There are some discrete variables that affect sensor performance. Here we will cover the common ones that affect the performance of both sensors (IR, and GPR):
1- Soil magnetic properties: The magnetic properties of the soil can strongly affect the received electromagnetic waves received in response from buried objects. This is a phenomenon verified in some type of soils, which are called magnetic soils. Variables taking this phenomenon into account could be magnetic susceptibility Ҳ of the soil, but interactions of magnetic properties with sensor measurements are still far to be reliably quantified by means of Ҳ. Therefore this variable is simply represented by a binary value (yes, no) standing respectively for highly magnetic soils and non-magnetic soils.
2- Soil moisture: The soil moisture influences the penetration depth of radar waves into the underground by attenuating them more quickly. Nevertheless, moderate level of soil humidity can improve images obtained from GPR sensors. The most suitable variable to take account of the soil moisture is the volumetric soil humidity percentage. Although it is a continuous value from 0% to 100%, its effect is evident for extremely dry or extremely saturated soils (more than 50 % humidity) by one side, and for moderately moist soils by the other.
3- Soil composition: This variable takes into account the fact that the particular materials (sand, clay, etc.) can enhance or attenuate thermal contrasts. The figure of merit of this variable will be a finite set of possible soil compositions, each one containing certain percentage of the main materials. Thus, a sandy soil will have 90% of sand and 10% of clay, for instance.
4- Soil uniformity: Soil uniformity takes into account the heterogeneity of underground materials composing the soil. Layers of different materials have different dielectric constants and this affects GPR measurements. Presence of soil non uniformities reflects in GPR images, which become hardly readable. Soil uniformity discrete variable has a binary value (yes, no), the former indicating presence of a multilayered heterogeneous soil, the latter a homogeneous soil.
5- Vegetation: Vegetation on soil is important for infrared images. In fact, it can cause non uniformity of solar heating, masking soil and mines signatures. It is therefore important to know if there is much vegetation over a soil zone.
6- Time of the day: The time of the day influences the contrast of thermal signatures. In fact, thermal anomalies are a diurnal cycle phenomenon. Following the time of the day it’s possible to have more or less evident signatures. Several trials have been made to establish a robust relation between hours of the day and maxima in temperature difference between bare soil temperature and above-mine soil temperature. Nevertheless, no precise and reliable results are available yet; therefore it is possible to give only wide range of hours representing levels of soil illumination, supposing operating hours for minefield clearance from 7 to 21.
7- Weather. Rain is an important factor for the thermal signatures. It reduces solar heating of the soil. A sunny weather favors solar heating and consequently image contrast. Prolonged time without solar heating will drive the necessity of active illumination. Weather condition is represented by a discrete variable assuming the values ‘clear’, ‘rain’ and ‘overcast’.