**2. Materials and methods**

*Military Engineering*

defense agencies [12].

as one of the currently used information collection methods. HUMINT refers to the collection of information by a trained HUMINT collector (military occupational specialty) [8], from people and their associated documents and media sources for the identification of elements, intentions, composition, strength, dispositions, tactics, equipment, personnel, and capabilities. Additionally, technology such as imagery intelligence (IMINT) can also be used for gathering information via satellite and aerial photography. Remote sensing techniques are quick, are easily manageable, and involve a wide variety of techniques where valuable information can be accessed remotely [9, 10]. Buried underground structures are difficult to detect, especially when they are fully covered by soil [11]. It is possible to detect such military underground structures by means of satellite images and aerial photographs. The concern about underground facilities (or "hard and buried" targets) is evident from the establishment of several purpose-dedicated components within various intelligence and

Underground structures such as military constructions and archaeological remains can affect their surrounding landscapes in different ways, such as localized soil moisture content and drainage rates [13], soil composition, and vegetation vigor [7]. Vegetation vigor could be observed on the ground as a crop mark, a spot which can be used to indicate the presence of underground structures [14]. Crop marks can be formed both as negative marks above concrete foundations and as positive marks

During the last decade, the improvement of sensor characteristics, such as higher spatial resolution and hyperspectral data, as well as the technological achievements

Additionally, in some cases, researchers seek not to find 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 (snow marks). 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 a proxy 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 the feature, crop

above the damper and more nutritious soil of buried pits and ditches [14].

in space technology, offers new opportunities for future applications [15].

vigor may be enhanced or reduced by buried archaeological features [16].

takes advantage of the application of UAVs [23].

area (lack of underground structure).

In comparing the two different kinds of marks, the positive crop marks are normally taller with darker green and healthy foliage than the negative crop marks, while negative crop marks tend to be paler green with lighter-colored appearance when monitored from the air [17]. Indeed, spectral remote sensing is widely used in several occasions for the detection of underground structures, such as agricultural

In addition, spectral remote sensing for the detection of underground military structures is considered to be very precise in detecting subsurface remains. Different geophysical processing techniques and equipment, such as GPR, magnetometer, and resistivity, are usually integrated to maximize the success rate of uncovering underground remains [19–22]. Moreover, the use of unmanned aerial vehicle (UAV, popularly known as a drone) for environmental remote sensing purposes has increased in recent years. Although the military has used UAVs for defense applications for decades, the scientific environmental sector increasingly

Also, this chapter investigates the possibility of applying satellite data using Landsat 8 sensor and comparing it with field data in an effort to distinguish between buried structure area (existence of underground structure) and vegetated

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remains [18].

This study proposes a methodology for detecting underground targets using remote sensing techniques. The basis of this methodology is the combination of the study of the vegetation phenology as a proxy for buried underground structures of significance to defense. Data acquisitions were used to identify any variations between the area over an underlying structure and over a reference area.

For this study, certain assumptions have been adopted. In the case of this project, phenological field observations were conducted in two test sites from 2016 to 2017 to determine the dates of completion of different phenological phases. For actual defense purposes, the characteristics of the area of interest are often not known. Furthermore, the cultivation of barley in the area is for investigative purposes and part of the experimental work for studying the impact of underlying structures on vegetation. Under real scenarios, different types of vegetation (if any) will be present.

#### **2.1 Study area**

The proposed methodology has been applied in Cyprus over a specific geographical area. The area is situated on a hill which provides clear viewing from airborne and spaceborne platforms, making the area ideal for remote sensing applications (**Figure 1**, left). Also, it is located within a fenced, abandoned military area (due to security and confidentiality issues, the specific area cannot be reported herein). The soil type of the area is leptosol which contains small amounts of gravel and with a very shallow depth.

**Figure 1.** *Overview of the study area (left); an example of a military bunker (right).*

**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.
