## 2 Survey and Data Analysis  
The Ground-Penetrating Radar (GPR) survey consists of sending electromagnetic microwaves under the subsurface, using a frequency range between 10 MHz and 2.5 GHz, with the aim of identifying possible permittivity contrasts in the underground by transmitting high-frequency radar pulses and recording the amplitude and delay time of the reflected pulses, e.g., [37]. Permittivity, along with magnetic permeability and conductivity, are the main parameters influencing the radiation of electromagnetic waves in the underground. Ground-Penetrating Radar is a highly valuable tool for shallow geophysical exploration, in terms of spatial resolution, portability, and costs. Volcanic deposits are suitable for the use of GPR, as they are typically thin (~100 m), shallow, and compositionally homogeneous and can be electrically resistive. These features may allow for good transmission of radar energy, e.g., [38]. The low-frequency, large-depth Loza GPR system (LDGPR) is equipped with two unshielded monostatic antennas, one Tx and one Rx, placed at the mutual distance of 6 and 10 m, respectively, for generating radar pulses at the central frequencies of 100, 25, and 15 MHz, e.g., [39]. The two antennas can be moved along the profiles using a parallel and/or an in-line array (Figure 2). More specifically, our LDGPR systems consists of (i) a high-power transmitter unit of 15 kV, 50 MW; (ii) a receiving unit to record broadband pulses with direct signal digitization, without stroboscopic conversion and 120 dB dynamic energy; (iii) two 6-15 m long Wu-King antennas without cable connection and with the radio trigger placed on foils made of high-density polyethylene resistant to rubbing (Figure 2) that are resistively loaded dipoles covered by a dielectric layer that avoids the dispersion of the transmitted signal; (iv) a controller connected to the receiver unit through a coaxial shielded multipolar cable. This system allows for digitizing signals with no requirement for multiple pulse repetition and waveform storage. The capability of direct digitizing allows the Loza instrument to be moved rapidly, with no loss in the quality of measurements with respect to traditional GPR systems. Figure 2: Acquisition of GPR data by the Loza system at Ischia Island. The transmitting (Tx) antenna radiates an electromagnetic signal into the probed medium, and a receiving (Rx) antenna gathers the portion of energy reflected/backscattered by the electromagnetic anomaly structures. A 2D image of the underground is obtained by moving Tx and Rx antennas along a line and by collecting, at each measurement point, the backscattered field as a function of the signal propagation time along the survey profile. This 2D image is also referred to as radargram or a B-scan and is composed of several A-scans, with uniform steps. The radargram provides a representation of the subsurface features, wherein localized objects appear as hyperbola, and material interfaces appear as constant signals. B-scans allow one to detect the levels of sharp variations in the received signal and significantly simplify the geological interpretation. As regards GPR resolution versus the depth of investigation, there is a trade-off between GPR vertical resolution and depth penetration. Higher-frequency antennas offer better vertical resolution but can only reach shallower depths. Achieving the optimal balance between resolution and depth is crucial for successful prospections using GPR. The GPR horizontal resolution is directly related to the size of this footprint. At shallow depths, the footprint is smaller, resulting in better horizontal resolution. As the depth increases, the footprint expands, leading to a decrease in horizontal resolution, e.g., [40].