Magnetic properties of soils have adverse effects on metal detectors, particularly hampering operations during clearance
of landmines and unexploded ordnance. Although there is well established research in soil magnetism and modeling
electromagnetic induction systems these have tended to exist in disparate disciplines. Hence, a workshop was organized
to bring together researchers, academics, stakeholders and manufacturers to discuss key priorities for research and
technology in a unique multidisciplinary environment. Key knowledge gaps identified include limited information on the
spatial heterogeneity of soil magnetic properties in 2D and 3D, whether current models describing soil responses are
appropriate for all soils and the need for compensation mechanisms in detectors to be improved. Several priorities were
identified that would maximize future developments for multidisciplinary research in soil magnetism and detector
technology. These include acquiring well constrained empirical data on soil electromagnetic properties and detector
response over the frequency range of detectors; development of predictive models of soil magnetic properties;
investigating variability of soil magnetic properties in two and three dimensions across a range of scales. Improved
communication between disciplines is key to effective targeting and realization of research priorities. Possible platforms
include a multidisciplinary pilot study at an appropriate site and the development of an online repository to assist
dissemination of results and information.
One of the Department of Defense's most pressing environmental problems is the efficient detection and identification
of unexploded ordnance (UXO). In regions of highly magnetic soils, magnetic and electromagnetic sensors often detect
anomalies that are of geologic origin, adding significantly to remediation costs. In order to develop predictive models for
magnetic susceptibility, it is crucial to understand modes of formation and the spatial distribution of different iron
oxides. Most rock types contain iron and their magnetic susceptibility is determined by the amount and form of iron
oxides present. When rocks weather, the amount and form of the oxides change, producing concomitant changes in
magnetic susceptibility. The type of iron oxide found in the weathered rock or regolith is a function of the duration and
intensity of weathering, as well as the original content of iron in the parent material. The rate of weathering is controlled
by rainfall and temperature; thus knowing the climate zone, the amount of iron in the lithology and the age of the surface
will help predict the amount and forms of iron oxide. We have compiled analyses of the types, amounts, and magnetic
properties of iron oxides from soils over a wide climate range, from semi arid grasslands, to temperate regions, and
tropical forests. We find there is a predictable range of iron oxide type and magnetic susceptibility according to the
climate zone, the age of the soil and the amount of iron in the unweathered regolith.
Modeling studies and experimental work have demonstrated that the dynamic behavior of soil physical properties has a significant effect on most sensors for the detection of buried land mines. An outdoor test site has been constructed allowing full control over soil water content and continuous monitoring of important soil properties and environmental conditions. Time domain reflectometry sensors and thermistors measure soil water1 content and temperature, respectively, at different depths above and below the land mines as well as in homogeneous soil away from the land mines. During the two-year operation of the test-site, the soils have evolved to reflect real field soil conditions. This paper compares visual observations as well as ground-penetrating radar and thermal infrared measurements at this site taken immediately after construction in early 2004 with measurements from early 2006. The visual observations reveal that the 2006 soil surfaces exhibit a much higher spatial variability due to the development of mini-reliefs, "loose" and "connected" soil crusts, cracks in clay soils, and vegetation. Evidence is presented that the increased variability of soil surface characteristics leads to a higher natural spatial variability of soil surface temperatures and, thus, to a lower probability to detect landmines using thermal imagery. No evidence was found that the soil surface changes affect the GPR signatures of landmines under the soil conditions encountered in this study. The New Mexico Tech outdoor Landmine Detection Sensor Test Facility is easily accessible and anyone interested is welcome to use it for sensor testing.
In this paper we present the results of recent field and laboratory studies of the mineralogy and magnetic properties of
young and/or weakly developed soils in Montana and California. The Chevallier Ranch UXO site in Montana is
characterized by a basaltic plug and radiating feeder dikes, which is found surrounded by shales of the Spokane
Formation. The site in California consists of an offset alluvial fan soil chronosequence of Little Rock Creek along the
Mojave section of the San Andreas fault. The fan sediments include significant amounts of mafic material. The fan ages
range from 16 to 413 thousand years. The results of magnetic susceptibility measurements and laboratory analysis of
mineralogy demonstrate that the magnetic susceptibility in these soils is predominantly correlated with parent material
and less with age or landscape position. Slow rates of soil forming processes lead to relatively low frequency dependence
in magnetic susceptibility as compared to similar-age soils in tropical environments. The magnetic character of the soils
can be accurately predicted with a previously developed model.
Ferrimagnetic minerals such as magnetite and maghaemite can affect ground-penetrating radar (GPR) signals. This may
lead to false alarms and missed targets when surveying for the detection of buried landmines and unexploded ordnance
(UXO). In most field situations ferrimagnetic mineral content is too low to affect GPR wave behavior. However, in soils
and sedimentary material with magnetite-rich parent material large concentrations of magnetite can be found. This paper
is a first systematic experimental effort to study the effects of large concentrations of magnetite for GPR detection of
subsurface targets. We study the effects of (i) different homogeneous mixtures of magnetite and quartz sand and (ii)
magnetite concentrated in layers (placer deposits), on the propagation behavior of GPR waves and reflection
characteristics of steel and plastic balls. The balls are buried in homogeneous mixtures of magnetite and quartz sand and
below a layer of pure magnetite. Important observations include that the simulated placer deposits did have a large effect
on the detectability of balls below the placer deposits and that homogeneous mixtures had no significant effect.
Magnetic soils can seriously hamper the performance of electromagnetic sensors for the detection of buried land mines and unexploded ordnance (UXO). Soils formed on basaltic substrates commonly have large concentrations of ferrimagnetic iron oxide minerals, which are the main cause of soil magnetic behavior. Previous work has shown that viscous remanent magnetism (VRM) in particular, which is caused by the presence of ferrimagnetic minerals of different sizes and shapes, poses a large problem for electromagnetic surveys. The causes of the variability in magnetic soil properties in general and VRM in particular are not well understood. In this paper we present the results of laboratory studies of soil magnetic properties on three Hawaiian Islands: O’ahu, Kaho’olawe, and Hawaii. The data show a strong negative correlation between mean annual precipitation and induced magnetization, and a positive correlation between mean annual precipitation and the frequency dependent magnetic behavior. Soil erosion, which reduces the thickness of the soil cover, also influences the magnetic properties.
In recent years it has become apparent that the performance of detection sensors for land mines and UXO may be seriously hampered by the magnetic behavior of soils. In tropical soils it is common to find large concentrations of iron oxide minerals, which are the predominant cause for soil magnetism. However, a wide range of factors such as parent material, environmental conditions, soil age, and drainage conditions control soil development. In order to predict whether magnetic-type iron oxide minerals are present it is important to understand the controlling factors of soil development. In this paper we present a conceptual model for predicting magnetic soil characteristics as a function of geological and environmental information. Our model is based on field observations and laboratory measurements of soils from Hawaii, Ghana, and Panama. The conceptual model will lead to the development of pedotransfer functions that quantitatively predict the occurrence and nature of magnetism in soils.
Electromagnetic sensors such as ground penetrating radar and electromagnetic induction sensors are among the most widely used methods for the detection of buried land mines and unexploded ordnance. However, the performance of these sensors depends on the dielectric properties of the soil, which in turn are related to soil properties such as texture, bulk density, and water content. To predict the performance of electromagnetic sensors it is common to estimate the soil dielectric properties using models. However, the wide variety of available models, each with its own characteristics, makes it difficult to select the appropriate one for each occasion. In this paper we present an overview of the available methods, ranging from phenomenological Cole-Cole and Debye models to volume-based dielectric mixing models, and (semi-) empirical pedotransfer functions.
In this paper we present the results of a study of some soil magnetic properties in Ghana. The soils sampled formed in different parent materials: Granites, Birimian rocks, and Voltaian sandstones. We discuss the role of environmental controls such as parent material, soil drainage, and precipitation on the magnetic properties. The main conclusion of this reconnaissance study is that the eight different soil types sampled have their own unique magnetic signature. Future research will have to confirm whether this conclusion holds for other soils in Ghana. If it does, the measurement of magnetic soil properties may become a viable complement for the investigation of soil erosion, land degeneration, and pedogenesis. The magnetic soil properties measured would probably not pose any limitations for the use of electromagnetic sensors for the detection of land mines and UXO.
The presence of magnetic iron oxides in the soil can seriously hamper the performance of electromagnetic sensors for the detection of buried land mines and unexploded ordnance (UXO). Previous work has shown that spatial variability in soil water content and texture affects the performance of ground penetrating radar and thermal sensors for land mine detection. In this paper we aim to study the spatial variability of iron oxides in tropical soils and the possible effect on electromagnetic induction sensors for buried low-metal land mine and UXO detection. We selected field sites in Panama, Hawaii, and Ghana. Along several horizontal transects in Panama and Hawaii we took closely spaced magnetic susceptibility readings using Bartington MS2D and MS2F sensors. In addition to the field measurements, we took soil samples from the selected sites for laboratory measurements of dual frequency magnetic susceptibility and textural characteristics of the material. The magnetic susceptibility values show a significant spatial variation in susceptibility and are comparable to values reported to hamper the operation of metal detectors in parts of Africa and Asia. The absolute values of susceptibility do not correlate with both frequency dependence and total iron content, which is an indication of the presence of different types of iron oxides in the studied material.
Previous modeling studies and experimental work have demonstrated that soil physical properties have a significant effect on most sensors for the detection of buried land mines. While a modeling approach allows for testing of the effects of a wide range of soil variables, most experimental work is limited to (field) soils with poorly known or controlled properties. With this in mind, we constructed a new outdoor test site with full control of soil water content and continuous monitoring of important soil properties and environmental conditions. In three wooden frames of 2 x 2 x 1 meter, filled with different soil types (sand, loam, and clay), we buried low-metal anti-tank and anti-personnel land mine simulants. We installed time domain reflectometry (for measurement of soil water content) and temperature probes at different depths above and below the land mines as well as in homogeneous soil away from the land mines. In this paper we document the features of this new test site and present results from the monitoring equipment.
In previous work we have shown that GPR signatures are affected by
soil texture and soil water content. In this contribution we will
use a three dimensional electromagnetic model and a hydrological
soil model to explore in more detail the relationships between GPR
signatures, soil physical conditions and GPR detection performance.
First, we will use the HYDRUS2D hydrological model to calculate a
soil water content distribution around a land-mine. This model has
been verified against measured soil water distributions in previous
work. Next, we will use existing pedotransfer functions (e.g. Topp,
Peplinski, Dobson, Ulaby) to convert the predicted soil water
contents around the land-mines as well as known soil textures and
bulk densities into soil parameters relevant to the electromagnetic
behaviour of the soil medium. This will enable a mapping between the
hydrological model and the electromagnetic GPR model. Using existing
and new laboratory and field measurements from the land-mine test
facilities at TNO-FEL we will make a first attempt to verify our
modelling approach for the prediction of GPR signatures in field
soils. Finally a detection algorithm is used to evaluate the GPR
detection performance with respect to changing environmental soil
Thermal signatures of buried land mines depend on a complex combination of environmental conditions, soil properties, and properties and burial depth of the land mine. Due to the complex nature of the problem most modeling and experimental efforts to understand thermal signatures of land mines have focused on the effects of one or a few variables. Of these variables, the effect of wind speed has received little attention in modeling and experimental studies. In this contribution we discuss the role of wind in the generation of thermal images and we present results of field experiments at the outdoor land mine detection test facility at New Mexico Tech. Here, several anti-tank and anti-personnel land mine simulants have been buried in sand, loam, and clay soils. During the measurements the environmental and soil conditions were continuously monitored using a fully equipped weather station and using probes for measurements of soil temperature and soil water content.
Thermal sensors hold much promise for the detection of non-metallic landmines. However, the prediction of their thermal signatures depends on a large number of factors. In this paper, an analytical solution for temperature propagation through homogeneous and layered soils is presented to predict surface temperatures as a function of soil heat flux amplitude, soil texture, soil water content, and thermal properties and burial depth of the landmine. Comparison with the numerical model HYDRUS-2D shows that the relatively simple analytical solution proposed here is reasonably accurate. The results show that an increase in soil water content has a significant effect on the thermal signature, as well as on the phase shift of the maximum temperature difference. Different soil textures have relatively little effect on the temperature at the surface. The thermal properties of the mine itself can play a significant role. It is shown that for most soils 10 cm is the maximum burial depth to produce a significant thermal signature at the surface.
Ground penetrating radar and thermal sensors hold much promise for the detection of non-metallic land mines. In previous work we have shown that the performance of ground penetrating radar strongly depends on field soil conditions such as texture, water content, and soil-water salinity since these soil parameters determine the dielectric soil properties. From soil physics and field measurements we know that the performance of thermal sensors also strongly depends on soil texture and water content. There is it critical that field soil conditions are taken into account when radar and thermal sensors are employed. The objectives of this contribution are (i) to make an inventory of readily available soil data bases world wide and (ii) to investigate how the information contained in these data bases can be used for derivation of soil dielectric and thermal properties relevant for operation of land mine sensors.