Magnetic behavior of soils can seriously hamper the performance of geophysical sensors. Currently, we have little understanding of the types of minerals responsible for the magnetic behavior, as well as their distribution in space and evolution through time. This study investigated the magnetic characteristics and mineralogy of Fe‐rich soils developed on basaltic substrate in Hawaii. We measured the spatial distribution of magnetic susceptibility (χlf) and frequency dependence (χfd%) across three test areas in a well‐developed eroded soil on Kaho'olawe and in two young soils on the Big Island of Hawaii. X‐ray diffraction spectroscopy, x‐ray fluorescence spectroscopy (XRF), chemical dissolution, thermal analysis, and temperature‐dependent magnetic studies were used to characterize soil development and mineralogy for samples from soil pits on Kaho'olawe, surface samples from all three test areas, and unweathered basalt from the Big Island of Hawaii. The measurements show a general increase in magnetic properties with increasing soil development. The XRF Fe data ranged from 13% for fresh basalt and young soils on the Big Island to 58% for material from the B horizon of Kaho'olawe soils. Dithionite‐extractable and oxalate‐extractable Fe percentages increase with soil development and correlate with χlf and χfd%, respectively. Results from the temperature‐dependent susceptibility measurements show that the high soil magnetic properties observed in geophysical surveys in Kaho'olawe are entirely due to neoformed minerals. The results of our studies have implications for the existing soil survey of Kaho'olawe and help identify methods to characterize magnetic minerals in tropical soils.

Large concentrations of magnetite in sedimentary deposits and soils with igneous parent material have been reported to affect geophysical sensor performance. We have undertaken the first systematic experimental effort to understand the effects of magnetite for ground-penetrating radar (GPR) characterization of the shallow subsurface. Laboratory experiments were conducted to study how homogeneous magnetite-sand mixtures and magnetite concentrated in layers affect the propagation behavior (velocity, attenuation) of high-frequency GPR waves and the reflection characteristics of a buried target. Important observations were that magnetite had a strong effect on signal velocity and reflection, at magnitudes comparable to what has been observed in small-scale laboratory experiments that measured electromagnetic properties of magnetite-silica mixtures. Magnetite also altered signal attenuation and affected the reflection characteristics of buried targets. Our results indicated important implications for several fields, including land mine detection, Martian exploration, engineering, and moisture mapping using satellite remote sensing and radiometers.

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.