KEYWORDS: General packet radio service, Signal attenuation, Dielectrics, Soil science, Data modeling, Sensors, Improvised explosive devices, Antennas, Land mines, Radar
On-site ground-penetrating radar (GPR) measurements with visual or acoustic real-time analysis cannot provide direct
information whether or not GPR is suitable for the site at all. However, the knowledge of the limitations of a technique is
of vital importance in the field in case of landmine, IED or UXO detection. For high-frequency (HF) GPR applications,
various electromagnetic (EM) loss mechanisms in the soil play a crucial role. We investigated the EM properties of
different soils using the coaxial transmission line (CTL) technique in the laboratory. We compared these results with
measurements based on time-domain reflectometry (TDR) and direct current (DC) electrical conductivity measurements.
We found that the absorption of EM energy in the soil cannot be described by DC electrical conductivity alone since
dielectric relaxation mechanisms prevail at high frequencies. In order to predict the soil-dependent performance of GPR,
we propose a conventional, relatively inexpensive, soil-moisture field sensor based on TDR as an alternative to the time consuming
laboratory measurements. The TDR probe was calibrated by means of the CTL technique and measures the
intrinsic attenuation as well as the relative dielectric constant. Comparisons between the GPR performance forecast
carried out by on-site TDR measurements and the experimental GPR performance shows a promising correlation.
KEYWORDS: Sensors, Magnetism, Soil science, Metals, General packet radio service, Sensor performance, Land mines, Dielectrics, Magnetic sensors, Data modeling
Metal detector has commonly been used for landmine detection and ground-penetrating radar (GPR) is about to be
deployed as dual sensor that is in combination with metal detector. Since both devices employ electromagnetic
techniques, they are influenced by magnetic and dielectric properties of soil. To observe the influence, various soil
properties as well as their spatial distributions were measured in four types of soil where a field test of metal detectors
and GPRs took place. By analyzing soil properties these four types of soil were graded based on the estimated amount of
influence on the detection techniques. The classification was compared to the detection performance of devices obtained
from the blind test and a clear correlation between the difficulty of soil and the performance was observed; the detection
and identification performance were degraded in soils that were classified as problematic. Therefore, it was demonstrated
that the performance of metal detector and GPR for landmine detection can qualitatively be assessed by geophysical
analyses.
Soil magnetic properties can seriously impede the performance of metal detectors used in landmine clearance operations.
For a proper planning of clearance operations pre-existing information on soil magnetic susceptibility can be helpful. In
this study we briefly introduce a classification system to assess soil magnetic susceptibilities from geoscientific maps.
The classification system is based on susceptibility measurements conducted on archived lateritic soil samples from 15
tropical countries. The system is applied to a soil map of Angola, resulting in a map that depicts soil magnetic
susceptibilities as a worst case scenario. An additional layer depicting the surveyed mine affected communities in
Angola is added to the map, which demonstrates that a large number of those are located in areas where soil is expected
to impede metal detector performance severely.
The small-scale variability of physical soil properties has a negative influence on ground exploration with physical
sensors. This particularly holds true for small target objects like landmines. Studies were carried out to determine
magnetic susceptibility, electric conductivity and dielectric permittivity of natural soils. The spatial variability of
the field data is quantitatively characterised by means of geostatistical analysis. We present field measurements
on different soils types in Germany and on former minefields in Mozambique. The spatial distribution of magnetic
susceptibility is governed by the mineral composition of the soil and its stone content. The correlation lengths
are in the range of a few meters. In contrast, electric conductivity and permittivity is mainly determined by
soil moisture. Due to the small-scale variability of topsoil water content, these two electric properties often
feature very small correlation lengths in the range of decimeters. By way of example, the influence of soil
variability on landmine detection is illustrated for radar sensors. Geostatistical simulation techniques are used
to generate random soil models which are used for realistic finite-differences (FD) calculations of electromagnetic
wave propagation. Permittivity variations appear to have a greater influence on radar detector performance than
conductivity variations and can mask the signals from the mines.
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