Efficient measurement and data inversion strategies for large scale geoelectric surveys

Over the past two decades the geoelectric method has played an increasingly important role in a wide range of engineering and environmental applications related to the characterization of the shallow subsurface. The success of the method can be attributed to the availability of (i) fast multi-electrode data acquisition systems, (ii) efficient 2D and 3D resistivity inversion programs and (iii) recently proposed novel experimental design techniques. Remarkable developments in all these three aspects have been reported, but there is still a need for further improvements – even with state-of-the-art technology it is still a challenging task to acquire and process large-scale 3D data sets. A powerful option for speeding up geoelectric data acquisition is that of parallel measurements, whereby two electrodes act as current source and sink, another electrode is employed as a reference potential and all the remaining electrodes measure the voltage differences with respect to the reference electrode. Building on an earlier experimental system designed and constructed in the Institute of Geophysics at ETH Zürich (ETH-DCMES ) a new, smart multi-electrode acquisition system has been developed (ETH-DCMES-II ) as part of this thesis research. It allows the parallel data collection task to be performed in a superior and more efficient manner. The most important modification concerns the individual data acquisition units (“DAUs”). They were modified such that the voltage waveforms recorded can be analyzed directly on the DAUs, which avoids the time-consuming download of the voltage waveforms over the serial data bus to the central computer. To allow the data acquisition system to be employed in rugged or difficult terrains not accessible by vehicles, the original bulky current source and its power supply were replaced by a newly developed light-weight and battery-driven source. Additionally, several other technical improvements have been realized to increase the overall system robustness and flexibility, as well as to ease the field handling. Besides improving the acquisition hardware, the measurement procedures were optimized. Finally, versatile and easy-to-use controlling-user software was written, that allows nonspecialists to operate the system. With all these modifications ETH-DCMES-II is now ready to be employed in large-scale 3D surveys including roll-along strategies. A new 2.5D/3D tomographic inversion program has been developed that includes