Dual mode detectors combining metal detection and ground-penetrating radar are increasingly being used during humanitarian demining operations because of their ability to discriminate metal clutter. There are many reports in the academic literature studying metal detector and ground-penetrating radar systems individually. However, the combination of these techniques has received much less attention. This paper describes the development of a novel dual modality landmine detector, which integrates spectroscopic metal detection with ground-penetrating radar. This paper presents a feature-level sensor fusion strategy based on three features extracted from the two sensors. This paper shows how the data from the two components can be fused together to enrich the feedback to the operator. The algorithms presented in this paper are targeted at automating the location of buried, visibly obscured objects; however, the system described is also capable of collecting information which could also be used for the potential classification of such items.

Antennas are an important component in ground penetrating radar (GPR) systems. Although there has been much research reported on the design of individual antennas, there is less research reported on the design of the geometry of bi-static antennas. This paper considers the effects of key parameters in the setup of a GPR head consisting of a bi-static bow-tie pair to show the effect of these parameters on the GPR performance. The parameters investigated are the antenna separation, antenna height above the soil, and antenna input impedance. The investigation of the parameters was performed by simulation and measurements. It was found when the bi-static antennas were separated by 7 cm to 9 cm and were operated close to the soil (2 cm to 4 cm), the reflected signal from a near-surface object is relatively unaffected by height variation and object depth. An antenna input impedance of 250 Ω was chosen to feed the antennas to reduce the late-time ringing. Using these results, a new GPR system was designed and then evaluated at a test site near Benkovac, Croatia.

Microwave ultra-wideband technology has been widely adopted in instrumentation and measurement systems, including ground-penetrating radar (GPR) sensors. Baluns are essential components in these systems to feed balanced antennas from unbalanced feed cables. Baluns are typically introduced to avoid issues with return signals, asymmetrical radiation patterns and radiation from cables. In GPR systems, these issues can cause poor sensitivity due to a reduction in radiated power, blind spots due to changes in the radiation pattern and additional clutter from common mode radiation. The different balun technologies currently available exhibit a wide variation in performance characteristics such as insertion loss, reflection coefficient and phase balance, as well as physical properties such as size and manufacturability. In this study, the performance of two magnetic transformer baluns, two tapered microstrip baluns and an active balun based on high-speed amplifiers were investigated, all up to frequencies of 6 GHz. A radio frequency current probe was used to measure the common mode currents on the feed cables that occur with poor performing baluns. It was found that commercially available magnetic transformer baluns have the best phase linearity, while also having the highest insertion losses. The active balun design has the best reflection coefficient at low frequencies, while, at high frequencies, its performance is similar to the other baluns tested. It was found that the active balun had the lowest common mode current on the feed cables.

Ground Penetrating Radar (GPR) is an important non-destructive tool to detect landmines. It radiates radar pulses to probe the ground that contains a variety of media, including landmine, sand, soil, clay, water, etc. and a large number of clutter items like burrows, cracks, discarded waste, branches and roots, metal wire, and so on. The subsurface is in such a complex and unidentified condition, that it will impact the performance of the GPR system considerably. This brings a big challenge for the system developers to control and understand the GPR system, especially during the early stage of the design. Therefore, a simpler and less time-consuming simulation method for GPR than the real-field test is essential for engineers to have an overview and a clear grasp of the whole GPR system. This paper uses gprMax, an open-source software using the Finite-Difference Time-Domain (FDTD) method, to explore the parameter tradeoffs for a transmitter-receiver pair of bowtie antennas operating at different spacing and heights above the ground for target at different depths.

This paper presents a comparison between solid and resisitively loaded bowtie antennas in GPR applications for the purpose of finding buried landmines. The resistively-loaded antennas were first evaluated over dry sand in a laboratory-based experiment with a landmine surrogate. Further testing, comparing the performance of the loaded antennas to that of solid bowtie antennas, was performed over real soil, with a real landmine. Using the loaded bowtie antennas, the system got a response from the target in the indoor experiment but not in the outdoor experiment. The system did detect the buried landmine in the outdoor experiment when using the solid bowtie antennas. It is likely that the losses in the real soil were too high compared to the losses in dry sand for the system to be able to get a response using loaded antennas. The GPR system tested in this paper forms part of a dual-modality system for humanitarian demining applications.