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The drillhole information from the Lontzen-Poppelsberg site has demonstrated three orebodies and has allowed the estimation of the extension of the lodes, their dip, and the location at the ground surface. The localisation of the lodes makes them excellent targets for further exploration with geophysics. This deposit is classified as a Mississippi Valley Type (MVT) deposit. It consists mainly of Pb-Zn-Fe sulphides that display contrasting values in resistivity, chargeability, density, and magnetic susceptibility, with regards to the sedimentary host rocks. The dipole-dipole direct current (DC) resistivity and induce polarization (IP) profiles have been collected and inverted to successfully delineate the Pb-Zn mineralization and the geological structures. Short-spacing EM34 electromagnetic conductivity data were collected mainly on the top of Poppelsberg East lode and have revealed a conductive body matching with the geologically modelled mineralization. Gravity profiles have been carried out perpendicularly to the lode orientation; they show a strong structural anomaly. High resolution ground magnetic data were collected over the study area, but they showed no anomaly over the ore deposits. The geophysical inversion results are complementary to the model based on drill information, and allow us to refine the delineation of the mineralization. The identification of the geophysical signatures of this deposit permits targeting new possible mineralization in the area.

19The acquisition of a full 3D survey on a large area of investigation is difficult, and from a 20 practitioner's point of view very costly. In high-resolution 3D surveys, the number of electrodes 21 increases rapidly and the total number of electrode combinations becomes very large. In this 22 paper, we propose an innovative 3D acquisition procedure based on the roll-along technique. It 23 makes use of 2D parallel lines with additional cross-line measurements. However, in order to 24 increase the number of directions represented in the data, we propose to use cross-line 25 measurements in several directions. Those cross-line measurements are based on dipole-dipole 26 configurations as commonly used in cross-borehole surveys. We illustrate the method by 27 investigating the subsurface geometry in a karstic environment for a future wind turbine project. 28We first test our methodology with a numerical benchmark using a synthetic model. Then, we given its sensitivity to electrical resistivity, where the 3D nature of the dissolution processes 72 makes the 2.5D hypothesis of the subsurface much weaker than for fault imaging for example. 73In most cases, the acquisition of a full 3D survey on a large area of investigation is difficult and, 74from a practitioner's point of view, very costly. In high-resolution 3D surveys, the number of 75 electrodes increases rapidly and the total number of electrode combinations becomes very large. 76Even with multi-channel acquisition systems, the time to acquire a complete data set is 77 prohibitive and even impossible as most systems can only handle a limited number of electrodes, 78and only through costly extensions to the main unit. In most applications, 3D surveys with a 79 substantial number of electrodes (more than 100) are not full 3D informative surveys but limit 80 the directions in which data is acquired to the two main directions and the cross-diagonal (e.g.,

a b s t r a c tThis paper investigates bedrock heterogeneity by applying three different geophysical approaches, in order to study the long-term behaviour and the interaction between closed-loop geothermal systems. The investigated site consists of four boreholes equipped with geothermal pipes on the campus of University of Liege, Belgium. The first approach includes acoustic borehole imaging, gamma-ray logging and cuttings observation and results to a detailed fracture characterisation, rock identification and layer dip angle determination. The second approach consists of measuring the thermal conductivity of cuttings at the laboratory. Study of cuttings thermal conductivity measurements can contribute to bedrock heterogeneity knowledge concerning the transition of one formation to another and the layer dipping. The third approach is based on high-resolution temperature profiles, measured during the hardening of the grouting material and the recovery phase of a Distributed Thermal Response Test. Through this approach a correlation of the temperature profiles to the geological characteristics of the surrounding bedrock is identified. The analysis of this correlation can provide information on fractured zones, alternation of different rock types and layering dipping. This latter approach can be easily applied on closed-loop geothermal systems to characterise the bedrock and investigate its heterogeneity as well as contribute to the their long-term behaviour prediction and to the optimisation of their efficiency.

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