**2.2 Methodology**

Spectral data are increasingly incorporated into process-based models of the Earth's surface and the atmosphere. The area of interest was determined first by identifying plots with a high probability of buried targets. Such areas can be determined from various sources, such as on-site irregular activities, personal communication, surveys, and crop marks.

In situ measurements were taken at two test areas: (a) vegetation area covered with vegetation (barley), in the presence of an underground military structure [hereafter denoted as area (a)], and (b) vegetation area covered with vegetation (barley), in the absence of an underground military structure [hereafter denoted as area (b)].

An SVC-1024 spectroradiometer of the Spectra Vista Corporation (SVC) with a spectral range of 350–2500 nm was used to measure reflectance values. The spectral resolution of the spectroradiometer is 1.0 nm. The measurements were taken between 11:00 AM and 13:00 PM (local time), under clear and overcast skies for diffuse light to minimize any variation of the incoming solar electromagnetic radiance. In addition, a calibrated Spectralon panel (with reflectance ≈99.996%) measurement was used as a reference, while the measurements over the crops were used as a target [12].

Then, the waveband reflectance values were used to calculate three vegetation indices (VIs), the normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and simple ratio (SR), as shown in **Table 1**. The waveband reflectance's were calculated from the RSR filter of the Landsat 8 sensor. The vegetation indices were plotted and statistically cross compared between the two areas of interest, namely, the "buried military structure" and the "nonmilitary structure."

In situ measurements were taken in a grid format which is the same as the dimension of the underground structure (13 m × 5 m), over the two study areas, and systematic targets were collected at each time to compile a representative sample that is statistically reliable. Due to the very close proximity of the two sites (<20 m), the analysis was based on the following two criteria: both study areas have similar soil and climatic characteristics [28]. Area (a) is the area over the underground structure itself and the area around it. Area (b) is the reference area in the absence of an underground military structure. The measurements were also made when the underground structure was covered with the existing natural soil which was subsequently cultivated and covered with vegetation (barley) in order to study possible differences of the spectral signature of vegetation, as a result of the existence of underground structures. During the campaign, 1740 measurements were taken using the SVC-1024 field spectroradiometer, with an average reference spectral signal as given for each of the six campaigns (random sampling) in **Table 2**.


**Table 1.**

*Vegetation indices used in this study, where pNIR, pRED, and pBLUE represent the atmospherically or partialatmospherically corrected surface reflectance values of the near-infrared (NIR), red (RED), and blue (BLUE) wavelengths, respectively [24].*

**95**

**Figure 2.**

*during phenological cycle for NDVI.*

*Detecting Underground Military Structures Using Field Spectroscopy*

**Figures 2-4** show the results for the vegetation indices shown in **Table 1** with reference to the phenological stages. The vegetation indices were applied to the barley crop over area (a) (red line) and area (b) (blue line). The response of VIs with respect to barley growth was evaluated by contrasting the minimum to the abovementioned areas. The results show that VIs display a distinct variation corresponding to the barley development and they could be used as cultivar-independent phenological indicators. It can be observed that there is a high correlation between the results of VIs. Indeed, VIs could be used in field spectroscopy for the detection of buried structures. The use of more than one VI for the detection of crop marks is suggested in order to enhance the final results. Furthermore, it is clear from these graphs that VI values vary from one phenological stage to another. Although the same dataset was used for all these vegetation indices, each of the VIs demonstrates

*Vegetation values for area (a) for (buried structure, red dots) and area (b) for (vegetated area, blue line)* 

**No. Date Phenological stage Number of measurements**

(a) 30-10-2016 Cultivation stage 120 (b) 11-12-2016 Tilling stage 120 (c) 23-01-2017 Flag Leaf Emerging stage 120 (d) 25-02-2017 Boot stage 460 (e) 05-03-2017 Head Emerging stage 460 (f) 16-03-2017 Flowering stage 460

*DOI: http://dx.doi.org/10.5772/intechopen.86690*

*Number of measurements in each phenological stage.*

**3. Results**

**Table 2.**

**3.1 Vegetation indices**

*Detecting Underground Military Structures Using Field Spectroscopy DOI: http://dx.doi.org/10.5772/intechopen.86690*


**Table 2.**

*Military Engineering*

**2.2 Methodology**

munication, surveys, and crop marks.

1. NDVI (Normalized Difference Vegetation Index)

**Figure 1** (right) shows a military storage bunker similar to the one that is in the focus of this research. The horizontal dimensions of the underground structure are 13 m × 5 m; it is a concrete storage bunker, located ~2 m below the ground surface.

Spectral data are increasingly incorporated into process-based models of the Earth's surface and the atmosphere. The area of interest was determined first by identifying plots with a high probability of buried targets. Such areas can be determined from various sources, such as on-site irregular activities, personal com-

In situ measurements were taken at two test areas: (a) vegetation area covered with vegetation (barley), in the presence of an underground military structure [hereafter denoted as area (a)], and (b) vegetation area covered with vegetation (barley), in the absence of an underground military structure [hereafter denoted as area (b)].

An SVC-1024 spectroradiometer of the Spectra Vista Corporation (SVC) with a spectral range of 350–2500 nm was used to measure reflectance values. The spectral resolution of the spectroradiometer is 1.0 nm. The measurements were taken between 11:00 AM and 13:00 PM (local time), under clear and overcast skies for diffuse light to minimize any variation of the incoming solar electromagnetic radiance. In addition, a calibrated Spectralon panel (with reflectance ≈99.996%) measurement was used as a reference, while the measurements over the crops were used as a target [12]. Then, the waveband reflectance values were used to calculate three vegetation indices (VIs), the normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and simple ratio (SR), as shown in **Table 1**. The waveband reflectance's were calculated from the RSR filter of the Landsat 8 sensor. The vegetation indices were plotted and statistically cross compared between the two areas of interest, namely, the "buried military structure" and the "nonmilitary structure." In situ measurements were taken in a grid format which is the same as the dimension of the underground structure (13 m × 5 m), over the two study areas, and systematic targets were collected at each time to compile a representative sample that is statistically reliable. Due to the very close proximity of the two sites (<20 m), the analysis was based on the following two criteria: both study areas have similar soil and climatic characteristics [28]. Area (a) is the area over the underground structure itself and the area around it. Area (b) is the reference area in the absence of an underground military structure. The measurements were also made when the underground structure was covered with the existing natural soil which was subsequently cultivated and covered with vegetation (barley) in order to study possible differences of the spectral signature of vegetation, as a result of the existence of underground structures. During the campaign, 1740 measurements were taken using the SVC-1024 field spectroradiometer, with an average reference spectral signal as given for each of the six campaigns (random sampling) in **Table 2**.

**No. Vegetation index Equation Reference**

2. EVI (Enhanced Vegetation Index) 2.5 (pNIR − pRED)/(pNIR + 6 pRED − 7.5 pBLUE + 1) [26] 3. SR (Simple Ratio) pNIR/pRED [27]

*Vegetation indices used in this study, where pNIR, pRED, and pBLUE represent the atmospherically or partialatmospherically corrected surface reflectance values of the near-infrared (NIR), red (RED), and blue (BLUE)* 

(pNIR − pRED)/(pNIR + pRED) [25]

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**Table 1.**

*wavelengths, respectively [24].*

*Number of measurements in each phenological stage.*
