# Autonomous Service Drones for Multimodal Detection and Monitoring of Archaeological Sites
## 2 Literature Review  
The term thermography indicates the recording and measurement of heat radiation using special sensors. In particular, the thermal cameras record the Thermal Radiation Infrared (TIR), which is a part of the electromagnetic spectrum included in the infrared range. Although the wavelength of visible light (i.e., perceptible to the naked eye) is between 0.4 and 0.78 \(\upmu\)m, that of the infrared is greater and ranges from 0.78 to 1000 \(\upmu\)m, as depicted in Figure 3.  
In turn, this range is divided into near infrared (NIR; 0.78-2.5 \(\upmu\)m), mid-infrared (MIR; 2.5-10 \(\upmu\)m) and far infrared (FIR; 10-1000 \(\upmu\)m). The TIR spans over the last two categories, positioning itself between 8-15 \(\upmu\)m [11]; it has been determined that this is the range in which the temperature of a ground, in the form of thermal radiation, without any interference from the air temperature [12]. As [PERSON] and [PERSON] in [13] indicate, soil temperature depends on solar radiation and exchange with the atmosphere. The temperature that we measure on the ground derives both from infrared thermal radiation emitted by the sun (that managed to cross-uncathed, without being reflected back, the gases that form the atmosphere) and the infrared thermal radiation that is absorbed and reflected by both the materials present above or near the ground. Indeed, the different physical and chemical components of a non-homogeneous soil lead to a different absorption and consequent emission of solar radiation, which is highlighted in a different thermal radiation measured on the surface. It is precisely based on this phenomenon that the analysis of thermal images with archaeological purposes relies on the materials that are above and below the ground surface and how they absorb, emit or reflect infrared rays in different ways, based on their own composition, density and moisture content [11; 14]. According to this principle, many archaeological elements can theoretically be identified in thermal images. The problems that led to the scarce use of this technique, outside of satellite imagery, were mainly related to the high cost and the poor spatial and radiometric resolution offered. Between the 1970-80s, a large and cooled scanning radiometer was used with liquid nitrogen that recorded thermal images on long rolls of film [11; 12; 13]. Despite the difficulty, the methodological foundation for the use of thermography in archaeology was developed in those early years. In particular, [PERSON] [13] and [PERSON] [15] conducted a series of controlled tests to quantify the thermal behavior of some archaeological elements  
Figure 3: Representation of the Electromagnetic Spectrum. with respect to the surrounding terrain. The work in [15] presents a summary and tests for soil thermodynamics, including abundant mathematical notation and physical properties used to understand the results, and how analysis can be applied to different archaeological contexts. This work is still considered the basis for archaeological thermography, where they demonstrate variations in thermal radiation between different elements or between parts of land with different composition, and contrast their thermal images to determine different physical properties that demonstrate substance origin. The four main physical properties that determine the variations of thermal radiation between different elements present on or in the ground are summarized as follows:  
1. Thermal conductivity: A physical property that measures the ability of an element to transmit heat (or infrared heat energy) through thermal conduction. That property depends on the nature of the material and is measured in watts per meter-kelvin (W/mK). 2. Volumetric heat capacity: The property that defines the amount of heat energy that must be added to a unit of volume of a material to increase its temperature by one degree.