Evaluation of different electrode arrays in delineation subsurface cavities by using 2D imaging technique

Abstract

The 2D imaging survey was conducted across a known cavity, called the Um El-Githoaa cavity, and it is located in (Hit area-Western Iraq). The synthetic sequences of electrodes of various electrode arrays were generated to select the suitable array parameters such as a- spacing and n- factor to survey. 2D measurements are collected along traverse above the cavity for Dipole-dipole with an n-factor of 6, Pole-dipole with an n-factor of 8, and Wenner- Schlumberger with an n-factor of 8, while the a-spacing equals 2m for all arrays. The inverse models clearly showed that the resistivity contrast between the anomalous part of cavity and background resistivity is about 700:100 Ωm, 550:100 Ωm, and 500:100 Ωm of Dipole-dipole, Pole-dipole, and Wenner- Schlumberger arrays, respectively. Therefore, these models indicated that all electrode arrays can detect the subsurface cavity with different shape and accuracy. But, the Um El-Githoaa cavity is well defined from 2D imaging with Dipole –dipole array. Another Dipole-dipole survey with n-factor value of 8 is done along the same traverse. The interpretation data shows that the results to be rather noisy, with increasing negative observed data, as well as the location and size of Um El-Githoaa cave being made different from the actual situation. So, it is not advisable to use the value of n-factor greater than 6 especially with shallow targets for Dipole-dipole array. We concluded that 2D imaging is a useful technique and more effective for determining and mapping subsurface cavities, when taken in consideration using the suitable a-electrode spacing and n-factor for each electrode array, especially with the Dipole –dipole array which provides the best subsurface cavity imaging