Underground structures are critical elements in the military arsenal of many countries. The importance of these structures is becoming an increasingly important part of the defence establishments, protecting critical governmental and military functions, thus contributing to victory during war, or at least make it more difficult for the adversary to destroy critical military capabilities (Sepp, 2000). During the 1960s, some of the remote sensing instruments originally developed for military reconnaissance and classified as defence secrets were released for civilian use as more advanced designs became available for military application (Campbell and Wynne, 2011). These instruments extended the reach of aerial observation outside the visible spectrum into the infrared and microwave regions. It was in this context that the term remote sensing was first used (Campbell and Wynne, 2011). Therefore, Remote Sensing is a rapidly developing scientific field that is applied in various fields of science, including the Military.

Remote sensing relies on observed spectral differences in the energy reflected or emitted from features of interest. Expressed in everyday terms, one might say that we look for differences in the “color” of objects, even though remote sensing is often conducted outside the visible spectrum, where “color”, in the usual meaning of the word, does not exist. This principle is the basis of multispectral remote sensing, the science of observing features at varied wavelengths, in an effort to derive information about these features and their distributions (Campbell and Wynne, 2011). The term spectral signature has been used to refer to the spectral response of a feature, as observed over a range of wavelengths (Parker and Wolff, 1965).

During the last decade, the improvement of sensor characteristics, such as higher spatial resolution and hyperspectral data, as well as technological achievements in space technology, offer new opportunities for future applications (Giardino, 2011). In this context, it should be noted that, in some cases, researchers seek to find not the target itself but rather to identify symptoms related to the topography (relief), crop characteristics (crop marks), soil characteristics (soil marks) or even changes in snow cover (Winton and Horne, 2010). For instance, archaeological structures buried beneath the soil (i.e., still un-excavated sites) can be detected through remote sensing images as stressed vegetation (crop marks) which can be used as proxies for the buried archaeological relics. Crop marks may be formed in areas where vegetation grows over near-surface archaeological remains. These features modify the moisture retention compared to the rest of the crop coverage of an area. Depending on the type of feature, crop vigor may be enhanced or reduced by buried archaeological features (Winton and Horne, 2010).

Underground constructions such as military structures, military bunkers, military bases, tunnel networks and archaeological remains can affect their surrounding landscapes in different ways, including changes in thermal inertia (Gunn et al., 2008), localized soil moisture content and drainage rates (Lasaponara and Masini, 2006), soil composition and vegetation vigor (Milton and Rollin, 2006); however, such changes could just be a sign which could also be attributed to other reasons (e.g., such as plant diseases). Vegetation vigor is often observed on the ground as a crop mark, a spot which can be used to denote the presence of underground structures. Crop marks can be formed both as negative marks above wall foundations and as positive marks above the damper and more nutritious soil of buried pits and ditches (Themistocleous et al., 2015).

This paper aims to study underground military structures over vegetated areas using both ground and satellite remote sensing data. Results obtained from a ground spectroradiometric campaign carried out at a specific area in Cyprus using a SVC-HR1024 field spectroradiometer are presented. Field spectroradiometric measurements were collected and analyzed in order to identify underground structures using the spectral profile of the vegetated surface over the underground target and the surrounding area for in situ observations, throughout the plants' development, with regard to its phenological cycle. Healthy vegetation shows an increase in reflectance in the visible region and a decrease of reflectance when the vegetation is under stress (Melillos et al., 2016b).

Figures 5–12 show the average values of the VIs, as these are mathematically shown in Table 2, during the crop's phenological cycle. In these figures, the VIs of the barley crop over the Structure Military Site (SMS) are shown by the red curves and over the Reference Site (RS) by the blue curves. The response of the VIs with respect to barley growth was comparatively evaluated in an effort to reveal significant differences between the above-mentioned two test areas. Indeed, the findings presented briefly below demonstrate that the eight VIs adopted in this research exhibit distinct differences, corresponding to barley development and between the two sites. They could be used as cultivar-independent phenological indicators. Indeed, VIs values could be used as single thresholds in field spectroscopy for the detection of military underground structures.

The use of more than one VIs for the detection of crop marks is recommended as it is considered to augment the final results (Agapiou et al., 2012). This procedure allowed to compare all VIs by applying the same reference (Mróz and Sobieraj, 2004). Using the VIs of Table 2, it may be seen clearly that there is a distinction between SMS and RS, in the head emerging and flowering stages (red and blue curves, respectively). Furthermore, for each phenological stage the results show that the most remarkable VIs during the phenological cycle from the tilling to the flowering stage are NDVI (Fig. 5), SR (Fig. 7), OSAVI (Fig. 10), DVI (Fig. 11) and MSR (Fig. 12).

Summarising the findings that can be deduced from Figs. 5–12 regarding the contrasting differences between the two sites at different phenological stages:

In the tilling stage (Table 3), NDVI (Fig. 5) obtained higher average values for RS (blue line) compared to SMS (red line). A similar situation was observed for SR (Fig. 7), RDVI (Fig. 8), OSAVI (Fig. 10) and DVI (Fig. 11). On the contrary, regarding the remaining VIs, SMS (red line) exhibited higher values compared to RS.

In this paper, an approach is proposed for detecting military underground structures throughout the phenological cycle of plant growth by using vegetation indices. Indeed, vegetation indices can corroborate areas of possible military underground structures. The advantages of using vegetation indices as proxy variables for inter-calibration among existing sensors are the low sensitivity to the uncertainties in atmospheric correction and the variation in the satellite viewing angle (Steven et al., 2003). In comparing the two test areas, the findings (Figs. 7 and 8) reveal substantial differences between them. The results show that the VIs in Table 2 are useful for determining areas where military underground structures are present. Spectroradiometric measurements can be used as an alternative approach to identify underground military structures, since they can provide accurate spectral signatures for a wide spectral region. Anomalies in the crop spectral signatures resulting from an existing underground structure can be recorded using spectroradiometer (Melillos et al., 2016b).

Within the framework of this research, the authors plan to expand this study by using additional techniques, such as surveying suitable areas with an unmanned aerial vehicle (UAV) with visible and near-infrared cameras, in order to generate

VIs for comparison with the in-situ spectroradiometric measurements (Melillos et al., 2016a). Also, the authors plan to investigate the response of the VIs by cultivating other crops and carry out a similar measurement scheme in an effort to reveal significant differences between the above-mentioned two test areas.