###### Abstract  
Proof of principle of object composition identification based on inductive measurements with an atomic magnetometer has been demonstrated in highly engineered laboratory conditions. Progress in the development of portable miniaturised magnetometers has encouraged on the parallel development of the measurement technologies involving this sensor, in particular concepts that would enable operation in complex test scenarios. Here, we explore the problem of material identification in the context of measurements performed with variable distance between the object and the primary radio-frequency field source and sensor. We identify various aspects of the measurement affected by variable distance and discuss possible solutions, based on the signal phase analysis, a combination of frequency and angular signal dependencies and the implementation of a pair of excitation coils. object surveillance; magnetic induction tomography; atomic magnetometer +
Footnote †: journal: Applied Sciences  
0000-0002-4000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-00000-0000  
In one of the first realizations of an inductive measurement with an atomic magnetometer, it was demonstrated that the frequency dependence of the inductive signal could facilitate the identification of object composition [11]. This measurement was enabled by the wide **tunability** of the magnetometer's operating frequency without compromising measurement sensitivity [8]. Although the demonstration was performed with materials with negligible magnetic permeability, this scheme can be expanded, e.g., as shown in this paper, to all categories of objects that produce inductive signatures. It has been shown that the result of inductive measurements depends on the relative orientation of the object and primary field direction [12], as illustrated in Figure 1. This is a consequence of the dominating surface character of the response created by eddy currents, even at relatively low operating frequencies, and the volumetric character of the response produced through object magnetisation. The problem was addressed by limiting observation to the component of the secondary field orthogonal to the primary field direction, demonstrating the successful identification of electrically conductive and magnetically permeable objects. These measurements were made possible by the presence of an **insensitive sensor axis**, enabling easy monitoring of the rf field components orthogonal to the primary field direction [10]. Recently, we demonstrated two object identification scenarios based on implementing the atomic magnetometer in an **active feedback loop** (so-called spin maser mode) [16]. The spin maser mode brings the benefit of high measurement bandwidth and the ability to track changes in the ambient magnetic field, removing the need for field stabilisation. The results presented here were recorded in the free-running mode (external drive for primary field), with active field stabilisation. Progress in the development of inductive measurement technology with the rf atomic magnetometers is complemented by the development of important aspects of the measurement, such as; the stabilisation of the bias magnetic field that defines the magnetometer operating frequency [17]; and the sensor components, i.e., silicon wafer vapour cells [18], miniaturisation of laser systems using Vertical Cavity Surface Emitting Laser diodes [19], and sophisticated magnetic coil designs that allow miniaturisation of the measurement unit [20]. Optimisation of the individual components of the magnetometer progresses the instrument towards more practical, real-life usage as opposed to lab-based operation. This increases the importance of studies that identify the technologies measurement limits, determining its commercial viability. While monitoring the angular dependence of the inductive signal discussed in ref.