**1. Introduction**

For decades, research on the detection of buried targets has led to the development of a variety of techniques for identifying buried structures [1, 2]. These techniques use a variety of geophysical instruments [3–6] that involve the use of ground-penetrating radar (GPR). Ground-penetrating radar is a sensitive technique for detecting even small changes in the subsurface dielectric constant. Consequently, the images generated by GPR systems contain a great amount of detail, much of it either unwanted or unnecessary for purposes of surveying for underground objects. A major difficulty, therefore, in using GPR for locating an underground structure concerns the present inability in the art to correctly distinguish return signals reflected by an underground object of interest from all of the signals generated by other subsurface features; the latter signals are collectively referred to as clutter [7].

Nowadays, a lot of attention is being paid to the development of new methods and instrumentation for the detection of buried targets. The detection of military underground structures is a major concern for military and national security agencies, as this is evident from the large budget [8] allocated for the detection and monitoring underground structures. National security agencies use human intelligence (HUMINT) 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 defense agencies [12].

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

During the last decade, the improvement of sensor characteristics, such as higher spatial resolution and hyperspectral data, as well as the technological achievements in space technology, offers new opportunities for future applications [15].

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

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 remains [18].

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 takes advantage of the application of UAVs [23].

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 area (lack of underground structure).

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**Figure 1.**

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

*Detecting Underground Military Structures Using Field Spectroscopy*

It is noted that in the literature, there is a gap in the monitoring of vegetation over military underground and ground structures throughout the plants' phenological development cycle; this paper aspires to contribute to the filling-in of this gap. Indeed, this chapter aims at presenting the results obtained from ground spectroradiometric campaigns, using an SVC-HR1024 field spectroradiometer, carried out in a specific area in Cyprus. For in situ observations, field spectroradiometric data were collected and analyzed to identify (known) underground structures using the spectral profile of the vegetated surface over the underground target and the surrounding area. Crop marks demonstrate the variations between the presence and the absence of military underground structures. The in situ measurements were resampled to the Landsat 8 sensor using the appropriate relative spectral response (RSR) filters.

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

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.

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.

between the area over an underlying structure and over a reference area.

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

**2. Materials and methods**

**2.1 Study area**

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

It is noted that in the literature, there is a gap in the monitoring of vegetation over military underground and ground structures throughout the plants' phenological development cycle; this paper aspires to contribute to the filling-in of this gap. Indeed, this chapter aims at presenting the results obtained from ground spectroradiometric campaigns, using an SVC-HR1024 field spectroradiometer, carried out in a specific area in Cyprus. For in situ observations, field spectroradiometric data were collected and analyzed to identify (known) underground structures using the spectral profile of the vegetated surface over the underground target and the surrounding area. Crop marks demonstrate the variations between the presence and the absence of military underground structures. The in situ measurements were resampled to the Landsat 8 sensor using the appropriate relative spectral response (RSR) filters.
