Ryan E. North

Variability of magnetic soil properties in Hawaii

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.

Mineralogy of Magnetic Soils at a UXO Remediation Site in Kaho'olawe Hawaii

Magnetic characteristics of soils can have a profound influence on electromagnetic sensors for the detection of unexploded ordnance (UXO) and may cause false alarms in the case of spatially variable concentrations. In particular, the performance of several electromagnetic sensors is hampered by viscous remanent magnetism, which is caused by the presence of ferrimagnetic iron oxide minerals of different sizes and shapes. Tropical soils formed on basaltic substrates commonly have large concentrations of iron oxide minerals. To improve detection and discrimination of UXO in these soils it is crucial to have a better understanding of the types of minerals responsible for the magnetic behavior, as well as their distribution in space. In this paper we present the results of recent field and laboratory studies of soil magnetic properties and soil mineralogy at the former Naval training range on Kaho’olawe Island, Hawaii. We discuss the role of environmental controls such as parent material, age and precipitation on the magnetic properties.

Coaxial line measurement and analysis of electromagnetic properties of soils for sensor applications

We report complex permittivity, conductivity, magnetic susceptibility, and attenuation for soils collected from a typical site in a current theater of operations. Our experimental setup consists of three network analyzers along with custombuilt sample holders and data reduction and analysis software. The sample holder has the advantage of large sample volume and a resulting higher signal to noise ratio. This system was developed to determine the electrical properties of soils over a wide frequency range from 100 Hz to 8 GHz. The lower frequencies are applicable to capacitive sensors for small shallow targets, while the higher frequencies are applicable to ground-penetrating radar (GPR) from 50 MHz to 2 GHz and beyond. S-parameter data is collected and reduced using a method, initially developed by Nicolson and Ross (1970)1, for the determination of dielectric permittivity, magnetic permeability, and loss tangent from measured Sparameter data. Experimental results are compared with site geology and mineralogy. Applications include detection of tunnels, land mines, unexploded ordinance (UXO), concrete reinforcements, and other shallow compact targets.

Development of an integrated soils laboratory for modeling and detection applications

The Geotechnical and Structures Laboratory at the US Army Corps of Engineers, Engineer Research and Development Center (ERDC) has developed a near-surface properties laboratory to provide complete characterization of soil. Data from this laboratory is being incorporated into a comprehensive database, to enhance military force projection and protection by providing physical properties for modelers and designers of imaging and detection systems. The database will allow cross-referencing of mineralogical, electromagnetic, thermal, and optical properties to predict surface and subsurface conditions. We present an example data set from recent collection efforts including FTIR in the Near-IR, MWIR, and LWIR bands, magnetic susceptibility (500 Hz to 8 GHz), and soil conductivity and complex permittivity (10 μHz to 8 GHz) measurements. X-ray data is presented along with a discussion of site geology, sample collection and preparation methods, and mineralogy. This type of data-collection effort provides useful constraint information of soil properties for use in modeling and target detection. By establishing real ranges for critical soil properties, we are able to improve algorithms to define anomalies that can indicate the presence of land mines, unexploded ordnance (UXOs), improvised explosive devices (IEDs), tunnels, and other visually obscured threats.

Comparison of two new portable magnetic susceptibility measurement systems

Magnetic soils confound both magnetometers and electromagnetic induction (EMI) sensors when these sensors are being used to detect landmines and unexploded ordnance (UXO). The amplitude of the magnetic susceptibility of the target is the problematic physical characteristic for magnetic detection; whereas the variation of magnetic susceptibility as a function of frequency, or magnetic viscosity, is the problematic physical characteristic that limits EMI sensors effectiveness for target detection. Quantifying the physical characteristics of the soils in which targets are located can potentially provide insight into new methods of detection. Two new production sensors, which measure magnetic susceptibility as a function of frequency, have been tested on paramagnetic salts and soil samples from sites that exhibit magnetic viscosity. The purpose was to document their response for comparison with other popular sensors such as the Bartington MS2 system or the Quantum Designs MPMS. One of the new sensors, the SM-100 sensor from ZH Instruments, measures magnetic susceptibility at five fixed frequencies (~400Hz - 8 kHz) and six field strengths (10-320 A/m). The MAGNASAT sensor, a recently developed tool from Queensland Magnetic Research, can measure over a wider frequency band (10 Hz to 100 kHz) at a single field strength (80 A/m). The MAGNASAT measures the both the real and complex components of the magnetic susceptibility, whereas the SM-100 only measures the real component. Both sensors are sensitive enough to measure diamagnetic materials such as water, however, which is useful in field settings.