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Landmine detection problem: The role of the soil DETERMINATION OF SOIL MOISTURE. Vlado Valkovi ć Department of Experimental Physics, Ruđer Bošković Institute, Bijenička c.54, 10000 Zagreb, Croatia .
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Landmine detection problem: The role of the soilDETERMINATION OF SOIL MOISTURE Vlado Valković Department of Experimental Physics, Ruđer Bošković Institute, Bijenička c.54, 10000 Zagreb, Croatia. Co-authors:Jasmina Obhođaš, Davorin Sudac, Karlo Nađ, Ruđer Bošković Institute, Zagreb, Croatia;Giancarlo Nebbia, Giuseppe Viesti, Istituto Nazionale di Fisica Nucleare,Sezione di Padova, Padova, Italy Work done under for Project: DIAMINE, Contract IST-2000-25237, under subcontract between Istituto Nazionale di Fisica Nucleare, Padova, Italy and Ruđer Bošković Institute, Zagreb, Croatia
INTRODUCTION • The purpose of this work was to collect data representing physical and chemical properties of different types of soils in Croatia, which have been investigated in order to provide the necessary background information for landmine explosive detection. • The most important topic investigated has been determination of soil moisture and its dynamical movement in different types of soils. The work presented here could be divided into two parts. One has been directed towards collection of representative soils in order to establish soil bank having the necessary physical and chemical properties determined, and other was directed towards daily and weekly in situ soil moisture measurements on several locations in different parts of Croatia. This also included daily soil moisture measurements in the test field having different types of soils from locations in Croatia. This was done in order to simulate behavior of different soils under same conditions.
SOIL BANK There are 19 soil samples collected for the soil bank available for testing procedures when capabilities of mine detection methodologies are going to be evaluated. These soils represent different soil types, vegetation, ground configuration and climate zones in landmine contaminated areas of Croatia. Preliminary determination of some basic physical and chemical properties (texture, silicate analysis, major and trace element analysis) for all soils has been performed.
Soil moisture measurements Soil moisture variations have been monitored at six locations in Croatia. Locations were chosen with the aim of covering different Croatian climate zones and representative types of soil in landmine affected areas. The locations are: Zagreb and Karlovac in the western part of Croatia, three locations in Zadar and its surroundings representing the south ofCroatia, and Križevci in the northern part of Croatia
Mesurement metodology • Since landmines in Croatia are supposed to be buried down to 30 cm it was decided to monitor soil moisture in profiles of -10, -20, -30 and -40 cm. • Considering this andthecriteria such as ease of insertion, minimal soil disturbance, ease of use, robustness, reliability, signal-to-noise ratio and cost, thetime-domain reflectometry (TDR), based on themeasurement of soil dielectric properties, was found to be the most suitable method. • Variations of soil moisture content in profiles of -10, -20, -30 and -40 cm have been measured with the Profile Probe (type PR1), constructed by Delta-T Devices Ltd. The probe has been calibrated for mineral soils, and theexpected error was< ± 0.1 m3.m-3. • For moisture measurements of thevital toplayer of soils, theTheta Probe (type ML2x) was used since Profile Probe is not designed for measurements lower than 10 cm. It operates on thesame principles as theProfile Probe. The expected error was estimated < ± 0.1 m3.m-3. • The results of measurements obtained with theProfile Probe and theTheta Probe were compared to gravimetric measurements as a part of QA/QC procedures.
Limitations of NBT • Atomic and nuclear physics based sensors might offer new possibilities in de-mining. Among the available nuclear techniques, the neutron backscattering technique (NBT), based on the detection of the produced thermal neutrons, is thought to be the most relevant for field applications. • Presented here are two limitations of NBT, both being related to the soil moisture: • First, the critical value of the soil moisture, reached when the density of the hydrogen atoms in the landmine is equal to that in the background soil, defines a condition for which the detection is not possible. Critical values are small for some of the landmine types thus suggesting the application of the method only to arid countries, where the soil moisture is lower than 10%. • Second, small scale variations of the soil moisture content, experimentally determined for different soil types, are found to be capable of generating false positive readings.
Surface soil moisture variability measurements methodology Surface soil moisture variability for different soil types at different locations were measured by using two methods: Theta probe (Manufacturer: Delta-T, UK), ML2, responding to dielectric properties of soil, and gravimetric method. Each time a surface area of 80 cm by 80 cm was investigated by measuring soil moisture at points separated by 20 cm. Probe ML2 contains 3 rods arranged in a circle around a central rod. This creates a defined cylindrical zone of measurement, 60 mm long x 26.5 in diameter (mass ~50 g). For the gravimetric method top 4 cm of soil were collected with plastic cup of diameter d= 55 mm ( mass ~100 g). Since the ML2 probe displays result in w% determination of soil density was required for comparison of two sets of data. The errors of the measurements were as follows: for ML2 probe the error was ≈5%, while for gravimetric method the relative error was estimated to be ≤10%.
Surface soil moisture (mass %) variations (locations Zagreb-graphs a. and b.; Zadar – Punta Mika, graphs c. and d.) as measured on undisturbed pseudogley with grass cover. (a and c) with ML2 probe, depth of measurements: 0-6 cm; calculated assuming w=1.2742 g/cm3. (b and d) with gravimetric method, depth of measurements: 0-4 cm.
Critical value estimation The problem of landmine detection and identification is a problem of detection and identification of theexplosive and its casing buried in the soil. Soil, due to its elemental composition and themoisture content, produces a background or noise in thespectroscopic measurements done for landmine detection and/or identification. The goal of spectroscopical methods is to maximize the signal to noise ratio. Since theneutron backscattering technique, NBT, measures the number of hydrogen atoms, we have introduced a concept of the critical value corresponding to the equality of the number of hydrogen atoms in the interrogated object and the surrounding soil. This critical value is characteristic of soil type and landmine type.
Characteristics of the landmines found in Croatia and critical soil moisture values for different landmine types
CONCLUSIONS Soil water content is the key attribute of soil in NBT landmine detection application. If the critical value of the soil water content is reached, the detection of landmine explosives is not possible. The critical value is reached when the density of the hydrogen atoms in the landmine is equal to that in the background soil. It is recommended that soil moisture content for NBT application do not exceed 10 kg.kg-1 (10 % Mass). According to preliminary results of soil moisture monitoring of different types of soils in different parts of Croatia, with the aim to cover versatile textures and structures of soils and versatile hydrometeorological regimes, the values of soil moisture often exceed the NBT soil moisture critical value. Considering this, the results presented here suggest that NBT is not suitable for landmine detection in Croatia, but it could be recommended to countries with arid climate were arid soils with soil moisture below 10 % Mass are quite common.
