Simulation of ERT surface-to-tunnel measurements.

Στην παρούσα εργασία με τη χρήση μίας δεξαμενής προσομοίωσης πραγματικών συνθηκών πραγματοποιήθηκαν μία σειρά πειραμάτων ώστε να μελετηθεί η εφαρμογή της διάταξης επιφανειακών-τούνελ ηλεκτροδίων. Στη δεξαμενή τοποθετήθηκε ειδικά κατασκευασμένη διάταξη για την τέλεση των πειραμάτων. Χρησιμοποιήθηκαν μίασειρά από ετερογενείς στόχους (αντιστατικούς και αγώγιμους) και η δεξαμενή πληρώθηκε με ομογενές μέσο (νερό). Για τη λήψη των μετρήσεων χρησιμοποιήθηκε πολυκάναλο όργανο. Ο αρχικός σκοπός των πειραμάτων ήταν να επιβεβαιωθεί η ορθότητα του κώδικα, ο οποίος προσμαρμόσθηκε ώστε να είναι εφικτές αυτού του είδους οι μετρήσεις. Επιπλέον εφαρμόσθηκαν καινούρια πρωτόκολλα, όπως επίσης και έγινε χρήση βέλτιστων μετρήσεων. Τέλος, μελετήθηκε η επίδραση των λανθασμένης θέσης των ηλεκτροδίων και η επίδραση του μεγέθους του τούνελ στα δεδομένα. The applicability of surface-to-tunnel electrical resistance tomography (ERT) measurements using a simulation tank for imaging subsurface targets is studied in this work. The tank was filled with water and inside of it was placed firmly a plastic construction where all targets were placed. Some of the targets that used were: a void plastic cylinder (resistive target) and metal plates (conductive target). Data were collected with a multichannel resistivity meter. The initial scope for these tests was to verify the reliability of developed inversionsoftware which was modified to cope with the specific measurements (surface-totunnel). Furthermore, during several experiments different electrode arrays (standard as well as new optimized) were validated. In our attempt to show the advantages of surface-to-tunnel measurements we analyzed the resolution for each configurationof traditional (surface) and surface-to-tunnel arrays. Studies also included testing the electrode displacement effect and also the effect of the tunnel size into the measurements

The inversion of data from very large three‐dimensional electrical resistivity tomography mobile surveys

New developments in mobile resistivity meter instrumentation have made it possible to survey large areas with dense data coverage. The mobile system usually has a limited number of electrodes attached to a cable that is pulled along behind an operator so that a large area can be covered within a short time. Such surveys can produce three‐dimensional datasets with hundreds of thousands of electrodes positions and data points. Similarly, the inverse model used to interpret the data can have several hundred thousand cells. It is impractical to model such large datasets within a reasonable time on microcomputers used by many small companies employing standard inversion techniques. We describe a model segmentation technique that subdivides the finite‐element mesh used to calculate the apparent resistivity and Jacobian matrix values into a number of smaller meshes. A fast technique that optimizes the calculation of the Jacobian matrix values for multi‐channel systems was also developed. A one‐dimensional wavelet transform method was then used to compress the storage of the Jacobian matrix, in turn reducing the computer time and memory required to solve the least‐squares optimization equation to determine the inverse model resistivity values. The new techniques reduce the calculation time and memory required by more than 80% while producing models that differ by less than 1% from that obtained using the standard inversion technique with a single mesh. We present results using a synthetic model and a field dataset that illustrates the effectiveness of the proposed techniques