *2.1.2 Station 2*

This station contains two different lithological units. The first is a basalt outcrop, and the second corresponds to white pumice. **Figure 5** shows the outcrops and the

#### **Figure 3.**

*Image of Isla Isabel composed of Google Earth images from June 23, 2015. The location of the radiometric stations corresponds to the white dots.* 


#### **Table 1.**

*Station location in UTM coordinates, number of spectral samples, and corresponding rock type.* 

measuring equipment, and **Figures 6** and **7** show the radiometric responses of basalt and the white tuff. As in Station 1, the basalt exhibits consistent radiometric responses in the four determinations. However, the response differs slightly from that of basalts in Station 1, since they show a plateau between 500 and 600 nm, where the former showed a broad reflectance peak.

The white pumice, as expected, shows radically different spectra from that of basalt. These determinations also show consistency between them, being characterized by a monotonous increase in reflectivity between 360 and 700 nm which evoque the response of a dry, bare soil followed by rapid variations at the end of the spectrum. *Radiometric Mapping of Hydrothermal Alterations in Isla Isabel, Mexico DOI: http://dx.doi.org/10.5772/intechopen.80530* 

#### **Figure 4.**

*Four radiometric records obtained at Station 1 in Isla Isabel. The station location is shown in* **Figure 3***, and its coordinates appear in* **Table 1***. The rock composition corresponds to basalt.* 

#### **Figure 5.**

*Measuring basalt with the spectrometer; the PC control and record measurements. At Station 2, two types of materials were sampled: Basalt and white tuff.* 

**Figure 6.**  *Four records of the radiometric response of basalt at Station 2-1.* 

**Figure 7.**  *Radiometric response of white pumice at Station 2-2.* 
