GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 3.2 105 Conclusions. The results show that the reliability of the inverse models depends significantly on the data acquisition pattern. The decrease of sensitivity with depth is lower for those acquisition patterns characterized by a high ratio a max / n max allowing to resolve deeper targets. Regarding the array type used, the information recovery and the resolution are overall better for WS and worse for DD. Anyway, they can be improved by considering data set with higher numbers of current dipoles but at the expense of the measurement times. In fact, the decrease in sensitivity with increasing depth is less evident for those patterns characterized by a high ratio a max / n max , that can consequently more easily allow to identify deeper targets. In this context, the MG array is often preferable because it provides comparable results, with a number of current dipoles substantially lower and, consequently a very short survey time. Tests on field data have allowed a comparison, although limited to the evaluation of the data misfit RMS%. These show higher values than on synthetic data, probably caused by a higher level of noise on the field data. However, the overall trend of the RMS% error as the pattern number is very similar to that shown by synthetic data. For a confirmation of this hypothesis, further simulations will therefore have to be performed with different noise levels on data. References Advanced Geosciences, Inc.; 2009: Instruction manual for EarthImager 2D, Version 2.4.0, Resistivity and IP inversion software , 139 pp. Dahlin T. and Loke M.H.; 1998: Resolution of 2D enner resistivity imaging as assessed by numerical modeling , Journal of Applied Geophysics, 38 , 237-249. Dahlin T. and Zhou B.; 2004: A numerical comparison of 2D resistivity imaging with ten electrode arrays . Geophysical Prospecting, 52 (5), 379–398. Dahlin T. and Zhou B.; 2006: Multiple-gradient array measurements for multichannel 2D resistivity imaging . Near Surface Geophysics, 4 (2), 113–123. Fiandaca G., Martorana R., Cosentino P.L.; 2005: Use of the linear grid array in 2D resistivity tomography . Near Surface 2005 , A023 Hennig T. and Weller A.: 2005. Two dimensional object orientated focusing of multi-electrode geoelectrical measurements .Proceedings of the 11 th meeting of the EAGE Near Surface Geophysics Conference , Palermo, Italy. Loke, M.H.; 2014: RES2DMOD ver. 3.01. Rapid 2D resistivity forward modelling using the finite-difference and finite-element methods . www.geotomosoft.com. Martorana R., Fiandaca G., Casas Ponsati A. and Cosentino P.L.; 2009: Comparative tests on different multi-electrode arrays using models in near-surface geophysics . Journal of Geophysics and Engineering, 6 (1), 1-20. Olayinka A.I. and Yaramanci U.; 2000: Use of block inversion in the 2-D interpretation of apparent resistivity data and its comparison with smooth inversion . Journal of Applied Geophysics, 45 , 63-82. Oldenburg D.W. and Li. Y.; 1999: Estimating depth of investigation in dc resistivity and IP surveys . Geophysics. 64 , 403-416. Fig. 3 – Trend of the average relative model sensitivity as a function of the depth of the target. Colored zones show the areas of variation of the parameter for each array, from the lower data.