## 1 Introduction  
Among the geophysical indirect methods for the preliminary assessment of the presence of underground structures for archaeological studies, multichannel GPR systems have been under the spottight for more than a decade [1, 2, 3]. Their peculiarity is to be composed of several radar antennas, both transmitters and receivers, placed next to each other. They, therefore, can collect a spatially dense amount of data, which easily allows for full 3D surveys, with improved logistics with respect to standard GPR single-channel systems [4, 5]. This means that very high resolution, usually of the order of few centimeters, can be achieved, making it possible to detect smaller targets [6] and making the interpolation process to more reliably produce horizontal depth-slices. Several examples of successful application of multichannel GPR systems for archaeological prospections are,therefore, available in the literature (e.g., [7; 8; 9; 10]) even in challenging and non-conventional applications (e.g., [11]). However, the data processing of multichannel GPR data can be significantly different, and more time consuming, from traditional GPR single-channel systems, given the large amount of data created [10]. Also, in some situations, the increased density of spatial sampling does not correspond to an increased quality of depth-slices. Small anomalies and wave scattering can disturb the data and reduce the visibility of more relevant targets of archaeological interest (i.e., walls or structures). Moreover, notwithstanding the benefits of multichannel systems, they are still affected by limitations intrinsic in the GPR wave propagation, particularly the data quality degradation in presence of even small quantities of clay within the investigated subsoil. Various studies on different multichannel GPR data processing approaches to exploit the full potentiality of these systems are available in recent literature. These include principal component analyses [12], studies on signal polarization and orientation [13], studies on filters for artifact reduction [14], and studies on positioning accuracy [15]. However, to the best of our knowledge, no study is specifically focused on improving multichannel GPR data quality over high-attenuating media, such as clayish soil. Also, few studies (e.g., [16; 17]) discuss appropriate configuration and measurement parameters for successful archaeological fieldwork. In this respect, profile spacing may play a key role in survey effectiveness: dense profile spacing (generally less than 0.5 m) is usually suggested. This can be easily obtainable with multichannel GPR systems. Denser spacing may, however, not always result in clearer imaging and can strongly increase investigation and processing times. As an example, a previous study [18] observed that, although many researchers suggest denser survey profiles, when mapping linear features, a coarser profile spacing can still be acceptable and effective depending on the aim of the surveys. In this context, we acquired a GPR multichannel dataset on the Greek-Roman archaeological site of Tindari (Sicily, Italy). This site is recognized to be very challenging with respect to GPR wave propagation, since high signal attenuation occurs due to the presence of highly electrically conductive clay and relevant moisture content [18]. In particular, the GPR survey, conducted in April 2023, was preceded by intense rainfalls, further compromising the data quality. The first results of the survey, obtained with standard processing approaches, were indeed not completely satisfactory and showed few of the attended targets, even where buried walls were confirmed to be present in the subsoil by previous excavations. Following the methodology of a pioneering study on GPR spatial density [16], we verified that for detecting main targets in archaeological prospection, a between-lines spacing of 25 cm is sufficient enough, and that higher spatial resolution does not provide substantial advantages.