GNGTS 2019 - Atti del 38° Convegno Nazionale

622 GNGTS 2019 S essione 3.2 The MultiSource System and data acquistion. The MS system (LaBrecque et al. , 2013a) is comprised of several independent transceivers. Each one has a microprocessor, multi-channel receiver channels and a global positioning (GPS) module for timing and position information. A 400-watt transmitter, powered by a high-discharge battery, is also built-in. The transceivers are controlled, via radio-link, with a command console wired to a field laptop. The novelty of this system is its ability to transmit current simultaneously with multiple transceivers. The simultaneous injection of currents lead to two major improvements: 1. increasing the signal-to-noise ratio without the need of using high-power motor-driven generators; 2. propagating more complex current flow patterns than simple, parallel dipoles that in noisy environments (the most common conditions) lead to a more robust estimation of the subsurface resistivity distribution. These peculiar features can also be used to optimize the resolution of deep targets and allow the system to achieve higher signal levels and larger depths of penetration. The resistivity survey targeted the west Mount Toc shoulder (facing the Piave Valley) comprised of outcropping Jurassic limestone and part of the landslide body above the western segment of the sliding plane. A total of 8 MS transceivers have been deployed, in a roll-along scheme, along a 2.1 km of resistivity profile using stations and 84 different 25-m spaced electrode positions (Fig. 1). Current injection was undertaken with 1x, 2x and 4x simultaneous transmitters. Passive potential measurements were collected using 9 recording units each one capable of handling two channels. Data sampling was set to 100 Hz. Results and discussion. The MS system, due to its peculiarity collects resistivity data either in dipole-dipole mode or in pole-dipole mode. In this experiment was used just the dipole- dipole array to gain a better insight in survey design and data processing for such type of data. The idea behind the current design was to gather the transceivers in two blocks of four units and move a block at a time along the survey line. Fig. 2 - Voltage drop down (on the left) comparison of voltages (on the right) obtained using a single transmitter (blue dots) and 2 transmitters (red and green dots). The measured potentials, with a single transmitter (1x), as expected drop down rather quickly as the dipole order increases (Fig. 2 on the left). The potential values lower below 1 mV as soon as the order reaches values of 7-8. Comparison of values collected with single transmitter (x1) and multiple transmitters (x2) clearly show how the potentials remain more or less stable as the geometric factor double its value. A slight difference could be also observed in the multiple transmitter dataset when receiving forward or backward as compared to the two transmitting dipoles. Monitoring of the potentials with the passive arrays (Fig. 3) was the key to estimate the minimum of potential values that could be used in the resistivity inversion and furthermore how to deploy the units in a resistive background such as the case of the study site.